Microstructural characterization of binary microstructure pattern in selective laser-melted Ti-6Al-4V

Selective laser melting (SLM) is an additive manufacturing process that offers efficient manufacturing of complex parts with good mechanical properties. For SLM, process parameters and post-processing are of importance as they affect the microstructure and consequently the mechanical properties. A feature in the microstructure, which is formed in SLM due to the fast cooling rate, is a binary microstructure pattern (BMP). The BMP is found in the horizontal plane and is formed with various laser scan angles between adjacent layers. The easiest distinguishable strategy is 90 degrees, which renders a shape similar to a chessboard. In this work, the BMP phenomenon was investigated in detail and a microstructural characterization was performed on the fine microstructure zone (FMZ) that separates the coarse microstructure zones (CMZ), by using light optical and scanning electron microscopes (SEM) that were equipped with electron backscattered (EBSD) and energy dispersive x-ray spectroscopy (EDS) detectors. Moreover, the effect of the process parameter hatch distance on the BMP was investigated and the overlapping between neighboring scan tracks in SLM was found to influence the size of the BMP, while the thickness of the FMZ remained constant. Different post-SLM heat treatments were performed and it was shown that the BMP retained unless the heat treatment temperature reached above the beta transus temperature. EBSD and beta grain reconstruction were performed as well to reveal the columnar beta grain orientations. The result showed that each CMZ and FMZ originates from a respective parent beta grains.