Friction stir welding method for strengthening low-cycle fatigue performance of high-strength aluminum alloy joint

Abstract

The invention relates to a friction stir welding method for strengthening the low cycle fatigue performance of a high-strength aluminum alloy joint. The friction stir welding method comprises the steps that a copper base plate with the heat conductivity being 300-500 W / m.K is laid in a tool groove used for friction stir welding; a pair of high-strength aluminum alloy plates to be welded are fixed in a tool groove, butt joint friction stir welding is conducted under the welding conditions that a stirring head rotates anticlockwise, the dip angle ranges from 2 degrees to 3 degrees, the rotating speed ranges from 300 rpm to 400 rpm, and the welding speed ranges from 60 mm / min to 120 mm / min, and the friction stir welding direction is consistent with the rolling direction of the high-strength aluminum alloy plates to be welded; in the friction stir welding process, the bottoms of butt welding seams of a pair of high-strength aluminum alloy plates to be welded are placed on a copper base plate, and the cooling rate during friction stir welding reaches 20-100 DEG C / s; the non-uniformity of the structure of the cross section of the welding seam is greatly reduced, namely, the strength and the low-cycle fatigue performance of the welding seam are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of friction stir welding, in particular to a friction stir welding method for high-strength aluminum alloy. BACKGROUND

[0002] 7XXX (Al-Zn-Mg-Cu series) aluminum alloy has a density of 2.74-2.86 x 10 3 kg / m 3 , and a tensile strength of 470-600 MPa, and has a high specific strength. It is one of the key structural materials in the fields of aviation, aerospace, weaponry and transportation, and plays a huge role in the field of aviation, and has received extensive attention in recent years, and is the focus of current material research. The wide application of high-strength aluminum alloy also means higher requirements for the material and more complex service environment. In order to achieve lightweight and high performance of structural components, advanced joining technology is used to manufacture components, and the joining method of high-strength aluminum alloy is continuously improved to improve the reliability of the joint, which has great theoretical and practical significance.

[0003] 7XXX high-strength aluminum alloy has relatively low weldability due to its high thermal expansion coefficient, high thermal conductivity, and high solidification shrinkage. In the traditional fusion welding process, the weld metal melts, and is prone to form porosity, hot cracks and other melting / solidification metallurgical defects, and produces contaminated smoke during welding.

[0004] Friction stir welding (FSW) invented by the British Welding Institute in 1991 is a new solid-phase welding method. In the FSW process, the temperature of the weld can dynamically self-adaptively balance, so the welded material always maintains a hot plasticized solid phase state. All series of aluminum alloys can be FSW, especially 7050 aluminum alloy which cannot be welded by traditional fusion welding, and the mechanical properties of the FSW joint are significantly better than those of traditional fusion welding, and the fracture strength of the optimized joint can reach about 80% of the base material, which is one of the best welding methods for 7050 high-strength aluminum alloy.

[0005] 7050 high-strength aluminum alloy is mainly composed of Zn, Mg and Cu. Although 7050 high-strength aluminum alloy has excellent mechanical properties, it is very sensitive to microstructure and highly dependent on the size and distribution of precipitates. During the FSW process of aluminum alloy sheet, the microstructure and properties of the joint are greatly affected by the thermal history, which includes heat generation and conduction. Most of the heat generated during welding is generated by the friction between the large-sized shoulder and the weld surface, and the heat input is different due to the different dwell time of the pin at the start and end of the welding process. During the heat conduction process, heat is transferred from the high-temperature region to the low-temperature region, i.e., from the weld to the base metal and from the top to the bottom of the aluminum sheet. Heat accumulation occurs during the movement of the stirrer along the welding direction, and the heat conduction efficiency changes with the change of the boundary contact medium, resulting in a large temperature gradient in the transverse direction (TD) of the weld. The microstructure of the base metal (BM), heat-affected zone (HAZ), thermo-mechanically affected zone (TMAZ) and weld nugget zone (WNZ) exhibits a high degree of non-uniformity, which leads to uneven changes in grain size and shape in the transverse direction. The microstructure of the BM is in the form of lath, the WNZ is in the form of fine equiaxed recrystallization, and there are a small amount of fine recrystallized grains in the TMAZ. The morphology and distribution of precipitates, precipitation behavior, texture component and distribution characteristics also change unevenly in the transverse direction, which leads to microstructure degradation such as abnormal grain growth, precipitate coarsening and texture anisotropy, resulting in a decrease in mechanical properties.

[0006] 7XXX high-strength aluminum alloy FSW joints are commonly used as important parts and structures (e.g., wing boxes) in the actual service environment of military strategic resources such as spacecraft, which are subjected to large load fluctuations and high stress and strain levels, which breaks the elastic limit of the structure itself and enters the plastic deformation range, and the main failure mode is low-cycle fatigue. In order to increase the service life of military strategic resources such as spacecraft, it is of great significance and urgency to improve the low-cycle fatigue performance of high-strength aluminum alloy welded joints.

[0007] It is well known that adjusting the welding process parameters can effectively control the temperature change of the weld during FSW: the welding temperature decreases with the increase of the welding speed, and increases with the increase of the rotation speed. Currently, related researches control the peak temperature, high-temperature residence time and cooling rate by optimizing the process parameters and external cooling conditions to regulate the temperature change during FSW and improve the microstructure uniformity to optimize the mechanical properties of the joint.

[0008] Some scholars have explored the effect of adding coolant on the temperature field during FSW. Hajinezhad et al. [Hajinezhad M, et al. Numerical analysis of effect of coolant on the transient temperature in underwater friction stir welding of Al6061~T6. Int. J. Adv. Manuf. Techn ol 83(5~8), 1241~1252(2016)] studied the effect of using liquid nitrogen for cooling after friction stir welding. Under air cooling conditions, the thermal cycle exhibits an elliptical profile, with peak temperatures at the bottom and top of the central region reaching 783℃ and 886℃, respectively. However, under rapid liquid nitrogen cooling conditions, this elliptical profile becomes significantly concave, the area of ​​the high-temperature region decreases, and the peak temperatures at the bottom and top of the WNZ drop to 767℃ and 824℃, respectively. In addition to the effect of external coolant on the temperature field, some scholars have also studied the effect of backing plates on the weld temperature field. Zhang et al. [Zhang Z, et al. Numerical analysis of effect of backplate diffusivity on the transient temperature in friction stir welding. J. Mater. Eng. Perform., 22(9), 2446~2450(2013)] systematically studied the influence of backplate thermal conductivity on the workpiece temperature field based on numerical simulation, and found that the backplate thermal conductivity significantly affects the final temperature distribution.

[0009] During the FSW process, as the thermal conductivity of the pad increases, the temperature increases slowly due to significant heat dissipation at the clamp and pad. The slope of the time-temperature curve gradually decreases, and the peak temperature gradually decreases, with the largest drop being approximately 50°C. Furthermore, the residence time above 200°C decreases almost linearly. This indicates that using a pad with higher thermal conductivity in FSW is a potentially feasible approach to control the temperature field.

[0010] However, the optimal peak temperature and cooling rate of different areas of the FSW joint are significantly different. Therefore, it is difficult for high-strength aluminum alloys to improve the microstructure inhomogeneity by controlling the temperature changes in the FSW process, simply by using a single cooling medium or replacing a single high thermal conductivity backing plate. It is necessary to control the temperature of the joint in different zones. Therefore, a combination backing plate can be used. However, since the combination backing plate itself has gaps, material flow will occur during the welding process, which will lead to a decrease in the low-cycle fatigue performance of the joint.

[0011] In summary, current research primarily focuses on improving the mechanical properties of joints, specifically optimizing their static load performance. Less attention has been paid to fatigue performance and low-cycle fatigue performance optimization, and the practical effects of using partitioned composite spacers are not satisfactory. Therefore, a method for controlling the FSW temperature to optimize the fatigue and low-cycle fatigue performance of FSW joints is urgently needed in this field and is of great significance for improving the service life of high-strength aluminum alloy FSW joints. Summary of the Invention

[0012] The purpose of this invention is to overcome the shortcomings of the prior art and provide an optimized friction stir welding method that improves the service life of high-strength aluminum alloy joints by reducing the HAZ of the friction stir welded joint, increasing the number and stability of dispersed precipitates, and thus enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints through replacing the high thermal conductivity pad.

[0013] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a friction stir welding method for enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints, comprising the following steps:

[0014] A copper pad with a thermal conductivity of 300–500 W / m·K is laid in the tooling tank used for friction stir welding;

[0015] A pair of high-strength aluminum alloy plates to be welded are fixed in the tooling groove. Under the welding conditions of counterclockwise rotation of the stirring head, tilt angle of 2-3°, rotation speed of 300-400 rpm, and welding speed of 60-120 mm / min, butt stir friction welding is carried out. The direction of stir friction welding is consistent with the rolling direction of the high-strength aluminum alloy plates to be welded.

[0016] During friction stir welding, the bottom of the butt weld of the high-strength aluminum alloy plates to be welded is placed on a copper pad, so that the cooling rate during friction stir welding reaches 20-100℃ / s.

[0017] This reduces the peak HAZ temperature of friction stir welding by 50–100°C, effectively reducing the dissolution and coarsening of nano-precipitates, increasing the η' phase retention rate by 20–30%, maintaining the η' phase size at 10–20 nm, and making the η' phase more stable and uniformly distributed, effectively enhancing the dislocation pinning effect and improving the mechanical properties of the material.

[0018] Meanwhile, it hinders the coarsening and continuity of grain boundary precipitates in the softened zone of friction stir welding, narrowing the width of the grain boundary precipitate-free zone by 30-50 nm, effectively delaying crack initiation; in addition, it reduces the diffusion time of solute atoms in the high-temperature stage, inhibits the growth of high-strength aluminum alloy grains in the HAZ, reduces the grain size by 35%-40%, and thus narrows and hardens the softened zone, ultimately significantly reducing the non-uniformity of the microstructure of the weld cross-section, and improving the weld strength and low-cycle fatigue performance.

[0019] Furthermore, the stirring head is an H13 steel stirring head with a shoulder diameter of 18-20mm, a threaded needle length of 6-12mm, a root diameter of 8-10mm, a tip diameter of 5-8mm, and a right-hand thread on the surface; used to weld a pair of high-strength aluminum alloy plates with a thickness of 6-20mm to be welded.

[0020] Furthermore, the high-strength aluminum alloy plate has a length of 700-900mm, a width of 80-100mm, and a thickness of 6-20mm.

[0021] Furthermore, the tooling groove is a tooling groove with a length of 900mm, a width of 75mm, and a thickness of 20mm.

[0022] Furthermore, the copper pad has a length of 700-900mm, a width of 75mm, and a thickness of 20mm.

[0023] Furthermore, the composition of the copper pad, by mass percentage, includes the following substances: Cu: ≥99.99%, O: ≤0.0005%, to ensure that the thermal conductivity of the copper pad is ≥398W / (m·K) at 25°C.

[0024] Furthermore, the copper pad is cut using a slow wire EDM machine, and then milled flat on both sides using a diamond-coated end mill. Finally, it is mechanically polished using wet sandpaper, successively from 600#, 1500#, 5000# to 7000#.

[0025] Furthermore, the high-strength aluminum alloy plate has a plate type of 7050-T7451, 7B50, 7A65, 2024, or 2219.

[0026] The composition of high-strength aluminum alloy 7050-T7451 by mass percentage is: Zn:≥5.7%, Cu:≥2.0%, Mg:≥1.9%, Mn:0.10%, Si:≤0.12%, Fe:≤0.15%, with the balance being Al and unavoidable impurities.

[0027] Furthermore, the procedure also includes the cutting of low-cycle fatigue specimens: using an electric spark wire cutter, low-cycle fatigue specimens are cut sequentially along the welding direction of the weld seam on the friction stir welded plate according to the ASTM-E8 standard. The gauge length of the specimen is 25±0.5mm and the specimen width is 10±0.5mm.

[0028] Furthermore, the friction stir welding machine tool is a CNC friction stir welding machine tool of model FSW-RL31-010.

[0029] The beneficial effects of this invention are as follows: Compared with the prior art, while maintaining optimal welding parameters and ensuring that the joint does not exhibit loose areas or void defects, this invention improves the heat dissipation efficiency of the weld through the high thermal conductivity (~400 W / m·K) of the copper backing plate. Compared with conventional joints, the low-cycle fatigue performance of the copper backing plate joint is significantly improved. The advantages of this invention are:

[0030] (1) This invention directly reduces the peak temperature by regulating the heat input. Compared with the traditional steel pad (thermal conductivity ~50W / m·K), the copper pad can reduce the peak temperature of HAZ by 50-100℃, which can reduce the dissolution and coarsening of nano-precipitates, increase the retention rate of η' phase by 20-30%, and keep the size of η' phase at 10-20nm (20-30nm in the traditional HAZ). Under low temperature conditions, η' phase is more stable and uniformly distributed, which can effectively enhance the dislocation pinning effect and improve the mechanical properties of the material. The reduction of peak temperature can reduce the coarsening and continuity of grain boundary precipitates, and narrow the width of the grain boundary non-precipitate zone (PFZ) from 50-100nm to 20-40nm, which can effectively delay crack initiation.

[0031] (2) The high thermal conductivity of the copper backing plate improves the heat dissipation efficiency of the weld and increases the cooling rate. Compared with conventional backing plate FSW, the cooling rate is increased from the conventional 5-20℃ / s to 20-100℃ / s, and the time that the HAZ is at high temperature (>350℃) is shortened to 5-10 seconds.

[0032] The coarsening precipitates in HAZ and PFZ serve as preferential initiation sites for fatigue cracks, significantly reducing fatigue life, especially under cyclic loading conditions. Rapid cooling can reduce the diffusion time of solute atoms at high temperatures, inhibit HAZ grain growth, control grain size to 15–25 μm (compared to 30–50 μm in traditional HAZ), and enhance grain boundary strengthening. Furthermore, it suppresses the precipitation of equilibrium phases (such as η and S phases), retains more fine metastable η' phases with a size controllable to 10–15 nm, and disperses them in the matrix, enhancing the dislocation pinning effect.

[0033] With increasing cooling rate, the nucleation sites of precipitates gradually shift from grain boundaries to intragranular defects (such as dislocations and subgrain boundaries), reducing the coarsening and continuity of grain boundary precipitates (GBPs). Furthermore, rapid cooling induces residual compressive stress on the surface (-150 to -200 MPa), offsetting the tensile stress component in cyclic loading, and can increase fatigue life by up to (N) under partial applied strain amplitude. f 50-100%. In addition, rapid cooling helps maintain the strong {001}<100> cubic texture formed during rolling, reduces anisotropy, thereby reducing local strain concentration during cyclic loading and delaying fatigue crack initiation.

[0034] Finally, the refined grains and uniform precipitates force the crack path to become tortuous, which can effectively reduce the crack propagation rate. Attached Figure Description

[0035] Figure 1 These are the welding parameters, fatigue specimen parameters, and FSW schematic diagram of the present invention;

[0036] Figure 2 This is a metallographic image of the cross-sectional microstructure of a conventional low-carbon steel gasket joint.

[0037] Figure 3 This is a metallographic image of the cross-sectional microstructure of the pure copper pad joint of the present invention;

[0038] Figure 4 This is a comparison diagram of the microhardness distribution cloud of the joint cross section of a low-carbon steel pad and a pure copper pad of the present invention.

[0039] Among them, (a) is the microhardness distribution cloud map of the cross section of a conventional low-carbon steel pad joint, and (b) is the microhardness distribution cloud map of the cross section of a pure copper pad joint.

[0040] Figure 5 This is a comparison diagram of the strain distribution cloud of "stress-strain" in room temperature tensile engineering of low carbon steel pad and pure copper pad joint of the present invention.

[0041] Among them, (a) strain distribution cloud map and curve of "stress-strain" in room temperature tensile engineering of conventional low carbon steel plate joint, and (b) strain distribution cloud map and curve of "stress-strain" in room temperature tensile engineering of pure copper plate joint.

[0042] Figure 6 This is a comparison of the lag hysteresis loops and strain distribution cloud diagrams for the 100th and 600th cycles of the low-carbon steel pad FSW joint under a strain amplitude of 1.2%.

[0043] Among them, (a) the zonal hysteresis loop and strain distribution cloud map of the conventional low carbon steel plate FSW joint at the 100th cycle under a strain amplitude of 1.2%; and (b) the zonal hysteresis loop and strain distribution cloud map of the conventional low carbon steel plate FSW joint at the 600th cycle under a strain amplitude of 1.2%.

[0044] Figure 7 This is a comparison diagram of the partitioned hysteresis loops and strain distribution clouds of the FSW joint of the present invention at the 100th and 600th cycles under a strain amplitude of 1.2%.

[0045] Among them, (a) the partitioned hysteresis loop and strain distribution cloud map of the pure copper pad FSW joint at the 100th cycle under a strain amplitude of 1.2%; and (b) the partitioned hysteresis loop and strain distribution cloud map of the pure copper pad FSW joint at the 600th cycle under a strain amplitude of 1.2%.

[0046] Figure 8 This is a comparison diagram of the partitioned hysteresis energy of the low carbon steel pad joint and the joint of the present invention at the 100th and 600th cycles under a strain amplitude of 1.2%.

[0047] Among them, (a) is a comparison of the lag energy of the conventional low carbon steel pad and the pure copper pad joint at the 100th cycle under a strain amplitude of 1.2%, and (b) is a comparison of the lag energy of the conventional low carbon steel pad and the pure copper pad joint at the 600th cycle under a strain amplitude of 1.2%.

[0048] Figure 9 This is a metallographic image of the cross-sectional microstructure of a steel-copper composite gasket joint.

[0049] Figure 10 This is a microhardness distribution cloud map of the cross-section of the steel-copper composite gasket joint;

[0050] Figure 11 These are life diagrams of the base material, the steel-copper composite gasket joint, and the conventional steel gasket joint under different strain amplitudes.

[0051] Figure 12 This is a comparison chart of the hysteresis loops of the steel-copper composite gasket joint and the conventional steel gasket joint under various strain amplitudes.

[0052] Among them, (a) is the hysteresis loop diagram of the steel-copper composite pad joint under various strain amplitudes, and (b) is the hysteresis loop diagram of the conventional steel pad joint under various strain amplitudes. Detailed Implementation

[0053] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0054] This invention controls the temperature field change of the weld by optimizing the thermal conductivity of the backing plate. The copper backing plate with high thermal conductivity rapidly reduces the temperature at the bottom of the weld, reduces the increase in grain size and the dissolution and coarsening of nano-precipitates, and improves the low-cycle fatigue performance of the welded joint.

[0055] Using optimized welding process parameters, the grain size at the bottom of the copper backing plate is reduced by 35% to 40% compared to the low-carbon steel backing plate. During the FSW process, WNZ undergoes dynamic recrystallization due to the combined effects of mechanical stirring and thermal cycling, achieving grain recrystallization and redistribution of precipitation strengthening phases. Generally, fine equiaxed grains are obtained. During the cooling stage, the grains grow. The higher the temperature and the longer the holding time, the larger the grain size. The lower the temperature and the shorter the holding time, the smaller the grain size.

[0056] During the FSW process, HAZ only experiences the effects of welding thermal cycling and is not subjected to mechanical stirring, thus retaining the original fibrous structure characteristics of the base material. However, some recrystallized grains grow due to heat input. During the recovery process of HAZ, some crushed grains may grow, while the original nanoscale η' phase inside the grains may dissolve or coarsen due to the aggregation of precipitated solute atoms into the undissolved precipitates. The needle-like η' phase transforms into columnar or disc-shaped precipitates after heating, and the precipitation density decreases.

[0057] Prolonged high-temperature residence time may promote the transformation of the metastable η' phase to the stable η phase (MgZn2), leading to material softening. The recovery process consumes dislocations stored within the grains, reducing the dislocation density in the microstructure. These phenomena are all detrimental to the joint's resistance to external deformation and the coordination of deformation between microstructures.

[0058] Therefore, the microstructural degradation of the HAZ makes it a weak area in terms of performance. The copper backing plate significantly reduces the peak temperature at the bottom of the weld and its high-temperature dwell time, effectively inhibiting the growth of WNZ and HAZ grains and reducing grain size.

[0059] The grain refinement effect delays crack propagation through the grain boundary strengthening mechanism, significantly improving the low-cycle fatigue performance of the FSW joint; the copper backing plate significantly reduces the peak temperature at the bottom of the weld and its high-temperature residence time, effectively inhibiting a series of evolutions such as the dissolution and coarsening of the metastable η' phase precipitated in HAZ, and hindering dislocation movement through the fine and dispersed precipitates, thereby improving the strength and low-cycle fatigue performance of the material.

[0060] To achieve the above objectives, the present invention provides the following specific embodiments:

[0061] Example 1: A friction stir welding method for enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints, comprising the following steps:

[0062] A copper pad with a thermal conductivity of 300–500 W / m·K is laid in the tooling tank used for friction stir welding;

[0063] The copper pad is composed of the following substances by mass percentage: Cu: ≥99.99%, O: ≤0.0005%, to ensure that the thermal conductivity of the copper pad is ≥398W / (m·K) at 25℃; it is cut using a slow wire EDM machine, and then milled flat on both sides using a diamond-coated end mill, followed by mechanical polishing with wet sandpaper from 600#, 1500#, 5000# to 7000#.

[0064] A pair of high-strength aluminum alloy plates to be welded are fixed in the tooling groove. Under the welding conditions of counterclockwise rotation of the stirring head, tilt angle of 2-3°, rotation speed of 300-400 rpm, and welding speed of 60-120 mm / min, butt stir friction welding is carried out. The direction of stir friction welding is consistent with the rolling direction of the high-strength aluminum alloy plates to be welded.

[0065] The composition of high-strength aluminum alloy by mass percentage is: Zn: ≥5.7%, Cu: ≥2.0%, Mg: ≥1.9%, Mn: 0.10%, Si: ≤0.12%, Fe: ≤0.15%, with the balance being Al and unavoidable impurities;

[0066] The stirring head is an H13 steel stirring head with a shoulder diameter of 18-20mm, a thread length of 6-12mm, a root diameter of 8-10mm, a tip diameter of 5-8mm, and a right-hand thread on the surface; it is used to weld a pair of high-strength aluminum alloy plates with a thickness of 6-20mm.

[0067] During friction stir welding, the bottom of the butt weld of the high-strength aluminum alloy plates to be welded is placed on a copper pad, so that the cooling rate during friction stir welding reaches 20-100℃ / s.

[0068] This reduces the peak HAZ temperature of friction stir welding by 50–100°C, effectively reducing the dissolution and coarsening of nano-precipitates, increasing the η' phase retention rate by 20–30%, maintaining the η' phase size at 10–15 nm, and making the η' phase more stable and uniformly distributed, effectively enhancing the dislocation pinning effect and improving the mechanical properties of the material.

[0069] Simultaneously, it hinders the coarsening and continuity of grain boundary precipitates in the softened zone of friction stir welding, narrowing the width of the grain boundary precipitate-free zone from 50-100 nm to 20-40 nm, effectively delaying crack initiation; in addition, it reduces the diffusion time of solute atoms in the high-temperature stage, inhibiting the growth of high-strength aluminum alloy grains in the HAZ, reducing the grain size by 35%-40%, thereby narrowing and hardening the softened zone, ultimately significantly reducing the non-uniformity of the weld cross-section, which improves the weld strength and low-cycle fatigue performance.

[0070] Example 2: Same as Example 1, except that the high-strength aluminum alloy plate is 700-900mm long, 80-100mm wide, and 6-20mm thick; the tooling groove is 900mm long, 75mm wide, and 20mm thick; and the copper pad is 700-900mm long, 75mm wide, and 20mm thick.

[0071] Example 3: Same as Example 1, except that the high-strength aluminum alloy plate is model 7050-T7451, 7B50, 7A65, 2024, or 2219.

[0072] Example 4: Same as Example 1, except that it also includes the step of cutting low-cycle fatigue specimens: Using an electric spark wire cutter, low-cycle fatigue specimens are cut sequentially along the welding direction of the weld seam on the weld plate of friction stir welding according to ASTM-E8 standard. The gauge length of the specimen is 25±0.5mm and the specimen width is 10±0.5mm.

[0073] Example 5: Same as Example 1, except that the friction stir welding machine tool is a CNC friction stir welding machine tool of model FSW-RL31-010.

[0074] like Figures 1-8 As shown, in order to further illustrate the technical solution and effects of the present invention, the present invention adopts the method provided in Examples 1-5, and conducts comparative experiments using a copper pad with a thermal conductivity of 300-500 W / m·K and a conventional tooling groove low carbon steel pad with a thermal conductivity of 30-60 W / m·K.

[0075] The low-carbon steel pad is 700-900mm long, 75mm wide, and 20mm thick. Its composition by mass percentage is: Fe: ≥99.5%, Mn: 0.30%-0.65%, C: 0.12%-0.20%, Si: ≤0.3%, S: ≤0.05%, P: ≤0.045%, and its thermal conductivity at 25℃ is ≥51W / (m·K).

[0076] The experimental results are as follows:

[0077] Combination Figure 1 This invention utilizes the high thermal conductivity of copper pads to regulate the thermal cycle of the FSW process. By improving the heat dissipation efficiency of the weld, the peak temperature is reduced, the cooling rate is increased, the diffusion time of solute atoms in the high-temperature stage is reduced, the growth of HAZ grains is suppressed, and the dissolution and coarsening of nano-precipitates and the coarsening and continuity of grain boundary precipitates are reduced, thereby shortening the width of the grain boundary precipitate-free zone.

[0078] Experiments have shown that the low-cycle fatigue life of FSW joints using copper pads is higher than that of conventional tooling slots using low-carbon steel pads under applied strain amplitudes of 1.6%, 1.4%, 1.2%, and 1.0%.

[0079] Figure 2 , 3 Metallographic photographs of various regions of the FSW weld metallographic specimens for two types of backing plates were taken from three locations (upper, middle, and lower WNZ) and five locations (advancing side of the heat-affected zone (TMAZ-AS) and retreating side of the heat-affected zone (TMAZ-RS).

[0080] The upper, middle, and lower parts of the WNZ are all subjected to localized high temperatures and intense mechanical stirring. However, due to differences in heat generation and dissipation conditions, the size of the equiaxed recrystallized grains varies in each region. During the FSW process, the upper part of the weld is in direct contact with the large-sized shoulder, resulting in high frictional heat generation and a high degree of plastic deformation of the weld metal. The WNZ grains in the upper part of the weld undergo sufficient dynamic recrystallization. However, the upper part of the weld has a high peak temperature and a long high-temperature residence time. The larger diffusion coefficient at high temperatures enhances atomic diffusion capabilities, making the upper WNZ more prone to grain boundary migration and grain growth. In addition, being closer to the surface, the cooling rate is slower, and the equiaxed grains formed by dynamic recrystallization eventually evolve into large-sized equiaxed grains.

[0081] The middle part is located in the core area of ​​the weld, and is subject to more uniform heat input and mechanical stirring. The grains are more uniform. Compared with the upper part of the weld, the peak temperature in the middle part of the weld is lower and the high temperature residence time is shorter. The growth of dynamic recrystallized grains is limited, and the equiaxed grains formed by recrystallization are smaller than those in the upper part.

[0082] Because the bottom is only subjected to frictional heat generation and mechanical stirring by the tip of the smaller diameter stirring needle, and because the pad has a higher thermal conductivity and dissipates heat the fastest, the peak temperature is the lowest and the high-temperature residence time is the shortest. As a result, the lower grains of WNZ are the finest.

[0083] exist Figure 2 , 3 The measurement software can be used to obtain, Figure 2 The grain sizes of the upper, middle, and lower parts of the conventional low-carbon steel pad WNZ are 6.74μm, 4.02μm, and 3.19μm, respectively.

[0084] Figure 3 The grain sizes of the upper, middle, and lower WNZ regions of the pure copper pad are 4.56 μm, 4.10 μm, and 2.13 μm, respectively, all of which conform to the rule that the grain size of WNZ decreases from top to bottom, and the grain size of the WNZ region decreases when the pure copper pad is cooled.

[0085] TMAZ is the transition region between WNZ and HAZ. During FSW, it is subjected to shear force from the stirring pin and extrusion by the plastic metal. The grains of TMAZ are significantly elongated. Due to the irregular shape and large variation in grain size of TMAZ grains, its quantitative calculation results lack statistical significance. However, it can be qualitatively seen that the TMAZ size of pure copper pads is smaller than that of conventional low carbon steel pads.

[0086] Figure 4(a) and (b) are microhardness distribution cloud diagrams of the cross-sections of FSW joints with conventional low-carbon steel backing plates and pure copper backing plates, respectively. The microhardness of the WNZ region in both joints is between 130 and 180 HV, slightly lower than that of BM, and shows a trend of higher hardness at the top and lower hardness at the bottom. The HAZ on both sides is between 100 and 135 HV, forming a low hardness zone (LHZ), which differs significantly from the microhardness of BM and is the area with the lowest hardness in the FSW joint. Compared with conventional low-carbon steel backing plates, copper backing plates reduce the impact of thermal cycling on the deterioration of the microstructure in the bottom HAZ. It can be observed that the microhardness of the LHZ is improved, increasing to 110–130 HV, while the area is also significantly reduced. In addition, the high thermal conductivity of copper backing plates also lowers the overall weld temperature, preventing the WNZ recrystallization grains from growing sufficiently, thus refining the grains. According to the Hall-Petch relationship, the smaller the grain size of the joint, the higher its mechanical properties, with the microhardness increasing from 160–125 HV to 180–140 HV.

[0087] Tensile properties of the joint were tested using an Instron 3382 electronic universal testing machine equipped with an LX500 laser extensometer at a tensile rate of 1 mm / min. The strain changes throughout the tensile process were recorded and calculated using an XTXIC-FLEX non-contact three-dimensional digital speckle dynamic strain measurement and analysis system (Digital Image Correlation, DIC). The tensile strain distribution cloud map of the joint cross section was obtained. According to the tensile property test, the tensile strength (UTS) of the pure copper backing plate joint was not much different from that of the conventional backing plate joint with the same welding parameters, while the yield strength (YS) and elongation were increased by 6.38% and 11.33%, respectively.

[0088] like Figure 5 As shown, six stress states were selected: 250 MPa, YS, the average of the intervals between YS and UTS (two points), UTS, and fracture, and the corresponding strain distributions were observed. In conventional plate joints, strain localization already occurs in the HAZ when the stress reaches 250 MPa. The degree of strain localization is slightly higher in the heat-affected zone on the advancing side (HAZ-AS). After the stress exceeds UTS, the strain in the HAZ-AS increases rapidly, reaching the fracture strain and leading to fracture failure.

[0089] The pure copper pad joint provided by the present invention exhibits significant strain localization in the upper part of HAZ-AS and HAZ-RS during yielding. As the stress increases, HAZ-AS undergoes partial deformation strengthening. The strain localization first shifts to the bottom of HAZ-RS and the top of WNZ, and then back to HAZ-AS. After the tensile stress reaches UTS, the strain localization remains in AS to HAZ, eventually leading to macroscopic necking deformation and fracture failure.

[0090] When subjected to tensile stress, the area of ​​strain localization in the copper-cooled joint is much smaller than that in the air-cooled joint with a low-carbon steel backing, and the degree of strain localization is also lower. This indicates that using pure copper backing for cooling reduces the peak temperature at the bottom of the weld and the high-temperature dwell time, effectively suppresses the softening of the HAZ structure, significantly reduces the area of ​​the LHZ, and improves the tensile properties of the joint.

[0091] Low-cycle fatigue tests were conducted on 7050-T7451 high-strength aluminum alloy FSW joints by applying tensile-compressive symmetrical cyclic loads using an Instron 8801 electro-hydraulic servo fatigue testing machine. Simultaneously, DIC technology was used to observe the strain distribution in different regions of the joint during each cycle of low-cycle fatigue, and the cyclic strain behavior of the two joints in HAZ and WNZ was analyzed.

[0092] Figure 6 , Figure 7 The stress-strain hysteresis loops of conventional pad and pure copper pad joints in a specific cycle are obtained when the plastic strain amplitude is 0.6%. At the same time, the joint gauge length is divided into 9 equally spaced regions along the loading direction from top to bottom to obtain the stress-strain hysteresis loops of 9 small regions. The two poles of each hysteresis loop and the strain distribution cloud map at the four intersection points with the coordinate axis are taken.

[0093] At 100 cycles, the shape of the hysteresis rings shows that the HAZ of RS and AS is wider than that of WNZ, and strain concentration is observed to begin in HAZ-RS. At 600 cycles, the strain distribution cloud map clearly shows that severe strain concentration occurs on the retreating side of the heat-affected zone (HAZ-RS), and the partitioned hysteresis rings on the RS side widen sharply. This strain concentration promotes the formation of fatigue crack initiation at this location, ultimately causing the specimen to fail.

[0094] In the pure copper backing plate joint of the present invention, from the 100th to the 600th cycle, as the number of cycles increases, the hysteresis rings in each region shrink, the plastic strain amplitude decreases, and cyclic hardening occurs. No obvious strain concentration zone is observed. That is, the microstructure and properties of the pure copper backing plate joint are more uniform in the weld cross-sectional direction.

[0095] Calculating the area of ​​the stress-strain hysteresis loop for each zone yields the hysteresis energy of each zone at different cycle counts. The hysteresis energy of each zone at each cycle life can quantify the plastic deformation occurring in each zone. For example... Figure 8 The joints exhibited the most severe HAZ softening, with the largest plastic deformation occurring in the HAZ-RS region. The peak hysteresis energy corresponds to the final fracture location. Based on the comparison of hysteresis energy in the 100th cycle, the pure copper pad joints showed better uniformity.

[0096] Furthermore, to further illustrate the inventiveness of this invention, an optimization experiment was conducted on the low-cycle fatigue performance of the steel-copper composite gasket described in the background art. The specific experimental results are as follows:

[0097] Combination Figure 1 This invention uses a combination of steel and copper backing plates to regulate the thermal cycle of the FSW process in different zones. By improving the heat dissipation efficiency of the weld seam through the copper backing plates, the peak temperature is reduced, the cooling rate is increased, the dissolution and coarsening of nano-precipitates are reduced, and the performance of the HAZ is improved. By using conventional steel backing plates, the performance improvement of WNZ due to grain refinement caused by rapid cooling is avoided, and the performance uniformity of the joint cross section along the transverse direction is maximized, thereby improving the low-cycle fatigue performance of the joint.

[0098] However, in the actual FSW process, due to the fit of the combined backing plate, material leakage will occur at the joint, and irregular protrusions will appear on the back of the joint. Since the smoothness of the joint surface seriously affects the low-cycle fatigue performance, in order to make the experiment comparable, an angle grinder and wet sandpaper are used to grind it as smooth as possible.

[0099] Experiments have shown that the low-cycle fatigue life of FSW joints using steel-copper composite pads is lower than that of conventional low-carbon steel pad joints under applied strain amplitudes of 1.2%, 1.0%, 0.8%, and 0.6%.

[0100] Figure 9 Metallographic images of various regions of the FSW weld seam specimen of the steel-copper composite backing plate were taken from three locations (top, middle, and bottom) of the WNZ and five locations (TMAZ-AS and TMAZ-RS). Measurement software revealed that the grain sizes of the top, middle, and bottom WNZ of the steel-copper composite backing plate were 4.58 μm, 4.15 μm, and 2.24 μm, respectively, consistent with the decreasing grain size trend from top to bottom. Because the overall cooling effect of the copper backing plate was slightly better than that of the steel-copper composite backing plate, it was compared with... Figure 2 Compared to steel-copper composite pads, pure copper pads have a slightly smaller grain size. The WNZ microstructure obtained by cooling copper pads has finer grains, resulting in better performance in this region, although the transverse uniformity of the joint cross-section is theoretically worse.

[0101] Figure 10The image shows the microhardness distribution cloud map of the cross-section of the steel-copper composite gasket FSW joint. The WNZ region exhibits a trend of higher hardness at the top and lower hardness at the bottom, with microhardness ranging from 140 to 180 HV. The HAZ on both sides forms the LHZ, with hardness values ​​ranging from 100 to 135 HV. Compared to conventional low-carbon steel gaskets, the steel-copper composite gasket shows a slightly smaller LHZ area and improved microhardness in all regions. Compared to pure copper gaskets, the pure copper gasket joint shows a significantly smaller LHZ area, with improved microhardness at the top of the WNZ but a smaller area of ​​high hardness. It is clearly evident that the uniformity of the WNZ and HAZ in the steel-copper composite gasket FSW joint is superior to that of conventional low-carbon steel gaskets and pure copper gasket joints, but the pure copper gasket joint shows a more significant optimization of the HAZ.

[0102] Figure 11 The failure cycles of low-cycle fatigue tests were conducted on the steel-copper composite pad FSW joint under four sets of specific applied strains. To avoid the randomness of the experimental results, the tests were repeated under the same experimental conditions. It can be seen that the low-cycle fatigue life of the steel-copper composite pad FSW joint is lower than that of the conventional steel pad joint under each applied strain.

[0103] Figure 12 The hysteresis loops of four sets of steel-copper composite pads and conventional steel pad FSW joints under specific applied strains are shown. During cyclic loading, each loading cycle consumes a large amount of energy, causing plastic deformation of the fatigue specimen. The energy consumed in a single cycle is approximately equal to the area of ​​the stress-strain hysteresis loop for that cycle, called hysteresis energy. Most of the hysteresis energy is dissipated as heat, while a small portion is absorbed by the material, causing micro-scale distortion within the material. The hysteresis energy of metallic materials originates from plastic deformation. Energy dissipation theory suggests that the hysteresis energy of a material accumulates continuously within the material during cyclic loading. When this energy reaches a critical value, the material will fail. Therefore, the larger the accumulated cyclic hysteresis loop area, the greater the plastic deformation during cyclic loading, and the more severe the fatigue damage. This area can be used to measure the degree of damage caused by plastic deformation in metallic materials during cyclic loading. Under the same conditions, the area of ​​the hysteresis loop of the steel-copper composite pad FSW joint is larger than that of the conventional iron pad.

[0104] Therefore, it is evident that using the most theoretically appropriate partitioned steel-copper composite pad to solve the technical problem of optimizing the low-cycle fatigue performance of the joint is currently impossible under the experimental conditions. Thus, this invention creatively abandons the most ideal solution and focuses on the weak area of ​​the joint—the heat-affected zone—to perform full-area and all-round control of the joint. High thermal conductivity pure copper pads are used for targeted control, and unexpected technical effects have indeed been achieved.

[0105] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A friction stir welding method for enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints, characterized in that, Includes the following steps: A copper pad with a thermal conductivity of 300–500 W / m·K is laid in the tooling tank used for friction stir welding; A pair of high-strength aluminum alloy plates to be welded are fixed in the tooling groove. Under the welding conditions of counterclockwise rotation of the stirring head, tilt angle of 2-3°, rotation speed of 300-400 rpm, and welding speed of 60-120 mm / min, butt stir friction welding is carried out. The direction of stir friction welding is consistent with the rolling direction of the high-strength aluminum alloy plates to be welded. During friction stir welding, the bottom of the butt weld of the high-strength aluminum alloy plates to be welded is placed on a copper pad, so that the cooling rate during friction stir welding reaches 20-100℃ / s. This reduces the peak temperature of the heat-affected zone of friction stir welding by 50–100°C, effectively reduces the dissolution and coarsening of nano-precipitates, increases the retention rate of the η' phase by 20–30%, maintains the size of the η' phase at 10–20 nm, and makes the η' phase more stable and uniformly distributed, effectively enhancing the dislocation pinning effect and improving the mechanical properties of the material. Meanwhile, it hinders the coarsening and continuity of grain boundary precipitates in the softened zone of friction stir welding, narrowing the width of the grain boundary precipitate-free zone from 50-100 nm to 20-40 nm, effectively delaying crack initiation; in addition, it reduces the diffusion time of solute atoms in the high-temperature stage, inhibits the growth of high-strength aluminum alloy grains in the heat-affected zone, reduces the grain size by 35%-40%, and thus narrows and hardens the softened zone, ultimately significantly reducing the non-uniformity of the weld cross-section, which improves the weld strength and low-cycle fatigue performance.

2. The friction stir welding method for enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints as described in claim 1, characterized in that, The stirring head is an H13 steel stirring head with a shoulder diameter of 18-20mm, a threaded needle length of 6-12mm, a root diameter of 8-10mm, a tip diameter of 5-8mm, and a right-hand thread on the surface; it is used to weld a pair of high-strength aluminum alloy plates with a thickness of 6-20mm to be welded.

3. The friction stir welding method for enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints as described in claim 1, characterized in that, The high-strength aluminum alloy plate has a length of 700-900mm, a width of 80-100mm, and a thickness of 6-20mm.

4. The friction stir welding method for enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints as described in claim 1, characterized in that, The tooling groove is 900mm long, 75mm wide, and 20mm thick.

5. The friction stir welding method for enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints as described in claim 1, characterized in that, The copper pad has a length of 700-900 mm, a width of 75 mm, and a thickness of 20 mm.

6. The friction stir welding method for enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints as described in claim 1, characterized in that, The copper pad is composed of the following substances by mass percentage: Cu: ≥99.99%, O: ≤0.0005%, to ensure that the thermal conductivity of the copper pad is ≥398W / (m·K) at 25℃.

7. The friction stir welding method for enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints as described in claim 1, characterized in that, The copper pad is cut using a slow wire EDM machine, then milled flat on both sides using a diamond-coated end mill, and finally mechanically polished using wet sandpaper at grits from 600#, 1500#, 5000# to 7000#.

8. The friction stir welding method for enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints as described in claim 1, characterized in that, The high-strength aluminum alloy plate has a plate type of 7050-T7451, 7B50, 7A65, 2024, or 2219. The composition of the 7050-T7451 high-strength aluminum alloy by mass percentage is: Zn:≥5.7%, Cu:≥2.0%, Mg:≥1.9%, Mn:0.10%, Si:≤0.12%, Fe:≤0.15%, with the balance being Al and unavoidable impurities.

9. The friction stir welding method for enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints as described in any one of claims 1-8, characterized in that, It also includes the procedure for cutting low-cycle fatigue specimens: using an electric spark wire cutter, low-cycle fatigue specimens are cut sequentially along the welding direction of the weld seam on the weld plate of the friction stir welded according to the ASTM-E8 standard. The gauge length of the specimen is 25±0.5mm and the specimen width is 10±0.5mm.

10. The friction stir welding method for enhancing the low-cycle fatigue performance of high-strength aluminum alloy joints as described in any one of claims 1-8, characterized in that, The friction stir welding machine tool is a CNC friction stir welding machine tool of model FSW-RL31-010.

Images