Precision casting process of an aluminum alloy automobile steering knuckle is designed and optimized to obtain a qualified aluminum alloy automobile steering knuckle precision casting process plan. Combining structural characteristics of aluminum alloy steering knuckle castings, casting material properties and casting experience, an inner gate is opened in main body and gooseneck of steering knuckle casting, and initial pouring scheme of aluminum alloy steering knuckle is designed; optimized pouring scheme of aluminum alloy automobile steering knuckle is given by setting shrinkage feeders in the area with serious casting defects in initial process plan and adding exhaust ducts on the top of casting. Based on ProCAST software, finite element models of two pouring schemes for aluminum alloy steering knuckle precision casting are established, filling process, solidification process, shrinkage and porosity characteristics of aluminum alloy steering knuckle precision casting are numerically simulated and analyzed. Initial pouring scheme of aluminum alloy steering knuckle casting has a relatively stable and smooth filling process. An isolated liquid phase area is formed during solidification process of casting. After complete solidification, there is a large area of shrinkage defects in the middle part of casting. Optimized pouring scheme can control flow of molten metal, filling sequence and solidification characteristics. The entire solidification process of casting is basically symmetrically distributed in the middle. Final solidification area is located inside feeding riser, and maximum shrinkage rate is controlled below 2%. Design of optimized pouring scheme is reasonable and effective, can effectively eliminate defects of aluminum alloy steering knuckle castings.
With continuous improvement of global environmental protection requirements, requirements for vehicle emission standards and fuel efficiency are also more stringent. Many studies have shown that automobile fuel consumption is related to its own weight, automobile lightweighting plays an important role in reducing fuel consumption. Development and use of low-density, high-strength, and excellent performance alloy materials to replace original automobile parts is one of effective ways to achieve lightweighting. Compared with traditional materials, aluminum alloys are widely used in automotive industry due to their excellent specific strength, low density, excellent plasticity/thermal conductivity and excellent corrosion resistance. As core component of automotive steering system, automotive steering knuckle is responsible for maintaining stable driving of vehicle and quickly transmitting driving direction. Many scholars at home and abroad have conducted relevant research on the forming process of aluminum alloy automotive steering knuckles. Li Zhi et al. took A356 aluminum alloy steering knuckles as research object, combined numerical simulation with optimization algorithms to study casting process parameters of differential pressure casting, and used intelligent algorithms to obtain optimal process combination parameters. Luo Yang et al. optimized gravity casting process of aluminum alloy automotive steering knuckles, took measures such as setting reasonable runners, risers and using insulation sleeves to achieve sequential solidification of castings and basically no defects inside castings. Luo Jixiang et al. studied extrusion casting process of aluminum alloy steering knuckles and compared and analyzed characteristics of horizontal and vertical extrusion casting machines. Results showed that production of steering knuckles with horizontal machines requires conformal cooling of thick and large parts of casting to eliminate shrinkage cavities and shrinkage defects. Production of steering knuckles with vertical machines is more advantageous, with uniform mechanical properties of castings, but prone to slag inclusions and pore defects. Chen et al.  optimized design based on structural simulation and casting process simulation, obtained a high-performance aluminum alloy steering knuckle by controlling all processes of semi-solid die-casting process. Das et al. studied rheological pressure die-casting process of A356 aluminum alloy, determined optimal pouring position, temperature and conditions through numerical simulation, obtained an aluminum alloy steering knuckle with ideal microstructure and mechanical properties. Research of many scholars mainly focuses on casting process design and optimization of pressure casting and sand casting of aluminum alloy steering knuckles, but there is little research on precision casting forming process of aluminum alloy steering knuckles. Based on this, this paper takes a certain type of automobile aluminum alloy steering knuckle as research object, studies and explores its aluminum alloy steering knuckle precision casting process, in order to provide a reference for precision casting process and lightweight design of automobile aluminum alloy steering knuckle.

Three-dimensional model of automobile steering knuckle casting is shown in Figure 1a. Basic outline size is 600 mm*275 mm*163 mm. It is mainly composed of two parts: goose neck and main body, which concentrates various structural features such as shafts, sleeves, and forks. Main body includes key components such as swing arms and connecting rods. There are many precision holes of different sizes on its surface. Structure is relatively complex, cross-sectional area of each part varies greatly. Goose neck is connecting part of steering knuckle, which is used to connect steering rod and wheel, and is responsible for transmitting steering force input by driver. Therefore, this part is thicker and larger, with a simple structure, but its cross-sectional area is larger. Thickness of steering knuckle casting is shown in Figure 1b. The thickest part of casting is located at junction of gooseneck and main body, with a thickness of about 29 mm. In general, the overall structure of steering knuckle casting is complex and uneven. It is a large-scale complex structure casting. Probability of formation of defects such as shrinkage holes and difficulty of post-processing inner runner should be fully considered during process design.
Material of automobile steering knuckle is A356 aluminum alloy, which has excellent liquid fluidity, a density of 2 680 kg/m3, a solidus temperature of 561 ℃, and a liquidus temperature of 616 ℃. It has excellent filling effect when casting castings with complex geometric shapes, and has a small solidification shrinkage rate, which can effectively avoid defects in casting during solidification process.

Fig.1 Three-dimensional model and thickness analysis of castings: a) three-dimensional model of castings; b) thickness of castings

Due to large number of blind holes and through holes in main body of automobile steering knuckle casting, flow resistance of molten metal is large during filling process, wall thickness difference of this part is large and cross-section changes are many, so it is very easy to form shrinkage holes, inclusions, cracks, insufficient pouring and other defects inside casting. At the same time, considering that flow path of molten metal through goose neck of casting is long, surface layer of casting and shell have a long time of heat exchange, which causes temperature of molten metal in contact part to drop faster than inner layer, which is easy to form surface oxide film, inclusions, cold shut and other defects.
Based on characteristics of high-strength cast aluminum alloy A356, combined with structural characteristics of automobile steering knuckle castings and casting experience, initial pouring scheme of aluminum alloy steering knuckle casting is obtained as shown in Figure 2a. Initial pouring system adopts a top pouring design with a simple structure and strong filling capacity. In theory, it can reduce probability of casting defects. In addition, an inner runner is opened in goose neck and main body to reduce defects such as cold shut and inability to pour. It ensures that temperature of upper part of casting is higher than that of lower part after filling, which is also conducive to solidification sequence of casting from bottom to top, is easy to post-process and cut to clean inner runner. Casting model with pouring system is imported into ProCAST software, and mesh is divided. Mesh value is 5 mm, and finite element mesh is automatically generated. The total number of generated surface meshes is 43,850, and the total number of body meshes is 217,473. Finite element model of initial process pouring system is shown in Figure 2b.

ProCAST software was used to perform numerical simulation on casting, and casting-related process parameters were set as follows: pouring temperature of aluminum alloy is usually about 100 ℃ higher than liquidus temperature, so pouring temperature in this paper is 700 ℃, mold shell preheating temperature is set to 400 ℃, and the entire pouring time is controlled at about 5 s. Heat exchange coefficient between mold shell and pouring system, and between mold shell and casting is 900 W/(m2·K)[21]. Natural cooling is adopted, heat exchange coefficient between mold shell and air is set to 10 W/(m2·K), and ambient temperature defaults to 20 ℃.

Filling process of initial pouring scheme is shown in Figure 3. It was observed that in the early stage of filling, molten metal entered cavity from two entrates at the same time, filling speed was low and uniform. When filling rate was 50%, flow rate of molten metal in cavity could reach up to 0.93 m/s, and no obvious splashing occurred. In the middle and late stages of filling, gooseneck was gradually filled from outer end to inside, and there was a slight air entrainment phenomenon. The entire filling process was relatively stable and smooth, molten metal temperature was higher than its liquidus temperature, there was no cold shut or insufficient pouring, and filling could be completed smoothly.

 

Fig.2 Initial pouring scheme and finite element model of castings: a) initial pouring scheme; b) finite element model

 

Fig.3 Filling process simulation: a) filling 30 %; b) filling 50 %; c) filling 90 %
Filling completion time of each part of knuckle casting is shown in Figure 4. It can be seen that liquid level rise heights in different areas of casting are inconsistent, gooseneck takes the shortest time to fill, and main body fills slowly. This phenomenon occurs because main body of casting is large in volume and complex in shape, while gooseneck is relatively small in volume and simple in structure, and filling speed is fast. At the same time, considering that molten metal flows from right inner runner to the bottom of impact cavity and is diverted to both sides, it eventually causes part of molten metal to flow into top of gooseneck first, so area near top of gooseneck is filled first.

 

Fig.4 Filling time of each casting part

Solidification process of steering knuckle casting is shown in Figure 5. As shown in Figure 5a, the total solidification time is 2057.5 s. In the early stage of solidification, top of goose neck and protruding area of main body begin to solidify first. Goose neck and main body as a whole solidify sequentially from outside to inside. Until both sides solidify to vicinity of inner gate, pouring system does not begin to solidify. This solidification sequence is conducive to solidification and forming of parts outside two inner gates of casting, and parts of casting between the two inner gates solidify after pouring system solidifies. This part is easy to form an isolated liquid phase area due to loss of shrinkage compensation effect of pouring system. Last solidification area is at junction of two parts, as shown in Figure 5b. This position is likely to form shrinkage defects.

During solidification process of casting, surface of casting solidifies first to form a surface hard shell. As solidification process proceeds, gate is closed. At this time, solid shrinkage of shell is less than shrinkage of molten metal in shell. Solidification shrinkage is not supplemented by external metal liquid, so a certain degree of vacuum is formed between surface layer of casting and internal liquid metal, and finally shrinkage defects are formed. Location and probability of shrinkage defects in castings can be determined according to Porosity criterion. Predicted distribution of shrinkage defects inside casting is shown in Figure 6. There is a large area of shrinkage defects at junction of gooseneck and main body of steering knuckle casting, which is consistent with analysis results of solidification process above. Another defect is located on runner, which is removed in post-processing and does not affect casting quality.

 

Fig.5 Simulation of solidification process of castings: a) solidification time of each casting part; b) final solidification zone

 

Fig.6 Casting defect prediction

Design quality of casting system is closely related to forming quality of casting. By analyzing initial casting scheme, it is found that main reason for shrinkage cavity of casting is that junction between goose neck and main body is far away from inner gate, and inner gate solidifies before this place, so casting system cannot compensate for shrinkage in time. Combined with above analysis, initial casting process is reasonably optimized, and shrinkage compensation risers are set in the area where casting defects are more serious in initial process to improve shrinkage compensation performance of casting system. In order to achieve effect of sequential solidification, designed riser size is 83 mm*70 mm*80 mm, of which riser neck size is 40 mm*20 mm*8 mm. At the same time, two exhaust channels with a diameter of 10 mm are added at the top of casting to reduce filling resistance and ensure filling quality of casting. Optimized pouring scheme is shown in Figure 7.

 

Figure 7 Optimized pouring scheme design
Fig.7 Optimized pouring scheme design

When initial process parameters and boundary conditions remain unchanged, optimized model is imported into ProCAST for solution, and filling process is shown in Figure 8. From filling process, it can be seen that liquid level rise height in each area of casting is basically same, and metal liquid flows relatively smoothly, which is conducive to forming castings with good surface quality, as shown in Figure 8a. Air entrainment of casting is shown in Figure 8b, where dark area represents gas, and it can be seen that there is no gas in casting, indicating that no air entrainment is generated inside casting cavity, indicating that exhaust of this process is good. Slag inclusion of casting is shown in Figure 8c. During filling process, as molten metal flows into cavity, slag inclusion floats above molten metal. At the end of filling process, slag inclusion is brought into riser and exhaust rod by molten metal to avoid slag inclusion defects inside or on the surface of casting.
Solidification process of optimized casting scheme is shown in Figure 9. As shown in Figure 9a, casting solidifies from both ends to middle connection in the entire solidification process, showing symmetry. Connecting part of ingates and casting solidifies almost at the same time, and feeding riser solidifies last, indicating that feeding riser and ingates can play a certain shrinkage effect. As shown in Figure 9b, after casting solidifies as a whole, feeding riser has not completely solidified. This solidification sequence is conducive to feeding effect of riser.
Shrinkage and porosity of optimized casting scheme are predicted. Figure 10a shows the total shrinkage rate of casting at a density of 98%. From simulation results, it can be seen that there are no shrinkage and porosity defects inside casting, and defects are concentrated inside feeding riser. Fig. 10b is a cross-sectional view of casting in x-y direction. It can be seen that there is no defect in the center of feeding riser, indicating that molten metal in this area is delayed until last solidification, ensuring that there is enough molten metal to feed original defective position of casting without defects.

 

Fig.8 Filling process of optimized pouring scheme: a) filling completion time of each part; b) prediction of gas entrapment; c) prediction of slag inclusion

 

Fig.9 Solidification process of optimized pouring scheme: a) solidification time of each casting part; b) final solidification zone

 

Fig.10 Casting defect prediction of optimized pouring scheme: a) shrinkage porosity of castings; b) x-y section diagram

Combining structural characteristics, casting material properties and casting experience of aluminum alloy steering knuckle castings, reasonable design of shrinkage feeder and opening of endogate can effectively reduce probability of defect formation. An optimized casting scheme was designed by adding shrinkage-compensating risers in areas where shrinkage holes are more serious in initial process plan. Simulation analysis found that aluminum alloy steering knuckle casting with optimized casting scheme showed a trend of solidifying layer by layer from both ends of casting to middle during the entire solidification process. Defects were mainly formed in casting system and riser parts. Shrinkage rate of steering knuckle casting was controlled below 2%, which met actual production requirements of aluminum alloy steering knuckle precision casting.