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The Quality Defects and Improvement Measures of Aluminium Alloy Die Casting Parts (Part Four)

Source: Junying   Author: Junying   Posted: 2017-07-31 10:32:21   Hits: 8
2.4 Crack
The substrate of aluminium alloy die casting parts is destroyed and disconnected, which forms the long and thin gap whose length is up to 50mm. It has irregular shapes of straight or wave lines. When the external force acts on it, it has the trend of extension. This kind of defect is called the crack.

2.4.1 Causes
The ingredients of alloy are abnormal. For example, the content of magnesium is too high, which increases stickness of alloy. At the time of ejecting the mold, the crack will appear. In the premise that the ingredients of alloy don't change, the crack also appears in the state of relatively high temperature. Structures around crack have obvious shrinkage cavities. When alloy is cooling and solidifying, outside areas shrink at first to produce outward tensile stress and cause the crack in the parts of shrinkage cavities because the order of cooling is different.

2.4.2 Improvement measures
Control the ingredients of alloy correctly. In some circumstances, add pure aluminium ingot to alloy so as to reduce the magnesium content of alloy, or add Al-Si master alloy to aluminium alloy so as to increase the silicon content. In order to relieve excessive heating of molds, increase cooling water circuits in the molds. Through water-cooling, the temperature of molds in this area can be lowered and thermal balance of molds can be kept. Change the structures of aluminium alloy die casting parts. Enlarge fillets. Change the pattern draft. Reduce the difficulty of mold unloading and differences of the wall thickness. Change the ejection position so as to make ejection force evenly distributed and eliminate excessive stress on the parts.

2.5 Strain
Along the direction of mold unloading, there is the linear strain mark on the surface of die casting parts. The mark has a certain depth. When it is serious, the whole surface is strained. Metal liquid and the surface of the mold bond together, which causes lack of materials on the surface of die casting parts.

2.5.1 Causes
There is damage on the surface of the mold cavity. The direction of mold unloading has no or too little inclination. Ejection is imbalanced. The mold loosens. The temperature of casting is too high or too low. If the temperature is too high, it will cause alloy liquid to adhere to the mold. The application effect of mold releasing agent is not good. The ferrum content in the ingredients of aluminium alloy is lower than 0.8%. The cooling time is too long or too short. Parallelism of the die casting machine is bad.

2.5.2 Improvement measures
Repair damage on the surface of molds. Correct inclination and increase surface finish of molds. Adjust the ejector pin so as to balance ejection power. Fasten molds. Control casting and mold temperature reasonably from 180℃ to 500℃. Change mold releasing agent or use anti-strained paint. Adjust the ferrum content of aluminium alloy. Adjust cooling time. Alter the ingate and change the direction of aluminium liquid. Adjust parallelism of the die casting machine.