SCHÜPBACH, Urs (Aazheimerhof 104, Neuhausen, CH-8212, CH)
KEMPA, Stefan (Rheingutstrasse 22, Feuerthalen, CH-8245, CH)
SCHÜPBACH, Urs (Aazheimerhof 104, Neuhausen, CH-8212, CH)
| CLAIMS 1. A process for joining a first member (1) comprising an aluminium alloy to a second member (2) comprising a titanium alloy and having at least one edge with a thickness e, comprising the steps of (i) chamfering said edge of said second member into a tapered truncated shape having on a first side a first tapering angle αl, on a second side a second tapering angle α2 and a minimum thickness t, wherein αl and α2 are greater than or equal to zero, the sum of αl and α2 is between 10° and 50° and t is between 0.05 e and 0.3 e, (ii) placing said first member and said chamfered edge of said second member (21) in an abutting relationship defining a geometry to be weld-brazed (3), (iii) heating the surface areas of said members adjacent the abutment to a temperature above the melting temperature of said aluminium alloy and below the melting temperature of said titanium alloy, in the presence of an inert gas (5) and of a filler metal (4) to obtain a weld-brazed joint. 2. A process according to claim 1 wherein the sum αl + α2 is between 20° and 40° and/or t is between 0,1 e and 0,2 e. 3. A process according to claim 1 or claim 2 wherein said geometry is selected in the group comprising a bevel butt with broad root face, a V butt with broad root face, a J- butt, a U-butt and combination thereof. 4. A process according to claim 3 wherein said geometry is a single-bevel butt with broad root face having an angle δ and a depth d, wherein said angle δ is between 40° and 80 ° and preferably between 50° and 70°, and said depth d is between 70% and 95% and preferably between 80% and 90% of said second member thickness. 5. A process according to claim 3 wherein said geometry is a double-bevel butt with broad root face having a first groove on one side with a first angle δl and a first depth dl and a second groove on the other side with a second angle δ2 and a second depth d2 and wherein said first and second angle δl and δ2 are between 40° and 80 ° and preferably between 50° and 70°, and wherein the sum of said first depth dl and said second depth d2 is between 70% and 95% and preferably between 80% and 90% of said second member thickness. 6. A process according to claim 3, wherein said geometry is a single- V butt with broad root face having an angle δ and a depth d and wherein said angle δ is between 100° and 140 ° and preferably between 110° and 130°, and wherein said depth d is between 70% and 95% and preferably between 80% and 90% of said second member thickness. 7. A process according to claim 3, wherein said geometry is a V- bevel butt with broad root face having a first V groove on one side with a first angle δl and a first depth dl and a second bevel groove on the other side with a second angle δ2 and a second depth d2, wherein said first angle δl is between 100° and 140 ° and preferably between 110° and 130°, wherein said second angle δ2 is between 40° and 80 ° and preferably between 50° and 70°, and wherein the sum of said first depth and said second depth is between 70% and 95% and preferably between 80% and 90% of said second member thickness. 8. A process according to any of claims 1 to 7 wherein said first and second members are selected from the group consisting of an extruded profile, a sheet and a plate. 9. A process according to any of claims 1 to 8 wherein said first and second members are fixed with a clamping device comprising means for maintaining a back pressure of said inert gas above atmospheric pressure, preferably higher than 110 kPa and wherein the finished weld-brazed assembly is obtained after a single welding run. 10. A process according to any of claims 1 to 9 wherein said aluminium alloy is a fusion weldable alloy from 2XXX, 5XXX, 6XXX or 7XXX alloy families 11. . A process according to claim 10 wherein said aluminium alloy is heat treatable. 12. A process according to claim 11 wherein said first member is in T3X or T4X temper during step and (iii), comprising a step following step (iii) (iv) aging said first member to a T6X or T8X temper. 13. A process according to any of claims 1 to 12 wherein heating is obtained from arc welding with a consumable electrode. 14. A process according to claim 9 wherein said clamping device (6) comprises an upper face (66) adapted to press against said members to be weld-brazed together, a channel (63), means (641), (642), (643) for tightening said members onto said upper face in order to obtain a substantially leak free pipe having said channel and members for walls, means (61) for introducing a gas (52) into said pipe and means (62) for obtaining a pressure of said gas in said pipe above atmospheric pressure so that said pressure forces the aluminium alloy melt onto said second member during weld-brazing. 15. A weld-brazed assembly obtainable according to the process of claim 10 comprising a first extruded member (1) comprising a fusion weldable alloy from 2XXX, 5XXX, 6XXX or 7XXX alloy families and a second extruded member (2) comprising a titanium alloy and having at least one edge with a thickness e and a tapered truncated shape having on a first side a first tapering angle αl, on a second side a second tapering angle α2 and a minimum thickness t, wherein αl and α2 are greater than or equal to zero, the sum of αl and α2 is between 10° and 50° and t is between 0.05 e and 0.3 e. 16. A weld-brazed assembly according to claim 15 wherein the ultimate tensile strength of said assembly is higher than 260 MPa and preferably higher than 280 MPa. 17. Use of a weld-brazed assembly according to claims 15 or 16 for the fabrication of structural elements for the aerospace industry. 18. An aircraft seat track comprising a weld-brazed assembly according to claims 15 or |
Field of the invention
The present invention relates generally to a joining process of aluminium alloys to titanium alloys, and in particular, to such process useful in the aerospace industry.
State of the art
It is generally known that when manufacturing semi-finished products and structural elements for aeronautical construction, certain required properties generally cannot be optimized at the same time independently of one another. The monolithic metallic structural elements with variable properties in space are thus very much in demand in the existing context in the aeronautical industry. Structural elements are subjected to a wide variety of contradictory constraints that require particular choices about materials and working conditions that can lead to unsatisfactory compromises. US 2005/156095 patent application explains that for manufacturing seat mounting rails of aircraft, it is advantageous to use a material highly resistant to corrosion such as titanium alloys. However, titanium alloys are more expensive and have a higher density than aluminium alloys, which is not advantageous with regard to the constant need for cost and weight reductions in the manufacture of commercial aircrafts. It is proposed to make a seat mounting rail with a lower section made of a first material, such as high strength aluminium alloy and an upper section made of a second material different from the first material, such as a titanium alloy. The first and second materials are interconnected by a homogeneous metallurgical interconnection or bonding.
Among welding techniques, two main families may be distinguished. In fusion welding processes, such as resistance spot welding, flash butt welding, laser welding, arc welding electron-beam welding, the weld is made above the melting point, in the liquid phase. In solid state welding such as friction welding, friction stir welding, or diffusion welding, the weld is made below the melting point, in the solid phase. Diffusion welding of titanium and aluminium has been reported in "Properties of diffusion welded hybrid joints titanium/aluminium, J. Wilden, JP Bergmann, S. Herz, Proceedings of the 3 rd International Brazing and Soldering Conference, April 24-26, 2006, Crowne Plaza Riverwalk Hotel, San Antonio, Texas, USA, pp338-343)". However, the strength of the assembly obtained is lower than 100 MPa. Regarding fusion welding techniques, two options may be considered in order to weld an aluminium alloys to a titanium alloy : a first option is to weld at a temperature above the melting temperature of the titanium alloy in order to have fusion of both the aluminium and the titanium alloys and a second option is to weld at a temperature above the melting temperature of the aluminium alloy but below the melting temperature of the titanium alloy, this later case will be referred to herein as "weld-brazing". The patent US 4,486,647 illustrates the first option: enough welding energy is provided in order to melt the aluminium and the titanium alloys at the melt boundary. However, when the melt solidifies, titanium-aluminium compounds are produced in large quantities, resulting in a poor mechanical strength of the joint, lower than about 100 MPa. The first option has also been reported in "Laser processing of aluminum-titanium tailored blanks, M. Kreimeyer, F. Wagner, F. Vollersten, Optics and Lasers in Engineering 43 (2005) 1021-1035". In this article a process is reported wherein the joining is achieved by melting the titanium base metal whilst heating the aluminium base metal through conduction. However, it appears again that a limited strength, around 200 MPa in this case, is obtained. The patent US 2,761,047 provides weld brazing conditions in order to join aluminium and aluminium alloys to titanium and titanium alloys with an inert gaseous arc torch. The process disclosed comprises a cleaning step which is said to be best accomplished when the torch has a non -consumable electrode and is of the ultra-high frequency alternating current type. Laser weld brazing of aluminum and titanium without filler metal is also reported in "Investigation of Laser-Beam Joined Titanium-Aluminum Hybrid Structures, Applied Production Technology APT'07, Bremen, September 17-19, 2007". Mechanical strength is improved compared to the first option, however it is still not higher than 242 MPa for a weld between a TiA16V4 alloy and a 6056 alloy in the T4 condition, aged after welding to the T6 condition. The reported joining speed at the conference was around 0.2m/mn and leads to a wide heat affected zone of around 20 mm.
Weld-brazing aluminium to titanium has proven difficult, the strength of the joint may be increased and the process output improved. There is a need for an improved method capable of weld-brazing aluminium alloy parts to titanium alloy parts, with a high output capable of providing high strength welding joints.
Object of the invention A first object of the invention is a process for joining a first member (1) comprising an aluminium alloy to a second member (2) comprising a titanium alloy and having at least one edge with a thickness e, comprising the steps of (i) chamfering said edge of said second member into a tapered truncated shape having on a first side a first tapering angle αl, on a second side a second tapering angle α2 and a minimum thickness t, wherein αl and α2 are greater than or equal to zero, the sum of αl and α2 is between 10° and 50° and t is between 0.05 e and 0.3 e, (ii) placing said first member and said chamfered edge of said second member (21) in an abutting relationship defining a geometry to be weld-brazed (3), (iii) heating the surface areas of said members adjacent the abutment to a temperature above the melting temperature of said aluminium alloy and below the melting temperature of said titanium alloy, in the presence of an inert gas (5) and of a filler metal (4) to obtain a weld-brazed joint. Another object of the invention is a weld-brazed assembly obtainable according to the process of the invention comprising a first extruded member (1) comprising a fusion weldable alloy from 2XXX, 6XXX or 7XXX alloy families and a second extruded member (2) comprising a titanium alloy and having at least one edge with a thickness e and a tapered truncated shape having on a first side a first tapering angle αl, on a second side a second tapering angle α2 and a minimum thickness t, wherein αl and α2 are greater than or equal to zero, the sum of αl and α2 is between 10° and 50° and t is between 0.05 e and 0.3 e.
Other objects of the invention include the use of a weld-brazed assembly of the invention for the fabrication of structural elements for the aerospace industry and aircraft seat tracks comprising a weld-brazed assembly according to the invention. Description of the figures
Figure 1 shows grooves defined by the various joint geometries and characterized by an angle δ and a depth d.
Figure 2 shows an embodiment of the invention. Figure 3 is an example of a specific shape of the chamfered edge according to the invention.
Figure 4 is an example of a specific shape of the chamfered edge according to the invention.
Figure 5 shows various abutting relationships of first and second members according to the invention.
Figure 6 shows two embodiments of extruded section weld-brazed according to the invention.
Figure 7 shows a clamping device according to the invention
Figure 8a and 8b show images of weld-brazed joints for trials number 1 and 4, respectively.
Figure 9 shows a hardness profile for trial number 1 and 2.
Detailed description of the invention
Unless otherwise indicated, all the indications relating to the chemical composition of the alloys are expressed as a mass percentage by weight based on the total weight of the alloy. Alloy designation is in accordance with the regulations of The Aluminium Association, known to those skilled in the art. A designation 6X56 includes any alloy having 6 as a first digit and 56 as third and fourth digits, such as for example 6056 and 6156. Titanium alloys are designated herein by using the chemical symbol for titanium, Ti, followed by the percentage number(s) and the chemical symbols(s) of the alloying element(s). For example, Ti-5Al-2.5Sn indicates that 5 weight percent aluminum and 2.5 percent tin alloying elements are present in the titanium alloy. The tempers of aluminium alloy products are laid down in European standard EN 515. Within the present description, T3X means any temper having 3 as a first digit, including for example T3, T39, T351 or T3511, mutatis mutandis for T4X, T6X and T8X. Unless otherwise indicated, the static mechanical characteristics, in other words the ultimate tensile strength (UTS, also designated as Rm), the tensile yield strength (TYS, also designated as YS or Rp0.2), the elongation at fracture A and the elongation at necking Ag, are determined by a tensile test according to standard EN 10002-1. Tensile tests on weld-brazed joints are determined according to EN 895: "Destructive tests on welds in metallic materials - Transverse tensile test". Unless otherwise indicated, the definitions given in the European Standard EN 12258-1 apply. The term "sheet" means a rolled product not exceeding about 6 mm in thickness. The term "plate" includes "medium plate" which is a rolled product from about 6 mm to about 30 mm in thickness, and "thick plate" which is a rolled product typically above about 30 mm in thickness. Joint geometries defined for welded joints in standard ISO 2553: 1992(E) are used herein to describe the preparation of edges to be welded or brazed. Grooves defined by the various joint geometries are characterized by an angle δ and a depth d, as illustrated in Figure 1. Thicknesses of extruded products are defined according to standard EN2066. According to the present invention, the process for joining a first member (1) comprising an aluminium alloy to a second member (2) comprising a titanium alloy and having at least one edge with a thickness e, comprises the steps of (i) chamfering said edge of said second member into a tapered truncated shape having on a first side a first tapering angle αl between the bevelled portion and the surface of said first side, on a second side a second tapering angle α2 1 between the bevelled portion and the surface of said second side and a minimum thickness t wherein the sum of αl and α2 is between 10° and 50° and t is between 0,05 e and 0,3 e, (ii) placing said first member and said chamfered edge of said second member (21) in an abutting relationship defining a geometry to be weld-brazed (3),
(iii) heating the surface of said members to a temperature above the melting temperature of said aluminium alloy and below the melting temperature of said titanium alloy, in the presence of an inert gas (5) and of a filler metal (4) to obtain a weld-brazed joint, αl and α2 are greater than or equal to zero. Figure 2 shows an embodiment of the invention wherein a first member (1) comprising an aluminium alloy is placed in an abutting relationship with a second member (2) having one edge chamfered into a tapered truncated shape (21). The first and second members are secured with a schematically represented clamping device (6). In this embodiment, the geometry of the abutting relationship (3) is a double-bevel butt with broad root face. A MIG torch (7) having a consumable filler wire (4) as electrode emits an inert gas which envelops the area of joining and provides the atmosphere (5) of inert gas, preventing oxidation of the surface areas adjacent to the abutting parts of the first and second members. The back portion of the weld (i.e, the portion of the weld located on the side opposite to the MIG torch) is also protected of oxidation by the inert gas (51). Means for providing the consumable electrode (8) are also present in this embodiment. The surface of the members is heated to a temperature above the melting temperature of the aluminium alloy and below the melting temperature of the titanium alloy to obtain a weld-brazed joint.
According to the present invention the chamfered edge of the second member comprising a titanium alloy must have a specific shape. Figure 3 and Figure 4 are two examples of specific shapes of the chamfered edge according to the invention. The second member has at least one edge with a thickness e. Typically, said second member is an extruded profile or a sheet although any edge with a plane-parallel shape can be of use. Preferably the first and second members are selected from the group consisting of an extruded profile, a sheet and a plate. As shown by Figures 3 and 4, the edge with a plane-parallel shape of thickness e has on a first side a first tapering angle αl between the bevelled portion and the surface said first side, on a second side a second tapering angle α2 between the bevelled portion and the surface said second side, and a minimum thickness t wherein the sum of αl and α2 is between 10° and 50° and t is between 0.05 e and 0.3 e. The tapered truncated shape of the invention provides a significant strength improvement of the welded joint and an improved process output. If the sum of αl and α2 is less than 10°, the tapered shape is too sharp compared to the edge thickness and the tapered face may not be entirely covered by the weld, which may cause loss of strength of the welded joint. If the sum of αl and α2 is more than 50°, the tapered shape is not sharp enough compared to regular square butt geometry, and it does not provide a significant strength improvement. If t is lower than 0.05 e, the truncated shape tip is too sharp and it may melt during weld-brazing, which should be avoided. If t is higher than 0.3 e, the truncated shape tip is not sharp enough compared to regular square butt geometry, and it does not provide a significant strength improvement. Preferably the sum αl + α2 is between 20° and 40° and/or t is between 0.1 e and 0.2 e. Although the present inventors are not bound to any specific theory, they believe that the improved strength of the weld-brazed joint according to the present invention is related to the increase of contact surface between aluminium and titanium alloys and/or to the improved flow of the melted alloy aluminium on the titanium alloy surface and/or to the absence of sharp contact angles between aluminium and titanium alloys, in relation to the tapered truncated shape. The tapered truncated shape is selected even for an edge thickness of less than 2,5 mm contrarily to the usual purpose of chamfered edges which is to reduce the thickness of the weld-brazing area.
The first member and the chamfered edge of said second member (21) are then placed in an abutting relationship defining a geometry to be weld-brazed. The abutting relationship comprises geometries wherein a limited joint spacing is left between the members to be weld brazed, typically less than 10% of the second member edge thickness e. Preferably, said geometry is selected in the group comprising a bevel butt with broad root face, a V butt with broad root face, a J- butt, a U-butt and combination thereof. By combination it is meant that, for example, a V butt with broad root face may be selected on one side whereas a bevel butt with broad root face is selected on the other side, providing a V-bevel butt with broad root face geometry.
In an embodiment of the invention, said geometry is a single-bevel butt with broad root face having an angle δ and a depth d, said angle δ being between 40° and 80 ° and preferably between 50° and 70°, and said depth d being between 70% and 95% and preferably between 80% and 90% of said second member thickness.. This embodiment is illustrated by Figure 5c.
In a preferred embodiment of the invention said geometry is a double-bevel butt with broad root face having a first groove on one side with a first angle δl and a first depth dl and a second groove on the other side with a second angle δ2 and a second depth d2 and wherein said first and second angle δl and δ2 are between 40° and 80 ° and preferably between 50° and 70°, and wherein the sum of said first depth dl and said second depth d2 is between 70% and 95% and preferably between 80% and 90% of said second member thickness. This embodiment is illustrated by Figures 5a, 5b, 5d and 5f. In another embodiment of the invention, said geometry is a single-V butt with broad root face having an angle δ and a depth d and wherein said angle δ is between 100° and 140 ° and preferably between 110° and 130°, and wherein said depth d is between 70% and 95% and preferably between 80% and 90% of said second member thickness. In yet another embodiment of the invention, said geometry is a V- bevel butt with broad root face having a first V groove on one side with a first angle δl and a first depth dl and a second bevel groove on the other side with a second angle δ2 and a second depth d2, wherein said first angle δl is between 100° and 140 ° and preferably between 110° and 130°, wherein said second angle δ2 is between 40° and 80 ° and preferably between 50° and 70°, and wherein the sum of said first depth and said second depth is between 70% and 95% and preferably between 80% and 90% of said second member thickness. This embodiment is illustrated by Figure 5e. V butt with broad root shape geometries on at least one side are advantageous when the aluminium alloy first member is too thick to be melted throughout its entire thickness in usual fusion welding conditions, typically this embodiment may be of use when the thickness of the first member is higher than 2,5 mm or even 5 mm.
Figure 6a and 6b illustrate weld brazed assemblies between extruded profile sections made of a titanium alloy (2) and of an aluminium alloy (1) joined by a weld-braze joint (10) according to the invention. When the weld-brazed assembly includes several aluminium alloys members as illustrated in Figure 6b, an unsymmetrical joint geometry such as for example single- bevel geometry (Figure 5c) or unsymmetrical double bevel geometry (Figure 5d) or single V geometry, is advantageous. On the other hand, symmetrical geometries such as for example double bevel geometry or double V geometry are advantageous for weld brazed assemblies including a single aluminium alloy member such as illustrated in Figure 6a.
The surface areas of said members adjacent the abutment are then heated to a temperature above the melting temperature of said aluminium alloy and below the melting temperature of said titanium alloy, in the presence of an inert gas (5) and of a filler metal (4) to obtain a weld-brazed joint. This operation is herein referred to as the weld-brazing step.
Fusion welding processes, such as resistance spot welding, flash butt welding, laser welding, arc welding such as TIG (Tungsten Inert Gas) or MIG (Metal Inert Gas), plasma welding, electron-beam welding and combinations thereof are preferred methods used for heating the surfaces of said members. In a preferred embodiment of the invention, an arc welding process with a consumable electrode is used. The advantage of the consumable electrode over the non-consumable electrode are in particular a reduced heat input which reduces the width if the heat affected zone and an easier process automation which enables a higher process output. According to the invention process, use of a filler metal is necessary to avoid hot cracks in the weld. Various filler metals made of aluminium alloys or zinc alloys are suitable for the present invention, the filler metal is usually selected to be compatible with the first member aluminium alloy. Examples of filler alloys are 2319, 4043, 4047, 4145, 5087 and 5183. Examples of inert gas suitable for the instant process are argon, helium, nitrogen and mixtures thereof. An oxidising gas such as oxygen or carbon-dioxide may be added to the inert gas. A suitable mixture is one that contains at least 95% of argon or helium and up to 5% of oxygen or carbon dioxide.
Any fusion weldable aluminium alloy can be of use for the process of the present invention. In particular, fusion weldable alloys of the 2XXX, 5XXX, 6XXX and 7XXX series are advantageous. Preferably the aluminium alloy first member is a heat treatable alloy, which means that it can be hardened by solution heat treatment and quenching. Among heat treatable aluminium alloys, 6XXX alloys or 2XXX alloys having at least 0.8 wt.% lithium are preferred. Weldable aluminium-lithium alloys having at least 0.8 wt.% lithium and further containing at least 0.1 wt.% silver ("Weldalite® alloys") are particularly suitable for the process of the present invention.
For artificially aged aluminium alloy products, the weld-brazing operation may be carried out before or after artificial aging. Strength is further improved when artificial aging is carried on the weld brazed assembly. In an embodiment of the present invention said first member is in a T3X or T4X temper during the weld-brazing step and the process comprises a step following the weld-brazing step consisting of aging the first member to a T6X or T8X temper. However, this additional process step may not be advantageous for the process output in some instances, in particular when the assembly has large dimensions, such as length of several meters. A weld-brazed assembly according to the invention is obtainable according a process of the invention andcomprises a first member (1) comprising a fusion weldable alloy from 2XXX, 5XXX, 6XXX or 7XXX alloy families and a second member (2) comprising a titanium alloy and having at least one edge with a thickness e and a tapered truncated shape having on a first side a first tapering angle αl between the bevelled portion and the surface of said first side, on a second side a second tapering angle α2 between the bevelled portion and the surface of said second side and a minimum thickness t, wherein αl and α2 are greater than or equal to zero, the sum of αl and α2 is between 10° and 50° and t is between 0.05 e and 0.3 e.
A weld-brazed assembly according to the invention has preferentially an ultimate tensile strength higher than 260 MPa, and preferably higher than 280 MPa. The weld-brazed assembly according the invention can be used for the fabrication of structural elements for the aerospace industry. For example, an aircraft seat track comprising a weld-brazed assembly according to the invention is advantageous.
Another improvement related to the instant process is a reduction of the width of the heat affected zone, which is typically lower than 15 mm or even lower than 10 mm. The process according to the present invention enables the use of high welding speed of preferably at least 0.5 m/min and even at least 1.0 m/min.
Usually, the finished weld brazed assembly is obtained after two welding runs : one on each side of the abutting members, which reduces the process output. The present inventors found that with a specific clamping device, it is possible to obtain a finished weld brazed assembly with a single welding run, which is highly advantageous for process output. Weld-brazing on one side is also advantageous when one side has low accessibility, such as in the geometry of Figure 6b. According to an advantageous embodiment of the invention, the first and second members are fixed with a clamping device comprising means for maintaining a back pressure of inert gas above atmospheric pressure, preferably higher than 110 kPa or 120 kPa and it is then possible to obtain a finished weld-brazed assembly after a single welding run. The back pressure of inert gas is the pressure of inert gas around the back portion of the weld. Unexpectedly, the pressure control obtained with the clamping device according to the invention enables to control the aluminium alloy melt flow and in particular to force the aluminium alloy melt onto the titanium alloy member, avoiding for example drop falling or excessive weld collar, during weld-brazing and obtain a substantially symmetrical weld brazed joint after a welding run on a single face, whatever the groove geometry. An appropriate clamping device for the process of the invention with means to control the inert gas back pressure is now described (Figure 7). The clamping device (6) useful for weld-brazing a first member comprising an aluminium alloy (1) to a second member comprising a titanium alloy (2), comprises an upper face (66) adapted to press against said members to be weld-brazed together, a channel (63), means (641), (642), (643) for tightening said members onto said upper face in order to obtain a substantially leak free pipe having said channel and members for walls, means (61) for introducing a gas (52) into said pipe and means (62) for obtaining a pressure of said gas in said pipe above atmospheric pressure so that said pressure forces the aluminium alloy melt onto said second member during weld-brazing. The means for tightening the member (1) and (2) onto the upper surface (66) are preferably channels (641) linked to a vacuum pump (643) through machined tubes (642). Mechanical clamping may also be used but it appeared to the present inventors that it is more difficult with this type of tightening means to obtain a substantially leak-free pipe between the channel and the members to be welded. By substantially leak free it is meant that it is possible to obtain a pressure within the pipe above atmospheric pressure without excessive inert gas input pressure. A diffuser may be used advantageously as a mean for obtaining an over pressure. The process according to the invention finds many uses, in particular in the mechanical construction of transportation vehicles in the car, railway, aerospace, shipbuilding industries. Fabrication of structural elements for the aerospace industry is a preferred application. The term "structural element" refers to an element used in a mechanical construction for which the static and/or dynamic mechanical characteristics are particularly important for performance and integrity of the structure, and for which a structural calculation is usually required or performed. It is typically a mechanical part, which if it fails will endanger the safety of the said construction, its users, passengers or others. For an aircraft, these structural elements include particularly elements making up the fuselage, such as the fuselage skin, stiffeners or stringers, bulkheads, circumferential frames, wings (such as the wing skin), stiffeners, ribs and spars, and the tail fin composed particularly of horizontal or vertical stabilisers, and floor beams, seat tracks and doors.
Extruded profiles weld brazed with a process according to the invention are particularly useful to make aircraft seat tracks.
EXAMPLES
Welded assemblies were prepared comprising 1.8 or 2 mm thick sheets made of an aluminium alloy and of a titanium alloy having the same thickness. The titanium alloy was Ti-6A1-4V. The aluminium alloy was either AA6X56 or AA2X98. AA6X56 was welded either in the T3X temper or in the T6X temper. When AA6X56 was welded in the T3X temper, the welded assembly was aged in order to obtain a T6X temper after welding. AA2X98 was welded either in the T3X temper or in the T8X temper. When AA2X98 was aged in the T3X temper, the welded assembly was aged in order to obtain a T8X temper after welding This process is referred to as PWHT (Post Welding Heat Treatment) whereas process wherein welding is carried out on an aged aluminium alloy member are referred to as HTBW (Heat treatment Before Welding).
MIG arc welding was used as a welding method. The trials were conducted with a CMT® welding machine from Fronius. Argon was used as an inert gas for welding. The filler metal was a 4043 alloy wire with a diameter of 1.2 mm. Unless otherwise mentioned the titanium sheet was chamfered with the geometry αl = oc2 = 15° and t/e = 0,15 (αl, α2 , t and e as defined in Figure 3). Unless otherwise mentioned the aluminium sheet and the chamfered edge of the titanium sheet were placed in an abutting relationship defining a double bevel butt with broad face geometry. Two clamping devices were used. Clamping device A is a standard clamping device described in Figure 2 and clamping device B is described in Figure 7. When clamping device B was used, weld-brazing was carried out only on one side of the abutting members. When clamping device A was used, welding was carried out on one side, then the welded assembly was turned upside down and welding was carried out in the other side. The trial conditions and mechanical properties of the welded joints are provided in Table 1.
Table 1. Trials conditions and results.
a : no chamfering of the titanium sheet, square butt geometry before weld-brazing b : Standard deviation of variants 1-4, 6 and 7 is < 10 MPa. For trial n°5 the deviation between the trials was too large (> 27 MPa) to provide a significant average value, maximum and minimum values are indicated.
Images of the weld-brazed sample from trial number 1 (two welding run) and from trial number 4 (single welding run) are presented in Figures 7a and 7b, respectively. Figure 8 is a hardness profile for weld-brazed samples from trial number 2 (HTBW) and from trial number 1 (PWHT). The hardness profile is useful to determine the width of the heat affected zone. For both samples, the width of the heat affected zone is around 10 mm, lower than 15 mm. The comparison of reference trial number 5 with trial according to the invention (1 to 4, 6 and 7) clearly shows the advantage of the process according to the invention : the strength of weld-brazed samples according to the invention is higher and also exhibits less deviation compared to reference weld brazed samples. The further improvement related to the post welding heat treatment process is observed in particular by comparing trial number 1 to trial number 2. Strength obtained with a single welding run (clamping device B) are similar than those obtained with two welding runs (clamping device A), the process output is thus doubled with clamping device B.