How to prevent and solve common defects (cracks, slag inclusions, and porosity) in aluminum melting and casting?

Aluminum melting and casting, as the core process in the production of aluminum alloy products, is prone to various quality defects due to its complex technology. Among these, cracks, slag inclusions, and porosity are the three most common issues, directly affecting the mechanical properties, appearance quality, and service life of aluminum alloy products. In severe cases, they can lead to product scrap and increased production costs for enterprises. As the aluminum melting and casting industry transitions towards precision and high-end production, and downstream industries such as automotive and aerospace continue to demand higher product quality, how to scientifically prevent and effectively address these three common defects has become a key issue for aluminum melting and casting enterprises to achieve high-quality development. Industry experts point out that the occurrence of these three defects is closely related to process operations, parameter control, and equipment configuration. Therefore, targeted prevention and control plans must be formulated to achieve "prevention first, combining prevention and control" in order to strengthen the product quality defense line.

According to research data from the Aluminum Processing Branch of the China Foundry Association, three types of defects—cracks, slag inclusions, and porosity—account for over 70% of the total defects in aluminum melting and casting products. Among them, porosity defects account for the highest proportion, reaching about 35%, while slag inclusions and cracks account for 25% and 15% respectively. Due to the lack of scientific prevention and control measures, some small and medium-sized enterprises face a product scrap rate of over 12% caused by these defects. Over time, enterprises in the industry have gradually formed a defect prevention and control system that is tailored to production practices through continuous practice and summarization, providing an effective path for solving the quality challenges in aluminum melting and casting.

Cracks are highly destructive defects in aluminum casting products, often appearing on the surface or inside the castings and manifesting as linear patterns. In severe cases, they can lead to casting fracture. Their occurrence is mainly related to factors such as improper temperature control, unreasonable molding processes, and uneven cooling rates. Excessive temperature fluctuations are the core cause of cracks. For example, high melting and pouring temperatures can increase the shrinkage stress of the casting, and stress concentration during cooling can easily lead to hot cracks; whereas too low holding and cooling temperatures, coupled with significant differences in cooling rates across various parts of the casting, can create thermal stress differences, leading to cold cracks. In addition, uneven wall thickness of the casting, too fast pouring speed, and unreasonable mold design can also cause stress concentration, inducing crack defects.

Regarding the prevention and resolution of crack defects, the industry has established clear practical solutions. On the prevention front, it is necessary to strictly control temperature throughout the entire process. The melting temperature should be maintained between 700℃ and 750℃, while the pouring temperature should be adjusted to 690℃ to 720℃ based on the wall thickness of the casting. During the cooling process, a gradient cooling method should be employed to avoid sudden temperature changes in localized areas, thereby reducing stress generation. Additionally, the structure of the casting design should be optimized to ensure uniform wall thickness as much as possible, avoiding structures that are prone to stress concentration, such as sharp corners and abrupt changes in wall thickness. The mold should be made of materials with uniform thermal conductivity to ensure even cooling. On the resolution front, minor cracks can be repaired through methods such as grinding and repair welding. During repair welding, it is necessary to control the welding temperature and speed to avoid secondary cracks. For severe cracks, they should be deemed scrap and avoided from entering subsequent processes. At the same time, the cause should be traced to optimize temperature control and molding processes.

The slag inclusion defect mainly manifests as the presence of impurity particles inside or on the surface of castings, which are mostly oxide inclusions, refractory material debris, unmelted metal particles, etc. Its occurrence is mainly related to factors such as incomplete purification of aluminum liquid, substandard raw material quality, and inadequate equipment cleaning. During the aluminum melting and casting process, aluminum liquid is prone to oxidize and form oxide inclusions. If the refining and filtration systems are not properly configured or operated, impurities cannot be completely removed and will enter the casting along with the aluminum liquid, forming slag inclusions. In addition, impurities mixed in the raw materials, refractory material debris falling off from the inner wall of the melting furnace, and floating sand in the flow channel and mold can also lead to slag inclusion defects.

To prevent and control slag inclusion defects, it is necessary to start from three dimensions: raw materials, purification, and equipment. At the prevention level, strict control of raw material quality is required, with the selection of qualified purity aluminum ingots and scrap aluminum. Before feeding, the surface of the raw materials should be cleaned of oil and impurities. The refining and filtration system should be optimized, adopting an "online refining + graded filtration" mode. The refining temperature should be controlled at 700℃~750℃, and suitable refining agents should be selected to fully remove oxide inclusions from the aluminum liquid. In the filtration process, a foam ceramic filter with 20PPI~30PPI should be used to intercept fine impurities. Regular cleaning of the melting furnace, flow channels, and molds is necessary to remove refractory debris and floating sand attached to the inner walls, thus preventing impurities from mixing into the aluminum liquid. At the solution level, surface slag inclusions can be removed through grinding and polishing; for internal slag inclusions, they need to be determined based on the size of the defect. Minor slag inclusions can be repaired through repair welding, while severe slag inclusions require scrap disposal. At the same time, the purification system and equipment cleanliness should be investigated, and timely optimization and adjustment should be made.

Porosity defects are the most common defects in aluminum casting products, manifesting as holes of varying sizes inside or on the surface of the castings. These defects reduce the compactness and mechanical strength of the products. Their occurrence is mainly related to factors such as high hydrogen content in the aluminum melt, entrapped air during the pouring process, and poor mold venting. During the melting and refining process of the aluminum melt, exposure to humid air or high moisture content in raw materials can lead to excessive hydrogen content. If hydrogen cannot be discharged in time during the cooling process, porosity will form. Too fast a pouring speed or improper pouring methods can cause air to be entrapped in the aluminum melt, leading to entrapped porosity. Poorly designed mold vent holes or inadequate venting can prevent air from being discharged when the aluminum melt fills the mold, resulting in porosity.

The core of preventing and controlling stomatal defects lies in reducing the hydrogen content in aluminum liquid, minimizing air entrainment, and ensuring smooth exhaust. At the prevention level, strict control of raw material and environmental humidity is essential. Raw materials should be properly moisture-proofed during storage and dried before feeding to prevent water-containing materials from entering the furnace. Refining processes should be optimized by using methods such as inert gas injection to fully remove hydrogen from the aluminum liquid. The refining temperature should be controlled between 720°C and 740°C to enhance degassing efficiency. Pouring processes should also be optimized by controlling the pouring speed and adopting a steady pouring method to avoid air entrainment. Additionally, mold design should be optimized, with exhaust holes reasonably arranged to ensure smooth air exhaust. At the solution level, small surface stomata can be repaired through grinding and repair welding. However, internal dense stomata can seriously affect product performance and should be deemed scrap. Meanwhile, the source of hydrogen and issues with the exhaust system should be traced to optimize refining and pouring processes.

Industry experts emphasize that the prevention and control of common defects in aluminum melting and casting is a systematic task that needs to be integrated throughout the entire production process, rather than focusing solely on a single aspect. Enterprises need to develop targeted prevention and control plans based on their own production line scale and product positioning, clarifying operational standards and parameter requirements for each step. They should strengthen operator training, enhance practical professionalism, standardize operations in key areas such as temperature control, refining and filtration, and pouring and cooling, to reduce human errors. Additionally, high-precision detection equipment should be equipped to conduct full-process inspection of castings, promptly detect defects, trace their causes, and continuously optimize process plans.

In practice, many enterprises have effectively reduced the occurrence rate of defects by optimizing prevention and control measures. For example, an aluminum processing enterprise has reduced the total occurrence rate of cracks, slag inclusions, and porosity defects by over 40% by optimizing temperature control processes, upgrading foam ceramic filtration systems, and standardizing raw material control, thereby increasing the product yield to over 95%. Another enterprise has introduced intelligent detection equipment to achieve real-time monitoring and precise tracing of defects, further enhancing prevention and control efficiency. These practical experiences demonstrate that scientific prevention and control measures, as well as standardized operating procedures, are key to addressing common defects in aluminum melting and casting.

With the continuous upgrading of aluminum melting and casting technology, defect prevention and control methods are also being continuously optimized. Relevant equipment companies are constantly developing high-precision temperature control equipment and efficient refining and filtering equipment to help enterprises improve their process control level. Aluminum melting and casting enterprises, on the other hand, are introducing intelligent control systems to achieve real-time monitoring and closed-loop management of the entire production process, precisely controlling parameters at each link and reducing defect generation from the source. In the future, with the application of technologies such as 3D printing and big data, aluminum melting and casting defect prevention and control will become more precise and efficient, better adapting to the production needs of high-end aluminum alloy products. This will help the aluminum melting and casting industry achieve quality improvement, efficiency enhancement, and green development, providing higher-quality aluminum alloy materials for downstream industries.