Nanoparticles Enabled Mechanism for Hot Cracking Elimination in Aluminum Alloys

Hot cracking has been a long-term challenge for the solidification processing of many high-performance aluminum alloys, such as 7075, 6061, and 2024 alloys. Nano-treating, by adding a low loading of ceramic nanoparticles into a metal matrix, can effectively reduce the hot cracking susceptibility of aluminum alloys during solidification processes such as casting, welding, and additive manufacturing. While previous studies have shown that ceramic nanoparticles enhance heterogeneous nucleation, inhibit grain growth, and modify secondary phases during solidification of various alloys, no systematic study has been conducted to investigate the underlying mechanisms of hot cracking elimination by different nanoparticles on different alloy systems. In this work, TiC and TiB2 nanoparticles have been incorporated into hot crack susceptible aluminum alloys 7075, 6061, and 2024, and a detailed thermal analysis and microstructure study were carried out to investigate the nanoparticle-enabled principal mechanisms of hot cracking elimination. It is discovered that the underlying mechanism is attributed to the unusual modification of both grains and intermetallic phases as well as a much higher liquid fraction at the final stage of solidification. More specifically, nanoparticles enable faster nucleation with a gradual latent heat release, an effective growth restriction of spherical aluminum α-grains (especially by TiC nanoparticles), a significant modification of the intermetallic phases, and a higher liquid fraction of non-equilibrium eutectic due to nanoparticle-induced diffusion blockage at the terminal stages of solidification.