Formation of globular structure in A357 aluminum alloy via a near-liquidus casting (NLC) route

A near-liquidus casting (NLC) route is applied to prepare A357 aluminum alloy with a globular structure. The method is cost-effective and based on the melt pouring with low superheat level followed by isothermal re-heating treatment in the semi-solid region. This study describes the effect of both casting temperature and re-heating time on the evolution of globular structure in A357 Al alloy. To do so, molten alloy is poured directly into a cylindrical mold at various temperatures of 625°C, 635°C, 645°C and 655°C. Then, the specimens are re-heated at 590°C for different time of 10, 20, 30 and 40 min. The microstructural development of specimens after reheating stage is investigated. The primary α-Al dendritic phase in the conventional cast alloy is transformed into globular shape in NLC-processed specimens only after re-heating reatment. Increasing both casting temperature and re-heating time change the size and shape factor of globules. The results show that the optimum combination of pouring temperature and re-heating time is found to be 625°C and 30 min, respectively. The mechanisms behind the microstructural evolution are proposed.