Effect of High Cooling Rate on the Solidification Microstructure of Al–Cu/TiB2 Alloy Fabricated by Freeze-Ablation Casting

In cast aluminum alloys, nanoparticle addition and increasing the cooling rate can enhance the strength of the alloy while improving its undesirable properties such as thermal cracking, shrinkage porosity, and casting micropores. Thus, the issues of nanoparticle distribution and shape size in alloys still need to be resolved. In this study, an innovative process known as freeze-ablation was introduced. This environmentally friendly, safe, and low-cost casting process could greatly increase the cooling rate of the melt. After pouring the Al–Cu-6wt%TiB2 alloy into a frozen sand mold, we used low-temperature fluid (such as water) to disperse the sand mold, and the cooling liquid could directly contact the casting to significantly increase the cooling rate. The results showed that the cooling rate of the alloy using the freeze-ablation process was nearly 40 times higher than that of ordinary resin sand molds, and its average grain size and secondary dendrite arm spacing (SDAS) decreased by 50% and 70%, respectively, compared to ordinary resin sand molds. At a high cooling rate, the TiB2 particles showed a more dispersed distribution and were smaller in size, with the smaller TiB2 particles exhibiting a better effect on grain refinement. With the addition of the TiB2 particles and solidification at a high cooling rate, the tensile strength of the TiB2/Al–Cu alloy reached 495.2 MPa under the freeze-ablation process, which was 15.2% higher than using a resin sand mold.