This article focuses on the wrinkling and shrinkage cavity defects that occur during the lost foam casting process of ductile iron reducer housings. It analyzes the causes of these defects in detail and presents effective solutions. The research findings have important guiding significance for improving the quality of lost foam castings and optimizing the casting process.
1. Introduction
Lost foam casting is a modern casting method with many advantages, such as good surface quality, high dimensional accuracy, and high process yield. It has been widely used in the production of various castings, including ductile iron reducer housings. However, during the production process, certain defects may occur, which affect the quality and performance of the castings. This article aims to study and address these issues.
2. Ductile Iron Reducer Housing and Lost Foam Casting Process
2.1 Ductile Iron Reducer Housing Characteristics
The ductile iron reducer housing requires high strength, toughness, wear resistance, and shock resistance. It has a specific chemical composition and mechanical properties to meet these requirements. For example, the material is often QT450 – 10, with a certain range of element contents as shown in Table 1.
| Element | Content Range |
|---|---|
| C | 3.5 – 3.8% |
| Si | 2.0 – 3.0% |
| Mn | ≤0.45% |
| P | 0.05% |
| S | 0.025% |
| Mg | 0.02 – 0.06% |
| RE | 0.015 – 0.04% |
The reducer housing has a certain weight (e.g., 112 kg) and varying wall thicknesses, from 14 mm to 54 mm, with geometric hot spots being relatively concentrated.
2.2 Lost Foam Casting Process Overview
Lost foam casting involves several key steps. First, a foam pattern is created using a suitable material. In this case, for ductile iron, a copolymer material is often chosen to balance the gas evolution and carbon content. The foam pattern is then coated, dried, and placed in a sand box. During pouring, the molten metal replaces the foam as it vaporizes, and a negative pressure is maintained to ensure proper filling and solidification.
3. Defects in Ductile Iron Reducer Housings
3.1 Wrinkling Defects
3.1.1 Appearance and Location
Wrinkling defects typically appear as an uneven surface, like an orange peel, on the upper surface of the casting after cleaning. They usually occur in areas where the metal liquid reaches last or in the “cold end” of the liquid flow. This includes the top, vertical sidewalls, or dead corners of the casting.
3.1.2 Causes
The original pouring system for the reducer housing was a top – pouring design, which was actually a modified mid – bottom pouring process using the cavity as a runner. In this process, the high – temperature iron liquid entered the top surface of the foam pattern from the inner gate and did not fill the cavity gradually from top to bottom. Instead, it directly penetrated the thick wall area at the top, used the top of the cavity as a runner, and then fanned down to the thin wall area and finally back up. This created a turbulent filling pattern, resulting in “cold end” areas in the thick and large dead corner regions at the top and sides, leading to wrinkling defects.
3.2 Shrinkage Cavity Defects
3.2.1 Appearance and Location
Shrinkage cavity defects are mainly found in the thick positions near the side processing holes of the reducer housing, which are caused by geometric hot spots.
3.2.2 Causes
The fundamental cause of shrinkage cavities in ductile iron castings is that during liquid contraction and solidification, a certain part of the casting (usually the hot spot that solidifies last) cannot receive timely liquid metal replenishment, resulting in irregularly shaped holes with rough walls. In the case of the reducer housing, the chemical composition and mechanical properties of the ductile iron meet the requirements, and factors such as spheroidization treatment temperature, pouring temperature, and negative pressure have been excluded as the main causes. The geometric hot spots are the key factor contributing to the shrinkage cavity defects.
4. Solutions to Defects
4.1 Solution for Wrinkling Defects
4.1.1 Pouring System Optimization
The wrinkling defect was mainly due to the 不合理 design of the pouring system, which caused turbulent flow of the iron liquid during pouring. To solve this problem, the pouring system was redesigned from a top – pouring to a bottom – pouring system. In the new system, the high – temperature iron liquid fills the cavity steadily from the bottom up. The front – end low – temperature iron liquid and the products of insufficient vaporization of the foam pattern stay at the machining allowance position on the top surface of the cavity, resulting in a sound surface of the casting.
4.1.2 Calculation and Design of the New Pouring System
- Pouring time calculation: (where specific values were calculated according to the actual situation)
- Bottom injection average static pressure head height calculation:
- Inner gate minimum cross – sectional area calculation:
Based on theoretical calculations and practical experience, the inner gate was designed as a central pouring with four inner gates, each with a cross – sectional dimension of . The total cross – sectional area of the four inner gates is within a certain range, and the length of the straight pouring channel was designed according to the required pressure head.
4.2 Solution for Shrinkage Cavity Defects
4.2.1 Traditional Solutions and Their Limitations
Traditional solutions for shrinkage cavity defects include setting risers at hot spots to provide the necessary metal liquid for volume change during casting formation, or using a chilling system such as cold irons to create an artificial end zone at the hot spot. However, for lost foam casting of reducer housings, these methods have limitations. Adding risers reduces the process yield of the reducer housing casting and increases the overall process complexity. Using cold irons may cause them to fall off during molding and lead to casting deformation, increasing the process difficulty and affecting the quality stability of the reducer housing, as well as increasing the casting cost.
4.2.2 The New Heat Dissipation Technology
A new heat dissipation technology was developed to solve the shrinkage cavity defect. The core of this technology is to change the casting structure, increase the heat dissipation surface area, and reduce the modulus at the hot spot. During the solidification stage, the negative pressure gas takes away a large amount of heat, achieving a chilling effect.
Specifically, foam sheets (heat dissipation sheets) are bonded at the hot spot positions of the casting. These heat dissipation sheets have a large specific surface area, which reduces the local modulus of the casting. Cold air flowing through the casting and the heat dissipation sheets during the negative pressure operation of the pump exchanges heat and takes away heat, forming a microchannel flow heat exchange with the local molding sand in contact with the casting and the heat dissipation sheets, creating a chilling zone with a large temperature difference. This makes the casting change from a local solidification mode to a similar sequential solidification mode, eliminating shrinkage cavities and shrinkage porosity defects.
5. Verification of Solutions
5.1 Verification of the Solution for Wrinkling Defects
After redesigning the pouring system to a bottom – pouring type and conducting batch production of 2000 pieces, the surface quality of the castings was verified. The results showed that the surface of the castings produced by the new casting process was qualified, and no batch wrinkling defects occurred.
5.2 Verification of the Solution for Shrinkage Cavity Defects
For the shrinkage cavity defect, after using the heat dissipation technology and bonding 12 heat dissipation sheets of a certain size at the hot spot region of the casting, and then conducting batch production of 2000 pieces, the machining verification showed that there were no quality problems in the bolt through holes of the reducer housing, and no batch shrinkage cavity defects occurred again.
6. Conclusion
In conclusion, through optimizing the pouring system, the wrinkling defect of the casting was solved by ensuring a stable filling process and using the process allowance to collect the iron liquid with impurities. For the shrinkage cavity defect caused by hot spots in lost foam casting of ductile iron, a new heat dissipation sheet technology was designed. By setting a certain number of heat dissipation sheets at the hot spots, the local heat dissipation speed was enhanced, the geometric hot spots of the casting were eliminated, and the shrinkage cavity defect was solved. The heat dissipation sheet technology has passed the practical verification of the reducer housing casting, proving that it can effectively solve the shrinkage cavity defect and is a new method for solving shrinkage cavity defects in lost foam casting. These research findings provide valuable references for improving the quality of lost foam castings and optimizing the casting process.
7. Future Research Directions
Although the current research has achieved certain results in solving the wrinkling and shrinkage cavity defects of ductile iron reducer housings in lost foam casting, there are still some areas that can be further explored.
7.1 Optimization of the Heat Dissipation Sheet Design
The current heat dissipation sheet design has proven effective, but there may be room for further optimization. For example, exploring different materials and shapes of heat dissipation sheets to improve their heat dissipation efficiency and adaptability to different casting geometries. This could involve conducting experiments with various materials such as composite materials with enhanced thermal conductivity and different geometric shapes such as curved or corrugated surfaces to better fit the contours of the casting and enhance heat transfer.
7.2 Study of the Influence of Process Parameters on Defect Formation
The formation of wrinkling and shrinkage cavity defects is affected by multiple process parameters. In the future, a more in – depth study could be conducted on how changes in parameters such as pouring temperature, pouring speed, negative pressure value, and coating thickness affect the formation of these defects. This could involve setting up a series of experiments with different parameter combinations and analyzing the resulting defect rates and characteristics. By understanding these relationships more precisely, it will be possible to further optimize the casting process and reduce defect rates.
7.3 Application of Advanced Simulation Technologies
Advanced simulation technologies such as computational fluid dynamics (CFD) and finite element analysis (FEA) could be applied to study the lost foam casting process. CFD can be used to simulate the flow of molten metal and the vaporization of the foam pattern during pouring, providing a more detailed understanding of the filling process and the formation of wrinkling defects. FEA can be used to analyze the stress and deformation during solidification, helping to predict and prevent shrinkage cavity defects. By integrating these simulation technologies into the casting process design and optimization, it will be possible to achieve more accurate control of the process and improve the quality of castings.
