During the solidification process, die-casting parts will experience volume shrinkage, forming shrinkage pores and porosity; Due to internal shrinkage of the casting, shrinkage defects may appear on the surface of the casting (see Figure 1). For die-casting parts, especially thick ones, shrinkage and porosity are common problems. This project analyzes and studies how to compensate for the shrinkage of the alloy liquid, how to prevent the formation of concentrated shrinkage pores in the alloy liquid, how to prevent the generation of gas and entrapment gas, and how to exhaust, in order to eliminate or reduce shrinkage defects and improve the yield of die castings.

Graphic and textual results

1. Improvement of casting structure

The thicker the wall thickness or the larger the hot spot of the casting, the greater the volume shrinkage, and the more difficult it is to expel gas, thus easily forming large shrinkage pores. This requires eliminating or reducing the hot spots of the casting as much as possible, and changing the sharp corners of the hot spot area of the casting to rounded corners; The wall thickness of the casting should be uniform, and at the connection where the wall thickness of the casting varies greatly, it should gradually transition smoothly; If the design of the casting inevitably involves hot spots, protrusions, bosses, etc., hollow structures, rib structures, etc. can be used to eliminate hot spots and reduce the quality of the casting.
Large hot spots and wide flat areas on castings are prone to generating eddy currents, making them prone to gas entrapment; Metal liquid is difficult to fill into the deep cavity of the mold, and blind holes and dead corners of the casting are difficult to exhaust, prone to gas entrapment, and prone to shrinkage and concavity defects. When designing this type of casting, it is necessary to consider the position of the internal gate and the overflow exhaust port.

2. Improvement of Die Casting Mold Structure

If the cross-sectional area of the inner gate is too small, it will cause the filling speed of the metal liquid to be too fast, resulting in spraying and promoting the metal liquid to wrap up a large amount of gas. Therefore, it is necessary to appropriately increase the cross-sectional area of the inner gate to prevent premature solidification of the metal liquid in the inner gate, thereby blocking the pressure boosting and contraction channel. To enhance the filling ability of the inner gate and extend the filling time. If the position of the inner gate is far from the hot spot where shrinkage or concavity occurs, it is necessary to appropriately change the position of the alloy liquid entering the inner gate, the number of inner gates, and the direction of the inner gates entering the mold cavity, so that the alloy liquid can be orderly filled in the mold cavity, pressure can be effectively transmitted, and the cavity gas can be effectively eliminated, achieving the purpose of effectively filling and shrinking the hot spot. Reasonably design the structural form of the connection between the inner gate and the mold cavity to achieve the ideal flow direction of the inner gate. To ensure that the alloy liquid first fills the deep cavity, thin wall, areas that require more alloy liquid, the middle of the large plane, areas that require higher surface quality of the casting, and areas that are difficult to form, the metal liquid should be filled in an orderly manner in the mold cavity to prevent vortex and entrainment of the metal liquid. Eliminating or reducing the frontal impact of molten metal on the mold wall can lower the mold temperature at the impact area. When changing the flow direction or position of the sprue, it is important to prevent premature solidification of the metal liquid that may block the parting surface or exhaust passage.

3. Improvement of Die Casting Process and Alloy Melting

By increasing the injection filling pressure and boosting pressure of the die-casting machine, increasing the opening of the boosting valve (shortening the boosting time), timely boosting and supplementing can be carried out, which can increase the flow and supplementing ability of the alloy liquid and improve the density of the casting structure. The content of gas trapped in the metal liquid can be reduced by reducing the low-speed and high-speed injection speed and delaying the starting position of the high-speed injection. High speed can also be used to achieve a speed of 60-120m/s for the inner gate, allowing the metal liquid to spray and fill the mold cavity in a mist like manner. Although this increases the total gas content in the casting, it will not wrap large bubbles and can significantly reduce the size of shrinkage and concavity, achieving the goal of hole size not exceeding the standard. Experiments have shown that low filling speed will result in large pores, but the quantity and total amount are also small; The filling speed is high, with a large number and total amount of pores, but the size is small. The higher the melting or pouring temperature of the die cast alloy, the more air is sucked into the alloy, the larger the crystalline grains of the casting, and the greater the shrinkage rate of the alloy liquid after solidification. During the melting and insulation process of the alloy liquid, it is necessary to control the temperature of the alloy liquid to avoid overheating, and to keep the alloy liquid at high temperature for a short period of time. If the alloy liquid needs to be shut down for more than 2 hours for insulation, the insulation temperature of ADC12 aluminum alloy should be reduced to 620-630 ℃. On the premise of ensuring that castings do not produce cold shuts or insufficient pouring, the pouring temperature of the alloy liquid should be reduced as much as possible, which can reduce the shrinkage and gas content of the alloy liquid.

4. Improvement of Die Casting Operation

The thickness of the remaining material cake in die casting is too thin, which can affect the transmission of boost pressure and make the final boost of the injection ineffective. Therefore, quantitative pouring should be used to ensure the thickness of the material cake in die casting. Excessive spraying of release agent on the mold cavity, high gas generation and concentration of the coating, and incomplete evaporation of moisture before pouring can all promote the generation of a large amount of gas entrained in the molten metal. If the gas is compressed between the surface of the mold and the interface of the molten metal, large pores will appear under the casting skin, and shrinkage will occur on the surface of the casting. The inner wall surface of the air shrinkage pores and holes caused by oxidation and pollution of the coating is mostly dark gray. This requires a low concentration, low dosage, and uniform spraying of the coating; After spraying paint, compressed air should be used to blow dry the moisture on the surface of the mold cavity, allowing the moisture to evaporate completely before closing the mold; Raise the temperature of the mold so that the moisture in the coating can quickly evaporate; If necessary, coatings with low gas generation can also be used.

5. Forced shrinkage method

Forced shrinkage is the use of external forces to drive the movement of liquid, semi-solid, and solid metals for shrinkage. There are two methods to achieve forced shrinkage of castings by external forces: one is local extrusion, and the other is forging shrinkage. The process of first die-casting and filling, followed by extrusion or forging is an effective way to solve the shrinkage and porosity defects of castings. On the basis of die-casting technology, adding forced local extrusion and shrinkage can adapt to the characteristics of die-casting technology and effectively solve the problem of local shrinkage and porosity in die-casting.

conclusion

Shrinkage and porosity defects are mainly caused by the volume shrinkage during the crystallization of the alloy liquid, the gas trapped in the alloy liquid, and the gas formed by the precipitation of hydrogen atoms dissolved in the alloy liquid. The main measure is to design castings that have no heat joints and are easy to fill; Try to accelerate the shrinkage of the alloy and reduce the mold temperature at the hot spot area; Refine the alloy liquid to reduce the chance of gas entrapment during the filling of the mold cavity. In short, it is necessary to comprehensively control from all aspects in order to truly prevent the occurrence of shrinkage and porosity defects.

Author of this article:

Liu Zunjian
Suzhou Youniang Precision Metal Manufacturing Co., Ltd
Liu Shudi
Technical Department of Shanghai Mitsubishi Electric · Shangling Air Conditioning Electrical Appliance Co., Ltd
Source of this article: Journal of Special Casting and Nonferrous Alloys