Heat treatment of laser powder-bed-fused Co–28Cr–6Mo alloy to remove its microstructural instability by massive FCC→HCP transformation

Laser powder bed fusion (L-PBF) of Co–28Cr–6Mo alloy is a powerful tool for many engineering applications but one has to be familiar with its microstructural instability in the as-produced state. In our previous study [Roudnicka et al., Additive Manuf. 44 (2021) 102025], we depicted the phenomenon of this instability when exposing the material to temperatures above 673 K. Microstructural changes bringing about hardening of the material were studied thoroughly. In the present study, we follow up by revealing the effect of this hardening on mechanical properties in tension and show the beneficial effect of heat treatment on removing the observed instability. Hardening associated with increasing the proportion of HCP ε-phase and with the precipitation of σ-phase has a detrimental effect on material plasticity and can be negatively affect the fatigue too. We thus applied a two-stage heat treatment comprising solution annealing at 1473 K/2 h and aging at 1173 K/5 h. Not only it induced material stability but also increased the material strength with not such a detrimental decrease in material plasticity as was brought by direct aging at 1173 K/5 h.