abstract

The theoretical simulation developed based on Finite Element Methods (FEM) is of great significance for studying the casting and forming of aluminum alloy materials. Theoretical simulation was used to study the low-pressure casting and gravity casting processes of ZL205A aluminum alloy cylinder castings. Based on the corresponding processes, the actual quality of the castings was obtained, and the ideal process suitable for the product was determined. The research results indicate that using an open gravity pouring process with a pouring system cross-sectional ratio of 1:4:5, the product's appearance fluorescence inspection and internal X-ray inspection meet the technical requirements, and the process yield reaches over 55%.

With the development of the aluminum alloy casting industry, the quality of medium wall thickness castings is increasingly improving, and they are widely used in aerospace, shipbuilding, new energy vehicles and other fields. Research on the application of thin-walled and thick wall castings is becoming increasingly urgent. Among them, cylindrical structural components are relatively more commonly used in thick wall casting structures. Currently, the quality performance of these types of castings is relatively poor, and the main defect types are large-area porosity and unformed casting.

This study focuses on thick walled cylindrical castings, which are cast using two methods: low-pressure casting and gravity casting. To a certain extent, it is widely believed in the industry that the internal organizational quality of low-pressure casting is better than that of gravity casting. However, there are many factors that affect the casting process (such as product structure, equipment and facilities, design of cold iron, design of riser, etc.), which should be comprehensively considered in order to design the optimal casting process that meets the needs of the product. The filling process of low-pressure casting is from bottom to top, and the macroscopic solidification sequence is mainly from top to bottom solidification (while solidification is relatively less commonly used); Gravity casting, as a widely used traditional casting method, has a filling process from top to bottom, and the macroscopic solidification sequence is mainly from bottom to top. This article mainly explores the applicability of low-pressure casting and gravity casting processes for thick walled cylinder castings.

1. Casting process
The casting material is ZL205A aluminum alloy, and the weight of the casting is 132 kg. The simulation shows that the liquidus temperature of the alloy is 650.5 ℃ and the solidus temperature is 548.3 ℃. Technical requirements for castings: The quality of castings meets the requirements of Class II castings in HB 963-2005; Mechanical properties: tensile strength ≥ 490 MPa, elongation after fracture ≥ 3%, Brinell hardness HBS ≥ 120; The chemical composition is shown in Table 1. The casting structure is shown in Figure 1.

1.1 Pouring System Design
Using FEM based Procast 2018 software for theoretical simulation, design the most ideal pouring scheme corresponding to low-pressure casting process and gravity casting process, and carry out product trial production of corresponding processes.
The low-pressure pouring process adopts a gap pouring process, with a vertical seam thickness designed to be 0.8 times the wall thickness of the casting, a vertical seam width designed to be 60 mm, and a vertical tube diameter designed to be 3.5 times the thickness of the vertical seam. Considering the thicker wall thickness of the casting, it is assumed that the effective filling distance of the vertical tube is designed to be about 100 mm, and the circumference of the casting is about 1110 mm. Therefore, it is necessary to design 6 vertical tubes and a lifting tube diameter designed to be 150 mm. The process design diagram is shown in Figure 2. The theoretical simulation surface mesh unit size is 3 mm x 3 mm, with approximately 2.72 million surface meshes and 12.64 million volume meshes. The pouring temperature is set to 680 ℃ and the pouring speed is set to 40 mm/s.

The simulation results of low-pressure pouring process show that the metal liquid is filled smoothly, and there is no splashing phenomenon in the riser. The solidification sequence is from top to bottom, first solidifying the casting part, then solidifying the vertical seam, and finally the vertical cylinder and transverse runner (Figure 3). The loosening results show that there is no obvious loose area in the casting part (Figure 4), and there is no isolated solidification phase. Analysis suggests that the process is theoretically feasible. The production rate of the proposed process is about 20%.

The gravity pouring process is designed as an open center pouring system, with a cross-sectional ratio of ∑ S sprue: ∑ S transverse sprue: ∑ S inner sprue=1:4:5. The design principle is that the flow rate of each inner sprue is uniform. The process is designed with two circular transverse sprues.
In order to achieve a bottom-up solidification sequence, a conformal cold iron is set below the casting, with a thickness of 1.2 times the corresponding casting wall thickness. A conformal riser is set above the casting process, with a height of 200mm. The process diagram is shown in Figure 5. The theoretical simulation shows that the mesh size of the surface is 3 mm x 3 mm, with approximately 260000 surface meshes and 1.61 million volume meshes. This plan is designed with a pouring temperature of 700 ℃ and a pouring speed of 40 mm/s.

The simulation results of gravity casting process show that the metal liquid is filled smoothly without obvious turbulence or other abnormal phenomena. The solidification sequence is relatively ideal, and the temperature field gradient is uniform, achieving the concept of bottom-up solidification sequence (Figure 6). The porosity results show that there is no obvious loose area in the casting part (Figure 7), and there is no isolated solidification phase. Analysis suggests that the process is theoretically feasible. The production rate of the proposed process is approximately 55%.

1.2 Style
The study adopts resin sand molding method, and the process parameters of key processes are shown in Table 2.

2. Experimental results and analysis
2.1 Low pressure pouring process
There were no obvious abnormalities during the pouring process. After pouring, when the mold was removed from the JM-083 low-pressure casting machine, it was found that the upper part of the mold riser tended to solidify. It is believed that the actual production process parameters are relatively consistent with the process concept.
After the precision cleaning of the casting, obvious shrinkage and porosity defects can be seen at the lower flange position (Figure 8). Analysis suggests that the cause of the defect at this location is poor aluminum liquid filling and contraction. The defect is located closer to the riser, which is the final solidification position of the casting (longer solidification time than the upper part of the riser), meaning that the riser cannot effectively compensate for this position. Analysis found that the diameter of the lifting pipe in the facility section is 150mm (the largest lifting pipe on the production site), which is smaller than the diameter of the vertical cylinder of 260mm. Within the limited crystallization pressurization time, the lifting pipe cannot effectively pressurize and shrink the casting. It is believed that this pouring process needs to be optimized.

2.2 Gravity pouring process
The filling time is extended by 5 seconds compared to the theoretical simulation value, and there may be some discrepancies between the manual pouring speed and the process parameters. The appearance quality of the casting is good (Figure 9a), and after surface cleaning, fluorescence detection shows no abnormalities (Figure 9b), The internal quality of the X-ray inspection meets the technical requirements of the casting, and the casting structure at the root of the riser is loose at level 2 (Figure 9c).

3. Conclusion
(1) The thick walled cylinder casting adopts gravity casting technology, which can meet the technical requirements of the casting. The area ratio of the pouring system in the process design is ∑ S vertical sprue: ∑ S horizontal sprue: ∑ S internal sprue=1:4:5, using the middle pouring method. The top riser of the casting is designed with an insulated riser, and the bottom of the casting is designed with a thickness of 1.2 times the wall thickness of the casting as a shaped cold iron. The production rate of the process is 55%.
(2) However, the castings cast using low-pressure casting technology have defects such as shrinkage and porosity, which cannot meet the technical requirements of the product. The main reason is that the diameter design of the lifting pipe is insufficient, and it is advisable to have a lifting pipe diameter of no less than 260 mm. The production rate of the process is only 20%. Considering all factors, this process design is not suitable for the trial production of this product.

author:
Ding Zhijie, Wei Yongli, Zhu Lilong
Shanxi Ruige Metal New Materials Co., Ltd

This article comes from: Foundry Magazine