Local pressurization during solidification of Al-Si-Cu alloys

Local pressurization is a squeeze method for eliminating the shrinkage porosity. In this study, local pressurization was carried out with a specially designed permanent mold to examine the effect of pressurization on the shrinkage porosity through the investigation of macrostructure, partial density variation under the pin tip for two types of aluminium. alloys of hypoeutectic (AC4B) and eutectic (ADC12). Then copper concentration at the different positions in AC4B casting was analysed in order to investigate segregation. And in order to estimate the flow of liquid or solid-liquid zone by local pressurization, the ratio of flowing volume of two alloys was measured with varied pressurization timing and different pressures. The solid fraction of flowing zone was calculated by the use of both the solidification simulation and the characteristic macrostructure change of flowing zone. The solid fraction indicates the fraction within which the present pressurization can induce interdendritic liquid flow. The results showed that the partial density of two alloys cannot be affected within the certain limits of pressurization timing for a given pressure and pressurizing speed, and does not increase obviously even if pressure increases, such as 64 to 128MPa for ADC12 casting. It was also found that the negative segregation of copper in AC4B casting occurred under the pin tip. The concentration of copper varied with distance from the pin tip. The degree of segregation was severe in the position 15mm distant from the pin tip. Opposite to the negative segregation, the positive segregation occurred in the upper zone around the pin. From the simulation, the maximum solid fraction of flowing zone, where interdendritic liquid moves, is shown to be about 0.46 and 0.53 for AC4B casting with pressure of 32 and 64Mpa, respectively.