Introduction
Aluminum alloy, as one of the lighter metal structural materials, has become a key focus of competition among military powers in the field of advanced military materials and metals. Compared to ordinary aluminum alloy materials, Al Cu alloy is a high-strength and high toughness material with excellent casting performance. The research results of this article will provide strong support for the application of Al Cu alloy in sand casting and semi closed shell casting fields. Sand casting is a common casting method in fields such as aerospace and automotive. In the process of sand casting, metal materials need to withstand various extreme conditions such as high temperature and high pressure. Therefore, high requirements have been put forward for the performance and stability of casting materials. Meanwhile, with the development of technology, the demand for sand casting materials is also constantly increasing. In this context, studying the application of Al Cu alloy in sand casting has important practical significance and practical application value. In terms of production technology, sand casting is used for Al Cu alloy casting, which provides excellent solutions for some Al Cu alloy casting due to its low cost and high flexibility. Taking the semi enclosed shell casting as an example, the process flow of Al Cu alloy sand casting is explained, and the quality of the casting is tested and analyzed, providing reference for the application of Al Cu alloy sand casting technology.
Casting process design
(1) Structural characteristics of castings. The three-dimensional structure of the semi enclosed cabin casting is characterized by: firstly, the thickness of the cover plate of the casting is 39 mm; Secondly, the cross-section of the casting is a rectangular cylindrical structure with a height of 690 mm; Thirdly, the wall thickness of the casting is 12 mm; Fourthly, the inner surface of the casting is not machined, and high dimensional accuracy is required for the casting.
(2) Technical requirements. Main technical specifications of castings: Welding repair is not allowed for castings. Additionally, thermal deformation correction is not allowed. The mechanical properties of the casting sample must meet the requirements of room temperature tensile strength ≥ 320MPa, yield strength ≥ 230MPa, and elongation ≥ 3%. The difference in wall thickness of castings shall not exceed ± 0.4mm, and after processing, the surface of castings shall not have defects such as shrinkage porosity, shrinkage porosity, sand inclusion, slag inclusion, etc.
In sand casting, differential pressure casting is commonly used to pour high-quality castings. During casting, the molten metal enters the mold cavity, and due to the high back pressure inside the cavity, the aluminum liquid is less likely to spray, splash, and smoothly fill the mold. At the same time, during pouring, the partial pressure of the alloy liquid and the gas in the upper part of the mold cavity are controllable, which is conducive to the crystallization and solidification of the alloy liquid under high pressure, resulting in shrinkage and greatly reducing pinholes, making the cast structure dense and the mechanical properties high. Adopting a bottom injection vertical gap pouring system, the thick end frame and thick cover plate of the casting are placed horizontally, which is advantageous The filling process of aluminum liquid is smooth and not prone to eddy currents, splashes, and impacts, which is also conducive to establishing reasonable sequential solidification conditions. Due to the thick cover plate of the casting, it shrinks during cooling and the sprue cannot be replenished. Therefore, a riser is added to the upper part of the cover plate. Castings are poured using differential pressure casting and a gap vertical casting system is employed. Design the casting pouring system by adding cold iron between adjacent gap runners. And, cold iron is also added to the lower part of the cover plate. By the rapid cooling effect of cold iron, the temperature gradient between the end of the casting and the riser and riser is forcibly increased, promoting the crystallization of the molten metal in a sequential solidification manner.
Sand casting molds belong to blank molds, and the castings produced must undergo mechanical processing to meet the size requirements for use. In addition to considering shrinkage rate, the design of sand casting molds also needs to consider factors such as machining allowance, casting fillet, and draft angle. Some parts with smaller dimensions must be filled in during design to ensure the quality and accuracy of the castings. Firstly, the design of the 3D rough model. When establishing a three-dimensional blank model of a semi enclosed shell, relevant factors have been taken into account to prepare for the design of the sand casting mold. Based on the casting material being Al Cu, the unmarked chamfer is R5~R8mm, the draft angle is 2 °, and the machining allowance of the machining surface is 5mm, each hole does not need to be cast here until mechanical processing is completed. Secondly, the design of the upper and lower sand cores. Select the company’s sand box based on the size requirements of the thick semi enclosed shell casting. Due to the features of convex platform, annular rib, and cover plate, the parting surface of the sand core mold for this casting is designed below the cover plate, otherwise the mold cannot be opened smoothly. In UG software, based on the shape characteristics of thick semi closed shell castings After designing the parting surface, divide the sand mold into two upper and lower sand molds for easy mold opening. Finally, the design and manufacturing of sand casting molds. According to the mold sand core and technical requirements, a semi closed shell casting mold is made, and the mold material is cast steel. The mold is divided into three parts: upper core box, lower core box, and outer mold.
In order to enhance the strength of the sand mold, it is necessary to perform low-temperature baking treatment on the dried sand mold. The baking temperature is set at 500 ± 5 ℃ and maintained at this temperature for 4-5 hours. The strength of the sand mold can be significantly enhanced. In the surface treatment process of sand mold casting, it is necessary to ensure that the working surface of the sand mold can be completely covered by the coating when applying the coating, and pay attention to the thickness of the coating. After sandblasting treatment, the surface of the casting is smooth and flat, without defects such as pits, burrs, and burrs from the pouring and casting surface quality. The castings were subjected to X-ray non-destructive testing and fluorescent penetrant inspection, and the results showed that there were no defects such as cracks, looseness, and shrinkage. The internal quality of the castings met the requirements of Class I parts in HB 963-2005 standard. After T6 heat treatment as required, the mechanical properties of the semi enclosed shell castings are tested on the test bars according to the technical agreement. Although there are dimensional deviations in the shell castings, they still meet the customer’s final processing and usage requirements. Analysis shows that the main reasons for the size deviation of shell castings are uneven shrinkage size and errors in the mold assembly process. Adopting differential pressure pouring method, combined with reasonable number of gap type runners, to avoid quality defects such as casting looseness and shrinkage. Reasonably set up end cold iron and riser in the corresponding parts of the casting. By analyzing the temperature field during filling and solidification processes, the sequential solidification of castings was ultimately achieved. After X-ray testing, fluorescence penetration testing, mechanical property testing, and dimensional inspection, the results show that the internal quality, mechanical properties, and surface accuracy of the semi enclosed Al Cu alloy shell casting developed meet the design requirements.
Conclusion
The sand casting process of semi enclosed Al Cu alloy shell was studied. Novacast software was used to simulate the filling and solidification behavior. Steel molds were used to produce sand molds, which were melted and poured in a differential pressure equipment furnace. The internal quality of the casting meets the requirements of Class I parts in HB 963-2005 standard, the mechanical properties meet the technical specifications, and the dimensional accuracy of the casting can reach the requirements of CT9 level. Therefore, this sand casting process can achieve the manufacturing of semi enclosed Al Cu alloy shell castings.