Influence of Aluminum Content on Grain Refinement and Strength of AZ31 Magnesium GTA Weld Metal

Gas-tungsten arc (GTA) welds were prepared on 3 mm thick sheets of continuous cast and rolled AZ31 magnesium alloy, welding over pre-placed die-cast inserts (AM20, AM50, AM60 and AZ91) in order to systematically vary the aluminum content in the weld metal.  Microstructure and compositional analysis of the weld metal was examined using optical and scanning electron microscopy (HR-SEM, EBSD and EDS/X).  The results showed that the average weld metal grain size decreased from 74 to 41 μm as the aluminum content increased from 2.4 to 5.0 wt.%.  This observed grain refinement is believed to be related to the increased constitutional undercooling associated with higher aluminum content.  Weld strength and hardness, and to limited extent ductility, were also observed to increase with aluminum content, as determined from cross-weld tensile tests.

Keywords: Magnesium Alloy AZ31, Weld Metal, Grain Refinement, Growth Restriction Factor, Variable Polarity Current, Weld Microstructure, Aluminum Content, Tensile Strength