Abstract
According to the structural characteristics of wheel core casting, the lost foam casting solidification process is simulated and analyzed in this paper. The simulation results, obtained using the numerical simulation software ProCAST, focus on the placement of the large flange of the wheel core facing downward. The findings reveal the presence of severe shrinkage cavity and porosity defects in the thick sections of the casting, aligning well with the outcomes of trial production. To address these defects, the wheel core casting is repositioned with the flange facing upward, resulting in a significant reduction in shrinkage cavity and porosity. Further optimization of the casting process involves placing chills at the inner and outer flanges and modifying the horizontal end face of the flange to a 30° inclined plane. Simulations demonstrate that the optimized process completely eliminates shrinkage cavity and porosity defects, establishing it as the optimal process scheme.
1. Introduction
Compared with traditional casting methods, lost foam casting offers advantages such as high production efficiency and reduced pollutant emissions. It exhibits significant benefits in the mass production of high-precision and complexly structured castings [1-3]. Lost foam casting differs significantly from traditional cavity casting, as the froth at the advancing front of the molten metal decomposes upon heating, absorbing a considerable amount of heat from the molten metal. This not only decreases the fluidity of the molten metal but also increases the likelihood of defects such as shrinkage cavity, porosity, and carbon slag in the casting [4-5].
To address the issue of shrinkage cavity and porosity defects in the lost foam casting process, researchers have employed various techniques, including increasing the pouring temperature of the molten metal [6], optimizing the gating system [7], strategically placing chills [8], and optimizing the design of risers [9-10]. These measures enhance the mold-filling capacity of the molten metal and improve the solidification sequence of the casting, effectively avoiding defects such as shrinkage cavity and porosity in lost foam casting. Structural components with ring-like shapes, similar to wheel cores, such as impellers [6], ring hubs [9], and ductile iron pipes [10], can eliminate shrinkage cavity and porosity defects in castings through the adoption of reasonable gating systems and large risers. This paper presents a simulation analysis of the lost foam casting process for ZG270-500 wheel cores. Based on both simulation analyses and production results, the casting process has been continuously optimized, ultimately resulting in an optimal casting process scheme free from defects such as shrinkage cavity, porosity, and carbon slag. This provides a foundation for the lost foam production of wheel core castings.
2. Wheel Core Structure and Simulation Parameters
2.1 Wheel Core Structure Analysis
The wheel core is a crucial structural component connecting the wheel and the drive axle, used in conditions involving vibration and shock. High strength requirements are imposed on the casting. The wheel core part studied. The red (darker) areas represent the machined surfaces after casting. These surfaces must be free from defects such as shrinkage cavity, porosity, and carbon slag after machining. The remaining areas do not require machining and retain the roughness after casting. The part features a ring-like structure with a casting height of 315mm, a primary wall thickness of 35mm, an outer diameter of φ300mm at the larger end with a 40mm thick flange, an outer diameter of φ180mm at the smaller end, and stepped inner cylindrical surfaces.
2.2 Simulation Parameter Settings
(1) The casting material is ZG270-500, with specific chemical compositions outlined in Table 1.
Table 1. Chemical Composition of the Casting
| Alloy Element | C | Si | Mn | S | P | Fe |
|---|---|---|---|---|---|---|
| Percentage Content w/% | 0.30~0.40 | 0.20~0.45 | 0.50~0.90 | ≤0.35 | ≤0.35 | Bal. |
(2) Simulations are conducted using ProCAST software with the Lost Foam module. The pouring temperature is (1545±15℃), the liquidus temperature is 1489℃, the pouring rate is 21kg/s, the negative pressure on the outer surface of the casting coating is 0.06MPa, the pouring pressure at the gate is 0.13MPa, and the pressure at the riser is 0.1MPa.
(3) The coating adopted is Sand permeable foam, with an interface heat transfer coefficient between the coating and the casting of 500W/(m2·K) [11].
3. Simulation Analysis and Production of the Original Scheme
3.1 Analysis and Calculation Model
The wheel core casting shares structural similarities with hub castings, both being ring-shaped components. Gao Chengxun et al. [9] employed an upside-down pouring process combined with coating process control to address shrinkage cavity and porosity defects in hub castings. Therefore, the initial casting process scheme for the wheel core casting adopts an upside-down placement, utilizing a bottom-pouring gating system. The gating system cross-section has dimensions of 30mm×50mm, and the riser dimensions are φ200mm×230mm. The analysis and calculation model for the original scheme, with a total of 1,552,471 tetrahedral elements.
3.2 Simulation Result Analysis
Due to the relatively simple structure of the wheel core casting, the solidification sequence and the location of isolated liquid regions are better observed using the solid phase field for analysis [12]. A cross-section of the mold is used for analysis, with the cross-section position shown in Figure 3. The solidification time starts from the beginning of pouring until the cooling and solidification are complete.
As shown in Figure 4(a), after the liquid metal fills the mold cavity, the metal temperature rapidly decreases under the chilling effect of the coating. At a solidification time of 196.84s, the solid fraction is 19.5%. As shown in Figure 4(b), when the solidification time reaches 466.84s, as the temperature decreases, the thinner H position of the casting solidifies faster, while the thicker K position solidifies slower, forming an isolated liquid region. At this point, the solid fraction is 39.9%. As shown in Figure 4(c), when the solidification time is 866.84s, the H position of the casting solidifies rapidly (solid fraction of 62.5%), making it difficult for the riser to feed the casting. At this point, the solid fraction is 54.1%. As shown in Figure 4(d), when the solidification time reaches 1446.84s, the solid fraction is 67.4%, the H position of the casting has completely solidified, and the riser can no longer feed the casting. An isolated hot spot forms at the K position of the casting, prone to shrinkage cavity and porosity defects [13-14].
Since the H position of the casting solidifies rapidly, the feeding channel from the riser is blocked, leading to the formation of shrinkage cavity and porosity defects in the casting. After the mold is completely solidified, the defects are mainly distributed at the thick K position of the mold, with additional defects found in the ingate and riser.
3.3 Trial Production
The lost foam material used is STMMA (styrene-methyl methacrylate), with the main manufacturing processes including preforming, aging, molding, and baking. The preforming is done using a DH-450 intermittent preforming machine with steam foaming at temperatures ranging from 90~115℃. The preformed beads are aged for 24 hours before the molding process. The coating adopted is a water-based lost foam steel casting coating, applied to the wheel core casting using a dipping method with three coats, resulting in a total coating thickness of 2mm. After coating, the casting is placed in a drying room with a temperature controlled at 50℃ and a relative humidity not exceeding 10% for drying for more than 5 days.