Optimization of pressurization process in low-pressure casting of a 5.4-ton gigantic C95800 copper alloy casting

During the low-pressure casting of extra-large size C95800 copper alloy components, traditional linear pressurization technique leads to a rapid surge of liquid metal inlet velocity at the regions where the mold cavity cross-section enlarges. This rapid increasement of liquid metal inlet velocity causes serious entrapment of gas and oxide films, and results in various casting defects such as the bifilm defects. These defects detrimentally deteriorate mechanical properties of the castings. To address this issue, an innovative nonlinear pressurization strategy timely matching to the casting structure was proposed. The pressurization rate decreases at sections where the cross-section widens, but it gradually increases as the liquid metal level rises. By this way, the inlet velocity remains below a critical threshold to prevent the entrapment of gas and oxide films. Comparative analyses involving numerical simulations and casting verification illustrate that the nonlinear pressurization technique, compared to the linear pressurization, effectively diminishes both the size and number of bifilm defects. Furthermore, the nonlinear pressurization method enhances the casting yield strength by 10%, tensile strength by 14%, and elongation by 10%. Examination through scanning electron microscopy highlights that the bifilm defects arising from the linear pressurization process result in the reduction of the castings' mechanical properties. These observations underscore the efficacy of nonlinear pressurization in enhancing the quality and reliability of gigantic castings, as exemplified by a 5.4-ton extra-large sized C95800 copper alloy propeller hub with complex structures in the current study.