1. Introduction
Evaporative pattern casting, also known as lost – foam casting, has gained significant popularity in the foundry industry due to its numerous advantages such as high – dimensional accuracy, complex shape casting capabilities, and reduced labor costs. When it comes to ductile iron castings, however, this casting method faces unique challenges. Ductile iron, with its excellent mechanical properties like high strength and ductility, is widely used in various industries. But during the evaporative pattern casting process, issues like shrinkage porosity, slag inclusion, and improper solidification can occur. This article aims to delve deep into the process design of evaporative pattern casting for ductile iron castings, analyze common defects, and propose effective prevention and improvement strategies.
2. Structure and Characteristics of Ductile Iron Castings in Focus
2.1 Material and Dimensions
The ductile iron casting under study is made of QT400 – 15 material. It has an outline size of 430mm×620mm×684mm and weighs 180kg. A single casting is produced per mold. The casting has several thick – walled areas, which pose challenges during the casting process. For example, a specific area with dimensions 183mm×40mm×59mm is prone to shrinkage defects. Table 1 summarizes the key characteristics of the casting.
| Characteristics | Details |
|---|---|
| Material | QT400 – 15 |
| Outline Size | 430mm×620mm×684mm |
| Weight | 180kg |
| Problematic Area Dimensions | 183mm×40mm×59mm |
| Number of Castings per Mold | 1 |
2.2 Special Requirements
The casting has two oil channels with a total length of 510mm. These oil channels require a high – pressure airtight test, and any internal defects are not allowed. This makes the casting process more demanding, as ensuring the integrity of the oil channels is crucial.
3. Design of Pouring Systems and Simulation Analysis
3.1 Pouring System Design Options
Four different pouring system designs were considered for the evaporative pattern casting of the ductile iron casting: side – bottom pouring, top pouring, stepped pouring, and bottom pouring. Each design has its own characteristics and potential impacts on the solidification process and defect formation.
- Side – bottom Pouring: In this design, the molten metal enters the mold from the side – bottom. As shown in Figure 1 (Simulation result of side – bottom pouring), during the solidification process, thinner areas solidify first, while thicker areas take longer to solidify. High – risk areas for shrinkage porosity include the intersection of the parallel plate and the upper end – face, the “U” – shaped bosses on both sides of the casting, and the bottom and boss – dense areas. The small slag – collecting pockets may have little impact on the solidification of the upper end – face, and their slag – collecting effect needs to be verified through experiments.
- Top Pouring: The top – pouring design allows the molten metal to enter the mold from the top. The simulation results (Figure 2: Simulation result of top pouring) show that the areas with a high risk of shrinkage porosity are similar to those in the side – bottom pouring design.
- Stepped Pouring: For stepped pouring, the molten metal enters the mold through multiple steps. The simulation (Figure 3: Simulation result of stepped pouring) indicates that the shrinkage – 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. After the inner gate solidifies, the upper end – face cannot be fed effectively.
- Bottom Pouring: In the bottom – pouring design, the molten metal enters from the bottom. Although the hot – spot area on the upper end – face can be fed by the riser during solidification, as shown in Figure 4 (Simulation result of bottom pouring), before the feeding channel closes, the feeding is incomplete, resulting in shrinkage porosity or shrinkage cavities in the hot – spot area of the upper end – face and inside the riser.
3.2 Simulation – based Comparison
A comparison of the four pouring system designs is presented in Table 2.
| Pouring System Design | Solidification Characteristics | High – Risk Areas for Shrinkage Porosity | Feeding Effect |
|---|---|---|---|
| Side – bottom Pouring | Thinner areas solidify first, thicker areas later | Intersection of parallel plate and upper end – face, “U” – shaped bosses, bottom and boss – dense areas | Need to verify slag – collecting effect, potential feeding issues in some areas |
| Top Pouring | Similar to side – bottom pouring | Similar to side – bottom pouring | Similar to side – bottom pouring |
| Stepped Pouring | Solidification in steps | Inside “U” – shaped bosses, intersection of parallel plate and upper end – face, upper end – face cannot be fed after inner gate solidification | Poor feeding for upper end – face |
| Bottom Pouring | Upper end – face hot – spot area fed by riser, but incomplete feeding | Upper end – face hot – spot area, inside riser | Incomplete feeding |
Based on the simulation results, it is clear that each pouring system has its own advantages and disadvantages. The choice of the pouring system needs to be carefully considered to minimize the occurrence of defects.
4. Process Experiment and Defect Analysis
4.1 Process Experiment
After analyzing the simulation results, the bottom – pouring system with risers was selected for the process experiment. The experimental setup is shown in Figure 5 (Bottom pouring with feeder). The purpose of this experiment was to verify the feasibility of the selected pouring system and to further analyze the defects that occurred during the casting process.
4.2 Main Defects
The main defect observed in the castings was shrinkage cavities on the upper end – face of the casting without feeding risers. As shown in Figure 6 (Shrinkage hole on the upper face of casting), these cavities were mainly located at the intersection of the parallel plate structure and the upper end – face. This defect not only affects the appearance of the casting but also its mechanical properties and performance.
4.3 Cause Analysis
4.3.1 Shrinkage Cavity Formation
The formation of shrinkage cavities in the casting is mainly due to the difference in solidification rates between thick and thin areas. In the casting, thick – walled areas solidify slower than the surrounding thin – walled areas. After the surrounding areas solidify, the thick – walled areas cannot be effectively fed with molten metal, resulting in the formation of shrinkage porosity or cavities.
During the solidification of ductile iron, which has a mushy – state solidification process, there is no hard shell formed on the surface in the initial stage. As graphite precipitates, the eutectic expansion pressure can cause two problems. First, the volume of the molten metal increases, and as the solidification range expands over time, the last – solidifying areas form shrinkage porosity or cavities. The volume shrinkage during the cooling process can also exacerbate the shrinkage porosity. Second, the expansion pressure can act directly on the mold surface, causing problems such as mold wall migration, mold expansion, and shrinkage porosity. Table 3 summarizes the factors contributing to shrinkage cavity formation.
| Factors | Description |
|---|---|
| Solidification Rate Difference | Thick – walled areas solidify slower than thin – walled areas, lacking molten metal feeding |
| Mushy – State Solidification | No initial hard – shell formation, eutectic expansion causes volume changes and pressure effects |
| Eutectic Expansion Pressure | Leads to volume increase, solidification range expansion, and mold – related problems |
| Cooling Shrinkage | Exacerbates shrinkage porosity |
4.3.2 Role of Risers
Risers play a crucial role in the casting process. In this experiment, risers were divided into slag – collecting risers and feeding risers. Feeding risers are mainly used to supplement molten metal and control pressure during the solidification process.
The pressure change in the riser and the mold during the casting process can be divided into three typical stages, as shown in Figure 7 (Three stages of pressure change in the feeder). After the inner gate solidifies, no more molten metal enters the mold, and the casting and the riser form an integrated system (Stage 1). When the liquid metal shrinks, the pressure in the riser reaches its minimum (Stage 2). As graphite and austenite precipitate, the liquid metal expands, filling the riser again (Stage 3). Throughout this process, the mold needs to have a certain rigidity to withstand the pressure changes.
When using the evaporative pattern casting method for ductile iron castings, it is necessary to select appropriate risers according to the casting structure. In addition, the negative pressure in the sand box during the pouring process cannot be ignored, as it can affect the filling and solidification of the molten metal.
5. Improvement Strategies and Their Effects
5.1 Improvement of Riser Parameters
Based on the traditional sand – casting riser design principle, two types of risers were designed for the evaporative pattern casting of ductile iron castings. Let the modulus of the hot – spot area of the casting be \(M_{S}\), the modulus of the riser be \(M_{R}\), and the neck modulus of the riser be \(M_{N}\).
- For 1# riser: \(M_{R}=M_{S}\), \(M_{N}=0.8M_{S}\)
- For 2# riser: \(M_{R}=1.5M_{S}\), \(M_{N}=0.6M_{R}\)
Except for the different riser sizes, all other controllable parameters were kept the same. Full – process trials were carried out for both types of risers, and process improvements such as anti – collision, anti – deformation, and pre – sand – filling treatments were added to verify the improvement effect of different risers on the shrinkage cavities at the hot – spot positions of the castings.
5.2 Improvement Effects
When the bottom – pouring system with 1# riser was used in the evaporative pattern casting, after rough turning of the casting, shrinkage cavity defects were found in the hot – spot area. As shown in Figure 8 (Test results using 1# feeder and bottom – pouring process), the scrap rate reached 37%.
When the bottom – pouring system with 2# riser was used, after rough turning, there were no obvious defects on the upper end – face of the casting, as shown in Figure 9 (Test results using 2# feeder and bottom – pouring process). There were no shrinkage porosity defects in the hot – spot area. Although discrete small – point defects were observed on the end – face, after full – process machining, these defects were removed, as shown in Figure 10 (End – face of parts after full – process processing). Table 4 compares the results of using different risers.
| Riser Type | Defects after Rough Turning | Scrap Rate | Defects after Full – Process Machining |
|---|---|---|---|
| 1# Riser | Shrinkage cavity defects in hot – spot area | 37% | N/A |
| 2# Riser | No obvious defects on upper end – face, discrete small – point defects on end – face | Low | No defects |
6. Overall Process Control for Quality Improvement
In addition to selecting the appropriate pouring system and riser design, overall process control is essential for improving the quality of ductile iron castings in evaporative pattern casting.
- Anti – collision Measures: During the handling and transportation of the casting mold and the castings, anti – collision measures should be taken to avoid damage to the mold and the castings. This can include using shock – absorbing materials and proper handling equipment.
- Coating Drying Control: The drying process of the coating on the pattern is crucial. If the coating is not dried properly, it can cause defects such as gas porosity in the casting. The drying temperature and time should be strictly controlled according to the coating requirements.
- Pouring Negative Pressure and Pressure – holding Control: The negative pressure in the sand box during pouring affects the filling of the molten metal. A proper negative pressure can ensure smooth filling and reduce the risk of defects. After pouring, pressure – holding for more than 15 minutes can help to improve the feeding effect and reduce shrinkage porosity. Table 5 summarizes the key points of overall process control. | Process Control Aspect | Control Measures | Purpose | |—|—|—| | Anti – collision | Use shock – absorbing materials, proper handling equipment | Avoid mold and casting damage | | Coating Drying | Control drying temperature and time | Prevent gas porosity | | Pouring Negative Pressure | Set appropriate negative pressure value | Ensure smooth molten metal filling | | Pressure – holding | Hold pressure for more than 15 minutes | Improve feeding effect, reduce shrinkage porosity |
7. Conclusion
Evaporative pattern casting of ductile iron castings is a complex process that requires careful consideration of many factors. Through the analysis of different pouring system designs, defect causes, and improvement strategies, the following conclusions can be drawn:
- The bottom – pouring system with appropriately designed risers is a suitable choice for evaporative pattern casting of ductile iron castings. By conducting comparative experiments based on the product structure, the best solution to solve the shrinkage porosity defect can be obtained.
- Overall process control, including anti – collision, coating drying, pouring negative pressure, and pressure – holding, is necessary to improve the internal and external quality of the castings. This comprehensive approach can effectively reduce defects and increase the casting qualification rate, making the evaporative pattern casting process for ductile iron castings more efficient and reliable.
In future research, further exploration of new materials and technologies in the evaporative pattern casting process for ductile iron castings can be carried out to further improve the quality and performance of the castings, meeting the growing demands of various industries.
