Filling and solidification process of original low-pressure casting process of a certain type of gearbox housing casting was numerically simulated to observe defective parts of casting that are prone to air entrainment, shrinkage, etc., and analyze reasons. According to simulation analysis results, optimization measures such as increasing pouring temperature and pouring speed, increasing size of top riser, and increasing number of chills were proposed. By numerically simulating, trial-producing and inspecting optimized casting process, feasibility of optimization measures was verified, defects were effectively avoided, and quality improvement goal was achieved.
Gearbox housing is one of core parts of mechanical transmission system, with a complex and irregular structure. It connects parts on transmission mechanism into a whole. With continuous improvement of China's rail transit technology level, process of replacing iron and steel with aluminum has been greatly accelerated. Gearbox material has changed from traditional ductile iron or gray cast iron to aluminum alloy, which can effectively reduce unsprung mass and improve running quality. Although low-pressure casting process of gearbox housing has been widely used, defects such as shrinkage will inevitably occur during casting process. Reason lies in selection and adjustment of parameters during casting process. At this stage, computer simulation technology has become very mature, which can intuitively and effectively show filling and solidification process of castings and predict distribution of defects. For example, Peng Lingling used FLOW-3D to simulate filling process of gearbox housing, predicted air entrainment of cavity, and made process improvements. Park Junjie and others simulated filling and solidification process of automobile bracket through ProCAST, took local pressurization optimization measures based on results to solve shrinkage and shrinkage defects of casting. Ma Yunan and others obtained filling speed, filling temperature, solidification liquid phase area and shrinkage defect distribution of box casting process through Anycasting, and carried out corresponding process optimization to eliminate internal defects. It can be seen that making full use of computer simulation technology to develop castings can effectively find location and cause of casting defects, so as to take corresponding process improvement measures.
This paper takes a certain type of aluminum alloy gear transmission housing as research object, uses simulation software to numerically simulate filling and solidification process of original low-pressure casting process, observes parts where defects such as air entrainment and shrinkage are prone to occur and analyzes reasons, so as to make corresponding process improvements, develop a high-quality and stable production process for transmission housing.

Three-dimensional model of housing is shown in Figure 1. Its appearance dimensions are 1457mm×601mm×411mm and its unit weight is 66kg. In terms of performance, tensile strength is required to be ≥297 MPa, yield strength is ≥230 MPa, and elongation is ≥11%; radiographic inspection requires that there are no defects such as holes in key parts; defects such as scale, cold shut, and cracks are not allowed on the surface of casting.

 

Figure 1 Three-dimensional diagram of casting
Material of this model of aluminum alloy gear transmission housing is A356 aluminum alloy, and aluminum alloy composition is shown in Table 1.

 

Table 1 Chemical composition of A356 aluminum alloy wB/%

As shown in Figure 2, casting has a complex structure and uneven wall thickness distribution. Minimum wall thickness is located at the outer edge of bottom, which is only 4 mm, maximum wall thickness is located at the bottom and side bosses and surface ribs, reaching 28 mm. Due to high requirements for mechanical properties and internal quality of casting (especially designated area), surface is not allowed to have defects such as slag inclusions and oxide scales. It is necessary to strictly control speed of filling process, exhaust, and solidification time of each part after filling is completed, so as to achieve purpose of stable filling and sequential solidification, thereby eliminating defects such as air entrainment, slag inclusions, shrinkage cavities and shrinkage.

 

Figure 2 Wall thickness of casting
Aluminum alloy gear transmission housing adopts a low-pressure casting process scheme (Figure 3). Pouring system is open, which has advantages of stable metal liquid flow and small scouring force on sand mold, which can effectively reduce generation of oxides. Cross-sectional area of sprue is 7,850 m2, cross-sectional area of runner is 12,448 m2, and cross-sectional area of endogate is 60,660 m2. Riser is designed to be a free-standing riser, and rest are blind risers. Chills are placed in thick-walled areas that cannot be compensated by risers. Exhaust channels are arranged at the corners of risers and casting cavities, and high-silica glass fiber filters are built into sprue.

 

Figure 3 Casting model

Casting body and chill materials are A356 aluminum alloy, and sand mold material is 40-70 mesh washed sea sand. Filling process and solidification process are numerically simulated based on simulation software. Thermophysical properties of A356 aluminum alloy are shown in Table 2, and thermophysical properties of washed sea sand are shown in Table 3.

 

Table 2 Thermophysical parameters of A356 aluminum alloy

 

Table 3 Thermophysical parameters of washed sea sand
Using overall average grid division method, the total number of grids is 5 802 060, and grid division is shown in Figure 4.

 

Figure 4 Grid division
Interface heat exchange coefficient between casting and sand mold and interface heat exchange coefficient between casting and chill are set as shown in Figure 5. Interface heat exchange coefficient between chill and sand mold is 500 W/(㎡·K); initial temperature of sand mold and chill is 30 ℃; filling process parameters are shown in Table 4, and casting cooling method is air cooling.

Table 4 Filling process parameters

 

Figure 5 Heat transfer coefficient

Numerical simulation results of filling process are shown in Figure 6, and casting cavity filling time is 31.56 s. As shown in Figure 6 (a), when filling time is 7.96 s, molten metal fills ingrowth under action of gas pressure, then begins to fill casting cavity. As shown in Figures 6 (b) and 6 (c), molten metal slowly fills upward in a laminar flow after passing through ingrowth. Process is smooth and orderly, and there is no obvious air entrainment. As shown in Figure 6 (d), when filling time is 30.26 s, temperature of some areas at the front of molten metal is lower than liquidus line. In order to ensure that molten metal has good fluidity during filling, pouring temperature and pouring speed need to be appropriately increased.

 

Figure 6 Numerical simulation results of filling process

Temperature field of casting at different solidification times is shown in Figure 7. Main body solidification takes 2242.42 s, and solidification process is slow. As shown in Figure 7 (a), the first solidification position is cold iron contact part and fan-shaped area on the side of casting. As shown in Figure 7 (b), when solidification process reaches 277.77 s, molten metal around top riser is already at solidus line, blocking shrinkage feeding effect of gate on the top. Combined with residual melt volume in Figure 8, it can be seen that a large isolated liquid phase area is generated in rib part of casting. After solidification, shrinkage cavities and shrinkage are prone to occur in this part, which directly affects final forming quality and mechanical properties of casting. As shown in Figure 7 (d), when solidification time is 2242.42 s, top riser has too little residual volume due to shrinkage feeding, which is easy to cause shrinkage defects inside casting.

 

Figure 7 Numerical simulation results of solidification process

 

Figure 8 Residual melt volume

According to defect location and characteristics, process improvement and optimization plan is as follows.
(1) Temperature is too low when original process is filled. Pouring temperature is increased to 700 ℃, filling pressure is increased to 42 kPa, and crusting pressure is increased to 44 kPa.
(2) Solidification speed of main body of casting varies greatly. Combined with Figure 2, it is found that number of chillers in casting is small and layout is scattered, resulting in uneven heat transfer of casting, and there are problems of local too fast or local too slow solidification. In addition, margin of top riser is too small, which is prone to shrinkage defects at corners and thick wall positions with large wall thickness differences. Optimization process takes measures to increase size and height of top riser, and at the same time, add conformal chillers at the far end of each riser and thick wall position at the bottom of casting. Thickness of chiller is twice wall thickness of contact part, which accelerates solidification speed of main body, shortens riser shrinkage compensation distance, and reduces or eliminates defects. Casting model of optimized casting is shown in Figure 9.

 

Figure 9 Casting model after optimization

It can be seen from Figure 10 that filling process of optimized casting is basically same as before optimization. Filling completion time is 16.26 s. Process is stable and there is no obvious air entrainment phenomenon. When filling is completed, temperature of molten metal is above liquidus line, indicating good fluidity.

 

Figure 10 Numerical simulation results of filling process after optimization

As shown in Figure 11, solidification time of main body of casting is shortened to 1058.22s after optimization. Combined with Figure 12, residual molten metal during solidification process of casting can be observed. It can be seen that solidification speed of each part of casting is accelerated, and no isolated liquid phase disconnected from riser or gate is generated during process. When solidification is completed, margin of each riser is normal, which is conducive to shrinkage compensation and reduces probability of shrinkage cavities and shrinkage defects.

 

Figure 11 Numerical simulation results of solidification process after optimization

 

Figure 12 Residual melt volume after optimization

In order to further verify feasibility of optimized process, optimized process was used for trial production. A356 alloy ingot was placed in a graphite clay crucible, heated and melted in a pit-type resistance furnace and heated to 740 ℃; HGJ-3 refining agent was used for refining treatment, and scum was skimmed off; then, melt was cooled to 720 ℃, Sr element (0.02%) was added to melt in the form of Al-10Sr master alloy for modification treatment, stirred and cooled to 700 ℃ for 10 min, and then surface scum was removed; casting was cast using parameters shown in Table 5.

Table 5 Trial production process parameters
After casting was filled and solidified, it was naturally air-cooled, as shown in Figure 13, and there was no obvious casting defect in appearance inspection; mechanical properties met tensile strength ≥297MPa, yield strength ≥230 MPa, and elongation ≥11%; it passed X-ray flaw detection GB/T 5677-2018 inspection standard, as shown in Figure 14, interior was good, no obvious shrinkage and shrinkage holes were found. Meet specified technical requirements.

 

Figure 13 Casting appearance

 

Figure 14 X-ray detection

Filling and solidification process of gearbox housing casting was numerically simulated by CAE simulation software, flow state and solidification trend of casting filling process were analyzed; locations where air inclusions, hot spots and isolated liquid phases may appear were predicted, corresponding process optimization was carried out to reduce probability of defects such as shrinkage cavities and shrinkage, and expected effect was achieved.
Pouring temperature was appropriately increased to 700~710 ℃, filling pressure was increased to 42kPa, crusting pressure was increased to 44kPa to ensure fluidity of metal liquid during filling process and improve filling quality. By increasing size of riser and number of chills, it is beneficial to accelerate cooling rate of hot spots of casting, enhance shrinkage compensation effect of riser, realize sequential solidification of casting, reduce probability of defects such as shrinkage cavities, shrinkage cavities and pores. Effectiveness and feasibility of above process improvements were verified through actual trial production.