Abstract: Structural characteristics of automotive headlight brackets were analyzed, die casting process was designed, and filling process, thermal balance and temperature field of aluminum alloy die casting were numerically simulated and analyzed using ProCAST software. Location and cause of defects were predicted, and mold structure was optimized based on prediction results. Actual production shows that optimized mold design improved casting quality.
Headlight brackets are an important part of automotive headlights. They need to have sufficient strength to support headlight housing, high stability and corrosion resistance to ensure that they can be firmly, accurately and stably fixed to vehicle body and ensure safety of driving at night. Aluminum alloys have been widely used in headlight brackets due to their good properties. However, due to uneven wall thickness and complex structure of headlight brackets, defects such as shrinkage cavities, porosity and cold shuts are easily generated during die casting process, which cannot meet requirements of safe driving of automobiles. To improve the overall yield of aluminum alloy automotive lamp brackets for a certain company, this study focuses on die-casting process of lamp bracket parts. CAE analysis of casting filling and solidification process was conducted using ProCAST, die-casting process and mold design were improved to address potential defects, thereby enhancing product quality.

Structure of lamp bracket casting is shown in Figure 1. Material is EN AC44300 aluminum alloy, and its chemical composition is shown in Table 1. Lamp bracket part has a relatively complex structure, generally plate-shaped, with some textures to ensure optical performance and many ribs. Main body of casting has a wall thickness of 3 mm, but there are significant differences in wall thickness in some areas. Minimum wall thickness in rib area is 1.2 mm, and maximum wall thickness is 8.4 mm. External dimensions are 190.3 mm * 191.88 mm * 74.22 mm, and weight is 613.5 g. Surface of casting is required to be free of burrs and scratches, and internal parts must be free of casting defects such as shrinkage cavities, porosity, cracks, and cold shuts, meeting strength requirements of bracket parts.

 

Fig. 1 Structure of car lamp holder casting
Table 1 Chemical composition of EN AC44300 aluminum alloy wB/%

 

According to structural characteristics of car headlight bracket casting, parting surface is selected on plane where top mounting hole with the largest projected area of casting is located.

Gating system not only plays an important role in controlling flow direction and state of molten metal in mold cavity, venting conditions, and pressure transmission of mold, but also can adjust filling speed, filling time, and temperature distribution of mold. In order to make flow of molten metal as short as possible and reduce unnecessary heat loss, to avoid molten metal directly impacting core, ingate is set at the top and straight edges of casting, as shown in Figure 2. Calculation of cross-sectional area of ingate:

 

Where: A_ingate is cross-sectional area of ingate, cm2; G is mass of molten metal passing through ingate, g; ρ is density of molten metal, g/cm3; v_filling is filling speed of molten metal flowing through ingate, m/s; t is filling time, s. Filling speed is 30 m/s, filling time is 0.04 s, and ingate wall thickness is 1.5 mm. Calculated cross-sectional area of ingate is 253 mm².
Structure and dimensions of sprue mainly depend on shape and size of die casting, shape, position, direction, and dimensions of ingate. Recommended formula for calculating sprue thickness is:

 

Where: D is sprue thickness, mm; T is ingate thickness, mm. Sprue thickness is chosen to be 8 mm. To facilitate demolding, draft angle of sprue is set to 15°.
A horizontal cold chamber die casting machine is selected, with a punch diameter of 70 mm and a sprue thickness of 18 mm.

During process of molten metal filling cavity, gas and cold molten metal at the front end should be removed as much as possible. By setting an overflow groove, thermal balance of mold is improved, thus improving quality of die casting. Area around casting holes is where molten metal converges, which easily generates eddies and traps gas. Therefore, overflow channels are placed on the outside of holes and at final filling point. For machining purposes, overflow channels are mainly placed on moving mold. Gating system for this part is shown in Figure 2.

 

Fig. 2 Gating and overflowing system design

Cooling system has a decisive influence on forming quality of die-cast products. Figure 3 shows internal cooling water channel arrangement of die-casting mold for this product. Channels are evenly distributed around casting, which is beneficial for uniform mold temperature.

 

Fig.3 Cooling channel distribution

Length and width of mold core of this set of molds are 350 mm and 300 mm, respectively. Mold core structure is shown in Figure 4. Fixed mold core includes insert 1, and moving mold core includes insert 2. Shape around casting is formed by 3 sliders. To facilitate maintenance and replacement and reduce costs, replaceable long pin cores are made in 17 deep hole areas.

 

Fig.4 Die and die core structures

Casting was preprocessed using HyperMesh software to obtain a high-quality surface mesh model, which was then input into MeshCAST module of ProCAST software to create a volume mesh. Mesh element sizes were set as follows: 1 mm for casting, and 2 mm for moving mold core, fixed mold core, slider, core, and water channels. The total mesh size was 19.54 million. Mesh models of casting and mold are shown in Figure 5.

 

Fig. 5 Mesh generation of casting and die

3.2.1 Initial and Boundary Conditions
Mold material was H13 steel, and die-casting process parameters are shown in Table 2. Heat transfer coefficient between mold and casting was set to 20,000 W/(m²·K), heat transfer coefficient between moving mold and fixed mold was 1,000 W/(m²·K), heat transfer coefficient between mold and air was 100 W/(m²·K), heat transfer coefficient between release agent and mold was set to 100 W/(m²·K). Heat transfer coefficient between cooling water and mold is 5,000 W/(m2·K), temperature of both cooling water and mold release agent is 20 ℃.
Table 2 Die casting process parameters

Table 3 Die casting forming cycle

3.2.2 Analysis of filling process
Figure 6 shows filling process of molten metal. The entire filling time is 0.042 s. At the beginning, molten metal fills thin plate of heat sink of casting first after passing through ingate, then enters cavity from both ends through ingates on both sides. After filling middle, molten metal flows to top until it is full, and finally fills overflow groove farthest from gate. However, during filling process of molten metal, due to problem of gate setting, some molten metal will fill thin plate of heat sink first through middle gate, then enter cavity through outer gate. There are multiple streams of molten metal converging, which can easily form cold shuts and air entrapment, as shown in circled area in Figure 6d. Gating system is basically reasonable, but there is still room for optimization and improvement.

 

Fig. 6 Mold filling process of car lamp holder casting
3.2.3 Mold Thermal Balance and Temperature Field Analysis
To meet requirements of product quality and production efficiency, thermal balance analysis is used to obtain temperature distribution and trend within mold cavity during die casting process. This helps to identify sensitive areas for temperature control, take measures to reduce temperature fluctuations, achieve uniform temperature field control in mold, and provide a reference for developing a reasonable temperature field plan. A uniform mold temperature field not only extends mold life but also improves casting quality; therefore, thermal balance and temperature field analysis of mold are crucial.
A point is selected on the surface of casting, fixed mold, and moving mold, as shown in Figure 7. Temperature-time curves are plotted, as shown in Figure 8. It can be seen that after 12 die casting cycles, mold has basically reached thermal balance.

 

Fig. 7 Points selected in casting, fixed die and moving die

 

Fig. 8 Temperature-time curves for 3 selected points
After mold reaches thermal balance, temperature field of next cycle is analyzed. As shown in Figure 9, from left to right, temperature distribution of moving mold, fixed mold, and fixed mold insert are shown in three stages: before filling, pressure holding and solidification, and after spraying release agent. Before filling, temperature field distribution of mold is relatively uniform. During filling of molten metal, surface temperature of mold cavity rises sharply. During pressure-holding solidification stage, heat exchange between mold and cooling water, as well as heat dissipation into air, causes temperature to gradually decrease. Due to complex cavity structure and non-uniform temperature field, with locally higher temperatures, solidification time varies across different parts of casting, but the overall temperature gradient on cavity surface is relatively small. When mold is opened and part is removed, mold surface comes into large contact with air, and under action of release agent, surface temperature of mold cavity drops rapidly, with most of mold cavity and insert surface temperatures falling below 500 ℃.
Analysis of mold temperature field shows that temperature field distribution on the surfaces of moving and fixed mold cavities is relatively uniform, but locally excessively high temperatures exist, indicating potential for optimization.

 

Fig. 9 Temperature fields of moving die, fixed die, and inserts in moving die at different times in one cycle
3.2.4 Defect Analysis
Figure 10 shows distribution of shrinkage cavities and porosity in die-cast bracket. It can be seen that shrinkage cavities and porosity in casting are concentrated at connection of rib structure of heat dissipation fins on bracket parts, and at locations with larger wall thickness around holes. Occurrence of these defects is mainly due to relatively high solidification temperature in these areas, the longer solidification time of molten metal, and uneven solidification rate. When metal is completely solidified, it cannot be replenished by molten metal, resulting in shrinkage cavities and porosity defects. In addition, due to complex structure of this casting and inadequate adjustment of cooling system and uneven heat dissipation, heat accumulation may occur, leading to excessively high internal temperature of casting, resulting in cold shut defects, which have a certain impact on product quality. Therefore, this solution still needs further improvement to ensure that produced bracket parts meet required performance requirements.

 

Fig. 10 Shrinkage cavity and porosity distribution of casting

In order to improve phenomenon of multiple streams of molten metal converging and forming cold shuts and air entrapment due to insufficient flow in gating channels at both ends of casting and small flow under influence of core on one side, three ingates were added at converging points at both ends and location where molten metal was insufficiently filled on one side, as shown in red circle in Figure 11, so that filling process was more stable and uniform; and since overflow grooves could not be set on heat dissipation thin plate, push rods were added to improve venting of ribs, eliminating cold shuts and improving forming quality of casting. Numerical simulation prediction of distribution of shrinkage cavities and porosity of casting after process optimization is shown in Figure 11. X-ray non-destructive testing was performed on casting, and results are shown in Figure 12. By comparison, it was found that testing results were basically consistent with numerical simulation results. Die casting had shrinkage cavities in the thickest part, plate corrugations and ribs did not have pores exceeding Ф0.3 mm, which met the quality requirements.

 

Fig. 11 Shrinkage cavity and porosity distribution of casting after process optimization

 

Fig.12 X-ray non-destructive detection result
After improving gating system of mold, practical application shows that mold has a high continuous production efficiency of 600 pieces/8 hours, a yield rate of 96%, and a mold life of 150,000 molds. Die-cast part with gating system is shown in Figure 13.

 

Fig.13 Real Die-casting part

Die-casting process and mold for aluminum alloy automotive lamp bracket castings were designed. CAE analysis was conducted on casting filling process, temperature field, and shrinkage defects. Based on analysis results, die-casting mold design was optimized, gating system and venting system were improved. Using optimized mold design, automotive lamp bracket castings that meet quality requirements were produced.