Abstract: Shape of truck fuel tank bracket is relatively complex and wall thickness is uneven. In die-casting process, if pouring system of mold is not designed reasonably, defects such as air entrainment, pores and cold shut are likely to occur. In view of above situation, pouring system of die-casting mold of truck mailbox bracket was designed, filling process was simulated, and die-casting process scheme was optimized. Production practice shows that optimized scheme reduces casting defects and scrap rate, which has certain engineering application value for improving casting quality and qualified rate.
In recent years, with continuous improvement of global environmental awareness and rapid development of automobile industry, society has paid more and more attention to lightweighting of automobiles. Through initial optimization of product structure, we found that die-casting process can significantly reduce wall thickness of product while ensuring structural strength, further improving lightweighting level of automobile. Die casting is a high-efficiency, high-quality and high-precision metal forming method. High-pressure casting has the highest production efficiency among many die-casting methods. Parts obtained under this process have excellent surface quality, highly precise dimensions and excellent mechanical properties. In die-casting process design, pouring system is extremely important for filling process and solidification process of molten metal. Whether design is reasonable directly affects quality of casting. However, due to its high-speed and high-pressure filling method, it is easy to involve gas in filling process, resulting in frequent pores and oxidation inclusions in die-casting. Therefore, design of die-casting pouring system is extremely important.
In order to obtain good die-casting quality, reasonable design of pouring system and overflow system is the key. Use of CAE technology to assist in optimization design of pouring system has become main means of current molding process design. This paper focuses on die-casting of aluminum alloy fuel tank bracket, carries out optimization design of pouring system, obtains a reasonable process plan, and provides a basis for mold structure design.

Three-dimensional structure of fuel tank bracket is shown in Figure 1. Maximum outline size is 720mm×640mm×120mm, main wall thickness is 4.3mm, maximum wall thickness is 18.1mm, structure is symmetrical, and blank mass is 5930g.

 

Sealing parts of casting must be completely free of defects such as cracks, looseness or bubbles, and surface is strictly prohibited from having problems such as cold shut, scratches, pores or inclusions. Alloy material is ADC12 aluminum alloy, and chemical composition is shown in Table 1.
Table 1 Chemical composition of ADC12 alloy

 

Reasonable setting of parting surface can optimize structural design of mold, improve production efficiency, and ensure performance of casting. Since casting has a complex structure and grooves, special attention should be paid to selection of parting surface to ensure that casting can be completely retained in moving mold. Maximum projection surface is used as parting surface, a slider insert is added to groove to facilitate removal of casting from mold, which is conducive to design of pouring system and overflow system. Using this method of parting can reduce damage of casting when demolding, thereby ensuring quality of casting. Parting scheme is shown in Figure 2.

 

A one-mold-one-cavity pouring system is adopted. Analyze main body contour and size of part, consider influence of length of aluminum liquid filling stroke, try to ensure that aluminum liquid does not directly hit core, ensure that it is filled in an orderly manner after entering cavity to ensure performance of casting and reduce possibility of pore defects. According to structure of casting, pouring system is consistent with parting surface, and inner gate is set at the center of casting to simplify mold structure. Cross-sectional area of inner gate is calculated according to formula (1).

 

Where: Ag is cross-sectional area of inner gate; G is weight of metal liquid flowing through inner gate (including part and overflow groove), which is 5.9kg; ρ is density of metal liquid, which is 2.7g/cm3; t is pouring time, which is 0.05s; V1 filling speed, which is 40m/s.
Table 2 Experience table of gate thickness of aluminum alloy castings

According to formula 1, Ag is calculated to be 1092mm2. Structure of aluminum alloy bracket is relatively complex. Referring to data in Table 2, thickness of gate is 1.5mm to 2.5mm. Wall thickness of some areas of casting is greater than 6mm, and internal quality requirements are relatively high, so gate thickness range is 2.4mm.

 

Based on theory and experience of gating system design, a high-pressure casting pouring system scheme is formulated as shown in Figure 3. Overflow trough size is: 45mm long, 25mm wide, and 15mm thick. It is arranged at the end of metal liquid filling and place where gas is easily rolled up in preliminary judgment, which can discharge gas in cavity, store contaminated metal liquid, control filling direction of metal liquid, reduce cold shut and insufficient pouring of casting.
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Grid size of metal liquid flow area is 2mm, and grid size is 1mm in thin wall such as gate. The total number of grids is 4.3 million, ensuring that at least two layers of grids are divided at the thinnest part of part. Casting material is ADC12 die-casting aluminum alloy, and mold material is H13. Pouring temperature is set to 650℃, and mold preheating temperature is 200℃. After calculation, slow pressure speed is 0.25m/s. When aluminum liquid reaches inner gate, lifting speed is 4.2m/s, and conversion time is 0.015s.

Figure 4 shows filling process of initial pouring scheme. It can be seen that aluminum liquid is filled in sequence after being injected into mold cavity. Aluminum liquid is sprayed upward along side wall and to both sides. Aluminum liquid on both sides fills middle and distal ends of casting at almost the same time. At 0.41s, cavity is completely filled with aluminum liquid.

 

During the entire filling process, aluminum liquid is carried out in an orderly manner from pouring system to cavity, proving that pouring and overflow system are properly designed. Figure 5 shows air entrainment in simulated casting. It can be clearly observed that air entrainment in the middle part and one side of casting is more serious: reason is that exhaust of aluminum liquid is not sufficient, and entrained air is not removed, resulting in pore defects. Reason for this situation is that exhaust of aluminum liquid is not sufficient, resulting in ineffective removal of entrained air, thus forming pore defects. Although overflow grooves are set at the far end and upper edge of casting, effect is not as expected, resulting in a high gas content in casting. Therefore, it is necessary to improve initial process plan to a certain extent to improve exhaust efficiency.

 

In order to remove gas in cavity as much as possible, according to characteristics of casting and air entrainment, overflow grooves are added to the far ends on both sides and middle part of casting. Optimized overflow system is shown in Figure 6.

 

Figure 7 is a simulation diagram of filling process after process improvement. It can be seen that aluminum liquid basically achieves sequential filling after entering cavity, and overflow groove is the last part to be filled. At 0.35s, aluminum liquid reaches gate and flows away from gate along bracket wall and reinforcement rib; at 0.37s, aluminum liquid on both sides fills middle and far ends of casting at almost the same time; at 0.39s, the entire casting is filled with aluminum liquid, leaving only overflow trough unfilled. Overflow trough is filled last, indicating that setting of overflow trough is reasonable; at 0.41s, filling is completed.

 

Compared with initial scheme, filling sequence is basically same, but more stable, and temperature difference of casting is also reduced. It can be seen from Figure 8 that distribution of entrained air is improved in optimized casting. After adding overflow trough, amount of entrained air in casting is significantly reduced, and almost all of it is transferred to the inside of overflow trough, which shows that improvement scheme is effective. Under optimized scheme, amount of entrained air in casting is greatly reduced, and impact on quality of casting is almost negligible.

 

According to simulation optimization results, die casting mold was designed and manufactured, and die casting production was carried out using process parameters described above. Figure 9 shows actual die castings produced. Casting surface is smooth, without defects such as pores, shrinkage, slag inclusions and deformation.

This paper analyzes structural characteristics of castings, designs initial pouring system and overflow system, analyzes flow and air entrainment sequence of filling results with assistance of numerical simulation technology, effectively identifies defect risks of actual production of product, optimizes pouring system or mold structure in time to reduce or even avoid and solve defect risks. According to improved design scheme, a feasible die-casting mold was designed and manufactured, and a reliable high-pressure casting process was developed to effectively ensure stability of production. At the same time, while ensuring quality of castings, number of scraps was successfully reduced, and production costs were saved.