Taking AlSi10MnMg die-cast aluminum alloy battery cover of new energy vehicles as research object, through numerical simulation analysis, we predict air entrainment defects during mold filling process and optimize design of die-casting mold overflow system. Through heat treatment test, artificial aging process parameters of product were determined. Tests have proven that numerical simulation can effectively control air entrainment. After artificial aging at 190℃ * 2 hours, battery cover meets mechanical property requirements of tensile strength 300MPa, yield strength 210MPa, and elongation 5%.
In recent years, due to demand for lightweight automobiles, aluminum alloy die-casting parts such as suspension beams, load-bearing beams, shock towers and wheel hubs have been increasingly used. AlSi10MnMg is a new type of high-strength and tough aluminum alloy die-casting material, which has characteristics of high tensile strength and elongation. During die-casting filling process, AlSi10MnMg alloy melt fills mold cavity at an extremely fast speed under pressure, which easily causes gas entrainment defects. Therefore, it is very important to analyze gas entrainment conditions during mold filling process. This study uses Anycasting software to conduct air entrainment numerical simulation analysis, and further improves mechanical properties of AlSi10MnMg thin-walled die castings through heat treatment process.

Figure 1 shows appearance of a new energy vehicle battery cover. Material is AlSi10MnMg. Product has an average wall thickness of 3 mm, an outline size of 152 mm * 142 mm * 22 mm, a casting projection area of 142 cm2, and a weight of 0.22kg. This product is assembled on upper cover of battery, with multiple sensors and bolts mounted on outer surface. Product function needs to protect internal battery pack and prevent liquid leakage. At the same time, it needs to withstand certain external impacts and resist long-term fatigue vibration. Internal control requires that tensile strength is greater than 300 MPa, yield strength is greater than 210 MPa, elongation is greater than 5%, processed surface cannot have shrinkage holes greater than 1 mm, and sealing test condition is that leakage rate is less than 8 mL/min under 10 kPa. As-cast tensile strength of this alloy is 240 MPa, yield strength is 140 MPa, and elongation is 5%, which cannot meet demand. Therefore, it needs to be improved from aspects of raw material composition, mold design and artificial aging.

 

Figure 1 New energy battery cover

AlSi10MnMg is based on Silafont-36 and complies with DIN EN 1706 standard. Chemical composition is shown in Table 1. Si content is slightly lower than AlSi eutectic alloy and has better fluidity. Low Fe content eliminates needle plate block of Al-Fe-Si phase, and die casting is less likely to crack under stress. A certain Mn content can weaken harm of Fe, prevent alloy from sticking to mold during die casting, and present a spherical phase in structure.

Table 1 Chemical composition of AlSi10MnMg wb/%
As can be seen from Table 1, in AlSi10MnMg aluminum alloy specified in standard, Mg content is 0.1%~0.6%, which is a relatively wide range. Wu Shusen et al. found that increase in Mg content is positively correlated with tensile strength and yield limit, and negatively correlated with elongation. Mg that is too low cannot produce sufficient strength and is not conducive to subsequent heat treatment, while Mg that is too high can lead to a reduction in elongation. Mg and Si form Mg2Si strengthening phase, which causes α-Al solid solution crystal lattice to be distorted, thereby causing to strengthen alloy. Mg content of 0.20%~0.40% can achieve better comprehensive mechanical properties.
Fe is a harmful element in die-cast aluminum alloys. It exists in flake or needle-like structure of FeAl3 and Al-Si-Fe, which reduces mechanical properties of alloy. Fe content should be less than 0.15%. Mn element can form a compound with Fe to further eliminate harmful factors of iron. At the same time, Mn can increase spherical crystal structure content of product and keep Mn content at 0.50%~0.80%.
Appropriately increasing Ti can significantly refine grain structure of aluminum alloys, improve mechanical properties of alloy, and reduce tendency of alloy hot cracking. Therefore, Ti content is controlled between 0.06% and 0.10%. Therefore, adjusted composition of die-cast aluminum alloy is shown in Table 2.

Table 2 Test material composition AlSi10MnMg wb/%

Figure 2 shows basic structure of battery cover pouring system. The total projected area is 290 c㎡, and the total pouring weight is 0.78 kg. UB350ic die-casting machine is selected, and clamping force of this equipment is 3500 kN. Design mold is 1 piece per mold, diameter of pressure chamber is ø60mm, and filling degree of material cylinder is 35%. Mold has five branch sprues, of which left branch sprue manufactures test piece 2 accompanying casting, and controls whether test piece needs to be manufactured through blocker 8. According to requirements of this product, components such as sensors are installed in "glasses hole" area on the lower left side of casting, "rectangular box" area on the lower right side, and "round hole" area at the end of casting. These three locations are through holes, all processed surface pores after processing in this area should be less than 0.6 mm. The four branch sprues of casting are in a comb-like structure. Inner gate length is 75 mm, gate thickness is 2.0 mm, which is 2/3 of solidification modulus, and the total gate cross-sectional area is 150 mm2, and calculated average flow velocity is about 40 m/s. Die-casting mold adopts FS438 die-casting hot work die steel, which has good hardenability, toughness, thermal strength, thermal fatigue performance, and small heat treatment deformation. Cooling medium is water, with programmable intermittent high-pressure point cooling of 10 kg/cm2 used locally, and natural exhaust. Position of slag bag and design of exhaust wave plate are determined through simulation analysis.

 

Figure 2 Battery cover pouring system
1. Battery cover 2. Test piece 3. Material handle 4. Sprue 5. Cross runner 6. Branch runner 7. Test piece slag cavity 8. Blocker
AnyCASTIN 6.0 software is used for simulation analysis, which can mainly carry out simulation analysis of mold filling, heat conduction, solidification process and stress field of casting. Simulation analysis objects are castings and die-casting molds, divided into 15.6 million grids. According to high-speed filling characteristics of thin-walled parts, initial boundary conditions are set as melt temperature 680 ℃, mold temperature 185 ℃, and punch filling speed 3.5 m/s, heat transfer coefficient between mold and casting is 2 000 W/(㎡.K), and heat transfer coefficient between molds is 1 000 W/(㎡.K).
Simulation analysis results are shown in Figure 3. Alloy liquid reaches inner gate at a low speed driven by injection punch, and aluminum liquid in the middle two gates is filled first. As can be seen from Figure 3a, high-speed injection starts at 0.015 s, average gate speed is about 42 m/s, aluminum liquid impacts mold and core, and fills cavity in a turbulent flow state. As can be seen from Figure 3b, "glasses hole" area is reached at 0.0175 s. This part of mold is for grinding and closing mold core. Aluminum liquid will impact core to generate vortex air entrainment. In order to ensure quality of this important part, design of eyeglass hole through-hole was changed to a 2 mm thick blind hole, that is, grinding of mold in this part is optimized to a 2 mm gap, air entrainment condition is significantly improved, and additional 2 mm of material is removed in subsequent processing. It can be seen from Figure 3c that the two streams of aluminum liquid reached ends of round hole and rectangular hole respectively at about 0.021 s of filling. Through simulation, it was found that obvious air entrainment occurred at the end of round hole and end of rectangular hole on opposite side of runner. Therefore, slag bag was designed to be placed in round hole and rectangular hole respectively. Inlet of slag bag was intersection of simulated aluminum liquid; it is filled to the end of casting at 0.024 s and flows to slag bag around casting, see Figure 3d. After multiple simulation analyses, eight slag bags were set up at filling ends of castings. X-rays after castings were produced showed that surrounding slag bags contained many shrinkage cavities and oxidized slag inclusions.

 

 

Figure 3 Battery cover filling and air entrainment simulation
Figure 4 shows velocity cloud diagram when filling 0.277 s. "π" type exhaust wave plate is to isolate a traditional exhaust wave plate in the middle while keeping ends connected. Generally, isolation length of left and right sides is 4 to 5 tooth buckles. This structure can not only ensure that gas discharge on the left and right sides does not interfere with each other, but also reduce space and save processing and manufacturing costs. As can be seen from Figure 4, at this moment, aluminum liquid on the left side reaches the first tooth button, and right side is about to reach the first tooth button. Using particle tracking technology, front end speed on the left side is measured to be 12 m/s, and on the right side is 13m/s. Since average gap between wave plates is 0.7 mm, speed of the two molten aluminum drops rapidly after passing through the first tooth button, and almost stops moving by the second tooth button. Final stop position of molten aluminum is related to viscosity of molten aluminum, mold temperature, casting pressure, etc.  Regarding design, there are four independent tooth buckles on the left and right sides of design, and fifth to eighth tooth buckles are connected. Practice has proved that design of "π" type exhaust wave plate is reasonable and production process is reliable.

 

Figure 4 Filling the 0.277 second velocity vector
According to simulation analysis results, UB350iC die-casting machine was used to produce this part. Injection punch size is ø60 mm, measured gap between punch and material cylinder is 0.07 mm. In order to adapt to filling of thin-walled parts, alloy liquid temperature of holding furnace is set to 680 ℃, actual mold temperature is controlled between 180 and 200 ℃, casting pressure is 80 MPa, the first-level injection speed is 0.2 m/s, and the second-level speed is 3.5 m/s. Casting is shown in Figure 5. Use XG-160S T/S X-ray real-time imaging machine to check internal quality of part. It can be seen that the overall outline of casting is clear, as shown in Figure 6. There are no visible pores, oxidized slag inclusions and other quality defects in important functional areas around eyeglass hole, square hole and round hole. Quality of pouring end is good, circumferential sealing annular groove meets design requirements. At the same time, accompanying test piece has a clear outline, no visible pores and other defects, as shown in Figure 7. A WDW-50 electronic universal testing machine was used to measure mechanical properties of accompanying specimens. Average tensile strength was 260 MPa, yield strength was 170 MPa, and elongation was 6%. Performance could not meet design requirements.

 

Take slices from three important areas of product, polish them and etch them in 10% caustic soda solution. Use an AX10 Zeiss metallographic microscope for structural observation, as shown in Figure 8. Most of bright α-aluminum is in the form of blocks or dendrites, with relatively coarse grain sizes, some of which are larger than 80 um. Small dark black Al-Si alloy is distributed relatively uniformly around α-aluminum, but small shrinkage pores are still visible, with a porosity of 3.5%. Slag bag in the round hole was taken for metallographic examination, as shown in Figure 9. It was found that there were multiple circular black pores and cavities, and there were densely distributed small holes around them. After image recognition and measurement, porosity in slag bag was greater than 9%, indicating that slag bag here realizes functions of gas collection and slag discharge. Gas is drawn into slag bag at high speed at front end of flow of aluminum liquid, and is compressed into holes under high pressure. This phenomenon is basically consistent with simulation analysis results, indicating that design of mold slag bag is more reasonable.

 

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 not only depends on composition and aging process of alloy, but also depends on defects of alloy during production process, especially number and distribution of vacancies and dislocations. It is generally believed that age hardening is result of segregation of solute atoms to form a hardened zone. AlSi10MnMg thin-walled die-casting parts can achieve better performance requirements under appropriate process parameters of T5 and T6. Since solution heat treatment process is more complex and higher temperatures can easily lead to out-of-deformation of thin-walled die-casting parts, thin-walled parts can be uniformly dissolved in 2 hours, artificial aging treatment is performed with temperature as variable.
Artificial aging temperature range is selected to be 170~210 ℃, with each interval of 10 ℃ as a group, and holding time is 2 h. An Sx2-12-6 resistance furnace was used for artificial aging, with a rated power of 12 kW and a furnace size of 550 mm * 550 mm * 450 mm; each group of 3 test pieces was suspended and placed in the middle of furnace, ventilated, and heated for 15 minutes to reach specified aging temperature, then automatically keep temperature for a timer. Finally, test pieces are manually taken out and cooled in the air.
Use WDW-50E universal tensile testing machine to conduct tensile testing at room temperature. Tensile testing process is based on national standard GB/T228.1-2010, as shown in Figure 10. Elongation rate is calculated by measuring values before and after stretching with a caliper. There are 3 pieces in each group. Test results are shown in Table 3.

 

Figure 10 Tensile test pieces at different aging temperatures

Table 3 Artificial aging parameters and mechanical properties table
It can be seen from Table 3 that after artificial aging heat treatment, yield limit and tensile strength have been improved to varying degrees compared to original cast state. At 190 ℃ *2h, they are 306 MPa and 222 MPa respectively, and then strength decreases slightly as temperature increases; At the same time, after T5 heat treatment, elongation first decreased and then increased. Elongation at 190 ℃ * 2h was 5.4%, but it was smaller than that of die-cast blank.
Heat treatment aging temperature was further selected to be 190 ℃ and heat preservation was held for 8 hours to de-age battery cover casting. After tensile test, tensile strength was 307 MPa and yield strength was 227 MPa, but elongation dropped to 2.6%, indicating that excessive heat treatment time has little effect on strength, but has a great impact on elongation.

Based on AlSi10MnMg die-cast alloy material, in view of mechanical performance requirements of new energy vehicle battery covers, a general method to solve this problem is proposed, namely, rational selection of alloy composition content, optimized design of die-casting molds, and correct configuration of artificial aging schemes. Results show that Mg content in alloy composition is 0.2%~0.4%, overflow system is optimized according to structural characteristics of part, artificial aging is selected at 190 ℃ and 2 h, and its performance can meet requirements.