Abstract
Impellers, as the core components of fans, are characterized by their thick nodular iron castings with uneven wall thickness distributions and complex hot joint configurations. The outer contours of these castings require full-body processing, leaving no room for casting defects, making the casting process highly challenging. This article delves into the impeller casting process optimization, aiming to address issues such as shrinkage porosity, slag defects, and casting fire running. Through comprehensive experimentation and simulation, a bottom gating system with optimized chillers and risers is proposed. The adoption of flat pouring with filter inserts significantly enhances iron melt purity, ensuring smooth filling without slag entrainment. Moreover, the optimized process effectively mitigates the risks of shrinkage porosity and casting fire running, meeting the strict requirements of MT (DIN EN 1369 Grade SM 3) and UT (DIN EN 12680-1 Grade 1) inspections.
Keywords: impeller casting, shrinkage porosity, bottom gating system, casting fire running, slag defects
1. Introduction
Impellers, essential components in fans, undergo continuous rotation within a cylinder to generate power. They must possess high strength and temperature resistance, making material selection and casting processes crucial. The complexity of impeller castings, including uneven wall thickness and intricate hot joints, coupled with the requirement for a flawless exterior finish, poses significant challenges in the casting industry. Traditional vertical pouring methods, though effective in ensuring casting quality, are associated with high production costs, low efficiency, and the risk of casting fire running.
This research aims to develop an optimized impeller casting process, focusing on the bottom gating system with flat pouring. By incorporating innovative techniques such as chillers, risers, and filter inserts, the study seeks to address the issues of shrinkage porosity, slag defects, and casting fire running, while enhancing production efficiency and reducing costs.
2. Literature Review
Previous studies on impeller casting have emphasized the challenges associated with uneven wall thickness, complex hot joints, and the need for flawless castings. The majority of casting enterprises worldwide adopt vertical pouring methods due to their ability to ensure stable filling, effective degassing and deslagging, and superior pressure feeding during solidification. However, these methods are hindered by high production costs, complex box assembly procedures, and the risk of casting fire running (Li et al., 2022).
Alternative approaches, such as horizontal pouring, have been explored to improve production efficiency and reduce costs. However, horizontal pouring poses challenges related to slag entrapment on the top surface of the castings due to the orientation of large flat surfaces (Wang & Li, 1998). Thus, the key to successful horizontal pouring lies in controlling the purity of the iron melt and the cleanliness of the mold cavity.
3. Experimental Materials and Methods
3.1 Experimental Materials
QT450-12 nodular iron was chosen as the material for the impeller castings, with dimensions of 1450mm × 850mm × 380mm and a weight of 1215kg. The castings were subjected to ultrasonic (UT) and magnetic particle (MT) inspections, ensuring the absence of visible defects after processing.
3.2 Experimental Setup
The experiment involved transitioning from the conventional vertical pouring setup to a flat pouring configuration. This modification aimed to enhance box assembly efficiency, reduce the risk of fire running, and address issues related to sand entrapment. Additionally, measures were taken to improve iron melt and mold cavity cleanliness, including the incorporation of a bottom gating system, filter inserts, and the use of ceramic tubes for the ingates.
3.3 Simulation and Inspection Methods
To predict and optimize the shrinkage porosity distribution, MAGMA solidification simulation software was utilized. Based on the simulation results, adjustments to the chillers and risers were made. The quality of the castings was verified through UT and MT inspections, adhering to the specified standards (DIN EN 12680-1 Grade 1 and DIN EN 1369 Grade SM 3).
4. Experimental Results and Analysis
4.1 Vertical Pouring Process
The vertical pouring process, as depicted in Figure 1(a), employs a bottom gating system with a circular ingate. This configuration ensures stable filling, effective degassing and deslagging, and superior pressure feeding during solidification. However, it is associated with complex box assembly procedures, high production costs, and a risk of casting fire running due to high static pressure heads and expansion forces during graphite formation.
Continuing the previous discussion on the impeller casting process, we will delve deeper into the experimental details and findings in English.
4.2 Verification of the Optimized Process
To validate the efficacy of the optimized process, three batches of experiments, labeled as “1+2+4,” were conducted. Following the completion of casting, the experimental pieces underwent Magnetic Particle Testing (MT) and Ultrasonic Testing (UT) as per the client’s technical specifications. The results demonstrated that all samples met the desired quality standards, exhibiting no visible casting defects upon final inspection. Subsequently, these samples were successfully delivered, and the optimized process is now being implemented for batch production. Figure 5 showcases the finished impeller products after processing.
5. Conclusions
Based on the comprehensive research and experimentation, the following conclusions can be drawn:
- Effective Resolution of Key Issues: The optimized bottom injection pouring process, coupled with refined arrangements of chillers and risers, has significantly addressed issues such as shrinkage porosity, slag defects, and casting runout in impellers.
- Theoretical Validation via Solidification Simulation: The utilization of solidification simulation software (e.g., MAGMA) has enabled theoretical verification of shrinkage porosity distribution, thereby minimizing the waste of production runs and enhancing process efficiency.
- Synergistic Effect of Multiple Techniques: The standalone use of risers was found insufficient in fully resolving shrinkage porosity issues. Instead, a combined approach incorporating chillers proved essential for achieving the desired results.
- Comprehensive Control of Inclusion: The elimination of impeller inclusion defects necessitated multifaceted strategies to enhance the purity of molten iron and the cleanliness of the mold cavity. This included minimizing oxidation during melting and pouring, installing filtration systems, and ensuring a smooth flow of iron into the mold.
In summary, this study has successfully demonstrated the feasibility of optimizing the impeller casting process through the adoption of a bottom injection pouring scheme coupled with refined cooling and feeding strategies. The results not only improve product quality but also enhance production efficiency and reduce costs, setting a benchmark for the industry in impeller casting technology.