Abstract: This study focuses on addressing the wrinkling and shrinkage defects encountered in the production of ductile iron reducer housings using lost foam casting. By optimizing the pouring system from top pouring to bottom pouring, the turbulent flow of molten iron during pouring is effectively minimized, resolving the issue of surface wrinkling. Furthermore, a novel heat dissipation technology is explored to tackle the shrinkage defects in the hot spots of the casting. The test results demonstrate that switching to a bottom pouring system completely eliminates surface wrinkling defects, while the new heat dissipation process resolves shrinkage defects, ensuring a straightforward process with high production yields.
1. Introduction
Lost foam casting, renowned for its high surface quality, dimensional accuracy, and high process yield, is ideal for producing ductile iron components. The reducer housing, which demands high strength, toughness, wear resistance, and shock absorption, is manufactured from QT450-10 material. With a weight of 112 kg and wall thicknesses ranging from 54 mm at the thickest point to 14 mm at the thinnest, the reducer housing presents a challenging geometry with concentrated hot spots. Based on the equipment capabilities and casting characteristics, lost foam casting was selected as the production method.
2. Original Casting Process and Defect Analysis
2.1 Original Pouring System
The original pouring system for the reducer housing featured a top-pouring design, with a process allowance of 4 mm on the end face. The tapping temperature ranged from 1,580 to 1,600°C, and the pouring temperature was between 1,370 and 1,440°C. The pouring was conducted under a negative pressure of -0.06 to -0.04 MPa, maintained for 900 seconds. The chemical composition of the QT450-10 reducer housing is outlined in Table 1.
Table 1. Chemical Composition of QT450-10 Reducer Housing (Mass Fraction, %)
| Element | C | Si | Mn | P | S | Mg | RE |
|---|---|---|---|---|---|---|---|
| Content | 3.5~4.0 | 2.0~3.0 | ≤0.45 | ≤0.05 | ≤0.025 | 0.02~0.06 | 0.015~0.04 |
Despite meeting the mechanical properties and spheroidization requirements, initial lost foam casting trials of the reducer housing exhibited defects such as wrinkling, porosity, and shrinkage. During small-batch production, a significant proportion of castings showed wrinkling on the end face and shrinkage defects in the geometrically hot spots.
2.2 Formation Causes of Wrinkling and Shrinkage
2.2.1 Wrinkling
Carbon defects are common in ductile iron castings with a carbon content typically ranging from 3.5% to 3.8%. Considering both gas evolution and carbon content, copolymer material was chosen for the foam pattern. This copolymer balances the gas evolution and solid carbon content of EPS and EPMMA, reducing the likelihood of carbon defects and porosity.
Wrinkling defects, appearing as an orange-peel texture on the cleaned casting surface, are often found in the last areas to be filled by molten metal or in “cold ends.” These defects typically occur on the top, vertical sidewalls, or dead-end areas of castings. In the case of the reducer housing, the original top-pouring design led to a turbulent filling pattern, creating cold ends on the top and side thick sections, resulting in wrinkling.
2.2.2 Shrinkage
The shrinkage defects in the reducer housing were primarily located in thick areas with machined holes, caused by geometric hot spots. The fundamental reason for shrinkage in ductile iron castings is the inability of the alloy to receive timely feeding of liquid metal during liquid contraction and solidification, resulting in irregularly shaped, rough-walled cavities.
3. Solutions and Verification
3.1 Solution for Wrinkling
The wrinkling defect in the reducer housing was attributed to the不合理的设计 of the pouring system, which created a turbulent filling pattern resembling a bottom-up fill from the middle. This caused wrinkling on the surface due to the turbulent flow of molten iron.
To address this, the pouring system was redesigned to a bottom-pouring configuration, ensuring a smooth, bottom-up filling process. This allowed the lower temperature molten iron and incompletely gasified foam pattern residues to accumulate in the process allowance at the top, producing castings with a flawless surface.
Theoretical calculations were conducted for the pouring system, including pouring time, average static pressure head height, and minimum cross-sectional area of the ingate. Based on empirical data, the minimum cross-sectional area of the ingate was set to between 3.5 and 12 cm², with each of the four ingates having a cross-sectional size of 70+1 mm × 40 mm, totaling 11.2~12.8 cm². The sprue length was designed as 480 mm with a head pressure of 200 mm.
After implementing the new casting process, 2,000 castings were produced for verification. The results showed that the surface quality of the castings was satisfactory, with no batch-wise wrinkling defects.
(a) Before Optimization (Wrinkled Casting) (b) After Optimization (Qualified Casting)
3.2 Solution for Shrinkage
Traditional methods to address shrinkage defects include the use of risers to provide compensating metal during solidification and chill systems to create a temperature gradient, promoting sequential solidification. However, these methods are not feasible for lost foam casting of reducer housings due to decreased process yield and increased complexity.
A new heat dissipation technology was developed specifically for lost foam casting. This technology involves attaching foam sheets (“heat sinks”) to the hot spots of the casting. During pouring and solidification, negative pressure drawing continuously pulls cold air through the sand box, enabling heat exchange between the casting, heat sinks, and surrounding sand.
The large surface area of the heat sinks reduces the local modulus of the casting, and the cold air removes significant heat, creating a microchannel flow heat exchange with the casting and heat sinks. This forms a large temperature difference, mimicking the effect of chill, resulting in sequential solidification and eliminating shrinkage and porosity defects.
(a) Planar Heat Dissipation (b) Heat Dissipation with Heat Sinks
Initial castings produced with the original process exhibited shrinkage defects in the bolt holes. Analyzing the casting structure revealed that these defects originated from geometric hot spots, consistent with the observed defects. Applying the heat sink process involved attaching 12 heat sinks, each measuring 50 mm × 30 mm × 7 mm, to the hot spot areas. Post-machining inspection confirmed that the bolt holes were defect-free. Subsequent batch production of 2,000 reducer housings showed no recurrence of shrinkage defects in the bolt holes.
4. Conclusion
This study successfully addressed the wrinkling and shrinkage defects in ductile iron reducer housings produced by lost foam casting. By optimizing the pouring system to a bottom-pouring configuration, turbulent flow during pouring was minimized, and surface wrinkling was eliminated. For shrinkage defects, a novel heat dissipation technology using heat sinks was developed, effectively managing the localized heat dissipation and resolving geometric hot spots. The heat sink process has been validated through practical applications, demonstrating its efficacy in solving shrinkage defects while maintaining high process yields. This represents a significant advancement in addressing defects in lost foam casting.