Improved strength-ductility synergy of directed energy deposited AZ31 magnesium alloy with cryogenic cooling mode

Abstract

Motivated by the effect of cooling rate on microstructure, standard air cooling (AC) and novel cryogenic cooling (CC) were employed in the wire-arc directed energy deposition process of Mg-alloy. The influences of cooling modes on macrostructure, defects, microstructure, mechanical properties and deformation behaviour were systematically investigated. Compared with AC component, CC component exhibits improved performance with yield strength, ultimate tensile strength and elongation increased by 50%, 58% and 174%, respectively. The improved strength-ductility synergy can be attributed to the decreased porosity, profuse nano-precipitates, finer equiaxed grain within inter-layer and the transformation of columnar to equiaxed grain within intra-layer. The enhancement of strain hardening rate by highly coherent nano-precipitates impeding dislocations glide, dynamic Hall-Petch effect of multiple twins and numerous 3D obstacle networks of dislocations should be responsible for the significant elongation improvement. This study thus introduces new insights into defect alleviation, microstructure modification and performance enhancement of additive manufacturing Mg-alloys.

Keywords

• wire-arc directed energy deposition
• cryogenic cooling mode
• defects
• microstructure evolution
• strength-ductility synergy