In recent years, due to demand for lightweight automobiles, aluminum alloy die-casting parts such as suspension beams, load-bearing beams, shock towers and hubs have been used more and more. As a new type of high-strength and toughness die-casting aluminum alloy, AlSi10MnMg alloy has characteristics of high tensile strength and elongation. During die-casting filling process, alloy melt fills mold cavity at an extremely fast speed under pressure, which is prone to entrainment defects, so it is very important to analyze entrainment status during filling process. This study takes AlSi10MnMg die-casting aluminum alloy battery cover of new energy vehicles as research object, uses AnyCasting software to carry out numerical simulation analysis of entrained air, predicts entrained air defects in filling process, optimizes design of overflow system of die-casting mold, further improves mechanical properties of AlSi10MnMg thin-walled die castings through heat treatment process, determines artificial aging process parameters of this product.

Figure 1 is a structural diagram of a new energy vehicle battery cover. Its material is AlSi10MnMg alloy. Average wall thickness of this product is 3mm, outline size is 152mm*142mm*22mm, projected area of casting is 142cm2, and weight is 0.22kg. Product is assembled on battery cover, and outer surface is equipped with multiple sensors and bolts. In terms of product function, it needs to protect internal battery pack to prevent liquid leakage, at the same time, it needs to withstand certain external shocks and resist long-term fatigue vibration. Internal control requires that tensile strength is greater than 300MPa, yield strength is greater than 210MPa, and elongation is greater than 5%. There should be no air shrinkage holes with a diameter greater than 1mm on processed surface. Leak rate is less than 8mL/min under condition of 10kPa. Tensile strength of as-cast alloy is 240MPa, yield strength is 140MPa, and elongation is 5%, which cannot meet demand. Therefore, it is improved from aspects of raw material composition, mold design and artificial aging.

 

Figure 1 New energy battery cover

Table 1 Chemical composition of AlSi10MnMg (%)

Table 2 Chemical composition of adjusted AlSi10MnMg alloy (%)
Figure 2 shows basic structure of gating system for battery cover. The total projected area is 290cm2, and the total casting mass is 0.78kg. UB350ic die-casting machine is selected, clamping force of this equipment is 3500kN. Design 1 mold and 1 piece, diameter of pressure chamber is ϕ60mm, and filling degree of material cylinder is 35%. Mold has 5 branch runners, of which left branch runner manufactures sample piece 3 accompanying casting, and controls whether sample piece needs to be manufactured through blocker 1 .

 

Figure 2 Battery cover gating system

1. Blocker 2. Branch sprue 3. Test piece 4. Slag collection bag 5. Battery cover 6. Runner 7. Sprue 8. Handle

 

Fig. 3 Air entrainment simulation of battery cover filling process

 

Figure 4 Velocity vector when filling 0.277s

 

Figure 5 casting

 

Figure 6 X-ray inspection diagram

 

Fig.7 Metallographic structure of different regions
Purpose of heat treatment of aluminum alloy castings is to improve mechanical properties of alloy, enhance corrosion resistance, improve processing performance, and obtain dimensional stability. Age hardening of aluminum alloys depends not only on composition of alloy and aging process, but also on defects of alloy during production process, especially number and distribution of vacancies and dislocations. AlSi10MnMg thin-walled die-casting parts can get better performance requirements under appropriate T5 and T6 treatments. Since solution heat treatment process is more complicated, the higher temperature is likely to cause deformation of thin-walled die-casting parts to be out of tolerance, and thin-walled parts can be uniformly dissolved within 2 hours, artificial aging treatment is adopted with temperature as a variable.

 

Fig.8 Results of tensile test pieces at different aging temperatures

Table 3 Mechanical properties of alloys in as-cast state and at different aging temperatures
Based on AlSi10MnMg die-casting alloy, according to mechanical performance requirements of new energy vehicle battery cover, a general method to solve this problem is proposed, that is, reasonable selection of alloy components, optimal design of die-casting molds, and correct configuration of artificial aging schemes. Results show that content of Mg in alloy is 0.2% to 0.4%, overflow system is optimized according to structural characteristics of parts, artificial aging is selected at 190℃×2h, and its performance can meet requirements.