1. Introduction
Investment casting is a crucial manufacturing method for producing high – precision components with complex geometries and varying wall thicknesses. It has found extensive applications in the automotive industry, especially for manufacturing engine parts. The engine pre – combustion chamber insert is a vital component that operates under high – temperature, high – pressure, and high – load conditions. Therefore, ensuring its high – quality formation is of utmost importance, with no allowable defects such as shrinkage cavities and pits on its surface.
1.1 Background
In the investment casting process of engine pre – combustion chamber inserts, challenges often arise in achieving the desired quality. The original horizontal pouring process scheme may lead to a relatively high rejection rate, with defects appearing on the top surface of the castings. This necessitates a detailed investigation and optimization of the casting process.
1.2 Objectives
The primary objectives of this study are to analyze the causes of casting defects in the existing process, design an improved pouring system, optimize the process parameters through simulation and orthogonal tests, and ultimately achieve a significant reduction in defects and an increase in the qualified rate of castings.
2. Materials and Methods
2.1 Casting Structure and Test Materials
The engine pre – combustion chamber insert is precision cast in one step, with only the inner runner requiring post – processing for cutting separation, while other parts need no additional machining. The three – dimensional model of the casting, constructed using SolidWorks software, has dimensions of 42 mm×42 mm×25 mm, with a top wall thickness of 7 mm and a skirt wall thickness of 2 mm, demonstrating uneven wall thickness and being classified as a precision small part.
The material used for the casting is 4Cr9Si2 martensitic heat – resistant steel. The actual chemical composition and content of the material were detected using a spectrometer and are presented in Table 1. The thermal physical properties of the material were calculated in ProCAST software, revealing that the thermal conductivity and enthalpy values vary significantly with temperature, with an overall increasing trend.
| Main Elements | C | Si | Mn | S | P | Cr | Ni | Fe |
|---|---|---|---|---|---|---|---|---|
| Specified Range | 0.35 – 0.50 | 2.0 – 3.0 | ≤0.70 | <0.030 | <0.035 | 8.0 – 10.0 | ≤0.60 | Remainder |
| Actual Content | 0.40 | 2.5 | 0.60 | 0.030 | 0.030 | 9.0 | 0.50 | Remainder |
2.2 Shell Preparation and Pouring Process
- Wax Model Preparation: Medium – temperature wax was used, and a double – station die – head hydraulic wax injection machine was employed to prepare wax models in the mold. The wax injection temperature ranged from 60 to 65 °C, the time was 12 – 14 s, and the injection pressure was 25 – 30 kg/cm². After cooling to room temperature, the wax models were trimmed and assembled using a manual bonding method.
- Shell Fabrication: The shell process consisted of four and a half layers, namely the surface layer, transition layer, two reinforcement layers, and a sealing layer. The surface layer slurry was prepared with a silica sol binder and zircon powder, with a viscosity of 33 – 45 s and 100 – 120 mesh zircon sand added. The transition layer and sealing layer slurries were prepared with a silica sol binder and mullite powder, with viscosities of 16 – 24 s and 12 s respectively, and 30 – 60 mesh mullite sand added for the transition layer. The reinforcement layer slurry had a viscosity of 14 – 16 s and 16 – 30 mesh mullite sand added. The drying and hardening conditions were as follows: a temperature of 20 – 25 °C, a humidity of 65% – 75% for the surface and transition layers, and a humidity of 45% – 65% for the reinforcement and sealing layers, with a drying time of 10 – 14 h.
- Wax Removal and Shell Firing: High – pressure steam was used for wax removal, with the internal pressure of the dewaxing kettle set to 0.6 – 0.8 MPa, the steam temperature to 150 – 160 °C, and the dewaxing time to 10 – 15 min. The shell was fired using a rotary regenerative energy – saving gas – fired kiln, with the firing temperature set according to the process scheme and a firing time of 30 – 60 min, and a holding time of not less than 45 min to ensure the removal of residual wax, organic matter, and moisture from the shell.
- Melting and Pouring: A 150 kg medium – frequency induction furnace was used for melting, and a Spectro optical emission spectrometer was used to analyze the alloy composition. Pouring was carried out after reaching the pouring temperature. After cooling to room temperature, the castings were post – processed, with the surface quality and geometric dimensions inspected to ensure a smooth and defect – free surface and compliance with the drawing requirements.
2.3 Process Parameter Setting and Orthogonal Test
The determination of casting process parameters takes into account various factors such as casting structure, material properties, quality requirements, and practical casting experience. For 4Cr9Si2 steel, the solidus temperature is 1152.6 °C, and the liquidus temperature is 1438.8 °C. Considering that the pouring temperature is generally 100 – 150 °C higher than the liquidus temperature of the metal, an initial pouring temperature of 1600 °C and a shell preheating temperature of 1100 °C were selected. The shell thickness was set to 6 mm, the interface heat exchange coefficient between the casting, pouring system, and shell was 500 W/(m²·K), and natural cooling was adopted with an ambient temperature of 20 °C. The pouring time for the original process scheme was calculated to be 3 s using the formula t=C√G, where C is a coefficient related to the relative density Kv of the casting (taken as 1.4) and G is the total mass of the molten metal.
Based on the original process parameter values, combined with the results of single – factor tests and practical casting experience, three key process parameters that significantly affect casting quality, namely pouring temperature (A), pouring time (B), and shell preheating temperature (C), were selected as test factors for an orthogonal test. The factor level distribution for the orthogonal test is shown in Table 2.
| Level | A (°C) | B (s) | C (°C) |
|---|---|---|---|
| 1 | 1570 | 3 | 1050 |
| 2 | 1600 | 4 | 1100 |
| 3 | 1630 | 5 | 1150 |
ProCAST software was used to set the relevant process parameters according to the orthogonal test arrangement for simulation calculations. The filling and solidification processes were simulated and analyzed, and the shrinkage porosity rate was used as a quality evaluation index for comparison and analysis. The shrinkage porosity rate yi was calculated using the formula yi=VP/VC*100%, where VP is the total volume of shrinkage cavities and porosity formed during the solidification of the molten metal, and VC is the total volume of the casting. Range analysis and variance analysis were used to process and analyze the orthogonal test data to determine the optimal set of casting process parameters and improve the product qualification rate.
3. Results and Discussion
3.1 Original Process Scheme and Defect Analysis
The original process scheme adopted a horizontal pouring design with 30 pieces per mold. The three – dimensional model was imported into ProCAST software, and the mesh was divided and repaired using Visual – Mesh. To ensure simulation accuracy and shorten the simulation time, the mesh unit size for the casting and inner runner was set to 1 – 2 mm, that for the sprue was set to 3 – 5 mm, and that for the runner and pouring cup was set to 8 – 12 mm, resulting in approximately 2.6 million meshes. After switching to the Visual – CAST module and setting the relevant process parameters, simulation calculations were carried out.
The defect simulation of the original process scheme revealed that most of the castings had defects, resulting in a relatively low casting yield. The calculated shrinkage porosity rate for this scheme was 3.358%, and the defects were generally observed in the area near the waist hole on the top of the insert. The main reasons for the defects are as follows:
- Volume Contraction of Cast Steel: The volume contraction of cast steel occurs throughout the casting process, including liquid contraction, solidification contraction, and solid contraction, which easily leads to shrinkage defects. The insert casting has a thin wall and small volume, and the feeding channel is obstructed by the waist hole structure, resulting in the formation of external shrinkage cavities and other defects on its top surface.
- Flow Turbulence during Filling: During filling, the molten metal impacts at the inner runner inlet, causing local flow turbulence, preventing gas from being expelled, and the small size of the inner runner inlet also contributes to the formation of defects.
Actual castings produced using the original process scheme showed concave pits on the edge of the top surface, as well as relatively severe shrinkage cavities and rat – tail indentations. The casting qualification rate was only 50%, and the simulation results were consistent with the actual defect distribution.
3.2 Improved Process Scheme and Simulation Analysis
3.2.1 Pouring System Design
A reasonable pouring system design can significantly reduce shrinkage porosity and other defects in castings. The design of the inner runner is one of the key aspects of casting process design. For areas with high casting quality requirements, inner runners should be avoided. Considering the high requirement for the surface roughness of the top surface of the engine pre – combustion chamber insert during use, the inner runner should be located away from the top surface. The casting has a cylindrical shape, and the skirt wall thickness is less than the top thickness. During solidification, the molten metal on the top of the casting will feed to the skirt, so slight shrinkage porosity and shrinkage cavities may appear on the top. At the same time, considering the small mass and large number of castings, it is inconvenient to add feeding risers due to the relatively small solidification contraction volume of a single casting. After comprehensive analysis, the inner runner was set on the side wall of the top of the insert to facilitate feeding to the top of the casting, and the cross – sectional area of the inner runner was increased to ensure a smooth feeding channel during solidification. The cross – sectional area of the inner runner was designed as a rectangle with a length of 10 mm for convenient cutting and processing.
The pouring position of the casting refers to its position in the mold during pouring and should ensure that the key surface of the casting is located at the bottom or side to ensure its forming quality. When assembling the casting and the pouring system, the wax models of the insert casting and the inner runner were tilted by about 45° so that the top surface of the insert was in a side position. The tilted inner runner can maintain a consistent flow direction of the molten metal during filling and ensure a smooth filling process, avoiding the direct intersection of the molten metal in the mold cavity to form turbulence. At the same time, the gas in the mold cavity and the inclusions in the molten metal can be quickly expelled through the upper part of the inner runner. Exhaust channels were set at both ends of the runner to expel the gas in the mold cavity during filling and reduce the filling resistance of the molten metal. The improved process scheme adopted a symmetrical vertical design of runner – sprue – inner runner, 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 s using the formula T=C√G, while other process parameters remained the same as those of the original process scheme. The simulation results of the filling process of the improved process scheme showed that most of the molten metal flowed from the sprue in the middle position to the bottom runner, then from the middle of the bottom runner to the two side sprue, and finally filled the mold cavity step by step upwards. The entire filling process lasted 3.704 s. In all the sprue, the liquid level of the molten metal was basically the same and rose steadily, enabling a complete filling of the mold cavity. At the beginning of filling, the casting pouring speed connected to the sprue in the middle position was relatively fast, reaching 0.513 m/s, while the flow of the molten metal in the other sprue was steady and the speed was lower, remaining below 0.385 m/s, which was beneficial for exhaust and slag removal.
The simulation results of the solidification process of the improved process scheme showed that the casting had the characteristics of a thin wall, small volume, and large surface area. The heat dissipation rate of the molten metal during pouring was relatively fast, so the castings arranged symmetrically on both sides and distributed up and down almost solidified simultaneously and all solidified before the inner runner. When the inner runner was completely solidified, the molten metal near the inner runner in the sprue gradually began to solidify. This solidification sequence from the inner runner to the sprue can effectively utilize the feeding function of the inner runner, ensuring that there is enough molten metal on the top of the casting to supplement the volume reduction caused by the solidification contraction of the molten metal, avoiding the formation of shrinkage porosity and shrinkage cavities in the casting. Taking the critical value of the feeding solid fraction as 70%, that is, when the solid fraction of the molten metal exceeds 70%, the feeding channel of the molten metal is closed and feeding cannot be carried out. From the simulation results, it can be seen that when the solid fraction of the casting is greater than 70%, the solid fraction of the inner runner is always less than 70%, indicating that the inner runner can still feed the casting. Therefore, the improved process scheme realizes the sequential solidification from the casting to the pouring system, which can significantly reduce the tendency of defect formation in the casting and is beneficial for the solidification forming of the casting.
The defect distribution of the improved process scheme showed that there were a few slight shrinkage cavities on the top area of some castings on both sides, and a large number of defects were generated in the sprue and the bottom runner. The defects in the sprue were evenly distributed in the middle position between adjacent inner runners. The main reason for this phenomenon is that during the solidification of the casting, the molten metal near the inner runner preferentially feeds the casting through the inner runner, while the molten metal in the sprue that is far from the inner runner (that is, the molten metal in the upper and lower regions where the sprue and the inner runner are connected) will flow towards the inner runner. At the same time, the inner runner and its adjacent area have already begun to solidify gradually, blocking the feeding channel of the molten metal in the sprue, resulting in the generation of defects in the sprue. The shrinkage porosity rate of this scheme was calculated to be 1.634%, which was 51.34% lower than that of the original process scheme.
3.3 Process Parameter Optimization
Although the defects of the castings in the improved process scheme were significantly reduced compared to those in the original process scheme, and the number of qualified castings increased, the qualification rate still did not meet the actual production requirements. Therefore, an orthogonal test method was used to explore the influence of various factors on casting quality and optimize the process parameters to improve the forming quality of the castings. A three – factor three – level orthogonal test table (Lg(3^3)) was used, and ProCAST software was used to simulate each group of tests and record the results.
| Test Number | A (°C) | B (s) | C (°C) | (%) |
|---|---|---|---|---|
| 1 | 1570 | 3 | 1050 | 1.724 |
| 2 | 1570 | 4 | 1100 | 1.572 |
| 3 | 1570 | 5 | 1150 | 1.953 |
| 4 | 1570 | 3 | 1100 | 1.629 |
| 5 | 1570 | 4 | 1150 | 1.897 |
| 6 | 1570 | 5 | 1100 | 1.483 |
| 7 | 1570 | 3 | 1150 | 2.060 |
| 8 | 1570 | 4 | 1100 | 1.513 |
| 9 | 1570 | 5 | 1150 | 1.367 |
Range analysis of the orthogonal test results was carried out to determine the primary and secondary influences of each factor. The mean values of the shrinkage porosity rates of each test factor and its various level combinations were calculated, and the range Rx was solved. The calculated results were RA=0.31021, RB=0.61064, and RC=1.34237. The influence degree of a test factor on the optimization target was judged by the size of its range value. A larger range value indicates a relatively greater influence on the optimization target, and vice versa. By comparing the range values of the three factors, the order of influence of each factor on the shrinkage porosity rate was determined as RC>RB>RA, that is, the shell preheating temperature had the greatest influence on the shrinkage porosity rate, and the pouring temperature.