Theoretical simulations developed based on FEM (Finite Element Methods) are of great significance to study of casting and forming of aluminum alloy materials. Theoretical simulation is used to study low-pressure casting and gravity casting processes of ZL205A aluminum alloy cylinder castings. Actual quality of castings is obtained based on corresponding processes and ideal process suitable for product is determined. Research results show that using open gravity pouring process with a pouring system cross-section ratio of 1:4:5, product appearance fluorescence inspection and internal X-ray flaw detection inspection all meet product technical requirements, and process yield rate reaches more than 55%.
With development of aluminum alloy casting industry, quality of medium-walled castings is increasing day by day, they are widely used in aerospace, shipbuilding, new energy vehicles and other fields. Research on application of thin-walled and thick-walled castings is becoming increasingly urgent. Among them, cylindrical structural parts are relatively used in thick-walled casting structures. At present, quality of this type of castings is relatively poor, main types of defects are large areas of looseness and unformed castings.
Object of this study is thick-walled cylinder castings. Casting methods are low-pressure casting and gravity casting. To a certain extent, it is generally believed in the industry that internal structure quality of low-pressure castings is superior to gravity casting. However, there are many factors that affect casting process (such as product structure, equipment and facilities, cold iron design, riser design, etc.), which should be considered comprehensively to design the best casting process that meets needs of product. Mold filling process of low-pressure casting is bottom-up filling, and macroscopic solidification sequence is mainly top-down solidification (at the same time, solidification applications are relatively rare); as a traditional and widely used casting method, gravity casting has a mold filling process from top to bottom, and macroscopic solidification sequence is mainly bottom-up solidification. This article mainly discusses applicability of low-pressure casting and gravity casting processes for thick-walled cylinder castings.

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

Table 1 Chemical composition of castings wB/%

 

Figure 1 Schematic diagram of casting

FEM-based Procast 2018 software was used for theoretical simulation, optimal pouring scheme corresponding to low-pressure casting process and gravity pouring process was designed, and product trial production of corresponding processes was carried out.
Low-pressure casting process adopts slot casting process. Thickness of vertical seam is designed to be 0.8 times wall thickness of casting. Width of vertical seam is designed to be 60 mm. Diameter of vertical tube is designed to be 3.5 times thickness of vertical seam. Considering thick wall thickness of casting, it is assumed that effective feeding distance of vertical cylinder is designed to be about 100 mm, and circumference of casting is about 1110 mm, which means that 6 vertical cylinders need to be designed in a process, and diameter of liquid riser pipe is designed to be 150 mm. Process design diagram is shown in Figure 2. Theoretical simulation surface grid unit size is 3 mm * 3 mm, number of surface grids is approximately 2.72 million, and number of volume grids is approximately 12.64 million. Pouring temperature was set to 680 ℃, and pouring speed was set to 40 mm/s.

 

Figure 2 Low pressure casting process diagram
Simulation results of low-pressure casting process show that metal liquid fills mold smoothly, and there is no splashing phenomenon in liquid riser pipe. Solidification sequence is from top to bottom, with casting part solidified first, then vertical joints, finally vertical tube and lateral runner (Figure 3). Porosity results show that there is no obvious porous area in casting part (Figure 4), and there is no isolated solidification phase. Analysis believes that process is theoretically feasible. Planned process yield rate is about 20%.

 

Figure 3 Solidification time chart of low pressure casting process

 

Figure 4 Loose defects in low-pressure casting process
Gravity pouring process is designed as a mid-cast open type. Cross-section ratio of pouring system is ∑S sprue: ∑S cross runner: ∑S inner runner = 1:4:5. Design principle is that flow rate of each inner runner is uniform, process is designed with two annular lateral sprues.
In order to realize bottom-up solidification sequence, a conformal cold iron is set below casting. Thickness of cold iron is 1.2 times wall thickness of corresponding casting. A conformal riser is set above casting process with a riser height of 200 mm. Process diagram is shown in Figure 5. Theoretical simulation surface grid unit size is 3 mm * 3 mm, number of surface grids is about 260,000, and number of volume grids is about 1.61 million. Pouring temperature of this solution is designed to be 700℃, and pouring speed is designed to be 40 mm/s.

 

Figure 5 Gravity pouring process diagram
Simulation results of gravity pouring process show that molten metal fills smoothly, with no obvious turbulence and other abnormal phenomena. Solidification sequence is ideal, temperature field gradient is uniform, and bottom-up solidification sequence concept is realized (Figure 6). Porosity results show that there is no obvious loose area in casting part (Figure 7), and there is no isolated solidification phase. Analysis believes that process is theoretically feasible. Planned process yield rate is approximately 55%.

 

Figure 6 Solidification diagram of gravity casting process

 

Figure 7 Loose defects in gravity casting process

Resin sand modeling method was used in study, and process parameters of key processes are shown in Table 2.

Table 2 Process parameters

There were no obvious abnormalities during pouring process. After pouring was completed, when mold was removed from JM-083 low-pressure casting machine, it was found that upper part of mold riser tube tended to solidify. It was believed that actual production process parameters were relatively consistent with process assumptions.
After casting is refined, obvious shrinkage holes and loose defects can be seen at lower box flange (Figure 8). Analysis believes that cause of defect at this location is poor feeding of aluminum liquid. Defective position is close to liquid riser tube, which is last position of casting to solidify (solidification time is longer than part above liquid riser pipe), that is, liquid riser tube cannot form effective feeding at this position. Analysis found that diameter of liquid riser pipe in facility part of 150 mm (the largest size of liquid riser pipe at production site) is smaller than diameter of vertical cylinder of 260 mm. Liquid riser pipe cannot effectively pressurize and feed casting during limited crystallization pressurization time. It is believed that this pouring process needs to be optimized.

 

Figure 8 Casting conditions of low pressure pouring process

Filling time is 5s longer than theoretical simulation value, and pouring speed of manual ladle may be somewhat different from process parameters. Appearance quality of casting is good (Figure 9a). No abnormalities were found in fluorescence detection after surface cleaning (Figure 9b). Internal quality of X-ray inspection meets technical requirements of casting. Among them, cast structure at riser root is level 2 loose (Figure 9c).

 

Figure 9 Situation of castings produced by gravity pouring process

(1) This thick-walled cylinder casting adopts gravity casting process to meet technical requirements of castings. Process design: Area ratio of pouring system is ΣS sprue: ΣS cross runner: ΣS inner runner = 1:4:5. Central pouring method is adopted, top riser of casting is designed with an insulating riser, and bottom of casting is designed with a conformal cold iron with a thickness of 1.2 times wall thickness of casting. Process yield rate is 55%.
(2) After casting using low-pressure casting process, castings have defects such as shrinkage cavities and porosity, which cannot meet technical requirements of product. Main reason is that diameter of riser tube is insufficiently designed, and diameter of riser tube should be no less than 260 mm. Process yield is only 20%. Taking all factors into consideration, this process design is not suitable for trial production of this product.