Vacuum Die Casting Magnesium Alloy Radiator Die Casting Performance

Magnesium alloys have the characteristics of low density, high specific strength and specific stiffness, strong electromagnetic shielding ability, strong shock absorption ability, excellent cutting performance, easy recycling, etc., and have been widely used in engineering. AZ91D magnesium alloy has been widely used in die-casting magnesium alloy due to its good casting properties and corrosion resistance, as well as certain mechanical properties. Vacuum die casting can reduce the generation of pores and oxide inclusions in magnesium alloy die castings, improve the compactness of die castings, reduce the filling resistance of molten metal in the mold cavity, and improve the mechanical properties of die castings at room temperature. Therefore, vacuum die casting can be used for the production of large and complex thin-walled parts with high mechanical strength and even heat treatment, such as radiators for LED lamps. The natural convection velocity of the air between the fins of the magnesium alloy radiator is large, which helps to improve the comprehensive heat transfer efficiency of the radiator. Therefore, the magnesium alloy can replace the aluminum alloy for the preparation of high-power LED radiators.

The key to the vacuum die-casting forming process is vacuuming, that is, how to quickly extract the gas in the cavity in a very short time during the filling process. The current vacuum pumping system has the problems of low pumping efficiency, low vacuum degree of cavity, complicated vacuum valve, high maintenance cost and poor reliability. Therefore, in view of the deficiencies of the prior art, this subject has designed a vacuum pumping system with main and auxiliary exhaust pipes, high reliability and low cost. Through the vacuum pumping system, combined with numerical simulation software, the forming properties of AZ91D magnesium alloy die castings under different vacuum degrees were studied.



1 Design of vacuum pumping system



Due to the short time of the vacuum die-casting forming process, generally shorter than 1 s, the response speed of the vacuum valve is required to be high. Aiming at the shortcomings of the existing vacuum exhaust system, a vacuum exhaust system including main and auxiliary exhaust pipes is designed, and its structure is shown in Figure 1a. The main working principle is that when the injection punch passes through the pouring port 5, the main valve 12 is controlled by PLC to open. At this time, the auxiliary exhaust pipe 11 and the main exhaust pipe 10 simultaneously pump the mold cavity. When changing from slow injection to fast injection, the induction switch is triggered, the normally open solenoid valve 9 is quickly closed, and the auxiliary exhaust pipe 11 continues to pump air into the cavity until the molten metal flows into the auxiliary exhaust pipe to cool and solidify. Obtain vacuum die castings. Through the main and auxiliary exhaust pipes, the cavity can be pumped at the same time, and the air in the cavity can be quickly pumped out to achieve an ideal vacuum degree.

The auxiliary exhaust pipe 11 is designed as a tooth-like structure, and the pipe is bent forward. The main purpose is to extend the flow distance of the molten metal entering the pipe to ensure that the molten metal cools and solidifies when passing through the narrow exhaust pipe. The auxiliary exhaust pipe In the whole die casting process has been in a state of pumping. The main exhaust pipe 10 is provided with a normally open solenoid valve 9, see Fig. 1b. Therefore, the air extraction control of the main exhaust pipe can be realized by controlling the normally open solenoid valve 9 . In the test, by changing the position of the induction switch, the closing time of the normally open solenoid valve 9 can be changed, thereby controlling the exhaust time of the main exhaust passage.

(a) Structure diagram of vacuum pumping system (b) Structure diagram of main exhaust passage

3. Vacuum tank 4. Vacuum pump 5. Feeding port 6. Punch 7. Pressure chamber 8. Cavity 9. Solenoid valve 10. Main exhaust pipe 11. Auxiliary exhaust pipe 12. Main valve

 Figure 1 Schematic diagram of vacuum pumping system

2 Simulation analysis



2.1 Analysis of die casting model

Solidworks 3D software is used to model the radiator in 3D. The radiator model (including gate and overflow groove) and mold structure are shown in Figure 2. According to the design principle of the vacuum pumping system of the main and auxiliary exhaust pipes, the mold structure of the radiator die-casting parts is made. The casting material is AZ91D magnesium alloy, and the thermophysical parameters are shown in Table 1.

Figure 2 The radiator model and mold structure diagram


2.2 Simulation analysis of gas entrainment in die castings

According to the characteristics of AZ91D magnesium alloy, after testing and verification, when the pouring temperature is 680 ℃, the optimal die-casting process parameters are selected, as shown in Table 2, and the air entrainment simulation analysis of the radiator is carried out.

The vacuum degree of vacuum die casting is between 0 and 101 kPa, and the smaller the value, the higher the vacuum degree. The process parameters in Table 2 were input into the FLOW-3D simulation software, and three groups of different cavity vacuum degrees were set at the same time. It can be seen that the entrained air volume of the die-casting part near the gate is lower than that far from the gate, and the central part of the die-casting part is filled and formed last, so the entrained air volume is the largest. The air volume of ordinary die castings with a cavity vacuum of 101 kPa is more than the other two groups of vacuum die castings, and it is mainly concentrated in the core of the die castings. When the cavity vacuum is 50 kPa, the air volume of the die castings decreases. However, the air entrainment area in the core is still relatively large. When the vacuum degree of the cavity is 10 kPa, the overall entrainment volume of the die-casting part is significantly reduced, and the entrainment in the core is also significantly improved. This shows that increasing the vacuum degree of the cavity can effectively improve the filling ability of the molten metal, can improve the porosity or shrinkage defects of the die casting, and is beneficial to the radiator die casting to obtain a die casting with a complete shape and a dense structure.

(a) 101 kPa (b) 50 kPa (c) 10 kPa

Fig. 3 Air entrainment results under different vacuum degrees

3 Test verification

Under the above optimal process parameters, the die-casting test was carried out using the vacuum pumping system of the main and auxiliary exhaust pipes. In the test, by changing the position of the induction switch, the pumping time of the main exhaust pipe is changed, so that the mold cavity can form different vacuum degrees, and the die-casting test is carried out respectively. Die castings with cavity vacuum of 101 kPa (No. 1), 50 kPa (No. 2), and 10 kPa (No. 3) were selected from the test for analysis, as shown in Figure 4. In terms of appearance, the three groups of die castings have relatively complete shapes. Combined with the results of air entrainment simulation analysis, the same heat sinks at the core of the radiator with the largest air entrainment were taken to prepare metallographic samples for comparative study, as shown in Figure 4b.

(a) Heat sink die casting                                              (b) Heat sink

Figure 4 Macro photos of die castings under different vacuum degrees

Figure 5 shows the microstructure of the magnesium alloy radiator under different vacuum degrees. It can be seen that there are more or less pores or shrinkage defects in both vacuum die casting and ordinary die casting. Compared with vacuum die casting, the pores of ordinary die castings have a wider distribution range and larger pore size. There are many oxidation inclusion defects; and when the cavity vacuum is 50 kPa, the size of the pores becomes smaller, the number of pores decreases, and the pore or shrinkage area is reduced by more than 50% compared with ordinary die casting, and the oxide inclusion defects are significantly improved; When the cavity vacuum is 10 kPa, the pore size and number of pores of the die casting are greatly reduced, the pore area is reduced by more than 70% compared with ordinary die castings, and the degree of segregation is significantly reduced. This shows that the vacuum degree of the cavity can improve the porosity or shrinkage defect of the die casting, and the higher the vacuum degree, the less the number of pores and the smaller the size of the die casting. At the same time, the structure of vacuum die casting is relatively dense. Combined with the finished product of the test, it is believed that vacuum die casting can reduce the filling resistance of the molten metal.

(a) Ordinary die castings (b) Die castings with a vacuum degree of 50KPa (c) Die castings with a vacuum degree of 10kPa

Fig.5 Microstructure of die casting



In order to facilitate the comparison of the mechanical properties of the die-casting parts under different vacuum degrees, the heat sinks at the same position were taken for testing, and the obtained mechanical properties are shown in Table 3. It can be seen that as the vacuum degree of the cavity increases, the tensile strength and elongation of the die-casting parts are also improved accordingly.



4 Conclusion



(1) High-vacuum die-casting can be realized by designing the vacuum pumping system of the main and auxiliary exhaust pipes, and cooperating with PLC for control. It has been verified by experiments that the vacuum pumping system can obtain magnesium alloy vacuum die castings with complete appearance, dense structure and better mechanical properties than ordinary die casting.

(2) Through the simulation software, the air volume of the magnesium alloy radiator under different vacuum degrees is simulated, and it is known through experiments that the vacuum degree of the cavity can change the filling ability of the molten metal. The molding ability is also improved accordingly, and the segregation degree of the casting composition is reduced, and the structure is denser. From the analysis of mechanical properties, it can be seen that the tensile strength of the vacuum die casting with a vacuum degree of 10 kPa is 9.2% higher than that of the vacuum degree of 50 kPa, and 17.1% higher than that of the ordinary die casting, and the elongation is higher than that of the vacuum degree. The 50 kPa die casting has an increase of 13.9%, which is 40% higher than that of the ordinary die casting.