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

Abstract

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°, 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 β transus temperature. EBSD and β grain reconstruction were performed as well to reveal the columnar β grain orientations. The result showed that each CMZ and FMZ originates from a respective parent β grains.

Martensite Formation and Decomposition during Traditional and AM Processing of Two-Phase Titanium Alloys—An Overview

“…In the case of titanium alloys (especially Ti-6Al-4V alloy), SLM technology seems to be the most often described in the technical publications among AM methods [17][18][19]23,[68][69][70][71][72][73]. According to their authors, the martensite formation and decomposition belong to the main factors determining mechanical properties of SLM-produced Ti-6Al-4V alloy.…”

“…Neikter et al [73] focused attention on the binary nature of the microstructure developed in a SLM-produced Ti-6Al-4V alloy. The binary microstructure was found in the horizontal plane and was formed with various laser scan angles between adjacent layers.…”

The Effects of Post Heat Treatment on the Microstructural and Mechanical Properties of an Additive-Manufactured Porous Titanium Alloy

“…Figure 5 shows the microstructures of the porous Ti64 samples before and after heat treatment. The microstructure of the as-built sample is mainly composed of acicular α martensite, because of the extremely high cooling rate during laser melting process, making the β domain phase transform to fine α martensite [21]. Some pores were found in the as-built sample, as shown in Figure 5a,b.…”

Tensile Properties of 21-6-9 Austenitic Stainless Steel Built Using Laser Powder-Bed Fusion

“…The solidification mainly depends on two parameters, i.e., thermal gradient G and the solidification growth rate R. G/R ratio defines the microstructure morphology, and G*R defines the cooling rate and hence the refinement of the microstructure; as the G*R value increases, the grain size becomes finer [ 5 , 28 ]. As shown in the IPF maps in Figure 6 , the typical morphology of the columnar microstructure of AM [ 29 ] is observed in the vertical cross-section, whereas a more equiaxed morphology is observed in the horizontal. All four investigated process parameters have a cumulative effect on the energy.…”