At present, how to reduce resource consumption and environmental pollution has become primary issue of human sustainable development. In order to effectively solve this problem, automobile lightweighting has received widespread attention. One of effective ways to achieve automobile lightweighting is to use new lightweight materials to replace traditional metal materials, and process new lightweight materials into automotive parts through advanced process means, which puts higher requirements on traditional automobile manufacturing industry. As the lightest metal structural material, magnesium alloy is widely used in die-casting production of automotive parts, but a large number of defects will be generated in actual die-casting production, resulting in scrapping of castings. Due to its low pass rate, development of magnesium alloy manufacturing industry faces huge challenges.
In order to further promote application of magnesium alloy in automotive lightweight technology, structural design, mold design and die-casting test of magnesium alloy automotive head-up display bracket (HUD) were carried out to explore feasibility of magnesium alloy application in automotive thin-walled structural parts, mainly involving computer simulation and die-casting process parameter optimization. Minimum entrained air volume and minimum shrinkage rate are proposed as optimization targets. CAE technology is used to simulate filling process of automobile HUD. Pouring temperature, mold preheating temperature and injection speed are theoretically analyzed and optimized through Minitab Taguchi test. Optimized process parameter combination is obtained, aiming to provide a reference for production of automobile HUD. Two pouring system schemes are designed for head-up display bracket (HUD) based on Magma software. Optimization scheme is given through numerical simulation analysis. On this basis, Taguchi test is used to study influence of pouring temperature, mold preheating temperature and injection speed on entrained air volume and shrinkage rate of automobile HUD during die casting, and optimized die casting process parameters are obtained.
Graphical results
Research material is AM60B alloy, and its chemical composition is shown in Table 1. Due to its high strength and good corrosion resistance, it is widely used in production of housings, thin or special-shaped brackets and other parts of electrical products. As a part of automobile head-up display bracket, AM60B alloy fully meets its performance requirements. Magnesium alloy HUD has high requirements for its processing accuracy and surface quality due to its thin wall thickness and complex structure. 3D model of HUD parts was designed using UG12.0 software, and two gating system schemes were designed, as shown in Figure 1.

 

Figure 1 3D model with gating system

 

Table 1 Chemical composition of AM60B magnesium alloy (%)

 

Figure 2 Temperature distribution of two schemes

 

Figure 3 Simulation results of filling speed of two schemes
It can be seen that temperature distribution of Scheme 1 is very uneven. There is a large area of low temperature near middle of casting, not exceeding 630℃, while temperature of other areas is above 645℃, forming a large temperature difference, which makes speed of this area inconsistent during solidification process, and there is a significant difference in solidification time, resulting in lack of metal liquid shrinkage in later solidified area. Serious shrinkage defects are prone to occur. Temperature distribution of Scheme 2 is more uniform, and temperature difference of main position of casting does not exceed 3℃. In terms of filling temperature, Scheme 2 is better than Scheme 1. When filled to 40%, filling speed of position A in Scheme 1 is too fast, reaching more than 50m/s, so that faster metal liquid will fill casting first, resulting in uneven filling of casting. When filling reaches 73%, because molten metal in area A is filled too quickly, an unfilled blank area will be formed when it is mixed with slower molten metal, as shown in area B in Figure 3c. This area is surrounded by two streams of molten metal and then slowly filled, which makes it easy for area to be suffocated. When filling reaches 90%, a large area that is easy to be suffocated appears, as shown in area C in Figure 3e. Compared with scheme 1, scheme 2 has a better filling speed simulation effect.

Table 2 Taguchi test factor-level table

Table 3 Taguchi orthogonal table and result statistics
For two response targets of air volume rate and shrinkage rate, they all meet expected small characteristics in quality characteristics of Taguchi test, so calculation formula of signal-to-noise ratio S/N is:

 

In the formula, n represents the number of tests; i represents i-th test.

 

Table 4 Signal-to-noise ratio calculation results
When only air entrainment rate is considered, it can be seen from Table 5 that C>B>A, that is, degree of influence of die-casting process parameters on air entrainment rate is from large to small: injection speed, mold preheating temperature, and pouring temperature. It can be obtained that when only air entrainment rate is considered, die-casting process parameter combination that meets maximum signal-to-noise ratio S/N1 is A2B1C3, that is, pouring temperature is 680℃, mold preheating temperature is 160℃, and injection speed is 6.5m/s.

 

Table 5 Extreme difference analysis table

Table 6 Variance analysis table
When only shrinkage rate is considered, it can be seen that A>B>C, that is, degree of influence of die-casting process parameters on shrinkage rate is from large to small: pouring temperature, mold preheating temperature, and injection speed. When only shrinkage rate is considered, die-casting process parameter combination that meets maximum signal-to-noise S/N2 is A1B2C1, that is, pouring temperature is 660℃, mold preheating temperature is 180℃, and injection speed is 4.5m/s.

Table 7 Variance analysis table

 

Figure 4 Comparison of air curling rate and shrinkage rate before and after optimization

 

Figure 5 HUD die casting
Conclusion
In die-casting process of AM60B magnesium alloy automotive HUD bracket, when only air curling rate is considered, injection speed has the greatest impact on it, followed by mold preheating temperature, and pouring temperature has the least impact. When only shrinkage rate is considered, pouring temperature has the greatest impact on it, followed by mold preheating temperature, and injection speed has the least impact. When air curling rate and shrinkage rate are considered comprehensively, optimal process parameter combination is: pouring temperature is 660℃, mold preheating temperature is 200℃, and injection speed is 6.5m/s.