Zinc die-casting is the process of filling molten zinc into the mold cavity rapidly and under high pressure. The time for liquid zinc to fill the cavity of the die-casting mold is very short, which is usually several hundredths of a second or several thousandths of a second. The pores formed in the process of die casting have smooth surface, and the shape is mostly circular or elliptical. They mostly exist on the surface or subcutaneous pores of the casting, and may also exist in the casting. The main source of pores is the gas involved in the process of die casting or the gas evolution of zinc liquid.
1、 Air entrainment during die casting.
1. The die-casting machine now basically adopts three stages of injection. In the first stage of injection, the injection punch advances at a relatively slow speed (usually within 0.3 m/s), which is conducive to extruding the gas in the pressure chamber; The second stage of injection is to press the structure and wall thickness of the casting to select the appropriate flow rate, and the inner gate speed is extremely fast (generally the punch speed is 1~6 m/s, and the speed of thin-walled parts, high air-tightness parts, and magnesium alloy parts may reach more than 8 m/s), so as to basically fill the mold cavity with liquid zinc. This level 1 is the key to produce pores in die castings. The higher the speed, the easier it is to produce eddy current and form pores.
In this process, the control of air holes in die castings is mainly achieved by controlling the first and second injection speed and the first and second switching points. 1、 The secondary speed should be as low as possible (but too low will affect the casting forming or surface quality, which should be determined according to the actual situation); The starting point of secondary injection can be selected after the position where casting blowholes are not allowed. We can choose different starting points for different castings. At the same time, with the continuous improvement and improvement of the working performance of the die-casting machine, such as the injection speed, pressure build-up time, and speed up time, the casting porosity will be less and less.
2. A good die-casting die should have a good gating system and overflow drainage system. In the process of die casting, it is necessary to try to keep multiple runners and the direction of zinc liquid flow consistent with the casting, and try not to collide with each other to produce eddy current and air entrainment due to filling confusion; In addition, when filling the cavity with multiple runners, attention should be paid to filling at the same time. It is not allowed to let one or several strands of zinc liquid reach the dead corner at the rear and then return to generate eddy current. The distribution of slag collecting ladle and exhaust duct on the die casting mould should be reasonable.
3. The temperature of die casting mould also has a key influence on the quality and porosity of castings. When the mold temperature is too high, the release agent volatilizes at high temperature and cannot form a dense skin film, which is easy to cause mucous membrane; If the mold temperature is too low, the film formed by the mold release agent has non-volatile moisture, which makes the mold release effect poor and leads to casting porosity. Generally, the mold preheating temperature is 150 ℃~180 ℃, and the working holding temperature is 220 ℃~280 ℃.
4. Gases from paint
A. First of all, the performance of the coating: too high volatile point and large gas generation have a direct impact on the casting porosity.
B. From the perspective of spray technology: too much spray is used and the spray time is too long, which is easy to cause a large amount of gas volatilization, and also causes the mold surface temperature to be too low. The moisture on the mold surface cannot evaporate for a while, and a large amount of gas is generated in the mold cavity after the mold is closed. In the production process, we should select the coating with good performance, low volatile point and low gas generation.
5. Because the characteristic of die casting is to fill the mold cavity at a very fast speed, and the liquid zinc solidifies rapidly in the mold to form products, there must be pores in the casting due to the entrapment of liquid zinc. However, the surface layer of the casting will also form a dense layer of fine grains due to rapid solidification. These fine grains have high mechanical properties. As long as the machining allowance of the casting is as small as possible, the physical properties of the casting can also be guaranteed. Excessive machining allowance will remove the surface dense layer, which will cause the exposure of internal pores and reduce the physical properties of the casting.