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 β matrix was predominantly by a combination of slip and α′′ martensite which formed as a primary product at columnar and sub-columnar grain boundaries. However, the operation of {332}〈113〉 and {112}〈111〉 β-twinning was also determined, howbeit with a very small surface fraction. This implies a small surface fraction of secondary α′′ martensite forming within β-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 α 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 β-grains were refined by a combination of precipitated α and deformation induced β-twins and martensite.