A novel technique to form gradient microstructure in AA5052 alloy

Abstract

In the present research, the microstructure and mechanical properties of AA5052 alloy with gradient microstructure produced by a novel technique based on hot compression bonding were investigated. It was found that a gradient microstructure with a strain-hardened core and a grain-refined shell formed by hot compression bonding. Some recrystallization mechanisms occurred near the surface of the processed samples including discontinuous dynamic recrystallization (DDRX), geometric dynamic recrystallization (GDRX), and particle stimulated nucleation (PSN). The ultimate tensile strength (UTS) strength of the AA5052 significantly increased after the hot compression bonding (280.7 MPa) without too much loss in ductility (18.9%). This was owing to the gradient microstructure (both grain boundary strengthening and strain hardening mechanisms). By comparison, the good combination of strength and ductility of the present AA5052 alloy distinguished it from the previously reported AA5052 alloy processed by other techniques. The Portevin-Le Chatelier (PLC) effect was clearly visible in the stress-strain curves of the initial and processed AA5052. There was an increase in the critical strain and stress with the increase in the number of passes. The formation of the gradient microstructure after the third, fourth, and fifth passes decreased the severity of the PLC effect. In this study, the recovery of strain hardening rate (RSHR) observed for the first time in the AA5052 alloy after the third, fourth, and fifth passes resulting in a good combination of strength and ductility. The fracture surfaces indicated that as the distance from the surface increased, the dimple depth in the fracture surface gradually increased. On the other hand, with increasing the number of passes, the size of dimples decreased owing to the formation of more recrystallized fine grains at the higher number of passes.