Effect of temperature field on the porosity and mechanical properties of 2024 aluminum alloy prepared by direct chill casting with melt shearing

Abstract

Porosity severely deteriorates the mechanical properties of aluminum alloys. Large-sized and aggregated porosity often occurs in the center of Direct-Chill (DC) cast ingot. The variation of the temperature field in the sump affects the formation of porosity. In the present work, high shear technology was introduced into the DC casting process and the rotation speed and position of the high shear device were precisely controlled based on the measured temperature fields of conventional DC casting to realize the control of the temperature field and study its effects on the microstructure and mechanical properties of DC cast ingot. The results showed that when the temperature of the melt near the shear site (TS) was close to the temperature of the melt near the center of ingot and at starting position of the slow cooling zone of conventional DC casting (TG), forced convection can affect the cooling zone which is below the graphite ring in the sump, resulting in uniform microstructure of the ingot. If TS was above the liquidus and close to TG (temperature field II), the microstructure shows rod grains with fine secondary dendrite arms, refined porosity and second phase particles. For the lower TS which is below the liquidus and close to TG, (temperature field III), the mixed microstructure of spherical grains and small equiaxed was formed. And the increase of size and content of porosity can decrease of mechanical properties of the ingot. The concomitant relationship between floating/spherical grains and porosity was also discovered and discussed.