Research on microstructure and mechanical properties of laser keyhole welding–brazing of automotive galvanized steel to aluminum alloy

Laser keyhole welding–brazing (LKWB) technique with fiber laser was used for butt joining of automotive galvanized steel to aluminum alloy using a filler wire of 4043 aluminum alloy (Al–5Si) without groove. The dissimilar joint was obtained both by welding the aluminum alloy with the filler wire in keyhole welding mode, and by brazing the molten aluminum alloy and filler wire to the solid galvanized steel. The results show that the galvanized steel was brazed by the molten aluminum alloys without melting. There was a thin layer of the intermetallic compositions (IMCs) on the interface between galvanized steel and welded seam. The thickness of IMCs layer was varied with joint zone ranged from 1.5 μm to 13 μm. The IMCs layer was composed of three different phases: α(τ5)-Al8Fe2Si, θ-Al13Fe4 and ζ-Al2Fe with microhardness ranged from ∼811 HV to ∼1060 HV, compared with ∼53.5 HV in brazed seam and ∼480 HV in galvanized steel substrate. Transverse tensile test results indicate that the maximum tensile strength of the butt joint reached 162 MPa and ductile fracture occurred at heat affected zone (HAZ) of aluminum alloy with appearance of necking in tension.