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Prevent surface defects of aluminum alloy die castings

Posted by: Chris Na 2021-05-31 Leave a comment

As the advantages of aluminum alloy die castings gradually appear, market demands continue to require processing plants to increase productivity, so as to upgrade the old iron casting products to aluminum alloys or magnesium alloys as soon as possible. As a result, the task of the processing plant has become heavy, and the quality must be maintained. Today, the editor will teach you how to prevent some common defects in aluminum alloy die castings.
One, die
Features and inspection methods: along the Mold opening direction, the surface of the casting presents strip-shaped scratch marks with a certain depth, and in severe cases, it is a surface scar. The other is the adhesion between the molten metal and the Mold, adhesion and strain, resulting in excessive or lack of material on the surface of the casting.
Reasons: 1. There is damage on the surface of the cavity (collapse or knock injury). 2. The inclination of the demolding direction is too small or inverted. 3. The ejector is unbalanced and the ejector is skewed. 4. Too high pouring temperature and high Mold temperature cause alloy liquid to adhere. 5. The effect of release agent is not good. 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. Repair the damaged parts of the mold surface, correct the demolding angle, increase the mold hardness (HRC45°~48°), and improve the mold finish. 2. Adjust the ejector rod to balance the ejector. 3. Replace the release agent with good release effect. 4. Adjust the iron content of the alloy. 5. Lower the pouring temperature and control the mold temperature to be stable and balanced. 6. Adjust the direction of the inner gate to prevent molten metal from directly hitting the core and wall.
Second, bubbles
Features and inspection methods: There are bulges of different sizes on the surface of the casting, or there are cavities under the skin.
Reasons: 1. The molten metal is filled too low in the injection chamber (control at 45%~70%), it is easy to produce entrainment, and the initial injection speed is too high. 2. Unreasonable mold casting system and poor exhaust. 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 not enough, the metal solidification time is not enough, the strength is not enough, the mold is opened too early, and the pressurized gas expands. 5. Excessive amount of release agent and injection head oil. 6. The blowing time after spraying is too short, and the water on the surface of the mold is not dried.
Preventive measures: 1. Adjust the switching point of die-casting process parameters, injection speed and high-pressure injection speed. 2. Modify the mold runner, add overflow groove and exhaust groove. 3. Reduce the mold temperature in the defect area, thereby reducing the pressure effect of the gas. 4. Adjust the smelting process. 5. Extend the mold retention time and adjust the blowing time after spraying. 6. Adjust the amount of release agent and injection oil.
三、Crack
Features and inspection methods: The surface of the casting has linear or irregular narrow lines, which tend to develop under the action of external force. Cold cracking—the metal at the cracking site is not oxidized. Thermal cracking—the metal at the crack is oxidized.
Reasons: 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 copper; aluminum-magnesium alloy contains too much magnesium. 4. The mold temperature is too low. 5. Where the wall thickness of the die casting changes drastically, shrinkage is hindered. 6. The mold staying time is too long and the stress is large. 7. Uneven force during ejection.
Precautionary 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 master alloys can be added to the alloy to increase the silicon content. 2. Change the structure of the casting, increase the fillet, increase the draft angle, reduce the wall thickness difference, 3. Change or increase the ejection position to make the ejection force uniform. 4. Shorten the time of mold opening or core pulling. 5. Increase mold temperature (mold working temperature 180°-280°).
Four, deformation
Features and inspection methods: The geometry of the die casting does not match the drawing. Overall deformation or partial deformation.
Reasons: 1. Poor casting structure design, causing uneven shrinkage. 2. The mold is opened too early, and the rigidity of the casting is not enough. 3. The die is deformed. 4. The ejector rod is set unreasonably, 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. Reasonably set the position and number of ejector pins. 4. Choose a reasonable method to remove the gate. 5. Eliminate the die factor.
Five, leaving marks and patterns
Features and inspection methods: visual inspection, there are stripes on the surface of the castings that are consistent with the flow direction of the molten metal, and there are obvious and non-directional lines that are different from the color of the metal matrix, and there is no development trend.
Reasons: 1. After the molten metal that first enters the cavity forms an extremely thin and incomplete metal layer, it is made up for the traces left by the subsequent molten metal. 2. The mold temperature is too low. 3. The cross-sectional area of ​​the inner gate is too small and the location is improper to cause splashing. 4. The pressure acting on the molten metal is insufficient. 5. Pattern: Excessive amount of paint and injection oil.
Preventive measures: 1. Increase the mold temperature. 2. Adjust the cross-sectional area or position of the inner gate. 3. Adjust the speed and pressure of the molten metal in the runner. 4. Choose appropriate paint, injection oil and adjust the amount of paint injection oil.

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