Microstructure Evolution During Laser Surfacing of Al 7075 Alloy

The present study investigates the microstructural evolution of the Al 7075 alloy through laser surfacing processes. The influence of varying laser parameters on microstructure formation is explored, and the impact of cooling rates on microstructural changes and defects is examined. The cooling rates during laser scanning ranged from 445 degrees C/s to 2350 degrees C/s, revealing a spectrum of microstructural changes. These microstructures encompass the development of columnar dendrites, equiaxed dendrites, dendrite fragmentation, and banded structures. Dendrite tip fragmentation is studied, revealing that cooling rates above 475 degrees C/s trigger fragmentation, notably within the columnar-to-equiaxed transition zone. For cooling rates below 475 degrees C/s, retraction was observed as the dominant mechanism for dendrite fragmentation. Moreover, the presence of porosity was identified as an impediment to the fragmentation process. Additionally, banded structures became apparent at cooling rates exceeding 1550 degrees C/s. Furthermore, by correlating calculated growth velocities with solidification microstructure, it was discerned that banded structures necessitated higher growth velocities compared to equiaxed dendrites. The ensuing discussion delves into these aspects, enhancing the comprehension of microstructural evolution during the laser surfacing of the Al 7075 alloy.