In the production of aluminum alloy die castings, porosity defects are among the most common internal quality issues. These defects are often difficult to detect through visual inspection, especially dense pinhole-type porosity, which typically requires X-ray inspection for identification. Porosity not only affects the appearance and performance of castings but can also severely impact the strength and durability of the product. Therefore, understanding the causes of porosity and implementing effective control measures are crucial for improving the quality of aluminum alloy die castings.
Porosity refers to spherical voids with smooth internal surfaces, typically formed during the cooling and solidification processes. When gas cannot escape from the liquid surface in time, it becomes trapped, forming bubbles within the casting. Porosity usually appears as round or elliptical voids, which may be located on the surface, just below the surface, or deep within the casting. Surface porosity is easier to detect, but internal porosity, especially pinhole-type defects, is more concealed and often has a greater impact on the mechanical properties of the casting.
The sources of porosity can be categorized as follows.
Raw Materials and Melting Process: The primary gas in molten aluminum is hydrogen, accounting for about 85% of the total gas content. Higher melting temperatures increase the solubility of hydrogen, but as the aluminum solidifies, the solubility of hydrogen drops significantly, causing it to precipitate and form porosity. Sources of hydrogen include moisture in the atmosphere, hydrogen absorption from humid air by the molten metal, and hydrogen content in raw materials, such as moisture on the surface of alloy ingots, contamination or oil on recycled materials, and damp tools or fluxes.
Gas Entrapment During Die Casting: During die casting, molten metal fills the mold cavity under high pressure and speed. If the metal flow is unstable, vortices can form, entrapping gas and leading to porosity. The extent of gas entrapment is closely related to die casting parameters (e.g., injection speed) and mold design (e.g., cavity structure).
Gas Generated by Mold Release Agent Decomposition: At high temperatures, the decomposition of mold release agents can release gas, increasing the gas content within the casting. Factors such as the gas generation rate of the release agent, spray volume, and spraying process can influence gas formation. Excessive spraying, overuse of plunger lubricants, or burning of the release agent can all contribute to gas generation.
Controlling porosity defects requires a comprehensive approach, including optimizing raw material selection, improving melting processes, adjusting die casting parameters, enhancing mold design, and refining spraying techniques.
Dry and Clean Alloy Materials: Using dry and clean aluminum alloy materials can effectively reduce gas absorption during melting by avoiding moisture and impurities.
Control of Melting Temperature: Properly controlling the melting temperature prevents excessive hydrogen dissolution. Degassing techniques can be employed during melting to reduce gas content in the molten metal.
Reduction of Moisture and Impurities: Strictly controlling the humidity of alloy ingots, recycled materials, tools, and fluxes can minimize the introduction of hydrogen or other gases.
Appropriate Selection of Die Casting Parameters: The choice of injection speed is critical for controlling porosity. Proper injection speed ensures smooth metal flow, reducing vortex formation and gas entrapment.
Adjustment of High-Speed Switching Point: Properly timing the high-speed switching point during die casting can prevent gas entrapment caused by rapid metal flow.
Sequential Filling: Sequential filling can effectively reduce gas retention in the mold cavity, ensuring smooth metal flow and gas expulsion.
Optimization of the Gating System: The thickness and direction of the gate should be adjusted according to the casting design to ensure uniform filling of the mold cavity and prevent gas accumulation at the gate.
Overflow and Venting Channels: Properly designed overflow and venting channels can effectively expel gas. The total cross-sectional area of overflow channels should be at least 60% of the total gate cross-sectional area to ensure effective slag and gas removal.
High-Performance Mold Release Agents: Selecting release agents with low gas generation and stable performance, while controlling the spray volume, can prevent excessive residue. Over-spraying can cause instant vaporization of aluminum, leading to porosity.
Improved Spraying Techniques: Reducing spraying time, optimizing nozzle direction, and extending blow-off time can minimize release agent residue and prevent porosity caused by contact between molten aluminum and moisture.
Reduction of Gas Content in the Mold Cavity: Dual-channel high vacuum technology can reduce the gas content in the mold cavity and pressure chamber to near-vacuum levels, thereby minimizing porosity.
Purification and Degassing Techniques: Combining high vacuum technology with purification and degassing techniques can further reduce gas content in the molten metal, decreasing the occurrence of porosity.
Although porosity defects are common in aluminum alloy die castings, the aforementioned process optimizations and technical measures can significantly reduce their occurrence. To further ensure product quality, the following inspection methods can be employed:
X-Ray Inspection: X-ray inspection is effective for detecting internal porosity, especially pinhole-type defects hidden within the casting.
Non-Destructive Testing (NDT): In addition to X-ray inspection, ultrasonic testing and CT scanning can be used to identify and repair internal defects in castings.
Porosity defects in aluminum alloy die castings not only affect appearance but can also negatively impact mechanical properties. By comprehensively analyzing the sources of porosity and implementing effective control measures—such as optimizing raw material selection, melting processes, die casting parameters, mold design, and spraying techniques—the occurrence of porosity defects can be significantly reduced. Advanced technologies, such as dual-channel high vacuum and optimized degassing processes, can further enhance the quality of aluminum alloy die castings. Through precise process control and inspection methods, the ultimate goal of improving the performance and reliability of die castings can be achieved.
