2.3 Shrinkage cavities
Shrinkage cavities refer to holes with irregular shapes on the thick sections of die casting parts
. The internal walls of holes are so rough that they may cause cellular structures in some parts of die castings and affect the strength of die castings
. The outer surfaces of die casting
aluminium alloy products have rather compact layers There are some tiny holes inside workpieces because of the shrinkage phenomenon. If the processing quantity exceeds the thickness of compact layer, holes will increase obviously.
In the process of die casting
, aluminium liquid begins to solidify after being pressed into and filling the cavity. Because the temperature of mold surfaces is relatively low and there is water-cooling, aluminium liquid begins to solidify from the surface which have the contact with the mold at first and forms a layer of crust in the outermost surface. Then it begins to solidify inwards gradually. Aluminium liquid shrinks and its volume becomes small along with decrease of temperature, but a layer of sealed crust has formed on the surface of die casting parts. Therefore, as aluminium liquid is solidifying gradually, there is some hollow space at the final solidified position. That is shrinkage. Oversize wall thickness causes low speed of inside cooling and solidification. After the cavity is full of the liquid metal, shrinkage cavities happen easily in the thin or thick die casting parts without getting enough supplementary of the liquid metal in the process of shrinkage.
2.3.2 Improvement measures
The method of eliminating shrinkage cavities can be reducing the wall thickness in the areas where shrinkage cavities exist, or optimizing the structures of die casting parts and molds, which can make solidification fast and even. Sometimes due to functions and structures of some areas, wall thickness can't be decreased. It can be considered to change the depth of die casting parts' processed prefabricated holes. From the angle of the technique, it is also an effective way to increase cooling water circuits in this area of the molds to strengthen cooling and speed up solidification rate of aluminium liquid so as to decrease the volume of shrinkage cavities and control shrinkage cavities within the unessential areas. Besides, the method of increasing pressure to improve compactness of structures can also be adopted. In general, shrinkage cavities and blowholes can't be eliminated completely. They can just be reduced or transferred.