Thermal behavior and microstructure control of Ti6Al4V alloy in laser directed energy deposition using synchronous gas cooling strategy

Synchronous gas cooling-assisted laser directed energy deposition is a novel method for the in-situ active regulation of the microstructure of Ti6Al4V alloy. This study systematically investigates the influence of different gas cooling flow rates on the grain evolution and phase structure characteristics of Ti6Al4V alloy fabricated by laser directed energy deposition, and reveals the thermal behavior mechanism through numerical simulation. The results show that, with increasing gas cooling flow rate, the α′ phase gradually transforms from an acicular morphology to a lath-like morphology, accompanied by a significant reduction in aspect ratio and a substantial improvement in size uniformity. Regarding β grain evolution, increasing the gas cooling flow rate suppresses the transition from columnar grains to equiaxed grains, promotes columnar grain growth, significantly reduces texture intensity, and increases dislocation density. However, the β grain size exhibits a trend of first decreasing and then increasing. Temperature-field analysis indicates that the cold jet induces a pronounced heat-sink effect through forced convective heat transfer, forming a distinctive temperature field characterized by lower temperatures in the cold-jet action zone and higher temperatures on both sides. Increasing the gas cooling flow rate not only reduces the peak temperature of the molten pool, but also increases the temperature gradient and cooling rate at the solidification interface, effectively narrowing the heat-affected zone and phase-transformation zone, while improving the temperature-field uniformity behind the center of the cold jet.