Effect of the heat treatment on the mechanical properties and microstructure of Scalmalloy® manufactured by Selective Laser Melting (SLM) under certified conditions

Abstract

Mechanical properties and microstructure of Scalmalloy® processed by Selective Laser Melting (SLM) under certified conditions are studied. Two types of samples are considered: as-built and heat-treated at 325⁰C for 4 hours. The heat treatment does not modify the grain size or the texture. SEM images show a typical bi-modal microstructure comprising equiaxed fine grained zones (FGZ) with ≈ 0.6 micron grain size and coarse grained zones (CGZ) with ≈ 7.5 micron grain size (longitudinal axis). According to electron backscatter diffraction (EBSD) analysis, FGZ have ≈ 0.8 micron equivalent circle diameter (considering grain boundaries those with misorientation > 15⁰) and contain areas with and without texture, while CGZ have ≈ 2.7 micron equivalent circle diameter, and most of the grains are oriented with [001] crystal direction along the building direction. On the contrary, the heat treatment produces a stress relief that is evidenced in EBSD by reductions in the low angle grain boundaries (LAGB) content, grain orientation spread (GOS), and dislocation density. Both samples show plastic deformation before fracture and the as-built condition features Portevin-Le-Chatelier (PLC) effect. The heat-treated samples exhibit the highest strength (yield strength = 454 ± 3 MPa; tensile strength = 509 ± 6 MPa) and hardness (165 ± 3 HV) while the lowest percentage of reduction of area (16 ± 6%) and elongation (11 ± 2%). Finally, the electrical conductivity of the material increases after the heat treatment which is associated with the redistribution of Mg-atoms out of the solid solution and the growth of precipitates