Effect Of Rolling On The Microstructure, Mechanical Properties Of AZ31 Mg Alloy Joints

Two different post-weld rolling processes are conducted to improve the poor mechanical prosperities of TIG welded AZ31 joints, which are regular rolling and partial rolling. The influence that rolling processes, deformation temperature and rolling reduction exert on mechanical prosperities of TIG welded AZ31 joints were investigated. The following results are obtained:① Effect of full rolling process on the microstructure, mechanical properties of AZ31 welded sheets.After annealing at 300℃ for 2 hours, welded sheets were rolled at 350℃ under different rolling passes and reductions of 10%,27%and 43%,respectively.Optical microscopy(OM), scanning electron microscopy(SEM), tensile tests and micro-hardness tests were carried out to study the changes of microstructure and mechanical prosperities during the rolling processes. It indicates that different degree of DRX has taken place in the FZ, HAZ and BM with the rolling reduction. Fully DRX has occurred and the average grain size of FZ, HAZ and BM became almost consistent with each other with a reduction of 43%.The UTS and the micro-hardness increases with the reduction. The UTS of welded joints reached 97% of the corresponding BM, which means the UTS of welded joint became closest to that of BM. Welded sheet with a reduction of 27% was annealed at 180℃for 1hour and only recovery was observed without obvious recrystallization. No microstructural change was seen before and after the annealing. The UTS decreased from 283 MPa to 261 MPa after the annealing, while the EL increased from 1.4% to 10.9%.② Effect of partial rolling process on the microstructure, mechanical properties of AZ31 welded sheets.After annealing at 300℃ for 2 hours, welded joints with excess weld bead were partially rolled at room temperature(cold rolling),250℃(warm rolling)and 400℃(hot rolling) with different reductions. were rolled at 350℃ under different rolling passes and reductions of 10%,27%and 43%,respectively.Optical microscopy(OM), scanning electron microscopy(SEM), tensile tests, micro-hardness tests, XRD and cup drawing tests were conducted. Some twins were observed in HAZ at different temperatures. However, DRX was initiated during warm and hot rolling with certain reduction. Some grains were refined and the average grain size decreased. The dominantstrengthening mechanism differs at different temperatures, while the UTS, EL and micro-hardness of the joints were improved as the rolling reduction increased. Once rolling reduction exceeds a critical value, tensile fracture location will be transferred from the FZ to the BM and the joints can achieve tensile properties roughly the same as those of the BM. It can be estimated that the CRR for the joints rolled at room temperature,250℃and 400℃ were about 20%,30%and 40%.The CRR increased as the rolling temperature increased and the basal texture intensity of the FZ increased. That is to say, rolling reduction is the dominant factor in affecting the macro-texture of the joint, even though high rolling temperature is favorable for weakening the basal texture during the rolling process. Welded sheets cold rolled with the critical reduction presents the maximum Erichsen value due to the fact the strong basal texture can resist the load along RD, but the formability was bad because the crack preferred to initiate and propagate.