Effect of Heat Treatment on Microstructure and Microhardness of Friction Stir-welded Joints for AZ31B Magnesium Alloy

Friction stir welding and postweld heat treatment were conducted on AZ31B magnesium alloy.The variation rule of microstructure and microhardness of welding joints before and after heat treatment was studied.The results show that as the annealing temperature increase,boundaries at both the advancing side and the retreating side of the welding joint disappeared gradually.During the process of heat treatment,energy obtained by welding nugget zone (WNZ) was mainly responsible for the grain growth,but more energy had been consumed during recrystallization in dilution zone of heat affected zone (HAZ) and thermo-mechanically affected zone (TMAZ),which caused the grain growing speed in WNZ was higher than that of in the dilution zone.When the annealing temperature was under 200 ℃,the microhardness of TMAZ was higher than that of WNZ.Moreover,the mircohardness at the advancing side of TMAZ was slightly higher than that of at the retreating side.When the annealing temperature was above 250 ℃,the microhardness of decreased distinctively.As the annealing temperature increase,the microhardness of HAZ decreased slightly.