Microstructure and properties of laser brazed magnesium to coated steel

A diode laser brazing procedure has been developed for joining AZ31B-H24 Mg alloy sheet to aluminum-coated, cold-rolled carbon steel sheet in the single flare bevel lap joint configuration using a Mg-Al based welding wire. In this process, the Mg-Al based filler metal and a shallow surface layer of the Mg alloy sheet were melted simultaneously by a diode laser beam, while no melting of the steel sheet occurred. The results of this study suggest that feasibility of this process depends strongly on the preexisting Al-12Si coating layer on the steel sheet that promotes wetting of the Mg-Al filler alloy as well as formation of a layer of θ-Fe(Al,Si) 3 intermetallic compound along the braze/steel interface. From the middle part of the braze/steel interface to the root of the joint, the Al-Si layer melted and mixed into the braze alloy and the intermetallic layer grew up to 8 μm thick. From the middle part of the braze/steel interface to the top of the joint, both the Al-Si and the intermetallic layer were dissolved. These two simultaneous phenomena led to an intermetallic layer with nonuniform thickness ranging from 0 to 8 μm along the braze/steel interface. The average fracture load of the joint was 767 N, representing a 72% joint efficiency relative to the steel sheet. Failure occurred when cracks propagated along the intermetallic layer starting at the root of the bevel joint and moved into the braze metal at the upper part of the joint.