Synchronous Ultrasonic Impact Treatment for Regulating Microstructure and Properties of Laser‐Directed Energy Deposited in Situ TiC/Ti6Al4V Composites

ABSTRACT In laser‐directed energy deposition (LDED) of TiC/Ti6Al4V composites, a low content of micron‐sized TiC provides a good strengthening effect on hardness and strength, but the improvement in plasticity remains a challenge. By introducing synchronous ultrasonic impact treatment (UIT) into the LDED process, an ultrasonic‐assisted LDED (UA‐LDED) technique was adopted to fabricate 1 vol% and 5 vol% TiC/Ti6Al4V composites. The results show that the UIT effectively reduces the lattice mismatch between TiC and α‐Ti, as well as the average grain sizes of TiC, prior β‐Ti (β p ), and α‐Ti. The ultrasonic waves interacted with the molten pool via cavitation and acoustic streaming, which could break TiC grains, promote the nucleation of β p , and suppress the growth of TiC and β p . Moreover, the ultrasonic waves also interacted with the high‐temperature deposited layer, which affected the formation of α‐Ti. Massive α‐Ti (α m ) grains observed at β p grain boundaries indicate the occurrence of massive α‐Ti transformation. UIT facilitated the massive transformation of 1 vol% TiC/Ti6Al4V and promoted the equiaxed growth of α m in 5 vol% TiC/Ti6Al4V. The α‐Ti grains inside β p grains were refined by UIT, and dynamically recrystallized grains were found. The microstructure regulated by UIT provides higher microhardness and greater elongation for the composites, while the tensile strength decreases slightly for the 1 vol% TiC/Ti6Al4V and increases for the 5 vol% TiC/Ti6Al4V. After UIT, the maximum plastic elongation of 1% TiC/Ti6Al4V increased from 1.09% ± 0.39% to 2.32% ± 0.86%, and that of 5% TiC/Ti6Al4V increased from 0.17% ± 0.02% to 0.88% ± 0.07%.