Die casting defect analysis
Die-casting defects refer to various imperfections that occur during the die-casting process or subsequent processing that do not meet design requirements. These defects include surface defects, internal defects, shape and dimensional defects, and more. These defects not only affect the appearance and performance of the die-casting but can also lead to product scrap and increase production costs. The occurrence of die-casting defects is closely related to factors such as die-casting mold design, die-casting process parameters, alloy material properties, and operating procedures. Accurately analyzing the causes of defects and implementing targeted measures are key to improving the pass rate of die-casting parts. Common die-casting defects include porosity, shrinkage, cold shut, under-giving, flash, cracks, and deformation.
Porosity is one of the most common internal defects in die-castings. It manifests as circular or oval holes within or on the surface of the die-casting, typically 0.1-2mm in diameter. In severe cases, it can be observed with the naked eye. Porosity is primarily caused by: air or gas being drawn into the molten metal during the injection process (e.g., an insufficiently filled die-casting chamber or excessively fast injection speeds leading to turbulent flow); poor mold venting (e.g., clogged or insufficiently deep venting slots); and excessive gas content in the molten metal (e.g., incomplete degassing during smelting). For example, in aluminum alloy die-casting, if the chamber is less than 40% full, the high-speed flow of the molten metal can easily form vortices, entraining large amounts of air and causing a porosity exceeding 5% within the die-casting. Porosity reduces the density and mechanical properties of die-castings and can lead to leaks in parts requiring tightness, such as hydraulic valve blocks. Solutions include: optimizing injection parameters (such as increasing the chamber filling degree to 60%-80% and reducing the high-speed injection speed); adding mold exhaust grooves (depth 0.05-0.1mm, width 5-10mm); and strengthening the degassing of the molten metal during smelting (such as using rotary jet degassing to reduce the hydrogen content to below 0.1ml/100g).
Shrinkage porosity (or shrinkage cavities) are loose, porous defects within die-cast parts caused by insufficient shrinkage feeding during solidification. They are typically found in thicker wall areas or hot spots (such as corners and protrusions). Shrinkage porosity is primarily caused by an improper solidification sequence (thick wall areas solidify last, lacking shrinkage feeding); insufficient injection pressure or short hold times, which prevent effective shrinkage feeding of the solidifying metal; and uneven mold cooling, resulting in slow cooling in certain areas. For example, in thick-walled flanges of automobile engine blocks, if the cooling channel is more than 30mm from the surface, slow cooling can easily lead to shrinkage porosity, resulting in airtightness test failure. Shrinkage porosity reduces the strength and sealing properties of die-cast parts and, in severe cases, can cause the part to fracture under stress. Solutions include: optimizing the mold cooling system (adding cooling water channels at hot spots, 20-25 mm away from the surface); increasing the holding pressure (specific pressure 80-120 MPa) and holding time (holding time increases by 0.5-1 second for every 1 mm increase in wall thickness); and adopting a sequential solidification process (controlling the solidification sequence through local heating or cooling).
A cold shut is a linear or irregular defect that appears on or within a die-cast part. It manifests as a distinct demarcation line between two strands of molten metal that is not fully fused, often accompanied by scale inclusions. Cold shuts are primarily caused by insufficient molten metal fluidity (e.g., low temperature, unsuitable alloy composition); slow filling speeds that result in excessive cooling of the molten metal during filling; and low mold temperatures that cause the molten metal to solidify rapidly upon contact with the mold. For example, in zinc alloy die-casting, if the molten metal temperature is below 400°C, the mold temperature is below 120°C, and the filling speed is below 2 m/s, cold shuts are more likely to occur in thin-walled areas. Cold shuts reduce the strength of die-cast parts and can become a source of fracture in stressed areas. Solutions include: increasing the molten metal temperature (e.g., to 650-680°C for aluminum alloys); increasing the mold preheat temperature (preheating the mold to 200-250°C for aluminum alloys); increasing the filling speed (for thin-walled parts, increase the filling speed to 5-8 m/s); and optimizing the gating system to avoid excessive molten metal diversion or excessively long flow rates.
Burrs are excess metal produced on die-cast parts, such as parting surfaces and slider mating surfaces. They appear as thin, flaky projections along the edges and are primarily caused by excessive or uneven mold clearance, insufficient clamping force, or excessive injection pressure. For example, if the mold parting surface has worn out due to long-term use, with a clearance exceeding 0.05mm, or if the clamping force is less than 1.2 times the injection force, flash can easily form on the parting surface. Burrs not only affect the appearance and assembly of the die-cast part, but can also cause scratches to operators and increase cleaning costs. Solutions include: adjusting the mold clearance by repairing the mold (keeping it within 0.01-0.03mm); increasing the clamping force (ensuring that the clamping force is ≥ injection force × 1.2); and reducing the injection pressure (while ensuring proper fill).
Cracks are fracture defects on or within die-castings, appearing as linear or network-like gaps. They can be categorized as hot cracks and cold cracks. Hot cracks occur at the end of solidification of the molten metal, when tensile stresses exceed the material’s high-temperature strength due to hindered shrinkage. They are commonly found at corners and thick-thin transitions in castings. Causes include unacceptable alloy composition (such as excessive levels of impurities), excessive mold constraints, and rapid cooling rates. Cold cracks occur after the die-casting cools to room temperature due to excessive internal stresses. They are commonly found in rigid areas. Causes include internal stress concentration, improper heat treatment processes, and insufficient material toughness. For example, magnesium alloy die-castings with an Fe content exceeding 0.01% are prone to hot cracking during solidification. Aluminum alloy die-castings may also develop cold cracks if the aging temperature is too high (over 200°C) during T6 heat treatment. Solutions include: strictly controlling alloy composition (e.g., Fe ≤ 0.005% in magnesium alloy); optimizing mold structure and reducing constraints (e.g., increasing fillets and avoiding sharp corners); rationally designing heat treatment processes; and eliminating internal stress (e.g., low-temperature annealing).
Methods for detecting die-casting defects include visual inspection, nondestructive testing (such as X-ray and ultrasonic testing), metallographic analysis, and mechanical property testing. By systematically analyzing the causes of defects and implementing improvements in mold design, process parameters, and material control, the defect rate can be effectively reduced and the quality of die-castings improved. For example, a die-casting of an automobile transmission housing had a pass rate of only 70% due to porosity and shrinkage. By optimizing the exhaust system, increasing hold time and pressure, and strengthening smelting degassing, the pass rate was increased to over 95%, significantly reducing production costs.
