As the advantages of aluminum alloy die castings gradually emerge, market demand continues to require processing plants to increase production in order to upgrade old iron casting products to aluminum alloys or magnesium alloys as soon as possible. As a result, the processing plant's tasks have become heavier, and quality must also be maintained. Today, the editor will teach us how to prevent some common defects of aluminum alloy die castings.
1. Drawing mold
Characteristics and inspection methods: Strip-shaped scratch marks appear on the surface of the casting along the direction of mold opening, with a certain depth, and in severe cases, they are planar scars. The other is that the molten metal adheres to the mold, causing adhesion and strain, resulting in excess or lack of material on the surface of the casting.
Reasons: 1. There is damage to the surface of the mold cavity (collapse or knock). 2. The slope of the demoulding direction is too small or inverted. 3. The ejection is unbalanced and the ejection is deflected. 4. The pouring temperature is too high and the mold temperature is too high, causing the alloy liquid to adhere. 5. The release agent is not effective. 6. The iron content of aluminum and gold components is less than 0.6%. 7. The cavity is rough and not smooth, and the mold hardness is low.
Preventive measures: 1. Modify the damaged parts of the mold surface, modify the demoulding angle, increase the mold hardness (HRC45°~48°), and improve the mold finish. 2. Adjust the ejector rod to balance the ejection. 3. Replace the release agent with good release agent. 4. Adjust the iron content of the alloy. 5. Reduce the pouring temperature and control the mold temperature to be stable and balanced. 6. Adjust the direction of the inner gate to prevent the molten metal from directly hitting the core and wall.
2. Bubbles
Characteristics and inspection methods: There are arches of varying sizes on the surface of the casting, or cavities under the skin.
Causes: 1. The coverage of the molten metal in the injection chamber is too low (controlled at 45%~70%), which is prone to air entrainment and the initial injection speed is too high. 2. The mold pouring system is unreasonable and the exhaust is poor. 3. The melting temperature is too high, the gas content is high, and the melt is not degassed. 4. The mold temperature is too high, the mold retention time is insufficient, the metal solidification time is insufficient, the strength cannot be opened prematurely, and the pressurized gas expands. 5. Excessive dosage of release agent and injection oil. 6. The air blowing time after spraying is too short, and the water on the mold surface has not been dried.
Precautionary measures: 1. Adjust the die-casting process parameters, injection speed and high-pressure injection speed switching point. 2. Modify the mold sprue and add an overflow groove and an exhaust groove. 3. Reduce the mold temperature in the defective area and then reduce the pressure of the gas. 4. Adjust the smelting process, 5. Extend the mold retention time and adjust the blowing time after spraying. 6. Adjust the dosage of release agent and injection oil.
3. Cracks
Characteristics and inspection methods: The surface of the casting has linear or irregular narrow textures, which tend to develop under the action of external forces. Cold cracking - the metal at the crack has not been oxidized. Thermal cracking - The metal is oxidized at the crack site.
Causes: 1. The iron content in the alloy is too high or the silicon content is too low. 2. The content of harmful impurities in the alloy is too high, which reduces the plasticity of the alloy. 3. Aluminum-silicon alloy: Aluminum-silicon-copper alloy contains too much zinc or too much copper; aluminum-magnesium alloy contains too much magnesium. 4. The mold temperature is too low. 5. Where the wall thickness of the casting changes drastically, shrinkage is blocked. 6. The mold retention time is too long and the stress is high. 7. The force is uneven when ejecting.
Preventive measures: 1. Correctly control the alloy composition. In some cases, pure aluminum ingots can be added to the alloy to reduce the magnesium content in the alloy; or aluminum-silicon center alloy can be added to the alloy to increase the silicon content. 2. Change the structure of the casting, increase the rounded corners, increase the draft angle, and reduce the wall thickness difference. 3. Change or increase the ejection direction to make the ejection force even. 4. Shorten the mold opening or core pulling time. 5. Increase the mold temperature (mold working temperature 180°-280°).
4. Deformation
Features and inspection methods: The geometry of the die-casting parts does not match the drawings. Total or partial deformation.
Causes: 1. Poor structural design of castings, causing uneven shrinkage. 2. If the mold is opened too early, the casting will not be rigid. 3. Die deformation. 4. The setting of the ejector rod is unreasonable and the force is uneven during ejection. 5. Improper gate removal method.
Preventive measures: 1. Improve the casting structure. 2. Adjust the mold opening time. 3. Set the position and quantity of ejector rods reasonably. 4. Choose a reasonable gate removal method. 5. Eliminate die drawing elements.
5. Traces and markings
Characteristics and inspection methods: Appearance inspection shows that there are stripes on the surface of the casting that are consistent with the direction of movement of the molten metal, and there are obviously non-directional textures that are different in color from the metal matrix and have no development trend.
Reasons: 1. The molten metal that first enters the mold cavity forms an extremely thin and incomplete metal layer, which is compensated by the subsequent molten metal and leaves traces. 2. The mold temperature is too low. 3. The cross-sectional area of the inner gate is too small and the position is improper to cause splashing. 4. The pressure acting on the molten metal is insufficient. 5. Patterns: Too much paint and injection oil are used.
Precautions: 1. Increase the mold temperature. 2. Adjust the cross-sectional area or orientation of the inner gate. 3. Adjust the speed and pressure of molten metal in the gate. 4. Select appropriate paint, injection oil and adjust the amount of paint injection oil.
