Method for assembling a set of at least three sheets, and the resulting assembly.

Abstract

The present invention relates to a method for assembling a set of at least three sheets, said set comprising a first metal sheet (10), referred to as the upper sheet, a second metal sheet (20), referred to as the lower sheet, and a sub-set of at least one sheet (30), referred to as the intermediate sheet, arranged between the first and second sheets, characterized in that it comprises at least the following steps: - cutting, preferably during a stamping step, at least one first opening (11) in the upper sheet, and at least one second opening (31) in the intermediate sub-set (30), - superimposing the upper, intermediate, and lower sheets, as well as the first and second openings, - performing a weld (50) by adding metallic material at the point where the first and second openings overlap. The invention also relates to the assembly thus formed. Figure 2

Description

Title of the invention: Method for assembling a set of at least three sheets, and resulting assembly.

[0001] The present invention relates to a method for assembling a set of at least three sheets made of different materials, in particular two steel sheets with an intermediate aluminium sheet, and also relates to the assembly thus produced.

[0002] In the automotive field and more particularly in the context of reducing energy consumption and therefore reducing CO2 emissions, it is desired to make motor vehicles as light as possible. Thus, with a view to lightening structures, it may be possible to consider replacing certain steel components with lighter aluminum alloy elements.

[0003] To assemble two components, one of which is made of steel and the other of aluminum alloy, mechanical assembly solutions known as conventional riveting, clinching, screwing are known and applicable. However, it may be necessary, for reasons of reducing the number of parts and rigidity, to make continuous connections between these elements.

[0004] The metallurgy of Fe-Al alloys being extremely complex, any fusion assembly between these two materials requires the use of specific technologies and the implementation of dedicated operating procedures.

[0005] Similarly, the studies carried out have demonstrated the need to adapt the design of welded joints to the specificities of steel / aluminium welding.

[0006] Indeed, steel and aluminum are materials that have very different physical properties, which makes any fusion assembly complex. The main differences are as follows:

[0007] The difference between the melting temperatures, Al of 660°C and Fe of 1536°C is considerable and increases further for aluminum alloys containing silicon. This difference, which is very penalizing in welding, where the aim is rather to melt the two materials, can however prove beneficial in the case of brazing; in this case the aluminum melts while the steel remains solid.

[0008] Furthermore, the thermal properties of aluminum and steel are also different, resulting in very different thermal behaviors of the two materials.

[0009] Thus, when a heat source is applied, the energy distribution presents a strong asymmetry on either side of the connection. The welding operating conditions are therefore always very complex.

[0010] Another difficulty in carrying out a steel / aluminium weld arises from the fact that it is impossible to obtain a ductile alloy between these two materials. Indeed, the solubility Since the aluminum content in steel is almost zero, fragile intermetallic compounds are formed very quickly, making any mechanical use of the alloys formed impossible. The equilibrium diagram predicts the formation of many defined or intermetallic compounds.

[0011] The solution commonly adopted for making an assembly is therefore not to weld, in the strict sense of the term, the steel and the aluminum, but to make a diffusion bond, similar to brazing. Thus, the aluminum is brought to a liquid state while the steel remains solid.

[0012] Although it is impossible to avoid the formation of intermetallic compounds, it is however possible to control their nature and layer thickness by adjusting the heat input to the assembly, i.e. by using CMT (Cold Metal Transfer) or laser arc welding technologies.

[0013] However, metallurgical analyses have confirmed the presence of intermetallic compounds between the filler metal and the steel. Thus, although it has been demonstrated that the thickness of the layer formed can be controlled by the use of CMT or laser technologies, the fact remains that this layer is fragile, and it is necessary to adapt the design of the welded joints to the specificities of this connection.

[0014] The object of the invention is therefore to propose a method which makes it possible to obtain an assembly of sheets made of different materials having different thermal behaviors, such as a steel sheet and an aluminum sheet, and which makes it possible to integrate changes of direction, if possible with smooth transitions, in order to distribute the normal and tangential stresses.

[0015] The invention relates in particular to a method for assembling a set of at least three sheets, said set comprising a first metal sheet called the upper sheet, a second metal sheet called the lower sheet, and a sub-set of at least one sheet called the intermediate sheet and arranged between the first and second sheets, characterized in that it comprises at least the following steps:

[0016] - cutting, preferably during a stamping step, at least one first opening in the upper sheet metal, and at least a second opening in the intermediate subassembly,

[0017] - superimpose all of the upper, intermediate and lower sheets as well as the first and second openings,

[0018] - carry out a weld by adding metallic material at the level of the superposition of the first and second openings.

[0019] Optional, complementary or substitutive features of the invention are set out below.

[0020] According to one embodiment of the invention, the welding step may be a plug welding step carried out using a MIG / MAG welding process.

[0021] According to another embodiment of the invention, the lower and upper sheets may be steel sheets, while the intermediate subassembly is a sheet of aluminum or composite material.

[0022] According to yet another embodiment of the invention, the lower and upper sheets may be aluminum sheets, while the intermediate subassembly is a steel sheet.

[0023] Preferably, the area of ​​the first opening is less than the area of ​​the second opening.

[0024] Advantageously, the at least first and second openings are circular openings, the diameter of the first opening being less than the diameter of the second opening.

[0025] Preferably, before the step of superimposing all of the upper, intermediate and lower sheets, and the first and second openings, a step of gluing at least one face of the intermediate subassembly is carried out.

[0026] Even more preferably, the gluing step uses a structural glue with an elastomer base, preferably loaded with glass beads.

[0027] The invention also relates to an assembly of a set of at least three sheets, said assembly comprising a first metal sheet called upper, a second metal sheet called lower, and a sub-assembly of at least one sheet called intermediate and arranged between the first and second sheets, characterized in that the first sheet and the intermediate sub-assembly are each respectively provided with at least one opening, said openings being superimposed and preferably circular, the surface area of ​​the first opening being less than the surface area of ​​the second opening, the assembly comprising a plug weld by adding metallic material at the level of the superposition of the first and second openings.

[0028] Advantageously, the intermediate subassembly is further fixed to the first and / or second sheet by means of an elastomer-based structural adhesive, preferably loaded with glass beads.

[0029] Other advantages and particularities of the invention will appear on reading the detailed description of implementations and embodiments which are in no way limiting, and the following appended drawings:

[0030] [Fig-1] This figure represents a schematic view of an assembly of three sheets before welding, in accordance with one embodiment of the invention.

[0031] [Fig.2] This figure represents a schematic view of an assembly of three sheets after welding, in accordance with an embodiment of the invention.

[0032] The assembly method according to the invention makes it possible to assemble a set of at least three sheets. Referring to [Fig.l] and [Fig.2], it can be seen that this set comprises a first metal sheet 10 called upper, a second metal sheet 20 called lower, and a subassembly of at least one sheet 30 called intermediate and arranged between the first and second sheets.

[0033] In the present case, the subassembly is composed of the sheet 30, but could just as well be composed of a stack of a plurality of sheets.

[0034] According to the method, a cutting step is first carried out, for example during a stamping step, of a first opening 11 in the upper sheet, and a second opening 31 in the intermediate subassembly 30. In practice, several openings are made in the upper sheet and as many in the intermediate subassembly, in order to multiply the welding points.

[0035] Then, a next step is carried out in which the upper, intermediate and lower sheets as well as the first and second openings are superimposed. In order to obtain the superposition of the openings, it is possible, for example, to jointly drill the first sheet and the intermediate subassembly already superimposed.

[0036] Finally, a weld 50 is produced by adding metallic material at the level of the superposition of the first and second openings.

[0037] Advantageously, the welding step is a plug welding step carried out using a MIG / MAG welding process. MIG-MAG welding is a semi-automatic welding process. The fusion of the metals is obtained by the heat energy released by an electric arc which breaks out in a protective atmosphere between a fusible electrode wire and the parts to be assembled.

[0038] The acronyms MIG and MAG stand for Metal Inert Gas and Metal Active Gas respectively. The difference between the two processes lies in the composition of the gas. The MIG process uses a neutral gas that does not react with the molten metal (argon or argon + helium), unlike the MAG process (a mixture of argon and carbon dioxide or oxygen in varying proportions depending on the metals to be welded). The gas is injected continuously onto the arc in order to completely isolate the molten metal from the ambient air.

[0039] Preferably, the surface area of ​​the first opening 11 is less than the surface area of ​​the second opening 31.

[0040] This difference in surface area prevents the intermediate subassembly from absorbing too much energy during welding and allows welding to be carried out in the best possible conditions. The plug weld makes it possible to block the intermediate subassembly sandwiched between the upper sheet and the lower sheet.

[0041] Advantageously, the at least first 11 and second 31 openings are circular openings, the diameter of the first opening 11 being less than the diameter of the second opening 31.

[0042] This circular shape allows a balanced distribution of thermal stresses during welding.

[0043] Advantageously, before the step of superimposing the assembly of the upper 10, intermediate 30 and lower 20 sheets, and the first 11 and second 31 openings, a step of gluing at least one face of the intermediate subassembly 30 is carried out.

[0044] This makes it possible to fix the intermediate subassembly to at least one of the upper and lower sheets.

[0045] A structural adhesive 40 of elastomer base, preferably loaded with glass beads 41 can advantageously be used.

[0046] The glass beads create an anchor in the sheets, which makes it possible to fix the stacking of sheets until the plug weld is made.

[0047] The assembly of this set of at least three sheets is then definitively completed with the crosslinking of the adhesive film during the baking of the assembly of sheets, once coated with a layer of paint.

[0048] The assembly of this set of at least three sheets is generally intended for the manufacture of bodywork parts.

[0049] The lower 20 and upper 10 sheets are generally steel sheets, while the intermediate subassembly 30 is a sheet made of aluminum or composite material.

[0050] The lower 20 and upper 10 sheets can also be aluminum sheets, while the intermediate subassembly 30 is a steel sheet.

[0051] Three tests were carried out, each time with a 6016 ALUMINIUM test piece, 1 mm thick, interposed between two STEEL test pieces, 2 mm thick, with G10 / 10 coating.

[0052] The welding method is of the CMT (Cold Metal Transfer) welding method type with an Alu Si 3 Mn filler wire, 12 / 10 diameter, under Argon with a flow rate of 15 L / min.

[0053] The wire feed speed is 4.5 m / min, the arc height correction value is 3%, the welding current is 72 amp, the welding speed is 300 mm / min.

[0054] The weld bead obtained is of very good quality and consists of a “spot” weld.

[0055] The three tensile tests to measure the degree of resistance to rupture of this weld, whether in shear or in pure tension, made it possible to establish values ​​for the shear tension respectively 2115, 2287 and 2256 daN.

[0056] These results are very encouraging, because by optimizing the welding parameters, we can hope to achieve the welding point values ​​in standard plugs which amount to 2500 daN.

[0057] In summary, this method of assembling three sheets makes it possible to join two outer metal sheets with a central sheet of a different nature, guaranteeing a hold equivalent to that of a standard plug or resistance welding point.

[0058] This process, which uses plug weld points, also has the advantage of only requiring access to one side of the assembly, unlike electric weld points.

[0059] This process also eliminates the need for riveting, clinching and screwing operations and parts, and requires very little filler metal, i.e. only welding wire (generally €2 / kg, or approximately €0.002 per plug weld point).

[0060] The embodiments described below being in no way limiting, it will be possible in particular to consider variants of the invention comprising only a selection of described characteristics, isolated from the other described characteristics (even if this selection is isolated within a sentence comprising these other characteristics), if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art. This selection comprises at least one characteristic, preferably functional without structural details, or with only a part of the structural details if this part only is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art.

[0061] Note that the various characteristics, forms, variants and embodiments of the invention may be associated with each other, in various combinations to the extent that they are not incompatible or exclusive of each other.

Claims

Claims

1. Method for assembling a set of at least three sheets, said set comprising a first metal sheet (10) called upper, a second metal sheet (20) called lower, and a sub-set of at least one sheet (30) called intermediate and arranged between the first and second sheets, characterized in that it comprises at least the following steps: - cutting, preferably during a stamping step, at least one first opening (11) in the upper sheet, and at least one second opening (31) in the intermediate sub-set (30), - superimposing all of the upper, intermediate and lower sheets as well as the first and second openings, - carrying out a weld (50) by adding metallic material at the level of the superposition of the first and second openings.

2. Assembly method according to claim 1, characterized in that the welding step is a plug welding step carried out using a MIG / MAG welding method.

3. Assembly method according to claim 1 or 2, characterized in that the lower (20) and upper (10) sheets are steel sheets, while the intermediate subassembly (30) is a sheet of aluminum or composite material.

4. Assembly method according to any one of claims 1 to 2, characterized in that the lower (20) and upper (10) sheets are aluminum sheets, while the intermediate subassembly (30) is a steel sheet.

5. Assembly method according to any one of the preceding claims, characterized in that the surface area of ​​the first opening (11) is less than the surface area of ​​the second opening (31).

6. Assembly method according to any one of the preceding claims, characterized in that the at least first (11) and second (31) openings are circular openings, the diameter of the first opening (11) being less than the diameter of the second opening (31).

7. Assembly method according to any one of the preceding claims, characterized in that, before the step of superimposing the assembly of the upper (10), intermediate (30) and lower (20) sheets, and the first (11) and second (31) openings, a step is carried out gluing at least one face of the intermediate subassembly (30).

8. Assembly method according to the preceding claim, characterized in that the gluing step uses a structural glue (40) of elastomer base, preferably loaded with glass beads (41).

9. Assembly of a set of at least three sheets, said assembly comprising a first metal sheet (10) called upper, a second metal sheet (20) called lower, and a sub-assembly of at least one sheet (30) called intermediate and arranged between the first and second sheets, characterized in that the first sheet and the intermediate sub-assembly are respectively provided with at least one opening (11, 31), said first (11) and second (31) openings being superimposed and preferably circular, the surface area of ​​the first opening (11) being less than the surface area of ​​the second opening (31), the assembly comprising a plug weld (50) by adding metallic material at the level of the superposition of the first and second openings.

10. Assembly of a set of at least three sheets according to claim 9, characterized in that the intermediate sub-assembly (30) is further fixed to the first and / or to the second sheet by means of a structural adhesive (40) of elastomer base, preferably loaded with glass beads (41).

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