Study on mechanical properties of nickel-based tungsten carbide cladding layer based on plasma cladding process

The Ni60+60%WC cladding layer was prepared on the surface of Q355 steel by plasma cladding process under different cladding currents. Through a metallographic microscope, microhardness test, shear strength test, and friction and wear test, the relationship between solidification structure and mechanical properties of Ni60+60%WC cladding layer was analyzed. Through experiments, it is concluded that when the current is 150-170 A, there is a good metallurgical bond between the cladding layer and matrix, and the bonding effect improves with the increase of the cladding current. The metallurgical bonding effect is poor when the current is 130 A or 140 A. When the cladding current is high, the hard tungsten carbide particles in the cladding layer show varying degrees of sinking phenomenon. Simultaneously, with the escalation of the cladding current, the tungsten carbide (WC) particles within the cladding layer decrease in size, and the ablation phenomenon becomes more obvious. When the cladding current is 130-160, the hardness of the obtained cladding layer increases to varying degrees compared to the matrix, with the highest hardness obtained with a current of 160 A. With an increasing distance from the cladding layer surface, there is an observed trend of hardness initially increasing and subsequently decreasing. In addition, as the cladding current goes up, the shear strength of the cladding layer goes down. The trend of abrasive resistance of cladding layers is the same as that of hardness. The abrasive resistance of the cladding layer increases with the increase of hardness.


Introduction
With the development of modern industry, many parts of construction machinery are often subjected to the compound superposition of various wear forms in the use process, and most mechanical parts will fail because of serious friction and wear.Then, they cannot continue to work [1] .Q235, Q355, and 16Mn steels are often used as manufacturing materials, but their abrasive resistance is low.To improve the abrasive resistance of this kind of steel, we improve the surface quality of parts, prolong the service life of parts, and reduce the processing cost.Plasma spraying, laser cladding, and plasma cladding are widely used at home and abroad.The alloy cladding layer, formed through plasma spraying, becomes mechanically bonded to the base metal, leading to a propensity for the detachment of hard particles and resulting in a relatively high powder wastage rate.Laser cladding is prone to crack defects due to its high processing power, concentrated energy, and large residual stress [2] .Based on this, using the above two surface treatment processes is limited.The plasma cladding technology compresses the plasma-free arc into a compressed arc with higher temperature, energy density, and ionization degree through mechanical, thermal, and magnetic compression effects.It uses this as a heating source to melt the alloy powder or welding wire and the surface of the matrix at the same time to form a molten pool.A surfacing layer characterized by excellent metallurgical bonding with the matrix is formed during the subsequent cooling process.Because of the swift heating and rapid cooling inherent in plasma cladding, heat becomes concentrated within a brief timeframe.This results in a cladding zone with a small heat-affected area and low stress.Simultaneously, the dilution rate between the cladding layer and the base metal is small, and there is a metallurgical bond between the two, so it has been increasingly widely used in practical engineering [3] .
Tungsten carbide possesses outstanding attributes, encompassing excellent corrosion and abrasion resistance, high hardness, exceptional thermal shock resistance, a high elastic modulus, robust oxidation resistance, a low coefficient of expansion, and various other advantageous qualities.Its wettability with nickel-based alloys is good.The cladding layer made by adding WC particles to nickel-based selffluxing alloy powder has the advantages of high abrasive resistance, hardness, and corrosion resistance.It is the best material to improve the performance of parts and components to make the cladding layer.Many scholars at home and abroad have reported the preparation of plasma cladding nickel-based tungsten carbide alloy cladding layer.For example, Peng et al. [4] used plasma cladding technology to clad WC/Ni-based composite coating on H13 die steel.The results show that the coating structure is dendritic, which is beneficial to improve the wear resistance and corrosion resistance of the cladding layer.Artur [5] prepared tungsten carbide particle reinforced nickel-based alloy coating on AISI 4715 by plasma transfer arc welding technology.The results show that the Ni-based cladding layer with uniformly distributed WC is formed, and the impact resistance is greatly improved, but its wear resistance is reduced.Ding Ziyang et al. [6] studied the influence of WC content on plasma cladding Nibased cladding layer.The experiment showed that with the increase of WC content, the macromorphology of cladding layer became worse, but its wear resistance and hardness gradually increased.In actual production, the WC content should be considered comprehensively to achieve the balance performance requirements.Mendez et al. [7] obtained that the main process of Ni-WC powder system is PTAW by studying the cladding material system and cladding process.In this process, the overheating effect of the molten pool is low, and the melting of hard particles is avoided.However, the plasma cladding process for high content tungsten carbide nickel-based alloy powder has not been reported yet.In this study, plasma cladding technology was employed to fabricate a nickel-based alloy cladding layer with a tungsten carbide content of 60% on the surface of Q355 steel.This study investigated the mechanical properties, including abrasive resistance, hardness, and adhesion, of tungsten carbide hard alloy cladding layers under varying current densities, providing practical guidance for the development of plasma cladding technology for high wear resistant alloy cladding layers.

Experimental materials
The base material used in this test is Q355 steel, and the cladding material included Ni60 powder alloy powder.The main chemical components are shown in Table 1.The cast tungsten carbide selected is 60% tungsten carbide alloy powder, and its main chemical composition is shown in Table 2.The particle size of cast tungsten carbide is 44-149 μm, the theoretical hardness of the welding layer matrix is HRC=54, and the hardness of hard tungsten carbide particles is HV0.1=2217.3.92 0.12 0.025

Sample preparation
The plasma cladding equipment adopted the LU-400TX-BW500RV-ROBOT20 plasma cladding machine, and the powder feeding method adopted synchronous coaxial powder feeding and single-pass cladding.The optimized preparation process parameters for the nickel-based tungsten carbide cladding layer were nonarc current 36.6 A, plasma cladding current 130-140-150-160-170 A, powder feeding speed 26 g/min, and powder feeding gas flow rate 6.5 L/min.Using high-purity nitrogen as the protective gas, the flow rate of the protective gas was 12.0 L/min, the distance between the laser nozzle and the workpiece surface was 5-10 mm, and the thickness of the cladding layer measured between 2.5 mm and 3 mm.The test was divided into five groups: A, B, C, D, and E. Apart from varying cladding currents, all other process parameters remained constant.Table 3 shows the experimental parameters used by each experimental group.

Analysis method of the cladding layer
After cladding was completed, the sample was cut into metallographic samples of 10 mm×10 mm×10 mm by WEDM equipment.The metallographic samples were polished with 80, 120, 180, 240, 320, 400, 600, 800, 1000, 1200, and 1500 mesh sandpaper until the surface of the samples was smooth and scratch free, and then polished to a mirror surface.Subsequently, the specimens were subjected to a 15-second etching process using aqua regia, composed of concentrated nitric acid and hydrochloric acid in a 1:3 ratio.The microscopic structure of the samples under different cladding currents was observed by Zeiss metallographic microscope and scanning electron microscope, and the macro-morphology of the cladding layer was observed.We compared the differences in macroscopic morphology of cladding layers under different cladding currents and analyzed the influence of different cladding currents on the formability, porosity, cracks, and other morphologies of the cladding layer.The samples were tested for Rockwell hardness by an HR-150A hardness tester, and the average spacing between two hardness testing points was 0.5 mm.Between the two hardness test points, a test point was randomly selected as the third hardness test point again for the hardness test.On this basis, the microhardness of the longitudinal section of cladding samples under different currents was measured.The hardness value of each point measured by each sample is resulted from three parallel measurements at this level, and the average value was obtained after summation.The shear strength test was tested on a universal testing machine; the wear test was carried out on a BD4603 wear test machine.The diameter of the wear surface of the sample is d=15.5 mm, and the Korean deer brand DEERFSO YA531+ model abrasive belt was used.The size is 100×912, the mesh number is 80, the test temperature is room temperature, the loading mass is 255 g, the rotational speed is 2980 r/min, and the wear time is 60 min.Figure 1 shows the macroscopic morphology of the cladding layer under different cladding currents.As can be concluded from the figure, when the current was small, the surface of the cladding layer was brighter, and there were fewer cracks.With the increase of cladding current, gray scum and cracks appeared gradually on the surface of the cladding layer.This might be due to the higher energy input at higher currents, resulting in hard tungsten carbide particles melting.Excessive temperature led to heightened fluidity of the molten pool, causing the dispersion of molten tungsten carbide within it.As a result, a significant amount of scum was formed during the subsequent cooling process.Under the action of a large current, the temperature gradient in the molten pool became larger after cladding, and the surface of the cladding layer after cladding contacts with air to produce a rapid cooling effect, the stress became larger, and more cracks were generated under the action of larger stress [6] .As shown in Figure 1, when the current was 160 A and 170 A, the crack ran through the entire cladding layer, and the appearance was poor. Figure 2 shows the micro-morphology of the cladding layer under different cladding currents.As can be concluded from Figure Ⅱ, when the current was 130 A and 140 A, the tungsten carbide particles were relatively completed and evenly distributed, and the sinking trend of tungsten carbide particles was not obvious.Still, there were many pore defects in the cladding layer.When the cladding current was 150-170 A, during the cladding process, the temperature at the top of the cladding layer was high, and the tungsten carbide particles were more likely to melt, resulting in fewer tungsten carbide particles on the top of the cladding layer [8,9] .When the current reached 150 A, it can be concluded that the tungsten carbide hard particles sank, and more defects appeared on the top of the cladding layer.When the current reached 160 A, the tungsten carbide hard particles sank more obviously, and the defects existing at the top of the cladding layer were arranged in a row and ran through the top of the whole cladding layer.When the current was 170 A, there were fewer tungsten carbide hard particles in the cladding layer, and the tungsten carbide particles were smaller.This may be due to excessive energy input; many tungsten carbide particles were severely ablated, and many of them were melted.During the cooling process of the cladding layer, a small amount of fine secondary precipitated tungsten carbide alloy particles was generated, and the cladding effect was not ideal.When the current rose to 150 A, the tungsten carbide hard particles in each sample cladding layer had a certain degree of subsidence; this might be because the density of tungsten carbide is higher than that of nickel (WC is about 15.7 g/cm 3 and Ni is about 8.4 g/cm 3 ), and the greater the current is, the more energy would be input, resulting in the better the fluidity of the molten pool, the more obvious the phenomenon of tungsten carbide particles sinking is [8] .Figure 3 shows the metallographic diagram of the fusion zone of the cladding layer and the matrix under different currents.As depicted in Figure 3, when the cladding current reached 130 A and 140 A, microcracks became evident at the interface, suggesting a suboptimal metallurgical bonding between the cladding layer and the matrix, which might be because the input heat was less under the condition of small current density, which was not conducive to the penetration of the cladding layer by plasma arc.When the current was 150 A, a fusion zone appeared on the contact surface, indicating that the cladding layer and the matrix were metallurgically bonded and had good adhesion.When the current was 160 A, the fusion zone was wider and clearer, and the metallurgical bonding effect reached its best.When the cladding current increased to 170 A, although the cladding and matrix were metallurgically bonded, it could be seen that there were fine tungsten carbide particles on the contact surface of the cladding layer and the matrix, which might have led to the decrease of the adhesion between the cladding layer and the matrix, which was not conducive to the improvement of the cladding layer performance.

Influence of cladding current on macroscopic and microscopic morphology of cladding layer
By analyzing the macro-morphology and microstructure of the cladding layer, a conclusion could be drawn that the macro-morphology and microstructure of the cladding layer showed opposite changes.With the increase of cladding current, the macroscopic morphology of the cladding layer gradually became rough, and the sinking phenomenon of tungsten carbide hard particles was more obvious.Still, the metallurgical bonding effect of the microstructure was better, so the cladding layer could not easily fall off.Figure 4 illustrates the hardness distribution within the cladding layer when exposed to varying cladding currents.As depicted in the figure, the hardness within each cladding layer exhibited an initial increase followed by a subsequent decrease as the distance from the cladding layer surface extended across five different cladding current settings, and various samples indicated characteristics: the surface of the cladding layer exhibited lower hardness, the middle section demonstrated higher hardness, while the bottom part showcased lower hardness levels [10] .When the current was between 130 and 150 A, as the cladding current rose, there was an upward trajectory in the hardness of the cladding layer, but the increased range was small.When the cladding current was 160 A, the hardness value was the largest.When the cladding current reached 170 A, the hardness decreased significantly, which was the minimum hardness of the sample among the five cladding current processes.Additionally, it is evident from Figure IV that under different cladding currents, the maximum hardness of the cladding layer appeared in different positions.When the cladding current ranged from 130 A to 150 A, the maximum hardness was observed in the middle and upper sections of the cladding layer.When the cladding current was 160-170 A, the maximum hardness was observed in the middle and lower regions of the cladding layer, possibly attributed to the varying distribution of tungsten carbide within the cladding layer.In conjunction with the microstructure of the cladding layer, it became evident that when the cladding current was set at 130 A or 140 A, tungsten carbide was uniformly dispersed in the cladding layer.There was no obvious sinking phenomenon, but there were more defects in the cladding layer, which hindered the increase of the hardness of the cladding layer, resulting in only a small increase in hardness.Small energy input was not conducive to plasma arc penetrating the cladding layer, so the hardness was relatively small at this time.When the cladding current was 150 A, the tungsten carbide sank obviously, which led to the change in the position of the maximum hardness of the cladding layer.Compared to the cladding current of 130-140 A, when the cladding current was 150 A, the tungsten carbide particles within the cladding layer were finer and were distributed throughout the central and lower regions of the cladding stratum, playing a dispersion strengthening role.Meanwhile, the smaller tungsten carbide in the cladding layer could prevent grain growth and refine the cladding layer structure [11] .When the cladding current was 160 A, the cladding layer hardness reached its maximum value.A larger energy input could decompose more tungsten carbide, and the decomposed W atoms and C atoms can form more high hardness compounds with the alloy elements released by Ni based melting in the molten pool, which promoted the improvement of the hardness of the cladding layer.Elements such as W, C, and Si, possessing higher concentrations within the molten pool, could dissolve into the Ni base.This dissolution process created an γ-Ni solid solution, effectively contributing to the reinforcement of the material via the mechanism of solid solution strengthening [5] .Considering the microscopic morphology, it was observed that at a cladding current of 160 A, tungsten carbide particles were distributed within the middle and lower sections of the cladding layer in a clustered configuration, which helped to increase the hardness of the cladding layer.When the cladding current was 170 A, combined with the metallographic structure, a large amount of tungsten carbide was decomposed under the action of the high-density current, and granular tungsten carbide particles appeared in the cladding layer.Meanwhile, higher current levels intensified the heating effect on the alloy powder because the arc directly heated the material.This, in turn, led to a significant ablation of a substantial quantity of alloy elements within the molten pool, ultimately resulting in a pronounced reduction in the hardness of the cladding layer.
For the samples after the above five kinds of current cladding, the hardness of the top and bottom of the cladding layer was low because the top of the cladding layer was affected by the direct plasma arc, and the alloy elements were ablated, resulting in a reduction in the hardness of cladding layer top.Under the cladding current, the matrix diluted the cladding layer, which made the hardness at the bottom of the cladding layer lower than that at other parts of the cladding layer [12] .4 shows the shear strength of the cladding layer under different cladding currents.As can be concluded from Table 4, when the cladding current increased, the shear strength of the cladding layer decreased.Combined with the macro-morphology and microstructure of the cladding layer after different cladding currents, it can be seen that the cracks on the surface of the cladding layer appeared with the increase of cladding current, and at the same time, there were more defects in the microstructure.When the cladding current reached 160 A or 170 A, the cracks ran through the entire cladding layer surface, causing the material to break along the cracks when it was sheared.When the cladding current was 170 A, the shear strength of the cladding layer was the lowest because there were many long cracks on the surface of the cladding layer under this current.There were fine tungsten carbide hard particles on the contact surface of the cladding layer and the matrix.This led to the lower bond strength between the cladding layer and matrix, resulting in lower mechanical properties and poor shear strength.The shear strength of the cladding layer was partially reliant on the hardness and rigidity of the cladding itself.Nevertheless, considering their influence on shear strength, the impact stemming from the macromorphology and micro-defects within the cladding layer could not be disregarded.Figure 5 and Figure 6 are graphs of the total wear amount and the wear amount per unit time of the cladding layer under different cladding currents, respectively as can be concluded from Figure 5 that the change trend of total wear of cladding layers treated by different cladding currents could be explained by hardness.Furthermore, there existed a correlation between hardness and the overall wear reduction.As hardness is elevated, the total wear diminishes.This inverse relationship signified that heightened hardness levels correspond to decreased total wear, thereby enhancing the wear-resistant characteristics of the cladding layer.To some extent, the enhancement of abrasive resistance relied on the morphology and distribution of tungsten carbide within the cladding layer.Combined with the microstructure diagram, it could be seen that when the current was in the range of 130 A-160 A, more tungsten carbide decomposed with the increase of the cladding current, forming smaller particle size tungsten carbide particles, which were dispersed and distributed within the cladding layer, playing a role in dispersion strengthening.The microscopic diagram showed that when the current reached 150 A, there were obvious melting marks around the tungsten carbide particles, which was beneficial to the bonding between the tungsten carbide particles and the cladding layer.In the friction and wear test, the tungsten carbide particles played a role as a wear-resistant skeleton, improving the abrasive resistance of the cladding layer.When the cladding current was 160 A, tungsten carbide particles were larger and finer, resulting in the best abrasive resistance.When the cladding current was 170 A, abundant tungsten carbide particles were ablated, and only a few granular secondary precipitated tungsten carbide alloy particles were retained in the cladding layer.Moreover, in the cladding layer under 170 A current cladding, there were fine tungsten carbide particles on the contact surface of the cladding layer and the matrix, which led to the lower bond strength between the cladding layer and the matrix, which was not conducive to the improvement of the abrasive resistance of the cladding layer, and the abrasive resistance was the worst.

Effect of cladding current on abrasive resistance of cladding layer
From Figure 6, it can be seen that the five samples treated with different cladding currents had relatively large unit wear during the initial wear stage.In contrast, the unit wear gradually flattened out in the subsequent wear stage.When the cladding current was 130 or 140 A, lower energy inputted, as indicated by the cladding current, led to insufficient penetration of the plasma arc, resulting in inadequate bonding strength between tungsten carbide particles and the matrix.This, in turn, led to weak adhesion between the cladding layer and the matrix, rendering the cladding layer susceptible to wear.However, due to the relatively bright macroscopic morphology of the cladding layer during 130 or 140 A current melting, there was no dross generation, and its surface friction coefficient was low.Therefore, the unit wear amount was relatively low during the initial wear stage.When the cladding current was 150-170 A, due to the poor macroscopic morphology of the cladding layer and the presence of a large amount of scum, the surface friction coefficient of the cladding layer was high.Based on the analysis of the hardness of various cladding layers, it was observed that the surface hardness of cladding layers obtained at different melting currents was relatively low, thus posing limitations in enhancing abrasive resistance.Based on the above two reasons, when the cladding current was 150-170 A, the unit wear during the initial wear stage was relatively high.
After the initial wear stage, the cladding layers under different melting currents entered a stable wear stage.When the cladding current was 130 A or 140 A, based on the microstructure of tungsten carbide and the poor penetration of plasma arc into the cladding layer, during the stable wear stage, the bonding strength between tungsten carbide particles and the matrix was insufficient, and tungsten carbide particles detach during the wear process.The softer matrix was exposed in the wear environment, exacerbating the wear amount.When the cladding current was 150 A or 160 A, the bonding strength between the tungsten carbide particles in the cladding layer and the matrix was relatively high.Tungsten carbide provided a good "protective effect" on the matrix.Simultaneously, a softer matrix facilitated a beneficial "supporting effect" on the hard tungsten carbide particles, impeding their detachment.Consequently, this significantly enhanced the overall abrasion resistance of the cladding layer, with a lower unit wear amount [10] .When the cladding current was 170 A, both the tungsten carbide and the matrix underwent severe ablation.Excessive current density led to excessive energy input, resulting in an excessively high dilution rate of the matrix to the melt pool, greatly reducing the hardness of the cladding layer.Meanwhile, due to small tungsten carbide particles on the contact surface of the cladding layer and the matrix, the bond strength between the cladding layer and the matrix was weak, and the cladding layer was prone to detachment under continuous wear.Therefore, the cladding layer obtained with a cladding current of 170 A had a higher unit wear amount during the stable wear stage.

Conclusions
This dissertation studied the influence of different cladding current densities on the properties of 60% tungsten carbide powder materials.It was found that low current led to poor cladding effect and weak bonding strength with the matrix.High current could lead to fine secondary tungsten carbide alloy precipitation, reducing abrasive resistance and the hardness of the cladding layer.The specific conclusions are as follows: a) At cladding currents of 130 A or 140 A, the sinking phenomenon of tungsten carbide particles was absent, and the distribution of these particles appeared to be relatively uniform.However, the bond strength of the cladding layer and the matrix was weak, and there was a phenomenon that the plasma arc could not penetrate the cladding layer well.When the cladding current was 150 or 160 A, there was a significant sinking phenomenon of tungsten carbide particles in the cladding layer, and there were obvious melting marks around the tungsten carbide particles.The bond strength between the cladding layer and the matrix was high, and the tungsten carbide particles were small.When the current was 170 A, the alloy elements in the cladding layer were seriously ablated, fine secondary tungsten carbide alloy particles were precipitated in the cladding layer, and granular tungsten carbide existed between the cladding layer and the matrix, which led to the decrease of the bonding strength between the cladding layer and the matrix.b) When the cladding current was between 130-160 A, the microhardness of the cladding layer showed a gradual increase.When the cladding current was 160 A, the cladding layer hardness reached the best.When the cladding current was 170 A, the cladding layer hardness was the lowest.c) As the cladding current increased, the shear strength of the cladding layer exhibited a downward trend.The shear of the cladding layer depended to some extent on the morphology and distribution of tungsten carbide in the cladding layer.Still, the influence of the macro morphology of the cladding layer on the shear strength also needs to be considered.
d) The abrasive resistance of the cladding layer followed the trend of hardness change.The higher the hardness is, the better the abrasive resistance of the cladding layer is.When the cladding current was 130 or 140 A, the unit wear amount was relatively low during the initial wear stage.When the cladding current was 150-170 A, the initial wear stage had a higher unit wear amount.When the current was 150 or 160 A in the stable wear stage, tungsten carbide hard particles played a good role as a wear-resistant skeleton, and the unit wear amount was relatively low.When the current was 130 or 140 A, the unit wear was higher.When the cladding current was 170 A, the friction and wear performance of the cladding layer was the worst.

Table 3 .
Experimental parameters of different experimental groups.

Table 4 .
Effect of different cladding currents on shear strength of cladding layers.