Microstructural properties, thermodynamics and kinetics of Al-Si semi-solid billet fabricated by liquid phase reaction sintering

Highlights: • Al-Si semi-solid billet was prepared directly from elemental powder. • Microstructure evolution was analyzed and a mechanism model was established. • The analysis and prediction system of liquid phase content was established. • A reaction dynamic model is established by thermodynamic and kinetic analysis. -- Abstract: This study proposes a method of preparing an Al-Si semi-solid billet by cold pressing and liquid phase reaction sintering with Al and Si elemental powders. The microstructural evolution and tensile properties of the billets were evaluated. Thermodynamic and kinetic analyses of the reaction were also conducted. The kinetic equation for the accurate prediction of the liquid phase content was derived. Al and Si elemental powders were reacted at 585 °C, 595 °C and 605 °C to form a liquid phase and were uniformly distributed in the billet. After reaction sintering, semi-solid billets with equiaxed Al particles that were uniformly distributed in the liquid phase were formed. Increases in Si content and reaction temperature contributed to the increase in liquid phase content after thermodynamic equilibrium. An increase in temperature and a decreased in porosity promoted the kinetic formation of the liquid phase. Meanwhile, the kinetic equations y_{6 wt%} = 86.61·exp(−25487.4/RT)·t^0.35 and y_{8 wt%} = 95.5·exp(−36154.2/RT)·t^0.4 described the relationship between the liquid phase formation ratio and the reaction temperature and time while accurately predicting the liquid phase content during the reaction. The increase in liquid phase content in the microstructure improved the strength of the billet but reduced its elongation. Moreover, the fracture mode of the billet changed from a ductile fracture to a brittle fracture. The billet fracture was mainly attributed to the generation and propagation of cracks in the liquid phase.