Abstract: This paper analyzes structural characteristics and forming quality requirements of bracket parts, addressing die casting defects such as uneven filling flow and porosity caused by uneven wall thickness in aluminum alloy connecting brackets for new energy vehicle motors. Filling and solidification process of bracket is simulated using Magma® software. Process areas of part are divided according to structural characteristics and subsequent machining requirements. Causes of local uneven flow during aluminum liquid filling are studied, and trends of defects such as porosity, cold shuts, and shrinkage are predicted, thereby optimizing design of die casting overflow system for connecting bracket. Based on pressure-flow relationship P-Q2 curve of die casting process, shape and size of ingate are improved, filling flow and filling pressure in each area are adjusted and balanced to improve internal density and production quality of casting.
Aluminum alloy connecting bracket parts for assembling electric motors and other power components in new energy vehicles are produced using high-pressure die casting. To achieve function of connecting electric motor with various power components of different shapes, inner surface of connecting bracket is designed with many uneven frames and thin-walled connecting ribs. This uneven frame and rib structure causes uneven filling speed and flow rate in various parts during aluminum alloy die casting, resulting in defects such as porosity, cold shuts, and incomplete filling in local parts, especially in areas requiring machining, affecting dimensional accuracy and realization of connection and installation functions. Magma® software was used to simulate and analyze die casting filling and solidification process. In response to defects such as shrinkage porosity and porosity generated by initial process design simulation, and combined with functional quality requirements of each area of casting, different process areas of part were divided. Differences in cooling and solidification speed and flow rate of molten metal in each area of part during filling process were studied. Uneven filling flow rate and speed distribution caused by uneven wall thickness of casting during die casting process, as well as porosity defects caused by local blind hole characteristics of casting. By simulating filling results, using P-Q2 pressure-flow rate curves and simulated air pressure values, this study investigates design of sub-gate size and shape for local runners. Shape and area of sub-gates are adjusted and optimized to coordinate filling flow rate and filling pressure relationship in casting, ensuring a balanced flow of molten aluminum into mold cavity. Simulation verification further improves defects such as shrinkage porosity and gas porosity, thereby enhancing density and quality of casting.

Figure 1 shows 3D diagram of connecting bracket parts and process area division. Material is A356.2 aluminum alloy, which has good fluidity, low density, and good corrosion resistance. During die-casting production, it exhibits low thermal cracking tendency, low linear shrinkage, and good airtightness. Product weighs 2.75 kg, has a volume of 1013.38 cm³, an average wall thickness of 3.5 mm, and outline dimensions of 319 mm * 209 mm. Die-casting mold is made of SKA61 mold steel. Based on geometry of support parts, it is divided into three areas: A, B, and C. Area A consists of thin-walled frame connecting ribs; Area B is connecting and fastening assembly part that connects to other parts, and will undergo machining after die casting, therefore requiring high dimensional accuracy and internal density; Area C is frame part with a more uniform wall thickness. In Area A, the outermost thin-walled ribs have a lower molten metal flow rate, making them prone to jetting, air entrapment, and cold shut defects, requiring balanced filling pressure and proper overflow channel design. In Area B, circles indicate subsequent machining forming holes, requiring a dense, non-porous internal structure in casting. Although region C is a relatively simple and fast-solidifying part, due to difference in filling flow rate and speed, defects such as porosity, cold shuts, and deformation are prone to occur at local intersection of three regions. Therefore, numerical simulation of die-casting filling and solidification process is required to determine a reasonable process design scheme.

 

Fig. 1 Process area division and 3D diagram of connecting bracket parts

Initial process scheme and parameter design for die-casting aluminum alloy connecting brackets must first ensure complete filling of each part of bracket components, as well as quality problems such as porosity, cold shuts, and deformation on machined surfaces. Based on equipment conditions of production enterprise, selected die-casting machine and initial process parameters are shown in Table 1.
Table 1 Initial process parameters for numerical simulation of pressure casting brackets

Based on basic principle that location of ingate should minimize flow of molten metal into cavity and prevent excessive heat loss during filling, thus avoiding die-casting defects such as cold shuts or flow marks, initial gating and overflow system design is shown in Figure 2. In initial process, inner gate uses 1 to 6 gates for simultaneous feeding. Cross-sectional shape and size of each gate are similar to ensure even and stable filling of molten metal, which is beneficial for venting and transmitting static pressure. Multiple overflow channels are also designed to accommodate flat and dispersed shape of connecting bracket.
Differentiated structures of three regions A, B, and C of bracket component may hinder even filling of molten aluminum. During filling, collisions at confluence of several molten metal streams can generate eddies and air entrapment. Furthermore, uneven wall thickness leading to differences in solidification timing can cause defects such as cold shuts and deformation. Therefore, multiple overflow channels (see arrows in Figure 2b) are designed to stabilize flow of molten metal and to accommodate cold, contaminated molten metal and improve venting. An overflow channel is added at thin-walled ribs of region A to avoid "dead-angle air trapping" caused by poor filling at this location.

 

Fig. 2 Initial design of gating system and overflow system of connecting bracket parts

Based on structural characteristics and quality requirements of connecting bracket parts, Magma® software was used to simulate temperature changes, filling speed, gas pressure changes, and shrinkage defects during die casting process of bracket. Figure 3 shows simulation of each stage of aluminum liquid filling. Through simulation comparison, it can be seen that there is an imbalance in filling speed and cooling solidification sequence of each process area during solidification process of bracket casting. As can be seen from Figure 3a, in initial filling stage of aluminum liquid, multiple vertical ingates are used for simultaneous filling, which makes heat conduction speed between aluminum liquid and mold asynchronous, and temperature change rate shows a certain difference.
Secondly, due to differences in wall thickness and shape of each area of part, volume of molten metal filling cavity and filling speed are quite different, and filling completion time is asynchronous. When area C is basically filled, there are still nearly 20% of cavity in some positions in areas A and B that are not filled. As shown in Figures 3c and 3d, filling process in areas A and B is slower than in other areas, uneven filling speed can cause aluminum liquid to converge and form trapped air. Simultaneously, temperature gradient distribution in different areas during filling process is very uneven, easily leading to shrinkage porosity, shrinkage cavities, and cold shut defects. Therefore, to ensure a balanced flow rate and speed of aluminum liquid entering mold cavity during filling, it is necessary to specifically increase area of ingate at corresponding locations, increase filling flow rate, and accelerate filling speed, especially for key areas in areas A and B of part, thereby reducing possibility of cold shut defects.

 

Fig. 3 Temperature simulation of connecting bracket parts during die-casting filling process

Considering dispersed structure of part frame and high requirements for solidification density of casting during subsequent machining, gas pressure change simulation and prediction of internal shrinkage cavities and porosity defects during filling process were also conducted, as shown in Figure 4. As shown in Figure 4a, at intersection of areas A and B in Figure 1a, black line indicates a higher gas pressure value, and aluminum liquid flow pattern shows air entrapment on machining surface, indicating possibility of internal porosity after machining. Meanwhile, a small amount of molten aluminum is trapped at ingate location. This is likely due to uneven feed flow and velocity of six ingates. When molten aluminum from different ingates converges at final filling position of casting with different velocities and temperatures, defects such as trapped aluminum, laminar flow, and flow marks are easily formed at metal confluence. Secondly, small amount of trapped molten aluminum near ingate can also lead to cold material accumulation, which may result in incomplete filling and delamination defects after machining.

 

Fig.4 Prediction and simulation of air entrainment and shrinkage defects during die-casting filling process
As shown in Figure 4b, liquid islands form hot spots at four corners of region B in Figure 1a. These hot spots will not appear after solidification. This is because wall thickness of region B is relatively thick, making it easy for hot spots to form locally at areas with uneven wall thickness. Furthermore, inability to effectively compensate for shrinkage from gating system can easily lead to a decrease in density of solidified structure, resulting in shrinkage defects and shrinkage cavities during solidification.

In initial process design of connecting bracket casting, six ingates were designed as a balanced structure with equal cross-sectional areas. However, wall thickness and structure of regions A, B, and C of bracket casting differed significantly, resulting in large differences in solidification results of molten aluminum in three regions. To address differences in wall thickness and solidification quality in regions A, B, and C, a reasonable ingate was designed to change imbalance in filling flow rate and filling pressure in each process region. Figure 5 shows optimized design diagram. To achieve a balance in flow rate and velocity between ingates 1-3 and ingates 4-6 on other side during filling, and to prevent low-temperature molten aluminum from entering mold cavity through faster-feeding ingates, thus avoiding accumulation of cold material, right-angle intersection of ingate 2 and main runner was changed to a rounded transition. This reduced tendency of molten metal from ingate 2 to spray when entering mold cavity. Stroke was also lengthened, ingate area enlarged, and feed angled to prevent negative pressure suction or air entrapment. Simultaneously, to reduce porosity defects, a dedicated overflow channel was added between ingates 2 and 3 for venting.

 

Fig. 5 Diagram of optimized scheme after shape modification of ingate

For die-casting production of aluminum alloy support parts with uneven wall thickness, in order to maintain a certain pressure energy at the end of die-casting filling process, gating system needs to maintain a high filling pressure and filling speed. This ensures that molten aluminum entering each area has sufficient, balanced filling pressure and flow rate, enabling synchronous filling and solidification of each part of support component. Following formula was used to simulate filling flow rate and filling speed:

 

Where Q is molten metal filling flow rate; Ag is ingate area; P is molten metal filling pressure; Cd is flow coefficient of aluminum alloy liquid; ρ is molten metal density; and g is gravitational acceleration.
Based on equipment and ingate design scheme, a P-Q2 relationship curve was generated to help designers verify ideal process region for relationship between filling pressure and mold energy, thereby verifying and judging rationality of ingate area design, as shown in Figure 6.

 

Figure 6 Rationality inspection of inner gate design by P-Q₂ relation curves
From P-Q2 relationship curve generated after changing area of ingate No. 2, it can be seen that when area of ingate No. 2 increases from initial theoretical design value of 2.35 cm2 to 3.35 cm2, simulated P-Q2 relationship process curve is in optimal region, proving that optimization scheme of changing area of ingate No. 2 is feasible. At the same time, based on P-Q2 relationship curve, parameters such as maximum filling pressure, cavity filling time, initial mold temperature of initial process were adjusted and optimized accordingly, as shown in Table 2.
Tab.2 Parameter comparison of primary and optimized scheme

To verify whether each area of casting achieved balanced filling pressure and flow rate, optimized simulation was performed again using software simulation.

Optimized scheme was analyzed by simulation of filling process pressure and solidification temperature. Pressure and temperature variation curves from liquid to solid phase during die casting process of optimized part were obtained, as shown in Figure 7. A comparison of gas pressure simulation at junction of molten metal and mold before and after optimization shows that initial design gas pressure was high, reaching 3557.979 kPa, while optimized gas pressure was reduced to 2302.127 kPa. This indicates that risk of vacuuming at this molten metal junction was significantly reduced.

 

Fig.7 Simulation comparison of pressure optimization during filling process
By selecting simulation sensors placed at locations where shrinkage porosity might occur during prediction, Magma® software can also generate heat exchange and temperature change curves between molten metal and mold at key locations to check whether temperature curve during solidification is smooth, as shown in Figure 8. It can be seen that optimized temperature curve of heat conduction between local mold and molten metal in casting shows a gradual decrease, proving that casting has a reasonable cooling and solidification rate during process from liquid to solid phase, avoiding defects such as shrinkage porosity caused by excessive temperature gradient between casting and mold.

 

Fig. 8 Temperature variation simulation of molten metal and mold during filling process

(1) Different wall thicknesses and shapes of aluminum alloy connecting bracket parts hinder equalization of speed and flow rate of molten aluminum during filling. Therefore, initial design of 6 equal ingates easily leads to large differences in solidification rate of each part, there are defects such as gas entrapment and cold shut at junction of molten aluminum.
(2) Differentiated ingates are designed for different wall thicknesses and structures in different areas of bracket casting to change relationship between filling flow rate and filling pressure in local area of casting. At the same time, addition of overflow grooves to ingates can improve venting effect, improve filling and solidification quality.