Microstructural Evolution, Hardness, and Alligatoring in the Mushy State Rolled Cast Al-4.5Cu Alloy and In-Situ Al4.5Cu-5TiB₂ Composite

A comparative study has been carried out on the microstructural evolution and hardness of the Al-4.5Cu alloy and in-situ Al-4.5Cu-5TiB₂ composite, either water quenched from, or subjected to, rolling at mushy zone temperatures. The cast alloy shows coarse α-Al dendrites, while the composite contains rosette-type grains with TiB₂ particles located at the grain boundaries. The plate-shaped specimens, in cast or pre-hot-rolled conditions, have been rolled at temperatures corresponding to 10, 20, or 30 vol pct liquid to thickness reductions in the range of 2.5 to 10 pct. While the alloy could be rolled defect free, the cast composites undergo alligatoring, except when thickness reduction is 5 pct or lower at 30 vol pct liquid. Alligatoring is caused by strain localization and shear failure, and is prevented partially on prior hot rolling, causing homogenization and equiaxed grain formation. Mushy state rolling leads to nearly equiaxed grains having bimodal size distributions. The surfaces with a larger volume fraction of finer grains exhibit higher microhardness than the center. Exposure to mushy zone causes grain growth in the alloy but not in the composite, due to Zener pinning by TiB₂ particles. On rolling at low liquid content to a large thickness reduction, the unmelted grains deform and work-harden significantly, causing an increase in hardness.