Study on columnar-equiaxed crystal transition of laser deposited TC4 titanium alloy induced by synchronous layer-by-layer ultrasonic impact

Abstract

In this work, synchronous layer-by-layer ultrasonic impact was proposed to induce columnar-equiaxed crystal transition during laser directed energy deposition of TC4 alloy. The microstructure characteristics and mechanical properties were thoroughly investigated to clarify the recrystallization process and strengthening mechanisms of deposited TC4 alloy. The results indicated that recrystallization occurred in the deposited sample following the application of ultrasonic impact, transforming the grain structure from columnar crystals extending through the deposited layer to fine equiaxed crystals. This process results in a grain size reduction of approximately 97 %. Additionally, the preferred orientation of the grains was significantly weakened, while the dislocation density was substantially increased. After applying ultrasonic impact, the internal phase structure of the sample experienced significant changes. The elongated acicular α-phase was gradually replaced by a lamellar net-basket α-structure, and increased retention of β-phases was observed within the grains, accompanied by the precipitation of ultrafine-grained β particles. Due to the grain boundary strengthening and dislocation strengthening with the contribution value of 49 MPa and 66 MPa, the overall mechanical properties of the alloys were markedly enhanced when the deposited samples were treated by ultrasonic impact. The excellent ultimate tensile strength, yield strength and elongation values were 1067 MPa, 896 MPa and 16.6 %, respectively. Generally, synchronous layer-by-layer ultrasonic impact offers a novel approach for improving the microstructure and enhancing the mechanical properties of TC4 alloy fabricated by laser directed energy deposition.