Aluminum alloy connector brackets used for assembling electric motors and other power components in new energy vehicles are produced using high-pressure casting. To achieve connection between motor and various power components, inner surface of connector bracket is designed with numerous uneven frames and thin-walled connecting ribs. This uneven frame and rib structure results in uneven filling speed and flow rate at various locations during aluminum alloy die-casting. This can lead to defects such as porosity, cold shuts, and under-casting in certain areas of part, especially those requiring machining. These defects affect dimensional accuracy and ability to achieve proper connection and installation. Magma® software was used to simulate, analyze die-casting filling and solidification processes. To address defects such as shrinkage and porosity generated by initial process design simulation, part was divided into different process zones, based on functional quality requirements of each region of casting. Differences in molten metal cooling, solidification rates and flow rates in each region during filling process were studied. Uneven filling flow and velocity distribution caused by uneven casting wall thickness during die-casting filling process was analyzed, as well as porosity defects caused by local blind holes in casting. Based on simulated filling results, P-Q2 pressure-flow curves, and simulated air pressure values, design of local runner gate size and shape was studied. Gate shape and gate area were optimized, relationship between local filling flow and pressure was coordinated to ensure a balanced flow of molten aluminum into mold cavity during filling. Simulations verified that defects such as shrinkage and porosity were improved, thereby enhancing casting's microstructure and production quality.

Figure 1 shows a 3D image of connecting bracket part and process zone divisions. Material is A356.2 aluminum alloy, which features good fluidity, low density, and excellent corrosion resistance. During die-casting, it exhibits low hot cracking tendency, minimal linear shrinkage, and excellent airtightness. Product weighs 2.75 kg, has a volume of 1,013.38 cm³, an average wall thickness of 3.5 mm, and outline dimensions of 319 mm * 209 mm. Die-casting mold is made of SKA61 die steel. Based on geometry of bracket part, it is divided into three regions: A, B, and C. Region A contains thin-walled frame connecting ribs, while Region B is fastening and assembly area connecting to other parts. This region undergoes machining after die-casting, therefore requires high dimensional accuracy and internal density. Region C is frame portion, which requires relatively uniform wall thickness. The outermost thin-walled ribs in Region A experience low molten metal flow, which is prone to jetting, air entrainment, and cold shut defects. Balancing filling pressure and properly designating an overflow channel are essential. Circles in Region B indicate holes that will be machined later, requiring a dense, pore-free internal structure. Although region C is relatively simple and solidifies rapidly, local intersection of three regions is prone to defects such as porosity, cold shuts, deformation due to differences in filling flow and speed. Therefore, numerical simulation of die-casting filling and solidification processes is necessary to determine a reasonable process design.
Initial process plan and parameter design for die-casting aluminum alloy connecting brackets must first ensure complete filling of all parts of bracket component and prevent quality issues such as porosity, cold shuts, and deformation on machined surfaces. Based on manufacturer's equipment availability, selected die-casting machine and initial process parameters are shown in Table 1.

 

Figure 1 Process Area Division and 3D Diagram of Connecting Bracket

Table 1 Initial Process Parameters for Numerical Simulation of Low-Pressure Casting Brackets
Based on principle that ingate position should minimize metal filling path and prevent excessive heat loss during filling, which can lead to die-casting defects such as cold shuts or flow marks, initial pouring and overflow system design is shown in Figure 2. In initial process, ingates were fed simultaneously through runners 1 to 6. Each runner had similar cross-sectional shapes and dimensions to ensure even and smooth filling of molten metal, facilitate degassing, and transfer static pressure. Multiple overflow troughs were also incorporated to address flat, dispersed shape of connecting bracket. Differentiated structures of three regions A, B, and C of bracket components could hinder even filling of molten aluminum. During filling, intersection of multiple metal streams could also generate vortices and air entrainment due to collisions. Furthermore, variations in solidification timing caused by uneven wall thickness could lead to defects such as cold shuts and deformation. Therefore, multiple overflow troughs (see arrows in Figure 2b) were designed to stabilize molten metal flow, contain contaminated cold metal, and enhance degassing. An overflow trough was added at thin-wall ribs on frame of region A to prevent "dead corner air trapping" caused by poor filling.

 

Figure 2: Initial Design of Die-Casting Gating and Overflow System for Connecting Bracket
Based on structural characteristics and quality requirements of connecting bracket, Magma® software was used to simulate temperature changes, filling speed, air pressure changes, and shrinkage defect prediction during bracket die-casting filling process. Figure 3 shows simulation of various stages of molten aluminum filling. Simulations reveal that filling speeds, cooling and solidification sequences of various process zones during solidification process of bracket casting are uneven. Figure 3a shows that during initial filling phase, multiple vertical ingates are used to fill mold simultaneously, resulting in asymmetric heat transfer between molten aluminum and mold, and a certain difference in the rate of temperature change. Furthermore, due to varying wall thickness and shape of different regions of part, volume of molten metal filling cavity and filling speed vary significantly, leading to uneven filling completion times. When region C is nearly full, nearly 20% of cavities in regions A and B remain unfilled. Figures 3c and 3d show that because regions A and B fill slower than other areas, uneven filling speeds can cause molten aluminum to converge and create air traps. Furthermore, temperature gradient distribution in various regions during filling process is highly uneven, making shrinkage, porosity, and cold shut defects more likely to occur. Therefore, to ensure balanced flow and speed of molten aluminum into mold cavity, it is necessary to increase ingate area at corresponding locations, increase filling flow rate, and speed up filling process, especially in key areas A and B of part. This reduces likelihood of cold shut defects.
Given dispersed structure of part frame and high requirements for solidification density of casting due to subsequent machining, we also simulated pressure changes during filling process, predicted internal shrinkage and porosity defects, as shown in Figure 4. Figure 4a shows that at intersection of areas A and B in Figure 1a, black box indicates a high pressure value. Flow pattern of molten aluminum indicates air entrapment on machined surface, suggesting possibility of internal porosity after machining. There is also a small amount of molten aluminum entrained air at ingates. This is likely due to uneven feed flow and speed of six ingates. When molten aluminum from different ingates converges at different speeds and temperatures at final filling point of casting, defects such as entrained air, laminar flow, and flow marks are easily formed at molten metal intersection. Furthermore, small amounts of molten aluminum entrained air near ingates can cause cold material accumulation, which can lead to underfilling and delamination defects after machining.

 

Figure 3: Temperature simulation of die-casting filling process for connecting bracket

 

Figure 4: Prediction of entrained air and shrinkage defects during die-casting filling
In initial process design for connecting bracket casting, six ingates were designed as a balanced structure with equal cross-sectional areas. However, wall thickness and structure of three regions A, B, and C of bracket casting vary significantly, resulting in significant differences in filling and solidification results of three regions. To address differences in wall thickness and solidification quality between regions A, B, and C, a rational gate design was developed to address imbalance in filling flow and pressure between various process zones. Figure 5 shows optimized design. To balance flow and speed between gates 1-3 and gates 4-6 on other side during filling, to prevent faster-feeding gate from allowing cold molten aluminum to enter mold cavity and cause cold material accumulation, right-angle intersection of gate 2 and main runner was changed to a circular arc. This reduced tendency of molten metal to spurt when entering mold cavity. Furthermore, stroke was lengthened, gate area was increased, feed was tilted to prevent negative pressure aspiration and air entrapment. Furthermore, to reduce risk of air porosity, a dedicated overflow trough was added between gates 2 and 3 for venting.
Based on equipment and ingate design, a P-Q2 relationship curve was generated to help designers identify ideal process region for relationship between filling pressure and mold energy, thereby verifying and assessing rationality of ingate area design (see Figure 6). P-Q2 relationship curve generated after changing area of ingate No. 2 shows that when area of ingate No. 2 is increased from initial theoretical design value of 2.35 cm2 to 3.35 cm2, simulated P-Q2 relationship process curve remains within optimal region, confirming feasibility of optimization plan for changing ingate No. 2 area. Furthermore, based on P-Q2 relationship curve, parameters such as initial process maximum filling pressure, cavity filling time, initial mold temperature were adjusted and optimized accordingly (see Table 2).

 

Figure 5: Optimized design after changing ingate shape

 

Figure 6: Checking rationality of ingate design using P-Q2 relationship curve

Table 2: Comparison of parameters between initial and optimized process schemes
A simulation analysis of pressure and solidification temperature during mold filling process for optimized solution yielded a simulated curve comparing pressure and temperature changes from liquid phase to solid phase during die casting process for optimized part (see Figure 7). A comparison of pressure simulations at intersection of molten metal filling point before and after optimization reveals that initial design pressure was high, reaching 3557.979 kPa, while optimized pressure was reduced to 2302.127 kPa. This indicates that risk of gas trapping at this intersection is significantly reduced. By selecting simulated sensors located at locations where shrinkage is predicted, Magma® software can also generate heat exchange and temperature curves between molten metal and mold at key locations to verify smoothness of solidification temperature profile (see Figure 8). It can be seen that heat conduction temperature curve between local mold and molten metal of optimized casting shows a gentle decline, which proves that casting has obtained a reasonable cooling and solidification rate in process from liquid phase to solid phase, avoiding defects such as shrinkage caused by excessive temperature gradient between casting and mold.

 

Figure 7 Comparative simulation diagram of air pressure optimization during filling process

 

Figure 8 Simulation diagram of temperature change of molten metal and mold during filling process

(1) Differentiated wall thickness shape of aluminum alloy connecting bracket parts hinders speed and flow balance of molten aluminum during filling. Therefore, initial design of 6 equal gates is likely to cause large differences in solidification speed in various parts, there are air entrapment and cold shut defects at intersection of molten aluminum.
(2) According to different wall thickness and structure of each area of bracket casting, differentiated gates are designed to change relationship between local filling flow and filling pressure of casting. At the same time, adding overflow grooves to gates can improve exhaust effect, improve filling and solidification quality.
(3) Comparison results of filling pressure simulation and P-Q2 relationship curves show that optimization scheme can improve defects such as air inclusion and shrinkage in castings.