Effect of build orientation in the SLM process on the microstructure, dislocation structure, and tribological properties of a CoCrWMo alloy

Abstract

This study investigates the effect of build orientation on the microstructure, mechanical response, dislocation behaviour, and wear mechanisms of CoCrWMo alloy produced by Selective Laser Melting. Samples were fabricated at seven build angles (0°–90°) and analysed using SEM, XRD, EBSD, nanoindentation, and ball-on-disc tribological testing. A strong correlation was obserwed between grain orientation and wear behaviour. Low-angle samples exhibited well-aligned columnar grains with high dislocation mobility and mild abrasive wear, whereas high-angle samples showed misaligned grains, reduced dislocation activity, and delamination-driven wear. Quantitative analysis revealed that microstrain decreased from approximately 0.0058 at 0° to 0.0031 at 90°, while crystallite size increased from 19 to 57.8 nm, indicating significant microstructural relaxation at higher build angles. EBSD analysis performed near plastically deformed regions revealed a strain-induced FCC→HCP phase transformation at higher build angles. The presence of the HCP phase leads to the formation of additional phase boundaries, which modify the local deformation behaviour. Dislocation mobility decreased by nearly an order of magnitude, from 4.48 × 10⁻⁹ m/s at 0° to 0.56 × 10⁻⁹ m/s at 90°, reflecting increasing constraints on plastic deformation. Despite similar wear volumes across orientations, SEM analysis of wear debris confirmed distinct mechanisms governed by microstructural anisotropy. Although hardness remained relatively constant (4.1–4.4 GPa), Young’s modulus decreased from 182 to 191 GPa to 176 GPa with increasing build angle, indicating pronounced elastic anisotropy. These findings highlight the critical role of grain boundary orientation and dislocation dynamics in controlling the tribological response of SLM-fabricated CoCrWMo alloy and provide new insight into proces-structure-property relationships in additively manufactured materials.