Altering microstructure and enhancing mechanical properties during direct energy deposition of Ti-6Al-4V via in-process laser heat treatments

In-process laser-based heat treatment (LHT) is explored as an alternative to post-process heat treatments such as annealing, in order to alter the hcp (α′ and αd) and bcc (β) phase content. Three samples were manufactured with a miniature laser metal deposition machine with the laser operating at 300 W. The reference sample was not subjected to any LHT. For the remaining samples, each deposited layer was subjected to an additional LHT at 100 W (LHT100) for one sample and 300 W (LHT300) for the other, without powder addition. LHT100 showed an overall improvement in the strength/ductility trade-off. Whereas LHT300 resulted in a lower strength but a much higher ductility and toughness than the other samples. Synchrotron X-ray diffraction analysis of as-built samples revealed an insignificant difference between hcp and bcc phase content between different samples. However, scanning electron microscopy uncovered a significant effect of LHTs on the fractions of different hcp phases. Along with the breakdown of α′ into αd and β, evidence of massive transformation from bcc β to hcp αm was also found in all samples. Results show promising improvement in the material strength/ductility trade-off, demonstrating the potential of in-process LHTs for altering microstructures and tailoring mechanical properties.