Investigation of microstructure evolution on different planes in laser welding of aluminum alloy

The solidification behavior of a molten pool is a critical factor affecting the mechanical properties of welded joints. This paper develops a multi-scale model combining the macroscale heat transfer and fluid flow model with the microscale phase field model for calculating the microstructure evolution on two different planes that are perpendicular to the thickness direction in the laser welding of the aluminum alloy. To obtain the time-varying temperature gradient (G) and solidification velocity (R) used in the simulation, a transient solidification conditions model is proposed. These models are validated by comparing the simulation results with the experimental results. The results indicate that G decreases, while R increases during solidification process. G/R decreases on both two planes, which results in the transformation of the microstructure from planar to cellular and then to the columnar grain. Additionally, it is found that the primary dendrite arm spacing of columnar grains on the lower plane is smaller, which is related to lower G−1/2R−1/4.