Effect of Cu/Ni foil interlayer on microstructure and mechanical properties of laser welded aluminum/steel joints

Laser welding of aluminum/steel dissimilar metals with Ni and Ni/Cu foils as intermediate layers was studied in this paper. The effect of Cu/Ni foil on microstructure and properties of dissimilar laser welded aluminum/steel joints was systematically investigated. Scanning electron microscope (SEM) and its own energy dispersive spectrometer (EDS) were used to observe and analyze the microstructure of the weld cross section and the element distribution in each area. X-ray diffractometer (XRD) was used to analyze the main phase of the welded joint, and tensile testing of welded joints using an electronic universal testing machine to characterize its mechanical properties. The results show that the tensile strength of the welded joint is greatly improved when Cu/Ni foil is added, compared with Ni foil as the interlayer only. The most obvious effect is that Cu foil with a thickness of 0.02 mm is added. The addition of Cu foil changes the phase composition, element distribution and microstructure of the aluminum/steel interface, and increases the fluidity of the molten pool. In the brittle phase of Fe-Al near the stainless steel side, some Fe atoms are replaced by Cu and Ni to form a new ternary ductile phase, which inhibits the generation of Fe-Al binary brittle intermetallic compounds and effectively improves the weldability of aluminum/steel. Therefore, the addition of Cu/Ni foil can effectively improve the metallurgical reaction in the process of aluminum/steel dissimilar laser welding, so as to improve the mechanical properties of welded joints.