Abstract
This study focuses on the defects of inclusion and shrinkage that appeared on the end face of a ductile iron reducer housing produced by lost foam casting. By increasing the allowance on the end face of the casting, the inclusion in the reducer housing was resolved. Furthermore, new heat dissipation technology and flexible chiller technology were explored, and the negative pressure during the casting process was strictly controlled to address the shrinkage hole defects in the hot spot areas of the casting. The results demonstrate that both the heat dissipation process and the flexible chiller process can effectively eliminate the shrinkage defects in lost foam casting ductile iron reducer housings. These processes are straightforward and result in high process yields.
1. Original Process of Ductile Iron Reducer Housing
Table 1. Chemical Composition of QT450-10 Reducer Housing (w%)
| Element | Composition Range |
|---|---|
| C | 3.5 – 4.0 |
| Si | 2.0 – 3.0 |
| Mn | ≤ 0.45 |
| P | ≤ 0.05 |
| S | ≤ 0.025 |
| Mg | 0.02 – 0.06 |
| RE | 0.015 – 0.040 |
Figure 1. Original Assembled Reducer Housing Pattern Showing Gating System
Description: The diagram illustrates the gating system of the original assembled reducer housing pattern.
2. Formation Causes of Inclusions and Shrinkage in Reducer Housing
2.1 Formation Cause of Inclusions
The formation of inclusions is related to the pyrolysis reaction of the foam pattern material used in lost foam casting. The pyrolysis of EPS (C8H8) and EP-MMA (C5O2H8) produces gaseous and solid products, which can lead to porosity and inclusion defects in the casting.
Table 2. Pyrolysis Reaction of EPS and EP-MMA
| Foam Material | Reaction | Gaseous Product | Solid Product |
|---|---|---|---|
| EPS | C8H8(s) → 8C(s) + 4H2(g) | H2 | C |
| EP-MMA | C5O2H8(s) → 3C(s) + 2CO2(g) + 4H2(g) | H2, CO2 | C |
2.2 Formation Cause of Shrinkage
Shrinkage defects in ductile iron castings occur when the alloy fails to receive timely liquid metal feeding during liquid contraction and solidification, especially in the hot spot areas where solidification occurs last.
3. Solutions and Verification for Defects in Reducer Housing
3.1 Solution for Inclusions
To address the inclusions, an additional 4mm allowance was added to the existing allowance on the reducer housing, resulting in a total of 8mm of allowance on the end face. This was verified through mass production, and the finished product rate increased from 88% to 97.96%.
Figure 2. Qualification Rate of Machined Reducer Housings
Description: The graph shows the improvement in the qualification rate of machined reducer housings after increasing the allowance.
3.2 Solution for Shrinkage Defects
Two new processes were developed for lost foam casting: heat dissipation and flexible chill technology.
3.2.1 Heat Dissipation Process
The heat dissipation process involves attaching foam fins to the hot spot areas of the casting. During casting and solidification, negative pressure draws cold air into the sand box, which exchanges heat with the casting and foam fins, reducing the modulus of the local area and forming a chilled zone.
Figure 3. Improved Casting Process with Heat Dissipation Fins
Description: The diagram shows the improved casting process with heat dissipation fins added to the hot spot areas.
Figure 4. Schematic Principle of Heat Dissipation Fins
Description: The schematic illustrates the principle of how heat dissipation fins work to create a chilled zone and eliminate shrinkage defects.
3.2.2 Flexible Chill Technology
Flexible chill technology uses steel shots instead of traditional chills. During the molding process, steel shots are placed in the hot spot areas and sealed with heat-resistant tape.
Figure 5. Using Flexible Chills
Description: The diagram demonstrates the use of flexible chills in the casting process.
4. Conclusion
- Inclusions: By increasing the allowance on the end face of the casting, inclusions can be concentrated within the allowance, allowing for the production of qualified castings without inclusion defects.
- Shrinkage: The heat dissipation process and flexible chill technology provide effective solutions for shrinkage defects in lost foam casting. The heat dissipation process, in particular, has been verified through mass production and has shown to meet design requirements.
This study demonstrates that with appropriate process modifications, lost foam casting can produce high-quality ductile iron reducer housings with minimal defects.