Factors affecting the porosity of die castings
The porosity of die-castings is a key quality indicator. It refers to the ratio of the volume of pores within a die-casting to the total volume of the die-casting. The presence of pores can reduce the mechanical properties, sealing, and corrosion resistance of the die-casting, and in severe cases, can even render the casting scrapped. Numerous factors influence the porosity of die-castings, including molten metal smelting, die-casting process parameters, mold structure, and alloy composition. In-depth analysis of these factors and the implementation of appropriate control measures are crucial for reducing porosity and improving product quality.
The molten metal melting process directly impacts the porosity of die-cast parts and is the primary step in controlling porosity. During the melting process, molten metal readily absorbs gases, primarily hydrogen, nitrogen, and oxygen. Hydrogen is the primary cause of porosity in aluminum alloy die-castings. The solubility of hydrogen in molten metal increases with increasing temperature. As the molten metal cools and solidifies, its solubility drops dramatically. If hydrogen is not expelled promptly, it can form pores within the casting. Therefore, effective degassing measures are essential during the melting process, such as introducing inert gases (argon, nitrogen) for refining. As inert gas bubbles rise, they absorb hydrogen from the molten metal, reducing the gas content. Furthermore, oxidation of the molten metal during the melting process can also produce oxide inclusions, which can become the nuclei of pores and promote their formation. Therefore, the melting temperature and time must be controlled to minimize oxidation of the molten metal. Refining agents should also be used to remove slag and improve the purity of the molten metal.
The selection and control of die-casting process parameters are key factors affecting the porosity of die-cast parts. Proper process parameters can effectively reduce the occurrence of porosity. Injection speed has the most significant impact on porosity. Excessively high injection speeds can cause turbulence in the molten metal during the filling process, entraining large amounts of gas and oxide film. If these gases cannot be discharged in time, they will form porosity within the casting. Therefore, the injection speed should be appropriately set based on the structure and wall thickness of the casting. While ensuring complete filling, the injection speed should be minimized to avoid turbulence. Injection pressure also affects porosity. Adequate injection pressure can compact the molten metal and reduce gas entrapment, but excessive pressure can increase turbulence and, in turn, increase porosity. Holding pressure and holding time are equally important. Insufficient holding pressure or a short holding time will prevent the molten metal from being adequately fed during solidification, allowing gas to remain within the casting and form porosity. Appropriate holding pressure and holding time can facilitate gas discharge and feeding, reducing porosity.
The rationality of mold structural design has a significant impact on the porosity of die-cast parts. Optimizing mold structure can effectively reduce the occurrence of porosity. The design of the runner system directly affects the filling state of the molten metal. An improper runner structure can lead to turbulent molten metal flow and gas entrapment. For example, improperly positioned gates can cause the molten metal to directly impact the cavity wall, generating vortices and gas entrapment. An undersized runner cross-section can increase the molten metal flow velocity and cause turbulence. Therefore, a rational runner system design and appropriate gate location and number must be selected to ensure smooth and orderly filling of the cavity by the molten metal. The design of the venting system is crucial for controlling porosity. Gas in the mold cavity primarily consists of air and gases introduced by the molten metal. If venting is not smooth, these gases will be trapped within the casting, forming pores. Sufficient venting grooves should be provided in areas prone to gas accumulation, such as the final filling point, corners, and deep cavities. The depth and width of the venting grooves must be precisely calculated based on the alloy type and die-casting process parameters to ensure smooth gas discharge while preventing molten metal from overflowing and forming flash.
Alloy composition and properties also affect the porosity of die-cast parts. Different alloys have varying gas absorption tendencies and gas solubilities. For example, aluminum alloys have a high solubility for hydrogen, making them prone to hydrogen pores. Zinc alloys, on the other hand, have a relatively low absorption tendency and generally exhibit lower porosity. Certain elements in the alloy also affect porosity. For example, magnesium in aluminum alloys increases the alloy’s absorption tendency, while silicon improves the alloy’s fluidity and reduces gas entrapment. Therefore, selecting the appropriate alloy grade is crucial for controlling porosity. While meeting the required casting performance, alloys with low absorption tendencies and good fluidity should be preferred. Furthermore, alloy fluidity is closely related to porosity. Alloys with good fluidity can fill the mold cavity at lower injection speeds, reducing turbulence and gas entrapment, thereby lowering porosity. Adjusting the alloy composition can improve fluidity. For example, increasing the silicon content in aluminum alloys can improve fluidity.
Mold temperature and cooling rate are also important factors affecting the porosity of die-cast parts. Excessively low mold temperature causes the molten metal to cool rapidly during the filling process, increasing viscosity and reducing fluidity, making it difficult for gases to escape and increasing porosity. Excessively high mold temperature prolongs the solidification time of the molten metal, allowing more time for gases to escape. However, excessively high mold temperatures can reduce production efficiency and increase shrinkage and deformation in the casting. Therefore, the mold temperature must be controlled within an appropriate range, typically determined by the alloy type and casting structure. For example, the mold temperature for aluminum alloy die-casting is generally controlled between 150°C and 250°C. The cooling rate’s impact on porosity is primarily reflected during the solidification process of the molten metal. Excessively rapid cooling prevents gases from escaping quickly, freezing them within the casting and forming pores. Excessively slow cooling results in coarse grains, affecting the casting’s mechanical properties. By properly designing the mold cooling system and maintaining a uniform cooling rate across all parts of the casting, the formation of pores can be effectively reduced and the quality of die-cast parts improved.
In summary, the factors that affect the porosity of die-castings are multifaceted and require comprehensive control across multiple links, including smelting, die-casting process, mold design, and alloy selection. By taking effective degassing measures, optimizing die-casting process parameters, rationally designing mold structures, and selecting appropriate alloy materials, the porosity of die-castings can be significantly reduced, the quality and reliability of die-castings can be improved, and the performance requirements of die-castings in different fields can be met. With the continuous development of die-casting technology, intelligent online monitoring and control technologies will play an increasingly important role in porosity control. By real-time monitoring of gas content and process parameters during the die-casting process and dynamically adjusting the production process, precise control of porosity can be achieved, promoting the high-quality development of the die-casting industry.
