Structural characteristics of automobile light bracket were analyzed, die-casting process was designed, ProCAST software was used to conduct numerical simulation analysis on filling process, die-casting mold heat balance and temperature field of aluminum alloy die-casting, predict location and causes of defects, and optimize mold structure based on prediction results. Actual production shows that use of optimized mold design improves quality of castings.
Car light bracket parts are an important part of car lights. They need to have sufficient strength to support car light housing, need to have high stability and corrosion resistance to ensure that they can be firmly, accurately and stably fixed on car body to ensure safety of driving at night. Aluminum alloy has been widely used in car light brackets due to its good properties. However, due to uneven wall thickness and complex structure of lamp bracket, defects such as shrinkage and cold insulation are prone to occur during die-casting production process, which cannot meet safe driving requirements of car. In order to improve the overall yield rate of a company's aluminum alloy car light brackets, this study focuses on die-casting forming process of car light bracket parts, conducts CAE analysis of casting filling and solidification process based on ProCAST, improves die-casting process and mold design for possible defects to improve product quality.

Structure of car light bracket casting is shown in Figure 1. Material is EN AC44300 aluminum alloy, and its chemical composition is shown in Table 1. Structure of car light bracket parts is relatively complex, and it is generally plate-shaped. There are some lines on plate to ensure optical performance, and there are also many rib structures. Main wall thickness of casting is 3 mm, but local wall thickness varies greatly. Minimum wall thickness of ribs is 1.2 mm, and maximum wall thickness is 8.4 mm. The overall 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 there are no casting defects such as shrinkage cavities, shrinkage porosity, cracks, and cold shuts inside, so as to meet strength requirements of bracket parts.

 

Figure 1 Casting structure of car light bracket

Table 1 Chemical composition of EN AC44300 aluminum alloy wB/%

According to structural characteristics of lamp bracket casting, parting surface is selected on plane where top mounting hole has the largest projected area of casting.

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

 

In formula: A is cross-sectional area of gate, c㎡; G is weight of molten metal passing through gate, g; ρ is density of molten metal, g/cm³; v is filling speed of molten metal flowing through inner runner, m/s; t is filling time, s. Filling speed is 30 m/s, filling time is 0.04 s, and wall thickness of gate is 1.5 mm. Calculated cross-sectional area of gate is 253 mm2.
Structural form and size of runner mainly depend on shape and size of die casting and shape, position, direction and size of ingate. Recommended calculation formula for thickness of runner is:
                                    D=(5~8)T(2)
In formula: D is thickness of lateral runner, mm; T is thickness of inner runner, mm. Thickness of lateral runner is taken to be 8 mm. In order to facilitate demoulding of casting, demoulding slope of lateral runner is set to 15°.
A horizontal cold chamber die-casting machine was selected, punch diameter was selected as 70 mm, and material handle thickness was set to 18 mm.

During process of filling mold cavity with molten metal, try to eliminate gas in cavity and cold metal liquid at the front end. By setting up an overflow tank, thermal balance of mold can be improved and quality of die castings can be improved. Area around hole in casting is where molten metal converges, it is easy to generate eddy currents and entrap gas. Therefore, overflow groove is set outside hole and at final filling place. At the same time, for processing, overflow groove will be mainly set on movable mold. Gating system for this part is shown in Figure 2.

 

Figure 2 Watering and drainage system design

Cooling system has a decisive influence on forming quality of die-cast products. Figure 3 shows layout of cooling water channels inside die-casting mold for this product. Water channels are evenly distributed around casting, which is conducive to a uniform mold temperature field.

 

Figure 3 Distribution of cooling water channels

Length and width of core of this mold are 350 mm and 300 mm respectively. Mold core structure is shown in Figure 4. Fixed mold core contains insert 1, and movable mold core part contains insert 2. Surrounding shape of casting is formed by three slide blocks. In order to facilitate maintenance and replacement, reduce costs, replaceable long pin cores are made in 17 deep holes.

 

Figure 4 Mold core structure

HyperMesh software is used to perform CAE pre-processing on casting to obtain a high-quality surface mesh model, which is then input into MeshCAST module of ProCAST software to create a volume mesh. Set grid unit size respectively: 1 mm for casting, 2 mm for movable mold core, fixed mold core, slider, core and water channel, the overall mesh number is 19.54 million, mesh model of casting and mold is shown in Figure 5.

 

Figure 5 Mesh division of castings and molds

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

Table 2 Die casting process parameters
Die-casting production cycle is divided into four stages, and corresponding times are shown in Table 3. Forming cycle is 50 s.

Table 3 Die casting forming cycle

Figure 6 shows filling process of molten metal. The entire filling time is 0.042 s. At the beginning, molten metal first fills thin plate part of casting heat sink after passing through ingate, then enters mold cavity from both ends through inner sprues on both sides. After middle filling is completed, molten metal flows to the top until it is full, and finally fills overflow tank farthest from sprue. However, during filling process of molten metal, there are still problems with setting of runner. Part of molten metal will first fill thin plate part of heat sink through middle runner, then enter mold cavity through outer runner. There are multiple strands multiple strands of molten metal flowing together, which can easily form cold insulation and air entrapment, as shown circled in Figure 6d. Gating system is basically reasonable, but there is still room for optimization and improvement.

 

Figure 6 Filling process of lamp bracket casting

In order to meet requirements of product quality and production efficiency, thermal balance analysis is used to obtain temperature distribution and change trend in mold cavity during die-casting process, which helps to determine sensitive areas of temperature control, take measures to reduce temperature fluctuations, achieve uniformity control of mold temperature field, and provide a reference for formulating reasonable temperature field planning. A uniform mold temperature field will not only extend service life of mold, but also improve quality of castings. Therefore, it is very important to conduct thermal balance analysis and temperature field analysis of mold.
Select a point on the surface of casting, fixed mold and movable mold, as shown in Figure 7, and draw temperature-time curve, as shown in Figure 8. It can be seen that after 12 die-casting cycles, mold has basically reached a thermal equilibrium state.

 

Figure 7 Selected points on casting, fixed mold and movable mold

 

Figure 8 Three-point temperature-time curve
After mold reaches thermal equilibrium, temperature field of next cycle is selected for analysis. As shown in Figure 9, from left to right are temperature distributions of movable mold, fixed mold and fixed mold insert in three stages before filling, pressure-holding solidification and release agent spraying. Before filling, temperature field distribution of mold is relatively uniform. When molten metal is filled, surface temperature of mold cavity will rise sharply as molten metal enters. During pressure-holding and solidification stage, heat exchange between mold and cooling water and heat dissipation into air cause temperature to gradually drop. Because cavity structure is complex, temperature field is not uniform, and local temperature is high, resulting in a certain difference in solidification time of each part of casting, but the overall temperature field has a small temperature gradient on cavity surface; When mold is opened and parts are taken out, mold surface is in large contact with air. At the same time, under action of release agent, surface temperature of mold cavity decreases rapidly, surface temperature of most of mold cavity and insert drops to below 500℃.
Judging from analysis results of mold temperature field, temperature field distribution on the surface of movable mold and fixed mold cavity is relatively uniform, but local temperature is too high and there are hot spots, so there is still room for optimization.

 

Figure 9 Temperature fields of movable mold, fixed mold and fixed mold insert at different times in one cycle

Figure 10 shows distribution of shrinkage cavities and shrinkage porosity in bracket die-casting. It can be seen that shrinkage cavities and porosity in casting are concentrated at rib structural connections of heat dissipation lamellae on bracket parts, and at locations with larger wall thickness around holes. Occurrence of these positional defects is mainly because temperature during solidification in these areas is relatively high, solidification time of molten metal is longer, and solidification rate is uneven. When it is completely solidified, molten metal cannot be fed, shrinkage cavities and shrinkage porosity defects appear. In addition, due to complex structure of this casting, improper adjustment of cooling system and uneven heat dissipation, heat accumulation may occur, causing internal temperature of casting to be too high, resulting in cold insulation defects, which will have a certain impact on product quality. Therefore, this solution still needs to be further improved so that produced bracket parts can meet required performance requirements.

 

Figure 10 Distribution of shrinkage cavities and porosity in castings

In order to improve phenomenon that flow of sprues at both ends of casting is insufficient and flow is small under influence of core on one side, resulting in confluence of multiple strands of molten metal to form cold isolation and air entrapment, a total of three in-runners are added at confluence at both ends and at location where molten metal is insufficiently filled on one side, as shown in red circle in Figure 11, to make filling process smoother and more uniform; and since overflow groove cannot be installed on heat dissipation sheet, in order to improve exhaust condition of ribs, an ejector rod is added, which eliminates cold separation and improves forming quality of casting.

 

Figure 11 Distribution of shrinkage cavities and porosity in castings after process optimization
After process optimization, numerical simulation predicts distribution of shrinkage cavities and shrinkage porosity in castings, as shown in Figure 11. X-ray non-destructive testing was performed on casting, and results are shown in Figure 12. Through comparison, it was found that flaw detection results were basically consistent with numerical simulation results. Die-casting parts had shrinkage holes in the thickest parts, and there were no pores exceeding Ф0.3 mm in flat corrugations and ribs, which met quality requirements.

 

Figure 12 X-ray non-destructive testing results
After improving pouring system of mold, practical applications show that continuous production efficiency of mold is high, 600 pieces/8 h, yield rate reaches 96%, and mold life reaches 150,000 molds. Die casting with pouring system is shown in Figure 13 shown.

 

Figure 13 Actual die casting

Designed die-casting process and mold for aluminum alloy lamp bracket casting, conducted CAE analysis on casting filling process, temperature field, shrinkage cavity and porosity defects. Based on analysis results, die-casting mold design was optimized, gating system and exhaust system were improved, optimized mold design was used to produce a lamp bracket casting product that met quality requirements.