Kinetic analysis of pore formation in die-cast metals and influence of absolute pressure on porosity

In vacuum die casting (V-HPDC), absolute pressure has a great influence on the porosity of castings. We used X-ray imaging, scanning electron microscopy, and optical microscopy to assess pore number and size in AlSi9Cu3 castings prepared with three different absolute pressures. As the absolute pressure decreased from 1013 mbar to 100 mbar, the average porosity reduced from 8.5% to 3.7%, and the pore number and size decreased significantly. From Magma simulation results, the local pressure of casting during the vacuum die casting process was significantly smaller than that of the ordinary die casting process (HPDC), and V-HPDC reduced gas entrapment significantly. Kinetic analysis of the influence of vacuum on the formation of vortices and pores showed that, in HPDC, entrapped gas in a vortex can flow only in a small range and cannot diffuse out. Thus, gas trapped in the liquid metal before solidification forms pores. V-HPDC reduces trapped gas in the molten metal before solidification, thus reducing the porosity of castings. The fluid velocity in V-HPDC filling process is larger than in HPDC. As the vacuum die casting speed, τw increases, (∂τw∕∂x)>0, flow separation conditions are prevented, in turn, preventing vortex formation during the die casting process. •The mechanism of gas pore formation during the vacuum die casting was analyzed.•With the reduction of absolute pressure, the number and size of porosity defects were significantly reduced.•Kinetic analysis was used to confirm the influence of vacuum on the formation of vortices and pores.