In order to improve the quality problems such as pores, microcracks, and residual stress generated in the traditional laser cladding preparation process and improve the mechanical properties of the cladding layer, laser peening (LP) was used to post-treat the iron-based cladding layer, and the microstructure evolution law and wear performance mechanism of the cladding layer before and after laser peening were compared and analyzed. The results showed that after LP treatment, no phase change occurred in the cladding layer, the (110) crystal plane diffraction peak produced a broadening effect, refined the surface grains, changed the complex residual stress field on the surface, and obtained a uniformly distributed residual compressive stress. The microhardness was 1.3 times that before laser peening, and the friction coefficient was reduced by 25% compared with that before laser peening, which effectively enhanced the wear performance of the cladding layer.

45 steel has excellent mechanical properties and good deformation resistance, and is widely used in key parts in the automotive manufacturing industry (such as gears, shafts, connecting rods and bolts, etc.); however, due to its low surface microhardness and poor wear resistance, the surface of the material is damaged, which seriously affects the service life of the parts. Therefore, improving the surface properties of 45 steel materials is of great practical significance for improving its service reliability and extending its service life. As an advanced material surface preparation technology, laser cladding technology has the characteristics of good economy, high efficiency, good flexibility, and pollution-free working environment. It is generally considered to be the preferred method for preparing surface coatings. However, due to the characteristics of its own manufacturing process, the temperature field is unevenly distributed during the cladding process, resulting in a high temperature gradient in the processing area, which is more likely to cause metallurgical defects such as pores, microcracks, and residual tensile stress, thereby affecting the preparation quality of the cladding layer. Therefore, a certain post-processing process is required to make up for the technical limitations of laser cladding in order to maximize the fatigue life of the cladding layer.

In recent years, researchers have begun to explore the use of post-processing technology to further improve the surface properties of laser cladding layers. Early research focused on optimizing laser cladding process parameters and using heat treatment technology to improve the microstructure and mechanical properties of the cladding layer. However, these technologies have certain limitations during use and cannot completely eliminate the metallurgical defects caused by cladding forming. Later research directions mainly focused on surface deformation strengthening, such as surface shot peening, surface mechanical grinding, ultrasonic impact treatment, etc., which reduce metallurgical defects, refine surface grains, and obtain surface residual compressive stress through plastic deformation on the metal surface, thereby improving surface-related mechanical properties. Fang Xiuyang et al. used shot peening technology to post-treat CuNiIn coating. The results showed that the coating after shot peening obtained a work-hardening layer, introduced a residual compressive stress field, and extended the service life of the material. However, the shot peening equipment consumed a lot of energy, the surface roughness after processing was low, the strengthening effect was uneven, and shot peening dead corners were easily generated. At the same time, new impurities were introduced. Cui et al. used ultrasonic impact to treat the laser cladding layer. The research results showed that the surface of the impacted material obtained a gradient structure, the grains were refined, and a residual compressive stress field was generated. However, the depth of the impact on the surface material was shallow, and the surface often produced uneven impact force due to the unevenness of the processed surface.

Laser peening (LP) is an excellent surface strengthening treatment technology for metal materials. It has the characteristics of high pressure (GPa), high energy (GW), and ultrafast (ns). It improves the microhardness, refines the surface grains, and improves the wear performance by peening the material surface at an ultra-high strain rate. At present, many scholars at home and abroad have studied LP technology. TAN et al. used shot peening, ultrasonic impact and laser shot peening to strengthen the surface of TC17 alloy and found that the surface roughness of ultrasonic shot peening was 0.04-0.12 μm, and the depth of influence on the surface was 0.8 mm. The surface roughness after laser shot peening was 0.36-0.12 μm, and the depth of influence on the surface was 1.5 mm. Laser shot peening had the best strengthening effect on the material. Zhao Kai et al. combined laser shock strengthening technology with cold spraying technology to study the changes in residual stress and microhardness of pure aluminum coating. The results showed that the residual stress state of the coating surface changed from tensile stress to compressive stress, and the surface microhardness increased by 34.49%. L. Hackel et al. performed laser shot peening post-treatment on laser directly formed parts and found that laser shot peening had a good effect on the changes in the microstructure of the formed parts, the closure of metallurgical defects, and the distribution of residual stress fields. At present, there are few reports on the use of laser shot peening for samples after laser cladding, and the exploration of material properties after laser cladding is still not sufficient. In order to further improve the comprehensive performance of metal materials, a composite strengthening method of laser cladding + laser shot peening is proposed. The laser cladding technology is first used to prepare a functional coating that meets the requirements of specific working conditions, and then laser shot peening is used to make up for the metallurgical defects caused by laser cladding. The technical advantages of the two are combined to maximize the performance of the material and extend the fatigue life of the material.

In this study, the iron-based cladding layer was prepared on the surface of 45 steel by laser cladding technology, and the surface of the cladding layer was post-processed by laser shot peening technology. By comparing and analyzing the changes in the phase composition, microstructure, residual stress, microhardness and wear performance of the cladding layer before and after laser shot peening, the influence of laser shot peening on the structure and wear performance of the cladding layer was explored, and its influence rules were analyzed and summarized.

2 Test materials and test methods
2.1 Test materials
The test material is 45 steel, the sample size is 100 mm×100 mm×20 mm, and its chemical composition (mass fraction) is as follows: C 0.42%～0.50%, Si 0.17%～0.37%, Mn 0.50%～0.80%, Cr≤0.25%, Ni≤0.25%, P≤0.035%, S ≤0.035%, Fe is the balance; the cladding powder is iron-based alloy powder, and its chemical composition (mass fraction) is as follows: C 15.0%, Cr 13.6%, Si 1.2%, B 1.6%, Mo 0.8%, Fe is the balance, and the particle size is 140～325 mesh. Before the test, SiC sandpaper was used to polish the substrate surface, acetone was used to remove surface oil and impurities, and the powder to be clad was placed in a 150 oC drying oven for drying to ensure the preparation quality of the cladding layer.

2.2 Experimental method
The laser cladding system mainly includes high-power fiber laser, laser cladding head, KUKA six-axis linkage industrial robot, and powder feeding system; according to the preliminary test results, the main process parameters of laser cladding are selected as follows: laser power 1 800 W, spot diameter 4.5 mm, overlap 2 mm, scanning speed 5 mm/s, powder feeding amount 30 g/min, coaxial powder feeding method, pure argon protection. Laser shot peening uses the Procudo200 surface strengthening equipment of LSPT Company in the United States. The main process parameters are: pulse laser energy 6 J, spot diameter 3 mm, pulse width 20 ns, wavelength 1 053 nm, frequency 5 Hz, overlap rate 50%; before the test, the surface of the cladding layer is mechanically ground to a smooth surface, and a 0.1 mm black tape is attached to the surface to be shot peened as an absorption layer, and 2 mm thick flowing water is used as a constraint layer.

The phase composition of the sample was analyzed by D8 multifunctional powder diffractometer; the cross-sectional morphology and wear scar morphology of the sample were observed by Quanta FEG250 scanning electron microscope; according to the national standard JB/T9394-2011, the residual stress on the sample surface was measured by X350-A stress tester; according to ASTM E92-2016 standard, the microhardness test of the sample was carried out by HVS-1000 Vickers hardness tester, with a loading load of 0.2 kg and a loading time of 15 s; the tribological test was carried out by a reciprocating friction tester, with CD15W-40 lubricating oil for oil pool lubrication. The upper sample was a SUS304 cylindrical pin with a diameter of 6 mm and a length of 20 mm, and the lower sample was a test sample. The contact mode was pin-slider reciprocating friction, with a load of 10 N, a stroke of 10 mm, and a frequency of 5 Hz. The three-dimensional morphology of the wear surface was observed by VK-X250K three-dimensional confocal microscope.

3 Results and discussion
3.1 Phase composition
Figure 1 shows the X-ray diffraction patterns of the cladding layer before and after laser shot peening. It can be seen from the figure that the phase composition of the cladding layer is mainly α-Fe martensite phase and precipitation phases such as M23C6, M7C3, FeCrB, etc. After laser shot peening, no new diffraction peaks appear in the cladding layer. The peak position of the diffraction peak has basically not changed, but between 40o and 50o. The peak intensity of the diffraction peak has increased significantly and become sharper; this shows that under the action of high-energy laser, elements such as Cr and C, The B element reacts chemically in the molten pool to generate carbides and borides. These hard phases are usually dispersed in the form of particles between crystals and grains, producing a dispersion strengthening effect, which is beneficial to improve the microhardness of the cladding layer and enhance the friction and wear performance; on the other hand, it shows that laser shot peening does not precipitate new phases in the cladding layer. From the literature, it is known that the phase structure of metal materials is mainly determined by the composition of the constituent materials and the cooling conditions during the solidification process. Laser shot peening only physically treats the surface of the cladding layer, and no other elements are introduced during the processing, so it will not affect the phase structure of the cladding layer. Figure 2 shows the half-height width of the (110) crystal plane diffraction peak before and after laser shot peening. It is obvious that the diffraction peak becomes wider and higher after laser shot peening, indicating that the microstructure of the cladding layer has lattice distortion and grain refinement. It can be inferred that laser shot peening induces significant changes in the surface structure of the cladding layer, refines the surface grains, and introduces residual compressive stress, so the (110) crystal plane diffraction peak has an enhancement effect and a broadening effect. 3.2 Microstructure Analysis
Figure 3 is a cross-sectional morphology of the Fe-based cladding layer before laser shot peening. As can be seen from Figure 3(a), the substrate and the cladding layer show good metallurgical bonding, with almost no obvious metallurgical defects such as pores and cracks. The microstructure at the interface between the cladding layer and the substrate is mainly plane crystals. This is mainly because the substrate surface temperature is low, the temperature gradient at the bottom of the molten pool is large, and the undercooling is small, resulting in slow crystal formation; the microstructure at the bottom of the cladding layer is mainly composed of coarse columnar crystals and dendrites that are almost perpendicular to the interface. This is because the temperature gradient is small and the undercooling is large, which accelerates the growth rate of the crystals, and most of them form columnar dendrites along the direction of maximum heat flow; due to the influence of the Marangoni effect, the heat dissipation rate of the molten pool surface is accelerated inside the cladding layer, which increases the undercooling and inhibits the preferential growth of dendrites, resulting in equiaxed crystals and relatively short dendrites in the microstructure of this area, as shown in Figure 3(b).

Figure 4 is a microstructure diagram of the bottom and inside of the cladding layer after laser shot peening. It can be observed that laser shot peening induced obvious plastic deformation in the cladding layer, and there were no continuous and slender dendrites in the structure. The size of the grains gradually increased with the distance from the laser shot peening surface. The columnar dendrites in the bottom area were fragmented and broken, decomposed into fine dendrites and equiaxed crystals, and the large-sized dendrites in the inner area disappeared, which was mainly composed of a large number of equiaxed crystals and refined dendrites. This is because the growth direction of columnar crystals and equiaxed crystals in the laser shot peening area is almost perpendicular to the direction of plastic deformation. They are easily broken after being subjected to the ultra-high strain rate laser shock wave. Since the plastic deformation is manifested in dislocation movement, when the dislocation moves to the vicinity of the grain, the grain boundary hinders the dislocation movement, resulting in the occurrence of dislocation pile-up. A large number of dislocation pile-ups form dislocation walls, which gradually evolve into subgrain boundaries. As the dislocation movement continues, the subgrain boundaries transform into grain boundaries, thereby forming fine grains. The surface of the cladding layer is greatly affected by the laser shock wave, and the dislocation movement is complex and along different directions, eventually forming randomly distributed equiaxed crystals and fine dendrites. Although the bottom area is less affected by the laser shock wave than the surface, and the dislocation movement is relatively weak, the coarse columnar crystals are still broken by the laser shock wave, forming a relatively refined dendrite structure.

3.3 Residual stress
Figure 5 shows the residual stress field distribution on the surface of the cladding layer before and after laser shot peening (5 different points on the surface are randomly selected for measurement). It can be clearly seen from the figure that the residual stress σx of the cladding layer without laser shot peening treatment ranges from −151 to 146 MPa, and σy ranges from −34 to 138 MPa; after laser shot peening treatment, the surface residual stress σx ranges from −707 to −635 MPa, and σy ranges from −584 to −417 MPa; the cladding layer without laser shot peening treatment has a complex residual stress field distribution, with both residual tensile stress and residual compressive stress, and the stress distribution is uneven and fluctuates greatly; after laser shot peening treatment, the residual tensile stress on the surface of the cladding layer is transformed into a larger compressive stress and the distribution is relatively uniform. The residual stress of the cladding layer mainly comes from three aspects: phase change stress during the solidification of the molten pool, thermal stress caused by uneven heating during the heating of the molten pool, and cold stress during the solidification of the molten pool; the phase change stress during laser cladding is relatively small and has little effect on the residual stress field in the cladding layer. It is mainly because the metal powder is constrained by the surrounding materials when it melts and expands on the substrate to form compressive stress. During the solidification process, the molten pool shrinks and is stretched by the edge materials to produce tensile stress. The uneven plastic deformation during melting and solidification is the main reason for the complex residual stress field distribution in the cladding layer. Laser shot peening is to apply high-energy shock waves to the area to cause ultra-high strain rate plastic deformation on the surface of the metal material, and the surface grains produce lateral volume expansion, which squeezes the surrounding tissues and reduces the surface crystal plane spacing. After multiple laser shot peening treatments, the grains are gradually refined, the surface hardness is improved, and a hardened plastic deformation zone is formed, which changes the stress state on the surface of the cladding layer and presents a large residual compressive stress field distribution.

3.4 Microhardness
Figure 6 shows the cross-sectional microhardness distribution of the cladding layer before and after laser shot peening (ten points were measured every 0.2 mm in the depth direction through the substrate and the cladding layer). It can be clearly observed from the figure that the microhardness of the cladding layer without laser shot peening treatment is significantly improved compared with the substrate. Its average microhardness is about 535.3 HV0.2, which is 2.8 times the microhardness of the substrate, indicating that the iron-based cladding layer significantly improves the microhardness of the substrate, but the microhardness of this area fluctuates to a certain extent. The cladding layer after laser shot peening forms a relatively uniform microhardness distribution, with an average microhardness of 692 HV0.2, which is 1.3 times the microhardness of the cladding layer before laser shot peening. The depth of the hardened layer formed reaches more than 1 mm, indicating that laser shot peening can significantly improve the microhardness of the cladding layer. On the one hand, this is attributed to the fact that the laser shot peening causes the surface material of the cladding layer to undergo plastic flow and work hardening, resulting in the strengthening of the surface structure density and the formation of a plastic deformation layer; on the other hand, the laser shot peening causes the grains in the surface layer to be squeezed and stretched, refines the surface grains, and increases the strain resistance of the surface material. According to Hall-Petch’s fine grain strengthening theory, the increase in microhardness is based on the combination of work hardening and fine grain strengthening. Laser shot peening produces more small-sized grains in the cladding layer. The increase in small grains increases the dislocation wall when the crystal undergoes dislocation movement, improves the deformation resistance of the cladding layer, and leads to an increase in yield strength, resulting in an increase in the microhardness of the cladding layer.

3.5 Friction and wear
3.5.1 Friction coefficient curve
Figure 7 shows the change curve of the friction coefficient of the cladding layer sample and the substrate with the friction and wear time before and after laser shot peening. It can be seen from the figure that with the change of friction and wear time, the change law of the friction coefficient of the sample surface is basically similar, mainly manifested in the running-in stage where the friction coefficient changes sharply at the beginning and the stable friction stage where the friction coefficient tends to be stable. The Hertz contact stress principle proves this phenomenon. The surface quality of the sample has a decisive influence on the volatility of the friction coefficient. According to the variation law of the friction coefficient in the figure, it can be found that the friction coefficient of the cladding layer without laser shot peening is generally stable, but its volatility is more obvious than that of the substrate and the sample after laser shot peening. On the one hand, it shows that the cladding layer has good forming quality and fewer voids; on the other hand, it shows that the laser shot peening effectively improves the forming quality of the cladding layer and reduces metallurgical defects. In the steady-state friction and wear stage, the friction coefficient of the substrate is stable at about 0.175, the friction coefficient of the cladding layer without laser shot peening remains at about 0.134, and the friction coefficient after laser shot peening remains at about 0.1. Compared with the cladding layer without laser shot peening, the friction coefficient after laser shot peening is reduced by 25%, indicating that the cladding layer after laser shot peening has better wear performance.

3.5.2 Confocal wear scar morphology scanning
In order to further obtain the morphological information of the friction and wear surface, the surface wear scar is photographed with high focus and its geometric characteristics are studied. It can be clearly observed from Figure 8 that the surface wear of the substrate is more serious than that of the other two samples, the friction and wear performance is the worst, the wear scar surface is the roughest, the cladding layer sample without laser shot peening is better, and the surface condition after laser shot peening is the best. The maximum wear scar width on the substrate surface is 939.335 μm, and the maximum furrow depth is 8.273 μm; the maximum wear scar width on the cladding layer surface without laser shot peening is 370.413 μm, and the maximum furrow depth is 5.484 μm; the maximum wear scar width on the cladding layer surface after laser shot peening is 235.972 μm, and the maximum furrow depth is 2.032 μm; it can be concluded that the wear scar width of the substrate, the cladding layer before and after laser shot peening gradually narrows and the depth gradually becomes shallower, which is consistent with the friction coefficient test results described in Figure 7.

Figure 9 is the SEM image of the wear scars of the three samples. It can be observed from the figure that the wear scar morphology of the three samples all show material delamination along the wear direction. This is mainly because the plowing effect on the wear parts during the friction and wear process compresses the material to transfer to both sides, resulting in a plow-shaped wear scar morphology, which is a typical abrasive wear mechanism. In Figure 9 (a), the substrate has deeper plowing grooves and large-area spalling pits. This is mainly due to the low hardness of the substrate material. The wear parts are pressed into the substrate under the action of the normal load, increasing the contact stress of the two interfaces. When the adhesion between atoms is greater than the shear force of 45 steel, shear failure occurs on the surface of the substrate, causing the surface material to fall off, showing adhesive wear; Figure 9 (b) shows shallower plowing grooves and fewer spalling pits compared to the substrate, indicating that the cladding layer is accompanied by slight adhesive wear. Since the Fe-based cladding layer has The higher the microhardness, the less likely the surface material will peel off during the wear process, thus reducing the occurrence of adhesive wear. In Figure 9(c), the adhesive wear is significantly reduced, mainly shallow furrows, indicating that the wear mechanism of the cladding layer surface after laser shot peening is mainly abrasive wear. Due to the laser shot peening treatment, a plastic deformation layer is formed on the surface of the cladding layer, and a uniformly distributed surface residual compressive stress is obtained, which increases the surface microhardness, improves the surface anti-micro deformation ability, reduces adhesive wear, and thus enhances the wear resistance of the cladding layer.

4 Conclusion
(1) After laser shot peening, the Fe-based cladding layer does not undergo phase change, the (110) crystal plane diffraction peak produces an enhancement effect and a broadening effect, and the structure in the cladding layer is fragmented and fractured, forming randomly distributed fine equiaxed crystals and refined dendrites. (2) After laser shot peening, the complex residual stress field on the surface of the cladding layer changed, showing a larger uniformly distributed residual compressive stress field; the microhardness of the cladding layer was improved, forming a relatively uniform microhardness distribution, and its average microhardness was 692 HV0.2, which was 1.3 times that before laser shot peening.
(3) After laser shot peening, under oil lubrication conditions, the friction coefficient of the cladding layer was reduced by 25% compared with that before laser shot peening, mainly due to shallow plowing (i.e., a small amount of abrasive wear), showing good wear performance.