Influence of heat accumulation on the distribution uniformity of microstructure and mechanical properties of laser additive manufacturing joint of 80 mm thick Ti6Al4V titanium alloy plates

Laser additive manufacturing (LAM) with directed energy deposition (DED) as the predominant foundation is an efficient and flexible joining technology that makes the fabrication of components with large thicknesses and complex structures possible. Heat accumulation phenomena caused by successive layer deposition strongly affect the microstructure and properties of components created with LAM. This study presents the conditions for component joining of constant process parameters and laser power gradient reduction process parameters to compare and investigate the influence of heat accumulation on the uniformity of mechanical properties and microstructure. The effects of the thermal cycle curve in different zones during LAM on the microstructure and mechanical properties of the top, middle and bottom of the deposition zone (DZ) under various parameters were investigated through finite element simulation and experiment. Results indicated that the cooling speed at the top of the DZ is slow and the heat accumulation effect is high. The aspect ratio of the α phase at the top of the deposition zone is high and high heat accumulation deteriorates mechanical properties. The heat accumulation effect is weakened when the gradient process parameters are used for joining. The content and size of the α phase at different joint heights are further homogenized. The tensile strength difference at various positions decreases and is higher than the tensile strength of the joint under constant parameters. This study promotes the LAM technology and provides a control reference for heat accumulation and microstructure regulation of large-thickness Ti6Al4V titanium alloy joints fabricated by LAM.