Improved strength and ductility of AZ31B Mg alloy sheets processed by accumulated extrusion bonding with artificial cooling

Abstract

In this study, accumulated extrusion bonding (AEB) process with application of artificial water cooling was successfully performed to fabricate fined-grains AZ31B Mg alloy sheets at 150°C, 200°C and 250°C. The resultant microstructure and mechanical properties are systematically investigated. It reveals that the processing temperature has an important effect on the microstructural evolution during extrusion. During AEB process at 150°C and 200°C, {10–12} tensile twinning was activated at early stage of extrusion, and subsequently continuous dynamic recrystallization (CDRX) occurred and dominated the further deformation. However, for the sample extruded at 250°C, hardly any twins can be observed, and new fined dynamic recrystallized grains were found along grain boundaries. Artificial cooling was utilized to reduce the rate of grain growth out of the extrusion die, resulting the grains significantly refined from 11 µm to 2.5 µm. Local high dislocation density region was also observed in the microstructure of sample processed at 150°C in artificial cooling condition, and the degree decreased with the processing temperature increase. The results summarized from tensile tests indicated that due to the grain refinement the strength and ductility was significantly enhanced (YS of 186 MPa vs. 145 MPa, UTS of 391 MPa vs. 336 MPa and FE of 31.5% vs. 24.5% compared with the as-received sample). Subsequently, annealing treatment at different temperatures was applied to eliminate the high dislocation density. The sample annealed at 200°C exhibited the best comprehensive mechanical property with YS of 179 MPa, UTS of 390 MPa and FE of 33.0%. As the annealing temperature increasing, the dislocation density was reduced by static recrystallization (SRX) and grain growth, leading to a decreased strength and ductility.