Microstructure of a modulated Ti-6Al-4V – Cu alloy fabricated via in situ alloying in laser powder bed fusion

In this work, Ti-6Al-4V was in situ alloyed with 15 wt% Cu using laser powder bed fusion (LPBF) to investigate the influence of the eutectoid alloying element Cu on microstructural features. The chemical inhomogeneities owing to in situ alloying are used to allow simultaneous solidification of differently composed material. After LPBF, the matrix material consists of metastable, equiaxed β-Ti and Ti₂Cu precipitates. Cu stabilizes β-Ti, and the unique solidification conditions with high thermal supercooling cause the equiaxed grain morphology and the formation of Ti₂Cu precipitates. Inclusions, assigned incompletely melted Ti-6Al-4V powder particles, consist of α’/α-Ti. In the transition zone between β-Ti and α’/α-Ti, orthorhombic α’’ is found as nano-sized crystals. For the first time, β-stabilization across a wide area through Cu with a composition of Ti_76.2Al_5.7V_3.2Cu_14.9 and the formation of α’’ with the composition of Ti_80.2Al_5.4V_3.3Cu_11.1 is shown. It is demonstrated that Cu is a very versatile alloying element in Ti-6Al-4V, significantly impacting microstructural development. Hence LPBF in situ alloying allows more than one alloy composition to be investigated for microstructural features in a single experiment, which could be an interesting approach to alloy development.