Abstract: This article focuses on the study of magnesium – based ceramic cores for stainless steel investment casting. It discusses the preparation process, properties, and the influence of various factors on the cores. Additionally, it analyzes the potential casting defects associated with the use of these cores and provides possible solutions. The research aims to improve the quality of stainless steel investment castings through a better understanding of ceramic cores.
1. Introduction
Investment casting is a widely used manufacturing process for producing complex and high – precision metal components, especially in the stainless steel industry. The use of ceramic cores in investment casting plays a crucial role in creating intricate internal cavities and shapes within the castings. However, the selection and optimization of ceramic cores are essential to avoid casting defects and ensure the quality of the final products.
1.1 Background of Investment Casting
Investment casting has a long history and has evolved over time to meet the increasing demands of modern manufacturing. It involves creating a wax pattern, coating it with a refractory material to form a shell, melting out the wax, and then pouring molten metal into the shell. The process allows for the production of components with high dimensional accuracy and complex geometries.
1.2 Importance of Ceramic Cores in Investment Casting
Ceramic cores are used to form internal cavities or passages in castings that cannot be easily achieved using other methods. They provide support during the casting process and are later removed to reveal the desired internal structure. In stainless steel investment casting, the choice of ceramic core material and its properties significantly impact the quality of the casting.
2. Materials and Methods
2.1 Raw Materials
The main raw material for the magnesium – based ceramic core is magnesium oxide (MgO), with a purity of over 98% and a particle size of 400 mesh. Calcium carbonate (CaCO₃) is used as a mineralizer, and wood flour is added as a pore – making agent.
2.2 Core Forming Process
The materials are weighed according to a specific ratio and then ball – milled for 2 hours at a speed of 350 r/min with a ball – to – material ratio of 1:1. The milled powder is then mixed with a 5% polyvinyl alcohol solution and ground evenly in a mortar. The resulting mixture is loaded into a mold, which is pre – coated with silicone oil to reduce friction. The core is then pressed at a pressure of 6 MPa for 2 minutes, resulting in a sample size of 60 mm × 10 mm × 6 mm.
2.3 Core Sintering Process
The pressed samples are sintered in a high – temperature furnace with a heating rate of 2 °C/min. The samples are held at various temperatures (100 °C, 200 °C, 400 °C, 600 °C, 800 °C, 1000 °C, 1200 °C) for 30 minutes each to ensure uniform heating, and finally held at 1340 °C for 30 minutes before being cooled with the furnace.
2.4 Core Performance Testing
2.4.1 Physical Properties Testing
- Flexural Strength: Measured using the three – point bending method on a CMT – 4503 universal material testing machine with a span of 30 mm and a loading speed of 0.5 mm/min. Six samples are tested for each group, and the average value is reported.
| Sample Group | Flexural Strength (MPa) |
| Pure MgO Core | 7.57 |
| 5% CaCO₃ Core | 24.03 |
| 10% CaCO₃ Core | — |
| 15% CaCO₃ Core | — |
| 20% CaCO₃ Core | — |
- Porosity: Determined using the drainage method.
| Sample Group | Porosity (%) |
| Pure MgO Core | — |
| 5% CaCO₃ Core | — |
| 10% CaCO₃ Core | — |
| 15% CaCO₃ Core | — |
| 20% CaCO₃ Core | — |
- Sintering Shrinkage Rate: Calculated based on the size change of the core before and after sintering.
| Sample Group | Sintering Shrinkage Rate (%) |
| Pure MgO Core | 0.22 |
| 5% CaCO₃ Core | 0.97 |
| 10% CaCO₃ Core | — |
| 15% CaCO₃ Core | — |
| 20% CaCO₃ Core | — |
2.4.2 Dissolution and Collapse Properties Testing
The dissolution and collapse properties of the ceramic core are evaluated by measuring the mass loss rate of the core after soaking in a 40% acetic acid solution at 80 °C for 10 hours.
| Sample Group | Mass Loss Rate (%) |
|---|---|
| Pure MgO Core | 100 |
| 5% CaCO₃ Core | 50.22 |
| 10% CaCO₃ Core | 45.17 |
| 15% CaCO₃ Core | 52.49 |
| 20% CaCO₃ Core | 55.34 |
3. Results and Discussion
3.1 Influence of CaCO₃ Content on Core Properties
3.1.1 Flexural Strength
As the CaCO₃ content increases from 0% to 5%, the flexural strength of the core increases significantly from 7.57 MPa to 24.03 MPa. However, as the CaCO₃ content further increases from 5% to 20%, the flexural strength gradually decreases. This is because CaCO₃ promotes sintering at low concentrations but causes a decrease in strength at higher concentrations due to the formation of pores and a change in the microstructure.
3.1.2 Porosity
The porosity of the core shows a different trend. Initially, with the addition of 5% CaCO₃, the porosity decreases compared to the pure MgO core. But as the CaCO₃ content increases from 5% to 20%, the porosity gradually increases. This is related to the decomposition of CaCO₃ and its effect on the microstructure of the core.
3.1.3 Sintering Shrinkage Rate
The sintering shrinkage rate of the pure MgO core is very low (0.22%). With the addition of CaCO₃, the shrinkage rate increases. When the CaCO₃ content is between 5% and 20%, the shrinkage rate remains relatively stable.
3.1.4 Dissolution and Collapse Properties
The pure MgO core completely collapses in the acetic acid solution after 10 hours. With the addition of CaCO₃, the core maintains its overall shape although the surface layer may peel off. The dissolution and collapse properties are affected by the sintering degree and porosity of the core.
3.2 Influence of Wood Flour Content on Ceramic Core Properties
3.2.1 Flexural Strength
As the wood flour content increases, the flexural strength of the core decreases significantly. For example, when the wood flour content increases from 0% to 5%, the flexural strength decreases from 22.19 MPa to 4.73 MPa. This is because the wood flour burns out during sintering, leaving pores and weakening the structure of the core.
3.2.2 Porosity
The porosity of the core increases with the increase in wood flour content. When the wood flour content is 5%, the porosity reaches a relatively high value. This is due to the volatilization of the wood flour during sintering, creating more pores in the core.
3.2.3 Sintering Shrinkage Rate
The sintering shrinkage rate of the core shows a complex trend with the increase in wood flour content. Initially, it decreases with the addition of 1% wood flour, then increases with further increases in wood flour content.
3.2.4 Dissolution and Collapse Properties
The dissolution and collapse properties of the core improve with the increase in wood flour content. When the wood flour content is 5%, the mass loss rate in the acetic acid solution reaches 70.12%, which is much higher than that without wood flour.
4. Casting Trials and Core Removal
4.1 Casting of Stainless Steel Samples
Stainless steel samples were cast using the prepared ceramic cores with different wood flour contents. The casting process involved using a silica sol shell, with a shell baking temperature of 1160 °C and a baking time of 60 minutes. The molten steel was poured at a temperature of 1570 – 1575 °C. After casting, the samples were cooled, the shell was removed by vibration, and the sand was removed using a crawler – type shot blasting machine.
4.2 Core Removal
The cast stainless steel samples with the ceramic cores were placed in a 40% acetic acid solution at 80 °C to dissolve the cores. The dissolution rate of the cores was measured over time. It was found that the core with 5% wood flour had the fastest dissolution rate of 0.78 mm/h, while the core with 3% wood flour had the slowest dissolution rate of 0.49 mm/h.
5. Casting Defects and Solutions
5.1 Common Casting Defects
5.1.1 Incomplete Filling
This defect may occur when the molten metal does not completely fill the cavity formed by the ceramic core. It can be caused by factors such as improper gating system design, insufficient pouring temperature, or poor fluidity of the molten metal.
5.1.2 Porosity
Porosity in the casting can be caused by the presence of gas bubbles trapped during the casting process. This can be due to improper degassing of the molten metal or the release of gas from the ceramic core during sintering.
5.1.3 Cracking
Cracking can occur in the casting due to thermal stress during cooling. If the ceramic core has a large difference in thermal expansion coefficient with the stainless steel, it can lead to cracking in the casting.
5.2 Solutions to Casting Defects
5.2.1 Optimization of Gating System
A well – designed gating system can ensure proper flow of the molten metal into the cavity formed by the ceramic core. This includes selecting the appropriate gate size, shape, and location.
5.2.2 Degassing of Molten Metal
Proper degassing of the molten metal can reduce the presence of gas bubbles in the casting. This can be achieved through methods such as vacuum degassing or using degassing agents.
5.2.3 Selection of Compatible Ceramic Core Material
Choosing a ceramic core material with a similar thermal expansion coefficient to the stainless steel can reduce the risk of cracking. Additionally, optimizing the properties of the ceramic core, such as porosity and sintering shrinkage rate, can also help in reducing casting defects.
6. Conclusion
The research on magnesium – based ceramic cores for stainless steel investment casting has provided valuable insights into the preparation and properties of these cores. The addition of CaCO₃ and wood flour has a significant impact on the core properties. The casting trials have demonstrated the feasibility of using these cores in stainless steel investment casting. However, to ensure high – quality castings, it is essential to address the potential casting.