Solidification under pressure of aluminum castings

Aluminum castings are widely used in automotive and other applications. These castings gained importance due to their high strength to weight ratio. However, aluminum castings are prone to porosity which significantly deteriorates the mechanical properties. Safety-critical components are subjected to hot isostatic pressing (HIPping), which heals the porosity. HIPping is a post-processing technique resulting in higher costs. An alternative technique is application of pressure during solidification.;The effect of application of 1.0132 MPa (150 psi) pressure during solidification on the cooling rate, soundness, pore characteristics and mechanical properties of aluminum castings was investigated in the current research. Wedge castings of A319, A356 and A206 alloys were made using lost foam and open cavity processes for this study. Samples with various cooling rates taken from the wedge castings were analyzed for density, ultrasonic velocity, tensile properties and metallurgical porosity. Castings were also made to analyze the effectiveness of pressure as a function of fraction solid present when the peak pressure was reached.;Pressure did not change the cooling rate significantly in A319 and A356 castings, while a slight increase in cooling rate is seen in A206 castings. The volume percent porosity decreased in all these alloys with pressure and the decrease in porosity was a function of alloy and cooling rate. Castings solidified under pressure exhibited a smaller mean diameter and mean porosity thickness.;Applied pressure increased the elongation and tensile strength in A206 castings increased by 100--750% and 12--55%, respectively, while no significant change in yield strength was observed. A slight increase in elongation of Al-Si alloys was achieved, while tensile and yield strengths were not significantly affected.;Ultrasonic velocity decreased with increasing volume percent porosity and decreasing mean porosity spacing in all the alloys. Ultrasonic velocity and mechanical properties were better correlated in the castings solidified without the application of pressure.;Theoretical analysis of porosity formation in A356 and A206 alloys showed that hydrogen bubbles formed faster in A206 compared to A356.