Objective To improve the friction and wear properties and corrosion resistance of GH5188 high-speed laser cladding coating, a nanocrystalline layer was prepared on the surface of GH5188 coating by ultrasonic rolling (UR). Methods Scanning electron microscope (SEM), X-ray diffractometer (XRD), energy dispersive spectrometer (EDS), Vickers hardness tester, high temperature friction and wear tester and electrochemical workstation were used to study the micromorphology, phase composition, microhardness, high temperature friction and wear properties and corrosion resistance of GH5188 coating under ultrasonic rolling. Results After ultrasonic rolling, the surface of GH5188 coating reached a mirror effect, and the roughness decreased by 58% compared with that of the unrolled coating. A nanocrystalline layer with a thickness of 18 μm was prepared. Compared with the H13 substrate, the surface microhardness of the unrolled coating increased by 21%, and the surface microhardness of the coating after ultrasonic rolling increased by 70%. Compared with the H13 substrate, the wear resistance of the unrolled coating increased by 69%, and the wear resistance of the coating after ultrasonic rolling increased by 81%. The electrochemical test results showed that compared with the H13 substrate, the corrosion resistance of the unrolled coating increased by 12%, and the corrosion resistance of the coating after ultrasonic rolling increased by 17%. Conclusion The dislocation density and grain boundary of the surface structure of the coating increased after ultrasonic rolling, and a nanocrystalline layer was obtained, which effectively improved the mechanical properties of the GH5188 coating, such as microhardness, wear resistance and corrosion resistance.

Studies have shown that about half of the mechanical energy is wasted due to erosion and wear caused by the relative movement between components [1]. Fatigue cracks usually occur on moving parts that are frequently subjected to cyclic loads[2]. The performance of the surface of mechanical parts is determined by the surface material. Therefore, it is very important to modify the surface material to obtain better performance in terms of surface roughness, hardness, microstructure, wear resistance, etc.[3]. Surface modification technology is usually represented by severe plastic deformation process, which produces a nanocrystalline layer through severe plastic deformation to improve the friction and wear performance, thermal fatigue life and corrosion resistance of the surface material[4]. According to Bertini et al.[5], ultrasonic rolling can improve the micro-fatigue performance of the material and increase the surface hardness by about 20%~25%. After ultrasonic shot peening treatment, the surface grains of Ti6Al4V are refined to a nanocrystalline layer of 17~25 nm; the reduced corrosion rate means improved corrosion resistance[6].

After quenching and tempering, H13 steel can easily obtain good hardenability, toughness, and thermal crack resistance, so it is widely used in hot working molds and die-casting molds. Its working environment is often subject to hot and cold cycles, and its surface is prone to defects such as thermal wear, high-temperature oxidation, and corrosion, which not only causes waste of resources, but also leads to an increase in processing costs. In order to improve and repair the surface performance of H13 steel, domestic researchers have proposed different cladding schemes. Deng Liqun et al. [7] used vacuum arc ion plating technology to prepare TiAlN composite coatings on the surface of H13 steel, which greatly improved the wear resistance compared with the substrate. Xu Peixin [8] selected cobalt-based and nickel-based alloy powders, supplemented with WC particles as reinforcement phases, and obtained composite cladding layers with high wear resistance and high oxidation resistance. Yao Shuang et al. [9] used Cr3C2 and Ti powders to prepare composite cladding layers with TiC particles as reinforcement phases generated by reaction at high temperature, which increased the hardness of the cladding layer to 2.21 times that of the H13 substrate. Qian Xingyue et al. [10] laser clad a high-performance cobalt-based alloy coating on the damaged part of H13 die steel, which significantly improved the microhardness and wear resistance of the H13 substrate. Yuan Xiao et al. [11] prepared iron-based and cobalt-based cladding layers on the surface of H13 steel, respectively. After comparative analysis, it was found that the cobalt-based cladding layer had a greater effect on improving the wear resistance of the substrate than the iron-based cladding layer.

Ultrasonic rolling technology is a surface modification technology based on severe plastic deformation, combining static extrusion and dynamic impact as rolling force [12]. The processing head of superhard materials can roll the metal surface under static extrusion. The ultrasonic energy field acts on the processing head through the ultrasonic exciter, and then acts on the metal surface, causing severe plastic deformation on the workpiece surface. Compared with traditional cold rolling, ultrasonic rolling produces a deeper surface residual compressive stress layer under low static pressure conditions [13]. Amanov et al. [14] treated the titanium alloy Ti6Al4Fe by ultrasonic rolling technology, and refined its grain size from 35.5 μm to 200 nm, and the surface hardness increased by about 1.4 times. Tan et al. [15] treated the TC17 alloy by ultrasonic rolling, and the surface roughness was reduced from 0.5~1.07 μm to 0.04~0.12 μm. The reduction in roughness reduced the friction coefficient and obtained a dense work-hardened layer. Zhao et al. [6] tested ultrasonic rolling under different process parameters on the titanium alloy Ti5Al4Mo6V2Nb1Fe, and found that the friction coefficient after ultrasonic rolling was reduced to 30% of the untreated sample. Ye et al. [16] used hard turning and ultrasonic rolling to process the samples respectively. After ultrasonic rolling, the surface roughness of the sample was reduced by 88.5%, the residual tensile stress was converted into residual compressive stress, the grain size at the top of the coating was refined, and the performance of the sample was significantly improved. Hao et al. [17] prepared Inconel 625 multilayer cladding layer on H13 die steel. Through ultrasonic rolling, its surface roughness decreased by 66%, hardness increased by 2.2 times, and high-temperature wear resistance was significantly improved.

At present, ultrasonic rolling technology has been widely used in surface treatment after surfacing, casting welding, and milling, but there are few reports on the application of high-speed laser cladding high-temperature alloy coating performance regulation. In this paper, ultrasonic rolling technology is used to strengthen and toughen the surface of high-speed laser cladding GH5188 high-temperature alloy coating, and the effect of ultrasonic rolling on the microstructure and mechanical properties of laser cladding cobalt-based high-temperature alloy coating is studied, providing a more effective process solution for H13 hot working die damage repair and surface strengthening.

1 Experiment

1.1 Coating preparation

First, the GH5188 coating is prepared using the ZKZM-6000 high-speed laser cladding equipment of Zhongke Zhongmei. The performance parameters are shown in Table 1. The system mainly consists of a semiconductor laser as a fiber-coupled continuous output laser and a coaxial center powder feeding device, and the powder feeder model is ZKZM‒DF. H13 steel with a size of 100 mm×80 mm×25 mm is selected as the substrate. The coating material uses GH5188 cobalt-based alloy powder with a particle size of 45~105 μm, and the chemical composition is shown in Table 2. Using Ar gas as the protective gas, coaxial center powder feeding is used to prepare multiple single-layer coatings on the surface of the H13 substrate.

After cladding, the coating surface is finely milled to remove impurities and oxide layers on the coating surface. Then, ultrasonic rolling treatment is performed at room temperature and repeated 3 times. Ultrasonic rolling process parameters: rolling ball diameter is 14 mm, ultrasonic vibration frequency is 28 kHz, amplitude is 10 μm, and applied static force is 400 N. The process of ultrasonic rolling is shown in Figure 1, and the processed sample is shown in Figure 2.

1.2 Microstructure and performance test

Comparative tests were conducted using a HT-1000 high-temperature friction and wear tester. Before the test, the surface defects of the wear sample were removed with sandpaper, and then cleaned with ethanol (mass fraction 75%). The test parameters were: heating to 600 °C and holding for 40 min, grinding ball rotation radius 2.5 mm, load 7 N, wear time 20 min, grinding ball diameter 5 mm, and friction ball Si3N4 material. The friction and wear sample was scanned and modeled using a DSX1000 digital microscope, and the sample was scanned with a depth of field camera to perform calculations and analysis on the wear section before and after rolling.

The cross section of the sample was ground to 1500 grit using an MTP-200 metallographic polisher, then polished with SiO2 paste and etched with aqua regia. Phase analysis was performed using an X-ray diffractometer (XRD, Bruker, D8-Advanced). The target used for the XRD test was a copper target, with a voltage of 40 kV, a current of 30 mA, and a diffraction angle of 20~100. Before observing the microstructure, each sample surface was first cleaned with ethanol (75% by mass) for 5 min using an ultrasonic cleaner, and then etched with aqua regia solution (HCl∶HNO3=3∶1) for 60 s. The etched sample was cleaned with ethanol (75% by mass) to remove residual solution and impurities. The microstructure and chemical analysis of the sample were tested and analyzed using a scanning electron microscope (SEM, Quanta 250) and an energy dispersive spectroscopy (EDS) detector.

The microhardness along the depth of the coating cross section was measured by a Vickers microhardness tester (HVS−1000) with a load of 3 N and a load holding time of 15 s. The microhardness distribution along the direction perpendicular to the coating was obtained from a set of test values. The vertical interval between adjacent test points was kept at 50 μm. Each test was repeated 3 times on the same horizontal line and the average value was taken. The possible errors in microhardness mainly come from the error in load, the control error in duration, the flatness error of the sample surface and the calculation error of the indentation area.

The electrochemical corrosion performance of the coating in NaCl solution (mass fraction of 3.5%) was analyzed by a CHI660D electrochemical workstation. A 212-type saturated calomel electrode, a 290-type platinum electrode and a cladding sample were selected as the reference electrode, auxiliary electrode and working electrode, respectively. Before the potentiodynamic polarization measurement, the open circuit potential (OCP) was recorded for 1 h to ensure stable potential, and the test time was 300 s.

2 Results and analysis

2.1 Microstructure analysis

Figure 3 shows that there is an obvious white bright area near the junction of the substrate, reflecting the good metallurgical bonding between the coating and the substrate during the cladding process. The bottom is mainly columnar crystals, and the middle and top parts are eutectic structures of columnar equiaxed crystals and cellular equiaxed crystals. Compared with the milling sample, it can be seen that under the action of ultrasonic rolling, the growth direction is toward the processing direction and finally parallel to the processing direction. The grain size of the top of the coating after rolling is significantly refined, and the cellular crystals and columnar crystals with disordered growth directions are squeezed and elongated, and the growth direction is parallel to the processing surface. In the plastic deformation area produced by ultrasonic rolling, the grain size is maintained in the range of 100~500 nm. As the depth increases, the plastic deformation effect caused by ultrasonic rolling decreases. In the non-plastic deformation area, the grain size ranges from 15 to 20 μm, with a maximum value of 30.1 μm. The grain size distribution of the coating surface is shown in Figure 4. According to calculations, the maximum depth of the nano-scale crystal layer formed on the surface parallel to the processing direction is about 18 μm. During the ultrasonic rolling process, the severe plastic deformation caused by high-frequency cyclic extrusion and rolling causes the coarse crystals to break into smaller grains. As the depth increases, the plastic strain decreases, and the degree of deformation gradually decreases from the surface, presenting a gradient deformation structure in the microstructure. In addition, it can be seen that the microstructure of the GH5188 coating near the surface is in the form of thin laths. This is because the GH5188 high-temperature alloy is a single FCC phase structure, which is prone to slip during the hardening process, resulting in a greater plastic deformation effect [18].

2.2 Phase composition

From the XRD diffraction pattern (Figure 5), it can be seen that the GH5188 coating is mainly composed of face-centered cubic γ-Co solid solution, Cr3C2 and Cr7C3. Due to the gradient melting of laser cladding and the high temperature tolerance of elements such as Ni and W, the proportion of γ-Co solid solution phase transformation during the solidification of the molten pool is reduced, forming a supersaturated solid solution rich in various alloy elements such as Cr, Ni, and W[19]. XRD analysis shows that there are more Cr3C2 and Cr7C3 carbides in the organization, and the spatial structure is hexagonal. Cr-containing carbides have the characteristics of high hardness and high temperature tolerance. In addition, W, Cr, Mn, Fe and other elements are dissolved in them to a limited extent. The alloy cementite formed has high valence, stronger covalent bonds and more stable cementite. These carbides are dispersed in the crystal and intergranular space in the form of particles [20], achieving fine grain strengthening and significantly improving hardness and wear resistance.

As can be seen from Figure 5, the diffraction peak in XRD has been significantly broadened. According to the literature [21], rolling treatment will cause the half-height width of the diffraction peak to change. Considering the relationship between grain size and half-height width of the diffraction peak, the size of nanograins can be calculated by the Scherrer formula, see formula (1) in the figure.

Where: D is the grain size, nm; K is the Scherrer constant, taken as 0.89; λ is the wavelength, nm; β is the half-height width of the diffraction peak, rad; θ is the half-diffraction angle, rad.

Table 3 shows the half-width of the crystal plane and the grain size of the sample obtained by JADE. Compared with the sample without ultrasonic rolling, the half-width of the diffraction peak of the sample after ultrasonic rolling becomes wider and the diffraction intensity peak decreases, indicating that the grain size of the surface structure of the coating is reduced and the grains in the structure are refined [22]. Figure 6 shows the element distribution diagram of the GH5188 cobalt-based high-temperature alloy coating. It can be seen that the coating elements are basically the same before and after the ultrasonic rolling process. According to the literature [23], the change in the phase structure of the metal material is attributed to the influence of the material composition and the cooling conditions of the metal solidification process. During the experiment, only the coating was physically treated and no other elements were introduced. Therefore, the application of ultrasonic rolling process has no effect on the phase composition of the coating.

3 Coating performance analysis

3.1 Microhardness

Figure 7 shows the microhardness values ​​of the H13 substrate, GH5188 coating and the coating after ultrasonic rolling. The average hardness of the GH5188 coating after fine milling is 320.12HV, which is 21% higher than the average hardness of the H13 substrate (264.68HV); the average hardness after ultrasonic rolling is 449.81HV, which is 70% higher. The main reason for the increase in hardness is that work hardening occurs during rolling due to plastic flow on the surface of the metal material. The tool head vibrates the coating surface at high frequency under a certain static load, resulting in the formation of a plastic deformation layer on the surface of the coating, and the surface structure density is strengthened. On the other hand, during the formation of the nanocrystalline layer, the grains in the layer are squeezed and stretched, thereby being refined and the strain resistance is enhanced.

Hardness can be understood as the ability of a material to resist plastic deformation. The smaller the grains in the material structure and the more grain boundaries there are, the more significant the ability to withstand plastic deformation [24]. Based on the Hall-Petch fine grain strengthening theory [13], the increase in hardness is due to the combined strengthening effect of work hardening effect and fine grain strengthening effect. The strengthening effect is given by formula (2) [25].

Where: σs is the yield strength of the material; σ0 is the friction force required to move a single dislocation; d is the grain size, nm; k is a constant. From formula (2), it can be seen that σs is inversely proportional to d. The top structure of the GH5188 coating is mainly columnar equiaxed crystals or cellular equiaxed crystals, and the growth direction is perpendicular to the direction of plastic flow. It is easier to break after high-frequency impact vibration. The grain size of GH5188 in the original sample and the size of the sample after rolling change significantly (as shown in Table 3). In the movement of crystal dislocations, smaller grains mean that their number is greater, the number of dislocation walls increases, the friction force required to overcome the deformation movement is greater, the yield strength is improved, and thus the hardness increases.

3.2 Analysis of high-temperature friction and wear behavior

Figure 8a is the FSEM image of the sample before ultrasonic rolling. It can be seen that there are fewer attachments on the wear marks of the GH5188 coating, and the scratches along the sliding direction are plow-shaped, indicating that abrasive wear occurs on the surface. As the wear time increases, the surface temperature of the sample increases, and oxidation occurs. A dense oxide film is formed on the coating surface under high temperature conditions, which can effectively reduce the friction coefficient between the coating and the friction ball, thereby improving the wear of the coating surface. Compared with the sample after ultrasonic rolling (Figure 8b), the oxide peeling area and depth of the unrolled sample increased significantly; abrasive wear resulted in more grooves, and the width and depth were more significant than those of the ultrasonic rolling sample. The grooves on the unrolled sample are always connected to the peeling area, while the grooves on the ultrasonic rolling sample are slender and independent of each other. Unlike continuous large-area peeling, the peeling area of ​​the ultrasonic rolling sample is more dispersed. This is because the microhardness of the rolled sample is higher and more uniform than that of the unrolled sample, which makes it difficult for the contact area to produce plastic deformation, thereby effectively protecting the substrate and improving the wear resistance of the coating.

The friction and wear sample was scanned in three dimensions using a DSX1000 digital microscope, and the wear volume before and after ultrasonic rolling was quantitatively analyzed. The three-dimensional profile scanning results are shown in Figure 9. The cross-sectional area of ​​GH5188 before rolling is 4909.31 μm2 and the cross-sectional area after rolling is 3024.06 μm2. The volume formula V=S×L can be used to calculate L=25133 μm (where the friction radius φ is 4 mm). The wear volume is shown in Figure 10. Compared with the H13 substrate, the wear resistance of the unrolled coating is increased by 69%. The ultrasonic rolling process reduces the wear of the coating by 34%. The wear resistance of the rolled coating is 81% higher than that of the substrate.

Figure 11 shows the surface roughness before and after ultrasonic rolling. After ultrasonic rolling, the surface roughness of GH5188 is reduced by 58%. This is because under the high-frequency vibration of the tool head, plastic flow occurs on the surface of the metal material, which makes the uneven surface achieve the effect of “cutting peaks and filling valleys”, thereby reducing the roughness value of the coating surface. The plastic flow of metal materials is conducive to obtaining ultrafine grains and increasing dislocation density. Dislocation slip, accumulation and rearrangement can lead to an increase in small-angle grain boundaries, thereby improving the mechanical properties of the material surface, such as microhardness and wear resistance.

As shown in Figure 12, the average friction coefficient of GH5188 before rolling is 0.39, and the average friction coefficient after ultrasonic rolling is 0.35, which is 19% and 27% lower than the friction coefficient of the H13 substrate (0.48), respectively. After ultrasonic rolling, the GH5188 coating shows better friction and wear properties.

The reason why the ultrasonic rolling process can improve the wear resistance of the coating is that during the ultrasonic rolling process, the ultrasonic vibration drives the carbide indenter to vibrate at high frequency on the surface of the sample. After the surface and subsurface are squeezed by high-frequency vibration, the grains are deformed and refined, causing the surface structure morphology and microstructure to change; under the combined effect of fine grain strengthening and work hardening, the surface hardness of the coating is increased and the friction coefficient is reduced. XRD and SEM analysis show that the improvement of high temperature friction resistance of GH5188 coating is mainly due to solid solution strengthening, supplemented by carbide dispersion strengthening; Mn, W and other elements in the alloy are dissolved between cellular equiaxed crystals and eutectic structures to achieve solid solution strengthening; in addition, Cr-containing carbides have high hardness and good high temperature tolerance, and are dispersed between grains, which improves the high temperature wear resistance of GH5188 alloy coating.

3.3 Corrosion behavior

Figure 13 is the potentiodynamic polarization curve of the sample, and Table 4 lists the values ​​of self-corrosion potential Ecorr and self-corrosion current density Jcorr measured from the Tafel region. Smaller Jcorr and relatively positive Ecorr reaction have stronger corrosion resistance. Among them, Jcorr is determined by deriving the linear part of the Tafel extrapolated
polarization curve. Therefore, Jcorr is the most accurate indicator for evaluating the corrosion resistance of the sample. Smaller Jcorr means stronger corrosion resistance. When the polarization potential exceeds +0.3 V, the corrosion current density fluctuates significantly and increases rapidly, mainly because the passive film on the coating begins to break. The GH5188 cobalt-based coating has a relatively small self-corrosion current density (6.49×10‒6 A/cm2) and a relatively positive self-corrosion potential (−0.374 V), indicating that the material dissolution rate of the GH5188 coating is small in the active dissolution stage and has excellent corrosion resistance.

The data in Table 4 show that compared with the H13 substrate, the corrosion rate of the unrolled coating is reduced by 12%, and the corrosion rate of the coating after ultrasonic rolling is reduced by 17%, indicating that the ultrasonic rolling process can improve the electrochemical corrosion resistance of the coating. The impact and extrusion generated by the working head during the ultrasonic rolling process cause plastic deformation on the coating surface, obtain a gradient nanocrystalline layer, and reduce the surface roughness. According to Fick’s first law, see formula (3).

Where: J is the amount of diffusion through the surface per unit time; A is the area of ​​the diffusion channel; d/dmt is the diffusion rate; “-” indicates the diffusion direction, from high concentration to low concentration; D is the diffusion coefficient; d/dCx is the concentration gradient. In the same medium, the diffusion coefficient D is the same, and the diffusion rate d/dmt is related to the area A and the concentration gradient d/dCx. Assuming that the electrolyte solution is the same in the experiment, the diffusion mode is steady-state diffusion, and the concentration remains uniform, the diffusion rate is only related to the surface area A. The greater the roughness, the more wrinkles on the surface, the larger the area exposed to the conductive medium, the faster the diffusion rate of Cl- in the electrolyte solution, and the more prone to pitting corrosion of the electrochemical working electrode. According to the literature [17], plastic deformation of the metal surface will accelerate the diffusion of oxygen into the metal and form an oxide film. Compared with the unrolled sample, it is obvious that the passivation film in the oxygen-rich area is thicker, which improves the electrochemical corrosion resistance.

4 Conclusions

1) Ultrasonic rolling causes the originally chaotic grain growth direction of the top to tend to the direction of plastic flow on the coating surface, and the grain growth direction of the surface and subsurface is parallel to the rolling direction, and the grain size is refined. The coarse columnar crystals and cellular equiaxed crystals in GH5188 are crushed into fine equiaxed crystals after extrusion, presenting a thin lath shape.

2) Compared with the H13 substrate, the surface microhardness of the coating without rolling increased by 2%, and the surface microhardness of the coating after ultrasonic rolling increased by 70%. The increase in hardness is attributed to the fine grain strengthening, which brings greater yield strength and bears more dislocation strain, which plays an important role in improving its fatigue resistance and wear resistance.

3) The phase strengthening mechanism of solid solution strengthening and carbide strengthening makes the GH5188 coating have excellent high temperature friction and wear resistance. Compared with the H13 substrate, the wear resistance of the GH5188 coating before and after rolling increased by 69% and 81%, respectively. After ultrasonic rolling, the surface roughness of the GH5188 coating was reduced by 58%, and a nanocrystalline layer with a thickness of 18 μm was obtained on the surface of the cladding layer.

4) After ultrasonic rolling treatment, the corrosion resistance of the GH5188 high-temperature alloy coating in 3.5% NaCl solution increased by 12%, and increased by 17% compared with the H13 substrate. The nanocrystalline surface formed by ultrasonic rolling has a thicker passivation film and lower roughness, which improves the electrochemical corrosion resistance of the coating.