Abstract
This paper focuses on the optimization of the precision casting process for engine pre-combustion chamber inserts. Initially, the engine pre-combustion chamber inserts were formed using a horizontal pouring process, which resulted in a high rejection rate due to defects such as shrinkage cavities and pits on the top surface of the castings. To address these issues, the causes of casting defects in the horizontal pouring process were analyzed, and a symmetrical vertical pouring scheme was designed. The process scheme was simulated using ProCAST software, and process parameters such as pouring temperature, pouring time, and shell preheating temperature were optimized through orthogonal experiments. The results indicate that under the optimized process parameters (pouring temperature of 1630°C, pouring time of 5 seconds, and shell preheating temperature of 1100°C), the defects in the castings were basically eliminated. After trial production verification, the qualified rate of the castings reached 91.67%. This study provides a reference for the casting process design and optimization of similar components.
1. Introduction
Precision casting, capable of producing castings with high dimensional accuracy, uneven wall thickness, and complex structures, has become one of the important methods in automotive component manufacturing. Engine pre-combustion chamber inserts are crucial structural parts within the engine, operating under harsh conditions of high temperature, high pressure, and heavy loads. Therefore, high forming quality is required, with no defects such as shrinkage cavities and pits on the surface. However, during the precision casting process of engine pre-combustion chamber inserts, defects such as shrinkage cavities and pits often appear on the top surface, leading to low production quality and qualification rates.
The design of the pouring system and process parameters in precision casting are considered key factors affecting the forming quality of castings. By adjusting relevant parameters, the solidification sequence of castings can be controlled to eliminate defects. The combination of computer numerical simulation and actual pouring experiments can quickly validate process schemes and reduce trial-and-error costs. Numerous studies have demonstrated the effectiveness of this approach in optimizing casting processes for various components.
This study aims to address the casting defects in the original casting scheme of a certain type of engine pre-combustion chamber insert. Based on the structural characteristics of the casting, the pouring system design was improved. ProCAST software was utilized to simulate the filling, solidification process, and defect distribution of the improved process scheme. Orthogonal experiments were conducted to adjust the casting parameters of the improved process scheme. After actual trial production verification, the forming quality and qualification rate of the castings met the requirements, providing a reference for the casting process design and optimization of similar components.
2. Experimental Process and Methods
2.1 Casting Structure and Experimental Material
The engine pre-combustion chamber insert is formed through a one-time precision casting process, with only the internal runner needing to be cut and separated during post-processing, and no additional processing is required for other parts. Therefore, high overall forming quality is required. SolidWorks software was used to construct a 3D model of the casting, with basic dimensions of 42 mm × 42 mm × 25 mm, a top wall thickness of 7 mm, and a skirt wall thickness of 2 mm. The casting is a precision small part with uneven wall thickness. the 3D model of the engine pre-combustion chamber insert.
The material of the casting is 4Cr9Si2 martensitic heat-resistant steel, primarily used for manufacturing components that operate in high-temperature and high-pressure environments . The actual chemical composition and content of the material were detected using a spectrometer, as shown in Table 1. Based on the measured composition of 4Cr9Si2 steel, the thermophysical properties of the material were calculated in ProCAST software. The analysis reveals that the thermal conductivity and enthalpy of 4Cr9Si2 steel vary significantly with temperature, overall showing an upward trend. High thermal conductivity promotes rapid solidification of the casting and the formation of fine grains, enhancing the strength and durability of the casting. High enthalpy indicates that the metal liquid loses less heat during filling, leading to strong fluidity and facilitating rapid filling .
Table 1: Main chemical compositions of 4Cr9Si2 steel
| Element | C | Si | Mn | S | P | Cr | Ni | Fe |
|---|---|---|---|---|---|---|---|---|
| Range | 0.35 | 0.50 | 2.0 | 0.030 | 0.035 | 8.0 | 10.0 | ≤0.60 |
| Content | 0.40 | 2.5 | 0.6 | 0.030 | 0.030 | 9.0 | 0.5 | Balance |
2.2 Shell Preparation and Pouring Process
Medium-temperature wax was used, and a double-station mold head hydraulic wax injection machine was employed to prepare the wax pattern in the die, with an injection temperature of 60-65°C, time of 12-14 seconds, and injection pressure of 25-30 kg/cm². After being cooled to room temperature with water, the wax pattern was trimmed and assembled manually.
The shell process consisted of four and a half layers: surface layer, transition layer, reinforcing layer (two layers), and sealing layer. The surface layer slurry was prepared from silica sol binder and zirconia powder, with a viscosity of 33-45 seconds and 100-120 mesh zirconia sand sprinkled on it. The transition layer and sealing layer slurries were both prepared from silica sol binder and mullite powder, with the transition layer slurry having a viscosity of 16-24 seconds and 30-60 mesh mullite sand sprinkled on it, and the reinforcing layer slurry having a viscosity of 14-16 seconds and 16-30 mesh mullite sand sprinkled on it. The sealing layer slurry had a viscosity of 12 seconds. The drying and hardening conditions were: temperature of 20-25°C, humidity of 65%-75% for the surface and transition layers, and humidity of 45%-65% for the reinforcing and sealing layers, with a drying time of 10-14 hours.
High-pressure steam dewaxing was adopted, with the internal pressure of the dewaxing kettle set to 0.6-0.8 MPa, steam temperature of 150-160°C, and dewaxing time of 10-15 minutes. Shell baking was performed using a rotary regenerative energy-saving gas baking furnace, with appropriate baking temperatures set according to the process scheme, a baking time of 30-60 minutes, and a holding time of no less than 45 minutes to ensure the removal of residual wax, organic matter, and moisture from the shell. Melting was conducted using a 150 kg medium-frequency induction furnace, and the alloy composition was analyzed using a Spark optical emission spectrometer. Pouring was performed after reaching the pouring temperature. The casting was cooled to room temperature before post-processing. The actual casting was used as the test object, and its surface quality and geometric dimensions were inspected, requiring a smooth and flat surface with no defects such as shrinkage cavities and pits, and geometric dimensions conforming to the drawing requirements.
2.3 Process Parameter Setting and Orthogonal Experiment
The determination of casting process parameters requires comprehensive consideration of factors such as casting structure, material properties, quality requirements, and actual casting experience. The solidus temperature of 4Cr9Si2 steel is 1152.6°C, and the liquidus temperature is 1438.8°C. Generally, the pouring temperature should be 100-150°C higher than the liquidus temperature of the metal, so the pouring temperature was set to 1600°C, and the shell preheating temperature was set to 1100°C. The shell thickness was 6 mm, the interface heat exchange coefficient between the casting, pouring system, and shell was 500 W/(m²·K), natural cooling was adopted, and the ambient temperature was 20°C. The original process scheme’s pouring time was calculated to be 3 seconds using the following formula :
t = C \sqrt{G} \] Where \( C \) is the correlation coefficient, related to the casting’s relative density \( K_v \), taken as 1.4; \( G \) is the total mass of the metal liquid. Based on the original process parameter values, combined with single-factor test results and actual casting experience, three key process parameters with a significant impact on casting quality were identified as experimental factors: pouring temperature (A), pouring time (B), and shell preheating temperature (C). Orthogonal experiments were conducted to optimize the process parameters with the goal of improving casting forming quality. The factor level distribution of the orthogonal experiments.
3. Experimental Results and Analysis
3.1 Original Process Scheme and Defect Analysis
The original process scheme adopted a horizontal pouring design with one mold producing 30 pieces. The 3D model was imported into ProCAST software, and mesh division and repair were conducted using Visual-Mesh. To ensure simulation accuracy and shorten simulation time, the mesh element sizes of the casting and internal runner were set to 1-2 mm, the mesh element sizes of the sprue were set to 3-5 mm, and the mesh element sizes of the runner and pouring cup were set to 8-12 mm, resulting in approximately 2.6 million mesh elements. The simulation was performed by switching to the Visual-CAST module after setting the relevant process parameters. The defect simulation of the original process.
It can be observed that most castings in the original process scheme had defects, leading to a low casting yield rate. The calculated shrinkage porosity rate for this scheme was 3.358%. Defects in the castings were generally distributed around the waist holes on the top surface. The main reasons for the defects are as follows:
① The volumetric shrinkage of steel castings occurs throughout the entire casting process, including liquid shrinkage, solidification shrinkage, and solid-state shrinkage, which can easily lead to shrinkage defects. The insert casting has thin walls, a small volume, and the feeding channel is obstructed by the waist hole structure, resulting in external shrinkage cavities on the top surface.
② During filling, the metal liquid impacts at the internal runner inlet, causing local flow turbulence. Gas cannot be discharged, and the small size of the internal runner inlet forms defects.
The actual defects of the engine pre-combustion chamber inserts cast using the original process scheme. It can be seen that there are pit defects on the edge of the top surface of the castings, as well as severe shrinkage cavities and mouse-tail indentation defects. The casting qualification rate was only 50%. The simulation results were consistent with the actual defect distribution.
3.2 Improved Process Scheme and Simulation Analysis
3.2.1 Pouring System Design
Reasonably designing the pouring system can significantly reduce defects such as shrinkage porosity in castings. The design of the internal runner is one of the key aspects of casting process design, and internal runner openings should be avoided in areas with high quality requirements for the casting . Considering that the top surface roughness of the engine pre-combustion chamber insert is required to be high during use, the internal runner opening should be avoided on the top surface. The casting has a cylindrical shape, with the skirt wall thickness being smaller than the top thickness. During solidification, the metal liquid on the top of the casting will feed towards the skirt, making the top prone to fine shrinkage porosity and cavities. At the same time, considering that the casting has a small mass and a large quantity, the solidification shrinkage volume of a single casting is relatively small, making it inconvenient to add feeding risers. After comprehensive analysis, the internal runner opening for this casting was designed on the side wall of the top of the insert to facilitate feeding of the top of the casting. The cross-sectional area of the internal runner was increased to ensure a smooth feeding channel during solidification.
The cross-section of the internal runner was rectangular, with a length designed to be 10 mm for easy cutting and processing. The pouring position of the casting refers to the position of the casting in the mold during pouring and should ensure that the critical surface of the casting is located at the bottom or side to ensure its forming quality . When assembling the casting and pouring system, the wax patterns of the insert casting and internal runner were inclined at about 45°, placing the surface of the top of the insert at the side position. The inclined internal runner can maintain a consistent metal liquid flow direction and stable filling during the filling process, avoiding direct intersection and turbulence of the metal liquid in the mold cavity. Additionally, gases and inclusions in the metal liquid can be quickly discharged through the top of the internal runner. Exhaust ports were set at both ends of the runner, enabling timely exhaust of gases in the mold cavity during filling and reducing the filling resistance of the metal liquid. The improved process scheme adopted a symmetrical vertical design with a runner-sprue-internal runner arrangement, with 60 pieces per mold, 30 pieces on each side.
3.2.2 Simulation and Analysis
The model of the improved process scheme was imported into ProCAST software, and the pouring time for this scheme was calculated to be 4 seconds using formula (1), with other process parameters remaining the same as the original process scheme. The simulation results of the filling process for the improved process scheme. It can be seen that most of the metal liquid flows into the bottom runner from the sprue in the middle position, then flows from the middle of the bottom runner to the sprues on both sides, and finally fills the mold cavity in sequence from the bottom up. The entire filling process lasted 3.704 seconds. The metal liquid levels in all sprues were basically the same and rose smoothly, enabling complete filling of the mold cavity. As shown in Figure 6a, at the beginning of filling, the casting connected to the sprue in the middle position had a faster pouring speed of 0.513 m/s, while the metal liquid flow in the mold cavities on other sprues was smooth and had a lower speed, remaining below 0.385 m/s, which is beneficial for exhaust and scum removal.
The simulation results of the solidification process for the improved process scheme. It can be seen that the casting has the characteristics of thin walls, a small volume, and a large surface area. During pouring, the heat loss rate of the metal liquid is fast, causing the castings arranged symmetrically on the left and right sides and in an up-down distribution to solidify almost simultaneously, and both began to solidify before the internal runner. When the internal runner completely solidified, the metal liquid in the sprue near the internal runner in the runner gradually began to solidify. This solidification sequence from the casting to the pouring system can effectively utilize the feeding effect of the internal runner, providing sufficient metal liquid at the top of the casting to compensate for the volume reduction caused by the solidification shrinkage of the metal liquid, avoiding the formation of shrinkage porosity and cavities inside the casting. The critical value of the feeding solid fraction was taken as 70%, i.e., when the solid fraction of the metal liquid exceeds 70%, the flow channel for feeding of the metal liquid is closed, and feeding is no longer possible. As shown in Figure 7b, when the solid fraction of the casting exceeded 70%, the solid fraction of the internal runner was always below 70%, indicating that the internal runner could still feed the casting. Therefore, the improved process scheme achieves sequential solidification from the casting to the pouring system. This solidification sequence can significantly reduce the tendency for defects to form inside the casting, which is beneficial for the solidification and forming of the casting.