Abstract:
The C95820 aluminum-bronze double-suction impeller, characterized by its large size, uneven wall thickness, and complex structure, faces challenges in traditional casting methods due to defects such as concentrated shrinkage porosity, oxide inclusions, and porosity. This research leverages the technical advantages of 3D printing sand molds, utilizing ProCAST software to optimize and design the gating and riser system. By integrating the design and printing of key blade sand cores and incorporating conformal air ducts, we successfully produced high-quality aluminum-bronze double-suction impeller castings. This paper presents a comprehensive study on the casting process, emphasizing the role of 3D printing in overcoming traditional casting limitations.
1. Introduction
The double-suction impeller is a turbine machinery equipment with two suction inlets, commonly used in high-flow and high-pressure fluid handling systems such as pumps and compressors. Compared to traditional single-suction impellers, double-suction impellers offer greater flow rates and higher efficiency, making them widely applied in industries like energy, petrochemicals, and water treatment.
Aluminum-bronze, with its narrow crystallization temperature range, good fluidity, and resistance to composition segregation and dispersed shrinkage porosity, is suitable for manufacturing dense castings. However, it is prone to oxide inclusions and porosity during melting and pouring, as well as significant shrinkage and the risk of concentrated shrinkage porosity and slag inclusion during solidification.
The advent of 3D printing technology has revolutionized traditional sand casting. Compared to conventional processes, 3D printing offers significant advantages in terms of high quality, short cycles, and low material consumption. It frees casting design from traditional molding methods, enabling flexible design of sand core schemes and gating and riser systems. The production process of sand molds becomes simplified, significantly reducing the production cycle and manufacturing costs of castings.
2. Structural Characteristics Analysis of the Impeller
The 3D model of the double-suction impeller studied. The impeller is made of C95820 aluminum-bronze, with a maximum outer diameter of 1015mm, a height of 630mm, and a central through-hole. The wall thickness is uneven, with a minimum of 5mm. The internal cavity features a complex structure, with two asymmetric blade layers connected by plates 15mm wide. The upper and lower cover plates are connected to the blade layer plates with a width of 19mm.
The large diameter and complex curved surface structure of the double-suction impeller, combined with the thin connecting plates between the two blade layers, make it challenging to avoid damaging the sand core during demolding and core extraction. Additionally, the aluminum-bronze’s susceptibility to oxide inclusions and porosity during casting necessitates a stable metal liquid level rise. Therefore, this research adopts the method of sand mold 3D printing for preparation.
Table 1: Chemical Composition of C95820 Aluminum-Bronze
| Element | wt.% |
|---|---|
| Cu | ≥77.5 |
| Al | 9.0-10.0 |
| Ni | 4.5-5.8 |
| Fe | 4.0-5.0 |
| Mn | ≤1.5 |
3. Casting Process Design
3.1 Gating System Design
Considering the symmetrical structure of the upper and lower cover plates of the double-suction impeller, the pouring position is selected as one of the cover plates,. This pouring position facilitates subsequent riser design, beneficial for overall shrinkage, and is conducive to sand mold 3D printing sand core assembly.
The casting adopts a bottom-pouring method to ensure a stable filling process and rapid filling of the mold cavity, preventing air inclusion, oxide formation, and cold shut defects. After optimizing various designs, the final gating system design. Four bottom-pouring ingates are directly connected to four blind risers. This design effectively controls the flow of molten metal and enhances the riser’s feeding ability, improving casting quality and integrity.
3.2 Riser Design
According to the simulation analysis of the part without risers, major shrinkage porosity defects are present in three thicker structural areas: the blade tips, the blade roots connected to the central plate, and the centers of the blades. Therefore, risers are added for feeding.
Based on the simulated defect locations and practical engineering experience, multiple preliminary casting process schemes were designed and analyzed using ProCAST software. The final optimized riser design. Six waist-shaped open risers are set at the upper ring opening position to feed the upper ring cover plate. The central hole is filled with a cylindrical open riser for feeding. Small cylindrical risers are designed at the tail of each blade, both for feeding and for drawing out impurities and gases in the molten metal, ensuring blade quality. Four symmetrical blind risers are designed at the bottom to feed the bottom ring cover plate. Molten metal enters the casting through the blind risers, with impurities and gases trapped in the blind risers, reducing defect formation.
3.3 Simulation of the Casting Scheme
During simulation, the mold material is resin-bonded sand, the pouring temperature is 1150°C, the initial sand mold temperature is 80°C, the heat transfer coefficient between the casting and the mold is 500 W/m²·K, and the pouring time is 120s. Other parameters are set to the software’s default values.
The molten metal enters the casting through the ingates into the blind risers, showing a stable rising state without metal liquid对冲or splashing, until the entire mold cavity is filled. The stable filling of the mold cavity effectively avoids oxide inclusions and porosity defects, ensuring the overall quality of the casting.
In the initial stages of solidification, the three connecting plates and blades, which have thinner wall thicknesses, begin to cool. During subsequent solidification, the casting gradually solidifies, with the open and blind risers solidifying later. The bottom cover plate, connected to the blind risers, solidifies in a later sequence. In the final stages of solidification, the central open risers are the last to solidify. The overall solidification process follows sequential solidification, meeting design expectations.
Shrinkage porosity is concentrated in the open and blind risers and the gating system. Although the bottom cover plate solidifies after the casting, the blind risers provide feeding, transferring defects to the blind risers. No defects are found on the casting, proving the feasibility of the casting process design and ensuring the overall solidification quality of the double-suction impeller.
4. Sand Mold Design for the Double-Suction Impeller
The sand cores for double-suction impellers are large in size and highly complex in structure, posing significant challenges in traditional molding processes. Traditional methods require the prior preparation of wooden patterns, which is time-consuming and inefficient. Furthermore, the molding of sand cores is limited by the size of the sand box, necessitating multiple sections, which increases the complexity of operations and the risk of sand core damage during the molding process. Even if the various parts of the sand cores can be integrated into a whole using adhesives, there may still be blade misalignment due to inadequate bonding or process errors, resulting in the loss of overall precision and potentially introducing casting defects such as porosity.
To address these issues, this study leverages the advantages of 3D printing technology. 3D printing technology is not limited by the complexity of sand core shapes and offers a printing accuracy far superior to traditional casting molding methods. Therefore, we designed the key sand cores of the double-suction impeller with a two-layer blade structure in separate sections to ensure the integrity and precision of the blades. Simultaneously, cleverly designed positioning devices and positioning lines on the main sand mold played a crucial role in the subsequent assembly process, ensuring that the dimensional accuracy was maintained.
When designing the sand mold, we paid special attention to the issue of ventilation. The selection of coated宝珠砂 (a type of sand with high gas emission) as the sand mold material increased the risk of porosity defects during the casting process. To effectively ventilate, we designed air channels within the sand mold. For the simple structure with a hub, we adopted a straight air channel design; for the curved structure with blades, we fully utilized the free-form design capabilities of 3D printing technology to create conformal air channels. The conformal air channels can better conform to the curved surfaces of the blades, achieving more effective ventilation and preventing porosity defects in the castings during the casting process.
In conclusion, the introduction of 3D printing technology has successfully addressed many challenges in the sand mold design of double-suction impellers, laying a solid foundation for subsequent efficient casting production.