In the production of high-precision aluminum die casting parts, internal porosity is a core defect affecting density. Its formation is closely related to factors such as gas entrapment during the molten metal filling process, excessive gas content in the molten alloy, and poor mold venting. To effectively eliminate porosity and ensure density, a comprehensive and systematic solution is needed, encompassing material processing, process optimization, mold design, and post-processing.
Material processing is the fundamental step in controlling porosity. During aluminum alloy smelting, if the raw material is damp or contains impurities, it easily decomposes at high temperatures, producing water vapor or hydrogen, leading to excessive gas content in the molten metal. Therefore, dry and clean alloy ingots must be selected, and the proportion of recycled materials must be strictly controlled. During the smelting process, nitrogen or argon is introduced for rotary degassing, utilizing bubbles to adsorb hydrogen in the molten metal and float to the surface, significantly reducing the gas content. Furthermore, using ceramic filters to filter the molten metal can remove oxide inclusions and reduce the formation of porosity nuclei.
Process optimization is a key means of reducing porosity. The matching of injection speed and pressure directly affects the filling state of the molten metal. If the injection speed is too fast, the molten metal is prone to turbulence, entraining gas and forming pores; if the speed is too slow, the molten metal may solidify prematurely, failing to adequately compensate for shrinkage. Therefore, the injection curve needs to be adjusted according to the structure of aluminum die casting high-precision cast parts, adopting a segmented injection process of "slow-fast-slow" to ensure that the molten metal fills the cavity smoothly. Simultaneously, increasing the boost pressure can enhance the compaction effect of the molten metal, reduce bubble volume, and increase density.
Mold design is crucial for pore control. The rationality of the venting system directly affects the gas venting efficiency. The mold needs to be equipped with overflow channels and venting channels at the end of the cavity, near the ingate, and in thick sections, utilizing the pressure difference during molten metal filling to guide gas to the venting area. The depth and width of the venting channels need to be optimized according to the casting material and structure to avoid clogging or jetting. In addition, adding vent plugs or permeable steel between the mold cores can further improve local venting capacity. For complex aluminum die casting high-precision cast parts, vacuum die casting is employed to remove gas from the mold cavity before molten metal filling, significantly reducing porosity.
The gating system design must balance filling and venting. The runner should employ a conical structure to gradually accelerate the molten metal during flow, reducing turbulence and gas entrapment. The cross-sectional area and position of the ingate need to be optimized to prevent the molten metal from directly impacting the core or cavity wall, thus preventing localized eddies. For aluminum die casting high-precision cast parts with uneven wall thickness, adding ingates or adjusting gate positions, and balancing the molten metal filling sequence, can reduce porosity caused by uneven shrinkage.
Post-processing can further eliminate residual porosity. Heat treatment, by controlling the heating rate and holding time, promotes the diffusion of internal gas to the surface and its expulsion through the oxide film. For localized porosity, localized extrusion or vibration aging processes can be used to close the pores through external force, increasing density. Furthermore, impregnation treatment, which uses vacuum pressure to infiltrate low-viscosity resin into the pores and then cures it, effectively seals micropores and meets airtightness requirements.
Production environment control is an easily overlooked aspect. Excessive air humidity accelerates hydrogen absorption by molten metal, especially during rainy seasons or in humid regions. Preheating and drying of smelting equipment, molds, and alloy materials are necessary to reduce surface moisture adsorption. Simultaneously, controlling workshop temperature and ventilation prevents gas precipitation due to decreased gas solubility caused by temperature drops during transfer.
Porosity control in aluminum die casting high-precision cast parts requires a comprehensive approach, encompassing materials, processes, molds, post-processing, and the environment. By optimizing material processing to reduce gas content, adjusting process parameters to reduce gas entrapment, improving mold design to enhance venting efficiency, combining post-processing to seal residual pores, and strictly controlling the production environment, the density of aluminum die casting high-precision cast parts can be significantly improved, meeting the stringent performance and reliability requirements of high-end applications.
