Technical field of the invention

[001] The present invention relates to a method for producing a welded joint between two welding target members being substantially made of dissimilar materials, such as plastic-based or metal-based members, and in particular to said method comprising a resistance welding step, and to a welded joint between said two target members.

Background of the invention

[002] Light-weight materials, like aluminium, aluminium alloys, most plastics, or composite materials, are becoming more and more important in a wide range of industries. For example, in the battery industry the use of lightweight materials may provide batteries and battery packs lighter in weight, while maintaining strength and electrical conductivity. In addition, in the automobile assembly, the relatively low specific density of aluminium alloys plays an important role in transitioning to the production of more efficient vehicles, lowering emissions during the lifetime of the product. Although automobiles with a car body fully constructed from aluminium alloys have reached commercial markets, in current practice, steel alloys play an important role in body-in-white production. Especially automotive parts requiring high stiffness and impact resistance are typically executed in (high strength) steel alloys.

[003] Hence, the use of multiple dissimilar materials and their combination within a wide range of applications, like for example in car body manufacturing, requires the need for robust joining methods for dissimilar materials.

[004] When reference herein is made to dissimilar joining or joining of dissimilar materials, reference may be made to the joining of welding target members substantially made of dissimilar materials, such as, for example, a first welding target member substantially consisting of iron-based material, and a second welding target member substantially made of an aluminium (alloy) based material, without being limited thereto.

[005] Resistance welding is a widely used technique to produce a joint between dissimilar materials, in particular between metal-based materials such as between steel and aluminium. Although different types of resistance welding exist, like resistance spot welding and projection welding, they all rely on applying resistive heating resulting in a metallurgical connection or joint. Indeed, electrodes (mostly copper-based electrodes) are disposed in direct contact with a stack comprising the members to be welded after which an electrical current is applied between the electrodes heating up at least a portion of the stack to form a welded joint, depending on the applied welding parameters.

[006] When performing resistance welding on target members (hereinafter also referred to as workpieces or welding target members) substantially made of similar materials (e.g. steels), sufficient heating will lead to the formation of a molten zone, also known as fusion zone, in between the members. After the welding current cycle, this molten zone, comprising material of both members, will solidify and ensure a joint between the two members.

[007] In the case of joining members substantially made of dissimilar material, for example a first welding member made of steel and a second welding member made of aluminium, two main difficulties may arise. On the one hand, differences in physical properties, such as electrical resistivity, thermal conductivity, melting temperature, etc. can prohibit the formation of a molten zone in both materials at the same time. For example, when the first material starts melting, the other material is not yet liquefied, or when the second material starts melting, the first material overheats (causing excessive expulsion) or boils, making welding impossible. On the other hand, the metallic structure of the two materials might be metallurgically incompatible due to limited (or absence of) solubility of one material (phase) in the other. This, for example, is the case when joining steel to aluminium. Multiple intermetallic compounds, typically formed in a layer-like morphology at the heated interface between the welding target members, are formed at the steel-aluminium interface due to heating during the joining process. These compounds form a layer in between the two welding members and ensure a connection between them. Nevertheless, due to the brittle behaviour of intermetallic compounds, such joints are typically characterised by their brittle failure.

[008] The most widely used process variant of resistance welding of dissimilar materials is resistance spot welding. This type of resistance welding is mostly applied to a welding stack of typically two, but possibly more, plate-like welding target members. A considerable amount of literature is available on the study of the properties (e.g. strength, fracture toughness) of the intermetallic compounds, formed when resistance welding essentially dissimilar materials. In order to gain control over the formation of the intermetallic layer, different researchers have presented methodologies altering its formation behaviour or omitting (or minimising) direct contact between steel and aluminium during the resistance spot welding process.

[009] For example, ARGHAVANI, M.R. et al. Materials & Design, Volume 102, 15 July 2016, Pages 106-114 proved that, in the case of galvanised steel, the presence of the zinc layer influences the formation of intermetallic phases or compounds at the interface between steel and aluminium. Especially at higher welding currents the effect would be beneficial, limiting the growth of the intermetallic compounds, hence resulting in a lower intermetallic layer thickness and increased strength compared to joints made in absence of a galvanised layer with identical joint size. The absence of significant changes to the joining process allows for uncomplicated implementation. However, improvements in joint properties achieved with this technique are limited and require an adaptation to the base material, i.e. one of the welding target members.

[010] Another adaption of the resistance spot welding technique to improve the brittle joint behaviour due to the intermetallic layer is related to the adding of an interlayer or transition material in between the two welding target members during the welding process, influencing or even omitting, direct contact between the steel- and aluminium- based welding target members. In order to improve the application of this interlayer or transition material during industrial application, a two-step approach has been applied wherein in a first step the interlayer is first joined to one of the welding target members. For example, LU, Y. et al Materials and Design, 165:107585, 2019, joined an intermediate material by ultrasonic welding to one of the welding target members to be joined before the resistance welding process. A variation of this methodology employs cold sprayed aluminium on the steel-based welding target members before joining it to aluminium- based welding target members, hereby omitting positioning the interlayer during the resistance welding process.

[Oil] Another known adaption of the resistance spot welding process of essentially dissimilar materials is related to the application of a cover plate between one (or both) of the welding target members and the welding electrode to influence the heat balance during welding. Although this cover plate does not directly influence the formation of an intermetallic layer in between the welding target members, it highly influences the heat balance during welding, hence formation of intermetallic compounds at the faying surface between the steel- and aluminium alloy-based welding target members. For example, QU I, R. et al., Journal of Materials Processing Technology, 209(8):4186-4193, 2009, applied this technique for joining aluminium-based welding target members to both carbon steel and stainless steel based welding target members. The experiments showed how the cover plate allowed formation of an increased aluminium nugget size, positively affecting the joint strength. It is, however, important to note that a permanent joint will be formed between the aluminium-based welding target member and the cover plate. This, in combination with the need to position the cover plate during the application of the welding technique, is a significant drawback of this adaption.

[012] Other prior art, like, for example, ZHANG, W. et al, Materials Design, 85: 461-470, 2015, aimed to improve the resistance spot welding method for joining dissimilar welding target members by modifying the geometry of the welding electrodes. In particular, ZHANG, W. et al. 2015 optimised the electrode geometry for joining a 6000-series aluminium alloy welding target member to a high strength steel welding target member, and reported strong dependency of the final joint strength on the electrode geometry. Indeed, similarly to the application of a cover plate between welding electrodes and welding target members, amending the welding electrode geometry may influence the formation behaviour of the intermetallic layer by altering the overall heat balance of the resistance welding setup, i.e. the welding electrode geometry may influence the electrode- and contact resistance, hence altering the heat generation at the interface between the welding electrode and the welding target member.

[013] An alternative applied method on the resistance spot welding process of materialbased dissimilar welding target members is resistance element welding, and variants thereof. Within this later mentioned welding technique, a dissimilar joint is achieved based on a metallurgical connection between relatively similar alloys. For example, LING Y.Z. et al., International Journal of Advanced Manufacturing Technology, 92(5-8):1923-1931, 2017, applied resistance element welding to join dual-phase steel to an aluminium alloy (in particular 6061-T6), using a S235 steel rivet. A comparison of these welded joints with the aforementioned resistance spot welded joints showed a significant increase in tensile shear resistance as well as fatigue life. For example, NIU, S. et al., Journal of Materials Processing Technology, 286(July):116830, 2020, applied resistance rivet welding, a variant of resistance element welding, having the advantage of omitting the punching step preceding resistance element welding. As such, resistance rivet welding combines features of resistance element welding and self-piercing riveting, achieving penetration of the steel rivet in the aluminium-based plate-like welding target member by resistance heating. Although resistance element welding and resistance rivet welding have both limited influence on the aesthetics of the steel-based welding target member and significantly increase the joint strength compared to the aforementioned resistance spot welding based processes, the implementation of a consumer material (i.e. rivet) is a drawback for (industrial) implementation of these welding techniques. Furthermore, resistance element welding needs an extra process step to punch through the aluminium plate, whilst resistance rivet welding still forms a significant region of brittle intermetallic phases.

[014] Aside from these resistance welding processes, mechanical joining could also be applied for joining dissimilar target members, for example, steel-based to aluminium-based joining target members. Examples of such processes are self-piercing riveting and mechanical clinching. These processes require the materials to be joined to allow sufficient plastic deformation. These processes often have the downside of significantly influencing the aesthetics of the product. Furthermore, self-piercing riveting has the downside of employing a consumer material. The application of adhesive bonding to join dissimilar materials has the disadvantage of cost of the consumer material and curing of the product, which might be suboptimal in a production line.

[015] Concluding, one may distinguish two types of optimisations for improving the properties of a resistance welding joint, for example, between a steel-based and aluminium alloy-based welding target member. On the one hand, techniques have been developed influencing the formation of the intermetallic layer - either by influencing the thermal balance during welding (cf. inter-layer, cover plate, electrode geometry) or altering the chemical composition at the faying surface (cf. galvanising, inter-layer). On the other hand, resistance welding process variants have been applied yielding a mechanical connection between the dissimilar welding target members, while metallurgically joining a third element to the steel workpiece. Overall, the aforementioned developments share one or more of the following drawbacks: an increased complexity and cost of implementation, limited improvements compared to conventional resistance spot welding (e.g. still brittle joint behaviour) and/or a negative impact on the aesthetics of the welding target member. [016] As a result of the aforementioned phenomena and the limitations of the optimisations investigated in academic literature, resistance spot welding of dissimilar welding target members, for example, steel-based welding target member to aluminium (alloy)-based welding target member, are hardly implementable in common industrial practice. Other alternative joining techniques such as mechanical clinching, self-piercing riveting or adhesive bonding also come with significant drawbacks.

[017] As an alternative to resistance spot welding, projection welding could be considered for joining, in particular welding, dissimilar metals. Whereas current concentration in resistance spot welding is determined by the geometry of the welding electrodes, this process variant uses a geometric feature protruding from an outer surface of the welding target member, wherein constriction of the welding current takes place. The latter has the advantage of increased control over heat generation and its location. As a result of this amended heat balance, the projection welding process variant is often characterised with lower welding times, compared to the resistance spot welding process. Although a vast amount of literature is available on dissimilar joining by resistance spot welding, investigating the formation and properties of joints, research on projection welding of this material combination is more scarce and, for steel-aluminium alloy joints, to the best of the author's knowledge, limited to the disclosures listed below.

[018] For example, ZVORYKINA, A. et al, Untersuchungen zur Herstellung von Stahl- Aluminium- Verbindungen durch das kombinierte Ultraschall- und Widerstandspunktschweifien, Leipzig, 2016, proposed a two-step welding procedure for steel to aluminium projection welding joints, whereby a first step consists in applying a tubular insert element to the aluminium-based welding target member by ultrasonic welding. This tubular element will act as the projection in the second projection welding step of the method. The second stage consists of projection welding this welding target member to the steel-based projection welding target member plate. Later, the method was adapted by exchanging the ultrasound welding by resistance welding, omitting the need for the former joining technique. This methodology has the advantage of limited influence on the aesthetics of the welding target members. However, the two-step welding procedure, as well as handling the insert elements could complicate industrial implementation, hence, considered as important drawbacks.

[019] ZHANG, G. et al, Journal of Manufacturing Processes, 44(June):19-27, 2019. ISSN 15266125 presented a "metallic bump assisted resistance spot welding", which may be considered as a projection welding variant whereby an AISi5 projection was applied by cold metal transfer. This method was employed for steel to aluminium joining. Both aforementioned researches, although classifiable as projection welding, mainly exploit the principles of inter layers, earlier mentioned. The final joint morphology is comparable with those of resistance spot welding of steel-aluminium joints, wherein an intermetallic layer plays the major role. Furthermore, the fact that these methods comprise two welding steps is a significant drawback.

[020] For joining sheet-like welding target members in automotive industry, employing embossed projection welding is common industrial practice. This is exemplified in the hemmed joints of car doors, wherein embossed projection welding is supplemented with adhesive bonding for sealing the inner door compartment. Developing a methodology for joining dissimilar materials using embossed projections would thus limit the changes needed to setups currently present in industry. Nevertheless, research on applying such embossed projections for joining steel to aluminium alloy material combinations is nonexistent.

[021] Hence, the issues outlined above illustrate the need in the art for a resistance welding based method for producing a welded joint between welding target members made of dissimilar materials to overcome at least part of the aforementioned drawbacks.

Summary of the invention

[022] It is an object of the present invention to provide good welding joints and methods for producing a welded joint between a first welding target member and a second welding target member, wherein the welding target members are essentially made of dissimilar materials overcoming at least part of the drawbacks listed before.

[023] It is an advantage of embodiments of the present invention that the welded joint between dissimilar materials is characterized in that they fail in a ductile mode rather than brittle.

[024] It is a further advantage of embodiments of the present invention that a mechanical connection is formed between the welding target members, wherein this mechanical connection may be formed during the resistance welding step.

[025] It is a further advantage of embodiments according to the present invention that an outer surface of the second welding target member opposed to the outer surface facing the first welding target member remains unaffected. [026] It is a further advantage of embodiments of the present invention that a welded joint may be formed between a metal-based welding target member and a non-metal based welding target, for example, a plastic, wherein the welded joint is characterized by a low temperature of plasticization.

[027] It is a further advantage of embodiments of the present invention that a welded joint may be produced between dissimilar relatively thin welding target members. Indeed, embodiments of the present invention may require low welding currents, allowing indirect or series welding configurations when using thin welding target members without overheating one of the welding target members.

[028] It is a further advantage of embodiments according to the present invention that no (additional) consumable materials are needed.

[029] It is a further advantage of embodiments according to the present invention that the method may easily be implemented in existing production lines.

[030] It is a further advantage of embodiments of the present invention that a welded joint may be formed without performing any modifications (surface, geometry) to the welding electrodes and that wear of the welding electrodes is limited.

[031] It is a further advantage of embodiments according to the present invention that a welded joint may be formed based on a good heat balance.

[032] It is a further advantage of embodiments of the present invention that the welded joint has a mechanical anchoring, possibly in combination with a metallurgical joint, hence resulting in an improved joint strength and ductility.

[033] It is a further advantage of embodiments of the present invention that the welded joint and method has limited influence on the aesthetics of the welding target members.

[034] According to a first aspect of the invention, the invention relates to a method for producing a welded joint between a first welding target member and a second welding target member within a welding region of a welding stack, wherein, when the welding region of the welding stack is subjected to resistance welding, the second welding target member reaching a softening point before the first welding target member, and wherein the first welding target member comprising a protruding portion having a first opening, and wherein the protruding portion extending away from an outer surface of the first welding target member towards the first opening, the method comprising the steps of: a) forming the welding stack by disposing the first and second welding target members together in physical contact, wherein, within the welding region of the welding stack, the first opening of the protruding portion of the first welding target member facing the second welding target member; b) heating, using resistance welding, the welding region of the welding stack such that at least a portion of the second welding target member becomes sufficiently soft to fill at least a portion of the protruding portion through the first opening of the first welding target member; and c) cooling the heated portion of the second welding target member such that a welded joint is produced between the first welding target member and the second welding target member within the welding region of the welding stack.

[035] According to a particular embodiment of the present invention, the inner diameter of the extending protruding portion converges towards the first opening.

[036] According to a particular embodiment of the present invention, the inner diameter of the extending protruding portion substantially equals the inner diameter of the first opening.

[037] According to a preferred embodiment of the present invention, the protruding portion extending along the major outer surface of the first welding target member.

[038] According to a preferred embodiment of the present invention, the protruding portion further comprising a second opening opposed to the first opening and facing away from the second welding target member within the welding region of the welding stack.

[039] According to a particular embodiment of the present invention, the first opening being a plurality of spaced apart openings.

[040] According to a preferred embodiment of the present invention, the protruding portion of the first welding target member is provided before, simultaneously or after placing the first and second welding target members in physical contact.

[041] According to a preferred embodiment of the present invention, the first and/or second opening of protruding portion is provided before, simultaneously or after the provision of the protruding portion of the first welding target member.

[042] According to a preferred embodiment of the present invention, the protruding portion and/or first opening of the first welding target member being provided by at least one of the group consisting of chemically etching, milling, laser cutting, punching, waterjet cutting, plasma cutting, electro discharge machining, turning or drilling.

[043] According to a particular embodiment of the present invention, the first and/or second welding target member comprising a plurality of layers.

[044] According to a specific embodiment of the present invention, at least two layers of the plurality of layers are made of a different material.

[045] According to a specific embodiment of the present invention, the first welding target member or a layer of the plurality of layers of the first welding target member is substantially made of an iron based material, preferably steel, and wherein the second welding target member or a layer of the plurality of layers of the second welding target member is made of an aluminium based material.

[046] According to a particular embodiment of the present invention, the first welding target member or a layer of the plurality of layers of the first welding target member is substantially made of a copper based material and wherein the second welding target member or a layer of the plurality of layers of the second welding target member is made of an aluminium based material.

[047] According to a particular embodiment of the present invention, the first welding target member or a layer of the plurality of layers of the first welding target member is substantially made of a titanium based material and wherein the second welding target member or a layer of the plurality of layers of the second welding target member is made of a magnesium based material.

[048] According to a second aspect of the invention, the invention relates to l.A welded joint between a first welding target member and a second welding target member disposed on each other defining a welding stack, and wherein, when the welding region of the welding stack is subjected to resistance welding, the second welding target member reaching a melting point before the first welding target member, the welded joint comprising, when viewed in a cross-sectional direction along the welding stack:

- a first protruding portion having a first opening defined by the first welding target member, and

- a second portion having a recessed portion receiving the first protruding portion and a second protruding portion received by the first opening and at least a part of the first protruding portion. [049] Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as 20 appropriate and not merely as explicitly set out in the claims

[050] Although there has been constant improvement, change and evolution of methods and devices in this field, the present concepts are believed to represent substantial new and novel improvements, including departures from prior practices, resulting in the provision of more efficient, stable and reliable joint and related production of this nature.

Brief description of the drawings

[051] FIG. la illustrates an isometric perspective cross-sectional view of a joint article comprising a welded joint according to embodiments of the present invention;

[052] FIG. lb illustrates an isometric perspective cross-sectional view of a first welding target member according to embodiments of the present invention;

[053] FIG.lbc illustrates an isometric cross-sectional front view of the first welding target member of FIG. lb;

[054] FIG.2a illustrates an isometric perspective view of a welding stack according to embodiments of the present invention to produce a welded joint of FIG. la;

[055] FIG.2ac illustrates an isometric cross-sectional front view of the welding stack of FIG.2a;

[056] FIG.2b illustrates an isometric perspective view of a welding stack with welding electrodes according to embodiments of the present invention to produce a welded joint of FIG. la;

[057] FIG.2bc illustrates an isometric cross-sectional front view of the welding stack with welding electrodes of FIG.2b;

[058] FIG.2c illustrates an isometric perspective view of a welding stack with welding electrodes and a welded joint of FIG. la according to embodiments of the present invention;

[059] FIG.2cc illustrates an isometric cross-sectional front view of the welding stack with welding electrodes and a welded joint of FIG.2c; [060] FIG.3a illustrates an isometric perspective cross-sectional view of a joint article comprising a welded joint according to embodiments of the present invention;

[061] FIG.3b illustrates an isometric perspective cross-sectional view of a first welding target member according to embodiments of the present invention;

[062] FIG.3bc illustrates an isometric cross-sectional front view of the first welding target member of FIG.3b;

[063] FIG.4a illustrates an isometric perspective view of a welding stack according to embodiments of the present invention to produce a welded joint of FIG.3a;

[064] FIG.4ac illustrates an isometric cross-sectional front view of the welding stack of FIG.4a;

[065] FIG.4b illustrates an isometric perspective view of a welding stack with welding electrodes according to embodiments of the present invention to produce a welded joint of FIG.3a;

[066] FIG.4bc illustrates an isometric cross-sectional front view of the welding stack with welding electrodes of FIG.4b;

[067] FIG.4c illustrates an isometric perspective view of a welding stack with welding electrodes and a welded joint of FIG.3a according to embodiments of the present invention;

[068] FIG.4cc illustrates an isometric cross-sectional front view of the welding stack with welding electrodes and a welded joint of FIG.4c;

[069] FIG.5a illustrates an isometric perspective cross-sectional view of a joint article comprising a welded joint according to embodiments of the present invention;

[070] FIG.5b illustrates an isometric perspective cross-sectional view of a first welding target member according to embodiments of the present invention;

[071] FIG.5bc illustrates an isometric cross-sectional front view of the first welding target member of FIG.5b;

[072] FIG.6a illustrates an isometric perspective view of a welding stack according to embodiments of the present invention to produce a welded joint of FIG.5a;

[073] FIG.6ac illustrates an isometric cross-sectional front view of the welding stack of FIG.6a; [074] FIG.6b illustrates an isometric perspective view of a welding stack with welding electrodes according to embodiments of the present invention to produce a welded joint of FIG.5a;

[075] FIG.6bc illustrates an isometric cross-sectional front view of the welding stack with welding electrodes of FIG.6b;

[076] FIG.6c illustrates an isometric perspective view of a welding stack with welding electrodes and a welded joint of FIG.5a according to embodiments of the present invention;

[077] FIG.6cc illustrates an isometric cross-sectional front view of the welding stack with welding electrodes and a welded joint of FIG.6c;

[078] FIG.7a illustrates an isometric perspective cross-sectional view of a joint article comprising a welded joint according to embodiments of the present invention;

[079] FIG.7b illustrates an isometric perspective cross-sectional view of a first welding target member according to embodiments of the present invention;

[080] FIG.7bc illustrates an isometric cross-sectional front view of the first welding target member of FIG.7b;

[081] FIG.8a illustrates an isometric perspective view of a welding stack according to embodiments of the present invention to produce a welded joint of FIG.7a;

[082] FIG.8ac illustrates an isometric cross-sectional front view of the welding stack of FIG.8a;

[083] FIG.8b illustrates an isometric perspective view of a welding stack with welding electrodes according to embodiments of the present invention to produce a welded joint of FIG.7a;

[084] FIG.8bc illustrates an isometric cross-sectional front view of the welding stack with welding electrodes of FIG.8b;

[085] FIG.8c illustrates an isometric perspective view of a welding stack with welding electrodes and a welded joint of FIG.7a according to embodiments of the present invention;

[086] FIG.8cc illustrates an isometric cross-sectional front view of the welding stack with welding electrodes and a welded joint of FIG.8c;

[087] FIG.9a illustrates an isometric perspective cross-sectional view of a joint article comprising a welded joint according to embodiments of the present invention; [088] FIG.9b illustrates an isometric perspective cross-sectional view of a first welding target member according to embodiments of the present invention;

[089] FIG.9bc illustrates an isometric cross-sectional front view of the first welding target member of FIG.9b;

[090] FIG.10a illustrates an isometric perspective view of a welding stack according to embodiments of the present invention to produce a welded joint of FIG.9a;

[091] FIG.lOac illustrates an isometric cross-sectional front view of the welding stack of FIG.10a;

[092] FIG.10b illustrates an isometric perspective view of a welding stack with welding electrodes according to embodiments of the present invention to produce a welded joint of FIG.9a;

[093] FIG.lObc illustrates an isometric cross-sectional front view of the welding stack with welding electrodes of FIG.10b;

[094] FIG.10c illustrates an isometric perspective view of a welding stack with welding electrodes and a welded joint of FIG.9a according to embodiments of the present invention;

[095] FIG.lOcc illustrates an isometric cross-sectional front view of the welding stack with welding electrodes and a welded joint of FIG.10c;

[096] FIG.11a illustrates an isometric perspective cross-sectional view of a joint article comprising a welded joint according to embodiments of the present invention;

[097] FIG. lib illustrates an isometric perspective cross-sectional view of a first welding target member according to embodiments of the present invention;

[098] FIG. llbc illustrates an isometric cross-sectional front view of the first welding target member of FIG. lib;

[099] FIG.12a illustrates an isometric perspective view of a welding stack according to embodiments of the present invention to produce a welded joint of FIG.11a;

[100] FIG.12ac illustrates an isometric cross-sectional front view of the welding stack of FIG.12a;

[101] FIG.12b illustrates an isometric perspective view of a welding stack with welding electrodes according to embodiments of the present invention to produce a welded joint of FIG.11a; [102] FIG.12bc illustrates an isometric cross-sectional front view of the welding stack with welding electrodes of FIG.12b;

[103] FIG.12c illustrates an isometric perspective view of a welding stack with welding electrodes and a welded joint of FIG.11a according to embodiments of the present invention;

[104] FIG.12cc illustrates an isometric cross-sectional front view of the welding stack with welding electrodes and a welded joint of FIG.12c;

[105] FIG.13 shows a cross-sectional view of a welded joint according to embodiments of the present invention;

[106] FIG.14 shows a detailed zoom of the molten zone of a welded joint according to embodiments of the present invention;

[107] FIG.15 shows a force-displacement diagram resulting from a lap shear test on a welded joint obtained according to embodiments of the present invention;

[108] FIG.16 shows a force-displacement diagram resulting from a lap shear test on a welded joint obtained from resistance spot welding according to the prior art;

[109] FIG.17 shows a cross-sectional view of the axisymmetric projection geometry according to an embodiment of the invention;

[110] FIG.18 shows a cross-sectional view of a welded joint according to embodiments of the present invention and according to the prior art;

[111] FIG.19 shows a force-displacement diagram resulting from a lap shear test on a welded joint obtained according to embodiments of the present invention and according to the prior art; and

[112] FIG.20 shows a force-displacement diagram resulting from a cross tension test on a welded joint obtained according to embodiments of the present invention and according to the prior art.

Description of illustrative embodiments

[113] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

[114] Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

[115] Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable with their antonyms under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

[116] It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. The term "comprising" therefore covers the situation where only the stated features are present and the situation where these features and one or more other features are present. Thus, the scope of the expression "a device comprising means A and B" should not be interpreted as being limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

[117] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may to a selection of embodiments. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[118] Similarly, it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

[119] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[120] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practised without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[121] As used herein, and unless otherwise specified, dissimilar materials refer to materials significantly differing in melting temperature, softening temperature, electrical conductivity, thermal conductivity and/or metallurgical properties.

[122] As used herein, and unless otherwise specified, welding target members refer to weldable workpieces, elements or components, irrespective of their shape, comprising at least one layer substantially made of a weldable material.

[123] As used herein, and unless otherwise specified, weldable materials refers to materials which may form a welded joint, for example, metal-based material like steel, aluminium, titanium or copper, as well as non-metal based materials like plastics, without being restricted thereto. [124] As used herein, and unless otherwise specified, a welding stack or stack refers to a structure wherein welding target members, in particular a first and a second welding target member, are disposed on each other along a stacking direction such that the welding target members of the stack are in physical contact.

[125] According to embodiments of the present invention, when a welding stack comprises a first and a second welding target member, the first and second welding target member may define the outer portions of the stack, i.e. limiting the welding stack. The outer portions of the stack define the outer surfaces of the stack and may be adapted to receive welding electrodes.

[126] A method for producing a welded joint between welding target members being made of dissimilar materials according to embodiments of the present invention will be hereinafter described with reference to the drawings.

[127] Referring to FIG. la, an isometric perspective cross-sectional view of a joint article 1 comprising a welded joint 2 between a first welding target member 3, e.g. a solid first workpiece as depicted in FIG. lb, and a second welding target member 4, e.g. a sheet- or plate-like member as illustrated in FIG. la. The cross-sectional view is taken along the stacking direction of the welding stack.

[128] FIG. lb and FIG.lbc illustrate, respectively, an isometric cross-sectional perspective and front view of the first welding target member 3 of FIG. la. The first welding target member 3 being a solid workpiece comprising a protruding portion 5 having a first opening 6, wherein the protruding portion 5 extending away from a major outer surface 7 of the first welding target member 3 towards the first opening 6. Referring to this example, the protruding portion 5 is cylindrical shaped having a substantially similar or equal inner diameter along the protruding direction towards the first opening 6.

[129] As it will become clear from the present disclosure and according to embodiments of the invention, the shape or geometry of the protruding portion is not limited to a cylindrical shape, but may be, longitudinal axisymmetric or any other tubular-like shape having at least a first opening, i.e. an opening or through-hole limiting the protruding portion along its extending direction away from the major outer surface and adapted to receive at least a portion of the by resistance welding heated material of a second welding target member, wherein the latter may also referred to as receiving at least a portion of the heated second welding target member. The through-hole may comprise a second opening opposed to the first opening and facing away from the second welding target member within the welding region of the welding stack.

[130] FIG.17 shows a cross-sectional view of the axisymmetric projection geometry according to an embodiment of the invention. In some embodiments, the projection diameter d _p is at least 2 mm and at most 15 mm, preferably at least 3 mm and at most 12 mm, preferably at least 4 mm and at most 10 mm, for example about 5, 6, 7, 8, or 9 mm. In some embodiments, the projection angle a is at least 0° and at most 60°, preferably at least 10° and at most 50°, preferably at least 20° and at most 40°, for example about 30°. In some embodiments, the projection wall thickness t _p is at least 0.10 mm and at most 1.00 mm, preferably at least 0.20 mm and at most 0.80 mm, preferably at least 0.30 mm and at most 0.60 mm, for example at least 0.40 mm and at most 0.50 mm, for example about 0.45 mm. In some embodiments, the projection height h _p is at least 1.0 mm and at most 2.5 mm, preferably at least 1.4 mm and at most 2.0 mm, preferably at least 1.6 mm and at most 1.8 mm, preferably about 1.7 mm. These values may be dependent on the scale of the final application.

[131] The welded joint 2 as illustrated in FIG. la may be obtained according to embodiments of the present invention of which the below-described steps are schematically illustrated by means of isometric cross-sectional perspective views, cf. FIG.2a-2c, and their associated isometric cross-sectional front views, cf. FIG.2ac-2cc.

[132] Referring to FIG.2a and FIG.2ac a welding stack 8 is formed by disposing the first welding target member 3, i.e. the solid workpiece, in physical contact with the second welding target member 4, i.e. the plate- or sheet-like welding member. Hence, the first welding target member 3 may also be referred to as the first stack member defining a first outer portion of the welding stack 8, whereas the second welding target member 4 may also be referred to as the second welding stack member defining a second outer portion of the welding stack 8. The first 3 and second 4 welding target members are disposed in physical contact such that the first opening 6 of the first welding target member 3 is facing the second welding target member 4. The second welding target member 4 reaches a softening point before the first welding target member 3 when the welding stack 8 may be subjected to resistance welding. [133] According to embodiments of the present invention, the protruding portion 5 may be provided before, simultaneously or after placing the first 3 and second welding target members 4 in physical contact.

[134] According to embodiments of the present invention, the first opening 6 of the protruding portion 5 may be provided before, simultaneously or after the provision of the protruding portion.

[135] Referring to FIG.2b and FIG.2bc, the formed welding stack 8 may be subjected to resistance welding by disposing welding electrodes 9a, 9b into contact with the welding stack 8. These welding electrodes 9a, 9b may be, without being limited thereto, copperbased welding electrodes. In this example, a first welding electrode 9a is disposed in direct contact with a first outer surface of the welding stack 8 corresponding with a major outer surface 41 of the second welding target member 4 facing away the welding stack 8. A second welding electrode 9b is disposed in direct contact with a second outer surface of the welding stack 8. This second outer surface of the welding stack 8 is opposed to the first outer surface of the welding stack 8, wherein this second outer surface of the welding stack 8 corresponds with a major outer surface of the first welding target member 3 opposed to the major outer surface of the first welding target member comprising the protruding portion 5.

[136] When in use, the welding electrodes 9a, 9b define a zone or region within the welding stack 8 subjected to resistance welding, also referred to as a welding zone or welding region. When resistance welding is applied on the welding stack 8, an electrical current applied by the welding electrodes 9a, 9b heats at least one of the welding target members 3,4, in particular the material of which the at least one welding target member 3,4 is essentially made of.

[137] The welding electrodes 9a, 9b may be adapted to clamp the welding target members 3,4 or to be disposed in electrical contact with at least one of the welding target members 3,4 in a direct, indirect, series or step configuration. The applicable welding electrodes configuration may ensure a compression force between the welding target members 3,4 at the location of the at least one protruding portion 5 and first opening 6, perpendicular to the contacting surfaces, hence according to the stacking direction. This compression force may also be provided by either one or more support structures whether or not in combination with the welding electrodes 9a, 9b. [138] Referring to FIG.2c and FIG.2cc, a current is applied between the welding electrodes 9a, 9b and an outcome of the step of heating the welding region of the welding stack 8 and filling at least a portion of the protruding portion 5 through the first opening 6 of the first welding target member 3 according to embodiments of the invention is illustrated. Due to resistance welding, and consequently the heating of at least a portion of the material of the second welding target member 4 such that it becomes sufficiently soft, at least a portion of the protruding portion may be filled. The step of passing an electric welding current through the welding region of the welding stack 8 requires defining a predetermined current density through at least one of the welding target members 3,4 such that heating at least a portion of the second welding target member 4, and hereby filling at least a portion of the at least a portion of the second welding target member 4 through the first opening 6 in the protruding portion 5 of the first welding target member 3 is accomplished.

[139] In contrast to most resistance welding applications, e.g. resistance spot welding and projection welding, known in industry, embodiments of the method according to the present invention do not necessarily require any of the welding target members to melt. Heating the second welding target member until it is sufficiently soft to fill at least a portion of the protruding portion through the first opening of the first welding target member is sufficient to form a welded joint between the welding target members.

[140] Referring to the example of the method for producing a welded joint according to embodiments of the present invention as illustrated in FIG. 2a-2c and FIG.2ac-2cc, the method further comprises a step of cooling the heated portion of the second welding target member 4 such that a welded joint 2 is produced within the welding region of the welding stack 8. This cooling step may be performed during and/or after the resistance welding, i.e. when the welding electrodes are still in contact with the welding stack and/or when the welding electrodes are not in contact anymore with the welding stack. Hence, the cooling of the heated portion may be initiated already when the current induced temperature within the welding region is sufficient to perform cooling of the heated portion and may continue once the welding electrodes are removed from the welding stack.

[141] Referring to FIG.3a, an isometric perspective cross-sectional view of another example of welded article 31 comprising a plurality of welded joints 32 according to a specific embodiment of the present invention is illustrated. [142] According to this example, the first welding target member 33, of which an isometric perspective cross-sectional view is illustrated in FIG.3b, comprises a protruding portion 35 having a first opening 36, wherein the protruding portion 35 extends away from and along a major outer surface 37 of the first welding target member 33 resulting in a longitudinal projection. This longitudinal projection may also be observed in an isometric cross-sectional front view of the first welding target member 33 as depicted in FIG.3bc.

[143] The first opening 36 may comprise a plurality of openings spaced away from each other. Although each opening of the plurality of openings face the second welding target member when methods according to embodiments of the present inventions are applied, the shape and/or inner diameters of these plurality of openings may be different. When in use, the first opening 36 or plurality of openings act as openings or through-holes adapted to receive at least a portion of heated material of a second welding target member 34 such that, after cooling, a welded joint or a plurality of welded joints 32 between dissimilar welding target members 33, 34 is or are produced.

[144] As illustrated in FIG.3a,3b,3bc, and according to embodiments of the present invention, the protruding portion 35 further comprises a second opening 361 opposed to the fist opening 36 and facing away from the second welding target member 34 within the welding stack 38. Hence, the second opening 361 may be disposed within the welding region of the welding stack. Indeed, as further explained below, the geometry and shape of the welding electrodes may be amended to bridge the second opening such that, when in use, the second opening may be disposed within the welding region.

[145] FIG.4a -4c and FIG.4ac-4cc schematically illustrate the formation of a welded joint between a first welding target member as shown in FIG.3b - 3bc and a second welding target member, e.g. a sheet- or plate-like welding target member. FIG.4a-4c and FIG.4ac- 4cc illustrate, respectively, isometric cross-sectional perspective and front views of this formation process as described in the following paragraphs.

[146] Referring to FIG.4a and FIG.4ac, a welding stack 38 is formed by disposing the first welding target member 33, i.e. the welding member having the longitudinal protruding portion 35, in physical contact with the second welding target member 34, i.e. the plate- or sheet-like welding member. When forming the welding stack 38, the plurality of openings 36 of the first welding target member 33 have to face the second welding target member 34. According to embodiments of the present invention, the protruding portion 35 may be provided before, simultaneously or after placing the first and second welding target members in physical contact. According to embodiments of the present invention, the first opening 36 may be provided before, simultaneously or after the provision of the protruding portion 35.

[147] Referring to FIG.4b and FIG.4bc, the formed welding stack 38 may be subjected to resistance welding by disposing the welding electrodes 39a, 39b, in contact with a predetermined portion of the outer surface of the welding stack 38. Referring to the present embodiment, a first welding electrode 39a is disposed in direct contact with a first outer surface of the welding stack 38 corresponding with a major outer surface 341 of the second welding target member 34 facing away from the welding stack 38. A second welding electrode 39b is disposed in direct contact with a second outer surface of the welding stack 38 opposed to the first outer surface of the welding stack, wherein this second outer surface of the welding stack corresponds with a major outer surface of the first welding target member 33 opposed to the major outer surface of the first welding target member comprising the protruding portion 35, i.e. longitudinal projection.

[148] FIG.4b - 4c and FIG.4bc - 4cc schematically illustrate that the shape and geometry of the welding electrodes 39a, 39b may be adapted to bridge the second opening 361 of the protruding portion such that, when in use, the second opening is disposed within the welding region.

[149] Next, a current may be applied between the welding electrodes 39a, 39b. An outcome of the step of heating the welding region of the welding stack 38 and filling at least a portion of the protruding portion 35 through the first opening 36, i.e. the plurality of openings according to this embodiment, of the first welding target member 33 according to embodiments of the invention, is schematically illustrated in FIG.4c and FIG.4cc. Due to resistance welding and consequently the heating of at least a portion of the material of the second welding target member 34 such that it becomes sufficiently soft, at least a portion of the protruding portion 35 may be filled through its first opening 36. The step of passing an electric welding current through the welding region of the welding stack 38 may require defining a predetermined current density through at least one of the welding target members 33, 34 such that heating at least a portion of the second welding target member 34, and thereby allowing a fill of at least a portion of the at least a portion of the second welding target member 34 through the plurality of openings 36 in the protruding portion 35 of the first welding target member 33, is obtained.

[150] Finally, the method further comprises a step of cooling the heated portion of the second welding target member 34 such that a welded article 31 comprising a welded joint 32 is produced within the welding region of the welding stack 38. As described before, this cooling step may be performed during and/or after the resistance welding, i.e. when the welding electrodes 39a, 39b are still in contact with the welding stack 38 and/or when the welding electrodes 39a, 39b are not in contact anymore with the welding stack 38. Hence, the cooling of the heated portion may be initiated already when the current induced temperature within the welding region is sufficient to perform cooling of the heated portion and may continue once the welding electrodes 39a, 39b are removed from the welding stack.

[151] Another example of a welded joint and related method according to embodiments of the present invention is schematically illustrated in FIG.5a, 5b, 5bc, FIG.6a-6c and FIG.6ac-6cc.

[152] FIG.5a,5b,5bc illustrate an isometric cross-sectional view of, respectively, a welded article 51 comprising a welded joint 52, and a first welding target member 53 according to embodiments of the present invention. The welded joint 52 is obtained between a first welding target member 53 comprising a round-shape protruding portion 55 having an inner geometry converging towards the first opening 56 of the protruding portion 55.

[153] According to embodiments of the present invention, the first welding target member 53 may comprise a plurality of similar shaped protruding portions. The protruding portion 55 further comprises a second opening 561 opposed to the first opening 56, wherein the inner diameter of the second opening is larger than the inner diameter of the first opening. The second welding target member 54 in this example is plate- or sheet-like shaped.

[154] Referring to FIG.6a-6c and FIG.6ac-6cc, isometric cross-sectional views, respectively perspective and front views, of different steps of the method to produce the welded article 51 and related welded joint 52 according to embodiments of the present invention are illustrated. In particular, as illustrated in FIG.6a and FIG.6ac, a welding stack 58 is formed by disposing the first welding target member 53, i.e. the welding target member having the converging round-shaped protruding portion 55, in physical contact with the second welding target member 54, i.e. the plate- or sheet-like welding member. Within the welding stack 58, the first opening 56 of the protruding portion 55 of the first welding target member 53 has to face the second welding target member 54.

[155] According to embodiments of the present invention, the protruding portion 55 may be provided before, simultaneously or after placing the first 53 and second 54 welding target members in physical contact. According to embodiments of the present invention, the first opening 56 may be provided before, simultaneously or after the provision of the protruding portion 55.

[156] Referring to FIG.6b and FIG.6bc, the formed welding stack 58 may be subjected to resistance welding by disposing the welding electrodes 59a, 59b into contact with the welding stack 58. According to this embodiment, a first welding electrode 59a is disposed in direct contact with a first outer surface of the welding stack 58 corresponding with a major outer surface 541 of the second welding target member facing away the welding stack. A second welding electrode 59b is disposed in direct contact with a second outer surface of the welding stack opposed to the first outer surface of the welding stack, wherein this second outer surface of the welding stack corresponds with a major outer surface of the first welding target member opposed to the major outer surface of the first welding target member 53 comprising the protruding portion.

[157] FIG.6b - 6c and FIG.6bc - 6cc schematically demonstrate that the shape and geometry of the welding electrodes 59a, 59b may be adapted to at least bridge the second opening 561 of the protruding portion such that, when in use, the second opening 561 is disposed within the welding region of the welding stack 58.

[158] Next, a current may be applied between the welding electrodes 59a, 59b. An outcome of the step of heating the welding region of the welding stack and filling at least a portion of the protruding portion 55 through the first opening 56 of the first welding target member 53 according to embodiments of the invention is schematically illustrated in FIG.6c and FIG.6cc. Due to resistance welding and consequently the heating of at least a portion of the material of the second welding target member 54 such that it becomes sufficiently soft, at least a portion of the protruding portion 55 may be filled. The step of passing an electric welding current through the welding region of the welding stack requires defining a predetermined current density through at least one of the welding target members such that heating at least a portion of the second welding target member, and hereby a filling of at least a portion of the at least a portion of the second welding target member 54 through the first opening 56 in the protruding portion 55 of the first welding target member 53, is accomplished.

[159] Finally, the method further comprises a step of cooling the heated portion of the second welding target member 54 such that a welded article 51 comprising a welded joint 52 is produced within the welding region of the welding stack 58. As described before, this cooling step may be performed during and/or after the resistance welding, i.e. when the welding electrodes 59a, 59b are still in contact with the welding stack and/or when the welding electrodes are not in contact anymore with the welding stack. Hence, the cooling of the heated portion may be initiated already when the current induced temperature within the welding region is sufficient to perform cooling of the heated portion and may continue once the welding electrodes 59a, 59b are removed from the welding stack 58.

[160] According to embodiments of the present invention, the resistance welding method step may also be applied in an indirect resistance welding configuration, resulting in a welded article 71 comprising a welded joint 72 as illustrated in, for example, FIG.7a and FIG.8cc. Although a different resistance welding configuration may be applied on the welding stack, this welded joint 72 may be essentially similar to the welded joint 52 discussed above in relation to FIG.5a and FIG.6c since the first welding target member 73 and the second welding target member 74 are similar as the one described in relation to FIG.5a and FIG.6c.

[161] The welded joint 72 may be obtained between a first welding target member 73 comprising a round-shape protruding portion 75 having an inner geometry converging towards the first opening 76 of the protruding portion 75, as illustrated in FIG.7b and 7bc.

[162] According to embodiments of the present invention, the first welding target member 73 may comprise a plurality of similar shaped protruding portions (not shown). The protruding portion 75 may further comprise a second opening 761 opposed to the first opening 76, wherein the innerdiameter of the second opening may be larger than the inner diameter of the first opening. As it may become clear from FIG.7b and FIG.7bc, the first opening 76 of the protruding section 75 is already provided. Nevertheless, according to embodiments of the present invention, the protruding portion 75 may be provided before, simultaneously or after placing the first 73 and second 74 welding target members in physical contact. According to embodiments of the present invention, the first opening 76 may be provided before, simultaneously or after the provision of the protruding portion. The second welding target member 74 in this example is plate- or sheet-like shaped.

[163] Different steps of this indirect resistance welding method are schematically illustrated in FIG.8a-8c and FIG.8ac-8cc, representing, respectively, isometric cross- sectional perspective of the welding stack and their related front views.

[164] Although the shape and geometry of the resulting welded joint may be similar to at least one of the aforementioned embodiments, the difference with the aforementioned embodiments is related to the positioning of the welding electrodes 79a, 79b in relation to the welding stack 78 as illustrated in FIG.8b, 8bc, 8c and 8cc. Instead of disposing the welding electrodes 79a, 79b at opposite outer surfaces of the welding stack, a first welding electrode 79a is disposed at a distance but aside the second welding electrode 79b such that, when a current is applied between the welding electrodes 79a, 79b, the protruding portion 75 and first opening 76 of the first welding target member 73 are disposed within the welding region. In order to ensure a compression force between the welding target members 73,74 at the location of the protruding portion 75 (also referred to as projection), perpendicular to the contacting surfaces between the first 73 and second 74 welding target members, one or more support structures 80 may be provided.

[165] A welded article 81 comprising a welded joint 82 according to a preferred embodiment of the present invention is schematically illustrated in FIG. 9a, 9b, 9bc, FIG.lOa-lOc and FIG.lOac-lOcc. These figures illustrate a welded joint and its formation starting from a T-shaped welding stack. Isometric perspective cross-sectional views of said welded article and joint and their related front view are, respectively, shown in FIG.9a, 9b, lOa-lOc, and FIG.9bc, lOac-lOcc.

[166] Referring to this embodiment, the protruding portion 85 of the first welding target member 83 extends from a minor side surface 831 resulting in a protrusion having a first opening 86 such that, when a welded joint 82 is formed within the welding stack 88, the first opening 86 faces the second welding target member 84. The protruding portion is further defined by two side openings spaced apart over a distance corresponding with the width of the minor side surface 831. Nevertheless, the protruding portion 85 has a to the first opening 86 converging inner geometry. The second welding target member 84 in this example is plate- or sheet-like shaped. [167] Different steps of the method to produce the welded article 81 comprising a welded joint 82 according to the one illustrated in FIG.9a, are illustrated in FIG.lOa-lOc and the corresponding FIG.lOac-lOcc.

[168] In particular, as illustrated in FIG.10a and FIG.lOac, a T-shaped welding stack is formed by disposing the first welding member 83, i.e. the welding member comprising the protruding portion 85, in physical contact with the second welding target member 84, i.e. the plate- or sheet-like welding member. The first opening 86 of the protruding portion 85 of the first welding target member 83 has to face the second welding target member 84 when disposing the first welding target member 83 in direct contact with the second welding target member 84 when forming the welding stack 88. Consequently, The first 83 and second 84 welding target members are defining a first and second outer portion of the welding stack 88.

[169] According to embodiments of the present invention, the protruding portion 85 may be provided before, simultaneously or after placing the first 83 and second welding target members 84 in physical contact. According to embodiments of the present invention, the first opening 86 may be provided before, simultaneously or after the provision of the protruding portion 85.

[170] Referring to FIG.10b and FIG.lObc, the formed welding stack may be subjected to resistance welding by disposing the welding electrodes into contact with the outer portions of the welding stack 88. In this example, a first welding electrode 89a is disposed in direct contact with a first outer surface of the welding stack corresponding with a major outer surface 841 of the second welding target member 84 facing away the welding stack 88. A second welding electrode 89b is disposed in direct contact with a second outer surface of the welding stack opposed to the first outer surface of the welding stack, wherein this second outer surface of the welding stack corresponds with a minor outer surface of the first welding target member opposed to the minor outer surface of the first welding target member comprising the protruding portion 85. FIG.10b - 10c and FIG.lObc - lOcc schematically demonstrate that the shape and geometry of the welding electrodes 89a, 89b are adapted to at least bridge the first opening 86 of the protruding portion 85 such that, when in use, the first opening 85 is disposed within the welding region.

[171] Next, a current may be applied between the welding electrodes 89a, 89b. An outcome of the step of heating the welding region of the welding stack 88 and filling at least a portion of the protruding portion 85 through the first opening 86 of the first welding target member 83 according to embodiments of the invention is schematically illustrated in FIG.10c and FIG.lOcc. Due to resistance welding and consequently the heating of at least a portion of the material of the second welding target member 84 such that it becomes sufficiently soft, at least a portion of the protruding portion may be filled. The step of passing an electric welding current through the welding region of the welding stack 88 requires defining a predetermined current density through at least one of the welding target members such that heating at least a portion of the second welding target member, and hereby filling at least a portion of the protruding portion 85 with at least a portion of the second welding target member 84 through the first opening 86 in the protruding portion 85 of the first welding target member, is accomplished.

[172] Finally, the method may further comprise a step of cooling the heated portion of the second welding target member 84 such that a welded article 81 comprising a welded joint 82 is produced within the welding region of the welding stack 88. As described before, this cooling step may be performed during and/or after the resistance welding, i.e. when the welding electrodes 89a, 89b are still in contact with the welding stack and/or when the welding electrodes 89a, 89b are not in contact anymore with the welding stack. Hence, the cooling of the heated portion may be initiated already when the current induced temperature within the welding region is sufficient to perform cooling of the heated portion and may continue once the welding electrodes are removed from the welding stack 88.

[173] Another example of a welded article comprising a welded joint according to a preferred embodiment of the present invention, and its related manufacturing method, are schematically illustrated in FIG.11a, lib, llbc, FIG. 12a-12c and FIG. 12ac-12cc. This example relates to the formation of a plurality of welded joints, and/or wherein the first welding target member comprising at least one protruding portion that is made of a conductive material, and wherein the second welding target member is made of a non- conductive material, or vice versa.

[174] FIG.11a depicts an isometric perspective cross-sectional view of a welded article 91 comprising a welded joint 92 between such welding target members 93,94. The first welding target member 93 comprises two protruding portions 95 comprising each a first opening 96. [175] As illustrated in FIG.llb-llbc, the first welding target member comprises a series of protruding portions. In this example, the protruding portions are similar to the aforementioned example illustrated in FIG.5a-5bc, FIG.6a-6c and FIG.6ac-6cc. Reference is made to the description of these figures for more information on the shape of the protruding portions. Nevertheless, the series of protruding portions are not limited to these geometries and shapes. According to embodiments of the present invention, the protruding portions may also be arranged according to a repeating or irregular pattern. In order to obtain a joint article, at least one, preferably all, protruding portions have to be disposed within a welded region defined by at least two welding electrodes.

[176] Referring to FIG.12a-12c, i.e. isometric perspective cross-sectional views of a welding stack 98 comprising a first 93 and a second 94 welding target member disposed on each other such that both members are in direct physical contact. The corresponding cross- sectional views are illustrated in FIG.12ac-12cc. The welding electrodes 99a may be disposed in direct contact with only the first welding target member 93 such that the protruding portions 95 and their first opening 96 are disposed within the welding region of the welding stack 98 when resistance welding may be applied. In according to provide a sufficient clamping or compressing force between the first 93 and second 94 welding target members at the location of the protruding portions (i.e. projections), preferably perpendicular to the contacting surfaces between the welding target members, a support structure 100 is disposed on the opposite side of the welding stack 98, i.e. in direct contact with the second welding target member 94.

[177] In embodiments of the present invention wherein the second welding target member 94 is not a conductive material (e.g. polymer), the welding configuration and related method steps as depicted in FIG.lla-llbc and FIG.12a-12c,12ac-12cc are still a plausible configuration. The welding current will flow through the first welding target member 93 only, heating this member. Through conduction, the second welding target member may heat up, allowing a (plastic) deformation of this member. Hence, the first welding target member 93 may penetrate the second welding target member 94 such that the at least one protruding portion 95 may at least be partly filled to form a welded article 91 comprising a welded joint 92.

[178] According to embodiments of the present invention wherein the first and second welding target member are made of a conductive material, the welding electrodes 99a may also be disposed in direct contact with the second welding target member 94 and the support structure may be disposed in direct contact with the first welding target member 93.

[179] Without being limited to the aforementioned examples, it should be stressed that the second welding target member may be made of a material such that, when resistance welding is applied, the material of the second welding target member reaching a melting point before the material of the first welding target member within the welding region. For example, without being restricted thereto, the second welding target member may consist essentially of aluminium or an aluminium alloy, whereas the first welding member may consist essentially of steel.

[180] According to other embodiments of the present invention, the second material may consist essentially of magnesium, magnesium alloys, or magnesium-based material, whereas the first welding target member may consist essentially of titanium or a titanium- based material.

[181] According to other embodiments of the present invention, the second material may consist essentially of aluminium, aluminium alloys, or aluminium-based material, whereas the first welding target member may consist essentially of copper or a copper-based material.

[182] The advantages of the present invention are achieved in a method for resistance welding two welding target members made of dissimilar materials, that includes the steps of:

(a) supplying a first welding target member comprising at least one projection comprising at least one opening;

(b) supplying a second welding target member reaching a melting point before the first welding target member during the resistance joining process;

(c) stacking the welding target members such that a contact point between the welding target members is defined by the location of the at least one protruding region;

(d) applying welding electrodes to one of the welding target members or both welding target members in a configuration such as direct, indirect, series, step, as known in resistance welding;

(e) ensuring a compression force between the welding target members at the location of the at least one projection, perpendicular to the contacting surfaces, either by welding electrodes, by a combination of one or more welding electrodes and one or more support structures, or by two or more supports;

(f) passing an electric welding current of predetermined density through at least one of the welding target members, heating at least a portion of the second welding target member, and hereby allowing flow of at least a portion of the at least a portion of the second welding target member into the opening in the projection of the first welding target member.

[183] In contrast to most resistance welding applications known in industry, the invention is not limited to the use of metals as material for the welding target members. However, at least one of the welding target members should be capable to conduct electrical current. In the case wherein one of the welding target members is not electrically conductive, the welding electrodes are applied on the conductive welding target member. In the case wherein both welding target members are electrically conductive, various welding electrode configurations known in resistance welding can be applied, such as direct, indirect, step or series.

[184] In contrast to most resistance welding applications known in industry, the presented method does not necessarily require any of the welded target members to melt. Heating the second welding target member until it is sufficiently soft to fill at least a portion of the protruding portion through the first opening of the first welding target member is sufficient to form a welded joint between the welding target members.

[185] In contrast to most resistance welding applications known in industry, the presented method does not necessarily produce a metallurgic joint between the welding target members.

[186] Heating of the second welding target member, allowing the flow of the at least a portion of the second welding target member in the opening in the projection in the first welding target member, can be either achieved by resistive heating in the second welding target member itself, resistive heating at the contacting face between the welding target members, or through conduction from heat generated in the first welding target member to the second welding target member.

[187] A weld joint of dissimilar materials produced by resistance welding according to the present invention comprises: a first component defined by the first welding target member, having at least one projection comprising at least one opening, not showing a significant molten region, a second component defined by the second welding target member, having a region that underwent a heating cycle, possibly up until melting temperature, a portion in the second component that, aided by increased temperature during the welding process, flowed into the at least one opening in the protruding region of the first welding target member.

[188] The geometry of the at least one protruding portion of the first welding target member can be longitudinal, axisymmetric or any other shape. The cross section of the protruding portion of the first welding target member is designed such that the thickness and angle with respect to the major outer surface of the first welding target member yield sufficient stiffness for the protruding portion not to fully collapse during the welding process.

[189] The cross section of the protruding portion of the first welding target member preferably has an inner geometry converging towards the top of the protruding portion. By so, the resulting joint will be characterised by an increased resistance to cross tension loading and behaving ductile when failure occurs in this loading direction.

[190] The opening or plurality of openings in the protruding region of the first welding target member has dimensions large enough to allow the at least a portion of the second welding target member to fill at least a portion of the protruding portion through the first opening of the first welding target member. The dimensions of this opening or plurality of openings will significantly influence the overall strength of the joint.

[191] The actuator force applied through the electrode(s) and/or the supporting structures on the welding region, comprising at least the hole in the protruding region of the first welding target member, should be sufficiently large to result in the at least a portion of the second welding target member to fill at least a portion of the protruding portion through the first opening of the first welding target member. The actuator force applied through the electrodes on the welding region, comprising at least the hole in the protruding region of the first welding target member, should not be as large that it results in such collapse of the protruding region in the first welding target member that it inhibits the at least a portion of the second welding target member to fill at least a portion of the protruding portion through the first opening of the first welding target member. [192] The welding current applied by the welding electrodes on at least one of the welding target members should be sufficiently large to yield softening of the at least a portion of the second welding target member whilst not be so large that it results in such collapse of the protruding region in the first welding target member that it inhibits the at least a portion of the second welding target member to fill at least a portion of the protruding portion through the first opening of the first welding target member.

[193] The presented method might be complemented with additional prior art practices, such as welding electrode materials, welding electrode geometries, cover plates and/or process tapes to influence the overall heat balance during welding.

[194] According to embodiments of the present invention, a welded joint may be provided comprising a first welding target member and a second welding target member disposed on each other defining a welding stack, and wherein, when the welding region of the welding stack is subjected to resistance welding, the second welding target member reaching a melting point before the first welding target member. The welded joint comprising, when viewed in a cross-sectional direction along the welding stack: a first protruding portion having a first opening defined by the first welding target member, and a second portion having a recessed portion receiving the first protruding portion and a second protruding portion received by the first opening and at least a part of the first protruding portion.

[195] Hence, a cross-section, along the welding stack direction, of a welded joint according to embodiments of the present invention is shown in FIG.13. This welded joint may be obtained according to embodiments of the present invention as described above, without being limited thereto. The welded joint in FIG.13 comprises a first welding target member 110 made of copper based material and a second welding target member 111 made of aluminium based material. The second welding target member did not melt in a region of significant size, the melting region comprised an extremely local region surrounding the protruding region of the first welding target member. The first welding target member has a thickness of 0.5 mm, the second welding target member has a thickness of 1mm, wherein the thickness is measured along the stacking direction. As it may be observed in FIG.13, the welded joint comprising a first protruding portion 110a having a first opening defined by the first welding target member 110, and a second portion having a recessed portion Illa receiving the first protruding portion and a second protruding portion 111b received by the first opening and at least a part of the first protruding portion 110a.

[196] A zoomed in view of a welded joint according to embodiments of the present invention is shown in FIG.14. The welded joint comprises a first welding target member made of copper material and a second welding target member made of aluminium based material. As can be seen, the molten zone is only of the order of magnitude of 10pm thick.

[197] A force-displacement diagram resulting from a lap shear test on a welded joint obtained according to embodiments of the present invention is shown in FIG. 15. The welded joint comprises a first welding target member made of a copper based material having a thickness of 0.5 mm. The welded joint further comprises a second welding target member made of an aluminium based material having a thickness of 1.5 mm. The graph depicts ductile joint behaviour. Values for force are depending on joint size, hence not given here.

[198] Another force-displacement diagram resulting from a lap shear test performed on a joint obtained from a prior art resistance spot welding process, is shown in FIG.16. The welded joint comprises a first welding target member made of copper based material and a second welding target member made of aluminium based material, with a thickness of 0.5 mm and 1.5 mm respectively. The graph depicts brittle joint behaviour. Values for force are depending on joint size, hence not given here.

[199] Welding experiments were performed on joints between a copper and aluminium alloy, representing the connection of copper conductors to an aluminium busbar. The process according to an embodiment of the invention was performed after applying an axisymmetric projection geometry, the cross section of which is schematically depicted in FIG.17, into the copper plate. Two projection geometries were used, differing in diameter. The projections had following dimensions: projection diameter of d _p = 5 mm (labelled LPW5) and d _p = 7 mm (labelled LPW7), projection angle a = 30°, projection wall thickness t _p = 0.45 mm and projection height h _p = 1.7 mm. Comparison was made to resistance spot welding as known in the art (labelled RSW).

[200] Table 1 below provides a summary of the studied welding configurations.

Table 1

[201] FIG.18 summarises macro cross sections of the studied welding conditions: top - LPW5; centre - LPW7; bottom - RSW. The cross section of the RSW joint shows a molten zone in the bulk of the aluminium workpiece, characterised by the presence of hot cracks and porosities. This zone is located slightly off-centre, reaching the common interface between the aluminium and copper workpiece. Furthermore, a thin molten zone can be observed at the surface where the electrode contacted during welding, resulting from the increased contact resistance at this location. As a result of this local melting, electrode degradation will take place rapidly. The interface between the copper and aluminium workpiece, where metallurgical bonding could be observed, is planar.

[202] The cross sections of the LPW5 and LPW7 joints show penetration of the copper projection into the aluminium, yielding a mechanical connection between the workpieces. In these samples, no molten zone is present in the bulk of the aluminium workpiece. A thin molten zone is observed contacting the penetrated projection. Some molten material is present in between the workpieces and on top of the aluminium within the joining region. High speed camera observations revealed that this molten material was expelled during welding.

[203] Table 2 summarises the lap shear and cross tension strength of the studied RSW and LPW samples. In addition, FIG.19 and FIG.20 depict the load-displacement curves for one specimen of each testing condition, approaching the mean of that sample, illustrating the difference in failure behaviour of the studied processes in this dissimilar joining configuration. FIG.19 shows a force-displacement diagram resulting from a lap shear test on a welded joint obtained according to embodiments of the present invention and according to the prior art; while FIG.20 shows a force-displacement diagram resulting from a cross tension test on a welded joint obtained according to embodiments of the present invention and according to the prior art.

Table 2 [204] From the values listed in Table 2, no statistically significant difference in lap shear strength of LPW and RSW with a nominal diameter of 7 mm can be observed, based on a 95% confidence level. Evidently, LPW with a smaller diameter, i.e, 5 mm, exhibits a lower lap shear strength. The cross tension strength of LPW, both with diameter 5 mm and 7 mm, is significantly larger compared to RSW. For joints of similar diameter, LPW showed an increase in cross tension strength by a factor of 5.

[205] The significant distinguishing characteristics of the resistance welding method of the present invention over the previously-described prior art are:

(a) the at least one projection in the first welding target member comprises at least one opening;

(b) at least a portion of the second welding target member flows through the at least one hole in the at least one protruding region during the welding process, yielding a mechanical connection between the dissimilar materials, in addition to the possible metallurgical connection between these materials;

(c) both low welding currents, in combination with higher welding times, as well as high welding currents, in combination with lower welding times, can be used, even when joining highly conductive dissimilar material combinations.

The characteristics of the presented method serve multiple advantages compared to prior art describing resistance spot welded joints and resistance projection welded joints between dissimilar material combinations. Firstly, the mechanical connection achieved by the presented method by the at least a portion of the second welding target member filling the at least a portion of the protrusion of the first welding target member through the opening in that protrusion allows material combinations to be robustly joined that would be characterised by brittle joint behaviour due to the presence of brittle intermetallic compounds without the presence of a mechanical joint when welded by conventional resistance welding processes. In conventional resistance welding processes, this brittle joint behaviour is sometimes avoided by producing exceedingly large joints. This way, the overall workpiece will fail outside the joining region. However, producing such large joints is not possible or not desirable in many cases, due to e.g. limited plate thickness, welding electrode degradation, conductivity of the base materials, damage to the workpiece. Secondly, and specifically in combination with the first advantage of having an opening in the protruding region, the presented method facilitates resistance welding of the welding target members without the necessity of heating any of the welding target members to its melting point. Limited heating of the welding target members results in limited influence of their initial mechanical properties compared to prior art describing resistance welding processes for joining dissimilar materials, whereby at least one, but often both, of the welding target members is partially heated to a temperature above the melting point. Furthermore, the fact that the presented method does not necessarily require any of the dissimilar materials to melt is an advantage for the energetic efficiency of the welding process, especially in the case of joining highly conductive materials.

Thirdly, due to the combined geometry of the protruding portion and the opening in the protruding portion, the method retains its advantage of possessing a mechanical lock between the welding target members when loaded in cross tension direction. This in contrast to earlier efforts of extending metallurgical bonds formed during resistance welding processes with a mechanical lock.

Fourthly, as an extension to the second advantage, as the method allows applying low welding currents, even for resistance welding highly conductive dissimilar material combinations, such as copper to aluminium alloys, the method is capable to be performed in welding configurations such as indirect welding, even in cases wherein the welding target members have a limited cross sectional area, without the risk of overheating any of the welding target members.

Fifthly, as the presented method strongly depends on the mechanical connection created between the welding target members, in addition to the possible presence of e.g. a metallurgical joint, application of the method is not limited to metals. It is known that resistance welding processes can be applied between metals and non-metals, e.g. polymers, when an adapted electrode configuration is used, e.g. series welding, indirect welding. It is an advantage of the presented method that it can yield a mechanical joint between metals and non-metals, e.g. polymers, by a resistance welding process. In a case wherein, for example, the first welding target member is the metal, the second welding target member is the polymer, and the electrodes contact the first welding target member in a series or indirect configuration. In this case, the heat is generated by resistive heating in the first welding target member and conducted to heat up the second welding target member.

Similarly, for example, the method can be applied to join ceramics to metals, whereby the ceramic is the first welding target member, the metal is the second welding target member, and the electrodes contact the second welding target member in a series or indirect configuration.

Sixthly, as the welding current is constricted in a protruding portion in the first welding target member during resistance welding, the electrode surface contacting the welding target members is not determining the current constriction in the welding target members. Hence, large electrode surfaces can be used contacting the welding target members. Consequently, the influence of the electrode contact pressure on the surface of the welding target members is negligible, resulting in the absence of any aesthetical influence on the face of the second welding target member opposite of the face of the second welding target member contacting the protruding portion of the first welding target member during resistance welding.

Seventhly, the presented method does not use a consumer material, compared to resistance welding methods in the prior art yielding a mechanical joint between the welding target members (e.g. resistance element welding).