In order to improve the service performance of remanufactured and repaired aluminum alloys, especially fatigue performance and corrosion resistance, the representative Al Zn-Mg alloy plate for rail transit was taken as the research object. The damage caused to the alloy by actual service conditions was simulated on the basis of studying the fatigue behavior of the alloy. The cracked sample alloy was repaired by different arc cladding processes and laser shock strengthening technologies, and the service performance differences of the alloy before and after repair were comprehensively compared. The study found that compared with traditional arc cladding, arc cladding combined with laser shock strengthening can significantly improve the fatigue resistance and corrosion resistance of the alloy. The samples of arc cladding combined with laser shock strengthening showed a lower fatigue crack growth rate in the low stress factor strength range, and maintained a low growth rate in the high stress factor strength range, which significantly improved the fatigue life, and also reduced the intergranular corrosion depth, and enhanced the corrosion resistance of the alloy. The research results can provide a theoretical basis and scientific guidance for the extension of the service life and corrosion control of aluminum alloy structural parts for rail transit. The opening and operation of the China-Europe Express, Jakarta-Bandung High-Speed ​​Railway, and China-Laos Railway provide strong support for strengthening regional cooperation, expanding cultural exchanges, and promoting common prosperity and development. Comfort, high speed and safety have become the hallmarks of China’s high-speed EMUs, demonstrating the strength of “Made in China”. Among them, aluminum alloy materials with high specific strength and excellent corrosion resistance are increasingly used in car body manufacturing, promoting the lightweight and carrying capacity of trains. However, the external environment of train operation plus its own static and dynamic loads may cause material damage, which means that after a certain service life, the components need to be repaired, maintained, replaced or scrapped.

The “14th Five-Year Plan for the Development of Circular Economy” and the “14th Five-Year Plan for Industrial Green Development” propose to make high-end intelligent remanufacturing a key research and development area for future remanufacturing, broaden the remanufacturing industry model, and implement high-quality development of the remanufacturing industry. Remanufacturing is a process that restores failed or scrapped products to a performance not lower than that of new products through a series of industrial processes such as disassembly, cleaning, repair or replacement, testing and assembly. It has been implemented in high-end equipment such as aircraft engine blades and shield machine bearings, creating huge value. How to apply advanced remanufacturing technology to the field of rail transportation, improve application reliability and extend service life is a problem that needs to be solved urgently.

Arc/laser cladding is a common remanufacturing repair technology. It uses the high temperature generated by the arc or laser as a heat source to quickly melt the wire or powder and solidify it on the surface of the parent material to achieve good metallurgical bonding, forming a highly protective coating with advantages such as high bonding strength and small heat-affected zone [1-3]. Laser shock peening, also known as laser shot peening, is an advanced surface treatment technology. Due to its significant improvement in fatigue, friction and wear, and stress corrosion resistance, it has been widely used in steel, titanium alloys, and aluminum alloys in recent years [4-7]. Compared with traditional mechanical shot peening, laser shot peening introduces higher residual compressive stress, and the decay rate is slower with increasing thickness, which can effectively prevent the initiation and expansion of corrosion cracks and enhance the corrosion resistance of the alloy [8-10]. However, domestic and foreign scholars rarely combine arc/laser cladding and laser shock peening. Whether the combination of the two advanced technologies can further improve the strength and corrosion resistance of aluminum alloys remains questionable.

Based on the above research background and current situation, this paper takes Al-Zn-Mg alloy plates, which are representative of rail transit, as the research object, conducts fatigue crack propagation experiments on prefabricated cracks, simulates the damage caused by actual service conditions to the alloy on the basis of studying the fatigue behavior of the alloy, repairs the cracked alloy by arc cladding and laser shock strengthening, and comprehensively compares the differences in fatigue performance and corrosion resistance of the alloy before and after repair, providing a reference for extending the service life and controlling corrosion of aluminum alloy structural parts for rail transit.

1 Experimental method
1.1 Arc cladding
Arc cladding is carried out at room temperature, and the cladding equipment is a Fronius welding machine with a KUKA six-axis robot arm. While having spatial freedom, the cladding speed can be controlled by controlling the moving speed of the robot arm. The robot arm is equipped with an automatic wire feeding device and a gas extraction device. The automatic wire feeding device controls the wire feeding speed, and the automatic gas extraction device can ensure that the sample is not oxidized and the smoke is removed in time.

During the cladding process, the wire and the workbench are connected to positive and negative electrodes respectively, and the arm controls the wire to approach or move away from the workbench to achieve arc starting and arc extinguishing during the cladding process. The working current is 171 A and the working voltage is 21.5 V. The cladding material of the Al-Zn-Mg alloy (original thickness 10 mm) sample uses 7055 aluminum alloy welding wire with a diameter of 1.2 mm, a cladding speed of 0.35 m/min, and a wire feeding speed of 8 m/min. Argon with a purity of 99.999% is selected as the shielding gas to protect the cladding process

To prevent oxidation, the process parameters such as processing and cladding pretreatment are adjusted during the arc cladding process. The specific cladding parameters are shown in Table 1.

1.2 Laser shock strengthening

Laser shock strengthening treatment is performed on the Al-Zn-Mg alloy components after cladding repair. Laser shock strengthening is carried out at room temperature, and its laser energy is Gaussian distributed in space. When the equipment is working, the high-energy laser beam generated by the high-energy pulse laser irradiates the sample surface, and the ablation layer on the sample surface quickly absorbs the laser energy to produce explosive gasification, generating a large amount of high-temperature and high-pressure plasma. The plasma continues to absorb the laser energy to generate a plasma shock wave, which impacts the sample surface under the constraint of the constraint layer, causing the sample to produce strong plastic deformation to achieve a strengthening effect. The ablation layer is selected as an aluminum foil tape with a thickness of 120 μm, and the constraint layer is a uniform thickness of 1~2 mm flowing water. The laser shock strengthening uses an energy of 2.4 J, a spot diameter of 2 mm, a lap rate of 50%, a cycle number of 3 times, a laser pulse width of 8.9 ns, and a wavelength of 1 064 nm.

1.3 Performance test method

The fatigue crack growth test was carried out at room temperature, using a compact C (T) tensile specimen, and the specimen size is shown in Figure 1. The thickness uses the original thickness of the plate. The fatigue test was carried out on a high-frequency fatigue testing machine (MTS-Landmark, maximum load 100 kN) according to ASTM E647 standard. A COD gauge was also clamped on the specimen to measure the crack tip opening displacement. The fatigue test used the same loading frequency (10 Hz) and the stress ratio (R=Kmin/Kmax) was 0.1. The fracture of the specimen was observed using a scanning electron microscope.

According to GB/T 7998-2023 “Evaluation Method for Intergranular Corrosion Sensitivity of Aluminum Alloys”, intergranular corrosion tests were carried out on 7003 aluminum alloy plates before and after repair. The test steps are as follows: After the Al-Zn-Mg alloy plates before and after repair were processed according to the above standards, the oil and impurities on the surface were cleaned with anhydrous ethanol, and the non-test surface was wrapped with epoxy resin. Then soak in NaOH solution (ρ=100 g/L, 10 mL NaOH+90 mL H2O) for 10 min, then use nitric acid (ρ=100 g/L, 12 mL HNO3+28 mL H2O) to clean the surface, and finally use distilled water to thoroughly clean the surface.

The sample was immersed in the corrosion solution (57 g NaCl+10 mL H2O2 solution+1 L H2O), the corrosion surface was 30 mm from the top of the liquid surface, the volume of the corrosion solution was 60 mL, and the temperature was controlled at 35±2 ℃ using the HH-2 digital constant temperature water bath, and the corrosion time was 6 h. After the corrosion is completed, the sample is cut perpendicular to the corrosion surface using an MNC-B-2 wire cutting machine, and then coarsely ground with 600#, 800#, 1000#, 1200#, and 1500# sandpaper, and then finely ground with 2000# sandpaper. Finally, it is mechanically polished with W1.5 polishing paste + woolen cloth, and then the surface is cleaned with an ultrasonic cleaner. After the sample surface is cleaned with alcohol and air-dried, it is observed using a GX51 inverted metallographic microscope.

2 Experimental results and discussion

2.1 Arc cladding morphology

Figure 2 is a photo of the Al-Zn-Mg alloy sample with crack C (T) after cladding. The cladding is performed on the front and back sides of the sample using a single-pass welding method. The cladding arc starts at A and extinguishes at B. As the cladding process progresses, the sample temperature continues to rise, and local collapse occurs at the arc-extinguishing end, causing the arc-extinguishing end weld height to be slightly lower than the arc-starting end height. The reason for this phenomenon is that the Al-Zn-Mg alloy sheet has high rigidity. Due to the short length of the prefabricated crack, the sample rebounds after unloading, and the crack is invisible to the naked eye, and cannot be accurately located and repaired. In addition, the sample thickness used in arc cladding is the original thickness of the sheet (10 mm), while the arc cladding depth is only about 1 mm, so it cannot penetrate the entire sheet, and cracks may still exist inside the sample after cladding.

Therefore, in order to solve the problem of hidden cracks and insufficient cladding depth, Scheme 2 uses an EDM wire cutting machine to cut a notch with a width of 0.18 mm on the plate to simulate the crack inside the plate, solve the problem that the crack is invisible to the naked eye and cannot be accurately located, and use a milling machine to process a 10 mm wide right-angle groove in the area near the crack on both sides of the sample, leaving only 1.5~2 mm plate thickness, and then arc cladding is performed on the right-angle grooves on both sides of the sample, and the right-angle grooves are filled with solder to achieve the purpose of repair. The actual component repaired by arc cladding using Scheme 2 is shown in Figure 3. As shown in Figure 3a, the sample has a local vacancy. This is because it is difficult to fill the entire right-angle groove during the cladding process due to the structural limitations of the C (T) sample itself. As shown in Figure 3b, local melting through occurs after cladding, and there is also a problem of insufficient local filling in some locations. The test results show that the effect of cladding is not good when the original plate thickness of only 1.5~2 mm is retained at a higher energy density.

The actual object using multi-pass arc cladding (Scheme 3) is shown in Figure 4. Figures 4a and 4b are the actual objects before and after the repair of Al-Zn-Mg alloy, respectively, and Figure 4c is the cross-sectional view of the weld after the repair of Al-Zn-Mg alloy. As shown in Figures 4a and 4b, the phenomenon of solder collapse at the edge of the sample is due to the high energy density during the cladding process. As can be seen from Figure 4c, there is still a problem of insufficient filling in the corners of the right-angle groove, and the overlap quality is poor. In addition, the overlap between the solder and the base material at the weld cannot be directly observed in multi-pass cladding, and the weld quality cannot be guaranteed.

The comparison before and after cladding after changing the right-angle groove to a trapezoidal groove (Scheme 4) is shown in Figure 5. After using a trapezoidal groove with arc transition and single-pass cladding, the gap before the repair can be well filled, but obvious thermal cracks appear at the cladding of the Al-Zn-Mg sample after the repair. This is because the thermal conductivity of Al-Zn-Mg alloy is high and the heat dissipation rate is too fast, which leads to excessive thermal stress and stress concentration, thus forming thermal cracks.

Figure 6 is a comparison of scheme 5 before and after cladding. As shown in Figure 6b, the repaired cladding layer has a good overlap with the base material and no defects are visible to the naked eye. This is because preheating and placing asbestos mesh can effectively reduce the heat loss efficiency and thermal cracking tendency.

2.2 Fatigue crack growth

Figure 7 is the fatigue crack growth rate curve of untreated Al-Zn-Mg alloy. As can be seen from Figure 7, the fatigue crack growth rate curve presents three typical fatigue crack growth stages, namely fatigue crack initiation stage (stage I), crack stable growth stage (stage II), and crack rapid growth stage (stage III). As the stress factor intensity range (ΔK) increases, the fatigue crack growth rate value increases monotonically. The crack of the original plate sample started to expand when ΔK=8.64 MPa·m1/2, and the sample completely broke when ΔK=41.9 MPa·m1/2, with 60 827 fatigue cycles.

Figure 8 shows the fatigue crack growth rate curves of Al-Zn-Mg alloy samples before and after repair, where Figure 8a shows the fatigue crack growth rate curves of Al-Zn-Mg alloy and arc cladding samples before repair, and Figure 8b shows the fatigue crack growth rate curves of Al-Zn-Mg alloy and arc cladding + laser shock strengthening samples before repair.

As shown in Figure 8, the fatigue crack growth rate curves of the samples before and after repair show three typical fatigue crack growth stages. With the increase of the stress factor intensity range (ΔK), the fatigue crack growth rate value increases monotonically. In the early stage of crack growth (ΔK<25 MPa·m1/2), the crack growth rate of the repaired Al-Zn-Mg alloy sample is lower than that of the sample before repair (see Figure 8a), and arc cladding + laser shock strengthening further reduces the fatigue crack growth rate of the alloy (see Figure 8b). However, when ΔK>25 MPa·m1/2, the crack growth rate of the repaired Al-Zn-Mg alloy sample is higher than that before repair. Compared with the arc cladding repaired sample, arc cladding + laser shock strengthening still reduces the fatigue crack growth rate of the sample in this range. The crack of the Al-Zn-Mg alloy sample began to grow at ΔK=8.64 MPa·m1/2, and the sample was completely broken at ΔK=41.9 MPa·m1/2, with 60 827 fatigue cycles. In the same range (27.3~73.4 MPa·m1/2), the arc cladding sample was completely broken after 38 223 cycles of fatigue loading, while the arc cladding + laser shock strengthening sample was completely broken after 153 001 cycles of fatigue loading. Therefore, laser shock strengthening can significantly improve the fatigue
performance of the alloy.

Figure 9 shows the fracture morphology of fatigue crack growth of the initial Al-Zn-Mg alloy sample. The crack extends from the prefabricated crack to the inside of the sample, and is not affected by other factors. The fracture shows a typical fatigue fracture radial morphology, and clear fatigue striations can be observed. The fatigue striation spacing is 0.24 μm in the crack initiation stage (near the crack source), and enters the stable expansion stage with the increase of fatigue crack growth rate, and the fatigue striation spacing increases to 1.23 μm. When the crack growth enters the rapid growth stage, the fatigue crack growth rate is fast, and the sample produces obvious necking.

Figure 10 shows the fracture morphology of fatigue crack growth of arc cladding Al-Zn-Mg alloy sample. The fatigue striation spacing near the crack source is 0.74 μm. There is a phenomenon of overlap cracking at the overlap of the cladding layer and the base material, and there are more impurities in the overlap crack. EDS spectrum detection shows that the impurities are mainly composed of Al and O elements, and there is a small amount of F element. In addition, it can be observed that there are many holes inside the solder.

Figure 11 shows the fracture morphology of fatigue crack propagation of Al-Zn-Mg alloy specimen after arc cladding + laser shock strengthening. As shown in Figure 11, the specimen starts to propagate from the prefabricated crack, and a certain amount of fine impurities can be observed 2~3 mm away from the crack source. EDS energy spectrum analysis shows that the impurities are mainly composed of Al and O elements, and a small amount of Na, Fe, and Zn elements. The fatigue striation spacing in the stable crack propagation stage is 2.04 μm.

2.3 Intergranular corrosion

Figures 12 to 14 are the intergranular corrosion profiles of Al-Zn-Mg alloy before repair, arc cladding, and arc cladding + laser shock strengthening, respectively. As shown in Figure 12, the intergranular corrosion depth of the original plate is 83~88 μm, and there are pitting pits with a depth of 186 μm in some parts (see Figure 12c). As shown in Figures 13a and 13b, after arc cladding, the depth of intergranular corrosion inside the cladding layer is about 49 μm, and can reach 68 μm locally. As shown in Figures 13c and 13d, the depth of intergranular corrosion in the parent material after arc cladding is about 43 μm, and there are pitting pits with a depth of 47 μm locally. As shown in Figures 14a and 14b, after arc cladding + laser shock strengthening, the depth of intergranular corrosion inside the cladding layer is about 32 μm, and the local corrosion depth is only 20 μm. As shown in Figures 14c and 14d, the depth of intergranular corrosion in the arc cladding + laser shock strengthening alloy parent material is about 28 μm, the local corrosion depth is only 16 μm, and no obvious pitting pits are observed.

Comparing the aluminum alloy samples before repair, arc cladding, and arc cladding + laser shock strengthening, the corrosion resistance of the plates is in the following order: arc cladding + laser shock strengthening > arc cladding > original plate. This is mainly because the dislocation density of the alloy is further increased after laser shot peening, and there are a large number of dislocation entanglements, dislocation cells and slip lines. Although a higher dislocation density may provide a potential pitting initiation site, it also accelerates the formation and repair rate of the passivation film of the alloy in the chloride solution. Therefore, the higher dislocation density and residual compressive stress of the laser shot peened alloy jointly enhance its corrosion resistance. Laser shot peening also enhances the bonding force between the second phase particles and the aluminum matrix by introducing residual compressive stress, making it difficult for pitting to initiate and expand around this phase. In addition, the residual compressive stress can make the alloy more resistant to pitting invasion along the thickness direction, so that pitting cannot enter deep into the aluminum matrix, reducing the depth of pitting.