At present, how to reduce resource consumption and environmental pollution has become primary issue for human sustainable development. To effectively solve this problem, automobile lightweighting has received widespread attention. One of effective ways to achieve lightweight automobiles is to use new lightweight materials to replace traditional metal materials and process new lightweight materials into automobile parts through advanced technology. This 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. However, a large number of defects will occur in actual die-casting production, resulting in scrapping of castings. Because of its low qualification rate, magnesium alloy manufacturing industry development faces huge challenges.
In order to further promote application of magnesium alloy in automobile lightweight technology, structural design, mold design and die-casting test of magnesium alloy automobile head-up display bracket (HUD) were carried out to explore feasibility of magnesium alloy application in automobile thin-walled structural parts, mainly involving computer simulation and die-casting process parameter optimization. It is proposed to optimize target of minimum air entrainment volume and minimum shrinkage rate, use CAE technology to simulate filling process of automobile HUD, conduct theoretical analysis and optimization of parameters such as pouring temperature, mold preheating temperature and injection speed through Minitab Taguchi test, and obtain an optimized process parameter combination, aiming to provide reference for production of automotive HUD. Two pouring system solutions were designed for head-up display bracket (HUD) based on Magma software, and optimization solution was given through numerical simulation analysis. On this basis, Taguchi experiments were used to study effects of pouring temperature, mold preheating temperature and injection speed on air entrainment volume and shrinkage rate of automobile HUD during die-casting process, and optimized die-casting process parameters were obtained.

Research material is AM60B alloy, and its chemical composition is shown in Table 1. Because of its high strength and good corrosion resistance, it is widely used in production of housings, thin or special-shaped brackets and other parts for electrical products. As a component of automotive head-up display bracket, HUD AM60B alloy fully meets its performance requirements. Magnesium alloy HUD has high requirements on processing accuracy and surface quality due to its thin wall thickness and complex structure. UG12.0 software was used to design three-dimensional mold of HUD parts, and two gating system solutions were designed, as shown in Figure 1.

 

Figure 1 Three-dimensional mold with gating system

Table 1 Chemical composition of AM60B magnesium alloy (%)

 

Figure 2 Temperature distribution diagram of two schemes

 

Figure 3 Simulation results of mold filling speed for two schemes
It can be seen that temperature distribution in Scheme 1 is very uneven. There is a large area near middle of casting where temperature is lower, not exceeding 630℃, while temperature in other areas reaches above 645℃, forming a large temperature difference. As a result, speed of this area during solidification process is inconsistent, and there is an obvious solidification time difference, resulting in that post-solidified area cannot be fed by molten metal. Serious shrinkage defects are prone to occur. Temperature distribution of Scheme 2 is relatively uniform, and temperature difference at main body of casting does not exceed 3℃. In terms of filling temperature, scheme 2 is better than scheme 1. When filling reaches 40%, filling speed at position A in Scheme 1 is too fast, reaching more than 50m/s, so that faster molten metal will fill casting first, resulting in uneven filling of casting. When filling reaches 73%, because molten metal in area A is filled too fast, an unfilled blank area will be formed when mixed with slower molten metal, as shown in area B in Figure 3c. Because this area is surrounded by two streams of molten metal and then slowly filled, this area is prone to suffocation. When filled to 90%, a large area where it is easy to hold one's breath 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 factors-level table

Table 3 Taguchi orthogonal table and result statistics
For the two response targets of air entrainment rate and shrinkage porosity, they are both consistent with small characteristics in Taguchi test quality characteristics. Therefore, calculation formula of signal-to-noise ratio S/N is:

 

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

Table 4 Signal-to-noise ratio calculation results
When only considering air entrainment rate, it can be seen from Table 5 that C>B>A, that is, influence of die-casting process parameters on air entrainment rate from large to small is: injection speed, mold preheating temperature, pouring temperature. It can be seen that when only considering air entrainment rate, die-casting process parameter combination that satisfies 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 Range analysis table

Table 6 Variance analysis table
When only considering shrinkage porosity, it can be seen that A>B>C, that is, influence of die-casting process parameters on shrinkage porosity from large to small is: pouring temperature, mold preheating temperature, injection speed. When only shrinkage porosity is considered, die-casting process parameter combination that satisfies 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 entrainment rate and shrinkage rate before and after optimization

 

Figure 5 HUD die casting

During die-casting process of AM60B magnesium alloy automobile HUD bracket, when only air entrainment rate is considered, injection speed has the greatest impact, followed by mold preheating temperature, and pouring temperature has the least impact. When only shrinkage porosity is considered, pouring temperature has the greatest impact, followed by mold preheating temperature, and injection speed has the least impact. When air entrainment rate and shrinkage rate are comprehensively considered, optimal process parameter combination is: pouring temperature is 660℃, mold preheating temperature is 200℃, and injection speed is 6.5m/s.