The formation of porosity in aluminum alloy die castings is mainly caused by porosity and shrinkage. Most of the pores are circular, and most of the shrinkage is irregular.
The causes of porosity in aluminum alloy die castings:
a. During the punching and solidification process, gas evaporated from the coating invades the molten metal, causing circular holes on the surface or inside of the casting.
b. If the gas content in the alloy liquid is too high, circular holes will also be formed during the solidification process.
The reason for shrinkage porosity:
a. During the solidification process of liquid alloys, shrinkage cavities are generated due to volume reduction or the inability of the final solidified portion to be filled with liquid metal.
b. During the heating process, due to uneven thickness or local overheating, the casting solidifies slowly at a certain location, causing volume shrinkage when it forms a concave surface with the casting.
Due to the presence of pores and shrinkage, the surface treatment of die-casting parts is very complicated. These pores may enter water. When spraying and baking after electroplating, the gas in the pores will heat up and expand, or the water in the pores will turn into steam, volume expansion, and volume expansion. When foaming on the surface of castings, appropriate methods should be used in production. Resolve the reasons for these drawbacks.
1. Aluminum alloy die casting:
The key to solving this problem is to reduce the amount of gas added to the casting. In an ideal state, the metal flow state of the alloy should enter the mold cavity through the nozzle through the dividing cone and runner, forming a smooth and uniform direction of metal flow. In the study of simulating the die-casting process of the filling system, it can be clearly seen that the sharp changes in the position of the runner and the increase in the cross-sectional area will cause turbulence and entrainment of the molten metal flow. Only when smooth liquid metal is conducive to gas entering the overflow and exhaust grooves from the flow channel and mold cavity, can it be discharged from the mold. The use of new ceramic filter materials eliminates the need for two nozzle materials and reduces impurities.
2. Regarding shrinkage:
Through reasonable nozzle design, inner gate thickness and position, mold design, mold temperature control, and cooling, ensure that all parts dissipate heat as evenly as possible during the solidification process, while also solidifying to avoid shrinkage. Pay attention to impurity elements in waste. Control the content of harmful impurities in alloy raw materials, especially lead content<0.002%.