Columnar-to-equiaxed Transition Mechanism and Remarkable Strengthening Effect in Additive Manufactured Pure Titanium Induced by Copper Addition

Additive manufacturing (AM) has unique advantages in the production of structural components with complex shapes. However, AM metallic materials face a common phenomenon of coarse columnar grains, leading to degraded mechanical properties. To overcome this problem in AM titanium (Ti) alloys, this study introduced alloying elements copper (Cu) into pure Ti. The effects of Cu addition on the grain morphology and mechanical properties of Ti matrix were investigated. It was revealed that with the addition of 6.5 wt% Cu, prior-beta grains of AM Ti completely changed from columnar grains larger than -200 mu m to equiaxed grains of -23 mu m. Simultaneously, alpha grains changed from columnar to laths shape, and co-precipitated with Ti2Cu to form a fine (Ti2Cu + alpha) eutectic structure. Consequently, Cu addition results in remarkable strength improvement of Ti matrix, viz. 205% and 222% increments in yield strength and ultimate tensile strength, respectively. The columnar-toequiaxed transition mechanism and strengthening behavior of the high-strength AM Ti-Cu alloy were discussed. This study may provide new insight into the fabrication of high-performance AM metallic components with fully equiaxed grains.