Aluminum alloy friction stir welding seam fatigue-free method based on rolling strengthening

Abstract

The invention belongs to the technical field of surface strengthening, and particularly relates to an aluminum alloy friction stir welding seam fatigue-free method based on rolling strengthening. The method comprises the following steps: (1) carrying out a microhardness test and a tensile test on an aluminum alloy matrix to obtain the hardness and tensile strength of the matrix; and (2) friction stir welding is conducted. The tensile strength after friction stir welding is not lower than 65% of the fatigue strength of a base body by adjusting friction stir welding process parameters; and (3) the surface of the friction stir welding part is treated. The surface of the welding seam is polished through an angle grinder and abrasive paper, so that the surface roughness of the welding seam reaches about Ra1.2 microns; and (4) rolling surface strengthening is carried out. Surface strengthening process parameters are adjusted, so that the surface hardness of a heat affected zone of the weld joint subjected to surface strengthening is not lower than the surface hardness of a matrix, the surface roughness Ra is not higher than 0.5 mu m, and the surface residual compressive stress is not lower than 100 MPa; and (5) carrying out a fatigue test.

Description

Technical Field

[0001] The invention belongs to the technical field of surface strengthening, and in particular relates to a fatigue-free method for aluminum alloy friction stir welds based on rolling strengthening. Background Art

[0002] Friction stir welding (FSW) has become one of the mainstream welding processes for aluminum alloys due to its advantages such as no welding defects and low strength loss. However, the welded joints of actual engineering components often have initial defects, which gradually develop into macro cracks during use and eventually lead to fatigue fracture. Studies have shown that compared with the parent material, the fatigue strength of aluminum alloy welded joints can lose about 50%, and may be reduced by up to 90% under certain harsh conditions of use.

[0003] Generally speaking, the main factors affecting the fatigue performance of aluminum alloy welded joints are: 1) weld defects such as pores, inclusions, and cracks; 2) microstructural softening of the heat affected zone (HAZ). Mature welding processes can effectively reduce or eliminate defects by adjusting welding parameters. However, it is inevitable that the fatigue life of welded joints will be reduced due to the softening of the microstructure in the heat affected zone. Researchers usually use post-weld heat treatment methods for welded joints to improve the fatigue performance of aluminum alloy welding. However, heat treatment of large welded structures is difficult to achieve in production. In addition to heat treatment, deformation strengthening is another effective strengthening mechanism for aluminum alloy welded joints. Fatigue cracks in aluminum alloy welded components often initiate on the surface, and their initiation life can account for more than 90% of the total life. Therefore, the fatigue performance of the entire welded component can be improved by surface strengthening alone. Among them, shot peening is widely used in aluminum alloy welded joints as a key process to improve the fatigue performance of aluminum alloy welded joints. However, the obvious disadvantage of shot peening is that it will greatly increase the surface roughness and even cause microcracks. Therefore, it is urgent to find a way to improve the fatigue performance of aluminum alloy welded joints. It would be best to increase the fatigue strength of the weld to the level of the matrix to achieve fatigue-free welds. Summary of the invention

[0004] In view of the poor fatigue performance of stir friction welds, the present invention aims to provide a fatigue-free method for aluminum alloy stir friction welds based on rolling strengthening. The design concept of dual adjustment of stir friction welding process parameters and surface strengthening process is adopted. Through the idea of ​​combining the strong and the strong, the fatigue strength of the weld is improved to the level of the matrix, thereby achieving fatigue-free welds.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A fatigue-free method for aluminum alloy friction stir weld based on rolling strengthening, comprising the following steps:

[0007] (1) Performing microhardness test and tensile test on the aluminum alloy matrix to obtain the hardness and tensile strength of the matrix;

[0008] (2) Friction stir welding

[0009] By adjusting the friction stir welding process parameters, the tensile strength after friction stir welding is not less than 65% of the fatigue strength of the matrix;

[0010] (3) Surface treatment of friction stir welded parts

[0011] Grind the weld surface with an angle grinder and sandpaper to a weld surface roughness between Ra2.0μm and Ra0.8μm;

[0012] (4) Rolling surface strengthening

[0013] By adjusting the surface strengthening process parameters, the surface hardness of the heat affected zone of the weld after surface strengthening is not lower than the surface hardness of the substrate, the surface roughness Ra is not higher than 0.5μm, and the surface residual compressive stress is not lower than 100MPa;

[0014] (5) Fatigue test

[0015] Fatigue strength tests were carried out on the friction stir welding samples after aluminum alloy matrix and surface strengthening.

[0016] The fatigue-free method for aluminum alloy friction stir weld based on rolling strengthening, in step (1), the strain rate of the tensile test is less than or equal to 10 -3 s -1 ,The surface hardness of the aluminum alloy substrate was measured by Vickers indentation method, using a standard instrumented Vickers indentation equipment.

[0017] In the fatigue-free method for aluminum alloy friction stir welds based on rolling strengthening, in step (2), the friction stir welding process parameters that can be adjusted include the friction stir welding tool, rotation speed and moving speed.

[0018] In the fatigue-free method for aluminum alloy friction stir welds based on rolling strengthening, in step (3), when the surface of the friction stir welded part is treated, the surface flatness is ensured, and the surface roughness should be measured using the same equipment and the same parameters.

[0019] In the fatigue-free method for aluminum alloy stir friction weld based on rolling strengthening, in step (4), the rolling surface strengthening process parameters include pressure, rotation speed and moving speed, and the three parameters jointly affect the surface integrity after surface strengthening. The moving speed is 0.1 mm / s; the pressure has a greater influence on the surface hardness and residual stress. When the hardness and residual stress do not meet the requirements, it is preferred to adjust the pressure; the rotation speed has a greater influence on the surface roughness. When the surface roughness does not meet the requirements, it is preferred to adjust the rotation speed.

[0020] The fatigue-free method of aluminum alloy friction stir weld based on rolling strengthening must meet the requirements of surface hardness, surface roughness and residual stress after surface strengthening treatment, and cannot undergo secondary processing.

[0021] In the fatigue-free method for aluminum alloy friction stir welds based on rolling strengthening, in step (4), the width and relative position of each welding area are accurately determined by hardness distribution, so as to judge whether the strengthening area meets the requirements.

[0022] The design concept of the present invention is:

[0023] The present invention improves the fatigue strength of aluminum alloy stir friction welding based on rolling strengthening. Before rolling strengthening, certain requirements are set on the tensile strength and surface roughness of stir friction welding. By adjusting the stir friction welding process parameters and the surface strengthening dual process, the fatigue strength of the weld is improved to the level of the substrate, solving the problem that large welded structures cannot achieve heat treatment. The present invention improves the fatigue strength of large welded structural parts and meets the requirements of engineering applications.

[0024] The present invention has the following beneficial effects and advantages:

[0025] 1. The present invention adopts the design concept of dual adjustment of stir friction welding process parameters and surface strengthening process, and proposes a fatigue-free method for aluminum alloy stir friction welds based on rolling strengthening through the idea of ​​combining the strong.

[0026] 2. The present invention solves the problem that fatigue performance of large welded structures cannot be improved through heat treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The figure is a flow chart of the method of the present invention.

[0028] Figure 2 It is the tensile curve in Example 1. In the figure, the abscissa Engineeringstrain is the engineering strain (%), the ordinate Engineeringstress is the engineering stress (MPa), AS-receivedBM represents the original matrix, and AS-weldedjoint represents the welded joint.

[0029] Figure 3 The figure is a histogram of tensile strength in Example 1. In the figure, the abscissa Sample is the sample, and the ordinate Tensilestrength is the tensile strength (MPa).

[0030] Figure 4 This is the surface roughness test diagram in Example 1. Among them, (a) is the surface roughness of the welded joint (As-weldedjoint), (b) is the surface roughness after rolling strengthening (SR-7KG-300); Sa represents the arithmetic mean deviation of the profile, Sq represents the root mean square of the profile, and Sz represents the maximum height of the profile. Sa, Sq and Sz are surface roughness, which characterize the profile on a certain surface; Ra, Rq, Rz are line roughness, which characterize the roughness on a certain straight line. Surface roughness can more comprehensively reflect the state of the component surface than line roughness.

[0031] Figure 5 It is a surface hardness test diagram in Example 1. In the figure, the horizontal axis Distance from weld center is the distance from the weld center (mm), the vertical axis Hardness is the hardness (HV), BM represents the basic mental, HAZ represents the heat affected zone (Heat Affected Zone), NZ represents the nugget zone (Nugget Zone), TMAZ represents the thermo-mechanically affected zone (Thermo-Mechanically Affected Zone), AS-welded joint represents the welded joint, and SRed joints represents the joint after rolling strengthening.

[0032] Figure 6 This is a residual stress test diagram in Example 1. In the figure, the horizontal axis Depthfromsurface is the depth from the surface (μm), the vertical axis Residualstress is the residual stress (MPa), AS-receivedBM represents the original matrix, AS-weldedjoint represents the welded joint, and SRedjoints represents the joint after rolling strengthening.

[0033] Figure 7 This is a fatigue test diagram in Example 1. In the figure, the horizontal axis Number of cycles to failure is the fatigue life (N f ), the ordinate Maximumstress is the maximum stress (MPa), AS-receivedBM represents the original matrix, AS-weldedjoint represents the welded joint, and SRedjoints represents the joint after rolling strengthening. DETAILED DESCRIPTION

[0034] like Figure 1As shown, the present invention proposes a fatigue-free method for aluminum alloy stir friction welds based on rolling strengthening, which includes the following steps: (1) performing microhardness test and tensile test on the aluminum alloy substrate to obtain the hardness and tensile strength of the substrate; (2) performing stir friction welding. By adjusting the stir friction welding process parameters, the tensile strength after stir friction welding is not less than 65% of the fatigue strength of the substrate; (3) treating the surface of the stir friction welded parts. The weld surface is polished by an angle grinder and sandpaper to make the weld surface roughness reach Ra1.2μm; (4) performing rolling surface strengthening. By adjusting the surface strengthening process parameters, the surface hardness of the heat affected zone of the weld after surface strengthening is not less than the surface hardness of the substrate, the surface roughness Ra is not higher than 0.5μm, and the surface residual compressive stress is not less than 100MPa; (5) performing fatigue test. Fatigue strength test is performed on the aluminum alloy substrate and the stir friction welding sample after surface strengthening.

[0035] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other examples improved or modified by ordinary technicians in this field belong to the scope of protection of the present invention. It should be understood that the embodiments of the present invention are only used to illustrate the technical effects of the present invention, rather than to limit the scope of protection of the present invention. In the embodiments, the methods used are conventional methods unless otherwise specified.

[0036] Example 1

[0037] In this embodiment, a fatigue-free method for aluminum alloy friction stir weld based on rolling strengthening includes the following steps:

[0038] (1) The 6061 aluminum alloy substrate was subjected to microhardness test and tensile test, and the hardness of the substrate was found to be 106 HV. Its tensile curve is shown in Figure 2 As shown, the tensile strength is 320 MPa; the strain rate of the tensile test is 3×10 -4 s -1 ,The surface hardness of the aluminum alloy substrate was measured by Vickers indentation method, using a standard instrumented Vickers indentation equipment.

[0039] (2) Perform friction stir welding. Figure 3 As shown, by adjusting the friction stir welding process parameters: the friction stir welding tool is truncated cone-shaped (large head diameter is 9.5 mm, small head diameter is 5.4 mm), the rotation speed is 800 r / min, the moving speed is 150 mm / min, and the tensile strength after friction stir welding is 215 MPa, which is 67% of the fatigue strength of the matrix.

[0040] (3) Treat the surface of the friction stir welded part. Grind the weld surface with an angle grinder and sandpaper to make the weld surface roughness Ra reach 1.062μm. Figure 4 shown.

[0041] (4) Rolling surface strengthening. Figure 5 As shown in the figure, by adjusting the rolling surface strengthening process parameters: pressure of 7kg, rotation speed of 300r / min, and moving speed of 0.1mm / s, the surface hardness of the heat affected zone of the weld after surface strengthening is not lower than the surface hardness of the substrate. Figure 4 As shown in (b), the surface roughness Sa after rolling surface strengthening is 0.293μm. Figure 6 As shown in Figure 1, the surface residual compressive stress after rolling surface strengthening is 100 MPa.

[0042] (5) Fatigue test: Fatigue strength test was conducted on the aluminum alloy matrix and the stir friction welding samples after surface strengthening.

[0043] like Figure 7 As shown, the fatigue strength of the 6061 aluminum alloy substrate is 167MPa, the fatigue strength of the stir friction welding sample is 145MPa, and the fatigue strength of the stir friction welding sample after being treated by the present invention is 167MPa. The present invention can increase the fatigue strength of the weld to the level of the substrate, achieving fatigue-free weld.

[0044] The implementation results show that the present invention effectively solves the problem that the fatigue performance of large welded structures cannot be improved through heat treatment. Through the present invention, the fatigue strength of the weld can be improved to the level of the matrix, achieving fatigue-free weld.

Claims

1. A fatigue-free method for aluminum alloy friction stir weld based on rolling strengthening, characterized in that: The following steps are involved: (1) Performing microhardness test and tensile test on the aluminum alloy matrix to obtain the hardness and tensile strength of the matrix; (2) Friction stir welding By adjusting the friction stir welding process parameters, the tensile strength after friction stir welding is not less than 65% of the fatigue strength of the matrix; (3) Surface treatment of friction stir welded parts Grind the weld surface with an angle grinder and sandpaper to a weld surface roughness between Ra2.0μm and Ra0.8μm; (4) Rolling surface strengthening By adjusting the surface strengthening process parameters, the surface hardness of the heat affected zone of the weld after surface strengthening is not lower than the surface hardness of the substrate, the surface roughness Ra is not higher than 0.5μm, and the surface residual compressive stress is not lower than 100MPa; (5) Fatigue test Fatigue strength tests were carried out on the friction stir welding samples after aluminum alloy matrix and surface strengthening.

2. The fatigue-free method of aluminum alloy friction stir welding based on rolling strengthening according to claim 1 is characterized in that: In step (1), the strain rate of the tensile test is less than or equal to 10 -3 s -1 ,The surface hardness of the aluminum alloy substrate was measured by Vickers indentation method, using a standard instrumented Vickers indentation equipment.

3. The fatigue-free method of aluminum alloy friction stir welding based on rolling strengthening according to claim 1, characterized in that: In step (2), the stir friction welding process parameters that can be adjusted include the stir friction welding tool, rotation speed and moving speed.

4. The fatigue-free method of aluminum alloy friction stir welding based on rolling strengthening according to claim 1, characterized in that: In step (3), when the surface of the friction stir welded part is processed, the surface flatness and surface roughness should be measured using the same equipment and the same parameters to ensure the surface flatness.

5. The fatigue-free method of aluminum alloy friction stir welding based on rolling strengthening according to claim 1, characterized in that: In step (4), the rolling surface strengthening process parameters include pressure, rotation speed and moving speed. The three parameters jointly affect the surface integrity after surface strengthening. The moving speed is 0.1 mm / s. The pressure has a greater impact on the surface hardness and residual stress. When the hardness and residual stress do not meet the requirements, it is preferred to adjust the pressure. The rotation speed has a greater impact on the surface roughness. When the surface roughness does not meet the requirements, it is preferred to adjust the rotation speed.

6. The fatigue-free method of aluminum alloy friction stir welding based on rolling strengthening according to claim 5, characterized in that: The surface hardness, surface roughness and residual stress requirements must be met after surface strengthening treatment, and secondary processing is not allowed.

7. The fatigue-free method of aluminum alloy friction stir welding based on rolling strengthening according to claim 1, characterized in that: In step (4), the width and relative position of each welding area are accurately determined by the hardness distribution to judge whether the strengthening area meets the requirements.

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