Effects of holding time during pre-sintering and post-forging annealing on the microstructure and mechanical properties of Ti parts fabricated by powder compact forging

Hydride-dehydride (HDH) Ti powder was consolidated into fully dense parts of rocker arms for internal combustion engines by powder compact forging at 1350 °C. The microstructure, tensile mechanical properties and fracture behavior of forged Ti parts were studied. It was found that fully dense forged Ti parts had a fine α lamellar structure, and increasing the holding time of the powder compact at 1350 °C from zero to 5 min and post-forging annealing at 550 °C for 6 h are two effective ways to improve the level of consolidation of the Ti parts by removing weakly bonded interparticle boundaries (IPBs), leading to a significant increase of their ductility. The former approach is a better way of improving the level of consolidation due to the fact that it does not cause significant microstructure coarsening and clear decrease of tensile strength of the Ti parts. The correlation between the yield strength of the forged Ti parts without and with annealing and their average α-Ti lamellar thickness shows that it follows the Hall-Petch relationship, and the sensitivity of the yield strength of α-Ti to the average lamellar thickness is clearly higher than its sensitivity to the average size of equiaxed grains.