Mechanism of columnar to equiaxed to lamellar grain transition during wire-laser directed energy deposition 205 C aluminum alloy utilizing a coaxial head: Numerical simulation and experiment

The microstructure under various conditions shows noticeable differences in the wire-laser directed energy deposition (DED) method using a coaxial head due to the intrinsic properties of aluminum alloy wire materials and the intricate heating process involved. The impacts of varied heat inputs and substrate preheating temperatures on solidification and microstructure evolution were explored in this study. A coupled model, including macroscopic finite element and microscopic cellular automata with interpolation, was employed to investigate the temperature field, thermal cycles, solidification parameters, and microstructure evolution under varied process parameters. This study revealed the transformation of four grain forms in the deposition layer, namely, lamellar grain, equiaxed grain, equiaxed fine grain, and columnar grain, through a comprehensive dynamic simulation of the microstructure. Quantitative research was conducted to determine the solidification region of equiaxed grains and the average length of columnar grain amid solidification under varied crystallization parameters. The findings demonstrated that under a lower heat input and preheating temperature, equiaxed fine grains are generated at the lower region of the deposition layer, while lamellar grains are formed above the equiaxed grains at the upper region of the deposition layer, which is similar to the morphology of a cast microstructure. The typical primary dendritic distance of columnar grain diminishes with the enlargement of the cooling rate and temperature gradient. The findings of this study offer a theoretical framework for regulating the microstructure of aluminum alloy deposition layers manufactured via wire-laser DED technology using a coaxial head.