Abstract:
The stainless steel bracket of a weeding machine was numerically simulated by investment casting using ProCAST software. The shrinkage defect of the casting was predicted, and the pouring system was improved. The process parameters of castings were studied through an orthogonal experiment, and the optimum scheme of process parameters was obtained by analysis. Through numerical simulation and actual casting, it was found that the casting defects were obviously reduced, which provides a reference for the production of stainless steel brackets.
1. Introduction
Stainless steel brackets are primarily used in automobiles, aerospace, agricultural machinery, and other fields as connectors between two important components, transmitting rotational speed and torque. Due to their complex external contours, stainless steel brackets are prone to shrinkage defects during the solidification process of investment casting and may encounter issues such as gas entrapment and inclusions during the mold-filling process. This paper utilizes ProCAST software to conduct numerical simulations of the original investment casting pouring process for stainless steel brackets and optimizes the pouring system and process parameters based on the simulation results to address shrinkage defects and improve casting quality, providing a reference for the actual production of stainless steel bracket castings.
2. Structure of Stainless Steel Bracket Casting
The stainless steel bracket serves as an important connecting part for transmitting rotational speed and torque on a weeding machine. The casting has an average wall thickness of 4 mm and a complex structure, consisting of irregular shapes such as circular faces, rectangular-like faces, and U-shaped shells. Its external dimensions are 141 mm × 81.8 mm × 60.84 mm, with a requirement for no internal and apparent casting defects. The casting material is 304L stainless steel, with a chemical composition (mass fraction, %): ≤0.03 C, ≤1.0 Si, ≤2.0 Mn, 18.0-20.0 Cr, 8.0-11.0 Ni, ≤0.03 S. The liquidus and solidus temperatures are 1461.9 °C and 1411.2 °C, respectively; the density is 7.93 kg/m³. A three-dimensional model of the stainless steel bracket for the weeding machine was created using Pro/E software.
Based on different pouring processes, the locations where shrinkage porosity and shrinkage defects are predicted to occur in this casting are at the maximum hot spots, located at the rectangular connection A, the U-shaped shell B, and the circular arc block C in contact with the side wall.
3. Investment Casting Process Design
3.1 Pouring Gate Selection
Analyzing the structural characteristics of the stainless steel bracket, the casting overall has an irregular shape, with the thinnest parts being the cylindrical section and the U-shaped shell, constituting a complex thin-walled structure. To improve production efficiency and ensure a smooth mold-filling process, according to actual production requirements, the weeding machine stainless steel bracket casting adopts a direct pouring gate and an inner pouring gate design, with two pieces per shell mold.
3.2 Determination of Mold-Filling Speed
The mold-filling speed of the casting was determined using the Kalkin formula in this experiment:
Where: v_{充} is the metal liquid mold-filling speed (cm/s); h is the height of the casting (cm); δ is the wall thickness of the casting (cm); T is the pouring temperature (°C). According to the formula, v_{充} is calculated to be 239.0534 mm/s, and the mold-filling speed is taken as 240 mm/s.
3.3 Process Parameters
According to related experiments, the liquidus temperature and solidus temperature of stainless steel 304L are 1461.9 °C and 1411.2 °C, respectively; the mold shell consists of 6 layers of materials made of colloidal silica + refractory quartz sand, with a thickness of approximately 8 mm; the casting adopts a top-pouring method and natural cooling in the negative Z direction of the gravity direction, with pouring and mold shell preheating temperatures of 1500 °C and 1000 °C, respectively, and a mold-filling speed of 240 mm/s.
4. Simulation Analysis of the Original Process Scheme
4.1 Three-Dimensional Modeling and Mesh Generation of the Pouring System
A three-dimensional model of the pouring system for the stainless steel bracket was created using Pro/E software, and imported into ProCAST software in .igs format to complete mesh generation. The total number of nodes is 12,706, and the total number of elements is 104,753. The three-dimensional finite element model of the pouring system for the stainless steel bracket.
4.2 Setting of Simulation Parameters
During investment casting simulation, numerous parameters need to be set, such as casting material, initial conditions, boundary conditions, etc. Accurate setting of relevant simulation parameters is necessary to obtain high-precision results. The following parameters were set in ProCAST: the casting material is 304L, with a pouring temperature of 1500 °C; the heat transfer coefficients between the casting and the mold shell, and between the casting and the air, are both 1000 W/(m²·K), and the heat transfer coefficient between the mold shell and the air is set to 50 W/(m²·K); the mold shell thickness is approximately 8 mm, with the outer surface of the mold shell set to air cooling, and the pouring method is top-pouring, using a velocity calculator to set the mold-filling speed to 240 mm/s, and the gravitational acceleration is set to 9.8 m/s².
4.3 Simulation Results and Analysis
The mold-filling status of the original process scheme for the weeding machine stainless steel bracket, the metal liquid rises smoothly in the mold cavity without impacting the mold cavity. At 1 second, the metal liquid fills the direct pouring gate and begins to enter the inner gate; at 2.5 seconds, the metal liquid fills the inner gate; at 3 seconds, approximately half of the mold-filling is completed; and at 4 seconds, the mold-filling is complete.
The solidification sequence of the casting directly affects the quality of the casting. The solidification process of the original process scheme for the weeding machine stainless steel bracket. during the entire solidification process, the casting basically solidifies from the outside to the inside and from the bottom to the top, which is conducive to obtaining high-quality castings. At 4 seconds, the outer thin wall of the casting begins to solidify, starting from the position away from the gate and proceeding towards the gate; at 1042 seconds, the main body of the casting has basically solidified completely.
The shrinkage porosity distribution diagram of the original process scheme for the weeding machine stainless steel bracket. Numerical simulation predicts that the shrinkage porosity rate (the ratio of the predicted shrinkage porosity volume in the casting body to the casting body volume) of the original process scheme for the weeding machine stainless steel bracket is 21.450%, and some shrinkage defects are distributed in the main body of the casting, leading to unqualified casting quality and subsequent scrapping, which is consistent with the actual situation, indicating good numerical simulation results. The causes of shrinkage defects are mainly due to unreasonable settings in the process scheme and process parameters, resulting in the casting being unable to receive feed during solidification due to volume contraction, thereby generating shrinkage porosity defects.