Stress corrosion cracking of gas-tungsten arc welds in continuous-cast AZ31 Mg alloy sheet

AZ31 Mg alloy sheet was welded using a gas-tungsten arc (GTA) process over inserts containing 2.3-9.3 wt.% Al. The welded specimens were susceptible to SCC in distilled water, with susceptibility increasing with decreasing weld metal Al (or beta particle) concentration. Primary stress corrosion cracks initiated at the weld metal-HAZ interface by stress-assisted localised dissolution and propagated through the weld and base metals by transgranular and intergranular H-assisted fracture (TG-HAF and IG-HAF) respectively. The IG fracture mode may be intrinsic to the texture imparted upon the base metal by rolling. The increase in SCC susceptibility with decreasing weld metal Al concentration is contrary to the purported roles of beta particles in promoting localised corrosion and as crack nucleation sites, but corresponds with increases in weld - base metal galvanic current density and weld metal localised corrosion susceptibility. (C) 2009 Elsevier Ltd. All rights reserved.