Introduction
The nodular cast iron DIFF case is an important component in automotive transmissions. Its casting process and quality directly affect the performance and reliability of the entire transmission system. In this article, we will introduce the casting process development of the nodular cast iron DIFF case, focusing on the prevention and control of casting defects.
Technical Requirements
The appearance of the split-type 9AT DIFF case is shown in Figure 1. The designed weight of the finished single piece is 2.32 kg, and the designed weight of the rough single piece is 3.32 kg. Unlike the conventional DIFF case design with 2 windows and 2 pin holes, this DIFF case is designed with 4 windows and 3 pin holes, and 3 asymmetric bosses are designed at the flange surface position (as shown in Figure 1). The material grade of the casting is QT600-M (enterprise standard), and the chemical composition is shown in Table 1. The dimensional tolerance and defect requirements are shown in Table 2, and the material requirements are shown in Table 3. The misalignment requirement of the DIFF case is ≤ 0.5 mm, which is relatively high. The diameter is relatively large, reaching ϕ161 mm. The thickness of the rough flange is 8.3 mm. The weight of a single piece is relatively light. Considering these factors, it is chosen to be produced on the DISA flaskless molding line.
During the development stage, the positions for metallographic, hardness, and porosity testing are shown in Figure 2, including the shaft head and the flange R corner position. At the sample stage, 100% X-ray inspection is required, and the X-ray inspection must meet ASTM E-446 ≤ level 2. At the same time, the customer will conduct CT inspection on the processed interior of the finished product, and the internal defects are required to meet the D3/1 requirement (that is, the defect area accounts for less than 3% of the maximum square area of the cross-section, and the maximum diameter of a single defect is less than 1 mm).
Table 1: Chemical Composition (mass fraction, %)
| Element | S | Si | Mn | Others | Ti | Sn | Mg | |
|---|---|---|---|---|---|---|---|---|
| Content | 0.02 | 1.8 – 3.0 | 0.2 – 1.0 | ≤0.06 | 0.2 – 1.0 | 0.06 | 0.06 | 0.027 – 0.06 |
Table 2: DIFF Case Dimensional Tolerances and Defect Requirements
| Tolerance | Casting Dimensional Tolerance/mm | Casting Contour Tolerance/mm | Defect Requirements |
|---|---|---|---|
| ISO 8062 – CT9 | 2 (±1) | 2 (±1) | Appearance Defects: Critical machining surfaces allow defects with a diameter ≤ 1 mm and a depth ≤ 1 mm, non-critical machining surfaces allow defects with a diameter ≤ 3 mm and a depth ≤ 1 mm; rough surfaces allow visible bottom hole-like defects with a diameter ≤ 4 mm and a depth ≤ 1 mm; Internal Defects: Porosity meets D3/1, X-ray inspection meets ASTM E – 446 level 2. |
Table 3: Microstructure and Mechanical Properties Requirements
| Property | Tensile Strength /MPa | Yield Strength /MPa | Elongation (%) | Hardness (HBW) | Spheroidization Rate (%) | Graphite Type | Pearlite (%) | Carbide and Phosphorus Eutectic (%) |
|---|---|---|---|---|---|---|---|---|
| Requirement | 650 | 405 | 3 | 200 – 265 | 80 | V + VI | 55 | 3 |
Process Scheme
Based on the characteristics of the thin flange design thickness, the large number of windows and pin holes, and the high requirements for internal porosity at the client end, the initial process design adopts a double-riser scheme, as shown in Figure 3. That is, a riser is set at the flange position corresponding to two pin holes on the casting. According to the weight and structural dimensions of the casting, rectangular risers are designed to improve the process yield. The edges of the rectangular risers are rounded with a large radius of R10 to reduce weight, and a shared riser is reasonably arranged in the middle position. Finally, a design scheme of 2 castings corresponding to 3 risers is adopted. Based on the multiple single-body simulation results, the size of the single-use risers on both sides is selected as 120 mm × 43 mm × 50 mm. To achieve the best feeding effect, the riser neck is designed to be maximized, removing the height of the separation stop of 0.5 mm. The design height of the riser neck is 7.7 mm, and the length of the riser neck is designed to be 55 mm, that is, the cross-sectional area of the riser neck reaches 423 mm². To reduce the weight of the single riser and meet the needs of liquid feeding, a 15 mm weight-reducing pressure groove is designed at the top of the riser, and a separation block is designed at the bottom of the riser. The height of the separation block is 15 mm. Finally, the designed single weight of the single-use risers on both sides is 2.1 kg, and the modulus is 6 mm. Similarly, the middle shared riser is designed with reference to the single-use riser, and the size of the shared riser is 120 mm × 55 mm × 50 mm, with a bottom pressure groove of 15 mm and a separation block height of 15 mm.
To improve the appearance quality of the DIFF case, the gating system adopts a lap joint design. The vertical cross runner and the horizontal cross runner are lap jointed once, and the vertical cross runners are lap jointed again through a 6 mm thin sheet. Finally, the ingate and the vertical cross runner are lap jointed again. Through multiple lap joints and the use of thin and wide runners, the effects of flow resistance and slag retention are achieved. Finally, the ingate enters the iron from the bottom of the riser, slowing down the impact of the molten iron flow velocity on the sand mold and avoiding the entry of impurities into the casting cavity during the filling process, achieving the goal of improving the appearance quality of the DIFF case.
To ensure that the shrinkage porosity at the shaft head position meets the defect requirements of D3/1, based on the actual development experience, for the shrinkage porosity and shrinkage at the isolated hot spot of the shaft head position of the DIFF case, the shaft head filling process is adopted. The shaft head does not need to be filled completely, only 60% is needed, that is, the entire shaft head height is 48 mm, and filling 29 mm can achieve the goal. To reduce the consumption of the sand core, a part of the inner cavity of the shaft head is formed by the outer mold, and the inner hole diameter is ϕ39 mm. For easy demolding, the corresponding position is designed with a 14 mm outer mold forming, and the demolding slope is designed to be 15°. The filling position is subsequently processed together with the shrinkage porosity and shrinkage through machining to achieve no shrinkage porosity and shrinkage defects at the shaft head position. Considering the concave shrinkage of the flange at the riser position, the corresponding flange position is thickened by 0.5 mm, which is also conducive to the feeding of the casting. At the same time, a cold pin is set at the pin hole position, and the size of the cold pin is ϕ6 mm × 25 mm, with a demolding slope of 3°.
The simulation results of shrinkage porosity and shrinkage defects are shown in Figure 4. The shrinkage porosity at the pin hole position is 2.6 mm³ and 11.0 mm³, the shrinkage porosity at the flange position is 0.7 mm³, and the shrinkage porosity at the blocked shaft head position can be processed away in the core. To meet the requirement of simulating the internal shrinkage porosity defect to 0, the scheme is continuously improved.
Sample Trial Production and Improvement
During the trial production using the above scheme, it is found that the pouring time is long and unstable, requiring 13 – 16 seconds, which affects the production rhythm on the spot. The samples are tested, and the appearance and dimensions are all qualified, the full-mold material and porosity tests are qualified, 100% X-ray inspection shows no internal defects, and the third-party CT inspection by the client is qualified.
In response to the problems existing in the trial production, the process is optimized: the difference between the simulated filling time and the actual pouring time is 5 – 8 seconds, which affects the production rhythm, and the process yield is relatively low, only 36.7%.
The analysis shows that the exhaust has an impact. Due to the large volume of the sand core of the DIFF case, the burning of the sand core during the pouring process will generate gas, which will affect the pouring time. The new process scheme adds an exhaust sheet in the horizontal cross runner for exhaust, and at the same time, the bottom of the exhaust sheet is made into a separation block for use.
To improve the yield, two sections of the vertical cross runner are canceled, and the horizontal cross runner is weight-reduced. The ingate directly enters the iron from the top of the riser, and the ingate entering the riser is lap jointed. The new improved process scheme is shown in Figure 7. The simulated pouring and filling time of the new improved process is 8.539 seconds, and the subsequent actual pouring and filling time in production is 10.2 – 10.3 seconds, which can meet the production rhythm on the spot. By reducing the runners and optimizing the gating system, the process yield is increased from 36.7% to 42.7%. The overall low process yield is mainly due to the large diameter and thin thickness of the casting, and risers are added for feeding to meet the internal defect requirements.
Conclusion
During the trial production, no shrinkage porosity and shrinkage defects are found in the internal X-ray inspection of the product, and the appearance is good. The casting has been mass-produced. In a certain month, 1,887 pieces were inspected for appearance, and 62 pieces were defective, with a qualification rate of 96.71%. Among them, 37 pieces had surface sand holes, with a defect rate of 1.96%; 18 pieces had bumps and scratches, with a defect rate of 0.95%; 7 pieces had unclear casting characters after shot blasting, with a defect rate of 0.37%; the processing defect rate at the client end is lower than 1%, achieving the goal of product development.
Future Research Directions
In the future, further research can be conducted on the following aspects to further improve the quality and performance of the nodular cast iron DIFF case:
- Optimization of the chemical composition and heat treatment process to improve the mechanical properties and microstructure of the casting.
- Development of more advanced simulation software and techniques to more accurately predict and control the casting process and defects.
- Exploration of new casting technologies and processes to improve the production efficiency and reduce the cost.
- Strengthening the quality control and inspection system to ensure the stability and reliability of the casting quality.
In conclusion, the development of the casting process for the nodular cast iron DIFF case is a complex and systematic project that requires comprehensive consideration of various factors. By continuously optimizing the process and controlling the casting defects, we can produce high-quality DIFF cases to meet the needs of the automotive industry.