Effect of trace rare earth La on microstructure and properties of Al-7%Si-0.6%Fe alloy

Al-Si alloys have been widely used in electronic information, communication, and other fields because of their high specific strength, excellent castability and good thermal conductivity. In recent years, with the rapid development of 5G communication technology, electronic communication equipment is gradually developing towards high integration and lightweight. The power of related equipment is higher and higher, which puts forward higher requirements for thermal conductivity and mechanical properties of materials. Si can improve the fluidity and strength of the Al-Si alloy, but a large amount of Si will aggravate the lattice distortion and increases amount of eutectic Si. This will reduce the plasticity of the alloy, increase the electron scattering and reduce the thermal conductivity. In order to improve the mechanical properties and thermal conductivity of Al-Si alloys, chemical inoculation is generally used. Sr is usually used as modifier and Al-B serves as grain refiner. However, the simultaneous addition of Sr and B into Al-Si alloy results in "poisoning" phenomenon, it becomes impossible to refine alpha-Al grains and modify eutectic Si simultaneously. In recent years, rare earth La has attracted more and more attention in improving the properties of aluminum alloys. However, previous studies mainly focused on the effects of La addition, consequently, the research on the effects of combined addition of La, Sr, B on the microstructure and properties of Al-7%Si-0.6%Fe alloy is lacking. In this work, solidification experiments are performed to investigate the effects of combined addition of La, Sr, B on the microstructure and properties of Al-7%Si-0.6%Fe alloy. The results show that the addition of trace rare earth La can effectively eliminate the poisoning effect of Sr and B, and enhance the modification effect of eutectic Si. Besides, the addition of La can promote the formation of alpha-Al heterogeneous nucleation substrate LaB6 and La can be used as a surfactant to reduce the undercooling of alpha-Al nucleation, thus it refines alpha-Al grains. The thermal conductivity of the alloy is significantly improved when the addition of La ranges from 0.02% to 0.06%; with the further increase of La addition, LaAlSi intermetallic compounds are formed in the alloy, leading the thermal conductivity of the alloy to decrease.