Study on CMT welding-brazing joint microstructure of 7A52 Al alloy-steel stud and influence of Ni coating

Q235 steel studs and 7A52 aluminum alloy were joined by cold metal transfer (CMT) welding method with Al-Si alloy wire (ER4043) as the filler metal. Good weld forming with stable welding process was obtained under the welding current of 115~135 A, arc voltage of 14.5~16.5 V, welding speed of 0.3 m/min. The microstructure of the joint was studied by means of scanning electron microscope (SEM) and energy dispersive x-ray (EDX). The results showed that fusion welding interface was formed in the 7A52 aluminum alloy fusion zone, and reaction layer was formed in the steel stud fusion zone which grew thinner from the root toe area to weld toe. The reaction layer was composed of Fe₂Al₅ near the steel stud and FeAl₃ near the weld metal. Nickel-rich zone was formed in the weld toe area of the fillet weld, which was composed by eutectic structures of Al and Al₃Ni, and a small amount of Al₃Ni₂. Comparing with the microstructure and mechanical properties of joints without Ni coating, the results showed that the properties of Ni coating during the welding process decreased the thickness of reaction layer and the brittleness of the joint. The highest shearing strength was 146.9 MPa, rising by 15%~19% as compared with joints without Ni coating, which had met the requirements of high strength Al alloy stud welding.