Effects of Cooling Rate and Solid Fraction on α-Al Phase Evolution in Rheo-Die Casting: Phase-Field Simulation and...

Area of Science:

• Materials Science
• Metallurgy
• Casting Technology

Background:

• Understanding semi-solid metal casting microstructures is crucial for material properties.
• High-solid-fraction processes present unique challenges in controlling microstructure.
• Existing models often lack dynamic simulation capabilities for complex cooling transitions.

Purpose of the Study:

• To investigate the dynamic microstructural evolution during high-solid-fraction semi-solid rheo-die casting.
• To develop and validate a phase-field model for simulating continuous solidification under varying cooling rates.
• To analyze the morphological evolution of α-Al phases in Al-7Si alloy under different processing conditions.

Main Methods:

• Developed a novel phase-field model coupling continuous cooling with explicit nucleation.
• Integrated the Swirled Enthalpy Equilibration Device (SEED) for slurry preparation.
• Conducted graded-cooling mold experiments to establish variable cooling rate and solid fraction conditions.
• Performed experimental and simulation investigations on Al-7Si alloy.

Main Results:

• Observed Ostwald ripening of the primary α1-Al phase during Stage I slurry preparation (0.1-0.3 K/s).
• Primary α1-Al continued growth under moderate cooling rates (15 K/s) in Stage II rheo-die casting.
• Secondary α2-Al formation is highly dependent on cooling rate and solid fraction.
• High cooling rates (150 K/s) led to explosive nucleation of α2-Al, while moderate rates (15 K/s) suppressed it.
• High cooling rates enhance solute trapping and constitutional undercooling, promoting secondary phase formation.

Conclusions:

• The developed phase-field model accurately captures microstructural evolution during two-stage cooling.
• Cooling rate and solid fraction are critical parameters controlling secondary α2-Al formation.
• High cooling rates favor secondary phase formation via solute trapping, whereas high solid fractions limit it by reducing solidification windows.