Copper and aluminum metal materials have good electrical conductivity, thermal conductivity and ductility, and excellent physicochemical and processing properties; It is widely used in electrical, machinery manufacturing, automobiles, new energy and other fields [1]. Whether the effective connection of copper-aluminum dissimilar materials can be realized [2] directly determines the application range of copper-aluminum metals. A large number of studies have proved that traditional fusion welding cannot achieve a good connection between copper-aluminum dissimilar metals [3], because the physical properties of copper-aluminum metals such as melting point, thermal conductivity, and linear expansion coefficient are quite different, and the two metals will produce a large number of intermetallic compounds during the solidification process, and the intermetallic compounds belong to the hard and brittle phase, which will not only reduce the strength of the weld but also increase the metal resistance value. The former Soviet scholar Ryabov [4] has carried out a series of fusion welding studies on copper and aluminum dissimilar metal materials. By opening a groove at a certain angle on the copper side, submerged arc welding and non-melting inert gas shielded gas welding (TIG) are used to weld copper and aluminum, and the comprehensive mechanical properties of the joint obtained are poor. If the mechanical properties of the welded joints are greatly improved by plating the composite metal (AG, ZN, SN, NI) on the copper surface in advance, which can basically meet the design requirements. For example, when the thickness of the coating is 60 m, the copper side is beveled at 75°, and a relatively ideal welded joint is obtained, and the analysis is that the coating reduces the formation of intermetallic compounds and reduces the thickness of the intermetallic interlayer, thereby improving the mechanical properties of the weld. However, due to the high cost of coated metal, and the increase in manufacturing difficulty and cost, the application of this process method has great limitations, and has not been applied and promoted on a large scale.
Solid-state welding requires less welding heat input because the base metal does not melt, and is more suitable for joining dissimilar metals than traditional fusion welding. Therefore, the solid-state welding technology has great advantages in the welding of copper-aluminum dissimilar metals, and the comprehensive mechanical properties of the welded joints obtained are also greatly improved compared with the traditional fusion welding. Friction stir welding, ultrasonic welding, electromagnetic pulse welding and other solid phase joining methods have become hot spots for scholars at home and abroad to study copper and aluminum metal dissimilar welding, and have achieved fruitful results. As an ancient joining technology, brazing is more suitable for welding between dissimilar metals, using a metal with a lower melting point than the base metal as the brazing metal, heating the weldment and brazing metal to a temperature lower than the melting point of the base metal, the liquid brazing metal will wet the base metal, fill the joint gap and diffuse with the base metal to form a weld, so as to achieve copper and aluminum welding. In recent years, with the maturity and progress of laser technology, laser welding has gradually been applied to copper and aluminum welding, especially in the field of high-quality and new energy power batteries.
Lee[5] et al. [5] used the traditional friction stir welding method to weld pure copper and 1050 aluminum alloy, and annealed it, and analyzed the influence of the growth law of intermetallic compounds in the weld on the resistivity and comprehensive mechanical properties of the welded joint. It is found that when the width of the intermetallic compound is 21 m, the resistivity is 45 cm. When the width of the intermetallic compound increases to 107At 5 m, the resistivity becomes 85 cm, the tensile strength decreases with the increase of the intermetallic compound, and the fracture position changes from the heat-affected zone on the aluminum side to the entire intermetallic layer. Ouyang et al. [6] used friction stir welding (FSW) to study the butt joint between copper and 6061 aluminum alloy, and found that the mechanical mixing area of the welded joint was mainly composed of CUL2, CUAL and CU9AL4. The range of microhardness in this region is HV02 136~760。Due to the formation of brittle intermetallic compounds, friction stir welding of copper-aluminum dissimilar metals is difficult. Ke Liming et al. [7] from Nanchang Hangkong University have also used friction stir welding (FSW) to study the butt joint between industrial pure copper T1 and anti-rust aluminum LF6, and the results show that the ratio of the rotation speed of the stirring head to the welding speed directly affects the compactness and microstructure of the weld. When the plate is thin, the welding process parameter window is large and the weld is well formed, because there is a strong plastic flow inside the joint, which reduces the formation of intermetallic compounds.
Electromagnetic pulse welding originated in the early 70s of the last century and belongs to a type of solid phase bonding, which is very suitable for welding between dissimilar materials, because it can achieve a good connection between metals with similar or dissimilar properties. Zhang Jie [8] tried to weld T3 pure copper with LF21 anti-rust aluminum by electromagnetic pulse welding, analyzed the microstructure of the weld joint, and tested the surface hardness value of the weld and the mechanical properties of the joint. The results show that the tensile strength of the welded joint is 137 MPa, which is 110% of the base metal LF21 anti-rust aluminum and 51% of T3 pure copper, respectively. The yield strength of the joint is 60 MPa, which reaches 102% of LF21 anti-rust aluminum and 88% of T3 pure copper, and the high-quality welded joint is obtained in terms of comprehensive mechanical properties of the joint. The maximum hardness of the two metal electromagnetic pulse welding joints appeared on the side of LF12 anti-rust aluminum, and the maximum value was HV 118, which was mainly due to the diffusion of T3 pure copper and LF21 anti-rust aluminum during the welding process, which produced a violent displacement reaction around the weld and generated -Al solid solution phase and Cual2 compound. In addition, the main components of the weld are the -Al solid solution phase, the Cual2 phase and the Cu phase.
In order to study the differences between electropulse welding and dissimilar metal welding, Raoelison et al. [9, 11] used different schemes to study this problem. They applied the same welding parameters and equipment to Cu Al and Al Al under the same environmental conditions, and the results showed that Cu Al dissimilar metals produced intermetallic compounds, while Al Al produced continuous metal bonding. Therefore, the comprehensive mechanical properties of Cu Al joints are low, and the weld is prone to brittle fracture, while Al A generally has good mechanical properties, and the ductile fracture is often due to plastic deformation during tensile.
Because the base metal does not melt, but relies on the brazing metal to connect the two metals, it is more suitable for welding between dissimilar metals. Sun et al. [12] studied the direct brazing of copper and aluminum, and analyzed the effects of brazing metal, flux, and process parameters on copper-aluminum brazing. The results show that under the premise of selecting the correct brazing metal and flux, the ideal brazing joint can be obtained by reducing the brazing temperature, reducing the brazing time, adjusting the weld gap and the surface finish of the base metal, and carrying out post-welding treatment. Xue Songbai et al. [13] of Nanjing University of Aeronautics and Astronautics used the flame brazing method to achieve a good connection between copper and aluminum in medium-temperature brazing, and selected improved non-corrosive flux and medium-temperature brazing metal to greatly reduce the formation of intermetallic compounds between copper and aluminum.
Koyama et al. [14] of Hiroshima University in Japan carried out a vacuum brazing study on Cu Al, and the brazing metal used was Alsimgbi alloy, and the results showed that the brazing temperature was the most suitable between 520 and 580 °C, and there were two kinds of intermetallic compounds in the weld, Cual2 and Cu3Al2, and the fracture of the weldment tensile test occurred at the intermetallic compound, and the strength of the joint was related to the type of intermetallic compound. Li Yajiang's research group at Shandong University has also studied the vacuum brazing process and joint microstructure of copper-aluminum dissimilar metals [15]. The results show that excellent copper-aluminum welded joints can be obtained by controlling the process parameters such as heating temperature, holding time and vacuum degree, and the interface area of the joint is mainly composed of the copper-side transition zone, the brazing seam and the aluminum-side transition zone, and a small amount of intermetallic compounds are generated, which have an important impact on the weld performance.
As a new welding method, laser welding has the characteristics of high energy density, narrow heat affected zone, large aspect ratio and low heat input compared with traditional welding. Xue Zhiqing et al. [16] studied the laser welding of copper-aluminum sheets, and analyzed the microstructure and mechanical properties of the joints. The results show that the microstructure of the joint is composed of a lath-like eutectic region, a lamellar eutectic structure and a dendrite subeutectic region, and a welded joint with excellent mechanical properties is obtained.
In terms of improving the tensile strength of copper-aluminum dissimilar metal joints, researchers have proposed different optimization methods, such as increasing the proportion of aluminum in copper-aluminum alloy welds and giving full play to the energy regulation role of laser welding power. Especially in recent years, the application of composite laser technology can accurately adjust the power density and distribution of the laser in the welding process, improve the welding stability and reduce the welding spatter.
Scholars from General Motors and Southern Methodist University in Texas [17] have conducted in-depth research on laser welding of copper-aluminum dissimilar metals. They used fiber lasers and disc lasers to lap weld copper and aluminum, and analyzed the microhardness and shear strength of the weld microstructure. In addition, the four-point test method was used to study the variation law of weld resistivity. The results show that the main factor affecting the quality and appearance of the weld is the type of base metal above, and when the aluminum is on the top, the surface of the weld will be obviously convex, and the penetration depth of copper and aluminum is shallow. When lap welded with copper-aluminum-copper or aluminum-copper-aluminum "sandwich" construction, the microhardness and resistivity of the joint are large, due to the formation of intermetallic compounds in the weld. They believe that the weld quality can be optimized by adjusting the welding process parameters, but further research is still needed to prove it, especially the formation mechanism of intermetallic compounds in the weld requires a lot of exploratory research work.
Some characteristics of copper and aluminum themselves determine that it is difficult to achieve a good connection by traditional fusion welding methods, while friction stir welding, electromagnetic pulse welding and other solid phase connection methods are more suitable for welding between copper and aluminum, and researchers at home and abroad have also done a lot of experimental research on this and achieved fruitful results. In addition, as long as the appropriate brazing metal and flux are selected, the ideal brazing joint can also be obtained. Finally, laser welding technology has greatly promoted the application of copper-aluminum connection, especially in the field of high-quality and new energy power batteries. Composite laser welding technology and annular spot welding are new types of laser welding in recent years, which have great advantages in improving welding stability and reducing welding spatter, and their welding mechanism and application results need to be further studied and improved.
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2.Tu Jiaxi, Wu Hongyan, Liu Guanpeng, Chen Yuhua. Design and welding process of resistance brazing device for parallel electrodes of multi-strand wires of microfilaments and heterogeneous materials. Precision forming engineering. 2023(08): 211-219 .
Article** — Metal World.