The appearance of the split-type 9AT differential housing is shown in Figure 1. The designed weight of a single finished product is 2.32 kg, and the designed weight of a single rough casting is 3.32 kg. Unlike the conventional differential housing design with 2 windows and 2 pin holes, this differential housing is designed with 4 windows and 3 pin holes, and at the same time, there are 3 asymmetric bosses at the flange surface position (as shown in Figure 1). The casting material is QT600-M (enterprise standard), 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 this differential housing is ≤ 0.5 mm, which is highly demanding; 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 various factors, it is chosen to be produced on the DISA boxless molding line.
In 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; and at the sample stage, 100% X-ray flaw detection is required, and the X-ray flaw detection must meet ASTM E-446 ≤ level 2. At the same time, the customer will conduct CT detection on the internal part of the processed finished product, and the detection requires that the internal defects 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).
Technical Requirements
Table 1: Chemical Composition (mass fraction, %)
| Element | C | S | Si | Mn | P | Ti | Sn | Mg |
|---|---|---|---|---|---|---|---|---|
| Content | 3.3 – 3.9 | 0.02 | 1.8 – 3.0 | 0.2 – 1.0 | ≤0.06 | 0.2 – 1.0 | 0.06 | 0.027 – 0.06 |
Table 2: Differential Housing Dimensional Tolerances and Defect Requirements
| Casting Dimensional Tolerance / mm | Casting Contour Tolerance / mm | Defect Requirements |
|---|---|---|
| ISO 8062 – CT9 | 2 (±1) | Appearance Defects: On key processing surfaces, defects with a diameter ≤ 1 mm and a depth ≤ 1 mm are allowed; on non-key processing surfaces, defects with a diameter ≤ 3 mm and a depth ≤ 1 mm are allowed; on rough surfaces, visible hole-like defects with a diameter ≤ 4 mm and a depth ≤ 1 mm are allowed; Internal Defects: Porosity meets D3/1, X-ray flaw detection meets ASTM E – 446 level 2. |
Table 3: Microstructure and Mechanical Properties Requirements
| Tensile Strength / MPa | Yield Strength / MPa | Elongation (%) | Hardness (HBW) | Spheroidization Rate (%) | Graphite Type | Pearlite (%) | Carbide and Phosphorus Eutectic (%) |
|---|---|---|---|---|---|---|---|
| 650 | 405 | 3 | 200 – 265 | 80 | V + VI | 55 | 3 |
Process Plan
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 individual simulation results, the size of the single riser on both sides is 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 reduction 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 design weight of the single riser 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 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 differential housing, the gating system adopts a splicing design. The vertical cross runner and the horizontal cross runner are spliced once, and the vertical cross runners are spliced again through a 6 mm thin sheet. Finally, the ingate and the vertical cross runner are spliced for the third time. Through multiple splices and the use of a thin and wide runner, the effects of flow resistance and slag retention are achieved; finally, the ingate enters the iron from the bottom of the riser to slow down the impact of the molten iron flow velocity on the sand mold and avoid impurities from entering the casting cavity during the filling process, achieving the goal of improving the appearance quality of the differential housing.
To ensure that the shrinkage porosity at the shaft head position of the differential housing meets the defect requirements of D3/1, based on the actual development experience, for the shrinkage porosity at the isolated hot spot of the shaft head position of the differential housing, the shaft head filling process is adopted. The shaft head does not need to be completely filled, 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 by machining to achieve no shrinkage porosity defects at the shaft head position. Considering the concave shrinkage at the flange position of the riser, 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 needle with a size of ϕ6 mm × 25 mm is set at the pin hole position, and the demolding slope is 3°.
The simulation results of shrinkage porosity 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 plan is continuously improved.
The casting is rotated by 90°, and two of the three bosses of the rotated casting are close to the riser position, and the riser is no longer directly facing the pin hole position of the casting, but facing the window position. At the same time, the cold needle is canceled, and the adjusted scheme is shown in Figure 5, and the simulation result is shown in Figure 6. It can be seen from Figure 6 that there is no shrinkage porosity at the pin hole and other positions of the casting, and the shrinkage porosity at the blocked shaft head position is 77.8 – 80.00 mm³, which can be removed by machining.
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 of the on-site molding. The samples are tested, and the appearance and dimensions are all qualified, the full mold material and porosity detection are qualified; 100% X-ray flaw detection shows no internal defects; the third-party CT detection 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 low, only 36.7%.
The analysis shows that the exhaust has an impact. Due to the large volume of the sand core of the differential housing, 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 for exhaust in the horizontal cross runner, 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 spliced. 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 on-site production rhythm. By reducing the runner and optimizing the pouring 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. To meet the internal defect requirements, risers are added for feeding.
Conclusion
During the trial production, no shrinkage porosity defects are found in the internal X-ray flaw detection 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, 62 pieces were rejected, and the qualification rate was 96.71%. Among them, 37 pieces had surface sand holes, with a rejection rate of 1.96%; 18 pieces had collision damage, with a rejection rate of 0.95%; 7 pieces had unclear casting characters after shot blasting, with a rejection rate of 0.37%; the rejection rate at the client end during processing is less than 1%, achieving the goal of product development.
In summary, through the development and improvement of the casting process for the differential housing, the requirements for internal defects and appearance quality are met, and the production efficiency and yield are also improved. This process demonstrates the importance of continuous optimization and innovation in the casting process to meet the increasingly high requirements of the industry.