Microstructure and mechanical behavior of as-built and heat-treated Ti–6Al–7Nb produced by laser powder bed fusion

Abstract

Ti–6Al–4V and Ti–6Al–7Nb were manufactured with laser powder bed fusion (LPBF). Microstructural comparison study between Ti–6Al–4V and Ti6Al–6Nb was used to understand processability similarities between two different titanium alloys. Quantitative similarities between two alloys revealed that Ti–6Al–4V processing parameters can be used for optimization of Ti–6Al–7Nb. The microstructure, processing, properties relationship and the influence of heat treatments were investigated for Ti–6Al–7Nb. The as-built microstructure was composed of a columnar prior β grains with fine acicular α′ martensite resulting in a yield strength of 1082 MPa and an ultimate tensile strength of 1160 MPa with an elongation of 9.7%. Solutionizing at 1055 °C and aging at 540 °C completely transformed the columnar structure of the prior β grains to equiaxed via phase transformation and grain growth, solutionized the α’ martensite into β and then created a fine lamellar α + β structure with air cooling. The resultant microstructure had reduced strength and hardness but increased ductility. The reduction in yield (871 MPa) and ultimate tensile (940 MPa) strength would be positive to minimize stress shielding of orthopedic implants. The improved elongation of 11.5% meets the requirements for biomedical applications which stipulates an elongation of at least 10% according to the ISO 5832-3 Standard.