Introduction
The lost foam casting process has gained significant popularity in the manufacturing industry due to its numerous advantages, such as high dimensional accuracy, excellent surface finish, and the ability to produce complex geometries. However, like any casting process, it is not without its challenges. Ductile iron castings produced through the lost foam casting process often encounter issues such as shrinkage porosity, slag inclusions, and other defects that can compromise the quality and integrity of the final product. This article aims to delve deep into the design of the lost foam casting process for ductile iron castings, analyze common defects, and propose effective prevention and improvement strategies.
Structure Analysis of Ductile Iron Castings
Material and Dimensions
The ductile iron casting under study is made of QT400 – 15 material. Its contour dimensions are 430mm×620mm×684mm, with a weight of 180kg. These specifications play a crucial role in determining the casting process parameters and potential defect areas. Table 1 summarizes the key material and dimensional information.
| Parameter | Details |
|---|---|
| Material | QT400 – 15 |
| Dimensions | 430mm×620mm×684mm |
| Weight | 180kg |
| Table 1: Material and Dimensional Information of the Ductile Iron Casting |
Thick – Walled Areas and Potential Defects
The casting has several thick – walled regions. For example, the area with dimensions 183mm×40mm×59mm is prone to shrinkage porosity. Additionally, it has two oil channels with a total length of 510mm, where high airtightness is required, and no internal defects are allowed. These structural features pose challenges during the casting process, as thick – walled areas tend to cool more slowly, leading to potential shrinkage defects. Figure 1 shows the structure of the casting, highlighting the thick – walled areas and oil channels.
Design of the Pouring System and Simulation Analysis
Pouring System Design Options
Four different pouring system designs were considered for the lost foam casting of ductile iron castings: side – bottom pouring, top pouring, step – by – step pouring, and bottom pouring. Each design has its own characteristics and potential impacts on the casting process. Table 2 provides an overview of these pouring system designs.
| Pouring System Design | Description |
|---|---|
| Side – bottom Pouring | Molten metal is introduced from the side – bottom of the mold |
| Top Pouring | Molten metal is poured from the top of the mold |
| Step – by – step Pouring | Molten metal is fed in a step – by – step manner through multiple gates |
| Bottom Pouring | Molten metal is introduced from the bottom of the mold |
| Table 2: Pouring System Design Options |
Simulation Results and Analysis
The MAGMA simulation software was used to analyze the solidification and feeding behavior of each pouring system design.
- Side – bottom Pouring: As shown in Figure 2, during the solidification process, thinner areas solidify first, while thick – walled areas take longer. High – risk shrinkage porosity areas are located at the intersection of the parallel plate and the upper end – face, as well as inside the “U” – shaped bosses on both sides of the casting. The bottom and boss – dense areas also have a relatively high risk of shrinkage porosity.
Top Pouring: The simulation results (Figure 3) show that the high – risk shrinkage porosity areas are similar to those of the side – bottom pouring method.
Step – by – step Pouring: Figure 4 indicates that the shrinkage porosity risk areas are concentrated inside the “U” – shaped bosses on both sides of the casting and at the intersection of the parallel plate and the upper end – face. Moreover, after the internal gate solidifies, the upper end – face cannot be fed.
Bottom Pouring: Although the hot – spot area on the upper end – face is fed by the riser during solidification (Figure 5), shrinkage porosity or shrinkage cavities may still form in the hot – spot area on the upper end – face and inside the riser before the feeding channel closes, indicating that the feeding effect is not ideal.
Based on the simulation results, it can be seen that each pouring system design has its own advantages and disadvantages in terms of solidification and feeding, and further optimization is needed.
Process Experiment
Selection of the Pouring System for Experiment
After analyzing the MAGMA simulation results, the bottom – pouring system with risers was selected for the process experiment. This system was chosen because it has the potential to improve the feeding of the casting, although the simulation showed some issues that needed to be addressed through experimentation. Figure 6 shows the bottom – pouring system with risers used in the experiment.
Experiment Results and Defect Observation
The experiment results showed that without a feeding riser, there were shrinkage cavities on the upper end – face of the casting, mainly concentrated at the intersection of the parallel plate structure and the upper end – face. This defect is a major issue for ductile iron castings and needs to be resolved to improve the quality of the products. Figure 7 shows the shrinkage cavities on the upper end – face of the casting.
Defect Analysis
Main Defects
The primary defect observed in the casting is shrinkage porosity, which appears as cavities on the upper end – face of the casting. This defect not only affects the surface quality of the casting but also may reduce the mechanical properties and airtightness of the product, especially in areas with high – pressure requirements such as the oil channels.
Cause Analysis
- Shrinkage Cavity Formation: During the solidification process of the casting, thick – walled areas solidify more slowly than the surrounding thin – walled areas. Once the surrounding areas solidify, the thick – walled areas lack the supply of molten metal, leading to the formation of shrinkage porosity or shrinkage cavities. In the case of ductile iron, its mushy – state solidification process further exacerbates this problem. During the initial stage of solidification, no hard shell is formed on the surface. As graphite precipitates, the eutectic expansion pressure can cause two phenomena: an increase in the volume of the molten metal, which may lead to the formation of shrinkage porosity or shrinkage cavities in the last – solidifying areas; and the expansion pressure acting on the mold surface, resulting in issues such as mold wall migration, mold expansion, and shrinkage porosity. Table 3 summarizes the causes of shrinkage cavity formation.
| Factor | Impact |
|—|—|
| Uneven Solidification | Thick – walled areas lack molten metal supply during solidification |
| Mushy – state Solidification of Ductile Iron | No hard – shell formation in the initial stage, eutectic expansion pressure causes problems |
| Table 3: Causes of Shrinkage Cavity Formation | - Role of Risers: Risers play a crucial role in the casting process. In this experiment, risers are divided into slag – collecting risers and feeding risers. Feeding risers are mainly used to supply molten metal and control pressure. The pressure change in the riser during the casting process can be divided into three typical stages, as shown in Figure 8. After the internal gate solidifies, no more molten metal enters the mold. When the molten metal shrinks, the pressure in the riser reaches its minimum. As graphite and austenite precipitate, the molten metal expands, filling the riser again. During this process, the mold needs to have a certain rigidity to withstand the pressure changes.
Improvement Scheme
Modification of Riser Parameters
Based on the traditional sand – casting riser design principle, two types of risers were designed. For the first type of riser (1# riser), \(M_{R}=M_{S}\) and \(M_{N}=0.8M_{n}\), while for the second type of riser (2# riser), \(M_{R}=1.5M_{S}\) and \(M_{N}=0.6M_{R}\). Here, \(M_{S}\) is the modulus of the casting hot – spot area, \(M_{R}\) is the modulus of the riser, and \(M_{N}\) is the neck modulus of the riser. All other controllable parameters were kept the same for a fair comparison. Table 4 shows the parameters of the two types of risers.
| Riser Type | \(M_{R}\) | \(M_{N}\) |
|---|---|---|
| 1# Riser | \(M_{R}=M_{S}\) | \(M_{N}=0.8M_{n}\) |
| 2# Riser | \(M_{R}=1.5M_{S}\) | \(M_{N}=0.6M_{R}\) |
| Table 4: Riser Parameters |
Improvement Effect
- 1# Riser: When the lost – foam casting process with the bottom – pouring system and 1# riser was used, after rough turning, shrinkage cavities were found in the hot – spot area of the casting, and the rejection rate reached 37%, as shown in Figure 9.
2# Riser: With the bottom – pouring system and 2# riser, after rough turning, there were no obvious defects on the upper end – face of the casting. The hot – spot area had no shrinkage porosity. Although there were discrete small – point defects on the end – face, these defects could be removed after full – process machining, as shown in Figures 10 and 11.
The results clearly show that the 2# riser is more effective in reducing shrinkage porosity defects in ductile iron castings produced by the lost – foam casting process.
Conclusion
- Optimal Pouring Process: Through comparative experiments, it is determined that for lost – foam casting of ductile iron castings, the bottom – pouring method combined with appropriately sized risers can effectively solve the problem of shrinkage porosity. By matching the riser size with the product structure, the best solution for improving the quality of castings can be obtained.
- Full – process Control: In addition to choosing the right pouring system and riser parameters, strengthening the full – process control of the lost – foam casting process is also crucial. This includes preventing bumps during handling, controlling the drying of the coating, and carefully regulating the pouring negative pressure and pressure – holding time. These measures can significantly improve the internal and external quality of the castings, ensuring the high – quality production of ductile iron castings.
In conclusion, by comprehensively considering the structure of ductile iron castings, optimizing the pouring system design, analyzing and preventing defects, and strengthening full – process control, the lost – foam casting process for ductile iron castings can be effectively optimized to produce high – quality products that meet the requirements of various industries.
