Abstract
The lifting arm, due to its complex structure, often encounters defects such as shrinkage and dispersed shrinkage during the investment casting process. This paper utilizes numerical simulation software, ProCAST, to analyze and optimize the casting process. Based on the simulation results of the initial process scheme, improvements are made to the gating system. Orthogonal experiments are conducted on three critical process parameters: pouring temperature, pouring speed, and shell preheating temperature. The optimal scheme A1B2C2 is obtained, which significantly enhances the process quality and production efficiency of the product.
Keywords: lifting arm; investment casting; gating system; ProCAST software; orthogonal experiment
1. Introduction
The lifting arm, an important automation machinery device, mimics human arm movements and is widely used in industrial production for capturing and transporting objects according to programmed trajectories and requirements. However, due to its complex structure, defects such as shrinkage and dispersed shrinkage frequently occur during the investment casting process. This paper analyzes the initial process scheme of a certain type of lifting arm and optimizes its investment casting process to reduce the shrinkage rate, providing a reference for the casting process design of similar products.
2. Structure of the Lifting Arm Casting
The lifting arm, a common casting product in industrial production, is widely used due to its good stability. It is made of ZG0Gr18Ni9Ti, a stainless acid-resistant steel casting. The chemical composition of ZG0Gr18Ni9Ti is shown in Table 1. The lifting arm casting has uniform wall thickness but a complex structure, with an outline dimension of approximately 70 mm × 45 mm × 57 mm and an average wall thickness of 6 mm. The overall structure of the casting can be divided into a stepped shaft part, a through-hole connection part, and a U-shaped claw working part. A three-dimensional solid model of the lifting arm is created using Pro/E software.
Table 1. Chemical Composition of ZG0Gr18Ni9Ti
| Element | Content (w B /%) |
|---|---|
| C | ≤0.08 |
| P | ≤0.045 |
| Cr | 17.0~20.0 |
| Mn | 0.8~2.0 |
| Ni | 8.0~11.0 |
| Si | ≤1.5 |
| S | ≤0.030 |
| Ti | 0.3~0.7 |
3. Investment Casting Process Design
3.1 Gating System Selection
The overall structure of the lifting arm casting is irregular, mainly consisting of a claw working area, a through-hole nut connection part, and a stepped shaft. To ensure a relatively smooth filling process and sequential solidification of the casting, the gating system design adopts a cross runner-sprue-ingate configuration, with six pieces per mold.
3.2 Pouring Model Establishment
Based on the three-dimensional model of the lifting arm, a three-dimensional model of its gating system is established, and a finite element model is obtained by meshing. The mesh value is set to 4 mm, automatically generating a finite element mesh with a total of 34,928 nodes and 249,485 elements. The initial process gating system scheme and its finite element model.
3.3 Simulation Parameter Settings
Accurate simulation parameter settings in ProCAST are crucial for obtaining precise simulation results. The simulation parameters involved in investment casting include heat transfer coefficients, boundary conditions (pouring speed, heat, temperature, etc.), initial conditions (shell, casting, air temperature), and material parameters. The simulation parameters are set as follows: the shell temperature during pouring is 850 ℃ and remains constant, with a shell thickness of approximately 6 mm. The pouring temperature ranges from 1,500 ℃ to 1,550 ℃, with 1,530 ℃ selected for the simulation. The gravity acceleration is set to 9.8 m/s², and the gravity direction is set to the negative Y-direction, aligning with the pouring direction and the gate surface. The heat exchange coefficient between the casting and air is set to 1,000 W/(m²·K), and the heat transfer coefficient is set to 500 W/(m²·K), with other settings using defaults.
4. Simulation Results and Analysis of the Initial Process
4.1 Determination of Pouring Speed
The pouring speed is calculated using the Karl Golden formula:
Where:
- v is the pouring speed of the molten metal (cm/s);
- h is the height of the casting (cm);
- δ is the wall thickness of the casting (cm);
- T is the pouring temperature of the alloy (℃);
- k is a constant.
Through calculation, the pouring speed v is 344.6167 mm/s. Considering the influence of parameter calculations, a pouring speed of 350 mm/s is used for simulation.
4.2 Main Process Parameters
The liquidus temperature of ZG0Gr18Ni9Ti stainless steel is 1,344.7 ℃, and the solidus temperature is 1,235.7 ℃. The entire shell, a closed entity composed of quartz sand refractory material and silica sol, is approximately 6 mm thick. Considering casting quality and cycle time, the entire pouring process is completed in the air using a top-pouring method, with the shell preheated to 1,000 ℃. Pouring is performed at a temperature of 1,530 ℃ with a pouring speed of 300 mm/s, and the casting is cooled naturally. The pouring direction is set with the gravity direction as the negative Y-direction.
4.3 Results and Analysis
The initial process scheme of the lifting arm exhibits different filling states during the filling process. The entire filling process is relatively smooth, with no violent impact on the inner wall of the shell by the molten metal, and no damage to the casting due to excessive pouring speed. The molten metal reaches the bottom in approximately 0.5 seconds, fills the sprue and enters the first set of workpieces before 1.0 second, and the filling is relatively flat. The first set of workpieces is completely filled at 2.5 seconds, with the second set half-filled, and the third set half-filled at 3.9 seconds.
The solidification of the casting directly affects its forming quality. The solidification rates at different times for the initial process scheme.
The initial process shrinkage distribution. The numerical simulation technology predicts the distribution of shrinkage pores with a volume fraction greater than 1.0. The simulation results show that shrinkage and dispersed shrinkage are mainly distributed in the sprue. Additionally, shrinkage and dispersed shrinkage are severe at the intersection of the shaft and connection parts of the components. This numerical simulation result is consistent with the causes of product defects in actual production under the initial process scheme, validating the accuracy of the simulation analysis.