"Method and apparatus for joining workpieces" The present invention relates to a new method of joining workpieces by means of a novel solid state bonding process called pin pressure joining and more particularly to solid state bonding of materials exhibiting different deformation behaviour characteristics and also to an apparatus to conduct such joining.

Several previously known methods have been applied to join dissimilar metal alloys, e. g. roll cladding and also friction stir welding and extrusion welding processes were tried to be adapted for this purpose, but different drawbacks are connected to these conventional joining methods.

In the roll cladding process two plates of an often dissimilar metal alloy are rolled together in a single deformation step between two rolls, with the combined deformation and pressure bringing the surfaces of the two plates so close together that subsequent diffusion between the contacting material of the two plates gives rise to a solid state bond. The obvious limitation of this joining method is that only (large) partially overlapping members, e. g. plates, can be joined and joining of side to side arranged members is not possible. Furthermore, the actually applied apparatus represents a substantial investment, and joining along more intricate joining lines is not possible. In the friction stir weld process (further called FSW) one circular rotating pin is urged in between two assembled adjacent to be joined workpieces of normally similar alloys along their joining line, creating a plasticized region in both work- pieces, their together deformation resulting in a mixed, solid state bonded region along the joining line in both workpieces. A rotating shoulder is being brought into contact with the to be joined members to add frictional heat input, aiding the plasticizing of these workpieces. Thus no heat nor deformation is generated as in conventional friction welding due to a relative motion between the workpieces to be joined. However, it is difficult and in some cases impossible to join dissimilar alloys or materials, due to differences in deformational behaviour between dissimilar alloys and metals.

In extrusion welding two separate streams of the same alloy are brought together under deformation (shear) and pressure at elevated temperatures first in the welding chamber and subsequently the opening (bearing channel) of an extrusion die so that diffusion occurs and the solid state bond between the two streams results.

Disadvantages here are that large equipment and expensive tooling is needed and the maximum size of the tooling limits the size of the members to be joined.

Different material properties between the joined to be materials, like e. g. melting point and deformation work, can severely hinder application of extrusion welding.

It is therefore an object of the present invention to provide a new joining process eliminating the present disadvantages and enabling two workpieces of either dissimilar alloys or entirely different materials to be solid state bonded along their joint line without the need for rolling or extruding them together. These and other objects of the present invention are met by provision of a novel joining/bonding method as defined in the attached patent claim 1, the preferred mode of conducting of the method according to dependent claims 2-5 and an apparatus to conduct such method according to claim 6.

The invention will now be described in details by way of examples and preferred operating modes in the following description referring to the attached drawings, Figs. 1-4, where Fig. 1 illustrates schematically the new principle of bonding of two members of dissimilar material, Figs. 2a, b show schematically a variant of the novel bonding method applying dual rotating pins, Figs. 3a, b, c illustrate schematically the novel bonding method applied on/providing lap welds, and Figs. 4a, b show a picture of a joint between steel and Al-alloy members provided by the novel bonding method.

Referring to Fig. 1 illustrating by way of an example the novel joining method, to be joined members, one made of soft material 1 and the second 2 of harder material, e. g. Al-alloy and steel, are assembled along their joining line. The rotating stir pin 3 attached to a shoulder 4 is urged into the soft alloy member adjacent to the joining line 5, but with its perimeter maintaining a small distance, t, from the edge of the hard workpiece, thus ensuring no or only very limited contact with the latter. The pin 3 is urged in so far that contact between the pin's attached shoulder (not shown in the Figure) and both workpieces is achieved for purposes of frictional heat generation to aid the plasticizing and subsequent diffusion processes.

Then the rotating pin and shoulder are traversed, with speed v, along the joining line in the soft alloy workpiece, while maintaining the pin perimeter distance t with the edge of the hard alloy workpiece. The preferential rotational direction of the pin w is against the traverse (joining) direction on the line of the joint, but in some combinations of alloys or metals and desired welding speeds the same directions can be envisaged as beneficial.

The preferable embodiment/mode of operation is to engage the softer material with the rotating pin, but in some cases it can be advantageous to engage the member with the harder material. Mechanical properties of the bond can be expected to be better when the degree of deformation of the harder material's contacting surface is the determining factor.

The novel method of providing the solid state bond between the two workpieces is now the deformation and movement of material of the softer alloy workpiece 1 around the pin in the narrow gap between the pin and the edge of the harder alloy workpiece, thus building up pressure (p2>p1) and increasing resulting deformation (shear z) with the objective to make the material of the softer alloy workpiece coming into contact with the edge of the harder alloy workpiece and matching its surface asperities to such an extent that diffusion occurs between the dissimilar materials, and thus a solid state bond is created. The gap t between the pin and edge of the harder alloy workpiece 2 is chosen such, and may under certain circumstances even be close to zero, that the resulting pressure in the gap exceeds the yield limit of the harder alloy, thus aiding the matching of the contacting surfaces, but it is not a necessity. The process may depend on a level of cleanliness of the edge of the harder alloy. The control of the width of the gap can either be position or force based. The width of the gap, t, in combination with the joining speed, rotational velocity w and diameter of pin determine pressure build-up and shear. A variant of the novel bonding method based on dual rotating pins is schematically shown in Figs. 2a, b as a top view 1a and a cross-sectional vertical view 2b taken along line A-A in Fig. 2a.

The principle differentiates in so far from principle one that a second pin 3' engages the second workpiece from the other side. This pin is also urged into the workpiece adjacent to the joining line but with its perimeter maintaining a small distance, t2, from the edge of the first workpiece, thus ensuring no or only very limited contact with the latter. The shoulder 4'is used for the same purposes as the shoulder attached to pin number one. Now both pins rotate against each other, building up pressure and increasing deformation in the narrow gap between them, made up of t+t2, thus allowing the joining lines of both workpieces to come into narrow contact while matching surface asperities, diffusion taking place thus forming a solid state bond. The choice of rotational direction is here also dependent on choice of welding speed and alloy combinations, but the most likely configuration is counter rotating pins as indicated by w1, w2 and in the Figure, thus forming small scale rolling equipment.

A variant of the novel bonding method to provide lap joining type of seams is illustrated in Figs. 3a, b as a top view 3a and a cross-sectional vertical view 3b taken along line B-B in Fig. 3a.

The principle differentiates in so far from from the above described principle one that the pin 3 penetrates the first workpiece 1 until a distance t from the second workpiece 2. The pin 3 is tilted as shown in Fig. 3b in relation to the perpendicular axis with the workpiece, in the plane between the perpendicular axis and the direction of welding. The amount of tilting depends on the resulting distance, the smaller t, the smaller the tilt angle can be chosen. The rotating, transverse moving pin will now compress material from front to back under the pin, thus creating the pressure increase resulting in deformation (shear T) with the objective to make the bottom material of the first workpiece coming into contact with the top of the second workpiece and matching its surface asperities to such an extent that diffusion occurs between the dissimilar materials, and thus a solid state bond is created. The tilt angle cc might be chosen opposite thus compressing material from the back of the pin to the front. This might aid heat input from the shoulder in front of the joint line.

A variant of the bonding method according to the present invention applying dual rotating pins 3,3' attached to their respective shoulders 4,4' is illustrated schematically in Fig. 3c. A second, tilted pin 3'engages the second workpiece from the other side. Now both pins 3,3' rotate building up pressure and increasing deformation in the narrow gap, made up of t+t2 between them, thus allowing the joining lines of both workpieces to come into narrow contact while matching surface asperities, diffusion taking place, thus forming a solid state bond. The pins 3,3' can be tilted at identical or different angles a, P.

Example Fig. 4 shows (an enlarged) picture of a novel solid state joint 7 between two materials exhibiting different deformation behaviour, workpiece 1 made of Al-alloy and workpiece 2 made of steel, respectively.

The workpieces have been joined by the novel method according to the present invention applying a rotating friction stir pin urged into workpiece 1 being a plate made of Al-alloy A6063 joined to a steel plate/member 2 of identical thickness (steel quality ST36 was applied). As shown in the pictures 4a and 4b showing in a top view and corresponding vertical cross-sectional view, respectively, the resulting joint a sound bonded seam 7 between the two different materials was achieved exhibiting an ultimate strength of 65 MPa as a result of tests conducted on the provided sample of bonded materials. Even if the novel method according to the present invention is described and illustrated by way of examples applying metal material exhibiting different characteristics, the inventive novel method is not restricted by the examples.

Other materials, e. g. thermoplastic members, can be advantageously joined applying the novel method and other rotating means than the described/illustrated rotating pins can be applied for plastizing of material.

Furthermore, in the case of special alloys it could be instrumental to plasticize the harder member. Coated workpieces/members, e. g. zinc coated steel members, can also be advantageously joined to other members.

It is also viable to conduct joining of non-deformable members, e. g. ceramics, to a plasticized material member (metal) which is presently considered to be a challenge covered normally by mechanical bonding means or gluing.

Additionally, the new joining method offers an advantage over conventional friction stir welding even when two materials of similar deformation behaviour are to be joined when plasticizing one of the members might not be feasible or desirable.

One such example is joining of dissimilar aluminium alloys, one of which has good corrosion resistance in special temper condition (AA7108 in T79). Friction stir welding leads to partial loss of this condition in the HAZ. Pin pressure joining can be applied in such a manner that the other material to be joined is engaged, thus suppressing the loss of special temper condition of the material not engaged.

Another example would be when the geometry of the second member does not allow the insertion of a rotating pin, the second member might even be non-plasticizable.

Still another advantage compared to friction stir welding is that the requirements on dimensional tolerances of the contacting surfaces of the to be joined members are less severe due to the inherent"adaption"of the plasticized material to the solid edge of the second member.