Zinc alloy die casting is a commonly used casting process, widely applied in industries such as automotive, electronics, and home appliances due to its excellent mechanical properties, good fluidity, and superior processing capabilities. However, during production, zinc alloy die-cast parts often face various defects that not only affect the appearance quality of the products but can also significantly reduce their mechanical performance and durability. This article will conduct an in-depth analysis of common defects in
zinc alloy die casting, exploring their causes, impacts, and improvement measures.
Inclusions refer to non-metallic particles mixed within the castings, which may originate from impurities in the casting alloy itself or external substances introduced during the production process. The most common inclusions in zinc die castings are intermetallic compounds formed by iron and aluminum. The presence of these inclusions often leads to defects on the surface or inside the casting, adversely affecting its mechanical properties and appearance.
Impact: Inclusions not only reduce the tensile strength and toughness of the casting but may also trigger cracks and localized corrosion, especially in high-temperature and corrosive environments. Moreover, during surface processing, inclusions can increase processing difficulty and surface roughness, affecting the final product's appearance.
Flow marks, also known as cold marks, are linear defects that appear on the surface of castings. These marks usually occur due to uneven cooling rates in different areas of the casting during solidification. Areas where flow marks form do not solidify completely during cooling, resulting in discontinuous lines on the surface.
Impact: Although flow marks have a minimal impact on the structural strength of the casting, they can significantly affect the market competitiveness of products where appearance quality is crucial, such as automotive exteriors and home appliance casings. Consumers’ sensitivity to product appearance makes this defect a critical concern in product design and production.
Intergranular corrosion is a type of corrosion that occurs within the boundary layers between metal particles. Zinc-aluminum alloys are particularly susceptible to intergranular corrosion, especially in high-temperature and high-humidity environments. An excess of certain impurity elements (such as chlorine and fluorine) can accelerate the onset of intergranular corrosion, which may also occur at room temperature and typical humidity levels.
Impact: Intergranular corrosion can lead to a significant reduction in the strength and stiffness of the casting, affecting its lifespan and reliability. Castings exposed to corrosive environments for extended periods require particular attention regarding their corrosion resistance. Measures to prevent intergranular corrosion include selecting appropriate alloy compositions and optimizing heat treatment processes to enhance the alloy's corrosion resistance.
Shrinkage porosity refers to voids that form on the surface or inside the casting due to volume shrinkage during the solidification of the zinc alloy. The formation of shrinkage porosity is usually closely related to casting process parameters, including mold design, molten metal temperature, and the characteristics of the injection system.
Impact: The presence of shrinkage porosity can lead to insufficient strength and an increased risk of fracture during use, negatively affecting the overall performance and safety of the product. To reduce shrinkage porosity, design should ensure uniformity in the filling process and minimize the volume of voids.
Gas porosity refers to voids formed within or on the surface of the casting, primarily including bubble porosity and shrinkage porosity. Bubble porosity typically occurs during high-pressure die casting when gases are entrained into the molten metal. These gases may come from impurities in the alloy or the mold, as well as from air trapped during the casting process.
Impact: Gas porosity not only decreases the sealing capability and strength of the casting but may also affect its fatigue resistance. For high-strength castings, such as aerospace and automotive components, the presence of gas porosity directly impacts product safety and reliability. Therefore, adequate gas venting and bubble control should be implemented during the design phase to enhance casting quality.
During the casting process, local deformation or surface cracks may occur in zinc alloy die-cast parts, typically due to uneven wall thickness, insufficient ejector pins, or misalignment of the ejector pin support plate. These factors lead to uneven stress on the casting, resulting in stress concentration that can cause deformation or cracks.
Impact: Deformation and cracks not only affect the appearance quality of the casting but may also lead to insufficient strength during use, increasing the risk of failure. To address this issue, it is essential to optimize the casting process, including adjusting the number of ejector pins and their placement to ensure uniform stress distribution on the casting.
In some cases, the die cavity of the zinc alloy die-cast part may not fill completely, resulting in some portions of the casting remaining unformed. This may be caused by poor mold venting, insufficient injection pressure, low molten metal temperature, or excessive injection speed.
Impact: Incomplete filling can lead to insufficient strength and defects in the casting, impacting its functionality and appearance. To resolve this issue, it is recommended to switch to a high-pressure die casting machine, increase the temperature of the mold and molten metal, and ensure sufficient filling during the casting process.
Blurry outlines of zinc alloy die-cast parts are typically caused by excessive thickness at the feed gate, insufficient pressure from the die casting machine, or improper gate placement. These factors hinder the flow of molten metal during filling, leading to irregular shapes.
Impact: Blurry outlines can directly affect product assembly and aesthetics, leading to customer dissatisfaction. To improve the appearance of the casting, adjustments should be made to gate placement, a higher pressure die casting machine should be used, and the thickness of the feed gate should be reduced to ensure smooth flow of molten metal into the mold.
To reduce the occurrence of defects in zinc alloy die casting, the following comprehensive measures are recommended.
Process Design Optimization: Improve the design of the pouring system, control the filling method effectively, and ensure smooth filling of the mold during the casting process to avoid the formation of bubbles and inclusions.
Temperature Control: Increase the temperature of the die and molten metal to ensure good fluidity and filling ability during die casting, thereby reducing the occurrence of defects.
Mold Venting Design: Ensure air can be effectively vented through proper mold venting design to reduce the occurrence of gas porosity and inclusions, ensuring the integrity of the casting.
Equipment Improvement: Use high-pressure die casting machines to provide sufficient injection ratios and pressures, ensuring the quality and performance of the casting.
Material Selection and Control: Select suitable zinc alloy materials and control the content of impurity elements in the alloy to enhance the casting’s corrosion resistance and strength.
Process Monitoring and Quality Control: Implement strict quality control during production, continuously monitor process parameters to ensure smooth progress at every stage, and timely identify and correct defects.
Defects in the zinc alloy die casting process significantly impact product quality and performance. By analyzing common defects such as inclusions, flow marks, intergranular corrosion, shrinkage porosity, gas porosity, deformation and cracks, incomplete filling, and blurry outlines, we can clarify their causes and effects, thus formulating effective improvement measures. Optimizing the casting process, controlling temperature, designing a reasonable mold venting system, selecting high-performance equipment, strictly controlling material quality, and enhancing process monitoring and quality management are key strategies for improving the quality of zinc alloy die castings.
