en 
fra 
de 
ru 
gle 
th 
ara 
it 
jp 
kor 
zh Common Manufacturing Defects in High - Pressure Die Casting
2025-01-14 15:02:57 hits：0
In the highly efficient and widely used metal forming process of high - pressure die casting, although high - precision and complex - shaped castings can be produced, due to the filling and solidification process of molten metal under high pressure and high speed, affected by the interaction of multiple factors, some manufacturing defects are inevitably prone to occur. A deep understanding of these common defects is crucial for optimizing the die - casting process, improving the quality of castings, and reducing production costs.
Gas Pores
Causes of Generation
- Gas Entrapment: During the die - casting process, the molten metal fills the cavity at an extremely high speed. This process is extremely rapid, and the air in the cavity often cannot be completely discharged in time. Thus, it is entrapped inside the casting by the fast - flowing molten metal, forming gas pores. For example, in the die - casting production of an automotive aluminum alloy wheel hub, when the filling speed of the molten metal reached 5 m/s, due to poor exhaust in the cavity, the defect rate of gas pores inside the casting suddenly increased from 5% when the filling speed was 3 m/s to 15%. When the filling speed of the molten metal reaches several meters per second or even higher, the possibility of gas entrapment increases significantly.
- Gas Evolution: The alloy material itself will gradually evolve gas during the solidification process. Taking aluminum alloy as an example, if its hydrogen content is too high, during the solidification stage of the casting, hydrogen atoms will combine to form hydrogen gas, which appears in the form of gas pores inside the casting. Generally, the solubility of hydrogen in aluminum alloy decreases as the temperature drops. When the temperature drops to a certain extent, supersaturated hydrogen will be released. Research shows that when the hydrogen content in aluminum alloy exceeds 0.6 ml/100 g, the probability of gas pores in the casting increases significantly. In the production of a certain aluminum alloy die - casting, due to improper control of the hydrogen content in the raw material, which reached 0.8 ml/100 g, ultimately 50% of the products had gas pore defects.
Influences
- Decline in Mechanical Properties: The presence of gas pores is like a hidden danger inside the casting. It reduces the density of the casting and disrupts the continuity of its internal structure. When the casting is subjected to pressure or external forces, stress concentration occurs around the gas pores, and cracks often initiate and expand from these weak gas pore areas, thus significantly reducing the overall strength and toughness of the casting. In the die - cast parts of automotive engines, if there are gas pores, under the high - speed operation of the engine, and when subjected to huge pressure and alternating loads, these parts are very likely to experience crack propagation from the gas pores, ultimately leading to part leakage or damage, seriously affecting the normal operation of the engine. Relevant tests show that for an aluminum alloy die - cast engine cylinder block with a gas pore volume fraction of 5%, its tensile strength is 20% lower than that of a product without gas pores, and its fatigue life is shortened by 30%.
- Air - Tightness Problems: For some castings with strict air - tightness requirements, such as components in the aerospace field and gas pipeline connectors, the presence of gas pores may directly lead to gas or liquid leakage, making the product unable to meet the usage requirements and even triggering serious safety accidents. In the die - casting production of an aero - engine fuel nozzle, once gas pores appear, even tiny ones, in the high - temperature and high - pressure fuel injection environment, fuel leakage may be triggered, leading to engine failure. According to statistics, aero - engine component failures caused by gas pores account for 10% of all failure causes.
Shrinkage Cavities and Porosity
Causes of Generation
- Solidification Shrinkage: During the solidification process of molten metal, its volume shrinks. If it does not receive timely feeding from the runner, riser, and other parts during the shrinkage process, shrinkage cavities or porosity will form in the last - solidifying parts of the casting. Generally, the thick - walled parts of the casting dissipate heat more slowly and have a longer solidification time than the thin - walled parts. In the later stage of solidification, they are more likely to have shrinkage cavities or porosity due to insufficient feeding. For example, when die - casting a large and complex structural part, due to the uneven wall thickness, the thicker areas have a large amount of shrinkage during solidification, while the surrounding thin - walled parts have already solidified and cannot provide sufficient molten metal for feeding. As a result, shrinkage cavities or porosity defects are likely to occur in the thick - walled areas. In the die - casting process of a large aluminum alloy support in the market, the cooling rate of the thick - walled part (wall thickness 20 mm) is three times slower than that of the thin - walled part (wall thickness 5 mm). Due to insufficient feeding in the thick - walled part, the defect rate of shrinkage cavities and porosity is as high as 30%.
- Solidification Mode: The solidification mode of the alloy also has an important influence on the formation of shrinkage cavities and porosity. Alloys with a layer - by - layer solidification characteristic have a greater tendency to form shrinkage cavities, while alloys with a mushy solidification mode are more likely to produce porosity. For example, tin - bronze alloy, because it is close to the layer - by - layer solidification mode, has a probability of about 25% of shrinkage cavity defects occurring during the die - casting process. Compared with the mushy - solidifying aluminum alloy, the shrinkage cavity problem is more prominent.
Influences
- Weakening of Internal Structure: The appearance of shrinkage cavities and porosity damages the compactness of the casting's interior, resulting in many small voids or porous areas inside the casting. This not only reduces the mechanical properties of the casting, making it prone to deformation and fracture when subjected to external forces, but also affects the fatigue life of the casting. In some parts that need to withstand repeated alternating loads, such as die - cast components in the automotive suspension system, shrinkage cavities and porosity will significantly reduce their fatigue strength, leading to premature failure of the parts. Experimental tests show that the fatigue life of an automotive suspension die - casting with porosity defects is 40% shorter than that of a normal part.
- Reduction in Pressure Resistance: For castings that need to withstand high pressure, such as hydraulic valve bodies and high - pressure pipe joints, shrinkage cavities and porosity may lead to a decrease in their sealing performance, causing leakage under high - pressure environments and making them unable to work properly. For example, in the die - casting production of a hydraulic valve body by an enterprise in the market, due to shrinkage cavity and porosity defects, the leakage rate of the product reached 15% during a 20 MPa pressure test, failing to meet the actual usage requirements.
Cold Shuts
Causes of Generation
- Poor Filling Conditions: During the filling process of molten metal, due to unreasonable gate design, the molten metal flows unevenly and slowly, or the temperature of the molten metal itself is too low, and heat is dissipated too quickly during the flow process, making the fluidity of the molten metal worse. When two or more streams of molten metal meet, they cannot fully merge, forming a gap on the surface of the casting that appears to be incompletely fused, namely a cold shut. For example, when die - casting a complex - shaped thin - walled casting, if the gate position is inappropriate, the temperature of the molten metal drops rapidly and the flow rate slows down when it flows through the thin - walled area. It is very easy to produce cold shuts at the intersections of the thin - walled areas or at the 对接 points of the molten metal streams.
- Uneven Mold Temperature: The surface temperature of the mold is uneven, and some parts have too low a temperature. When the molten metal contacts these low - temperature areas, it will quickly cool and solidify, causing subsequent molten metal streams to be unable to merge well with it, thus generating cold shuts. In the production of a certain mold, due to a cooling system failure, the temperature of a local area of the mold was 50 °C lower than the normal operating temperature. The defect rate of cold shuts in the castings produced in this area was as high as 40%.
Influences
- Damage to Appearance Quality: Cold shuts directly affect the appearance quality of the casting, making its surface have obvious discontinuous traces, reducing the aesthetics and overall quality of the product. In some products with high requirements for appearance, such as electronic product housings and ornaments, cold shut defects will directly render the product scrapped. In the production of an aluminum alloy housing for an electronic product by a foundry in the market, due to cold shut defects, the product non - conforming rate reached 10%, resulting in a direct economic loss of hundreds of thousands of yuan.
- Performance Degradation: The metal bonding strength at the cold shut is relatively low. This not only reduces the strength of the casting, making it easy to break from the cold shut when subjected to external forces, but also affects the sealing performance of the casting. For some castings that need to be sealed, such as automotive engine cylinder blocks and pump bodies, cold shuts may lead to liquid or gas leakage, affecting the normal operation of the equipment. Research shows that the sealing performance of an automotive engine cylinder block with cold shut defects is reduced by 30%, and it is more likely to leak under high - pressure tests.
Flash
Causes of Generation
- Excessive Mold Clearance: During the die - casting process, if the clearance between the parting surface of the mold or between the slide block and the cavity is too large, under the action of high - pressure molten metal, the molten metal will overflow from these clearances, forming flash. During long - term use of the mold, due to the scouring of high - pressure molten metal, the mechanical impact of mold opening and closing, and thermal expansion and contraction, the parting surface will gradually wear, increasing the clearance. In addition, if the mold manufacturing accuracy is not high and there is a problem of excessive clearance at the initial stage, it is also easy to cause flash. Through experiments, after a certain die - casting mold was used 5000 times, the clearance of the parting surface increased from the initial 0.05 mm to 0.15 mm, and the defect rate of flash increased from 2% to 10%.
- Insufficient Clamping Force: During die - casting, sufficient clamping force is required to resist the pressure generated by the molten metal in the cavity. If the clamping force is insufficient and cannot effectively prevent the parting surface of the mold from opening, the molten metal will overflow from the parting surface under the pressure difference, forming flash. For example, when die - casting large castings or thin - walled castings, due to the large filling pressure of the molten metal, if the clamping force is improperly selected, it is very easy to have flash. When die - casting a large aluminum alloy casting with a size of 500 mm × 300 mm, when the clamping force is less than 80% of the theoretical requirement, the defect rate of flash soared from 5% to 25%.
Influences
- Increased Post - Processing Workload: The presence of flash greatly increases the subsequent cleaning and processing workload of the casting. Removing flash requires a large amount of manpower, material resources, and time, increasing production costs. Common methods for removing flash include manual grinding, mechanical processing, chemical corrosion, etc. However, no matter which method is used, additional processes and resource inputs are required. According to statistics, in a certain die - casting factory, the post - processing cost increased by 20% due to flash defects, resulting in an additional annual cost of about 500,000 yuan.
- Dimensional Accuracy and Assembly Problems: Flash may cause the casting size to exceed the designed tolerance range, affecting its assembly accuracy with other parts. In some assembly occasions with extremely high requirements for dimensional accuracy, such as the assembly of aero - engine components, the dimensional deviation caused by flash may lead to the inability to complete the entire assembly, and even affect the performance and safety of the product. During the assembly process of a certain aero - engine blade, due to flash on the blade causing a dimensional deviation of 0.1 mm, the assembly failure rate increased from 1% to 10%, seriously affecting the production progress.
Surface Scratches
Causes of Generation
- Frictional Effect: During the die - casting process, there is a large frictional force between the casting and the mold surface. When the mold surface is not smooth enough and there are small protrusions, scratches, or 拉伤痕迹，the surface of the casting is easily scratched by the defects on the mold surface during the demolding process, forming surface scratches. In addition, uneven coating application, which fails to form an effective lubricating isolation layer between the casting and the mold surface, will also increase the frictional force and lead to surface scratches. For example, in areas with too little coating, the casting contacts the mold directly, increasing the frictional force and making it easy to produce surface scratches. In a certain die - casting production, when the surface roughness Ra value of the mold increased from 0.8 μm to 1.6 μm, the defect rate of surface scratches on the casting increased from 3% to 10%.
- Unreasonable Process Parameters: Unreasonable settings of die - casting process parameters, such as die - casting speed and draft angle, will also increase the risk of surface scratches. Excessive die - casting speed will instantaneously increase the frictional force between the casting and the mold surface, and too small a draft angle will increase the resistance when the casting is demolded, both of which may lead to surface scratches. In a certain die - casting process, when the die - casting speed was increased from 3 m/s to 5 m/s and the draft angle was reduced from 3° to 1°, the defect rate of surface scratches on the casting increased from 5% to 20%.
Influences
- Reduction in Surface Quality: Surface scratches directly damage the surface integrity of the casting and reduce its surface quality. This not only affects the appearance of the casting, making its surface rough and uneven, but also reduces the corrosion resistance of the casting. In a humid or corrosive medium environment, the scratched parts are prone to become the starting points of corrosion, accelerating the corrosion damage of the casting. Through experiments, in a salt - spray corrosion environment, the corrosion rate of a casting with surface scratch defects is 50% faster than that of a normal casting.
- Stress Concentration and Shortening of Fatigue Life: The scratched areas will form stress concentration points. When the casting is subjected to external forces or alternating loads, these stress concentration points are prone to initiate and expand cracks, thus reducing the fatigue life of the casting. For some parts that need to withstand long - term alternating loads, such as the crankshafts and connecting rods of automotive engines, surface scratches will significantly shorten their service life. Research shows that the fatigue life of an automotive engine connecting rod with surface scratch defects is 35% shorter than that of a normal connecting rod.
Understanding the common manufacturing defects in high - pressure die casting and their causes and influences is the key to solving and preventing these defects. Through multi - aspect measures such as optimizing mold design, improving die - casting process parameters, strengthening mold maintenance and upkeep, and enhancing operator skills, the occurrence of manufacturing defects can be effectively reduced, and the quality and reliability of high - pressure die - castings can be improved to meet the needs of different industries for high - quality castings.
