Effect of Nickel Foil Thickness on Microstructure and Microhardness of Steel/Aluminium Alloy Dissimilar Laser Welding Joints

Laser lap welding between DP980 steel and 6061 aluminum alloy was carried out using IPG YLS-6000 fiber laser, nickel foil with different thickness (0.10, 0.20 and 0.30 mm) was used as interlayer. The microstructure was observed by SEM, EDS was used to determine the distribution of elements and intermetallic compounds (IMC) types, and the hardness was determined by HV1000IS. The results showed that the microstructure in weld seam (WS) and fusion line (FL) of welded joint without nickel foil was δ ferrite and lath martensite (LM). For welded joints with nickel interlayer, the δ → γ phase transition was promoted by nickel elements. Thus, with the thickness of the nickel foil increased, the content of nickel elements in weld pool increased, resulting in decrease of δ ferrite content in WS and FL. Meanwhile, the microstructure in WS and FL was full LM when the thickness of the nickel foil was 0.30 mm. Meanwhile, the Ni_xAl_y IMC was found in interface, which inhibited Fe–Al metallurgical reaction and reduced the thickness of Fe–Al IMC. Therefore, with nickel foil thickness increased, the content of nickel elements and the Ni_xAl_y IMC increased. Moreover, the maximum thickness of the IMC layer was able to reduce to 10.66 when the thickness of the nickel foil was 0.10 mm. With the reduction of δ ferrite content in WS and FL, the microhardness in WS increased with the thickness of the nickel foil increased. However, because the NixAly IMC was a soft phase, the microhardness in interface decreased, significantly.