Automotive steering gear castings studied are formed from ADC12 aluminum alloy, with brand name YZAlSi11Cu3, and its chemical composition is shown in Table 1.

Table 1 Chemical composition of ADC12 aluminum alloy (mass fraction)
Figure 1 shows structure of casting. Casting is a long tubular shape. Diameter of tube body is φ53.5 mm and length is 557 mm. Wall thickness of casting is uneven. Structure of two ports is relatively complex. There are through holes with different diameters and different spatial directions. There are two nozzles on the right end that are perpendicular to each other. The larger diameter through hole on the right end is connected to main pipe hole. Lateral core pulling mechanism is required in many places during subsequent mold design. In order to ensure that subsequent design process proceeds smoothly, UG software is used to analyze castings.

 

Figure 1 Casting structure
Through UG software using rolling ball calculation method, Figure 2 shows casting wall thickness cloud chart. Measured average wall thickness of casting is 4.88 mm, and maximum wall thickness is 21.72 mm, which is consistent with actual thickness of casting. Measured volume of casting is 5 496.72 cm3 and weight is 4.3 kg. There are no special surface quality requirements.

 

Figure 2 Casting wall thickness cloud chart

Casting is a long tube with complex shapes of two ports and thick neck wall, which makes it difficult to form. Injection pressure ratio during die casting is 85 MPa. In order to avoid shrinkage holes and cracks on the surface and inside of casting due to excessive shrinkage during cooling, casting temperature is 670 ℃ and mold preheating temperature is 150 ℃. In order to make internal structure density of casting higher and achieve expected mechanical properties, a holding time of 5s is selected, and filling speed is approximately 0.22 seconds.
DCC800 cold chamber die-casting machine is selected as die-casting equipment, and its main technical parameters are shown in Table 2.

Table 2 DCC800 cold chamber die casting machine parameters

As shown in Figure 3(a), according to structure of casting, mold parting surface is linear, with upper half being a fixed mold core and lower half being a movable mold core. Parting mold here makes mold structure simpler, arrangement of pouring system and selection of internal gate position are also more convenient, exhaust conditions of cavity and effect of overflow system are also better. Blank space shown in Figure 3 (b) is a slider. From Figure 3 (c), it can be seen that its parting surface is not completely flat, but has a curved surface. Because cavity structure here is relatively complex, it is difficult to demold with a flat parting. Changing it to a curved parting here is not only conducive to demolding, but also reduces pressure of molten metal flowing through this place during filling, so that molten metal can better fill complex cavity structure of this port.

 

Figure 3 Mold parting surface

Combined with mechanical characteristics of casting, mold is designed as a 1-cavity structure. Material shrinkage rate is 0.55%. Casting has a deep cavity in axial direction. A core pulling mechanism needs to be set. A core pulling mechanism is set at each end, pulling stroke of left core pulling mechanism is 525 mm, and pulling stroke of right core pulling mechanism is 110 mm. An inclined core pulling mechanism is also required at the right end, and pulling stroke of inclined core pulling mechanism is 120 mm. Position distribution of core pulling mechanism is shown in Figure 4.

 

Figure 4 Position distribution of core pulling mechanism

Side gate pouring system is adopted and arranged on outer side of casting according to structural characteristics of casting. In this way, mold structure can be kept compact and thermal balance can be maintained, and gate condensate can be easily removed. Select inner gate type as side gate, with a total of 4 gates. Sprue is primary part for transmitting pressure and is the key to smoothly introducing molten metal into lateral runner and controlling filling conditions of molten metal. Connecting channel from the end of sprue to the front end of ingates is cross runner. Its function is to introduce molten metal from sprue into inner gate, preheat mold with the help of molten metal in cross runner, to compensate for shrinkage and transfer static pressure when casting cools and shrinks. Gate position is shown in Figure 5.

 

Figure 5 Gating system
Die-casting machine used for castings is a horizontal cold chamber die-casting machine. In order to ensure smooth demoulding of castings, inclination of sprue is set to 10°. Its structure is shown in Figure 6.

 

Figure 6 Sprue for horizontal cold chamber die casting machine

Overflow system consists of an overflow tank and an exhaust tank, which can enable molten metal to promptly discharge gas, inclusions and other impurities in mold cavity during process of filling mold, which is conducive to filling and solidification, reduces and prevents occurrence of pore defects in casting. It is produced to ensure quality of castings and can also make up for some defects caused by unreasonable design of gating system. Place multiple slag bags at complex shape of left port, and place slag bags evenly on remaining pipe body. Arrangement model of slag bags, overflow tank and exhaust slot is shown in Figure 7. An exhaust channel is opened at the tail of overflow groove to enhance overflow and exhaust effect.

 

Figure 7 Overflow system

Cooling system design is shown in Figure 8, which is distributed on fixed mold and movable mold along position of casting perpendicular to parting surface. There is a cooling water path at each end of nozzle. In order to better cool depth of nozzle cavity, there is a parallel cooling water path on both sides of sprue. There is a cooling water path parallel to parting surface on fixed mold, and there is only a cooling water path perpendicular to parting surface on movable mold. Reason is that casting on the side of movable mold has a complex shape and a protruding nozzle, which is not suitable for arranging parallel water paths. .

 

Figure 8 Cooling system

Assemble designed mold base, core, sprue sleeve, diverter cone, support column, pad, push rod, support plate, push plate and core pulling mechanism. The overall assembly of mold is shown in Figure 9.

 

Figure 9 Overall assembly of mold

Method of combining actual production and numerical simulation is used to determine reasonable process parameters. MAGMAsoft casting simulation software is an efficient optimization tool for improving quality of metal castings, optimizing process conditions and reducing production costs. It can determine existing casting materials and casting processes on the market. Appropriate process parameters and optimal casting process conditions play an important role in actual production process of die-casting process. In following, MAGMAsoft software is used to simulate casting process of automobile steering gear, and particle tracking function of filling process in software is used to obtain streamlines, flow velocity and other information in flow field.

(1) Analysis of filling process. Casting filling process was analyzed, and material traces were observed from the moment molten metal entered gate until cavity was completely filled. As shown in Figure 10, casting filling process was smooth and cavity was fully filled.

 

Figure 10 Filling process

(2) Analysis of molten metal flow velocity. During die-casting process from beginning of mold filling to the end of mold filling, flow velocity of molten metal at four gate positions was analyzed, as shown in Figure 11. At the beginning of filling, metal flow rates at four gates are 60.560, 76.527, 97.232, and 84.424 m/s, respectively. Metal flow rate at each gate is relatively high because cavity is empty at the beginning of filling, resistance of metal liquid entering cavity is small, and metal liquid is injected into cavity at high pressure, so flow rate is relatively fast. At the end of mold filling, flow rates of molten metal at four gates corresponding to beginning of mold filling dropped to 24.514, 20.345, 47.890, and 46.026 m/s respectively. At the end of mold filling, inside of mold cavity has been fully filled, and resistance of molten metal entering mold cavity at the gate is large, resulting in a decrease in flow rate of molten metal.

 

Figure 11 Metal liquid flow rate

(3) Analysis of solidification process. It can be seen from Figure 12(a) that when solidification begins, temperature reduction part first starts from position of exhaust plate and extends along direction of exhaust groove and overflow groove. Solidification direction is opposite to flow direction of molten metal during filling. As can be seen from Figure 12(b), tube body located in the middle of cavity solidifies first. Wall thickness here is small and uniform, so solidification is faster. After solidification of casting pipe body is completed, two ports have not yet solidified. Unsolidified volume on the left side is larger and volume on the right side is smaller. Shape of pipe mouth at the two ports is more complex. In order to make casting meet strength requirements, wall thickness is thicker. As a result, heat dissipation in these two places is uneven and they are final solidification parts. Hot joints may appear in these two places, leading to shrinkage cavity defects. Subsequent analysis of hot joints and shrinkage cavities is required.

 

Figure 12 Casting temperature distribution

(4) Defect analysis. A hot spot is a node or local area in casting that solidifies slower than surrounding metal during solidification process of molten metal. It is also final part to cool and solidify. Defects such as shrinkage cavities and shrinkage cracks will appear in hot joints, causing strength of casting to decrease and fail to meet usage requirements. As can be seen from Figure 13, there are hot spots at through hole of left port, and shape of right port is complex, with multiple hot spots appearing. There are many factors that affect thermal node of castings: geometric factors such as structure, shape, size of casting, shape and size of casting mold; process factors such as pouring temperature, pouring time, type and position of pouring and riser system; thermal factors such as path and flow rate of molten metal in casting mold. As shown in Figure 14, hot spot causes two small shrinkage holes on the left and right ports, which affects the overall structural strength of casting and requires further optimization of mold structure. Because shrinkage hole defects are relatively small, there is no need to optimize the overall structure of mold. It is planned to add cooling water channels at location where hot spot appears on core to accelerate solidification and cooling of casting in this area, then use MAGMAsoft to test optimized results.

 

Figure 13 Hot spot analysis

 

Figure 14 Shrinkage cavity analysis

Structure of mold cooling system is optimized, as shown in Figure 15. For location where hot node appears on the left end, a cooling water path perpendicular to parting surface is arranged on fixed mold, which is close to hot node. For location where hot section appears on the right end, so range of hot section is large, two more cooling water channels perpendicular to parting surface are arranged on movable mold, one goes deep into side nozzle of tube body, and the other is close to hot section. After optimization, numerical simulation was performed, and no defects such as shrinkage cavities were found.

 

Figure 15 Cooling system optimization
In order to further verify whether optimized die-casting process plan and mold structure are feasible, casting trial production was carried out based on optimized process parameters and molds through numerical simulation. Trial casting entity is shown in Figure 16. After inspection, left and right ports of casting were fully filled. There are no defects such as shrinkage holes internally and on the surface, the overall structural strength and performance meet usage requirements, indicating that optimized process parameters are reasonable, gating system and cooling system are reasonably designed. Numerical simulation of die-casting process has greatly shortened product development cycle, saved a lot of manpower and material resources, and provided an effective reference for actual production of products.

 

Figure 16 Two ports of casting