Abstract: This article comprehensively reviews the research progress of materials for investment casting molding shells. It begins with an introduction to the significance of investment casting and the importance of molding shell materials in achieving high-quality castings. The refractory materials, binders, and additives used in molding shells are discussed in detail, including their types, properties, applications, and recent advancements. The article also highlights the challenges and future directions in the development of molding shell materials, with a focus on the pursuit of improved performance, environmental friendliness, and intelligent manufacturing. Through a thorough analysis of the current state of the art, this review provides valuable insights for researchers and practitioners in the field of investment casting.
1. Introduction
1.1 Investment Casting Process and Significance
Investment casting, also known as precision casting, is a manufacturing process with a long history and significant importance in modern industry. The process involves creating a wax pattern of the desired part, coating it with a refractory material to form a shell, dewaxing the shell, and then pouring molten metal into the cavity. This method allows for the production of complex and high-precision components with excellent surface finish and dimensional accuracy. It is widely used in various industries, especially in aerospace, automotive, and medical fields, where the demand for high-quality and intricately shaped parts is crucial.
1.2 Importance of Molding Shell Materials
The quality of the molding shell directly affects the quality of the final casting. The molding shell serves as a container that holds the molten metal during casting and determines the shape, surface quality, and dimensional accuracy of the casting. It must possess several important properties, such as high temperature resistance, good mechanical strength, low thermal expansion coefficient, and excellent chemical stability. The selection and development of appropriate molding shell materials are, therefore, essential for achieving high-quality investment castings.
2. Refractory Materials
2.1 Types and Properties of Refractory Materials
2.1.1 Oxide Ceramic Materials
Oxide ceramic materials are commonly used in investment casting due to their excellent high-temperature properties. Zirconium oxide (ZrO₂) is a popular choice for its high melting point, good chemical stability, and low thermal expansion coefficient. Calcium oxide (CaO) is another oxide material with unique thermodynamic stability, although it requires careful handling due to its reactivity with moisture and carbon dioxide. Yttrium oxide (Y₂O₃) is known for its high-temperature resistance, corrosion resistance, and low thermal conductivity, making it suitable for casting titanium alloys. Alumina (Al₂O₃) -based materials, such as electro-fused corundum, have good thermal shock resistance and slag resistance, and are often used in casting high-carbon steels.
2.1.2 Non-oxide Materials
Non-oxide materials, such as graphite and silicon carbide (SiC), have also been studied for their potential use in investment casting. Graphite has good thermal conductivity and lubricity, but its oxidation resistance at high temperatures is a concern. SiC has high hardness and excellent thermal stability, but its brittleness and reactivity with some molten metals limit its application. In general, non-oxide materials are less commonly used as refractory materials compared to oxides, but they can offer unique properties in certain applications.
2.2 Application and Research Progress of Refractory Materials
2.2.1 Zirconium-Based Refractory Materials
Zirconium-based refractory materials, such as zircon sand and zirconia (ZrO₂), are widely used in the investment casting industry. Zircon sand is favored for its good fluidity and high coating coverage, which helps to prevent shell deformation during high-temperature processing. ZrO₂, with its high melting point and chemical inertness, is often used to improve the performance of composite refractory materials. For example, the addition of ZrO₂ to magnesia (MgO) refractory materials can enhance their corrosion resistance and thermal shock resistance. Research has also shown that zirconium-based refractory materials can effectively reduce the reaction between the shell and the molten metal, resulting in improved casting quality.
2.2.2 Calcium Oxide (CaO) and Related Materials
CaO is a refractory material with unique properties, especially in vacuum casting applications. However, its reactivity with moisture and carbon dioxide requires special handling and storage conditions. To overcome these limitations, researchers have developed calcium zirconate (CaZrO₃) by sintering CaO and ZrO₂ powders. CaZrO₃ has better high-temperature chemical stability and can be used as a substitute for ZrO₂ in some cases. Additionally, the addition of CaO to other refractory materials, such as barium zirconate (BaZrO₃), can improve their thermodynamic stability and reduce the reaction with titanium-rich alloys.
2.2.3 Yttrium Oxide (Y₂O₃) and Its Applications
Y₂O₃ is highly regarded for its excellent high-temperature properties and chemical stability. It is an ideal refractory material for casting titanium alloys, as it has minimal reaction with molten titanium. However, the high cost of Y₂O₃ limits its widespread use in industrial production. To address this issue, researchers have explored the use of Y₂O₃ as an additive in other refractory materials. For example, adding Y₂O₃ to MgO refractory materials can improve their sintering and thermal shock resistance, as well as reduce the grain size of the sintered material. This not only enhances the performance of the refractory material but also reduces the overall cost.
2.2.4 Electro-Fused Corundum and Alumina-Based Materials
Electro-fused corundum is a traditional refractory material with good thermal shock resistance and slag resistance. It is commonly used in the production of molds for casting high-carbon steels. In recent years, researchers have focused on improving the performance of alumina-based refractory materials by adding various additives. For example, the addition of nano-carbon black, carbon nanotubes, and graphene to Al₂O₃ – C refractory materials can enhance their mechanical properties and thermal shock resistance. The development of Al₂O₃ – MgO – CaO refractory materials has also attracted attention, as they offer a combination of good high-temperature resistance, lightweight properties, and energy-saving benefits.
2.3 Comparison and Selection of Refractory Materials
The selection of refractory materials depends on several factors, including the type of casting alloy, the required casting quality, and the cost considerations. For high-temperature alloy castings, zirconium-based refractory materials are often preferred due to their excellent performance and compatibility with the molten metal. When casting titanium alloys, Y₂O₃ – based materials or composites containing Y₂O₃ are more suitable to minimize the reaction between the shell and the alloy. In the case of casting high-carbon steels, electro-fused corundum and alumina-based materials are commonly used. Additionally, the cost of the refractory material is an important factor, especially for large-scale industrial production. Therefore, a careful balance between performance and cost is necessary when selecting refractory materials for investment casting.
The following table summarizes the properties and applications of different refractory materials:
| Refractory Material | Properties | Applications |
|---|---|---|
| Zirconium-based (ZrO₂, zircon sand) | High melting point, good chemical stability, low thermal expansion coefficient, good fluidity | High-temperature alloy casting, surface layer of mold shell |
| Calcium oxide (CaO) | Thermodynamic stability | Vacuum casting (with limitations due to reactivity) |
| Calcium zirconate (CaZrO₃) | Improved high-temperature chemical stability compared to CaO | Cast steel shell refractory (non-alkaline conditions) |
| Yttrium oxide (Y₂O₃) | High-temperature resistance, corrosion resistance, low thermal conductivity, stable chemical properties at high temperature | Casting titanium alloys (as refractory or additive), improving properties of other refractories |
| Electro-fused corundum (Al₂O₃-based) | Good thermal shock resistance, slag resistance | Casting high-carbon steels, surface layer of mold shell |
| Al₂O₃ – MgO – CaO | Good high-temperature resistance, lightweight, energy-saving | Casting steel shell refractory |
3. Binders
3.1 Types and Properties of Binders
3.1.1 Silica Sol
Silica sol is a widely used binder in investment casting due to its excellent properties. It can produce mold shells with high surface quality, good dimensional accuracy, and high-temperature strength. Silica sol is an environmentally friendly binder as it does not require the addition of hardeners and emits fewer pollutants during the casting process. However, its long drying time can affect the production efficiency. To overcome this limitation, researchers have developed fast-drying silica sols, such as FS – III and ZF – 801, which can significantly reduce the drying time and improve the overall performance of the mold shell.
3.1.2 Ethyl Silicate
Ethyl silicate is a traditional binder with a long history of use in investment casting. It has good wetting ability and slurry fluidity, which results in a short shell-making cycle. However, ethyl silicate has some drawbacks, such as long gelation time, poor air permeability, and the need for air purification in the workshop due to volatile gases. Despite these limitations, its unique properties, such as good deformability, make it suitable for certain applications. In recent years, research has focused on improving the performance of ethyl silicate binders, especially in terms of hydrolysis stability and gelation time.
3.1.3 Water Glass
Water glass is a cost-effective binder that offers short shell-making cycles and rapid hardening. It is commonly used for casting carbon steels and low-alloy steels where high dimensional accuracy is not required. However, the use of water glass as a binder is associated with environmental pollution issues due to the use of ammonium chloride as a hardener. To address these concerns, composite binders combining water glass and silica sol have been developed. These composite binders can reduce the environmental impact while maintaining the advantages of both binders.
3.2 Application and Research Progress of Binders
3.2.1 Silica Sol in Investment Casting
Silica sol has been the preferred binder in many investment casting applications, especially for high-quality castings. The development of fast-drying silica sols has revolutionized the shell-making process by significantly reducing the production time. For example, the FS – III silica sol developed by researchers can reduce the drying time of each shell layer to 1 – 2 hours, while the ZF – 801 silica sol can improve the surface quality of the casting and reduce the rejection rate. Additionally, modifications of silica sol, such as the addition of carbon – nylon hybrid fibers, have been studied to enhance the mechanical and thermal properties of the mold shell.
3.2.2 Ethyl Silicate and Its Modifications
Although ethyl silicate has some limitations, its unique properties make it a valuable binder in certain cases. Research has been conducted to improve its performance, such as the study of ethyl silicate 50, which has better hydrolysis stability compared to other grades. The combination of ethyl silicate with silica sol has also been explored to take advantage of the strengths of both binders. For example, alternating the use of ethyl silicate and silica sol as binders can reduce the drying time and improve the overall quality of the mold shell.
3.2.3 Water Glass and Composite Binders
Water glass – silica sol composite binders have gained popularity in recent years due to their ability to balance cost and environmental concerns. These composite binders can produce mold shells with improved surface quality and dimensional accuracy compared to water glass alone. The development of new hardening agents and modification techniques for water glass has also been an area of research focus. For example, the use of aluminum chloride as a hardener in the back layer of the mold shell can reduce the ammonia emissions and improve the strength of the shell.
3.3 Comparison and Selection of Binders
The selection of binders depends on various factors, including the casting requirements, production efficiency, and environmental considerations. Silica sol is an excellent choice for high-quality castings that require good surface finish and dimensional accuracy, especially in applications where environmental impact is a concern. Ethyl silicate is suitable for applications where good wetting ability and deformability are required, although its drawbacks need to be carefully considered. Water glass is a cost-effective option for less demanding applications, but the use of composite binders can help to overcome its limitations. In general, a combination of different binders may be used to optimize the performance of the mold shell based on specific casting needs.
The following table compares the properties of different binders:
| Binder | Advantages | Disadvantages | Applications |
|---|---|---|---|
| Silica Sol | High surface quality, good dimensional accuracy, high-temperature strength, environmentally friendly | Long drying time | High-quality castings, aerospace and automotive industries |
| Ethyl Silicate | Good wetting ability, short shell-making cycle, good deformability | Long gelation time, poor air permeability, volatile gases | Some applications where deformability is important |
| Water Glass | Short shell-making cycle, cost-effective | Environmental pollution (due to hardener), low dimensional accuracy | Casting carbon steels and low-alloy steels |
| Water Glass – Silica Sol Composite | Reduced environmental impact, improved surface quality compared to water glass alone | – | Applications where a balance between cost and performance is required |
4. Additives
4.1 Types and Functions of Additives
4.1.1 Refiners
Refiners are additives used to improve the surface quality and mechanical properties of castings. They work by promoting the nucleation and growth of fine grains in the molten metal, which helps to reduce casting defects such as porosity and cracks. Commonly used refiners include cobalt aluminate (CoAl₂O₄), which is widely used in the casting of high-temperature alloys. New types of refiners, such as the W powder – alloy powder mixture developed by researchers, have also shown promising results in improving the surface quality of castings.
4.1.2 Defoamers
Defoamers are essential in the slurry preparation process to prevent the formation of bubbles, which can cause defects in the mold shell. They work by reducing the surface tension of the slurry and promoting the coalescence and rupture of bubbles. Common defoamers used in investment casting include octanol and polyoxypropylene glycerol ether (GP). The choice of defoamer depends on factors such as the type of binder and the desired effect on the slurry properties.
4.1.3 Mineralizers
Mineralizers are added to the mold shell slurry to enhance the sintering process and improve the mechanical properties of the shell. They promote the formation of new phases and improve the bonding between the refractory particles. For example, the addition of Al – Si – Ca and Al – Si – Mg mineralizers to alumina-based coatings can significantly increase the strength of the mold shell at high temperatures. The proper selection and dosage of mineralizers are crucial to achieve the desired effect without introducing unwanted impurities.
4.2 Application and Research Progress of Additives
4.2.1 Refiners in Casting Process
Refiners have been widely studied and applied in the investment casting of various alloys. The addition of CoAl₂O₄ to the slurry has been shown to effectively refine the grain structure of high-temperature alloys, leading to improved mechanical properties and reduced casting defects. The new type of refiner developed by Wang Haiwei has shown great potential in improving the surface quality of castings by providing a better lattice match with the alloy and promoting homogeneous nucleation.
4.2.2 Defoamers and Their Effects
Defoamers play a crucial role in ensuring the quality of the mold shell slurry. Octanol is commonly used to reduce the surface tension of water glass binder slurries, improving the coating quality. GP, on the other hand, has a purification function and is more suitable for applications where environmental friendliness is a concern. The proper use of defoamers, including the correct dosage and mixing method, can significantly reduce the occurrence of bubbles in the slurry and improve the integrity of the mold shell.
4.2.3 Mineralizers for Shell Performance Enhancement
Mineralizers have been extensively investigated to improve the performance of mold shells. The addition of specific mineralizers can increase the strength and thermal stability of the shell, making it more suitable for high-temperature casting applications. For example, the use of ASM – Cr mineralizer in mullite-bonded corundum shells can enhance the toughness of the matrix by modifying the expansion behavior of mullite. However, the use of mineralizers requires careful consideration of their effects on other properties of the shell, such as porosity and chemical stability.
4.3 Importance of Additive Optimization
The optimization of additives is crucial for achieving high-quality investment castings. The proper combination and dosage of additives can have a significant impact on the properties of the mold shell and the final casting. For example, the choice of refiner depends on the alloy being cast and the desired grain refinement effect. The use of defoamers must be carefully controlled to avoid over-defoaming, which can affect the slurry rheology. Mineralizers should be selected based on the specific requirements of the shell, taking into account factors such as sintering temperature and desired mechanical properties. Overall, additive optimization is an ongoing area of research to improve the performance and reliability of investment casting processes.
The following table summarizes the functions and applications of different additives:
| Additive | Function | Application |
|---|---|---|
| Refiners (e.g., CoAl₂O₄, W powder – alloy powder mixture) | Promote nucleation and growth of fine grains, improve surface quality | High-temperature alloy casting, improving casting quality |
| Defoamers (octanol, GP) | Reduce surface tension, prevent bubble formation, improve slurry coating quality | Slurry preparation for mold shell, ensuring shell integrity |
| Mineralizers (Al – Si – Ca, Al – Si – Mg, ASM – Cr) | Enhance sintering process, improve mechanical properties of shell | Alumina-based coatings, mullite-bonded corundum shells, increasing shell strength and thermal stability |
5. Future Trends and Challenges
5.1 Development Trends of Molding Shell Materials
5.1.1 High-Performance and Environmentally Friendly Materials
The future of investment casting will see a continued emphasis on the development of high-performance and environmentally friendly molding shell materials. This includes the search for new refractory materials with improved thermal and mechanical properties, as well as the development of binders and additives that are less harmful to the environment. For example, the use of bio-based binders and recycled refractory materials is an area of active research. Additionally, the optimization of material combinations and processing techniques will be crucial to achieve the desired performance while minimizing the environmental impact.
5.1.2 Intelligent Manufacturing in Investment Casting
Intelligent manufacturing is set to play an increasingly important role in the investment casting industry. This involves the integration of advanced technologies such as artificial intelligence, machine learning, and sensor networks into the casting process. For example, the use of computer-aided design (CAD) and computer-aided manufacturing (CAM) systems allows for more precise design and fabrication of wax patterns and molds. Real-time monitoring of the casting process using sensors can provide valuable data on temperature, pressure, and flow, enabling better control and optimization of the process. Machine learning algorithms can be applied to analyze this data and predict potential defects or process variations, allowing for proactive adjustments to be made. Additionally, automated handling and inspection systems can improve production efficiency and reduce human error. The development of intelligent manufacturing technologies will not only enhance the quality and consistency of investment castings but also enable more efficient use of resources and reduced waste.
5.2 Challenges in Molding Shell Material Research
5.2.1 Meeting Stringent Quality Requirements
As industries such as aerospace and automotive continue to demand higher quality and more complex castings, the challenge of meeting stringent quality requirements becomes more prominent. This requires a deeper understanding of the relationship between molding shell materials and casting quality. The need for precise control of material properties, such as thermal expansion coefficient and chemical stability, is crucial to ensure that the final casting meets the required dimensional accuracy and surface finish. Additionally, the development of new testing and characterization methods is necessary to accurately evaluate the performance of molding shell materials under different casting conditions.
5.2.2 Cost Reduction and Process Optimization
Cost reduction is a perpetual challenge in the manufacturing industry. In investment casting, finding ways to reduce the cost of molding shell materials while maintaining or improving their performance is essential for competitiveness. This involves exploring alternative raw materials, optimizing manufacturing processes, and improving the efficiency of material usage. Process optimization also includes reducing the cycle time for shell making and casting, minimizing energy consumption, and reducing waste generation. Balancing cost reduction with quality improvement is a complex task that requires interdisciplinary research and collaboration between material scientists, engineers, and manufacturers.
5.3 Research Directions and Prospects
5.3.1 Novel Materials and Composites
The exploration of novel materials and composites for investment casting molding shells holds great promise. This includes the development of advanced ceramic materials with enhanced mechanical and thermal properties, as well as the design of composite materials that combine the advantages of different components. For example, the combination of ceramic and metallic phases may result in materials with improved toughness and thermal conductivity. Nanostructured materials and coatings are also an area of active research, as they can offer unique properties such as improved surface hardness and wear resistance. The development of these novel materials and composites will require a multidisciplinary approach, involving materials science, chemistry, and physics.
5.3.2 Sustainable and Green Casting Technologies
With the growing emphasis on environmental sustainability, the investment casting industry is moving towards more sustainable and green casting technologies. This includes the use of renewable and recyclable materials, as well as the development of energy-efficient and low-emission casting processes. The research and application of bio-based binders and refractory materials derived from waste or by-products are gaining attention. Additionally, the optimization of casting processes to reduce energy consumption and waste generation, such as the use of advanced cooling and solidification techniques, is an important research direction. The development of sustainable casting technologies will not only benefit the environment but also contribute to the long-term viability of the investment casting industry.
6. Conclusion
In conclusion, the research and development of materials for investment casting molding shells have made significant progress in recent years. The continuous improvement of refractory materials, binders, and additives has enabled the production of high-quality castings with complex geometries and excellent performance. However, there are still many challenges and opportunities in this field. Future research should focus on the development of high-performance and environmentally friendly materials, the integration of intelligent manufacturing technologies, and the optimization of casting processes. By addressing these challenges and seizing the opportunities, the investment casting industry can continue to meet the growing demands of various industries and contribute to the advancement of manufacturing technology.