Process for increasing residual compressive stress in aluminum alloy bar and applications

Abstract

The application discloses a process method for increasing residual compressive stress of aluminum alloy rods and application, and comprises the following steps: quenching and drawing after solid solution treatment of the aluminum alloy rods; and the cumulative deformation of the drawing is not higher than 15%. The application provides a process method for increasing residual compressive stress of aluminum alloy rods, and pre-stretching is performed on an aluminum alloy extrusion blank by using a drawing process, so that the internal stress state of the aluminum alloy extrusion blank changes and is redistributed. Small deformation drawing is selected, plastic deformation of the drawing only occurs on the surface of the rod, shrinkage of the surface layer of the rod is limited by the inside, and thus the surface layer of the rod is in residual compressive stress in the axial direction.

Description

Technical Field

[0001] The present invention relates to the technical field of aluminum alloys, and in particular to a process method for increasing the residual compressive stress of an aluminum alloy bar and its application. Background Art

[0002] Aluminum alloy, a lightweight, high-strength, and corrosion-resistant material, is widely used in aerospace fasteners such as bolts, screws, nuts, and rivets, becoming an indispensable component of aerospace structures. However, with increasing technological demands, the performance requirements for aluminum alloys are also constantly increasing. Due to the compositional characteristics of Al-Zn-Mg-Cu alloys, it exhibits a high sensitivity to stress corrosion cracking (SCC). Fasteners such as bolts and nuts are used in a single aircraft, ranging from hundreds of thousands to millions, and their total weight can account for 5-6% of the aircraft's total weight. Aluminum alloy fasteners often suffer from stress corrosion cracking (SCC) during use in aerospace vehicles, hindering their safe, efficient, and reliable operation. Residual stress is a key factor affecting the stress corrosion (SCC) performance of aluminum alloys. Research has shown that residual compressive stress can reduce the SCC susceptibility of specimens, while residual tensile stress can increase it. Aluminum alloy bar blanks typically undergo extrusion forming and subsequent solution and quenching and aging heat treatments before processing. These pretreatments affect the alloy's residual stress, thereby impacting its SCC resistance. During the extrusion deformation process, due to the uneven metal flow, additional tensile stress is generated on the surface of the extruded material, which will increase the sensitivity of the alloy to stress corrosion. Moreover, the alloy needs to be solution treated at a high temperature above 450°C and then rapidly quenched. Relevant studies have shown that aluminum alloys produce large residual stresses during solution quenching, even close to the yield strength of the material. This residual stress will also have a significant impact on stress corrosion. Therefore, it is necessary to optimize the bar preparation process to effectively eliminate the surface residual tensile stress or convert it into residual compressive stress. There are currently four main methods for eliminating residual stress:

[0003] The first method is natural aging, which relieves stress by allowing the workpiece to sit in a natural environment. This method is time-consuming, often requiring months or even years, and has low production efficiency. The second and most traditional and popular method is thermal aging, which involves placing the workpiece in a thermal aging furnace for heat treatment to relieve stress at high temperatures. However, if the aging temperature is low, the residual stress changes minimally, while higher aging temperatures can reduce the alloy's strength. Therefore, this method is difficult to operate, requires stringent equipment and process requirements, and its effectiveness is difficult to guarantee. The third method is stress relief using sub-resonance. However, the process parameters of this method are highly dependent on the workpiece shape, requiring customized processing parameters for different workpiece shapes. Complex workpieces require skilled professional technicians to operate, and only 23% of the workpiece stress can be relieved. The fourth method is vibration aging, which uses mechanical assembly to form a complete stress relief system. It can achieve stress relief in a short period of time and can cover all workpieces requiring stress relief. However, the mechanism of the vibration aging process remains unclear, and its suitability for airborne equipment remains controversial both domestically and internationally.

[0004] Therefore, it is necessary to propose a process that is simple to operate, highly feasible, and can improve the stress state of aluminum alloy bars. Summary of the Invention

[0005] The present invention aims to address the aforementioned technical problems existing in the prior art. To this end, the present invention proposes a process for increasing the residual compressive stress in aluminum alloy bars. By pre-stretching the aluminum alloy extruded blank using a drawing process, the internal stress state is altered and redistributed. A small amount of deformation is selected during drawing, resulting in plastic deformation only on the surface of the bar. This limits the shrinkage of the surface layer of the bar to the internal components, resulting in residual compressive stress on the surface layer in the axial direction.

[0006] The present invention also proposes an application of the above process method in the preparation of aerospace components or automobile components.

[0007] According to one aspect of the present invention, a process for increasing the residual compressive stress of an aluminum alloy bar is proposed, comprising: solution treating the aluminum alloy bar, followed by quenching and drawing;

[0008] The cumulative deformation of the drawing is no more than 15%.

[0009] The principle of the present invention is:

[0010] Drawing with small deformation increases the surface residual compressive stress. The residual stress distribution of the drawn bar has the following three situations:

[0011] ①During drawing, the entire cross section of the rod undergoes plastic deformation, residual tensile stress is generated in the outer layer of the rod, and residual compressive stress appears in the center layer.

[0012] ②During drawing, plastic deformation occurs only on the surface of the bar, resulting in residual compressive stress on the surface of the bar and residual tensile stress in the center.

[0013] ③ If plastic deformation during drawing does not penetrate the center of the bar, the residual stress distribution in the drawn product should be intermediate between the first two cases. After drawing, the outer layer of the bar will have residual tensile stress, the center layer will also have residual tensile stress, and the middle layer will have residual compressive stress. In these three cases, we use small deformation passes to ensure that plastic deformation occurs only on the surface of the bar, thereby generating residual compressive stress on the bar surface.

[0014] According to the embodiments of the first aspect of the present invention, there are at least the following beneficial effects:

[0015] Compared with the prior art, the beneficial technical effects of the present invention are: ① obtaining the effect of reducing the diameter of the aluminum alloy bar; ② reducing the residual stress of the original aluminum alloy bar, and being able to convert the residual tensile stress into residual compressive stress.

[0016] This invention primarily focuses on methods for increasing the residual compressive stress in aluminum alloy bars. It studies how the drawing process improves the residual stress distribution. Compared to extrusion, drawing can process thin-walled tubes and very fine materials. The metal undergoes plastic deformation through the die orifice, resulting in a product with the same shape and size as the die orifice. Furthermore, due to reduced friction, the uneven deformation of drawn products is much less than that of extruded products. Compared with existing technologies, this invention achieves the effect of reducing the diameter of aluminum alloy bars; simultaneously, it reduces the residual stress in the original aluminum alloy bar and converts the residual tensile stress into residual compressive stress.

[0017] The aluminum alloy rod of the present invention does not take alloy composition into consideration.

[0018] In some embodiments of the present invention, the solution temperature is 350-550°C.

[0019] In some embodiments of the present invention, the solid solution time is 1 to 5 hours.

[0020] Under the above solid solution conditions, overburning will not occur and the internal structure can reach a uniform solid solution state.

[0021] In some embodiments of the present invention, the quenching method includes water-cooling quenching.

[0022] In some embodiments of the present invention, the water cooling quenching time is no longer than 10s.

[0023] In some embodiments of the present invention, the drawing comprises multi-pass drawing.

[0024] In some embodiments of the present invention, in the multi-pass drawing, the deformation of a single pass is no more than 10%.

[0025] In some embodiments of the present invention, in the multi-pass drawing, the deformation amount of a single pass is 8-10%.

[0026] In some embodiments of the present invention, the aluminum alloy rod comprises a 7075 aluminum alloy rod.

[0027] In some embodiments of the present invention, the diameter of the aluminum alloy rod is 10 to 20 mm.

[0028] According to another aspect of the present invention, application of the process method in the preparation of aerospace components or automobile components is proposed. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the metallographic photograph of sample 1 in the test example;

[0030] Figure 2 This is a metallographic photograph of sample 2 in the test example;

[0031] Figure 3 This is a metallographic photograph of the sample drawn to 11.5 mm in Example 1;

[0032] Figure 4 This is a metallographic photograph of the sample drawn to 14 mm in Example 2;

[0033] Figure 5 This is a metallographic photograph of the sample drawn to 11 mm in Comparative Example 1;

[0034] Figure 6 This is a metallographic photograph of the sample drawn to 10 mm in Comparative Example 2. DETAILED DESCRIPTION

[0035] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with the embodiments to fully understand the purpose, features and effects of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0036] Example 1

[0037] This embodiment provides a process for increasing the residual compressive stress of an aluminum alloy bar, and the specific steps are as follows:

[0038] In this embodiment, a 7075 aluminum alloy extruded bar with a diameter of 12 mm is selected, and its alloy composition is as follows: Si 0.03, Fe 0.11, Cu 2.18, Mn 0.043, Mg 2.04, Cr 0.033, Zn 8.06, Ti 0.02, Zr 0.12, and the balance is aluminum;

[0039] S1. Select a 50cm long original bar of the above composition, turn one end to 9mm Φ and 75mm in length. First, perform a solution treatment at 470°C for 60 minutes, followed by water quenching.

[0040] S2. The solid solution 7075 aluminum alloy was deformed to Φ11.5 mm by cold drawing in one pass, with a deformation of 8.16%.

[0041] S3. Finally, the surface residual stress of the original Φ12 mm bar and the sample drawn to Φ11.5 mm was measured using X-ray diffraction technology.

[0042] After a secondary cold drawing to Φ11.5mm, the residual stress value on the surface of the sample decreased from 48.5MPa of the original bar to a residual compressive stress of -25.54MPa, and the residual stress state changed from tensile stress to compressive stress.

[0043] It can also be seen from the metallographic photos that the degree of deformation of the rod is small, the elongation of the grains along the drawing direction is not obvious, and the equiaxed shape of the original billet is basically maintained, which to a certain extent indicates that the plastic deformation has not completely entered the center layer.

[0044] Example 2

[0045] In this embodiment, a 7075 aluminum alloy extruded bar with a diameter of 15 mm is selected, and its alloy composition is as follows: Si 0.12, Fe 0.15, Cu 1.62, Mn 0.15, Mg 2.49, Cr 0.22, Zn 6.04, Ti 0.025, Zr 0.031, and the balance is aluminum;

[0046] S1. Select a 50cm long original bar of the above composition, turn one end to 9mm Φ and 75mm in length. First, perform a solution treatment at 470°C for 60 minutes, followed by water quenching.

[0047] S2. The solid solution 7075 aluminum alloy was deformed to Φ14 mm by cold drawing in one pass, with a deformation of 12.89%.

[0048] S3. Finally, the surface residual stress of the original Φ15mm bar and the sample drawn to Φ14mm was measured using X-ray diffraction technology.

[0049] This embodiment provides a process method for increasing the residual compressive stress of an aluminum alloy bar. The difference between this embodiment and Example 1 is that a different diameter of the original bar is selected. The Φ15 mm original bar is cold drawn to Φ14 mm in one pass, and the deformation in one pass is 12.89%. The residual stress value on the surface of the sample changes from -32.01 MPa of the original bar to -127.92 MPa, and the residual compressive stress increases by approximately 100 MPa.

[0050] Comparative Example 1

[0051] This comparative example provides a process for increasing the residual compressive stress of an aluminum alloy bar, specifically:

[0052] In this comparative example, a 7075 aluminum alloy extruded bar with a diameter of 12 mm was selected, and its alloy composition was as follows: Si 0.03, Fe0.11, Cu 2.18, Mn 0.043, Mg2.04, Cr 0.033, Zn 8.06, Ti 0.02, Zr 0.12, and the balance was aluminum;

[0053] S1. Select a 50cm long original bar, turn one end into Φ9mm, and the length is 75mm. First, it is solution treated at 470℃ for 60min and then water-quenched.

[0054] S2. The first cold drawing process of the solution-state 7075 aluminum alloy was performed to a deformation of Φ11.5 mm, with a deformation of 8.16% per pass.

[0055] S3. The aluminum alloy is then drawn and deformed to Φ11.5mm, and then cold-drawn to Φ11mm in the second pass, with a deformation of 8.51% and a cumulative deformation of 15.97%;

[0056] S4. Finally, the residual stress of the Φ11 mm drawn bar was measured using X-ray diffraction.

[0057] Comparative Example 1: Based on Example 1, the second cold drawing process was performed to deform the sample to Φ11 mm, while other parameters remained unchanged.

[0058] After two passes of cold drawing, the residual stress value on the surface of the Φ11mm sample decreased from 48.5MPa of the original bar to 36.76MPa, and the residual stress state did not change.

[0059] Comparative Example 2

[0060] This comparative example provides a process for increasing the residual compressive stress of an aluminum alloy bar, specifically:

[0061] In this comparative example, a 7075 aluminum alloy extruded bar with a diameter of 12 mm was selected. The alloy composition, by mass percentage, is as follows: Si 0.03, Fe 0.11, Cu 2.18, Mn 0.043, Mg2.04, Cr 0.033, Zn 8.06, Ti 0.02, Zr 0.12, and the balance is aluminum.

[0062] S1. Select a 50cm long original bar, turn one end into Φ9mm, and the length is 75mm. First, it is solution treated at 470℃ for 60min and then water-quenched.

[0063] S2. The first cold drawing process of the solution-state 7075 aluminum alloy was performed to a deformation of Φ11.5 mm, with a deformation of 8.16% per pass.

[0064] S3. The aluminum alloy is then drawn and deformed to Φ11.5mm, and then cold-drawn to Φ11mm in the second pass, with a deformation of 8.51% and a cumulative deformation of 15.97%;

[0065] S4. The aluminum alloy, which is then drawn and deformed to Φ11mm, is deformed to Φ10mm by the third cold drawing pass. The pass deformation is 17.36%, and the cumulative deformation is 30.57%;

[0066] Comparative Example 2 is based on Comparative Example 1 and undergoes a third cold drawing process to deform the steel to Φ10 mm, while other parameters remain unchanged.

[0067] After three cold-drawn passes to a diameter of 10 mm, the residual stress on the surface of the sample decreased from 48.5 MPa in the original bar to 18.36 MPa, with no change in the residual stress state. Metallographic photographs show the presence of slip bands in the center of the sample bar, indicating that plastic deformation has entered the center from the surface.

[0068] Test Case

[0069] In this test example, samples of the original bars of the above embodiments and comparative examples were taken and residual stress was tested using X-ray diffraction. The test results are listed in Tables 1 and 2:

[0070] Table 1 X-ray diffractometer measurement results of surface residual stress of original 7075 aluminum alloy bar

[0071] sample Slope Standard deviation Residual stress / MPa Sample 1 Φ12mm -0.387 0.312 48.5±27.1 Sample 2 Φ15mm 0.193 0.269 -32.01±19.98

[0072] The residual stress on the surface of the original bar and the drawn sample was obtained by X-ray diffractometer. The incident light is at 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40° and 45°, and the peak values ​​of different diffraction angles are obtained. The data points with low intensity are discarded and the statistical results are presented in each table. The slope M is obtained by linear fitting with the peak angle, and then the residual stress value σ=K×M is obtained according to the stress constant K.

[0073] The residual stress value of the original 7075 aluminum alloy extruded bar was determined by X-ray diffraction method to be 48.5 MPa, and the initial state was tensile stress state.

[0074] Figure 1 This is a metallographic photograph of the original 7075 bar with a diameter of 12 mm, test example 1. The original 7075 bar with a diameter of 12 mm has fine grains in the center layer, mostly equiaxed.

[0075] Figure 2 This is a metallographic photograph of the original 7075 bar (15 mm diameter) in Test Example 2. The central grains are small, nearly equiaxed grains with a radially layered morphology.

[0076] Table 2 Surface residual stress of 7075 aluminum alloy after drawing to different diameters

[0077]

[0078] The data in the analysis table show that for a 7075 bar with an original diameter of 15 mm, the cumulative deformation is 12.89% when drawn to 14 mm in a single pass, and the residual compressive stress reaches 127.92 MPa, an increase of approximately 95 MPa. For a 7075 bar with an original diameter of 12 mm, the first drawing pass has the best effect on improving residual stress, and the residual stress state also changes from the tensile stress of the original bar to compressive stress, with the compressive stress on the bar surface reaching 25.54 MPa. At this time, plastic deformation occurs only on the surface of the bar, with residual compressive stress on the surface and residual tensile stress in the center. As the drawing continues, the plastic deformation enters the center layer of the bar, the residual compressive stress on the surface of the bar decreases significantly, and gradually transforms into residual tensile stress.

[0079] It can be seen that when the cumulative deformation is less than 15%, the drawing process has the best effect in reducing the residual stress of the original aluminum alloy bar, and the residual tensile stress on the surface of the original bar is converted into residual compressive stress.

[0080] Table 3 Different incident angles Statistical table of diffraction angle θ values ​​under

[0081] serial number φ angle <![CDATA[sin 2 f]]> Peak Angle 1 0 0.000 148.424 2 5 0.008 148.390 3 10 0.030 148.237 4 15 0.067 148.350 5 20 0.117 148.187 6 25 0.179 148.259 7 30 0.250 148.033 8 35 0.329 148.339 9 40 0.413 148.134 10 45 0.500 148.188

[0082] Table 3 corresponds to the test example sample 1 (the original bar in Example 1), the peak value of the 7075 original bar Φ12mm at different diffraction angles was measured by the side tilt method, and the effective results were analyzed. The slope was obtained by linear fitting with the peak angle, and then multiplied by the stress constant K = -125.19 to obtain the surface residual stress value of 48.5 MPa.

[0083] Table 4 Different incident angles Statistical table of diffraction angle θ values ​​under

[0084]

[0085]

[0086] Table 4 corresponds to test example 2. The peak values ​​of 7075 original bar Φ15mm at different diffraction angles were measured by the tilt method. The effective results were analyzed. The slope was obtained by linear fitting with the peak angle, and then multiplied by the stress constant K = -165.72 to obtain the surface residual stress value of -32.01 MPa.

[0087] Table 5 Different incident angles Statistical table of diffraction angle θ values ​​under

[0088] serial number φ angle <![CDATA[sin 2 f]]> Peak Angle 1 0 0.000 156.286 2 5 0.008 156.287 3 10 0.030 156.303 4 15 0.067 156.358

[0089] Table 5 corresponds to Example 1. The side tilt method is used to measure the peak values ​​of 7075 bar Φ12mm single-pass drawn to 11.5mm. The effective results are analyzed at different diffraction angles. The slope was obtained by linear fitting with the peak angle, and then multiplied by the stress constant K = -174.02 to obtain the surface residual stress value of -25.54 MPa.

[0090] Figure 3 This is a metallographic photograph of a 7075 aluminum alloy bar with a diameter of 12 mm drawn to 11.5 mm in a single pass, Example 1. The center of the bar contains slender grains and some grains that have been elongated along the drawing direction. Parallel slip bands appear on the surface of the bar, and plastic deformation is evident.

[0091] Table 6 Different incident angles Statistical table of diffraction angle θ values ​​under

[0092]

[0093]

[0094] Table 6 corresponds to Example 2. The side tilt method is used to measure the peak values ​​of 7075 bar Φ15mm single-pass drawn to 14mm. The effective results are analyzed at different diffraction angles. The slope was obtained by linear fitting with the peak angle, and then multiplied by the stress constant K = -165.72 to obtain the surface residual stress value of -127.92 MPa.

[0095] Figure 4 This is Example 2, the metallographic structure of a 7075 aluminum alloy bar with a diameter of 15 mm after single-pass drawing to 14 mm. The center is a mixture of deformed grains and dynamically recrystallized grains, the white area is the recrystallized region, and the surface of the bar is a coarse-grained layer. A small amount of slip bands appear on the surface, but do not extend into the center of the bar.

[0096] Table 7 Different incident angles Statistical table of diffraction angle θ values ​​under

[0097] serial number φ angle <![CDATA[sin 2 f]]> Peak Angle 1 0 0.000 139.158 2 5 0.008 139.105 3 10 0.030 139.091 4 15 0.067 139.073 5 20 0.117 139.056 6 25 0.179 139.043

[0098] Table 7 corresponds to Example 1. The side tilt method is used to measure the peak value of 7075 bar Φ12mm after multiple passes drawing to 11mm. The effective results are analyzed at different diffraction angles. The slope was obtained by linear fitting with the peak angle, and then multiplied by the stress constant K = -174.02 to obtain the surface residual stress value of 36.76 MPa.

[0099] Figure 5 For comparative example 1, the metallographic photograph of 7075 aluminum alloy with a diameter of 12 mm drawn in multiple passes to 11 mm shows that more slip bands appear on the surface of the rod and more fine grains appear in the center layer, indicating that dynamic recrystallization has occurred.

[0100] Table 8 Different incident angles Statistical table of diffraction angle θ values ​​under

[0101] serial number φ angle <![CDATA[sin 2 f]]> Peak Angle 1 0 0.000 139.116 2 5 0.008 139.118 3 10 0.030 139.110 4 15 0.067 139.105 5 20 0.117 139.102 6 25 0.179 139.093 7 30 0.250 139.086 8 35 0.329 139.067 9 40 0.413 139.059 10 45 0.500 139.073

[0102] Table 8 corresponds to Example 2. The side tilt method is used to measure the peak value of 7075 bar Φ12mm after multiple passes drawing to 10mm. The effective results are analyzed at different diffraction angles. The slope was obtained by linear fitting with the peak angle, and then multiplied by the stress constant K = -174.02 to obtain the surface residual stress value of 18.36 MPa.

[0103] Figure 6 This is a metallographic photograph of a 7075 aluminum alloy bar with a diameter of 12 mm drawn in three passes to 10 mm. It can be seen that the center of the bar includes slender grains and some recrystallized grains, and parallel slip bands also appear, indicating that the center layer has also undergone strong plastic deformation.

[0104] While the embodiments of the present invention have been described in detail above, the present invention is not limited to the embodiments described above. Various modifications may be made within the scope of knowledge possessed by a person skilled in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof may be combined with one another unless there is a conflict.

Claims

1. A process for increasing the residual compressive stress of an aluminum alloy bar, characterized in that: For the following steps: S1. A 50 cm long original bar was selected, one end was turned to Φ9 mm and a length of 75 mm, and then solution treated at 470°C for 60 min and water quenched to obtain a solid solution state of 7075 aluminum alloy. The original bar is a 7075 aluminum alloy extruded bar with a diameter of 12 mm, and its alloy composition is as follows: Si 0.03, Fe 0.11, Cu 2.18, Mn 0.043, Mg 2.04, Cr 0.033, Zn 8.06, Ti 0.02, Zr 0.12, and the balance is aluminum; S2. Solution-state 7075 aluminum alloy was cold-drawn in one pass to a diameter of 11.5 mm, with a deformation of 8.16% per pass. S3. Finally, the surface residual stress of the original Φ12 mm bar and the sample drawn to Φ11.5 mm was measured using X-ray diffraction technology; After a secondary cold drawing to Φ11.5mm, the residual stress value on the surface of the sample decreased from 48.5MPa of the original bar to a residual compressive stress of -25.54MPa, and the residual stress state changed from tensile stress to compressive stress.

2. Use of the process according to claim 1 in the preparation of aerospace components or automobile components.

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