Understanding the refinement of grains in laser surface remelted Al–Cu alloys

The coarse-grained microstructure of laser surface remelted (LSR) Al–x wt.%Cu (x = 2, 5 & 7) is significantly refined by Al–3Ti–1B grain refiner. Epitaxial columnar grain growth at the melt pool boundary is suppressed. A reduction in grain size of about 80% is achieved and more than 25% of added TiB particles appear to be activated compared to less than 0.1% being activated during casting. The increased nucleation rate is explained in terms of an increase in thermal undercooling, activating a larger number of nucleants of lower potencies, and a reduction in the size of the nucleation free zone