In order to solve problems such as pores and shrinkage holes during die-casting process of high-pressure die-cast aluminum alloy differentials, finite element software ProCAST was used to numerically simulate high-pressure die-casting process of differential housing, die-casting defects were predicted by solving flow field, temperature field, and velocity field, and die-casting process was optimized through orthogonal experiments. Results show that there are air entrainment problems in bottom cylinder and bracket areas on both sides during filling process; during solidification process, temperature gradient of molten metal is large, casting will solidify unevenly, runner will solidify prematurely, and shrinkage defects will form when it is completely solidified. Based on influence of die-casting parameters on solidification time, air entrainment volume and shrinkage cavity volume, optimal die-casting parameters are pouring temperature of 650℃, mold temperature of 220℃, and injection speed of 5 m/s. Die-casting test verified reliability of die-casting parameters.
Differential housing is a large and complex housing with large size, uneven wall thickness, and complex overall structure. It is mainly used for power transmission in four-wheel drive SUVs. During vehicle operation, output power is high and starting torque is large. Differential housing needs to adapt to harsh service environments such as heavy loads, high impact, and high stress. However, traditional differential housings are generally made of ductile iron or gravity casting, which has problems such as large overall mass, low dimensional accuracy, long production cycle, and large pouring risers.
At present, differential housing has undergone a lightweight structural design of "ribs instead of solids", which can effectively reduce weight of differential housing while ensuring structural strength. Relevant parts are made of high-strength aluminum alloy instead of traditional ductile iron. Dual weight reduction of lightweight materials and structures enables lightweight design of differential housing to be met. However, lightweight design has brought about problem of difficult coordinated control of shape of differential housing. Therefore, seeking advanced processes for high-efficiency and high-quality production of differential housings has become focus of current research.
This article uses lightweight material A380 aluminum alloy and advanced manufacturing technology to form high-pressure die-casting, and proposes a high-pressure die-casting forming process for aluminum alloy differential housing. However, structure of differential housing is complex. During die-casting process, eddy currents will entrap air and molten metal will be difficult to uniformly feed, resulting in defects such as pores and shrinkage holes inside casting. Therefore, this paper establishes a finite element model of high-pressure die-casting differential housing, conducts numerical simulation of differential housing die-casting process, uses solution of flow field, temperature field and velocity field to predict casting defects. Through orthogonal experiments and data statistical analysis, effects of pouring temperature, mold temperature, and injection speed on filling quality of castings were studied, reliability of optimal process parameters was verified through die-casting experiments, providing certain guidance for lightweight production of differential housings.

Import designed three-dimensional model of differential housing (Figure 1a) into ProCAST simulation software as an igs format file, and mesh differential in mesh module of ProCAST. Taking into account different wall thicknesses of castings, pouring systems, and overflow systems, different grid sizes are used for castings, pouring systems, overflow systems, and molds. Set casting mesh size to 1.5 mm, pouring system and overflow system mesh size to 3 mm, and mold mesh size to 10 mm. After meshing, number of meshes is approximately 17.5 million, and finite element model is shown in Figure 1b.

 

Figure 1 High-pressure die-casting finite element model
Differential housing material is A380 aluminum alloy, and its chemical composition is shown in Table 1. Thermal physical properties parameters of A380 aluminum alloy are provided by software, where solidus temperature is 508 ℃ and liquidus temperature is 589 ℃. Mold material is H13 alloy steel. During die-casting numerical simulation process, die-casting mold only plays a role in heat exchange and does not participate in flow field calculation. Die-casting process involves a variety of heat transfer phenomena, mainly including heat conduction between castings and molds, molds and molds, heat convection between air and outer surface of the mold. Heat transfer coefficient between mold and mold is set to 1 000 W/(㎡·K), heat transfer coefficient between mold and air is 10 W/(㎡·K), transfer coefficient between casting and mold adopts heat transfer coefficient in the form of a curve (Figure 2). Initial process parameters of high-pressure die casting: pouring temperature 660 ℃, mold temperature 220 ℃, injection speed 4 m/s.

Table 1 Chemical composition of A380 aluminum alloy wB/%

 

Figure 2 Heat transfer coefficient of A380-H13

Casting filling process is shown in Figure 3. It can be seen from figure that when mold filling rate is 30%, molten metal in the middle runner begins to fill cavity through inner runner, while runner on both sides is far away from punch, and molten metal has not yet reached inner runner. When filling rate is 50%, because molten metal in the middle runner fills mold quickly, after reaching far runner, molten metal in right runner will flow back into cylindrical area at the right end of mold cavity, so there may be more defects in cylindrical area at the right end. When mold filling rate is 70%, molten metal in the middle runner completes filling middle part of cavity, and molten metal in runner on both sides fills brackets on both sides. When filling rate is 80% to 90%, filling of upper bracket on the right side is relatively chaotic. This is the last place where molten metal reaches in cavity, requiring a longer filling distance. At this time, molten metal loses more heat, resulting in reduced fluidity. At the same time, due to fact that local areas in wall thickness of casting are not filled in time, molten metal will backflow after reaching end of mold cavity, and air entrainment will occur at wall thickness. Backflow and air entrainment will lead to formation of pore defects in late solidification stage of casting.

 

Figure 3 Metal liquid filling process
Molten metal filling time and temperature distribution are shown in Figure 4. According to time distribution of filling process in Figure 4a, filling process of differential housing can be divided into four parts: blue, green, yellow and red from first to last. Blue area mainly includes sprue and casting near sprue; green area mainly includes bottom of brackets on both sides; yellow area mainly includes platform of casting and upper right bracket far away from inner sprue; red area is mostly located in overflow groove of casting, and a small part is located in the area where molten metal backflows at wall thickness after reaching the end of cavity. Overall, filling order of differential housing mainly follows rule from near to far, but upper bracket on the right side of casting is filled late, resulting in poor filling quality there. Figure 4b shows temperature distribution of casting filling process. During mold filling process, molten metal in mold cavity always maintains a high temperature. Bottom cylinder of casting and brackets on both sides are areas with the lowest temperature of mold cavity, with a temperature of approximately 625℃. At the end of mold filling, temperature of molten metal in mold cavity is above A380 liquidus line (598℃), and no premature solidification occurs. During the entire mold filling process, temperature of molten metal in cavity is relatively high, and temperature of molten metal at the far runner is slightly lower than that at the near runner. However, temperature gradient is small, the overall distribution is reasonable, and fluidity of molten metal is good.

 

Figure 4 Filling process time and temperature distribution

Figure 5 shows temperature distribution and solid phase fraction changes during solidification process of casting. It can be seen from Figure 5a that when solidification state reaches 60%, temperature of casting with thin walls drops rapidly, and solid-liquid phase coexists at this time, while temperature of casting with thick walls drops slowly and is still in liquid phase. When solidification state reaches 80%, temperature of molten metal in most areas of cavity has reached below solidus line, but temperature level of molten metal in wall thickness area is relatively high and maintains a large temperature gradient (90℃) with surrounding area, which is prone to thermal stress concentration, leading to formation of hot cracks and shrinkage cavity defects in wall thickness of casting.

 

Figure 5 Temperature distribution and solid phase fraction during solidification process of castings
Change of solid phase fraction during solidification process of casting is shown in Figure 5b. Distribution of solid phase fraction of casting follows rule that solid phase fraction is high in thin-walled area and low in thick-walled area. Solid phase fraction of upper bracket on the right side of casting and wall thickness is lower than that of surrounding area. When solid phase fraction in surrounding area reaches 0.8, solid phase fraction in wall thickness area is only 0.5, which is even lower than that of ingate. According to metal solidification theory, when solid phase fraction reaches a certain level, dendrites grow to form a closed skeleton, cutting off pressure transmission and liquid phase feeding channels. This uneven solidification phenomenon leads to isolated liquid phase areas during solidification process, eventually shrinkage porosity and shrinkage cavities are formed in wall thickness of casting.
Solidification time of casting is shown in Figure 6. It can be seen from figure that solidification sequence of castings follows law from far to near. Thin-walled areas of casting and far sprue solidify first, overflow tank and brackets on both sides of casting solidify second, and thick-walled area of casting solidifies last. This is because thin-walled area will solidify first due to its lower temperature and sufficient heat exchange with mold, while thick-walled area will solidify last due to its large wall thickness and slow heat transfer.

 

Figure 6 Casting solidification time

Distribution of entrained gas inside casting is shown in Figure 7a. It can be seen from figure that there is a lot of gas in overflow groove of casting, which effectively plays the role of removing gas, proving rationality of overflow system design. Average air entrainment volume in casting is about 0.000 3 g/cm³, but air entrainment volume in upper end of right bracket and left bracket area of casting is about 0.000 6 g/cm³. This is due to entrainment and backflow of molten metal in this area, resulting in poor mold filling in this area.

 

Figure 7 Defect prediction results
Distribution of shrinkage cavities inside casting is shown in Figure 7b. It can be seen from figure that distribution of defects is consistent with analysis of solidification process of casting. Most of shrinkage holes in casting are concentrated at the thickest wall thickness, a small number are located at upper bracket and bottom cylinder on the right side. Use VE software to measure shrinkage cavity volume of casting. Defect volume is about 3.146 cm³. Since upper bracket on the right side of casting and bottom cylinder are parts with higher quality requirements, shrinkage cavity defects are not allowed inside. Therefore, shrinkage defects need to be eliminated by optimizing die-casting process.

This study selected pouring temperature, mold temperature, and injection speed as main influencing factors in orthogonal test, and designed a factor-level table for orthogonal test, as shown in Table 2.

Table 2 Orthogonal test factor level table
Orthogonal experiment was designed with three factors and three levels. According to rules for using orthogonal tables, L9 (3³) orthogonal table was selected, and a total of 9 sets of orthogonal experiments were conducted. Numerical simulation analysis was performed on each group of test plans in turn, and three quality indicators of solidification time, shrinkage volume, and air entrainment volume in simulation results were used as test results. Test results of each group are shown in Table 3.

Table 3 Orthogonal test plan

In order to determine order and change pattern of impact of three factors on solidification time, range analysis was performed on simulation results. Table 4 is solidification time range analysis table, and Figure 8 shows main effect diagram of die-casting process parameters on average solidification time. According to Table 4, it can be seen that influence of die-casting process parameters on solidification time is: pouring temperature (A) > mold temperature (B) > injection speed (C). As for solidification time, pouring temperature and mold temperature have a significant impact on it, while injection speed has a limited effect. This is because temperature parameter is too high, resulting in a longer time for molten metal to reach solidus line, while injection speed has little effect on temperature change of molten metal. Therefore, within a reasonable range of process parameters, temperature parameters can be selected at a lower level to improve die-casting production efficiency. It can be seen from Figure 8 that when pouring temperature is A1, mold temperature is B1, and injection speed is C3, solidification time of casting is minimized. Therefore, when solidification time is used as single evaluation criterion for casting quality, optimal die-casting process parameters for castings are A1B1C3.

Table 4 Solidification time range analysis table

 

Figure 8 Main effect diagram of die-casting process parameters on mean solidification time
In order to determine primary and secondary order and changing pattern of influence of three factors on internal air entrainment volume of casting, a range analysis was performed on simulation results. Table 5 shows range analysis table of air entrainment volume, and Figure 9 shows main effect diagram of die-casting process parameters on mean value of air entrainment volume. According to Table 5, it can be seen that influence of die-casting process parameters on air entrainment volume is: pouring temperature (A) > injection speed (C) > mold temperature (B). As for air entrainment volume, pouring temperature and injection speed have a significant impact on it, while mold temperature has a limited effect. This is related to fact that reasonable pouring temperature and injection speed can provide good fluidity of molten metal. Within a certain range, the higher pouring temperature of molten metal, the faster injection speed, and the better its fluidity. It can be seen from Figure 9 that when pouring temperature is A3, mold temperature is B3, and injection speed is C3, air entrainment volume of casting is minimized. Therefore, when air entrainment volume is used as single evaluation criterion for casting quality, optimal die-casting process parameters for castings are A3B3C3.

Table 5 Volume air volume range analysis table

 

Figure 9 Main effect diagram of die-casting process parameters on mean air entrainment volume
In order to determine order and change pattern of impact of three factors on internal shrinkage cavity volume of casting, a range analysis was performed on simulation results. Table 6 shows positive difference analysis table of shrinkage cavity volume. Figure 10 shows main effect diagram of die-casting process parameters on average shrinkage cavity volume. According to Table 6, it can be seen that influence of die-casting process parameters on shrinkage cavity volume is: mold temperature (B) > pouring temperature (A) > injection speed (C). Pouring temperature and mold temperature have a very significant impact on shrinkage rate, while injection speed has a limited impact on shrinkage rate. This is because shrinkage defects inside casting are caused by untimely feeding of molten metal during solidification process, good temperature parameters can improve solidification process of casting, while injection speed has a small effect on improving solidification process of casting. It can be seen from Figure 10 that when pouring temperature is A2, mold temperature is B3, and injection speed is C2, shrinkage cavity volume of casting is minimized. Therefore, when shrinkage cavity volume is used as single evaluation criterion for casting quality, optimal die-casting process parameters for castings are A2B3C2.
Combining above analysis, it can be concluded that when solidification time is used as evaluation index, optimal process parameters are pouring temperature 630 ℃, mold temperature 180 ℃, and injection speed 5 m/s; when air entrainment volume is used as evaluation index, optimal process parameters are pouring temperature of 670 ℃, mold temperature of 220 ℃, and injection speed of 5 m/s; when shrinkage cavity volume is used as evaluation index, optimal process parameters are pouring temperature 650 ℃, mold temperature 220 ℃, and injection speed 4 m/s. According to selection principle of actual production process, it is necessary to improve production efficiency while ensuring product quality. Therefore, order of importance of quality indicators in orthogonal test is: shrinkage volume, air entrainment volume, and solidification time. After comprehensive analysis, it is finally determined that optimal die-casting process parameters for differential housing are pouring temperature 650 ℃, mold temperature 220 ℃, and injection speed 5 m/s.

Table 6 Analysis table of positive difference of shrinkage cavity body

 

Figure 10 Main effect diagram of die-casting process parameters on average shrinkage cavity volume

A Bühler 1 300 t die-casting machine was used to conduct high-pressure die-casting differential test verification, and optimal die-casting process parameters were selected: pouring temperature 650℃, mold temperature 220℃, and injection speed 5 m/s. Two high-pressure die-cast differential castings were obtained as shown in Figure 11. After trimming process, overflow groove, exhaust groove and flow channel are removed. Net weight of part is 4.4 kg. Surface of differential housing is smooth, outline is clear, and there are no defects such as cracks, flash edges, cold isolation, etc., and quality is good. X-ray flaw detection was performed on upper bracket, lower bracket and bottom cylinder of differential with high quality requirements. From X-ray image, it can be seen that there are no obvious pores and shrinkage defects inside casting, which verifies that die-casting process optimization plan meets production requirements.

 

Figure 11 Physical picture and X-ray flaw detection of differential housing die-casting
Mechanical properties were tested on upper end of differential housing casting bracket, lower end of bracket and bottom cylinder, as shown in Table 7. It can be concluded from table that there are differences in mechanical properties at different locations, but mechanical properties of samples near runner are better than those of samples far away from the runner, mechanical properties of samples with thin walls are better than those with thick walls. During casting forming process, as extrusion force gradually increases, size of primary grains will decrease, thereby significantly improving mechanical properties of casting. From above flow field analysis, it can be seen that upper bracket and bottom cylinder of sample are located near sprue, pressure is at a high level, and mechanical properties are good, while lower bracket is located at far sprue, and pressure will be lost during transmission process. It is at a low level and has poor mechanical properties. However, the overall mechanical properties of differential housing casting are good and meet production requirements.

Table 7 Mechanical properties of differential housing castings

(1) Based on ProCast software, a finite element model of high-pressure die-cast differential housing casting was established to simulate filling and solidification process of high-pressure die-casting. By solving flow field, temperature field, and velocity field, location and causes of internal defects in casting were predicted.
(2) During filling process of differential housing, there is an air entrainment problem in the bottom cylinder and bracket areas on both sides, especially in upper end of right bracket and left bracket area of casting. Air entrainment volume is higher than average air entrainment volume by 0.000 3 g/ cm³; During solidification process, maximum internal temperature gradient of casting is 90℃, which results in uneven solidification of molten metal and existence of multiple isolated liquid phase areas in local areas of casting. Therefore, shrinkage defects appear in the thicker parts of casting and upper bracket on right side. At the bottom cylinder, defect volume is about 3.146 cm³, it is necessary to optimize die-casting process to eliminate air entrainment and shrinkage cavity defects.
(3) Combining finite element orthogonal test and high-pressure die-casting test, optimal die-casting process is: pouring temperature 650 ℃, mold temperature 220 ℃, and injection speed 5 m/s.