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Text | Afang
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preface
As a high-strength super-hard aluminum, 7050 aluminum alloy has high specific strength, fracture toughness and fatigue strength, and is widely used in the manufacture of various aviation structural parts. However, with the continuous improvement of the performance requirements of structural parts in the aviation field, the modification of aluminum alloy properties has always been the focus of many scholars.
In recent years, laser shock (LS), as a new treatment process, has become an effective means of surface strengthening of aluminum alloys. The results show that after laser impact, not only the high compressive stress is formed on the surface layer, but also the microstructure of the surface layer of the material is improved, which can significantly improve the service performance of the parts, especially the fatigue resistance.
With the development of cryogenic technology and the diversification of material experiments, cryogenic treatment has gradually extended from ferrous metals (iron and steel materials) research to non-ferrous metals and composite materials. Among them, the research on cryogenic treatment of aluminum alloy has become a hot spot in recent years. Studies have shown that the process method combining plastic deformation and cryogenic treatment can further improve the comprehensive properties of materials. Therefore, the composite treatment method using cryogenic-laser impact has attracted widespread attention.
Therefore, let's study experimentally what impact laser impact and cryogenic treatment will have on the residual stress and mechanical properties of 7050 aluminum alloy.
One. Experimental materials and methods
The experimental material in this paper is 7050 aluminum alloy in rolled state, and the chemical composition is shown in Table 1. The specimen is processed separately into a residual stress test specimen (fig. 1(a)) and a standard tensile specimen (fig. 1(b)) as shown in fig. 1. The aluminum alloy specimen is subjected to the composite treatment process of "laser impact-cryogenic-supplemental low-temperature tempering".
SGR-Extra-10 laser strengthening device is used to laser impact the sample, and the impact position is shown in the black area in Figure 1, in which the laser wavelength output of neodymium glass pulsed laser is 1064nm, pulse width is 30ns, frequency is 0.1Hz, and the spot diameter of the laser beam in the strengthening area is 4mm.
Before laser impact strengthening, the aluminum foil is attached to the surface of the specimen to be strengthened to protect the surface of the material; A uniform deionized water film with a thickness of about 1 mm is formed at the strengthening position to suppress excessive plasma expansion and increase shock wave pressure. The laser impact energy and the number of impacts were 15J, (1, 3, 5), 25J (1 time) and 30J (1 time), respectively. The laser impact treatment site and specimen clamping method are shown in Figure 2.
SLX-30 program control deep freezer to the laser impact of the specimen for cryogenic (C)-supplementary low temperature tempering (A) treatment, the detailed process is: at a cooling rate of 2 °C/min to cool the specimen to -190 °C, and then take out the specimen and immediately immerse it in liquid nitrogen to keep warm for 12h, and then heat the specimen to 120 °C and keep warm for 4h at a heating rate of 2°C/min, and finally take out the specimen and air cool it to room temperature. According to the different treatment processes, the experimental grouping and corresponding numbers in this paper are shown in Table 2.
The PRISM laser speckle interferometer test system was used to detect the residual stress of the specimen after different process treatment. EM-5L microhardness tester (measurement error range±2%) was used to test the microhardness of samples after different processes (on the residual stress test sample); MTS-SANSCMT5000 electronic universal tensile testing machine was used to test the tensile properties of samples after different processes.
The microhardness and tensile test results were taken as the average of the three specimens as the final result. After the specimen is treated by different processes, small specimens of 10mm× 10mm×5mm are prepared in the laser impact area of the residual stress test specimen to characterize the microstructure perpendicular to the laser impact surface. The samples were ground by 400, 600, 800, 1000, 1500 and 2000 mesh sandpaper, and then the polishing surface was immersed in a newly formulated etching agent (2vol%HF+3vol%HCl+5vol%HNO3+190vol% distilled water H2O) for 30s.
Finally, microstructure changes of specimens treated by different processes were characterized under light microscopy (OM) and scanning electron microscopy (SEM).
Two. Residual stress
In order to explore the effects of cryogenic treatment and supplemented low-temperature tempering on the surface residual stress of aluminum alloy after laser impact treatment, the residual stress distribution of aluminum alloy surface in four different states was compared with the original state (UL-UC), laser impact treatment (L-UC), cryogenic treatment (L-C) after laser impact treatment, cryogenic treatment and supplementary low-temperature tempering treatment (L-CA) after laser impact treatment, and the results are shown in Figure 3.
It can be seen that the residual stress of the original state specimen is evenly distributed with depth and is close to 0MPa numerically. This is due to the natural aging of the specimen for up to 60 months, so the residual stress release of the material is relatively uniform, which also reflects the accuracy of the stress test results from the side.
After laser impact, the compressive stress on the surface of the specimen increases significantly, and the extreme value of the compressive stress appears at a depth of 0.2mm, which is about -260MPa. However, cryo-supplemental low-temperature tempering after laser impact can reduce the compressive stress on the specimen surface, where the extreme value of compressive stress at a depth of 0.2 mm is about -200MPa.
It can be seen that the surface compressive stress is reduced by about 23% after increasing cryogenic-low temperature tempering treatment. As the depth increases, the level of compressive stress inside the specimen is slightly lower than that of a single laser impact specimen. The results show that the cryogenic-supplemental low-temperature tempering treatment after laser impact can release the compressive stress on the surface of 7050 aluminum alloy after laser impact treatment.
However, a single cryogenic treatment after laser impact has little effect on the residual stress distribution on the specimen surface. When the laser impact energy is 15J, the influence of different impact times on the residual stress distribution of 7050 aluminum alloy specimen is studied, and the results are shown in Figure 4.
It can be seen that compared with single and 5 laser impacts, the compressive stress value of the specimen surface after 3 laser strokes is higher, and the stress layer depth is larger. This shows that the influence of the number of laser impacts on the residual compressive stress on the surface of the 7050 aluminum alloy specimen is not a linear increase. Due to the limited degree of plastic deformation of the material, there is an optimal number of laser impacts for surface compressive stress.
It is worth noting that although supplementing the low-temperature tempering treatment will reduce the surface compressive stress of the specimen after laser impact, the surface compressive stress can still reach a high level by adjusting the number of laser impacts.
In this study, the maximum compressive stress value of the specimen surface (at 0.2mm) was about 230MPa after three laser impacts, which was close to the maximum compressive stress value of 260MPa for a single laser impact. The change of residual stress with depth after laser impact of 7050 aluminum alloy specimen with different energies is shown in Figure 5.
It can be seen that at a depth of 0.2mm, the compressive stress on the surface of the specimen increases with the increase of energy. However, as the depth increases, the compressive stress inside the specimen after the 30J impact is slightly smaller than when the energy magnitude is low. Among them, when the energy magnitude is 25J, there is a high residual compressive stress on both the surface and the inside of the specimen. In general, the influence of energy within a certain range on the residual stress of the laser impact surface is relatively small.
Three. microhardness
The effect of cryo-supplemental low-temperature tempering treatment on the microhardness of 7050 aluminum alloy specimen after laser impact is shown in Figure 6. On the whole, whether the cryogenic-supplemented low-temperature tempering treatment is carried out after laser impact has little effect on the microhardness value. However, compared with the single laser impact, the microhardness of the specimen fluctuates significantly with the change of detection depth after laser shock-cryogenic-supplemented low-temperature tempering.
The results show that the cryogenic-supplemental low-temperature tempering treatment after laser impact can homogenize the hardness of the surface and matrix of the aluminum alloy, thereby improving the overall performance of the material.
Four. Tensile properties
Figure 7 shows the effect of 15J laser impact on the tensile properties of 7050 aluminum alloy. Among them, the tensile strength of the 7050 aluminum alloy specimen after 15J laser impact is shown in Figure 7(a), where Figure 7(b) is the stress-strain curve of the specimen when it is stretched. As can be seen from Figure 7(a), the tensile strength and yield strength of the specimen after laser impact are improved compared to non-laser impact.
The tensile strength was further improved after the cryogenic treatment, while the tensile strength and yield strength did not change significantly with the addition of cryogenic and supplemented low-temperature tempering treatment. In addition, the elongation of the specimen after laser impact decreases significantly, and the elongation will be further reduced by the addition of cryogenic treatment.
The microstructure can be seen from the experimental results of residual stress and mechanical properties, and the cryogenic treatment after laser impact can further improve the tensile strength of aluminum alloy specimens, and at the same time has little effect on the surface residual stress.
Therefore, this paper selects a single laser impact and cryogenic treatment after laser impact for microstructure comparison. The metallographic structure of the 7050 aluminum alloy specimen after different process treatment is shown in Figure 8.
As can be seen from Figure 8(a), the original structure (rolled state) of the specimen consists of a matrix phase and a second phase of particles. After laser impact cryogenic treatment, the microstructure of the specimen showed a significant gradient structure, and a plastic deformation layer with a width of about 500 μm appeared near the surface, while the matrix structure did not change significantly (Figure 8(c)).
From the relationship between material strain and stress, it can be seen that the volume shrinkage caused by plastic deformation will produce large compressive stress in the microstructure, so the compressive stress on the surface of the specimen increases significantly after laser impact. The high degree of compressive stress and plastic deformation is conducive to dislocation proliferation, and the dislocation density increases with the extension of the holding time of cryogenic treatment, and exists in the microstructure of aluminum alloy by tangling.
In addition, a large number of refined grains are found in the deformation layer. The addition of grain boundaries promotes the formation of defects, so the ability to block the passage of dislocations during deformation is enhanced, resulting in an increase in the tensile strength of the 7050 aluminum alloy specimen after laser impact. To further investigate changes in microstructure, SEM is used for characterization.
Figure 9 shows the SEM structure of 7050 aluminum alloy specimen after different process treatment. Comparing Figure 9 (a) and (b), it can be seen that cryogenic treatment after laser impact can aggravate the surface plastic deformation of aluminum alloy specimens, and the area close to the matrix also shows certain deformation. In addition, compared with a single laser impact, the addition of cryogenic treatment significantly increases the defects in the microstructure.
This is caused by the reduced plastic deformation resistance of aluminum alloys at low temperatures. Local magnification of the feature area is shown in Figures 9(c), (d). It can be seen that the addition of cryogenic treatment after laser impact causes fine subgrain boundaries to appear in the microstructure, indicating that the grains are further refined, so the strength of the aluminum alloy is improved.
Five. conclusion
1. Laser impact can significantly increase the surface compressive stress of 7050 aluminum alloy, and cryogenic treatment alone after laser impact treatment has little effect on the residual stress of the sample, but the combination of cryogenic treatment and supplemented low-temperature tempering has a certain release effect on the surface compressive stress. Three laser impact treatments can compensate for the release of surface compressive stress by cryogenic and supplemental low-temperature tempering treatment after a single laser impact.
2.After the 2.7050 aluminum alloy is treated by the composite process of "laser impact-cryogenic-supplementary low-temperature tempering", the hardness fluctuation between the matrix and the surface is reduced, and the overall hardness of the material is homogenized, but there is no obvious effect on the strength. However, only cryogenic treatment after laser impact treatment can improve the strength of 7050 aluminum alloy.
3. The laser impact causes obvious plastic deformation of the surface layer of 7050 aluminum alloy, thereby increasing the surface compressive stress. After the addition of cryogenic treatment, the deformation degree of the surface layer of the material intensifies, the subgrain boundary increases, and the grain is refined. Therefore, after this process, 7050 aluminum alloy has the highest strength.