Magnesium alloy has characteristics of high specific strength and specific stiffness, good impact resistance, excellent electromagnetic shielding and thermal conductivity, and easy recycling. It also has good casting performance and corrosion resistance, is widely used in aviation, aerospace, 3C and other industries. For example, magnesium alloy can replace aluminum alloy and be used to prepare high-power LED radiators. To obtain magnesium alloy die-casting parts with dense structure and good performance, vacuum die-casting is one of main forming methods. In view of shortcomings of current vacuum die-casting such as low cavity pumping efficiency, unreliable vacuum valve closing, slow vacuum valve response, and high price, a vacuum pumping system containing full-process and half-process exhaust channels was designed. When performing vacuum die casting, reasonable process parameters can obtain good quality die castings. Preliminary research found that fast injection speed has a great impact on structural properties and mechanical properties of vacuum die castings. Therefore, it is of positive significance to study influence of different fast injection speeds on quality of magnesium alloy vacuum die castings. Taking AZ91D magnesium alloy as research object, three sets of vacuum die-casting tests were conducted using a self-designed vacuum pumping system to study effects of different fast injection speeds on mechanical properties and structure of vacuum die-casting parts.

A large AZ91D magnesium alloy LED lamp radiator is used as the target product, and Solidworks software is used to conduct a three-dimensional solid modeling of radiator. Its structural diagram is shown in Figure 1. Size of casting is 220mm*130mm*170mm, and thickness is uneven. The thickest is 8mm, the thinnest is 1.8mm, and average thickness is 4.5mm. Heat sink is the thinnest, a cylindrical push rod position is set at an appropriate position on each heat sink to ensure that the entire die casting is evenly stressed during demoulding process and facilitates smooth demoulding. Gating system is designed according to structural characteristics and forming method of casting.
Test was conducted on a DM400 horizontal cold chamber die-casting machine. In order to protect mold, improve forming efficiency of castings, and reduce scrap rate, an AODE oil circulating mold temperature machine was used to preheat mold to 200℃. Schematic diagram of vacuum exhaust system is shown in Figure 2. Working principle of vacuum pumping system containing full-process and half-process exhaust channels is as follows: before start of die-casting, main valve is closed, and vacuum pump begins to continuously pump air from vacuum tank to achieve a preset vacuum level. When die casting starts, half-process exhaust solenoid valve is open. When molten metal enters pressure chamber through pouring port, injection punch starts to inject. When injection punch crosses pouring port and touches induction switch, main valve opens, vacuum tank simultaneously evacuates mold cavity through full-process exhaust channel and half-process exhaust channel. When injection punch pushes metal liquid forward and triggers fast injection induction switch, half-process exhaust solenoid valve is quickly closed by electromagnetic force, while full-process exhaust channel continues to pump air into cavity, as shown in Figure 2b. When metal liquid fills mold cavity, enters curved and narrow exhaust channel to cool and solidify, pumping stops. At this time, a vacuum die-casting test is completed and a vacuum die-casting part is obtained.

 

Figure 1 3D model of die casting

Table 1 Thermophysical parameters of AZ91D magnesium alloy

 

Figure 2 Schematic diagram of vacuum pumping system

1. Static mold 2. Moving mold 3. Vacuum tank 4. Vacuum pump 5. Pour port 6. Punch 7. Pressure chamber 8. Cavity 9. Solenoid valve 10. Half-process exhaust channel 11. Full-process exhaust channel 12.Master valve

Table 2 Die casting process parameters
Vacuum die casting is shown in Figure 3. Observation shows that vacuum die-casting parts of three sets of radiators are complete in appearance, there is no phenomenon of insufficient charging, and there is almost no difference in appearance. Careful observation revealed that part of heat sink in L1 has a cold insulation, as shown in Figure 4. Therefore, when product is subjected to static load or cyclic stress, it is easy to crack or even break, which seriously affects the safety of the product. In addition, surface flow marks also appear. This is due to low injection speed of fast injection mold. Molten metal that first enters die-casting mold cavity forms a thin and incomplete metal layer, which is covered by subsequent molten metal and leaves traces, thus affecting surface quality of die-casting part. See Figure 4b. No obvious cold insulation and surface flow mark defects were found in L2 heat sink; due to high injection speed of L3, when molten metal quickly fills cavity, it has a great impact and is prone to flash, which increases cost and time of product machining, causes material waste, and shortens life of mold.

 

Figure 3 Radiator vacuum die casting

 

Figure 4 L1 heat sink surface defects
A number of ordinary die-casting parts and vacuum die-casting parts were produced using L1~L3 process parameters. Ordinary die-casting parts and vacuum die-casting parts produced in the three groups of experiments were sampled respectively. Sampling locations were divided into two parts: second part of heat sink and bottom plate, as shown in Figure 2. Metallurgical crystalline samples and tensile samples were taken using wire-cut electric discharge machines, and JHY-5000 electronic universal testing machine was used to test mechanical properties of heat sink. It can be seen that L1 heat sink has larger shrinkage cavities and a wider distribution of shrinkage porosity; L2 heat sink has a smaller shrinkage range and smaller size; L3 heat sink has larger shrinkage cavities and shrinkage porosity. L1 base plate has smaller shrinkage cavities but they are widely distributed; L2 base plate has a small amount of shrinkage porosity and smaller shrinkage holes; L3 base plate has multiple shrinkage porosity and shrinkage holes. Analysis shows that using a self-designed vacuum pumping system can extract most of gas from mold cavity, so internal pores of vacuum die-casting parts are reduced. However, due to different fast injection speeds, vacuuming time is different, final air pressure of mold cavity is also different, and flow state of metal in cavity is also different. Therefore, shrinkage porosity and shrinkage holes of vacuum die-casting parts produced by three groups of experiments are also different.

Table 3 Mechanical properties of heat sink

 

Figure 5 Microstructure of vacuum die-casting heat sink

 

Figure 6 Microstructure diagram of vacuum die-cast radiator bottom plate

(1) A new vacuum pumping system was used to conduct a vacuum die-casting test. Results show that magnesium alloy radiator vacuum die-casting parts have a complete appearance, less cold insulation, and a reduced number of cracked parts. Compared with ordinary die-casting parts with same process parameters, average tensile strength is increased by 12% and elongation is increased by 72%.
(2) When fast injection speed is high, vacuum die castings are prone to casting defects such as shrinkage and shrinkage holes; when fast injection speed is low, casting defects such as cold shuts and surface flow marks are prone to occur, thus affecting final quality of product.
(3) Under conditions of fast injection speed of 4m/s, slow injection speed of 0.2m/s, slow injection distance of 120mm, and injection specific pressure of 84MPa, magnesium alloy vacuum die-casting parts with complete appearance, dense structure and good mechanical properties can be obtained.