Die castings will shrink in volume during solidification process, forming shrinkage cavities and shrinkage porosity; due to internal shrinkage of castings, shrinkage defects will appear on the surface of castings (see Figure 1). For die castings, especially thick die castings, shrinkage cavities and shrinkage porosity are common problems. This topic analyzes and studies several aspects such as how to compensate for shrinkage of alloy liquid, how to prevent formation of concentrated shrinkage cavities in alloy liquid, how to prevent generation of gas, encapsulation gas, how to exhaust to eliminate or reduce shrinkage cavities and shrinkage defects, thereby improving yield rate of die castings.
Graphical results

 

Figure 1 Shrinkage cavity defect map

The thicker wall thickness of casting or the larger hot section, the greater volume shrinkage, and the more difficult it is to discharge gas, so it is easy to form large shrinkage cavities. This requires that hot joints of casting be eliminated or reduced as much as possible, and sharp corners of hot joints of casting should be changed to rounded corners; wall thickness of casting should be uniform, transition should be gradual and smooth at joints where wall thickness of castings differs greatly; if hot joints, lugs, bosses, etc. are unavoidable in casting design, hollow structures, rib structures, etc. can be used to eliminate hot joints and reduce quality of casting.
Larger hot spots and wide flat surfaces on castings are prone to eddy currents, which easily trap gas; it is difficult to fill deep cavity of mold with molten metal, it is difficult to vent blind holes and dead corners of casting, easily trap gas, easily cause shrinkage cavities and shrinkage defects. When designing this type of casting, location of inner gate and overflow exhaust port must be considered.

If cross-sectional area of inner gate is too small, filling speed of molten metal will be too fast and spray will occur, causing molten metal to envelop a large amount of gas. Therefore, cross-sectional area of inner gate should be appropriately increased to prevent molten metal in inner gate from solidifying prematurely, thereby blocking pressurization and feeding channel. It is necessary to enhance feeding capacity of inner gate and extend feeding time. If position of inner gate is far away from hot spot where shrinkage cavities or dents occur, it is necessary to appropriately change position where alloy liquid is introduced into inner gate, number of inner gate, and direction in which inner gate enters mold cavity, so that alloy liquid can be filled in mold cavity in an orderly manner, pressure can be effectively transmitted, cavity gas can be effectively eliminated, so as to achieve purpose of effectively feeding hot joint. Reasonably design structural form of connection between inner gate and cavity to achieve ideal flow direction of inner gate. Alloy liquid must first be filled into deep cavities, thin walls, locations that require more alloy liquid, middle location of a large plane, locations that require high surface quality of castings, and locations that are difficult to form, etc., so that molten metal can be filled in cavity in an orderly manner to prevent molten metal from generating eddy currents and air entrainment. Eliminating or reducing frontal impact of molten metal on mold wall can reduce mold temperature at impact site. Pay attention to changing flow direction or position of inner gate to prevent molten metal from solidifying prematurely and blocking parting surface or exhaust channel.

Increase injection filling pressure and boosting feeding pressure of die-casting machine, and increase opening of boosting valve (shorten boosting time), so that boosting and feeding can be carried out in a timely manner, which can increase flow and feeding capacity of alloy liquid, improve density of casting structure. Content of molten metal wrapping gas can be reduced by reducing low-speed and high-speed injection speeds, delaying starting position of high speed. You can also use a very high speed to make inner gate speed reach 60~120m/s, so that molten metal can be sprayed into mold cavity in a mist form. In this way, although the total gas content in casting is increased, larger bubbles will not be encapsulated, size of shrinkage cavities and shrinkage dents can be significantly reduced, so that size of holes does not exceed standard. It was learned from test that when filling speed is low, large pores will appear, but number is small, and the total amount is small; when filling speed is high, number of pores will appear, the total amount is large, but size is small. The higher melting or pouring temperature of die-casting alloy, the more air will be absorbed inside alloy, the larger crystal grains of casting will be, and the greater shrinkage of alloy liquid after solidification. During melting and heat preservation process of alloy liquid, temperature of alloy liquid must be controlled not to overheat, and alloy liquid must not be kept at a high temperature for too long. If alloy liquid needs to be shut down for more than 2 hours to maintain heat, heat preservation temperature of ADC12 aluminum alloy should be reduced to 620~630℃. On the premise of ensuring that casting does not produce cold shut or insufficient pouring, pouring temperature of alloy liquid should be reduced as much as possible, which can reduce shrinkage and gas content of alloy liquid.

If thickness of die-casting remaining material cake is too thin, it will affect transmission of booster pressure, making final pressurization of injection unable to have an effective feeding effect. Therefore, quantitative pouring must be used to ensure thickness of die-casting material . If too much release agent is sprayed into cavity, paint will emit a large amount of gas, paint concentration will be high, and water in paint will not evaporate completely before pouring, it will cause a large amount of gas to be involved in molten metal. If gas is compressed between surface of mold cavity and metal liquid interface, large pores will appear under skin of casting, and shrinkage will appear on the surface of casting. For shrinkage cavities that are oxidized and contaminated by paint, inner wall surface of cavity is mostly dark gray. This requires a low concentration of paint, a small amount, and even spraying; after spraying paint, use compressed air to dry moisture on the surface of mold cavity and allow moisture to evaporate cleanly before closing mold; increase mold temperature so that moisture in paint can evaporate quickly; if necessary, you can also use paint with a small amount of gas. .

Forced feeding is use of external force to push liquid, semi-solid and solid metals to move for feeding. There are two ways to achieve forced feeding of castings by external force: one is local extrusion, and the other is forging feeding. Adopting process of first die-casting and filling, then extrusion feeding or die forging feeding is an effective way to solve defects of shrinkage cavities and shrinkage porosity in castings. On the basis of die-casting process, adding forced local extrusion feeding can adapt to characteristics of die-casting process, can well solve problems of local shrinkage cavities and shrinkage porosity in die-casting parts.
In conclusion
Shrinkage cavities and shrinkage porosity defects are mainly caused by volume shrinkage of alloy liquid during crystallization, gas entrapped in alloy liquid, and gas produced by precipitation of dissolved hydrogen atoms in alloy liquid. Main measures are to design castings that are easy to fill without hot spots; try to speed up shrinkage of alloy and reduce mold temperature at hot spots; refine alloy liquid to reduce chance of gas entrapment when alloy liquid fills cavity, etc. In short, only through comprehensive control from all aspects can occurrence of shrinkage cavities and shrinkage porosity defects be truly prevented.