Effect of preheating on thermal gradient and solidification rate of melt pool in laser metal 3D printing

Abstract

This work investigates the effect of preheating on temperature gradient and solidification rate of melt pool in laser metal 3D printing. An analytic model as well as particle-level numerical simulation was employed to reveal the correlation between the process variables and the thermal phenomena for Ti-6Al-4V. First, three-dimensional numerical simulation was performed and the results were compared with those by using an analytic solution. As the analytical model proposed by Rosenthal predicted the thermal behavior successfully, it was mainly employed to identify the solidification behavior depending on laser power P and scan speed V. As the most critical parameters to characterize a process, the temperature gradient G and the solidification rate R at the bottom of the melt pool were examined under different conditions. When there was no preheating, G and R decreased with the linear energy density P/V. In all the conditions of P and V, the solidification regime determined by the temperature gradient G and solidification rate R was in the regime to form columnar grains in the microstructure. Particular at the bottom the melt pool, which is the most critical location that determines the microstructure of 3D printed parts, no combination of P and V could produce the thermal condition G and R to form equiaxed grains.ii Secondly, the effect of preheating temperature on temperature gradient G and solidification rate R was investigated. High preheating temperature, e.g., 1500oC, reduced the temperature gradient significantly, so that thermal conditions to form equiaxed grains could be obtained at high linear energy densities. However, when the preheating temperatures  was less than 800 ℃, the temperature gradient was too large to produce equiaxed microstructure regardless of the linear density. This work demonstrated that thermal conditions to yield equiaxed grains in the microstructure of Ti-6Al-4V could not be obtained by adjusting the input energy level without preheating. It was also shown that preheating the system could achieve the thermal condition to form equiaxed grains but the temperature had to be impractically high. Therefore, this work suggests that high-temperature local preheating/reheating of the melt pool using an additional laser beam or a focused infrared should be incorporated to achieve the optimal thermal condition to produced fine equiaxed grains in laser 3D printing of Ti- 6Al-4V part