Heat treatment of a Beta-Ti21S alloy produced by additive manufacturing

Metastable β-Ti alloys are a class of heat-treatable materials increasing research interest nowadays. Their mechanical properties make them very suitable for high-end applications, such as racing and aerospace. In this study, the heat treatment behaviour of β-21S (Timetal 21S) fabricated by laser powder bed fusion (L-PBF) was investigated, to evaluate its influence on microstructure and mechanical properties. Solution heat treatment (SHT) at 930 °C x for 30 min allowed an equiaxed microstructure with no massive grain coarsening. Artificial ageing (AA) at 590 °C x 8h following SHT resulted in higher strength, compared to that achieved by direct ageing, and in α precipitation shifted to higher temperatures with a triangular arrangement. The suitability of two different direct ageing treatments, namely AA (590°C for 8h), and double ageing (DA) at 690°C for 8h plus 650°C for 8h, could be confirmed for the AM alloy. Both treatments provided strength values comparable to literature findings but also in an incredibly high elongation at break. AA provided a yield strength of about 1200 MPa and preserved the elongation at break of the as-built material (21%). On the other hand, the outstanding fracture elongation after DA (34%) led to a slightly lower strength (970 MPa). On the other hand, SHT+AA resulted in a higher strength (1286MPa) but also in very limited ductility (7%) due to the grain boundary precipitation of alpha phase.