Effect of Si content on the interfacial reactions in laser welded-brazed Al/steel dissimilar butted joint

Abstract

6061-T6 aluminum alloy and DP590 steel were joined successfully by a laser welding–brazing process with pure Al, AlSi5, and AlSi12 filler metals. Interfacial microstructure of the joints with highest tensile strength (under laser powers of 2000 W, 2000 W and 2500 W for joints obtained with pure Al, AlSi5 and AlSi12 filler metals, respectively) were selected for comparison. The interface produced with the pure Al filler metal consisted of a thick layer of η-Fe2Al5 with scattered rod-shaped θ-FeAl3, with average thicknesses of 10.1 μm in the bottom region and 16.7 μm in the top region. When 5 wt.% Si was added to the filler metal, the interfacial intermetallic compound (IMC) components remained similar, whereas the interface thickness decreased to 3.8 μm in the bottom region and 7.5 μm in the top region. On further increasing the Si addition to 12 wt%, the interfacial IMC acquired 1.2 μm-thick τ5-Fe2Al8Si in the bottom region with 5.6-μm-thick θ-Fe(Al,Si)3 and τ5-Fe2Al8Si in the top region. The chemical potential of Si in the Fe-Al-Si ternary system was lower at the Fe-Al side comparing with weld seam and steel substrate, which led to a preferentially diffusion of Si to the Fe/Al interface and finally caused the aggregation of Si in the Fe/Al interface. Si addition improved the joint strength and reduced the required laser power for a suitable joint. The joint produced with the AlSi5 filler metal had the highest tensile strength and largest fracture displacement.