The excellent deformation hardening ability of high manganese steel makes it widely used under strong impact load conditions. However, with the long-term service of high manganese steel parts, their surfaces will gradually show wear failure. Therefore, in order to maintain the excellent deformation hardening ability of high manganese steel parts, the laser wire feeding cladding technology of high manganese steel coating is proposed. High manganese steel coating was prepared on Mn13 steel plate using laser wire feeding cladding technology, and the surface of the cladding layer was deformation hardened by ultrasonic rolling technology. The microstructure, phase composition and mechanical properties of the high manganese steel cladding layer before and after ultrasonic rolling were analyzed, and the ultrasonic rolling hardening mechanism of the high manganese steel cladding layer was revealed. The results show that the microstructure of the laser wire cladding high manganese steel coating is a dendrite structure, and there is a component segregation of Mn and C elements between the dendrites; no phase change occurs during ultrasonic rolling, and the hardness and wear resistance of the coating are greatly improved after ultrasonic rolling; the dendrite segregation, dislocation and twins of C and Mn in the initial high manganese steel coating seriously hinder the movement of dislocations during ultrasonic rolling, thereby increasing the density of dislocations; due to the twin-induced plastic deformation effect of high manganese steel, a large number of deformation twins will be generated inside the coating after ultrasonic rolling, and the interaction between the deformation twins further enhances the deformation hardening ability of the high manganese steel cladding layer. Laser wire cladding provides a technical basis for high-performance repair of the surface of large high manganese steel parts, which is of great significance.

High manganese steel has excellent deformation hardening ability, and its surface can harden rapidly under the action of impact load, thus showing excellent wear resistance. Therefore, high manganese steel is often used in workpieces such as excavator teeth, crushers, and railway frogs that serve under strong impact load conditions. However, during long-term service, the surface of high manganese steel parts will gradually show a certain degree of wear, thus shortening its service life. Therefore, repairing the surface of high manganese steel parts is extremely important for extending their service life.

At present, surfacing welding is commonly used in engineering to repair high manganese steel parts. However, due to the high carbon content in high manganese steel, its weldability is poor. During the welding process, it is very easy to precipitate network carbides at the austenite grain boundary and then crack. Therefore, when welding high manganese steel, it is usually necessary to use a low current, low speed, and intermittent welding method. At the same time, water cooling and hammering are also required during the welding process. This method of repairing high manganese steel parts by surfacing welding not only has low welding efficiency and poor welding quality, but also greatly increases the repair cost of high manganese steel parts and reduces the production efficiency of enterprises.

As a more popular surface modification technology in recent years, laser cladding technology has been widely used in metal parts with poor weldability due to its low heat input and rapid cooling rate. At present, domestic and foreign scholars have carried out a series of studies on laser cladding technology on the surface of high manganese steel parts. HE et al. used laser cladding technology to prepare particle-reinforced iron-based composite coatings on high manganese rail steel and found that the microhardness of laser-strengthened rail steel reached 800HV, and its wear resistance was improved by about 8 times compared with untreated rail steel. Pi Ziqiang et al. used laser cladding technology to prepare Fe-WC composite cladding layers with different WC contents on high manganese steel substrates. The study found that the hardness and wear resistance of the composite cladding layers increased with the increase of WC addition. Wang Guofa et al. used nano-carbide ceramic composite powder materials to prepare a cladding layer with high strength, high toughness and high wear resistance on high manganese steel frogs by laser cladding technology, which greatly reduced the wear of high manganese steel frogs.

At present, the research on laser cladding on the surface of high manganese steel parts mainly uses non-high manganese steel materials as cladding materials. The cladding layer cannot have the same excellent deformation hardening ability, so it has certain limitations in the repair and remanufacturing of large high manganese steel parts. There are also reports on low-carbon high-manganese steel in the field of laser additive remanufacturing. Park used laser directional energy deposition technology to prepare low-carbon high-manganese steel samples (Fe-12Mn-5Cr-1Ni-0.4C) on the surface of Inconel718 and conducted wear tests on them. The results showed that the high-manganese steel samples prepared by laser directional energy deposition technology have excellent wear resistance, confirming the applicability of high-manganese steel as a coating material in industry. However, the current laser cladding and remanufacturing methods for high-manganese steel are all powder-feeding, which leads to low material utilization during the cladding process, greatly limiting the application of laser cladding technology on large high-manganese steel parts. Compared with powder-feeding laser cladding, the material utilization rate of wire-feeding laser cladding is greatly improved, almost 100%, so it is more suitable for large high-manganese steel parts and has broad application prospects.

However, high manganese steel can only show excellent deformation hardening ability under strong impact load conditions, and its deformation hardening ability is weak under low impact conditions. In order to improve the applicability of high manganese steel coating under various working conditions, surface pre-hardening treatment of high manganese steel can effectively improve the deformation hardening ability of high manganese steel under low impact conditions. Ultrasonic rolling is a new type of surface pre-hardening technology. It combines ultrasonic vibration with traditional rolling technology to improve the properties of the material surface, thereby enhancing the performance of the material. For high manganese steel, a material with excellent deformation hardening ability, ultrasonic rolling treatment of high manganese steel cladding layer can effectively achieve deformation hardening of the cladding layer, thereby improving its wear resistance.

In this paper, laser wire feeding cladding technology is used, high manganese steel wire is used as cladding material, and Mn13 is used as matrix to prepare high manganese steel coating. On this basis, ultrasonic rolling is used to deform harden the surface of the cladding layer. The microstructure and mechanical properties of the high manganese steel cladding layer before and after ultrasonic rolling are studied, and the ultrasonic rolling hardening mechanism of the high manganese steel cladding layer is deeply analyzed. This technology provides an important technical basis for high-performance repair of large high-manganese steel parts, which is of great significance.

1 Test method

1.1 Sample preparation
The high-manganese steel matrix uses Mn13 wear-resistant high-manganese steel plate produced by Shanghai Jiugang New Materials Group Co., Ltd. Its main components (mass fraction) are as follows: 1.13% C, 12.84% Mn, 0.47% Si, 0.052% P and 0.009% S, and the rest is Fe. The steel plate is produced by electric arc furnace plus vertical continuous casting process and rolled to 12 mm thick. Wire cutting to 100 mm×100 mm×12mm, rust removal with sandpaper, alcohol ultrasonic cleaning to remove oil and dirt, and then drying. The welding wire is a high manganese steel welding wire with a diameter of Φ1.2 mm. Its main components (mass fraction) are as follows: ≤1.10%C, 11.00%-18.00%Mn, 0.30%-1.30%Si and ≤2.50%Mo, and the rest is Fe. It is produced by Tianjin Bridge Welding Material Group Co., Ltd. A three-dimensional CNC mobile platform equipped with a HWL-1500 semiconductor laser was used for laser wire feeding cladding experiments. The welding wire was fed to the edge of the laser spot on the surface of the substrate through a wire feeder, and the protective gas was argon. After a large number of preliminary experiments and explorations, the following laser process parameters were used: laser power of 1400W, scanning speed of 5mm/s, wire feeding angle of 45°, wire feeding speed of 10mm/s, spot diameter of 2 mm, overlap rate of 45%, and a single-layer high manganese steel coating was prepared. The coating thickness was about 800μm. The single-layer high manganese steel coating was ultrasonically rolled under different rolling pressures using an H+VM850 ultrasonic processing machine. The rolling pressure F was 150N and 300N respectively. The schematic diagram of laser wire cladding and ultrasonic rolling is shown in Figure 1. The relevant process parameters of ultrasonic rolling treatment are as follows: the impact frequency is 40kHz, the amplitude is A=5μm, and the ultrasonic rolling area size is l1=4mm, l2=15mm, and l3=0.02mm.

The sample was cut along the cross section of the cladding layer using an electric spark wire cutting machine (DK7735). The cross section was smoothed by metallographic grinding and polishing. Then, the cross section was etched with 4% nitric acid alcohol solution for microstructure observation and composition detection.

1.2 Performance test and structural characterization
The microstructure of the cross section of the cladding layer after corrosion was observed using a field emission scanning electron microscope (FE-SEM, MAIA3 LMH). The element distribution of casting structure and cladding structure was analyzed by electron probe microscope (EPMA, SHIMADZU EPMA 8050G). The phase composition of cladding structure before and after impact was analyzed by X-ray diffractometer (XRD, Bruker advance D8), voltage 40 kV, current 30 mA, scanning speed 5 (°)/min, scanning range 20°~100°. The Vickers hardness of cladding layer and substrate before and after ultrasonic rolling was tested by HVSA-1000A microhardness tester, load 300 g, loading time 15 s. The wear test of high manganese steel coating was carried out by pin-disc rotary friction and wear tester (MPX-3X), the test selected Al2O3 grinding ball with hardness of 2000HV, friction radius of 5mm, spindle speed of 240r/min, applied load of 50N, and test time of 60min. An Olympus DSX1000S digital microscope was used to observe the surface morphology of the wear, and the width, depth and average cross-sectional area of ​​the wear scar were measured. Electron backscatter diffractometer (EBSD, Oxford Nordly max3) was used to collect the microstructural and crystallographic information of the cladding structure before and after ultrasonic rolling, operating at an accelerating voltage of 20 kV and a beam current of 8 nA with a step size of 1.4 μm. The test area was treated with electrolytic polishing before the test, and the EBSD data was analyzed based on AztecCrystal. Transmission electron microscopy (TEM, JEM2100) was used to study the bright field image, dark field image, diffraction pattern and high-resolution image of the microstructure of the coating structure before and after ultrasonic rolling. The ion thinning technology was used to treat the test area before TEM testing, and the Digital micrograph software package was used to process the TEM data.

2 Results and discussion

2.1 Microstructure analysis
Figure 2 shows the cross-sectional microstructure SEM images of different regions of the single-layer high manganese steel coating, and the local magnified images of each region are shown in Figure 2d-f. It can be seen from the figure that there is a relatively obvious fusion zone boundary at the junction of the bottom of the cladding layer and the substrate. The substrate structure below the boundary is coarse equiaxed austenite grains, while from the bottom to the top of the cladding layer, its structure is composed of columnar dendrites and equiaxed dendrites. The microstructure of the high manganese steel cladding layer is significantly different from that of the substrate, which is caused by the lower heat input and extremely fast cooling rate during the laser cladding process. During the rapid solidification of the alloy, its microstructure morphology mainly depends on the temperature gradient G and the growth rate R. Depending on the difference in G/R, its microstructure shows different morphologies. Due to the lower thermal conductivity of high manganese steel, the temperature gradient at the bottom of the cladding layer is the largest and the growth rate is the smallest. As the solidification process continues, the distance from the fusion zone becomes larger and larger, the temperature gradient continues to decrease, and the growth rate continues to increase, which leads to a decrease in its G/R. The microstructure at the bottom and middle is columnar dendrites, while the G/R at the top reaches the minimum value, so the top of the coating is equiaxed dendrites.

In addition, during the laser cladding process, due to the rapid cooling rate of the molten pool, no network cementite will be formed at the grain boundary, so no microcracks will be generated inside the cladding layer, which will affect the performance of the cladding layer.

Figure 3 is the EPMA element distribution diagram of the high manganese steel substrate and the cladding layer. The element distribution of the high manganese steel substrate is shown in Figure 3a. It can be seen that the element distribution in the high manganese steel substrate is relatively uniform. The distribution of elements in the cladding layer is shown in Figure 3b. It can be seen that unlike the high manganese steel matrix, the Fe, Mn, and C elements in the coating are unevenly distributed. The Mn element is enriched between the dendrites, while the Fe and C elements are enriched inside the dendrites. This uneven distribution of elements is consistent with the weld structure of laser-welded high manganese steel. From the element line scan in Figure 3, it can be seen that the element changes ΔFe and ΔMn of the cladding structure are significantly higher than those of the casting structure. This is because the temperature gradient of the liquid high manganese steel is small, the cooling and solidification rate is slow, and the element diffusion time is sufficient, so that the solid high manganese steel has no obvious microsegregation and the structure is equiaxed crystal. During the laser cladding process, the temperature gradient of the molten pool along the depth direction is large and the solidification rate is fast, resulting in insufficient time for the elements to diffuse, and the diffusion rates of each element are inconsistent, which makes the cladding high manganese steel element enriched, dendrite segregation occurs, and the structure is uneven. In addition, the high manganese steel coating has a high content of C element. Due to the strong affinity between C element and Mn element, C element always co-precipitates with Mn element between dendrites, and then produces C-Mn-rich areas between dendrites, forming a large number of C-Mn solute dipoles. These C-Mn solute dipoles can strongly interact with the strain field of dislocations, pin and store dislocations between dendrites, increase dislocation density, and thus make the high manganese steel cladding layer have better deformation hardening ability.

2.2 Phase composition and mechanical properties analysis
The hardness data of the high manganese steel substrate and coating before ultrasonic rolling and after ultrasonic rolling at different rolling forces F are shown in Figure 4a. Before ultrasonic rolling, the initial hardness of the cladding layer was 273.7 HV0.3, which was higher than 249.62 HV0.3 of the high manganese steel substrate. This shows that the coating has a higher initial hardness than the high manganese steel substrate, which is due to the sub-grain microstructure of the cladding layer. After ultrasonic rolling of the high manganese steel substrate and the coating surface, the hardness of both was greatly improved, which shows that deformation hardening occurred on their surfaces after ultrasonic rolling. Under a rolling pressure of 150N, the hardness of the high manganese steel substrate is 463.63 HV0.3, an increase of 185%, and the hardness of the coating is 531.41 HV0.3, an increase of
194%. As the rolling pressure increases to 300N, the hardness of the high manganese steel substrate and coating reaches 486.67 HV0.3 and 590.06 HV0.3, respectively, which are 187% and 215% higher than those of the non-ultrasonic rolling samples. The above results show that the high manganese steel coating has better deformation hardening ability than the high manganese steel substrate, and the deformation hardening ability becomes more significant with the increase of rolling pressure.

Figure 4b shows the XRD spectrum of the high manganese steel coating before and after ultrasonic rolling treatment under a rolling pressure of 300N. As can be seen from the figure, the phase structure of the high manganese steel cladding layer is a single γ-Fe phase before and after rolling, which indicates that the high manganese steel coating did not undergo martensitic phase transformation during ultrasonic rolling. Compared with before rolling, the diffraction peak intensity of the (111) crystal plane of the coating after rolling increased significantly, while the diffraction peak intensity of the (220) and (311) crystal planes decreased slightly, indicating that the grains inside the coating deflected during the rolling process, resulting in a large number of (111) crystal planes with preferred orientation. The (111) crystal plane is the slip plane and twin plane of high manganese austenitic steel. In the early stage of the plastic deformation process, the slip system starts with the movement of dislocations. When the slip stress gradually reaches the critical twin stress of high manganese steel, twinning begins to appear inside, resulting in a large number of deformation twins, which in turn leads to a significant increase in the diffraction peak intensity of the (111) crystal plane.

In order to study the effect of ultrasonic rolling on the wear resistance of high manganese steel coating, we tested the friction and wear properties of high manganese steel coating before and after ultrasonic rolling treatment under a rolling force of 300N. The wear scar morphology is shown in Figure 5. It can be seen that compared with the cladding layer without ultrasonic rolling, the depth, width and cross-sectional area of ​​the wear scar of the coating after ultrasonic rolling are greatly reduced, which shows that ultrasonic rolling greatly improves the wear resistance of the coating, which is attributed to the high residual compressive stress formed by the high-frequency impact and rolling action caused by ultrasonic vibration on the coating surface during ultrasonic rolling. In addition, there is a slight accumulation of material at the edge of the wear scar of the sample without ultrasonic rolling, but this phenomenon does not occur in the sample after ultrasonic rolling. This is because the hardness of the sample without ultrasonic rolling is low. During the wear process, the extrusion of the grinding ball on the surface will produce severe plastic deformation, which will lead to bulges on both sides of the wear scar. Since the hardness of the sample after ultrasonic rolling is high, no bulges will be produced due to the extrusion of the grinding ball.

In order to more intuitively evaluate the wear resistance of the high manganese steel coating before and after ultrasonic rolling, we calculated the volume wear rate of the cladding layer after wear. The formula of volume wear rate is shown in formula (1)-(3) (see the formula in the figure).

In formula (1)-(3), η is the volume wear rate, ΔV is the volume loss of the cladding layer, Fload is the vertical load applied during the wear process, Save is the average cross-sectional area of ​​wear, L is the moving distance of the grinding ball on the surface of the cladding layer during wear, T is the wear time, SR is the spindle speed, and R is the friction radius. The corresponding parameters are shown in Table 1. It can be seen from Table 1 that the volume wear rate of the cladding layer before and after ultrasonic rolling is 3.1×10-4mm3/N·m and 1.7×10-4mm3/N·m respectively. The ultrasonic rolling treatment improves the wear resistance of the cladding layer by 45%, which shows that ultrasonic rolling treatment can effectively improve the wear resistance of the coating.

Figure 6a and b are SEM images of the surface morphology of the high manganese steel coating before and after ultrasonic rolling. It can be seen that the surface of the sample without ultrasonic rolling treatment is rough and uneven, while the grooves and pits on the surface of the sample after ultrasonic rolling treatment are filled, which is due to the microscopic impact force brought by the ultrasonic vibration during the ultrasonic rolling process. These microscopic impact forces can effectively reduce surface defects such as cracks and holes, thereby improving the flatness of the coating surface.

During the ultrasonic rolling process, due to the presence of ultrasonic vibration, the strain rate of plastic deformation is between 103 and 104/s, which is a typical high strain rate plastic deformation, which will cause severe plastic deformation on the surface of the material and form a significant severe plastic deformation layer. Figure 6c is a microstructure diagram of the coating cross section after ultrasonic rolling. It can be seen that after ultrasonic rolling, a severe plastic deformation layer of about 5μm appeared on the surface, and the microstructure in the severe plastic deformation layer was corrugated, which indicates that the microstructure of the high manganese steel layer has undergone complex changes during the ultrasonic rolling process, which will lead to the formation of nanocrystals and twins. In addition, the observed corrugated texture also indicates that the grain orientation on the surface of the cladding layer has changed after ultrasonic rolling. As shown in the XRD spectrum of Figure 4b, ultrasonic rolling leads to the appearance of a high-intensity (111) crystal plane diffraction peak, which reflects that the grain orientation has been redistributed in order to adapt to the high stress generated by the high-frequency vibration of the ultrasonic wave.

2.3 Ultrasonic rolling hardening mechanism
In order to further reveal the ultrasonic rolling deformation hardening mechanism of high manganese steel coating, we conducted EBSD and TEM tests on the samples before and after ultrasonic rolling treatment under 300N impact force. Figure 7 shows the EBSD-band comparison diagram (BC) and geometric dislocation (GND) density diagram of the high manganese steel cladding layer before and after ultrasonic rolling. It can be observed from Figure 7c that a hardened layer with a depth of about 200μm is formed on the surface of the cladding layer after ultrasonic rolling. The reason is that the high-frequency vibration generated by the ultrasonic vibration during the ultrasonic rolling process will produce continuous microscopic impact force on the material surface. Under the action of the microscopic impact force, the material surface will produce greater plastic deformation. Combined with the mechanical hardening process, a deeper hardened layer will be produced on the surface of the cladding layer. A large number of black lines are also observed inside the hardened layer. These black lines are generally regarded as deformation twin bundles. Due to the resolution limitation of EBSD, the software cannot fully identify these black lines. Therefore, a detailed analysis of deformation twins will be carried out with the help of TEM in the subsequent part of this article. Figure 7b and d are the EBSD-GND images of the cladding layer before and after ultrasonic rolling, respectively. It can be seen that the hardened layer of the sample after ultrasonic rolling is brighter and darker than that before ultrasonic rolling. Figure 7e shows the distribution of GND density values ​​before and after ultrasonic rolling. It can be found that the GND density before ultrasonic rolling is 0.21×1014/m2, while the GND density after ultrasonic rolling increases sharply to 0.48×1014/m2, an increase of 128%. The reason for the sharp increase in dislocation density is that the severe plastic deformation process caused by the high strain rate conditions during ultrasonic rolling causes violent dislocation movement inside the cladding layer. Due to the dendritic structure of the coating, on the one hand, the subgrain boundaries formed between dendrites can capture and store dislocations. On the other hand, the C-Mn solute dipoles between dendrites can also act as a barrier to dislocation movement, strongly hindering the movement of dislocations, and then causing dislocations to accumulate at subgrain boundaries. The accumulation of dislocations causes local stress concentration at the subgrain boundaries, and when these stresses reach or exceed the critical stress of twins, they will promote the nucleation and growth of deformation twins.

Figure 8a is the EBSD-inverse pole figure (IPF) of the coating before ultrasonic rolling. Its microstructure is characterized by EBSD as coarse columnar crystals, which are much larger than the grain size in Figure 2. This is because EBSD can measure the orientation difference between grains. It stipulates that the grain boundary orientation difference of 15° is used as the basis for grain division, but within a single columnar crystal, the orientation difference between dendrites is extremely small, showing the same color in the IPF inverse pole figure, so it is regarded as the same grain in EBSD. The growth direction of columnar crystals is vertical as a whole, which is attributed to the large temperature gradient in the laser cladding process, which promotes the strong epitaxial growth of grains along the heat flow direction. Figure 8b is the IPF diagram of the cladding layer after ultrasonic rolling, and Figure 8c is the corresponding pole figure (PF) before and after ultrasonic rolling. It can be observed that there is no obvious preferred orientation in the cladding layer before ultrasonic rolling, but after ultrasonic rolling, there is an obvious preferred orientation of the (111) plane, which is consistent with the substantial increase in the intensity of the diffraction peak of the (111) crystal plane after ultrasonic rolling in Figure 4b. Figures 8d and e are partial enlarged views of the white frame area in Figure 8b. It can be seen that the color inside the columnar crystal after ultrasonic rolling has changed significantly compared with that before ultrasonic rolling, showing a wavy color distribution. This is because the drastic plastic deformation during ultrasonic rolling leads to the generation of a large number of dislocations, as shown in Figure 7e. These dislocations interact and influence each other to form dislocation cell substructures, which have small orientation differences and then form subgrain boundaries, as shown in Figure 8f. After ultrasonic rolling, the subgrain boundaries formed by dislocation cells inside the high manganese steel coating can hinder the movement of dislocations like grain boundaries, capture and store dislocations, so that the high manganese steel cladding layer has higher hardness and enhanced deformation hardening ability.

Figure 9 is a TEM image of the microstructure of the high manganese steel coating before ultrasonic rolling. The rapid cooling and solidification of the molten pool produces tensile stress, causing plastic deformation, leading to dislocation movement and the formation of twins. As shown in Figure 9a, it can be clearly seen that the straight high angle grain boundaries (HAGB) are distributed between the two grains, and the dislocation movement on the right side of the HAGB forms dislocation lines (DLs) and dislocation walls (DW). After the dislocation moves to the HAGB, it is hindered by the HAGB and accumulates on its right side, resulting in a low dislocation density on the left side to form a dislocation free zone (DFZ). The phase can be determined to be γ-Fe by selected area electron diffraction pattern (SAEDP) calibration, which is consistent with the XRD analysis shown in Figure 4b. In Figure 9b, the HAGB crossing reduces the mean free path of dislocation slip, resulting in increased resistance to dislocation movement, forming dislocation cells (DCs) and dislocation tangles (DTs). The dislocation accumulation is more significant than that in Figure 9a. At the same time, the hindering effect of HAGB also causes the formation of DFZ on the left. Figures 8c and 8d are bright field images, dark field images, high resolution TEM (HRTEM) and micro-area inverse Fourier transform of twins. The symmetrical arrangement of the matrix and twin atoms in Figure 9d shows that the bright white band in Figure 9c is a twin. At the same time, it can be seen that the movement of the twin is blocked at the HAGB, and the transmission stops, which leads to the refinement of the grains on the left side of the HAGB, and the resistance to dislocation movement is greater than that on the right side of the HAGB, resulting in an increase in dislocation density and serious dislocation accumulation. This shows that there are high-density dislocation structures and twins in the high-manganese steel cladding layer, which is one of the sources of its deformation hardening.

The TEM image of the microstructure of the coating after ultrasonic rolling is shown in Figure 10. Figures 10a and 10b are the dark field and bright field images of deformation twins, respectively. It can be seen that after ultrasonic rolling, a high-density deformation twin bundle is generated on its surface. The SAEDP in Figure 10c also confirms the existence of deformation twins. Since the high manganese steel coating has a high density of dislocations before ultrasonic rolling, the introduction of ultrasonic vibration during ultrasonic rolling increases the activity of dislocations at the microscopic level. The strong plastic deformation caused by it will lead to the accumulation of higher density dislocations inside the grains, and then stress concentration will occur inside the grains, promoting the nucleation of deformation twins [15,22]. In addition, it can be seen in Figure 10b that some twins have a certain degree of deflection, which may be caused by further strain after the formation of deformation twins. In other areas, we also observed the emergence of multi-level twin systems. As shown in Figure 10d and e, a large number of dislocations were found around the primary twins and secondary twins, indicating that the twin boundaries can act as obstacles to dislocation movement during plastic deformation, resulting in local stress concentration and strain hardening. In addition, the interaction between the primary twins and the secondary twins formed a rhombus-shaped area, refined the grains, and produced a dynamic Hall-Petch effect, which is beneficial to improving the deformation hardening ability of the high manganese steel cladding layer. Figure 10f is the HRTEM image of the yellow frame area in Figure 9e, showing the atomic arrangement at the intersection of the primary twin and the secondary twin. It can be found that there is a serious lattice distortion at the intersection, which indicates that the interaction between the primary twin and the secondary twin leads to the dislocation reaction and thus produces additional deformation hardening locally.

In summary, in the high manganese steel coating, due to the presence of a large number of subgrain boundaries and C-Mn atomic pairs formed by the segregation of C and Mn elements at the subgrain boundaries, the laser cladding high manganese steel has a stronger dislocation capture and storage ability than the traditional cast high manganese steel, which makes the nucleation of deformation twins easier. After the deformation twins are generated, the dislocation movement is further hindered due to the existence of deformation twin boundaries, and the deformation twins will also interact with each other, refine the grains, and reduce the grain size. The total stress of high manganese steel is as follows: See formula (4) in the figure.
In the formula, ρd is the dislocation density, l is the twin spacing, D is the grain size, M is the average Taylor factor, G is the shear modulus, β is a constant, b is the Burgers vector, σ0 is the flow stress, and α0 is the geometric factor. It can be seen that the high dislocation density and deformation twins generated after ultrasonic rolling deformation hardening of high manganese steel are the main reasons why the high manganese steel coating has excellent deformation hardening ability.

3 Conclusions
Aiming at the wear and repair problems of high manganese steel parts, a laser wire cladding technology for high manganese steel coating was proposed, and the prepared coating was subjected to surface deformation treatment by ultrasonic rolling technology. The microstructure and mechanical properties of the high manganese steel coating before and after ultrasonic rolling were studied, and the deformation hardening mechanism was analyzed, and the following conclusions were drawn:
(1) The microstructure of the laser wire cladding high manganese steel coating is composed of columnar dendrites and equiaxed dendrites; due to the high cooling rate during the laser wire cladding process, no network cementite is formed; the microstructure of the cladding layer has the phenomenon of elemental inhomogeneity: Fe is enriched in the dendrites, Mn and C are enriched between the dendrites, and the segregated Mn and C between the dendrites can form C-Mn solute dipoles to hinder the movement of dislocations, thereby improving the deformation hardening ability of the cladding layer.
(2) The high manganese steel coating was subjected to ultrasonic rolling treatment, and the phase composition, hardness and friction and wear properties of the cladding layer before and after ultrasonic rolling were analyzed. The results showed that no phase change occurred during the ultrasonic rolling process, and the hardness and wear resistance of the coating were greatly improved after ultrasonic rolling.
(3) The ultrasonic rolling hardening mechanism of the laser wire cladding high manganese steel coating was revealed: there were dendrite segregation, dislocations and twins of C and Mn in the initial high manganese steel coating, which led to serious obstruction of dislocation movement during the ultrasonic rolling process, resulting in a higher density of dislocations; due to the TWIP effect of high manganese steel, a large number of deformation twins will be generated inside it after ultrasonic rolling, and these deformation twins will interact with each other, further improving the deformation hardening ability of the high manganese steel coating.