Mechanical properties and microstructure of gas tungsten arc welded magnesium AZ91D plates

Gas tungsten are (GTA) welds were performed on 4-mm thick plates, machined from as-cast magnesium AZ91D ingots. The microstructure and defect formatio n was investigated by optical and scanning electron microscopy. Mechanical properties were determined by standard tensile tests on small-scale specime ns. A wide heat affected zone (HAZ) (> 3 mm) was created adjacent to the fu sion line that consisted of two regions: (1) a partially melted zone (PMZ), created near the fusion line; and (2) a wide region, which was heat affect ed without melting. It was found that, after resolidification, a continuous Al12Mg17 phase existed along the PMZ grain boundaries that markedly reduce d the joint strength to below that of fully annealed Mg-AZ91D alloys. In co ntrast, specimens machined from welded metal exhibited improved strength an d ductility that resulted from the microstructural refinement caused by the rapid cooling during resolidification of the fusion zone. When the creatio n of a large PMZ was prevented, as in electron beam welding, the joint stre ngth was comparable to that of the base metal. (C) 2001 Elsevier Science B. V. Ai rights reserved.