Development of microstructure and high hardness of Ti6Al4V alloy fabricated using laser beam powder bed fusion: A novel sub-transus heat treatment approach

The present work aims to study the effect of novel sub-transus heat treatment on the microstructure evolution and hardness of Ti6Al4V alloy fabricated using laser based powder bed fusion. The results re-vealed that the as-built microstructure is alpha ' martensite which decomposes to alpha + beta during sub-transus heat treatment at 970 degrees C for different soaking times. The formation of a bi-modal structure with approximately 25% globular primary alpha and transformed beta is observed. The development of globular primary alpha is at-tributed to thermal grooving and boundary splitting. The resultant coarsening of these alpha/beta grains was as-sociated with a time-temperature combination and orientation relation (OR) to 45 degrees [100]. Moreover, precipitation of fine Ti3Al intermetallic (alpha 2) in plate form is observed predominantly inside the primary alpha grains. Owing to the faster cooling rate post soaking time resulted in Ti3Al precipitates which is the con -version of alpha to alpha 2 precipitates. The average micro-hardness of the specimen heat-treated for 12 h at 970 degrees C is found to be 45% higher in comparison to the as-built specimen. Whereas, the micro-hardness differs be-tween the primary alpha and transformed beta region due to the alloy element partitioning effect. The average micro-hardness of 592 HV is observed in primary alpha grains in the specimen heat-treated for 12 h at 970 degrees C. The prior results are associated with the formation of intermetallic nano-sized Ti3Al (alpha 2). (c) 2022 Elsevier B.V. All rights reserved.