New Insights into the Formation Mechanism of Solid-Liquid Segregation in the Semi-Solid Processing: Macro-Micro Linkage Theory

Solid-liquid segregation is a typical problem in multiphase flow, especially in the die casting of metallic alloys. The avoidance of phase segregation in High Pressure Die Casting (HPDC), and Semi-solid Processing (SSP) for suspended flow is a benefit for improving product performance. At present, there is not a unified statement on the cause of the solid-liquid segregation defect. The formation mechanism of phase segregation is investigated by theoretical analysis from the perspective of hydrodynamic and rheology. The relationship between hydrodynamic parameters and characteristic rheological index is established. Maps of critical velocity, power-law index n, and consistency coefficient k are depicted for the semi-solid slurries. Then experiments and corresponding multiphase simulations were conducted to verify this theory. The macroscopic and microscopic linkage mechanism was proposed based on the theoretical discussion. This investigation not only provides important insights into the flow map of the semi-solid slurry, but also offers a key criterion of filling velocity for the avoiding of phase segregation for shear-thinning non-Newtonian fluids.