This article mainly studies the application of A356.0 aluminum alloy semi-solid slurry technology in gravity casting process. Selecting thin-walled aluminum alloy valve body castings with complex structures as the research object, aluminum liquid with a certain solid phase ratio prepared by air blowing pulping equipment was used for trial production of tilting gravity casting process and conventional liquid aluminum liquid tilting gravity casting process. An experimental plan was developed and multiple sets of process data were collected. Multiple data such as semi-solid and liquid flowability, metallographic structure, and product performance were compared and analyzed in each set of experiments, laying the foundation for the application of semi-solid pulping in aluminum alloy gravity casting.
Aluminum alloy gravity casting is a common casting process for aluminum alloys, which is suitable for products with harsh working conditions and high mechanical performance requirements. However, due to the fact that its forming mainly relies on gravity, there are certain product structural limitations in practical applications. For products with thin walls (average wall thickness not exceeding 5 mm), complex structures, high mechanical properties, and pressure requirements, defects such as porosity, shrinkage, looseness, and poor forming are prone to occur during gravity casting, resulting in low product yield, increased manufacturing costs, and difficulty in achieving mass production. The semi-solid pulping technology is a professional method to obtain a solid-liquid mixture slurry with a certain non dendritic primary solid phase near the liquidus line of the metal. Due to the particularity of its preparation method, semi-solid slurry has finer grains, increased flowability, and reduced deformation resistance due to the presence of a certain amount of liquid phase, which improves the forming ability of gravity poured complex parts. This study proposes using A356.0, a common aluminum alloy casting material, to perform tilting gravity casting under certain process conditions using semi-solid slurry technology and conventional liquid metal states. The results of the two processes are compared and analyzed to improve the qualification rate and quality level of structurally complex thin walled aluminum alloy gravity castings.
Compared with the conventional aluminum alloy gravity casting process, this process only requires the addition of a semi-solid pulping process, and has improved product qualification rate, low manufacturing cost, and shortened process cycle, making it suitable for large-scale production and a topic of concern in the industry.
1 Experimental process
This study selected a typical workpiece with complex structure, thin wall, and high performance requirements - the valve body of the automotive processing system booster valve as the research object. In the same set of gravity casting molds, the same aluminum alloy casting material A356.0 was used for testing. Experiments were conducted on semi-solid slurry obtained using semi-solid pulping technology under two production process conditions: tilting gravity casting and conventional liquid tilting gravity casting. Three sets of process parameters were designed for each production process, with 10 molds (20 pieces) produced in each set. The products were compared and studied by group.
1.1 Performance requirements and structural analysis of test products
The material of the test valve body is A356.0 aluminum alloy, and its chemical composition requirements and mechanical performance requirements are shown in Table 1 and Table 2, respectively.
The valve body structure is complex, with air passages and inserts in the inner cavity. The three-dimensional shape of the valve body is shown in Figure 1.
The valve body is irregularly shaped and has multiple thick walled protrusions. One side of the pipe mouth is a diamond shaped flange, and the other side is a rectangular flange with protrusions. The outer contour dimensions are 179 mm × 87 mm × 131 mm, with an average wall thickness of 4.37 mm, a maximum wall thickness of 23.21 mm, and a minimum wall thickness of 2.8 mm. The wall thickness analysis is shown in Figure 2.
The valve body is required to undergo a sealing test, with water as the test medium and a test pressure of 0.3 MPa. The appearance of the product is not allowed to have cracks, pores, penetrating shrinkage or looseness.
1.2 Product Casting Process and Mold Design
Select the tilting gravity casting process based on product technical requirements and structural characteristics. When designing the pouring system, a comprehensive analysis of shape factors and wall thickness factors combined with the principle of sequential solidification, considering the effect of hot spot shrinkage, is carried out. The feeding is selected at the thickest part of the casting wall thickness, and the shell core is used to assist in forming at the airway, inner cavity, and diamond flange. Since the product is small, in order to ensure the temperature of the mold, the structure of the first mock examination and two cavities is set. The casting process design scheme is shown in Figure 3, and the mold structure design scheme is shown in Figure 4.
1.3 CAE simulation analysis
CAE analysis is based on the liquid gravity casting process, and the analysis results are shown in Figure 5. From the four dimensions of filling analysis, entrainment analysis, solidification analysis, and porosity analysis, it can be seen that the filling process is relatively stable without obvious entrainment phenomenon. During the solidification process, there is an isolated liquid phase and an increase in gas pressure in the inner cavity airway. The risk probability of gas shrinkage pores in the thinnest airway wall due to isolated liquid phase and trapped gas factors is about 2%. The simulation results indicate that the casting process needs to focus on controlling trapped gas and solidification temperature fields.
1.4 Verification of Gravity Casting Process for Valve Body
When verifying the gravity casting process for the valve body, the same tilting gravity casting machine and the same set of molds are used to verify both casting processes. The two schemes follow the same process flow: tilting pouring → shaking sand → removing the pouring system → removing the flash edge → heat treatment → shot blasting. The heat treatment process is carried out according to the process parameters of solid solution (535 ± 5) ℃× 6 h+aging (170 ± 5) ℃× 6 h. The samples produced by the two schemes are placed in the same workstation equipment in the same way and enter the same heat treatment furnace for heat treatment.
Option one uses conventional liquid aluminum for tilting gravity casting, while option two uses a semi-solid pulping machine to make semi-solid pulp of liquid aluminum before tilting gravity casting.
1.4.1 Scheme 1: Conventional liquid aluminum tilting gravity pouring
The conventional tilting pouring of liquid aluminum is based on the parameter conditions set during CAE simulation, combined with the simulation results, and validated according to the three sets of process parameters in Table 3. During the validation, 10 molds (20 pieces) were produced for each set of parameters, and the group numbers were marked at designated positions using a needle marking machine. Production was carried out according to the process flow. After the production process was completed, visual inspection, X-ray inspection, and machining airtightness verification were carried out, and the results were statistically analyzed. The statistical results are shown in Table 4.
According to the product inspection results, the main appearance defects are insufficient pouring and air streaks. The X-ray inspection results show that the main defect is the tissue shrinkage and penetration of the wall thickness between the airway and the inner cavity. The main airtightness defect is the leakage of the inner cavity, which is consistent with the location of the X-ray inspection defect.
1.4.2 Scheme 2: Semi solid pulp material tilting gravity pouring
After semi-solid pulping of liquid aluminum using the GISS semi-solid pulping machine, tilting gravity pouring was carried out. The experiment was divided into three groups and carried out according to the test parameters in Table 5. During verification, 10 molds (20 pieces) were produced for each parameter group, and the group numbers were marked at designated positions using a needle marking machine. Production was carried out according to the process flow. After the production process was completed, visual inspection, X-ray inspection, and processing airtightness verification were carried out, and the results were statistically analyzed. The statistical results are shown in Table 6.
According to the inspection results, the appearance of the product is well formed, and X-ray inspection of a very small number of products shows slight shrinkage and looseness in the tissue between the airway and the inner cavity wall thickness.
1.5 Verification of Mechanical Properties of Valve Body
Randomly select 3 parts from each of the 3 and 6 groups that have passed the airtightness test after processing, and take samples at the designated location of the sample body. The sampling area is shown in Figure 6, and 1 piece is taken from each sample and processed to the specified size requirement. The size of the test bar is shown in Figure 7. The experiment was conducted using the WD-P4104 electronic universal testing machine, with a measurement error of ± 0.5% in the testing machine system. The results of the tensile test are shown in Table 7.
2 Results and Discussion
2.1 Comparative analysis of process data
Comparing Tables 4-6, analyzing the process parameters and product inspection results of the two schemes, it can be found that semi-solid gravity casting requires lower aluminum pouring temperature, shorter solidification time, higher mold temperature, and higher overall product qualification rate.
2.2 Comparative analysis of mechanical properties of products
According to the comparison of mechanical properties in Table 7, it can be found that compared with conventional liquid aluminum gravity castings, semi-solid slurry gravity castings have higher mechanical properties.
2.3 Comparative analysis of product quality
Randomly select one product from each of the three and six process parameters for appearance and internal quality comparison (CT scanning method) and metallographic structure analysis. The comparison results are shown in Figures 8-11.
Figure 11 Microstructure of semi-solid slurry casting samples and conventional liquid casting samples
From Figures 8-9, it can be observed that compared to conventional liquid aluminum gravity castings, the surface flow patterns and insufficient pouring of semi-solid slurry gravity castings are significantly reduced.
From the CT scan results in Figure 10, it can be observed that compared to conventional liquid aluminum gravity castings, semi-solid slurry gravity castings have smaller micro shrinkage in the wall thickness zone.
From the metallographic results in Figure 11, it can be observed that compared to conventional liquid aluminum gravity castings, the grain size of semi-solid slurry gravity castings is smaller and finer.
3 Conclusion
(1) Semisolid gravity castings are easier to form and can effectively solve casting defects such as insufficient casting. And it can effectively solve casting defects such as insufficient pouring and air streaks caused by trapped gas.
(2) During the solidification process of semi-solid gravity casting technology, the shrinkage rate is lower and the internal structure is denser.
(3) The semi-solid gravity casting process has better mechanical properties.
(4) The semi-solid gravity casting process has lower melting and insulation temperatures, lower energy consumption per ton of molten metal, and can significantly reduce manufacturing costs. The gravity pouring process has a lower pouring temperature and shorter solidification time, which can effectively reduce the process cycle and improve production efficiency.
Author:
Hu Lijun, Zhu Yuxiao, Zhang Haichao
Wuxi Bell Machinery Co., Ltd
This article comes from: Foundry Magazine
