There are various second phase particles in the die cast ADC10 aluminum alloy, especially the second phase containing iron, which can significantly reduce its ductility and affect its structural application. By appropriately modifying the cast microstructure of ADC10 alloy, the mechanical properties of the alloy can be improved. This article investigates and analyzes the effect of friction stir processing (FSP) on the microstructure and mechanical properties of die cast ADC10 alloy: FSP can effectively refine the as cast structure of the alloy, decompose coarse Si particles, and α- Uniform and fine equiaxed Si particles were formed in the Al matrix. In addition, friction stir machining eliminates defects such as pores that serve as crack sources and shatters second phase particles, thereby improving the mechanical properties and ductility of the alloy.
In the past few decades, industries such as automobiles and machinery have been deeply cultivating the transition from relatively heavy black alloys to lighter cast aluminum alloys to improve fuel efficiency and driving range, and reduce carbon emissions. Die cast aluminum alloy has attracted widespread attention due to its light weight, good fluidity, and excellent mechanical properties. Among them, ADC10 aluminum alloy is widely used in automotive die cast components and other fields. However, the porosity, iron containing needle like phase, uneven distribution of second phase particles, and coarse Si particles in die cast aluminum alloys severely limit their mechanical properties such as tensile strength and elongation. In order to improve the mechanical properties of die cast ADC10 aluminum alloy, it is usually necessary to start from refining the structure and reducing porosity. Friction stir welding (FSP) is a hot working technique that utilizes the machining principle of friction stir welding (FSW) to locally modify the microstructure of castings. It has the potential and ability to improve the mechanical properties of die cast ADC10 alloy. Stirring friction machining involves applying significant plastic deformation to the surface of a workpiece using a rotating tool, which helps to alter the microstructure of die cast aluminum alloys and eliminate pores. The tool speed and lateral displacement speed are two important process parameters that affect the degree of microstructure refinement in friction stir machining. Research has shown that friction stir machining has the advantages of decomposing large-sized Si particles, eliminating porosity in die cast A356 alloy, and improving investment casting A319 alloy α- The ability of Al dendrite structure; For A319 and A356 castings, friction stir machining also decomposes the second phase particles in the stirring zone and distributes them uniformly on the substrate in a relatively uniform shape. The combined effect of these microstructural changes can improve the strength and ductility of die cast aluminum alloys, with a decrease in porosity leading to a reduction in crack sources, thereby improving the mechanical properties of the alloy and enhancing its fatigue performance. Therefore, this article conducted a stir friction machining treatment on the die cast ADC10 aluminum alloy and studied the effect of machining on typical porosity defects such as porosity and shrinkage porosity in the alloy. In addition, its influence on microstructure refinement was also studied, and finally, the mechanical properties differences between as cast materials and stir friction processed materials were compared.
1. Test materials and methods
The stirring friction machining test was conducted on a 4.5mm thick die cast ADC10 aluminum alloy sheet, and the alloy composition is shown in Table 1. The size of the stirring head for the ADC10 alloy in die casting is shown in Figure 1a, and Figure 1b shows the working principle of stirring friction processing. A single pass processing is carried out using vertical force. The rotation speed of the stirring head (usually composed of a stirring needle and shaft shoulder) is 650 r/min, the movement speed of the stirring head is 0.35 m/min, and the inclination angle is 5 °. The forward and backward sides of the processing area are marked in Figure 1b, and the stirring friction processing depth is maintained at 3.25 mm. The processing starts from the edge of the die cast ADC10 sheet at 50mm, with a travel distance of 200mm, and the sheet size is 300mm x 30mm. The temperature of the mixing head and plate is measured by a thermocouple inserted into the ADC10 alloy plate and threaded pin. The position of the thermocouple is shown in Figure 2, and it is 10 mm away from the machining centerline. Extract metallographic specimens from ADC10 alloy sheets perpendicular to the direction of stir friction machining. Subsequently, the sample was embedded with epoxy resin, polished and polished, and etched with Keller reagent. A metallographic photo of the ADC10 alloy sample was taken using an optical microscope, and its pore distribution was observed. Finally, the mechanical properties of ADC10 alloy were measured in the die-cast state and after stir friction processing. Samples were cut from the melt zone of the stir friction processed plate using wire cutting. The length of the tensile sample was 25 mm, the width was 5.76 mm, and the thickness was 4.5 mm. The sampling location is shown in Figure 2. The tensile test was conducted using a universal testing machine (Zwick proline) at room temperature, with a tensile speed set to 0.01 mm/s. The porosity was detected using a metallographic microscope (EOC-BA310Met) and built-in software, with a detection area of 100 m ².
2. Experimental results and discussion
2.1 Microstructure
As shown in Figure 3, the stirring friction processing zone is a typical bowl shaped structure. The sampling position on the cross-section is 130 mm away from the starting point of travel. Figure 4 shows the temperature records and vertical pressure variation curves measured by each thermocouple during the friction stir machining process. It can be seen from the records that the temperature of the threaded pin gradually increases with the machining process, and ultimately remains at around 425 ℃. Furthermore, from the thermocouple readings at a distance of 100mm and 200mm from the starting point of travel, it can be seen that the peak temperature of the ADC10 alloy sheet at a distance of 10mm from the machining centerline is around 245 ℃. The vertical force during the stirring friction machining process is 10 kN. The metallographic photo of the low magnification microstructure of the die cast ADC10 alloy is shown in Figure 5a. The skin of the die cast sheet is relatively dense and there is no significant distribution of pores. However, there are a large number of pore defects in the central area of the die cast sheet, mainly including curling and shrinkage, with pore sizes ranging from a few micrometers to tens of micrometers. The metallographic structure is shown in Figure 5b, and the microstructure of ADC10 exhibits dendritic morphology α- Composed of Al phase, eutectic Si particles, Cu rich Al2Cu particles, and Fe containing second phase. As shown in Figures 5b and 5c, α- The size and distribution of Al phase and Al Si eutectic vary from the wall end to the center. The Al Si eutectic structure is densely distributed in the edge region, which contains round/spotted eutectic Si particles. However, as shown in Figure 5c, the Al Si eutectic structure has a lower density in the middle part of the plate. In Figure 5c, the presence of needle shaped coarse Si particles can be clearly observed. α- The interdendritic arm spacing of Al phase also varies with the distance from the edge α- The average size of Al is 6 μ Around m, and the average size in the central area is 8 μ About m. As shown by the arrow in Figure 5d, there are also Al2Cu and Fe rich second phase particles in the microstructure of the die cast ADC10 alloy. The needle shaped Fe rich phase will deteriorate the mechanical properties of the alloy. By analyzing multiple metallographic images of the die cast state, the porosity volume fraction of the alloy was calculated to be around 1.354%. The porosity shows a gradient distribution from the center to the edge, with a large standard deviation. The porosity can reach up to 6.025% near the edge.
As shown in Figure 6, friction stir machining has a significant effect on eliminating pores in die cast ADC10 alloy. After stir friction processing, the volume fraction of porosity decreased to 0.073%. The stirring effect of the stirring head in friction machining and the significant plastic deformation of the base material can effectively eliminate the micropores in the as cast structure. As shown in Figure 6, friction stir machining will produce as cast α- Complete modification of Al dendritic tissue. Needle shaped eutectic silicon particles are broken into smaller, almost equiaxed particles. In the melt nucleation zone of friction stir machining, Si particles are uniformly distributed in the Al matrix. In addition, compared with the die cast ADC10 alloy material, Al2Cu and Fe containing second phase particles (as indicated by the arrow in Figure 6) are also decomposed into smaller particles. Therefore, friction stir machining successfully refined the microstructure of die cast ADC10 alloy, significantly reducing the porosity of the alloy, and it also played a significant role in improving the various mechanical properties of ADC10 alloy.
2.2 Mechanical properties
Based on the study of modifying the microstructure of die cast ADC10 alloy after stir friction machining, the room temperature mechanical properties of the die cast ADC10 alloy and the alloy after stir friction machining were tested. The tensile test sample is cut from the fusion zone of the plate after stir friction processing, and the length direction is aligned with the centerline of the direction of stir friction processing. The sampling diagram is shown in Figure 2. Five samples were taken under the conditions of die-casting and stir friction processing, and subjected to tensile tests on a universal testing machine. The average value was finally compared with the die-casting alloy. Table 2 shows the tensile strength and elongation test results of die cast specimens and stir friction processed specimens. As shown in Table 2, the tensile strength of the die cast ADC10 alloy is 312 MPa, and the yield strength is 157 MPa. After stir friction processing, the tensile strength of the ADC10 alloy decreases to 301 MPa, but the yield strength is improved to 186 MPa. The elongation of ADC10 alloy has increased from 3.4% in die cast state to 5.7%.
The fracture morphology of die cast ADC10 alloy and stir friction processed ADC10 alloy samples was analyzed by scanning electron microscopy. The scanning electron microscope image of the fracture surface of the die cast specimen shows the presence of large pores inside the alloy (Figure 7a), and the cleavage mode of the fracture indicates low ductility. On the contrary, the fracture morphology of the alloy sample after stir friction processing shows that there are no obvious pores in the microstructure of the alloy after stir friction processing, and the fracture mode of the alloy is typical dimple fracture (Figure 7b). The comparison of fracture morphology results shows that the ductility of ADC10 alloy material after stir friction processing is significantly improved. The die cast ADC10 alloy has a high porosity and dendritic structure, as well as the presence of needle like Si phase and larger second phase particles. This structural composition leads to lower strength and elongation of the alloy, and friction stir processing significantly reduces the porosity of the alloy, which damages it α- The Al dendrite structure also refines the needle like Si phase and second phase particles, and the distribution is more uniform. The refinement of this microstructure significantly improves the yield strength and elongation of ADC10 alloy. The fracture of die cast ADC10 alloy mainly occurs in areas with high porosity and low ductility, while friction stir processing modifies the microstructure of the alloy, ultimately resulting in better plasticity.
3. Conclusion
(1) The solidification structure of die cast ADC10 alloy has a high porosity α- The dendritic structure of Al, needle like Si phase, and larger second phase particles reduce the strength and elongation of the alloy.
(2) Friction stir machining significantly reduces the porosity of the die cast ADC10 alloy and damages it α- The Al dendritic structure effectively refines the needle like Si phase and larger second phase particles in the die cast ADC10 alloy, with a more uniform distribution along the fusion zone, resulting in a finer microstructure of the alloy.
(3) Friction stir machining significantly improves the yield strength and elongation of ADC10 alloy. The fracture of die cast ADC10 alloy mainly occurs in areas with high porosity and low ductility, while stirring friction processing modifies the microstructure of the alloy, ultimately resulting in better mechanical properties and plastic characteristics.
author
Song Fulin
changsha aeronautical vocational and technical college
Kuang Yiming
Hunan University of Information Technology
This article is from the Journal of Foundry
