Research on forming mechanism of digitalized frozen sand mold green casting

Abstract

Purpose

This study aims to promote the sustainable green development of the casting industry.

Design/methodology/approach

This paper presents an innovative frozen sand mold green casting technology that utilizes water as a novel alternative to conventional organic or inorganic binders in the sand mold production process. The sand is solidified through cryogenic treatment and subsequently transformed into complex, high-quality structures utilizing Computer Numerical Control machining or 3D printing methods.

Findings

In contrast to conventional sand mold manufacturing technology, this method has advantages in environmental conservation and sustainable production practices, especially its ability to minimize the emission of harmful gases and dust to nearly zero throughout the casting process. Additionally, the high thermal conductivity of the frozen sand molds significantly enhances the performance of the castings. Furthermore, by implementing an economic and efficient thermal dissipation circulation, it is possible to directly recycle more than 90% of the casting sand. This approach helps to avoid tedious procedures associated with traditional sand mold recovery methods that rely on physical and chemical techniques.

Social implications

This technology not only facilitates the green transformation of the contemporary casting industry but also establishes a new technology standard for more sustainable and environmentally friendly production processes.

Originality/value

This paper presents an innovative frozen sand mold green casting technology that utilizes water as a novel alternative to conventional organic or inorganic binders in the sand mold production process.

1. Introduction

Environmental conservation has emerged as a fundamental concern for nations, especially industrialized countries, as the concepts of “carbon neutrality” and “peak carbon dioxide emissions” underscore the urgency of this matter (Chen et al., 2022; Mallapaty, 2020; Wang et al., 2021a; Liu et al., 2022; Wu et al., 2022). In the field of mechanical manufacturing, casting technology and equipment are of paramount importance. However, conventional casting methods encounter difficulties related to increased energy consumption, increased expenses and reduced efficiency. Although China's foundry industry has strong growth momentum and currently leads the world in casting production, this achievement is accompanied by serious environmental pollution and high energy consumption costs. The current critical concern revolves around accelerating the enhancement and optimization of casting processes and the iterative improvement of forming equipment to attain efficient energy utilization and ensure effective environmental protection. Furthermore, high-performance complex castings play a crucial role in the manufacturing of high-end equipment, serving as the basis for such equipment. To achieve high-quality and sustainable development in the casting industry, essential pathways are needed including the adoption of environmentally friendly materials, process intensification and equipment digitization (Kazmi et al., 2021; Shan et al., 2022; Ranade et al., 2021).

To promote the sustainable green development of the casting industry, it is necessary to innovate and develop new casting principles, methods, processes and equipment. Traditional sand casting techniques have historically relied on organic or inorganic binders such as phenolic resin, furan resin, water glass or clay. These materials produce large amounts of solid waste and emit polluting gases throughout the molding and casting process. In the foundry workshop, unpleasant odors are released during the molding and core-making processes, mainly due to the presence of free formaldehyde, free phenol and free furfuryl alcohol in the resins. Among them, free formaldehyde and phenol present comparatively higher risks to human health. Most casting binders consist of organic chemicals and are a major source of harmful gas emissions, thereby demonstrating limited potential for environmentally friendly and sustainable progress. Numerous scholars (Mondejar et al., 2021; Lee et al., 2017; Qosim et al., 2020; Nishihora et al., 2018) have extensively researched green casting, covering fundamental theories, specialized materials, unique processes and system equipment. Among them, various advanced casting technologies such as binder-free lost foam casting (Egilegor et al., 2020; Dong et al., 2023), V-process casting (Prasath and Vignesh, 2017), water glass sand mold casting (Holtzer and Kmita, 2020), moldless composite forming casting (Zheng et al., 2023; Wang et al., 2021b; Shan, 2017) and sand mold 3D printing (Upadhyay et al., 2017; Sivarupan et al., 2021; Zhao et al., 2018; Mitra et al., 2019; Snelling et al., 2015; Walker et al., 2018) have experienced significant development and extensive utilization. Huazhong University of Science and Technology has pioneered a novel vacuum low-pressure lost foam molding technique. This method involves the compaction of dry sand particles under a vacuum environment, without the need for binders. The process offers superior insulation properties and is particularly advantageous for the production of thin-walled components (Jiang and Fan, 2018; Singh et al., 2018). A&B Corporation and Harmony Castings in the USA have collaborated to develop a V-process casting technology for aluminum alloys. This innovative technology enables the efficient production of intricate and thin-walled aluminum parts. China Academy of Machinery Science and Technology has introduced a moldless composite forming technology and equipment for intricate castings using 3D Computer Aided Design (CAD) models. This method enables the fast creation of sand molds and cores by cutting sand blanks directly, resulting in a 50–80% reduction in the processing cycle and approximately one-tenth decrease in processing costs. Consequently, manufacturing expenses are lowered by 30–50%, facilitating quick adaptability for diverse small-scale productions and high-performance complex castings. The digital sand mold 3D printing technology, which originated from Massachusetts Institute of Technology (MIT), encompasses sand mold selective laser sintering and sand mold micro-droplet jet forming (Sivarupan et al., 2019). EOS GmbH Electro Optical Systems (German), Voxeljet (German), ExOne (USA), Kocel (China) and Nanjing University of Aeronautics and Astronautics (China) have conducted comprehensive research aiming at enhancing materials, structural design, process integration and system equipment for foundry 3D printing. This has led to the development of a range of large-scale sand mold 3D printers, facilitating the industrial implementation of digital sand mold 3D printing technology (Hodder and Chalaturnyk, 2019; Del et al., 2020). The institutions mentioned above have conducted comprehensive and advanced investigations to promote green casting technology. They have overcome various common technical obstacles, introduced many innovative concepts into the field of green casting and broadened the application scope of related processes (Shan et al., 2023; Yang et al., 2023). Figure 1 shows the innovative development of sand casting technology. In 2020, Green Intelligent Manufacturing Technology and Equipment Team at Nanjing University of Aeronautics and Astronautics introduced a novel digitalized frozen sand mold green casting technology and equipment (Shan et al., 2020). This innovative approach utilizes water as a binder and employs low-temperature Computer Numerical Control (CNC) machining or printing to consolidate sand particles into molds, enabling the production of high-quality complex structure sand molds. Furthermore, this method facilitates the full-cycle recovery of sand mold materials. This approach efficiently addresses the severe environmental pollution, challenges in recycling waste sand and increased dust emissions associated with conventional sand casting techniques employing organic or inorganic binders. It provides a novel and effective strategy for promoting environmentally friendly and sustainable practices within the casting sector.

Scholars have conducted exploratory research in the field of frozen sand mold green casting technology, offering significant theoretical basis and guidance for the development and implementation of this technology. In 1970, W.H. Booth Company in the UK introduced frozen casting technology and proposed a novel concept using water as a binder instead of resin. Sand particles are mixed with an appropriate amount of water, placed in a mold and subsequently solidified in a low-temperature environment not exceeding −15 °C. Once the sand mold reaches adequate strength, it is removed so that the casting can be poured and solidified (Kita and Hino, 1980; Yan et al., 2004). This process presents several advantages, including reduced dust generation during molding, automatic disintegration of the sand mold after pouring, elimination of unpleasant odors and the possibility of reusing the sand (Fox et al., 2012). In 1980, the Japanese Industrial Technology Research Institute conducted strength tests on frozen sand molds. The results indicated that the compressive strength of the frozen sand molds peaked at 1.7 MPa–3.3 MPa at temperatures ranging from −50 °C to −60 °C (Omura and Tada, 2012). This result further confirms the feasibility of using frozen sand molds in casting applications. In 2005, Kansai University in Japan introduced a negative pressure frozen sand mold casting technique, which enabled the rapid solidification of the frozen sand molds (Nakayama et al., 2009). The conventional method of producing frozen sand molds predominantly utilized metal or wooden molds for the molding process. This approach encountered challenges related to limited digitalization, significant resource waste and poor dimensional accuracy. These issues have hindered the rapid development and industrial implementation of frozen sand mold green casting technology.

The digitalized frozen sand mold green casting forming technology proposed in this study is driven by 3D CAD model of complex sand mold structures. This method combines the principles of CNC machining and a 3D printing to directly cut or print frozen sand blanks, eliminating the need to make physical prototypes or molds. Therefore, this approach can prevent the sand mold precision from being degraded during the molding and/or demolding process. It can effectively shorten the casting time and realize green, flexible, high-precision and efficient production of various complex metal structures (Dong et al., 2009; Xu et al., 2013; Shan and Zhu, 2018; Shi et al., 2023).

In this paper, the forming method of digitalized frozen sand mold green casting, the technology for precise solidification of multi-alloy castings and the establishment and construction of a digitalized frozen sand mold green casting demonstration line were discussed in detail. Moreover, this paper explains and enhances the disciplinary framework and development trends in sand casting theory, thereby making significant contributions to the development of novel casting technologies and the improvement of the green casting theoretical framework. Figure 2 illustrates the comprehensive strategy developed by the research team, which is based on digitalized frozen sand mold green casting technology and corresponding equipment. This strategy includes the selection of environmentally friendly materials, digitalization of processing technologies, control of casting performance, automation of testing processes, utilization of intelligent forming equipment and implementation of recyclable manufacturing cycles. These initiatives are aimed at achieving integrated innovation in frozen sand mold green casting technology.

2. Principles and methods of frozen sand mold forming

The digitalized frozen sand mold green casting technology includes CNC machining forming and 3D printing of frozen sand molds. CNC machining for the formation of frozen sand molds relies on subtractive manufacturing principles implemented by three-axis or five-axis machines to achieve near-net shaping of complex sand molds. Quartz sand or specialized sand raw particles are pre-mixed with an appropriate amount of water, stirred and compacted and then solidified into a desired shape in a low-temperature environment ranging from −5 °C to −20 °C. The complex frozen sand molds are divided into manageable units for easier processing. The frozen sand blanks are processed through CNC machining based on 3D CAD model to create multiple unit sand molds (cores). These molds are then precisely assembled using a specified connection structure to obtain superior quality frozen sand molds. Subsequent ultra-low-temperature overcooling treatment in a low-temperature environment (≤–20 °C) results in enhanced strength, increased surface hardness and improved precision in frozen sand molds. This process is conducted under negative pressure and involves the pouring of multi-alloy system metal liquids. The utilization of low-temperature forming and overcooling through CNC machining ensures that the sand molds meet casting specifications. This method provides several benefits, including rapid response speed, high forming accuracy, excellent surface quality and suitability for mass production.

3D printing forming of frozen sand molds, also known as sand mold freezing printing technology, entails the precise spraying of a water-based adhesive onto a layer of initial sand particles. This process results in the production of complex sand molds through an in situ low-temperature solidification process. Using a piezoelectric micro-droplet ejector, the water-based adhesive is deposited onto the surface of the sand particles, creating a cross-sectional representation of a water-containing porous medium that is regulated by slice layer information programming. Simultaneously, a low-temperature gas rapidly flows over the area to induce freezing and solidification, resulting in the formation of an initially robust solidified layer. Given the relatively low bonding strength after the initial printing process, it is necessary to place 3D structure of the frozen sand mold in a low-temperature environment for ultra-low-temperature freezing solidification. This step aims to improve the final bonding strength of the sand mold. This method is applicable for producing frozen sand molds with complex structures, presenting new opportunities for small-scale green casting of complex components using frozen sand molds. Figure 3 depicts a schematic diagram illustrating the forming principle of digitalized frozen sand mold green casting. This process consists of various stages, including the low-temperature pre-cooling blanking process, low-temperature CNC machining process, low-temperature 3D printing process, ultra-low-temperature overcooling process, low-temperature casting process and sand particle recycling process.

3. Disruptive advantages of frozen sand mold green casting

3.1 Green binder – water

The utilization of water as a binding agent in sand mold casting represents a disruptive innovation within the casting industry. The strength and permeability of frozen sand molds play a critical role in determining their suitability for casting applications. The strength originates from the bonding bridges formed by ice crystals and the interfacial binding interaction with sand particles. The strength and other critical casting properties of frozen sand molds are impacted by the shape, distribution pattern, size and film thickness of the ice crystal bonding bridges. The frozen sand mold is composed of individual sand particles connected by ice crystal bonding bridges, which collectively bear and transmit loads when subjected to external forces. Therefore, it is crucial to investigate the microscopic fracture mechanisms of frozen sand molds under external forces to elucidate their macroscopic mechanical properties. This study employed Olympus microscopes and Zeiss micro-computed tomography (CT) to investigate the development, solidification and fracture micromorphology of ice crystal bonding bridges under low-temperature loading conditions using pure water and 70/140 mesh silica sand. The results suggest that before freezing, the water film exists in a free state within the porous medium formed by sand particles. Subsequently, during the process of freezing at low temperatures, the water film solidifies in situ, forming ice crystal bonding bridges, which impart strength in different temperature ranges (Figure 4(a)). In situ freezing CT analysis indicates that the water film within silica sand particles mainly appears as bulk shapes, whereas in zircon sand particles, it appears as consistent thin film morphology (Figure 4(b and c)). Further statistical analysis indicates that as the sand particle size decreases, there is an increase in the dispersion of water films among particles, leading to a higher proportion of small-sized water films. This phenomenon results in an increased formation of ice crystal bonding bridges at lower induced temperatures. This leads to a larger effective freezing bonding interface between the ice crystal bonding bridges and sand particles, resulting in higher sand mold strength. Furthermore, zircon sand exhibits higher surface energy, relatively lower specific surface area and better particle morphology, which enhances the wetting properties of the unbound water film on the surfaces of sand particles. This facilitates its uniform distribution along the contours of the sand particles. After the phase transition at low temperatures, the effective contact area between the ice crystal bonding bridges and sand particles within zircon sand exceeds that of silica sand. This results in increased tensile and compressive strength under identical water content and freezing conditions. Figure 4(d) depicts the process of liquid-solid phase transition of ice crystal bonding bridges within moist sand particles at low temperatures. The interfaces of the ice crystal bonding bridges between sand particles display a crescent shape. The formation process of ice crystal bonding bridges involves nucleation and crystal growth, which are affected by the temperature gradient among sand particles. In the process of fractures due to external forces, the tensile fracture mechanism of frozen sand molds is mainly manifested as internal fractures within the ice crystal bonding bridges and interfacial fractures at the junctions between sand particles and ice crystals. When the ice crystal bonding bridges are firmly attached to the sand surfaces, tensile fractures mainly occur within the ice crystals. When the bonding strength of the interface between the sand particle and the adhered ice layer is lower than the strength of the ice crystals themselves, fractures are more likely to occur at the interface layer between the sand particles and ice crystals (Figure 4(e)). Mechanical tests conducted on silica sand, chromite sand and zircon sand within multi-material frozen sand molds indicate that the tensile strength of these molds rises with decreasing freezing temperatures. This phenomenon can be attributed to the enhanced stability of the crystal structure of ice particles at lower temperatures. The increased stability significantly improves the strength of the ice and subsequently reinforces the overall structure of the sand mold. This relationship is depicted in Figure 4(f). When the water content of the sand mold ranges from 4 to 5 wt.% and the freezing temperature is equal to or less than −20 °C, the tensile strength of the frozen sand mold is at least 1.0 MPa and the permeability is at least 70, which meets the performance requirements of sand mold casting. However, when the water content is greater than or equal to 5 wt.%, despite the additional enhancement in the strength of the frozen sand mold, there is a potential risk of increased gas emission and porosity defects in the castings.

3.2 High-performance forming – superior organizational performance

In the green sand casting process of frozen sand molds, the precise formation of complex castings depends significantly on key factors such as determining the interfacial heat transfer coefficient (IHTC) and ensuring strength retention. The investigation of the heat exchange dynamics between frozen sand molds and castings, as well as the establishment of a coherent mapping correlation between various parameters, are essential steps towards attaining accuracy in casting formation. The physical properties of frozen sand molds are significantly different from traditional resin sand molds, making traditional air gap theories inapplicable. The variation pattern of the IHTC in green casting of frozen sand molds is determined by the conjugate gradient method, taking into account both air and geometric models. The temperature field distribution of complex alloys and sand mold structures is computed considering various moisture levels and temperatures of frozen sand molds. This calculation is then followed by IHTC's calibration of the sand molds using a back-calculation program. As the solidification temperature decreases during the casting process, the shrinkage rate of the castings decreases gradually. Simultaneously, the expansion rate of the frozen sand molds exceeds the metal's shrinkage rate, resulting in a reduction in the number and size of gaps between the casting and the sand mold. This change affects the heat transfer conditions, causing IHTC to gradually increase. Subsequently, as the internal temperature of the frozen sand mold increases, the water near the interface of the casting and the sand mold vaporizes and penetrates the interface area. This penetration results in an increase in both the number and size of the gaps, leading to a decrease in IHTC. As the temperature difference between the casting and the frozen sand mold diminishes further, the number and size of the gaps tend to stabilize, consequently resulting in a gradual stabilization of IHTC (Figure 5(a)). Using the back-calculated thermophysical parameters and IHTC of the frozen sand mold, a numerical simulation was conducted to analyze the temperature field and solidification time of A356 aluminum alloy in both frozen sand molds and resin sand molds. The results indicate that after 600 s, a large amount of the solids begins to form at the peripheries of the frozen casting specimens that are in direct contact with the sand mold. However, the majority of the resin sand castings still remain in the liquid phase. By 2,400 s, the frozen casting's temperature reaches approximately 276 °C, while the resin sand casting's temperature hovers approximately 318 °C. These results show that frozen casting can accelerate the solidification process of the castings. The solidification time for the outer edge of A356 aluminum alloy wheel hubs in frozen casting is approximately 340 s (at 100% solid fraction) and approximately 510 s for the inner edge. In resin sand casting, the solidification time for the outer edge of A356 aluminum alloy wheel hubs is approximately 660 s and approximately 700 s for the inner edge. Compared to resin sand casting, frozen casting reduces the solidification time of the aluminum alloy wheel hubs by approximately 27.1–48.5%. During the casting process, the parts in direct contact with the sand mold solidify rapidly, causing the solidification interface to gradually move towards the casting center and away from the sand mold. When the temperature of the frozen sand mold surface rises above 0 °C, the ice crystal bonding bridges start to melt, leading to a reduction in the initial strength of the sand mold. If the temperature exceeds 100 °C, the water within the sand mold initiates the evaporation process. If the surface of the high-temperature melt fails to solidify completely before the frozen sand mold reaches a temperature exceeding 100 °C and thus cannot offer adequate support, the sand mold will be at risk of collapsing. This situation poses a technical challenge to the design of the gating system and the optimization of mold filling (Figure 5(b)). Through the analysis of the microstructure and hardness distribution of AZ91D magnesium alloy materials under resin sand mold and frozen sand mold casting processes, it is found that due to the substantial temperature gradient between the high-temperature melt and the frozen sand mold, as well as the endothermic effect caused by the melting and vaporization of water at the interface between the sand mold and melt during the pouring process, the heat transfer between the castings and the frozen sand mold is enhanced, the solidification rate of the casting is increased and the grains are refined. The surface and core hardness of castings produced in frozen sand molds is increased by approximately 23% compared to those in conventional resin sand molds. This enhancement in hardness is attributed to the increase in water content and the reduction in freezing temperature, which is positively correlated with the micro-hardness augmentation in the frozen sand mold castings (Figure 5(c)).

3.3 Green and recyclable sand-mold recovery and near-zero emissions of pollutant gases

The pursuit of environmentally friendly casting processes and sustainable production practices represents a forefront area of interest and practical application within contemporary casting technology development. While pursuing the maximization of casting output and economic benefits, it is essential for foundries to consider the ecological sustainability of the casting process (Shi et al., 2024; Benromdhane, 2018). The emission of solid waste accounts for over 90% of the total emissions generated during casting production. The resultant gas and dust pollution not only has an adverse impact on the environment but also poses a threat to human health. Therefore, achieving zero emission of solid waste is crucial for achieving an environmentally friendly foundry. With the increasingly stringent global environmental regulations, the recycling of foundry waste sand has been widely recognized and applied in the industry. Conventional methods for recycling waste foundry sand typically include screening, separation, crushing, grinding, vacuuming and pickling. These processes may increase secondary pollution and result in suboptimal sand recovery rates. This paper discusses the utilization of the frozen sand molds green casting process for typical components of multi-alloy system, focusing on research related to pollutant gas emissions. Calculations indicate that the direct sand recovery rate of A356 aluminum alloy automotive steering knuckles is 91.1% (Figure 6(a)); the direct sand recovery rate of AZ91D magnesium alloy lightweight wheels and aerospace impeller components is 97.6% (Figure 6(b)); furthermore, for HT250 gear housing covers, the direct sand recovery rate reaches 97.9% (Figure 6(c)). During the solidification process of multi-alloy system castings, the introduction of molten metal results in the generation of environmentally friendly water vapor, thereby minimizing the emissions of pollutant gases. It is important to mention that components made of AZ91D magnesium alloy have low reactivity with water, thereby reducing the possibility of intense heat exchange, combustion or explosion. Therefore, the development of the frozen sand molds green casting process for large magnesium alloy components is a crucial forthcoming endeavor for the research team. Continuous advocacy for the engineering application of frozen sand mold green casting technology in the production of large aluminum alloy, magnesium alloy, titanium alloy and high-temperature alloy components is intended to guide the casting industry towards a more environmentally friendly and sustainable track.

4. Typical application cases

To promote the practical application and industrialization of digitalized frozen sand mold green casting technology, this project has designed a series of high-precision and high-efficiency frozen sand mold CNC machining forming equipment. These machines are available in vertical dimensions of 2,000 × 1,700 × 600 mm and horizontal dimensions of 1,200 × 1,200 × 600 mm. The cutting temperature can be adjusted continuously within the range from room temperature to −25 °C while maintaining a repeat positioning accuracy of ±0.05 mm. Furthermore, a prototype has been produced based on the frozen sand mold 3D printing principles. Research has been conducted to address the challenges related to integrated manufacturing, assembly accuracy and casting performance control of complex castings for aerospace, rail transportation and power machinery. This study focuses on developing strategies for multi-material frozen sand mold adaptive partitioning, digital machining and high-precision assembly forming. This study offers efficient solutions for the high-precision machining and assembly of complex asymmetric rotary body structures. It also improves the recycling efficiency of multi-material sand molds, contributing to sustainable green development in the casting industry. Figure 7(a and b) depicts the process of partitioning, assembling and forming multi-material frozen composite sand molds for aerospace components. In contrast, Figure 7(c) shows the splitting and assembling process of multi-material composite sand molds of space capsules, and Figure (d) demonstrates the pyrolytic recycling process of multi-material frozen composite sand molds used in the production of construction machinery components.

According to the foundation of digitalized frozen sand mold green casting forming systems and equipment, the Green Intelligent Manufacturing Technology and Equipment Team at the Nanjing University of Aeronautics and Astronautics (NUAA) has improved the development of essential components and application demonstration lines for digitalized frozen sand mold green casting at the NUAA International Innovation Harbor. These advancements incorporate essential systems such as cryogenic vacuum molding, equipment for low-temperature processing, zoned gradient refrigeration, vacuum pouring devices and logistics transfer systems. Innovatively, a digital twin system has been introduced for typical green casting application scenarios. This demonstration line aims to achieve precise, reliable, efficient, intelligent and environmentally friendly production of complex castings using aluminum alloy, magnesium alloy and cast iron. Figure 8(a and b) depicts the apparatus utilized in the digitalized frozen sand mold green casting process. In contrast, Figure 8(c and d) shows the digital twin system and the demonstration line application of the prospective factory designed for digitalized frozen sand mold green casting. This initiative aims to promote the modernization and environmental transformation of casting technology, thereby creating new opportunities for the sustainable growth of the sand casting industry.

5. Conclusion

This paper presents an innovative study on the mechanisms, processes and equipment involved in the digitalization of frozen sand mold green casting. The study extensively explores the essential scientific and engineering basis of digitalized frozen sand mold green casting technology and equipment, involving topics such as the digitalized low-temperature forming mechanism of frozen sand molds, the precise solidification mechanism of green casting, CNC machining equipment for frozen sand molds and future green casting factory research. The primary results are as follows:

• A method for digitizing the formation of green binders using frozen sand molds was developed. By using water as a binder, this technique brings a revolutionary change in the realm of sand mold casting. The study elucidates the process of solid-liquid phase transition and the mechanism of interface control involved in the formation of ice crystal bonding bridges in the porous media of frozen sand molds. By investigating the formation and fracture evolution of ice crystal bond bridges at macro and micro scales, the study optimized the water binder content and freezing temperature range suitable for sand mold casting. This approach enables precise control of fundamental casting properties, including tensile strength and permeability of multi-material frozen sand molds.
• The specific mechanism of heat transfer during the formation of frozen sand mold green casting was established. This involves the development of an accurate prediction model for the temperature distribution in green sand casting, as well as a back-calculation model to determine IHTCs. These efforts aim to achieve accurate predictions of the temperature distribution in the frozen sand mold green casting processes. Experiments involving A356 aluminum alloy, AZ91D magnesium alloy and HT250 cast iron within multi-alloy systems have shown that frozen sand mold green casting can refine the alloy structure and improve the performance, thereby achieving a higher greening degree compared to conventional sand mold casting.
• A prospective facility for frozen sand mold green casting was established, and a 2,000 × 1,700 × 600 mm digital frozen sand mold system equipment was developed, integrating the key units and application demonstration lines for digitalized frozen sand mold green casting. The critical subsystems are frozen vacuum molding, low-temperature processing equipment, zoned gradient refrigeration, vacuum pouring device and logistics transfer systems. The proposed facility has also pioneered the development of multi-material full frozen sand molds for applications in aerospace, rail transportation and marine automotive industries, thereby promoting the development of the foundry industry towards sophisticated, intelligent and environmentally friendly practices.

6. Future work

In future research endeavors, the team aims to delve deeper into the solidification heat and mass transfer mechanisms within frozen sand mold green casting. The study will focus on investigating the theory and methods for accurately characterizing the various performance aspects of frozen sand molds. Additionally, the team plans to explore the composite high-flexibility collaborative processing techniques for frozen sand molds. The research will contain integrated precision control of structural performance in low-temperature casting of complex castings, online monitoring studies of lightweight structure forming of frozen sand molds, investigations on high-performance manufacturing processes involving multi-field coupling in frozen sand molds, exploration of hybrid additive and subtractive manufacturing models for frozen sand molds and the establishment of evaluation standards for frozen sand molds casting-recycling-recasting systems. These endeavors are directed towards promoting the utilization of environmentally friendly materials in sand mold production, reducing the weight of structures, improving the precision of castings, integrating online testing processes and merging equipment systems. These initiatives are propelling the application of high-performance flexible manufacturing technology for solidifying sand molds.

This work was funded by the National Key R&D Program of China (No. 2021YFB3401200); the Jiangsu Provincial Basic Research Program (Natural Science Foundation) Youth Fund (No. BK20230885); the Special Technical Project for Equipment Pre-research (No. 30104040302) and the International Joint Laboratory of Sustainable Manufacturing, Ministry of Education and the Fundamental Research Funds for the Central Universities (No. NG2024012).

Figure 1

Evolutionary milestone in sand casting technology

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Figure 2

Integrated strategy for materials, processes, performance, testing, equipment and recycling in frozen sand mold green casting technology

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Figure 3

Schematic of digitalized frozen sand mold green casting principles

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Figure 4

Digitalized frozen sand mold forming mechanism and performance analysis (a) evolution of water freezing process; (b) in situ micro-CT scan analysis of water freezing; (c) statistical analysis of water film profiles; (d) phase transition mechanism of ice crystal bonding bridges; (e) fracture mechanism of ice bonding bridges and (f) mechanical performance variations in frozen sand molds

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Figure 5

Relationship between thermal physical parameters of frozen sand molds and macro-microscopic performance of castings: (a) determination of IHTC in green casting with frozen sand molds; (b) filling and solidification behavior of frozen sand molds and (c) macro-microscopic performance analysis of magnesium alloy castings in frozen sand molds

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Figure 6

Green casting experiments of multi-alloy metal components using frozen sand molds: (a) A356 aluminum alloy castings; (b) AZ91D magnesium alloy castings and (c) HT250 cast iron

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Figure 7

Innovative pathways in frozen sand mold green casting – manufacturing, precision assembly and recycling: (a) servo cabin frozen sand mold partitioning and assembly; (b) precision scanning of multi-material fully frozen sand mold composite forming for tail cabin; (c) splitting and assembling process of multi-material composite sand mold of space capsule and and (d) assembly and natural disintegration process of multi-material frozen sand molds

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Figure 8

Future factory for digitalized frozen sand mold green casting equipment: (a) and (b) digitalized frozen sand mold green casting forming equipment; (c) digital twin of future factory for frozen sand mold green casting and (d) application demonstration line for frozen sand mold green casting

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