Study On The Mechanism Of Dissimilar Joining Aluminum Alloy To Pure Copper By Laser Welding

Aluminum-copper dissimilar connection is currently widely used in new energybatteries and some electronic products.However,due to the differences in thermal-physical properties between aluminum and copper,brittle intermetallic compounds will be formed in the welded joints.Laser welding has the characteristics of lower heat input and fast cooling speed,which is especially suitable for the welding of such dissimilar metals.In view of the above application scenarios,the products have strict requirements on volume and weight.The thickness of aluminum and copper dissimilar materials is usually only dozens to hundreds of microns.Thermal input of traditional laser is too large,and nanosecond pulse laser thermal input is small and can be controlled more accurately.At the same time,aluminum and copper are high reflectivity metals,and the peak power density of nanosecond pulse laser can overcome this limitation.At present,nanosecond laser is a kind of short pulse laser,which is mainly used in laser marking,laser drilling,laser rapid prototyping and other aspects.It is rarely used in welding.The process,microstructure analysis and performance test of nanosecond laser welding of dissimilar metals still need to be further studied.Because the size of a single welding joint formed by nanosecond laser is toosmall to form a reliable connection,spiral welding track is formed by scanning laser galvanometer,and the macroscopic morphology is similar to the circular welding spot.Based on the above experimental equipment,the average power of nanosecond pulse laser,the scanning speed of laser and the gap of scanning track were set as parameters to weld the two superposition forms of copper on aluminum and aluminum on copper,and the influence of parameters on the welding forming was studied.The experimental results show that the process window of the sample under aluminum and copper is narrow and optimal forming only can be achieved within part of parameters,while the sample under copper on aluminum is consistent.Therefore,the sizes of different characteristic regions of typical joint forms are measured and compared.By comparing the forming of copper on top of aluminum and aluminum on top of copper,it is found that when copper on top of aluminum,the welding seam consists of various small welding seams with a large depth-width ratio that are"nail"shaped through copper and aluminum,while when aluminum on top of copper on bottom,the metal on the aluminum side completely melts,and part of aluminum invades the copper base material to form a"V"shaped small welding seam.The microstructure of nanosecond pulsed laser welding joints of aluminum-copper dissimilar metals was studied.Physical phase and composition analysis of two kinds of joints with different superposition forms of copper on top of aluminum and copper on top of aluminum were carried out respectively.The experimental results showed that there was no significant difference in the microstructure under different forms of superposition,and from the copper side to the aluminum side,four characteristic regions were presented,namely?₂-Cu₉Al₄,(+Eutectic(?+?),Lump(?-Cu Al₂)),Eutectic(?-Al/?-Cu Al₂),and subeutectic(?-Al,Eutectic(?+?)).The main difference is the distribution of different tissues in the weld.Finally,tensile test and fracture analysis were carried out on the welded joints,and the properties and fracture mechanism of the joints were characterized.In addition,nanoindentation test was carried out on the micro-area of the joints.The results show that the main fracture form of joints is interface fracture,and the composition at the fracture point indicates that it is mainly the concentration distribution of the?-Cu Al2 at the interface,which breaks under the action of shear stress.Nanoindentation test showed that macroscopically,the hardness of the weld zone was higher than that of the base material,and microscopically,Lump(?-Cu Al₂)>?₂-Cu₉Al₄>+Eutectic(?+?)>Eutectic(?+?)>?-Al+Eutectic(?+?).