Effect of thermomechanical processing on compressive mechanical properties of Ti-15Mo additively manufactured by laser metal deposition

A bidirectional powder deposition strategy was employed to additively manufacture Ti-15Mo wt% using laser metal deposition. The as-built alloy was subsequently subjected to post-fabrication uniaxial thermomechanical processing at strain rates of 0.00055s(-1), 0.0011s(-1), 1s(-1), and 4s(-1), with strains of 20 % and 40 %. Experiments were conducted at room and elevated temperatures. Phase identification, elemental and microstructural characterisation were conducted using x-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. The three distinct zones, namely the fusion, remelted and heat affected zones, identified in each deposited layer of the as-built microstructure were retained after thermomechanical processing. After processing, electron backscatter diffraction was used to analyse deformation mechanisms. Deformation accommodation in beta matrix was predominantly by a combination of slip and alpha '' martensite which formed as a primary product at columnar and sub-columnar grain boundaries. However, the operation of {332}< 113 > and {112}< 111 > beta-twinning was also determined, howbeit with a very small surface fraction. This implies a small surface fraction of secondary alpha '' martensite forming within beta-twins in the deformed microstructure. Compressive mechanical properties showed a strong dependence on strain rate as higher flow stress and compressive strength were obtained at higher strain rates. Grain structure homogenisation was not achieved after thermomechanical processing as there were alpha dominated regions as well as martensite/twin dominated regions which implies that an-isotropic tensile properties would emerge after tensile deformation on multiple pre-TMCPed samples. However, columnar beta-grains were refined by a combination of precipitated alpha and deformation induced beta-twins and martensite.

Automated summary

This study employed laser metal deposition to additively manufacture Ti-15Mo wt% alloy, and subsequently subjected it to post-fabrication uniaxial thermomechanical processing. The results showed that different zones in the microstructure remained after processing, and deformation mechanisms mainly involved slip and martensite formation. The compressive mechanical properties were found to be dependent on strain rate, with higher flow stress and compressive strength observed at higher strain rates. Grain structure homogenisation was not achieved, leading to anisotropic tensile properties.