Effects of heat treatment on powder surface conditions and rheological behavior of Ti alloy powders

• A clean Ti64 powder system without changing particle size or shape was successfully established via heat treatments. • Ti64 powders heat-treated at 773 K exhibited a high band gap due to the formation of crystalline surface layer containing alumina nanostructures. • The smaller band gap contributed to more hydroxyl groups and worse powder flowability. • The relationship between surface hydroxyl groups and powder cohesion of Ti alloy powders was first clarified. The flowability of feedstock powders is crucial for producing reliable metallic parts using laser powder bed fusion (L-PBF). However, the effects of the powder properties on the powder rheology remain poorly understood because of the complexity of the contributing factors. In this paper, a facile heat treatment approach is proposed to modify the surface conditions of Ti-6Al-4 V alloy powders without altering their particle size or morphology. This “clean powder system” enables a direct investigation of the relationship between the surface conditions and powder flowability, minimizing interference from other variables. Specifically, Ti-6Al-4 V alloy powders intentionally oxidized in air at the processing temperatures of 573 and 773 K were referred to as Ti64-573 and Ti64-773, respectively. Microstructural observations revealed that an amorphous layer with a higher oxygen content was formed on the surface of the Ti64-573 powder, whereas a crystalline layer containing alumina nanostructures was formed on the surface of the Ti64-773 powder. Consequently, the Ti64-773 powder exhibited the highest band gap (3.2 eV), which contributed to the reduced cohesive strength and, in turn, improved the powder bed packing owing to the presence of fewer hydroxyl groups and hydrogen bonds.