What is the reason of shrinkage cavity in Dalian aluminum alloy die casting?

Dalian aluminum alloy die casting is mainly used in electronics, automobiles, motors, home appliances and some communication industries. Some aluminum alloy products with high performance, high precision and high toughness are also used in industries with high requirements such as large aircraft and ships.

Shrinkage cavity in aluminum alloy die-casting refers to irregular holes in the thick section of the die-casting. The inner wall of the hole is rough, which may even lead to honeycomb structure in the die-casting part, affecting the strength of the casting, and exposing the product processing hole.

There is a fairly dense structure on the outer surface of die cast aluminum alloy products, and some small holes will appear inside the workpiece due to shrinkage porosity. If the processing amount exceeds the thickness of the dense layer, the holes will increase significantly.

(1) Cause

During the die casting process, the aluminum liquid begins to solidify after it is pressed into the mold cavity and filled. Because the temperature of the mold surface is low and accompanied by water cooling, the aluminum liquid starts to solidify from the surface in contact with the mold, forms a layer of hard shell on the outside, and then gradually starts to solidify inward.

As the temperature decreases, the aluminum liquid gradually shrinks and the volume becomes smaller, but a layer of sealed hard shell has been formed on the outer surface of the casting, so as the aluminum liquid gradually solidifies, some hollow spaces, namely shrinkage cavities, will be formed at the solidification position.

Excessive wall thickness causes slow internal cooling and solidification speed. After the cavity is filled with liquid metal, it can not be supplemented sufficiently during the shrinkage process, which is easy to occur on the castings with uneven thickness.

(2) Improvement measures

The method of eliminating shrinkage cavity can be realized by reducing the wall thickness of the area where the shrinkage cavity is located so that it can solidify rapidly and uniformly, or by optimizing the casting and mold structure.

Sometimes, due to the function and structure of a certain area, the wall thickness cannot be reduced, so it can be considered to increase the depth of casting processing preformed holes to change the processing area. From the perspective of technology, add cooling water path on the mold in this area, strengthen cooling, and accelerate the solidification speed of aluminum liquid to reduce the volume of shrinkage cavity, and control the shrinkage cavity in the non important area;

It can also increase the compactness of the pressure lifting structure. Shrinkage and porosity can not be eliminated generally, and can only be reduced or transferred, or even sometimes it is just a problem of changing the processing to produce shrinkage cavity.