Research on Sand Casting Process of Semi-enclosed Al-Cu Alloy Shell

1. Introduction

Aluminum alloy, as one of the lighter metal structural materials, has become the focus of competition among military powers in the field of advanced military materials. Compared with ordinary aluminum alloy materials, Al-Cu alloy is a high-strength and high-toughness material with excellent casting properties. The research results of this paper will provide strong support for the application of Al-Cu alloy in the fields of sand casting and semi-enclosed shell castings. In the fields of aerospace and automobile, sand casting is a common casting method. During the sand casting process, metal materials need to withstand various extreme conditions such as high temperature and high pressure. Therefore, high requirements are put forward for the performance and stability of casting materials. At the same time, with the development of science and technology, the demand for sand casting materials is also increasing. In this context, the study of the application of Al-Cu alloy in sand casting has important practical significance and practical application value. In terms of production process, the selection of sand casting for Al-Cu alloy casting has the characteristics of low cost and high flexibility, providing an excellent solution for some Al-Cu alloy casting. This paper takes the semi-enclosed shell casting as an example to describe the process flow of Al-Cu alloy sand casting and analyze the quality of the casting, providing a reference for the application of Al-Cu alloy sand casting technology.

2. Casting Process Design

2.1 Casting Structure Characteristics and Technical Requirements

2.1.1 Casting Structure Characteristics

The three-dimensional structure of the semi-enclosed cabin body casting is shown in Figure 1. The main characteristics of this casting are as follows: First, the thickness of the casting cover plate is 39 mm; second, the cross-section of the casting is a rectangular cylindrical structure, and the height of the casting is 690 mm; third, the wall thickness of the casting is 12 mm; fourth, the inner surface of the casting is not processed, and the dimensional accuracy requirements of the casting are high.

2.1.2 Technical Requirements

The main technical indicators of the casting: the casting is not allowed to be repaired by welding. In addition, thermal deformation correction is not allowed. The mechanical properties of the casting sample need to meet the requirements of room temperature tensile strength ≥ 320MPa, yield strength ≥ 230MPa, and elongation > 3%. The wall thickness difference of the casting is not allowed to exceed ± 0.4mm, and defects such as shrinkage porosity, shrinkage cavity, sand inclusion, and slag inclusion should not appear on the surface of the casting after processing.

In fact, when designing the actual process, the number of vertical cylinders should comprehensively consider the characteristics of the casting material, the control filling range, the feeding range of a single vertical simple gap runner, and the wall thickness of the casting. Therefore, the determination of the value of n also needs to comprehensively consider the material, casting method, the position of the thicker boss, and the pouring method. The number of casting gap runners is set to 8 according to the diameter of the casting, which ensures that the gap runners can not only guide the flow but also effectively feed the casting. Therefore, according to the characteristics of the casting structure, a slot-type gating system is designed.

2.2 Process Design

2.2.1 Determination of Pouring Method

At present, in sand casting, in order to pour high-quality castings, differential pressure pouring is usually used. When pouring, the molten metal enters the mold cavity. Due to the high back pressure in the cavity, the aluminum liquid is not easy to spray, splash, and fill the mold smoothly. At the same time, the partial pressures of the alloy liquid and the gas in the upper part of the mold cavity are controllable during pouring, which is conducive to the crystallization and solidification of the alloy liquid under high pressure, greatly reducing shrinkage porosity and pinholes, making the casting structure dense and the mechanical properties high.

2.2.2 Determination of Pouring Position

A bottom-pouring vertical slot-type gating system (see Figure 2) is adopted. The thick end frame and thick cover plate of the casting are placed horizontally, which is conducive to the smooth filling process of the aluminum liquid, not easy to cause eddy currents, splashes, and impacts, and also conducive to establishing reasonable sequential solidification conditions. Since the cover plate of the casting is relatively thick and shrinks during cooling, the runner cannot feed, so a riser is added above the cover plate.

2.2.3 Design of Pouring System

The casting is poured by differential pressure pouring and uses a slot vertical cylinder type gating system. When designing the slot-type gating system, the number of vertical cylinders can be determined by referring to the following empirical formula:




where n is the number of vertical cylinders, S is the outer perimeter of the casting, δ is the wall thickness of the casting, a is the slot thickness, b is the slot width, and D is the diameter of the vertical cylinder.

2.2.4 Design of Chills

The pouring system of the casting is designed, and chills are added between adjacent slot runners. In addition, chills are also added under the cover plate. Through the chilling effect of the chills, the temperature gradient between the end of the casting and the vertical cylinder and riser is forcibly increased, prompting the molten metal to crystallize in a sequential solidification manner.

3. Casting Simulation Analysis

In order to ensure the effectiveness of the casting process design, Novacast software can be used for simulation experiments. Through Novacast, the casting is meshed, parameters are set, and analysis and calculation are carried out to obtain the filling and solidification processes, as shown in Figure 3. The pouring temperature is set to 680±5℃. By observing the chilling effect of the end chills, the temperature gradient between the far end of the casting and the slot runner is greatly increased, and a solidification mode of sequential crystallization from the far end to the slot runner is clearly formed. The casting can be sequenced very well, and there is no underpouring or entrained air phenomenon. The software predicts shrinkage porosity and shrinkage cavities (see Figure 4). The cover plate of the casting is relatively thick and requires a large amount of feeding during the cooling process, and the riser just plays a good feeding role. There is local shrinkage porosity in the casting, but no shrinkage cavity. Considering comprehensively, it is preliminarily judged that the casting process meets the requirements and can be poured.

4. Design of Sand Casting Mold

The sand casting mold belongs to the blank mold, and the produced casting must be machined to meet the size requirements for use. In addition to considering the shrinkage rate, the design of the sand casting mold also needs to consider factors such as machining allowance, casting fillet, and draft angle. Some small parts must be filled during the design to ensure the quality and accuracy of the casting. First, the design of the three-dimensional blank model. When establishing the three-dimensional blank model of the semi-enclosed shell, relevant factors have been considered to prepare for the subsequent design of the sand casting mold. According to the casting material of Al-Cu, the unmarked chamfer is R5~R8mm, the draft angle is 2°, and the machining allowance of the machined surface is 5mm. The various holes do not need to be cast here and will be completed by machining. Second, the design of the upper and lower sand cores. According to the size requirements of the thick semi-enclosed shell casting, the company’s sand box is selected. Since the casting has features such as bosses, annular ribs, and cover plates, the parting surface of the sand core mold of the casting is designed under the cover plate, otherwise, the mold cannot be opened smoothly. In UG software, after reasonably designing the parting surface according to the shape characteristics of the thick semi-enclosed shell casting, the sand mold is divided into two upper and lower sand molds that are convenient for mold opening (see Figure 5). Finally, the design and manufacture of the sand casting mold. According to the mold sand core and technical requirements, the semi-enclosed shell casting mold is made, and the mold material is cast steel. The mold is divided into three parts: the upper core box, the lower core box, and the outer mold.

5. Formation of Sand Mold

The pre-prepared mold is used for sand filling and molding operations. During the molding process, it is necessary to ensure that the resin sand is evenly distributed and compacted. Then, the sand mold is naturally dried, and the entire drying cycle is 49~50h. In order to enhance the strength of the sand mold, the dried sand mold needs to be baked at a low temperature. The baking temperature is set to 500±5℃ and maintained for 4~5h, and the strength of the sand mold can be significantly enhanced. During the surface treatment of the sand mold, 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 the surface treatment is completed, the sand mold can be assembled and prepared for pouring.

6. Melting and Pouring

Melting and pouring are carried out in a vacuum differential pressure pouring furnace, and the ingots that have been remelted twice and passed the sampling test are used.

7. Development Results

7.1 Internal Quality

After sandblasting treatment, the casting has a smooth and flat surface without pits, flash, or burrs from the perspective of pouring and casting surface quality. X-ray non-destructive testing and fluorescent penetration testing were carried out on the casting, and the results showed that the casting had no cracks, porosity, shrinkage cavities, and other defects, and the internal quality of the casting met the requirements of Class I parts in HB 963-2005.

7.2 Mechanical Properties

After the casting is heat-treated according to the requirements of T6, the mechanical properties of the test bars of the semi-enclosed shell casting are tested according to the requirements of the technical agreement. The test results are shown in Table 1. It can be seen from the table that the mechanical properties of the test bars of the developed semi-enclosed shell casting all meet the design requirements.

7.3 Dimensions

The casting is detected by a three-dimensional scanner, and more than 90% of the dimensions can meet the CT9 level standard of GB/T6414 specification. Although there are dimensional deviations in the shell casting, it still meets the final processing and use requirements of the customer. The analysis shows that the main reasons for the dimensional deviation of the shell casting are the uneven shrinkage of the casting and the errors in the assembly process of the mold.

8. Conclusion

The differential pressure pouring method is adopted, and a reasonable number of slot runners is used to avoid the formation of quality defects such as porosity and shrinkage cavities in the casting. The end chills and risers are reasonably set at the corresponding positions of the casting. By analyzing the temperature field during the filling and solidification processes, the sequential solidification of the casting is finally realized. Through X-ray inspection, fluorescent penetration inspection, mechanical property testing, and dimensional inspection, the results show that the internal quality, mechanical properties, and surface accuracy of the developed semi-enclosed Al-Cu alloy shell casting all meet the design requirements.

In the future, further research can be carried out to optimize the casting process, improve the quality and performance of the casting, and expand the application range of Al-Cu alloy in the field of sand casting. At the same time, attention should also be paid to the research and development of new casting materials and technologies to meet the requirements of the manufacturing industry.

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