1) What’s the shrinkage porosity?
Shrinkage and shrinkage porosity usually occur inside the casting, which is one of the common defects in the formation of aluminum alloy die castings. Shrinkage porosity of aluminum alloy die-castings generally occurs in thick parts at the root of the riser near the inner runner, wall thickness transition points, and thin walls with large flat surfaces. The fracture of aluminum castings in the as-cast state is gray, light yellow, and grayish-white, light yellow, or gray-black after heat treatment. On the X-ray film, it is cloudy and severely filamentous. Shrinkage can be found by X-ray.
2) The reason for shrinkage porosity:
During the solidification process, the casting cannot be supplemented from the aluminum distributing liquid and form holes.
The pouring temperature is too high, and the mold gradient distribution is unreasonable.
The injection ratio is low and the boost pressure is too low.
The inner gate is thin, the area is too small, and it solidifies prematurely, which is not conducive to pressure transmission and liquid metal feeding.
There are hot spots or cross-sections in the casting structure that change drastically;
The amount of molten metal pouring is too small, the remaining material is too thin, and it cannot be used for feeding.
3) How to improve the shrinkage porosity:
Lower the pouring temperature and reduce the shrinkage.
Increase the injection pressure and boost pressure to improve the compactness.
Modify the inner gate to make the pressure better, which is conducive to the feeding effect of liquid metal.
Change the structure of the castings, eliminate the metal gathering parts, and make the wall thickness as uniform as possible.
Speed up the cooling of thick parts.