In-situ analysis of incipient melting of Al2Cu in a novel high strength Al-Cu casting alloy using laser scanning confocal microscopy

Incipient melting in a novel Al-Cu alloy was investigated. Precipitation hardenable aluminum alloys are typically heat treated to further enhance their in-service properties. However, over heating during heat treatment leads to incipient melting thereby compromising mechanical properties. Advanced microscopy and real time imaging techniques such as a high temperature laser scanning confocal microscopy (LSCM) enable a novel direct in-situ observation of incipient melting and phase transformations. In this research, isothermal analysis was carried out using LSCM at elevated temperatures to analyze incipient melting with time. The predominant phase of this alloy was Al2Cu in both blocky and eutectic morphologies. The results showed that incipient melting was accompanied by a clustering of liquid micro-droplets followed by a complete melting of Al2Cu at approximately 548 degrees C. The in-situ analysis also revealed that eutectic Al-Al2Cu particles were found to melt prior to blocky Al2Cu particles. Further, localized melting was also observed to occur at a random sequence for eutectic Al-Al2Cu. As a result of incipient melting, the microstructure consisted of a proliferation of defects such as brittle ultra-fine eutectic clusters and porosity. The mechanisms of incipient melting were discussed using interfacial energies, diffusion of Cu, and particle coarsening.