Evaluation of Grain Refinement and Mechanical Properties of Additive Friction Stir Layer Welding of AZ31 Magnesium Alloy

This work examines the grain refinement and mechanical properties of additive friction stir layer welding (AFSLW) of a Mg Alloy. The AFSLW process was used to additively fabricate a seven-layer tall build using 5-mm-thick plates of AZ31 Mg alloy, with each layer containing three parallel welding passes. This multipass AFSLW process demonstrates the ability of this additive method to produce tall and wide structures free of volumetric defects. While the AFSLW material was found to have lower monotonic tensile and fatigue strengths as compared to wrought Mg AZ31-H24, this result was expected due to the heat input from the additive layer welding process that relaxed the cold work strengthening mechanisms. Metallography cross sections and analysis via scanning electron microscopy revealed that the AFSLW build did not contain any detectable defects in any of the 21 weld passes. Furthermore, postmortem fractography found fatigue crack initiation mechanisms to be similar in both the AFSLW and wrought AZ31-H24. The results of this study indicate that large, defect-free builds with predictable mechanical properties can be achieved using the AFSLW process.