Effects of Post-Weld Heat Treatment on the Microstructure and Mechanical Properties of Automatic Laser-Arc Hybrid Welded AZ31B Magnesium Alloys

Abstract

The aim of this study was to determine the microstructural evolution, tensile characteristics, and strain-hardening response of AZ31B magnesium alloy welds as influenced by post-weld heat treatment (PWHT). Thus, the AZ31B alloy was welded by using a low-power pulsed Nd:YAG laser-arc hybrid welding equipped on the six-axis welding robot in the present study. Microstructure, mechanical properties and strain-hardening behaviors of the AZ31B joints under various post-weld heat treatment (PWHT) temperatures were characterized. As the heat treatment temperature increases, the grain size of the welded joint gradually increases, and the amount of β-Mg17AI12 phase noticeably decreases. The mechanical properties of the welded joint specimens showed a significant enhancement when subjected to heat treatment at 300 °C and 350 °C for 20 min. Especially, after 350 °C heat treatment for 20 min, the ultimate tensile strength (UTS) and elongation (EL) of specimen were 339.6 MPa and 20.1%, respectively, which were up to 99.5% and 98.5% of the AZ31B base material (BM). The strain-hardening capacity of specimens is significantly influenced by the grain size. Due to having the largest grain size, the 400–20 min specimen exhibited the highest hardening capacity and strain hardening exponent. In Kocks-Mecking type curves, both stage III and stage IV were observed in BM and joint specimens. At higher net flow stresses, the strain hardening rate in the 400–20 min joint specimen was higher due to the larger grains, which allowed for more dislocation accommodation and improved the capacity for dislocation storage.