Texture, microstructure and mechanical anisotropy in SLM processed superalloys

Abstract

Samples of Nickel‐based (IN738LC and Hastelloy X) and Cobalt‐based (Mar‐M509) alloys were built by selective laser melting (SLM). To evaluate the anisotropy in the mechanical behavior of the material due to layer‐wise build up, specimens were built with their cylinder axis (loading direction) oriented either parallel to the building direction, or perpendicular to the building direction. After building up the specimens by SLM, they were investigated either under the “as‐built” condition or after heat treatment. The analysis of microstructural anisotropy in SLM made specimens was done by using EBSD, EDX and X‐ray texture analysis methods, and then correlated with anisotropic material behavior observed during tensile and creep testing at room temperature and 850°C. All Ni‐based samples possess the same general texture, with the majority of grains forming one single component of a cube texture with one of the cubic axes parallel to the building direction, and another cubic axis parallel to the laser scanning direction. The Young's modulus determined during tensile testing is lowest parallel to the building direction and parallel to the laser scanning direction. By applying suitable laser scanning strategies, the possibility to switch from transverse anisotropic to transverse isotropic properties and reverse is demonstrated for triple layered tensile samples. While the Ni‐based alloys exhibit coarse and elongated grains with a sharp texture and thus a pronounced mechanical anisotropy, the Co‐based alloy shows smaller grains with only moderate structural and mechanical anisotropy. These differences are discussed with respect to different recovery and recrystallization behavior of the two groups. Furthermore, recrystallisation heat treatment leads to a weakening of the texture and a reduction in mechanical anisotropy. The anisotropy of Young's modulus was modeled based on the single crystal elastic tensor and the measured crystallographic preferred orientations, and compares well with the data from tensile tests.