Unraveling the inherent anisotropic properties of in situ alloyed copper-modified titanium alloys produced by laser powder bed fusion

Abstract

Preventing anisotropy in materials produced by laser powder bed fusion (LPBF) has become a major challenge. In this work, we achieved good mechanical properties in LPBF-produced titanium by promoting the columnar to equiaxed transition (CET) though alloying with copper to produce a Ti-5 wt% Cu alloy. Horizontal specimens at different build heights all consisted of mainly equiaxed prior-β grains (∼ 13 µm) with ultrafine α lamellae (∼ 0.19 µm width) that exhibited weak textures. Thermal cycling along the build direction (BD) not only promoted the segregation of Cu from top to bottom, but also converted the compressive residual stress (−109.0 ± 22.2 MPa) to tensile residual stress (10.4 ± 9.7 MPa). The friction coefficient, hardness, and wear resistance found in different sections of the build were all very similar. The wear mechanism of the specimens was a combination of abrasive and delamination wear. The yield strength, ultimate strength, and elongation of specimens from the top section of the build were 993 ± 18 MPa, 1145 ± 60 MPa, and 8.6 ± 0.2%, respectively. While the strength of the middle and bottom specimens remains at the same level as that of the top specimen, a reduction in elongation was observed due to the segregation of Cu. Additionally, the strength of vertical specimens was about 50–70 MPa higher than that of horizontal specimens, which is thought to be caused by residual stress gradient along the BD.