Influence of friction stir processing and aging heat treatment on microstructure and mechanical properties of selective laser melted Mg-Gd-Zr alloy

Abstract

The low ductility of Mg alloy fabricated by selective laser melting (SLM) technique usually restricts its application in several fields. Friction stir processing (FSP) is a promising post-treatment method to solve this problem. A detailed study of how FSP affect pore defects, molten pool boundary, grain morphology, aging precipitates and mechanical properties of SLMed Mg-10Gd-0.2Zr (G10K, wt.%) alloy was conducted. The results show that FSP can lead to porosity reduction from 0.779% to 0.015%, vanishment of molten pool boundary, columnar to equiaxed transition and grain refinement. Thus, a significant enhancement of room temperature tensile properties is reported with a remarkable increase of elongation (EL) from 2.2% to 7.5% without the sacrifice of strength after FSP (while yield strength (YS) and ultimate tensile strength (UTS) increased from 180 MPa, 228 MPa to 202 MPa, 272 MPa respectively). After peak aging heat treatment, the plate aspect ratio and area number density of β' aging precipitates in the FSP-T5 G10K alloy are higher than those of SLM-T5 G10K alloy, resulting in a stronger aging hardening response (hardness increment 28.3 HV versus 22.7 HV). As a result, the FSP-T5 G10K alloy exhibits YS of 285 MPa, UTS of 356 MPa and EL of 1.3%, while those of the SLM-T5 G10K alloy are only 243 MPa, 260 MPa and 0.3%. These findings demonstrate that FSP is very effective for modifying microstructure and enhancing mechanical properties of the SLMed alloys.