WO2005082569A1

WO2005082569A1 - Method for welding similar metal joining partners, in particular, aluminium-copper connecting points

Info

Publication number: WO2005082569A1
Application number: PCT/EP2005/001875
Authority: WO
Inventors: Ihor Mys; Michael Schmidt; Gerd Esser; Manfred Geiger
Original assignee: BLZ Bayerisches Laserzentrum Gemeinnützige Forschungsgesellschaft mbH
Priority date: 2004-02-27
Filing date: 2005-02-23
Publication date: 2005-09-09
Also published as: DE102004009651A1; DE102004009651B4

Abstract

The invention relates to a method for welding similar metal joining partners comprising the following steps: preparing one of the joining partners (1, 8) made from a first metal material, preparing the other joining partner (2, 9) made from a second metal material, layering a layer of a welding additional substance (4, 11), made of a third metal material, between both of the joining partners (1, 8; 2, 9), and welding both of the joining partners (1, 8; 2, 9), by integrating the welding additional material (4, 11), with the aid of an energy beam, preferably a laser beam (7).

Description

Process for welding dissimilar metallic joining partners, especially aluminum-copper joints

The invention relates to a method for welding dissimilar metallic joining partners and in particular aluminum-copper connection points.

With regard to the background of the invention, it should be noted that an increasing interest in joining dissimilar metallic workpieces is being promoted by special fields of application, such as, for example, mechatronic systems in automotive engineering. The currents required for the actuation of such systems typically range from 10 to 100 amperes in the usual range of generators and power distributors, but these currents occur in complex structures of the mechatronic systems. The traditional electrical engineering material copper with its low-loss current carrying capacity and high heat dissipation is well suited to meet the relevant boundary conditions there, but aluminum, in particular, also offers good reasons for use in such systems in terms of costs and other technical properties such as low weight. In order to be able to make an optimal choice of material for different elements of the systems, there are inevitably many transitions between base materials such as copper (alloys) and aluminum (alloys). The assembly and connection technology often requires joining techniques on sufficiently large cross-sections of such material combinations under limited space. The problems associated with this can often be solved by laser welding. The mentioned excellent properties of the materials copper and aluminum with regard to electrical and thermal conductivity are based on their high electron mobility. However, this also leads to strong reflection or low absorption in the infrared range, in which classic welding lasers, such as an Nd: YAG laser, work. As a result, high powers and energies are required for laser beam welding of these materials in order to successfully carry out the process.

Another problem is that the materials copper and aluminum show great differences in the melting temperature and thermal expansion, which on the one hand lead to segregation of high and low melting phases during fusion welding and on the other hand can cause the build-up of residual stresses in the joining zone.

After all, both materials show complete solubility in one another in liquid state and limited solubility in solid state. With a copper content of less than 1 mass% or an aluminum concentration of less than 8 mass%, slow cooling leads to mixed crystal formation, while in the intermediate concentration range the structure contains parts of intermetallic compounds in the equilibrium case. The microhardness of such intermetallic phases that can occur in the Cu-Al system is significantly greater than that of pure aluminum or copper. Higher hardnesses of intermetallic compounds cause a reduction in ductility and lead to a notch and crack sensitivity of the connections. Because of their non-purely metallic nature, intermetallic phases also show a fundamentally higher specific resistance than the base materials. In summary, the mechanical and electrical properties of connection points between different metallic base materials, such as copper and aluminum, depend very much on the distribution of the intermetallic phases formed during welding. In particular in the form of coherent strips, these so-called "intermetallics" impair the deformability of the connection and the electrical resistance more than in the form of small, separate deposits. Overall, the generally good basic properties of the individual materials are often significantly deteriorated by the intermetallics.

Based on the problems described, the invention has for its object to provide a method for welding dissimilar metallic joining partners and in particular aluminum-copper connection points, which achieves improved mechanical and electrical properties of the transition.

According to the characterizing part of claim 1, this object is achieved in that a layer of a welding filler consisting of a third metallic material is layered between the two joining partners from different metallic materials. The two joining partners are then welded to one another using the welding filler material with the aid of an energy beam, preferably a laser beam.

The invention is based on the knowledge that when welding dissimilar metals, intermetallic phases can be reduced by using additional materials that are used with both base materials. Substances have a certain solubility. The filler material acts as an enriched melt between the two joining partners, so that the "incompatible" components are no longer in pure contact with each other.

Nickel, silver and tin and their lightly doped alloys are preferred as an additional material in the copper-aluminum system. In principle, zinc is also conceivable, but due to its high vapor pressure, it will hardly be usable in fusion welding.

Various basic concepts are possible for introducing the additional material layer between the joining partners. For this purpose, the filler material layer can be formed by a separate film layer with a thickness order of magnitude in the micrometer range from a few μm to, for example, about 100 μm, which is inserted between the two joining partners before welding.

The provision of the filler material layer by a coating on at least one of the two joining partners is particularly elegant in terms of process technology, since this eliminates the need to position and fix the intermediate film.

Due to the small thickness dimension of the filler material layer in the form of a film or coating, this filler material is essentially built into the alloy formed by welding from the two welding base materials, ie primarily a copper-aluminum alloy, as doping. Consequently, there is at least a ternary alloy in the melting range, which is in contrast to the prior art. There are flat additional materials in the joining area of such a thickness that only binary alloys are formed between the filler material as the central layer and the adjacent joining partners.

Another reason for the known intermediate layers of filler materials of comparatively greater thickness is that these filler material layers are intended to serve as a diffusion barrier between the two adjacent joining partners.

Different strategies are possible for the application of the welding beam depending on the dimensions and shape of the joining partners. Thus, flat joining partners with the filler material layer in between can be butt-welded to one another at one or more individual points or a point seam by means of a preferably pulsed laser beam at their exposed edge edges. The use of a film or coating layer in the additive material has the advantage of uniformly stable welding results, since a “natural” concentration gradient of the additive material is created from one joining partner to the other over the entire area of the melting zone.

The second application strategy is particularly suitable for connections between a flat and a wire-shaped joining partner, such as are produced, for example, in electronics when contacting copper wires on solid aluminum bodies. The laser beam is applied to the elongated joining partner from the direction facing away from the filler material layer and the flat joining partner.

A special feature of the method according to the invention is that which is also provided according to a preferred embodiment compared to the Application of the laser beam offset between the joining partners. Its focus is thus shifted towards one of the two joining partners, which means that the strength of the welded joint can be increased even without the interposition of a filler material layer compared to an undisplaced laser application. Offset paths in the order of magnitude of the thickness of the additional material layer or alternatively approximately half the diameter of the laser beam, in particular in the case of thin additional material layers, are to be assumed as preferred values.

Further features, details and advantages of the invention will become apparent from the following description, in which welding tests and the measurement results resulting therefrom are explained in more detail with reference to the accompanying drawings. Show it:

1 and 2 is a side view and plan view of a welding assembly,

3 shows an enlarged detail of detail III from FIG. 1,

4 shows a diagram to show the stability and resistance of welded connections between two joining partners with the inclusion of various additional material layers and laser beam displacement paths,

Fig. 5 is a graph showing the tensile strength of welds between two joining partners Integration of silver foil as an additional material of different thickness,

Fig. 6 is a graph showing the tensile strength of welded connections between two joining partners with the incorporation of different galvanic coatings as filler layers, and

7 and 8 are schematic perspective representations of joining partners with filler material layers with an alternative laser beam application.

As is clear from FIGS. 1 and 3, two strip-shaped joining partners, namely an aluminum strip 1 and a copper strip 2, are laid one on top of the other in an overlap area 3. A film layer 4 made of a filler metal is positioned in this overlap area 3. As is clear from FIG. 1 and in particular 3, the aluminum layer 1, copper strip 2 and foil layer 4 with their longitudinal edge edges 5 are exposed. The length of the two strips 1, 2 is approximately 50 mm, their thickness 1.2 mm. Test strips made of nickel or silver foil with a thickness of 0.1 mm are used for the foil layer 5. These foils consisted of technical silver or nickel with a purity of 99.9 percent.

On the edge edges 5 facing away from one another, welding points 6 are set at a distance of approximately 5 mm with the aid of a Nd.YAG laser (not shown) by applying a single pulse. The focus diameter of the Laser beam 7 is 400μm, the pulse duration is 10 ms and the laser power is 5.2 kW.

In addition to the symmetrical application of the welding points 6 to the joining surface plane F defined by the film layer 4 shown in FIGS. 1 to 3, it is also possible to apply the laser beam 7 offset in relation to this joining surface plane F in the direction of a joining partner. As shown in Fig. 4, a beam offset x in the direction of the aluminum strip 1 towards 0.1 mm and 0.2 mm was chosen when using nickel and silver foil for test purposes. The stability of the welded connections made with the test geometry according to FIGS. 1 to 3 was assessed with the aid of tensile tests, the result of which is plotted in FIG. 4. At the same time, the contact resistance for the copper-aluminum transition is recorded there. The corresponding values were determined using the 4-point measurement method that is usual for conductivity measurements.

As can now be seen from FIG. 4, the maximum tensile force is significantly increased by using foils as an intermediate layer - when using silver foils - between 40% and 70%. At the same time, the contact resistance drops by a corresponding amount.

During the tensile tests, it was also found that the weld samples did not fail at the joint, as was the case with conventional weld spots, but rather in the cross-section of the aluminum samples, which was weakened by the notch effect of weld spots. Here, a reduction in the laser power with a corresponding reduction in the size of the weld notch can lead to a further improvement in the strength of such connections. It can also be seen from FIG. 4 that an offset x of the laser beam 7 into the aluminum strip 1 by 0.1 mm or 0.2 mm, in particular in the case of the silver foil, leads to significantly increased strength values and reduced contact resistance. It is remarkable that, contrary to the usual "welding philosophy", that in the case of unequal joining partners, the energy beam is directed at the higher-melting partner, the direction of offset towards the low-melting aluminum is aimed here.

The improvement in the welding results due to the offset is presumably due to a targeted change in the mixing and cooling ratios due to the offset. In the special aluminum-copper system, the offset in the direction of the aluminum is also advantageous in that the copper surface shows little coupling for the laser beam 7 due to its high reflectivity. Appropriate welding results would therefore only be achieved by increasing the laser power.

With the specified welding offsets of 0.1 mm or 0.2 mm in the direction of aluminum, the copper is only partially melted directly by the laser beam, but also indirectly by the aluminum melt. After the tensile tests were carried out, there were relatively high values of the standard deviation, which can be explained by random, extremely strong connections with a strength of 700 to 800 N as well as by the enlargement of the sliding zone due to the exothermic nature of this connection formation.

The above statements regarding the gain in strength of the welded connection caused by the laser beam offset are supported by further re welding and tensile tests confirmed, the results of which are shown in the diagram in FIG. 5. On the one hand, it can be seen from this that by using an Ag film as a filler, an enormous increase in strength can be achieved with film thicknesses of 0.025 mm, 0.050 mm and 0.100 mm. The rates of increase compared to welding without the addition of materials can be seen to reach a factor of around 2.5. Furthermore, the diagram according to FIG. 5 shows that with the specified film thicknesses of the additional material, an optimal beam offset is in the range between 0.1 mm and 0.2 mm. From this it can be deduced that the beam offset should correspond to approximately half the beam diameter.

FIG. 6 shows a diagram with which the strengths of copper-aluminum weldings are shown using different galvanic coatings as an additional material. All spot welds on the edge were carried out with a beam offset of 0.2 mm in the direction of the aluminum strip 1. The galvanic coatings were nickel, silver and tin layers with a thickness of 3 μm and 10 μm.

It is clear from the diagram according to FIG. 6 that the use of 10 μm thick coatings significantly increases the strength of the welded joints. When using silver and tin coatings, the strength can be doubled, whereby the highest strength values are achieved with a tin coating with a thickness of 10 μm. The strength of the welded connection reaches values that are close to the strength when using a silver foil with a thickness of 100 μm. Another advantage of tin coatings is, besides their high strength, the spread of the material in electronics. Tin top layers are the most common and cheapest coatings used in today's state-of-the-art electronics manufacturing.

The welding tests on which the diagram according to FIG. 6 is based were carried out with an Nd: YAG laser (power: 9 KW) under argon protective gas. Rectangular pulses with a period of 9 ms were used.

7 shows an alternative method for welding dissimilar metallic joining partners. Thus, a foil layer 4 made of silver in the form of a short, strip-shaped piece of foil is placed on an aluminum base 8 and a copper wire 9 with its end 10 is positioned thereon. The two joining partners 8, 9 are welded by applying a laser beam 7 from the side of the copper wire 9 facing away from the aluminum base 8 and the foil layer 4, so that the silver foil layer 4 and the aluminum base 8 are heated and melted melted in the joining area and connected to each other.

8 shows a similar configuration in which the welding filler material is applied to the aluminum base 8 in the form of a strip-shaped silver coating 11. Due to the longitudinal expansion of the silver coating 11 compared to the silver strip 4 in FIG. 5, the copper wire 9 can be spot welded over a greater length at its end to the aluminum base 8 with the aid of several welding spots (not shown).

Claims

claims

1. Method for welding dissimilar metallic joining partners, in particular aluminum-copper connection points, characterized by the following method steps: providing one joining partner (1, 8) from a first metallic material, providing the other joining partner (2, 9) from a second metallic one Material, layers of a layer of a welding filler Material es (4, 11) made of a third metallic material between the two joining partners (1, 8; 2, 9), and welding the two joining partners (1, 8; 2, 9) underneath Integration of the filler metal (4, 11) using an energy beam, preferably a laser beam (7).

2. The method according to claim 1, characterized in that the two joining partners (1, 2; 8, 9) consist of aluminum or copper and the filler material is silver, nickel, tin or a low-doped alloy of these filler materials.

3. The method according to claim 1 or 2, characterized in that the additional material layer is formed by a separate film layer (4).

4. The method according to claim 3, characterized in that a film layer (4) with a thickness in the μm range, preferably from 20 μm to 100 μm, is used.

5. The method according to claim 1 or 2, characterized in that the filler layer by a coating (11), preferably a galvanic coating of silver, nickel or tin with a thickness in the order of 2 microns to 15 microns, on at least one of the two joining partners (8) is formed.

6. The method according to any one of the preceding claims, characterized in that the flat joining partners (1, 2) and the filler layer (4) by means of a preferably pulsed laser beam (7) butt at their exposed edge edges (5) with one another via one or more Single points or a point seam can be spot welded.

7. The method according to claim 6, characterized in that the laser beam (7) is applied offset relative to the joining surface plane (F) between the joining partners (1, 2) in the direction of one of the two joining partners (1).

8. The method according to claim 7, characterized in that the laser beam (7) is applied with an offset (x) in the order of magnitude of the thickness of the filler material layer (4).

9. The method according to claim 7, characterized in that the laser beam (7) is applied with an offset (x) in the order of magnitude of its half beam diameter.

10. The method according to any one of claims 1 to 5, characterized in that on a flat joining partner (8) with the interposition of the additional material layer (4, 11) an elongated, preferably wire Shaped joining partner (9) is welded by means of a laser beam (7), the laser beam (7) being applied to the elongated joining partner (9) from the direction facing away from the filler material layer (4, 11) and the flat joining partner (8) ,