As energy shortages and environmental protection issues become increasingly prominent, magnesium alloys, as the lightest "green" metal structural materials, have gradually become one of hot spots in research and development of lightweight materials. Magnesium alloys are widely used in electronic products, transportation and biomedical fields due to their high specific strength, good damping properties and degradability. As main load-bearing structural component of car seats, seat cushion frame must meet requirements of strength while also taking into account lightweight. AE44 magnesium alloy has strong corrosion resistance and excellent shock absorption effect, which can improve human body comfort. In addition, its good creep resistance can reduce fatigue damage during service and help extend service life of seat. Currently, magnesium alloy parts for cars are mainly formed by die-casting. However, long development cycle of die-casting molds and high test costs limit application of die-casting magnesium alloys. Cushion frame is a large-sized, thin-walled structural part, and existing die-casting process poses a great challenge to its forming. Numerical simulation technology has characteristics of saving test costs, improving research and development efficiency. Therefore, CATIA software is used to draw three-dimensional solid shape of cushion skeleton and surface mesh is divided; ProCAST software is then used to numerically simulate cushion skeleton die-casting filling and solidification process, skeleton pouring system and key die-casting process parameters are optimized to provide a reference for its application.

Outline size of die-cast magnesium alloy seat cushion frame is 500mm*400mm*100mm, which is a large thin-walled complex casting. CATIA software is used to draw three-dimensional shape of seat cushion (see Figure 1), then ProMesh module of ProCAST software is used to implement meshing. Minimum wall thickness of cushion skeleton is 3mm. Taking into account both calculation efficiency and number of meshes, a 3mm mesh is used for division, then MeshCAST module is used for fine mesh division and mesh repair. Final number of surface meshes is 115168, and number of volume meshes is 2197614.

 

Figure 1 3D diagram of car seat cushion frame

Table 1 Simulated die-casting process parameters

 

Figure 2 Radial runner design and mold filling simulation results
(a) Radial runner design (b) Mold filling simulation results
According to principle of simultaneous arrival, principle of decreasing cross-sectional area and principle of flow channel heat dissipation, pouring system of cushion frame is optimized. Thickness of cushion frame is thin, which causes molten metal to flow slowly. It will first fill central area of mold cavity, then flow to surrounding areas, and finally reach edges. For this reason, consider setting up an overflow system and a feeding system at the edge of casting to facilitate discharge of residual gas and inclusions in melt. After many optimizations, sprue system was designed in a radial shape (see Figure 2).

 

Figure 3 Comparison before and after optimization of overflow tank
(a) Before optimization (b) After optimization

 

Figure 4 Simulation filling results of groups A to F
For die casting filling process, filling sequence should be ensured to reduce melt gas content at casting location. Simulation results show that unfilled area (gray) should remain connected instead of being wrapped by melt, and based on this, it is better for liquid level to be stable. In this way, comparing simulation results of groups A, B and C, it is found that filling sequence under parameters of group B is better. When low-pressure pressure is increased from 0.15MPa to 0.20MPa, between 0.0213s and 0.0443s in Group D (Figure 4m~Figure 4o), a gray closed area appears in casting, and "gas entrainment" occurs. The filling sequence is not as good as that of Group B. This may be related to fact that when low pressure is too high, flow rate of molten metal is too fast and turbulence occurs.

 

Figure 5 Comparison of coagulation time between groups E and F

 

Figure 6 Filling time of groups A, B, D and E
(a) Group A (b) Group B (c) Group D (d) Group E

 

Figure 7 Shrinkage cavities and porosity distribution diagrams of Group B and Group E

 

Figure 8 Filling time chart of any section of the Y-axis and X-axis of Group B castings
(a)Y axis (b)X axis

(1) Designed and optimized casting and overflow system of magnesium alloy seat cushion frame, set overflow groove at the edge and dead corner, set exhaust hole size to 10mm*10mm, improved shape and size of sprue, which solves defects such as air entrainment and insufficient pouring that are easy to occur during simulation process, and achieves sequential solidification.
(2) Process parameter improvements and simulation results show that optimal die-casting process parameters for AE44 magnesium alloy cushion frame are: pouring temperature of 670℃, mold temperature of 190℃, injection speed of 6m/s and low pressure of 0.15MPa. Under these process parameters, molten metal flows smoothly, casting is completely filled, and there are fewer casting defects.