Due to needs of environmental protection and energy saving, lightweight vehicles have become a development trend in automotive industry. Original production method of a certain automobile gearbox suspension end cover is aluminum alloy gravity casting. Restricted by casting process, its basic wall thickness is 15mm and its appearance is rough, making it difficult to accurately control each feature data. Assembly features require mechanical processing to ensure, resulting in low output and high cost per part. In order to meet requirements of lightweight vehicles, improve production efficiency, and reduce costs, process was reformed and die-casting was used for production.
After adopting die-casting process, wall thickness of part is reduced to 5mm, structural strength remains unchanged, subsequent machining process margin is small, production efficiency is improved, cost is significantly reduced, and dimensional control accuracy is high. However, due to its high-speed and high-pressure filling mode, gas is easily involved during filling, resulting in existence of pores and oxidized inclusions. Basic wall thickness of this part is 5mm, outline size is 194mm*190mm*72mm, and part weight is 770g. Parts requirements: dimensional tolerance is ±0.1mm, no defects on the surface, and allowable shrinkage standard refers to VW50093-5%-Ф2. For large molds, since guide posts and guide bushes are arranged at a larger distance from edge of mold to the center, expansion amount will be different when dynamic and static molds are heated under different conditions. It is required that static load is 8kN and pressure test is 72 hours, and parts have no cracks, fractures, or plastic deformation; assembly environment is simulated, and when impact load is 20kN, parts have no cracks, fractures, or plastic deformation, and number of impacts is 36 times.

Shape and structure of parts are shown in Figure 1. Size requirements of this part are high. Therefore, while ensuring accuracy of mold processing, deformation and shrinkage rate of parts during ejection during forming stage, shrinkage deformation of part itself, shrinkage rate of part, etc., must be taken into consideration, accurate judgment and prevention should be made in the early design process to avoid failure to meet the later size requirements. In view of appearance requirements of parts, it is necessary to ensure that there are no die-casting defects such as punching and straining. Porosity standards are extremely strict. If porosity does not meet expectations, part strength test will fail. On the premise of ensuring porosity, it is necessary to accurately control positions of pores and shrinkage porosity in parts to avoid shrinkage, porosity positions and pores appearing at key nodes of structural connections, which will have a great impact on the overall performance of part and make impact test of part unsuccessful. When suspension end cover is fixed at three points, force direction is 8.24°. Fixed position is shown in Figure 2, force direction and angle are shown in Figure 3. It can be seen that force of fixed point 3 of part is much higher than that of fixed point 1. No pores or shrinkage holes are allowed near fixed point 3 during X-ray flaw detection and CT inspection, and coarse grains are not allowed during slice inspection.

 

(a) Front                                           (b) reverse side
Figure 1 Part shape and basic structure

 

Figure 2 Suspension end cover fixed points

 

Figure 3 Suspension end cover stress surface and angle
After discussing plan, preliminary mold design plan was determined as follows: ① Ensure that each area is reasonably configured when filling parts, exhaust smoothly, and reduce gas involvement as much as possible; ② Considering special requirements of parts, pouring and drainage system needs to be set near fixed point 3 , ensuring that this area is filled preferentially, and at the same time, boost pressure transmission effect of runner will ensure density of this area; ③ Since injection cylinder of die-casting machine has a certain diameter, when using a small piston, pressure generated will be higher. Use a piston with a diameter of 70mm to maximize pressurization effect of die-casting machine; ④ Improved pressurization effect of die-casting machine requires higher clamping force. In order to avoid flying material phenomenon caused by insufficient mold clamping force to offset thermal expansion of mold and uneven expansion force during mold production, thermal balance and force balance need to be considered during design. Thermal balance requires that temperature of each area of mold tends to be consistent during mass production, avoiding uneven expansion caused by excessive temperature differences in each area, ensuring good contact between mating surfaces during static mold closing; force balance requires that expansion force of each area of mold tends to be consistent during dynamic die-casting to avoid unilateral stress on mold, causing parting surface to deflect slightly and cause flying materials.

 

Figure 4 Front of pouring and drainage system

Table 1 Pouring and drainage system parameters

 

Figure 5 Axial side view of upper mold waterway arrangement
Water channel arrangement of upper mold adopts a side annular cooling + water well structure, which together form a simple conformal cooling according to shape of part. At the same time, with help of annular cooling pipelines, targeted water wells are set up in thick slag bag area to control heat and ensure heat balance in each area of upper mold; water path 1 adopts an annular cooling layered design + water well structure. Incoming water first cools main channel, and each branch channel is cooled through water well; water path 2 adopts an annular cooling + water well structure to cool side of part to avoid die-casting defects such as cracks and punching caused by direct impact of gate; water channel 3 plays the role of cooling second half of parts. Since parts have different shapes, cooling wells are added to ensure even cooling of parts. Annular cooling is limited by processing and cannot ensure good temperature control in all areas. In areas where annular cooling cannot be involved, high-pressure point cooling is added to supplement cooling. Design of lower mold cooling system is shown in Figure 6.

 

Figure 6 Axial view of lower mold waterway layout

 

(a) Before modification                              (b)After modification
Figure 7 Schematic diagram of pouring system adjustment

Table 2 Parameters of die casting machine and mold pouring system

 

(a) Charging time is 0.0842s                               (b)Charging time is 0.1316s

 

(c)Charging time is 0.1558s                                (d)Charging time is 0.1809s
Figure 8 Simulation diagram of filling process
Casting material is AlSi12Cu1Fe, pouring temperature is 660℃, mold material is SKD61, preheating temperature is 120℃, and operating temperature is 200℃. Piston diameter is 70mm, low speed is 0.38m/s, high speed is 2.98m/s, and water cooling inlet temperature is 25℃. Mold is tested according to this design. Parts were inspected after subsequent machining and shot blasting. Dimensions were confirmed to be qualified and there were no defects on the surface of parts. When CT equipment and X-ray flaw detection were used, pores in some areas of parts exceeded specified requirements. Main locations of pores are shown in Figures 9 and 10. Since this is location of gate, stroke of die-casting machine will have a greater impact. According to communication with site, after die-casting process was adjusted, porosity has improved, but it still cannot meet requirements, see Figure 11.

 

Figure 9 Position of air hole area of part

 

Figure 10 Schematic diagram of location of air hole area of part

 

Figure 11 Close-up of location of air hole area of part
Based on test results and combined with on-site conditions, discussion and analysis concluded that following problems mainly existed.

During die-casting, it was found that aluminum stuck to core, parts were strained, and hole size did not meet requirements, so injection speed and injection pressure were reduced during die-casting. Simulation of impact core at feed port is shown in Figure 12.

X-ray flaw detection shows that location of shrinkage holes is back area of mold core. Mold flow analysis also shows that mold core blocks flow of aluminum liquid, causing obvious insufficient pouring in back area, which may lead to shrinkage and porosity. Simulation of shrinkage cavities forming and core blocking flow of aluminum liquid is shown in Figure 13.

Wall thickness of this part exceeds 15mm, and filling dead space on part structure causes air entrapment. Even on gate side, pressurization effect of die-casting machine still cannot guarantee density here.

This area is the area with the thickest wall thickness of part. It may be due to incomplete internal cooling, which causes inside of part to continue to cool and shrink after side wall of part is cooled, forming shrinkage cavities. According to mold flow analysis results, this area is indeed final cooling area. Analysis results are shown in Figure 14.

 

Figure 12 Simulation of direct impact of feed port on core

 

Figure 13 Simulation of core blocking flow of aluminum liquid

 

Figure 14 Mold flow analysis cooling sequence

 

Figure 15 Working status of extrusion pin

 

Figure 16 Extrusion pin structure
After above scheme was adjusted, it was verified by mold flow analysis. Verification results are shown in Figure 17. Simulation analysis results show that after process modification, liquid phase distribution during solidification process of casting is more reasonable, and probability of shrinkage defects is significantly reduced. However, tendency of shrinkage defects in some thick and large parts is still high (see Figure 17). Therefore, in subsequent die-casting production, cooling water flow rate is accelerated, a cooling tower is added to cooling cycle, and cooling water temperature is controlled. Then test mold again. During mold test, following measures are taken to ensure that actual production is as close to simulation effect as possible: ① Strictly control production process. After problem of aluminum liquid impacting core is solved, speed and slow injection parameters of die-casting machine must comply with simulation results and only be fine-tuned based on on-site conditions; ② Separate assembly control of spot cooling in slower cooling areas, strictly control inflow water temperature, and increase water flow in these areas; ③ Die-casting machine is equipped with an extrusion pin structure, and local extrusion technology is used during die-casting .

 

(a) Liquid limit distribution at a certain time                 (b) Prediction of hole defects
Figure 17 Mold flow analysis and verification after modification of pouring and drainage system

 

(a) Front   (b) Reverse side
Figure 18 Qualified parts
After modifying process and mold, parts were tested again and passed CT inspection. It has passed load test and is in normal mass production.