6DS180T-1702015-YDC cover is a part of operating device of a bus transmission. As shown in Figure 1, its outline dimensions are 150 mm * 304 mm * 60 mm, with an average wall thickness of 4 mm. Casting is a flat plate made of ADC12 aluminum alloy, produced using a die-casting process on a DCC1600T cold chamber die-casting machine. Sealing performance requirement is that maximum leakage should not exceed 10 mL/min under a pressure of 0.1 MPa. Since cover shell is installed on the outside of transmission and is an appearance part, its surface must not have defects such as flow marks or cold shuts.

 

Figure 1. Cover Structure
Through tracking production of 4,375 covers and analyzing scrap, a total of 280 scrap pieces were found, with a scrap rate of 6.4%. Analysis and statistics of defect types in scrap products (see Figure 2) show that cold shut defects accounted for 186 pieces (66.43%), black skin defects after processing accounted for 52 pieces (18.57%), shrinkage defects accounted for 20 pieces (7.14%), porosity defects accounted for 16 pieces (5.71%), and other defects accounted for 6 pieces (2.14%). These defects exceeded company's required defect ratio range and could not be resolved through process adjustments. Therefore, simulation analysis and optimization of mold structure and die-casting process are necessary to reduce scrap rate.

 

Figure 2 Typical defects of top cover

As core tool in die-casting production, rationality of mold structure determines quality of casting. By analyzing mold structure and working process, influence mechanism of mold structure on casting defects was derived, targeted optimization was performed. Figure 3 shows mold structure and working process. Die-casting mold working process can be simply divided into four stages.

 

Figure 3 Mold Structure and Working Process
1. Moving Mold Platen 2. Moving Module 3. Fixed Module 4. Fixed Mold Platen 5. Screw 6. Casting 7. Sprue Sleeve 8. Diverter Cone 9. Push Rod 10. Push Rod Fixing Plate 11. Push Plate 12. Moving Mold Base Plate
(1) Spraying Release Agent: Spraying release agent is a key step in die casting production, connecting mold, process, and casting quality. A uniform and appropriate amount of release agent helps to obtain castings with smooth surfaces and clear outlines. If spraying is uneven or excessive, it can lead to defects such as cold shuts, watermarks, and holes in casting.
(2) Mold Closure and Die Casting: After mold is closed, molten aluminum quickly fills mold cavity through ingate. Air and cold material in cavity are discharged through overflow system, ensuring that there are no air holes inside casting and no cold shut defects on the surface.
(3) Cooling and Solidification: After die casting is completed, molten aluminum cools rapidly in sealed cavity. Internal circulating water system controls solidification sequence, ensuring that solidification starts from position furthest from gate and proceeds towards gate. During cooling and solidification process, die-casting head maintains pressure and continuously replenishes aluminum liquid to prevent shrinkage porosity inside casting.
(4) Mold opening and part removal: After casting has solidified, moving mold retracts to set position, and die-casting machine ejection mechanism pushes casting out of moving mold through push rod. This process requires overcoming clamping force of casting on moving mold, which can easily cause deformation or even cracking of casting.

Mold temperature is an important parameter in die-casting process. In addition to spraying release agent, circulating water system is main way to control mold temperature. Circulating water system can enable die-casting mold to reach a better thermal balance state, improve sequential solidification of casting, facilitate pressure transmission, improve internal and surface quality of casting, and stabilize dimensional accuracy of casting. Temperature of moving mold during production was measured by an infrared thermometer. As shown in Figure 4, mold temperature in cold shut and dense porosity area at far end of casting was 100~132 ℃, but process requirement was that mold temperature should be (200±50) ℃, which did not meet process requirements.

 

Figure 4 Mold circulating water circuit layout and actual temperature measurement

Gating system of top cover is shown in Figure 5. Gating system is channel for molten metal to fill cavity under pressure. It plays a controlling and regulating role in flow direction of molten metal, overflow and venting conditions, pressure transmission, filling speed, mold temperature distribution and filling time. Gating system not only determines state of molten metal flow, but is also an important factor affecting casting quality. Filling process of top cover was simulated using MAGMA software. Based on experience of similar castings, simulation conditions were set as follows: 68 million grids were divided, a 1.6*104 kN die casting machine was used, pouring temperature was 660 ℃, mold temperature was 150 ℃, and casting pressure was 80 MPa. Two sets of die-casting process parameters were set for simulation. First set of parameters: high-speed injection starting point 450 mm, high-speed injection speed 3.5 m/s. Simulation results are shown in Figure 6. Second set of parameters: high-speed injection starting point 580 mm, high-speed injection speed 3.5 m/s. Simulation results are shown in Figure 7.

 

Figure 5 Top Cover Gating System

 

Figure 6: Simulation of First Filling Process
(a) 40% Filling (b) 60% Filling (c) 80% Filling (d) 100% Filling

 

Figure 7: Simulation of Second Filling Process
(a) 40% Filling (b) 60% Filling (c) 80% Filling (d) 100% Filling
MAGMA simulation analysis revealed that during filling process, due to high flow rate of molten aluminum, a slag pocket forms at far end of formed casting, preventing cold material at far end from being discharged smoothly. The smaller slag pocket in the middle also hinders effective discharge of cold material. Second group of aluminum liquid filled faster, with each gate having a similar filling speed. Encapsulation phenomenon was less pronounced than in the first group. Therefore, a high-speed injection starting point of 580 mm and a high-speed injection speed of 3.5 m/s were chosen as subsequent die-casting process parameters. Analyzing casting structure and gating system, casting was generally a flat plate with uneven gate distribution. Right side was simpler than left, resulting in a faster aluminum liquid filling speed on the right side, making it prone to encapsulation with left side's aluminum liquid at far end.

(1) Cold Shut Analysis: After measuring mold surface temperature (see Figure 4(c)), formation of cold shut defect at far end of casting was directly related to excessive release agent leading to an excessively low mold surface temperature. By adjusting release agent spraying amount and opening of cooling water flow control valve, mold temperature was brought within required process range.
(2) Analysis of Porosity: Analysis of MAGMA simulation results (see Figures 6 and 7) shows that aluminum melt coils at far end during filling, and air pressure in defect area is high. Slag collection bag at far end cannot completely expel coiled air and cold material from mold cavity, resulting in porosity defects in formed casting in this area.
(3) Analysis of Shrinkage: Combining mold's circulating water circuit layout and measured temperature in this area, large cooling water flow at far end of casting leads to excessively low mold temperature, resulting in cold shut defects. Core-pulling mechanism of mold is overheated due to lack of release agent, and core-pulling part has a large wall thickness, resulting in shrinkage defects during solidification.
(4) Analysis of Deformation: Because top cover casting has a flat plate structure with a large surface area, cooling rates differ between center and edges, between different wall thicknesses, leading to uneven thermal shrinkage and varying thermal stresses inside casting, causing deformation and resulting in black skin defects after processing.

Based on MAGMA simulation results of second set of process parameters and combined with mold cooling water channel distribution, it is proposed to either increase size of distal slag baffle to facilitate smooth discharge of cold material or optimize gate structure to make the entire filling process more stable and smooth, avoiding cold shuts. Therefore, two optimization schemes are adopted: Scheme 1: Increase size of distal slag baffle, as shown in Figure 8(a), increasing cross-sectional area of three slag baffle openings from 45.16 mm² to 61.79 mm², and increasing volume of these three slag baffles from 8064 mm³ to 15503, 15459, and 14058 mm³ respectively; Scheme 2: Increase size of three distal slag baffles while simultaneously increasing cross-sectional area of ingate from 67.9 mm² to 78.7 mm².

 

Figure 8 Optimization Scheme
Setting process parameters: high-speed injection starting point 580 mm, high-speed injection speed 3.5 m/s, simulations were performed on Scheme 1 and Scheme 2. Results are shown in Figures 9 and 10. Changing gate structure cannot effectively improve filling process of molten metal; molten aluminum still accumulates at far end, and casting in Scheme 2 has a larger air pressure in cold shut area. Comparing simulation results shown in Figures 6 and 7, changing gate carries a greater risk.

 

Figure 9 Simulation of filling process and air pressure distribution in Scheme 1
(a) 80% filling (b) 100% filling (c) Air pressure distribution

 

Figure 10 Simulation of filling process and air pressure distribution in Scheme 2
(a) 80% filling (b) 100% filling (c) Air pressure distribution
Cooling water channels marked on moving and fixed mold sides in Figure 4 were removed, and release agent spraying program was optimized to reduce amount of release agent sprayed at defect locations, increase mold temperature, thus increase molten metal temperature to eliminate cold shut defects. After measures were implemented, mold temperature was measured again. Results are shown in Figure 11. The overall temperature of moving mold increased significantly, and temperature of cold shut area increased to 155~188 ℃. Cold shut defect of casting was basically eliminated. Flaw detection results were significantly better than before improvement. Porosity and shrinkage defects were significantly improved.

 

Figure 11 Effect after implementation of scheme
(a) Actual temperature measurement of mold (b) Flaw detection of casting (c) Surface quality of casting
Cold shut is caused by formation of an oxide film in the front section of molten metal. When two molten metals with lower temperatures meet, they cannot fully fuse, resulting in fine and irregular sunken lines on the surface of casting. Due to long flow of molten metal, temperature of aluminum liquid drops significantly when it reaches far end of mold, resulting in cold shut. By increasing mold temperature, increasing size of slag collection bags in cold shut area, and increasing high speed starting point from 450 mm to 530 mm, cold shut and porosity were basically eliminated. To further verify improvement effect, processing results of 14,000 castings produced after implementation of measures were tracked and statistically analyzed. 235 castings were found to be defective, reducing the overall scrap rate to 1.68%. Porosity and black scale defects on machined surfaces were improved, as shown in Figure 12.

 

Figure 12: Casting processing results after process optimization
(a) Elimination of porosity on machined surfaces (b) Elimination of black scale defects