Abstract: The interest in steel/aluminum structure parts is continuously increasing in modern transportation tools, however, such the technical problems as the generation of Fe-Al brittle intermetallic compounds need to be solved urgently for steel and aluminum dissimilar metal welding quality. Deep penetration laser welding test with Si powder is carried out on the DP590 dual phase steel with thickness 1.4 mm and the 6016 aluminum alloy with thickness 1.2 mm. By using optical microscopy, scanning electron microscopy, X-ray diffraction and tensile test, the microstructure, fracture morphology, interface element distribution, main phase and mechanical properties of joints are studied. Metal vapor/plasma shape of welding process is simultaneously shooted by using high-speed cameras, and emission spectrum of metal vapor/plasma is also collected by using spectrometer. The results indicate that the morphology of welding surface is well, and some defects such as pore and crack is not seen when the welding power is 2 000 W, welding speed is 50 mm/s and the defocus distance is + 2 mm, and Ar gas as the protection gas and the flow rate is 15 L/min. The average linear load of the welding sample with Si powder reaches 108 N/mm. Compared to that without Si powder, the average linear load of joint increases by 8%. The laser energy is more easily absorbed by the sample, which leads to the weld penetration increasing, while there is no obvious change for the Al content on both sides of weld width, the grain in the weld zone is not coarsened, the fluidity of steel and aluminum molten pool is improved, the thickness of the intermetallic layer is decreased, in addition, Al_0.5 Fe₃Si_0.5 phase with better ductility is obtained by the metallurgical reaction among the Si, Fe and Al elements, which retard and reduce the generation of Fe-Al brittle intermetallic compounds. Hence, the mechanical properties of joint can be improved with Si powder.

Key words: deep penetration laser welding, metallurgical reaction, Si powder addition, steel/Al dissimilar metal