Microstructure and Properties of CoCrFeNiMnTi x High-Entropy Alloy Coated by Laser Cladding

: In this study, laser cladding of CoCrFeNiMnTi x ( x is the proportion of the mass of a material, x = 0.0, 0.2, 0.4, 0.6, 0.8) high-entropy alloy (HEA) composite powder coating on 45 steel substrate was studied by using the method of preplaced powder. The phase composition, morphology, microhardness, corrosion resistance and wear properties of CoCrFeNiMnTi x high entropy alloy were analyzed by XRD, SEM, microhardness tester, electrochemical workstation and reciprocating friction wear tester, respectively. The influences of Ti concentration on structure and properties of CoCrFeNiMn HEA laser cladding coating were discussed. The macromorphology of CoCrFeNiMnTi x HEA coating layer becomes worse with the increase in Ti quantity. The coating layer is a face-centered cubic solid solution phase. The microstructure of the coating layer is dominated by dendrites and equiaxed crystals. The average microhardness of the coating layer grows with the increases in Ti content, and CoCrFeNiMnTi 0.8 can reach 823 HV. The friction coefficient of the cladding coating gradually reduces and the wear resistance adds as Ti content rises; the friction coefficients of CoCrFeNiMnTi 0.6 and CoCrFeNiMnTi 0.8 cladding coating are similar, at 0.835 and 0.828, respectively. Adhesive and abrasive wear are the two basic types of cladding coating wear. In 3.5 wt.% NaCl solution, the corrosion potential of cladding coating increases with increases in Ti content, the corrosion potential of CoCrFeNiMnTi 0.8 is about 244 mV higher than that of CoCrFeNiMnTi 0 , and the density of corrosion current drops to 3.41 × 10 − 6 A/cm 2 from 7.17 × 10 − 5 A/cm 2 .


Introduction
Wear and corrosion are identified as the primary causes of mechanical parts failures in the process of their working, and approximately 80% of mechanical parts failures are attributed to their surface wear and corrosion [1].The 45 steel is a widely used material in the modern industry, with uses in machines (spindles, gears, shafts, connecting rods), vehicles (piston rings, transmission shafts, brake pads), aerospace (engines, crankshafts, rams), and chain sprocket wheels, because of its strong and resistant to deformation mechanical qualities [2,3].Nevertheless, 45 steel's application under wear, friction and corrosion conditions is seriously restricted due to its low surface hardness, low corrosion resistance and poor abrasion resistance performance [4].Manufacturing mechanical parts with high-performance materials resistant to wear and corrosion entails higher costs.Thus, the preparation of a wear-resistant and corrosion-resistant cladding layer on conventional 45 steel surfaces with high-performance alloy powder presents an effective solution to this issue.The use of diverse surface alteration methods with different cladding alloy powders allows an economic material with a protective layer for improving surface properties and functionality [5].The traditional cladding alloy powder materials typically involve nickel-based compositions with added reinforcing phases such as TiC [6], WC [7], Al 2 O 3 [8], and SiC [9].Yeh introduced HEA where the liquid or random solid solution states compared to traditional alloys, have noticeably greater mixing entropies [10].An alloy comprising five to thirteen primary elements and a molar ratio of 5% to 35% for each element is known as HEA [11].Compared to other alloys, HEAs offer a series of advantages including strong wear resistance [12][13][14], outstanding corrosion resistance [15], and superior resistance to high-temperature oxidation [16], which have garnered increasing attention from researchers.Rakesh et al. [17] fabricated two new mechanically alloyed AlCoCrFeMoW and AlCoCrFeMoV coatings.Low pores and a precise microstructure were displayed by HEA coatings.Microhardness results revealed that the maximum mean hardness of around 561 ± 63 HV was attained by AlCoCrFeMoV HEA coatings.Yang et al. [18] sintered the FeNiCrCoMoW coatings by plasma surface alloying technology with different holding temperatures (1150 • C, 1200 • C, 1250 • C, 1300 • C) and studied the microstructure morphology evolution of HEA coatings.All FeNiCrCoMoW coatings consist of two layers: one for deposition and the other for diffusion, creating a gradient structure HEA coating.The phase of FeNiCrCoMoW HEA coatings is entirely composed of FCC phase + HCP phase.The FeNiCrCoMoW HEA coating has good application prospects.
Laser cladding technology offers a method to form a high-performance cladding alloy layer, which utilizes high-density energy laser beams to melt cladding materials onto the surface of substrates.In comparison to other surface cladding techniques like flame spraying and plasma cladding, laser cladding technology presents several advantages such as a smaller heat-affected zone, lower coating dilution rate, formation of alloy bonding with the substrate, dense microstructure, and a wide range of applications [19][20][21], leading to its increasingly widespread use.Lv et al. [22] produced FeCoNiCrAl coatings on Q235 steel substrate by using the laser cladding process.The steel surface's resistance to wear is enhanced.FeCoNiCrBSi coating on the surface of a stainless-steel exhibits exceptional resistance to corrosion [23].Liu et al. [24] discussed the relationship between the frictional properties and alloy material element design, secondary phase doping, and process treatment.Meanwhile, Zhou et al. [25] have elaborated on the effects of elements, laser processing technology, post-treatment, and service temperature on the resistance to corrosion.Both studies suggest that the addition of specific elements to a certain HEA system can effectively enhance its wear resistance and corrosion resistance.The addition of small amounts of other elements in HEA systems can induce lattice distortion, thereby enhancing the comprehensive performance of the alloy [26][27][28][29].Excellent metallurgical bonding surfaces between the substrates and the Al-modified FeCoNiTiAl x and FeCoCrNiAl x coatings were observed [30,31].The microhardness of CoCrFeNINb x first grew remarkably with an increase in Nb concentration, and it thereafter remained high [32].Nevertheless, there has been little research done to far on the impact of the Ti element on HEA coatings, and the underlying mechanisms remain unclear.It is essential to carry out methodical investigation and research.
Based on this, in this study, the new CoCrFeNiMnTi x (x = 0, 0.2, 0.4, 0.6, 0.8) HEA was designed, and prepared on 45 steel substrates by using the pre-placed powder laser cladding process.The influence of Ti addition on the microstructure, friction resistance and corrosion properties of CoCrFeNiMnTi x coatings were investigated systematically.Our objective is to produce HEA coatings and enhance the 45 steel's resilience to corrosion and surface wear, hence increasing the material's use in a variety of technical applications.

Experiment Materials
The 45 steel with dimensions of 110 mm × 110 mm × 10 mm was chosen as an instance of the substrate.The substrate sample surface was prepared by sandblasting to remove rust and impurities, followed by polishing with 200#, 400#, 600#, 800#, and 1000# grit sandpaper to achieve a smooth surface.Subsequently, the surface was wiped clean with anhydrous ethanol and air-dried for further processing.CoCrFeNiMn alloy powder (purity 99.9%, particle size 250 mesh) and pure Ti powder (purity 99.9%, particle size 250 mesh) were mixed according to the molar ratios using a planetary ball mill to ensure uniform blending.The powder ratios are listed in Table 1.

Experiment Method
The mixed powder of CoCrFeNiMnTi x was prepared into a paste-like consistency by using a 5% polyvinyl alcohol solution.The paste mixed alloy powder was preplaced onto the surface of the substrate by using a 1 mm thick acrylic template.Then, the substrate samples with preplaced powder were dried at 60 • C for 24 h in preparation for laser cladding.The laser cladding process was carried out using a laser cladding system with the model of RFL-4000 manufactured by Raycus of China (Wuhan Raycus Fiber Laser Technologies Co., Ltd., Wuhan, China).The optimal laser cladding process parameters for the CoCrFeNiMnTi x coating were determined based on the macroscopic quality of the cladding coating surface.The specific process parameters of laser cladding are listed in Table 2.After laser cladding, single and multi-pass cladding samples were cut into different sizes using an electrical discharge wire cutting machine (Model of EDM 450, Suzhou Baoma Numerical Control Equipment Co., Ltd., Suzhou, China) for various performance tests and characterizations.Then, the main tests and characterization were as follows: (1) The samples were ground using sandpaper, grinding started at 180#, and then proceeded at 400#, 800#, and 2000# sandpaper.Then they were polished using a 0.5 µm aluminum oxide suspension, and then etched by a 4% hydrofluoric acid solution.Then the microstructure of sample was observed by SEM (SEM, Model of SEM3200, CIQTEK Co., Ltd., Hefei, China), and composition analysis was performed using the energy spectrometer of Oxford Explorer 30 (Oxford Instruments, Oxford, UK).
(2) The phase of the HEA cladding coating was checked with an X-ray diffractometer produced by Bruker in the Oberkochen, Germany.
(3) Microhardness of the cladding coating was measured using the HXS-1000AY Vickers hardness tester (Shanghai Haowei Optoelectronics Technology Co., Shanghai, China).Ten seconds was the holding time, and the load weighed 0.2 kg.Measurements were taken at intervals of 0.1 mm.Four sets of measurement data were collected in the same direction of each sample, and the mean value of each set was calculated as the final values of corresponding sample's microhardness.
(4) The GF-I reciprocating friction and wear tester, made by Lanzhou Zhongke Kaihua Technology Development Co., Ltd.(Lanzhou, China), was used to perform the friction and wear tests on the cladding coating.In this experiment, the friction pair was the 4 mm-diameter GCr15 steel ball, which was offered by Lanzhou Zhongke Kaihua Technology Development Co., Ltd.(Lanzhou, China).100 mm/s was the sliding line speed, and 5 N was the test load.
(5) Electrochemical corrosion experiments were performed using the RST5000 electrochemical workstation made by Zhengzhou Shiruisi Instrument Technology Co., Ltd.(Zhengzhou, China).The non-test surface of the sample was uniformly coated with silicone Coatings 2024, 14, 620 4 of 14 gel to insulate it, leaving an exposed area of approximately 0.785 cm 2 of cladding coating for testing.After being submerged in a 3.5 weight percent NaCl solution for an hour, the prepared samples were connected via copper wires to the electrochemical workstation's working electrode.The scan rate was set to 5 × 10 −4 V/s, the sampling interval was 1 × 10 −4 V, and the rest time was 10 s.After immersing the various samples in a 3.5 weight percent NaCl solution for 72 h, the corrosion mechanism was examined.

Cladding Coating Morphology Analysis
Figure 1 illustrates the macroscopic morphology of the multi-pass laser-cladded CoCrFeNiMnTi x HEA coating.From the Figure 1, it is evident that the morphology of the alloy cladding coating without added Ti content appears comparatively bright and smooth, exhibiting better formability.However, with an increase in the Ti content, the formability of the cladding coating gradually deteriorates.The result is in accordance with Zuo et al. [33].According to the authors of [33], the addition of Ti content leads to an increase in the contact angle between the CoCrFeNiMnTi x high-entropy coating with the substrate.Consequently, as the Ti content increases, the wettability between the cladding coating with the substrate and both the substrate and adjacent cladding coatings diminishes.The increase in surface tension results in the formation of more ripples and unevenness on the cladding coating surface, leading to an overall decrease in formability.
1 × 10 −4 V, and the rest time was 10 s.After immersing the various samples in a 3.5 weight percent NaCl solution for 72 hours, the corrosion mechanism was examined.

Cladding Coating Morphology Analysis
Figure 1 illustrates the macroscopic morphology of the multi-pass laser-cladded CoCrFeNiMnTix HEA coating.From the Figure 1, it is evident that the morphology of the alloy cladding coating without added Ti content appears comparatively bright and smooth, exhibiting better formability.However, with an increase in the Ti content, the formability of the cladding coating gradually deteriorates.The result is in accordance with Zuo et al. [33].According to the authors of [33], the addition of Ti content leads to an increase in the contact angle between the CoCrFeNiMnTix high-entropy coating with the substrate.Consequently, as the Ti content increases, the wettability between the cladding coating with the substrate and both the substrate and adjacent cladding coatings diminishes.The increase in surface tension results in the formation of more ripples and unevenness on the cladding coating surface, leading to an overall decrease in formability.

Phase Analysis
Figure 2 illustrates the XRD patterns of the CoCrFeNiMnTi x (x = 0, 0.2, 0.4, 0.6, 0.8) HEA cladding coating.Notably, in the absence of Ti element, the phase composition of the CoCrFeNiMn cladding coating primarily comprises a single face-centered cubic (FCC) solid solution phase.The composition phase of the cladding coating still consists of a single FCC solid solution phase even after the Ti components are added and the Ti content is increased.The position of the FCC phase diffraction peaks in the cladding coating basically remains unchanged, but the intensity of the diffraction peaks gradually increases.This is because there are more constituent elements in the CoCrFeNiMnTi x HEA, and the close atomic percentage of each element makes the mixing entropy in the system high, which promotes the generation of random disordered solid solution phases with a simple and stable structure.This result is in accordance with Wang et al. [34].Meanwhile, it may be concluded that an increase in Ti content further promotes the formation of the FCC solid solution phase.

Phase Analysis
Figure 2 illustrates the XRD patterns of the CoCrFeNiMnTix (x = 0, 0.2, 0.4, 0.6, 0.8) HEA cladding coating.Notably, in the absence of Ti element, the phase composition of the CoCrFeNiMn cladding coating primarily comprises a single face-centered cubic (FCC) solid solution phase.The composition phase of the cladding coating still consists of a single FCC solid solution phase even after the Ti components are added and the Ti content is increased.The position of the FCC phase diffraction peaks in the cladding coating basically remains unchanged, but the intensity of the diffraction peaks gradually increases.This is because there are more constituent elements in the CoCrFeNiMnTix HEA, and the close atomic percentage of each element makes the mixing entropy in the system high, which promotes the generation of random disordered solid solution phases with a simple and stable structure.This result is in accordance with Wang et al. [34].Meanwhile, it may be concluded that an increase in Ti content further promotes the formation of the FCC solid solution phase.

Microstructural Morphology and Structural Analysis
Figure 3 shows the microstructural morphology of the CoCrFeNiMnTix HEA coating.It can be seen from the Figure 3a-e that the grain structures of all CoCrFeNiMnTix cladding coatings are mostly dendrites and equiaxed crystals, and the amount of equiaxed crystal increases with the increase in Ti content.The rationale behind this phenomenon can be attributed to two primary factors.Firstly, laser cladding, characterized by its energy concentration and rapid cooling rate, facilitates swift crystallization of crystals, thereby exerting a moderating influence on the anisotropic growth of crystals.Secondly, the higher melting point of Ti element relative to other elements within the alloy system plays a pivotal role.As Ti content escalates, the alloy system's capacity for heat absorption is augmented, subsequently amplifying the alloy's solidification rate and refining the grain size.There are some small pores in the coating Figure 3a.This is primarily because the laser cladding technology's quick heating and cooling cycles lead to inadequate

Microstructural Morphology and Structural Analysis
Figure 3 shows the microstructural morphology of the CoCrFeNiMnTi x HEA coating.It can be seen from the Figure 3a-e that the grain structures of all CoCrFeNiMnTi x cladding coatings are mostly dendrites and equiaxed crystals, and the amount of equiaxed crystal increases with the increase in Ti content.The rationale behind this phenomenon can be attributed to two primary factors.Firstly, laser cladding, characterized by its energy concentration and rapid cooling rate, facilitates swift crystallization of crystals, thereby exerting a moderating influence on the anisotropic growth of crystals.Secondly, the higher melting point of Ti element relative to other elements within the alloy system plays a pivotal role.As Ti content escalates, the alloy system's capacity for heat absorption is augmented, subsequently amplifying the alloy's solidification rate and refining the grain size.There are some small pores in the coating Figure 3a.This is primarily because the laser cladding technology's quick heating and cooling cycles lead to inadequate dispersion during the solidification process.As a result, as the molten pool solidifies, fewer pores are created and a tiny amount of gas is prevented from escaping [35].

Microhardness Analysis
Figure 5 illustrates the microhardness distribution curves of laser-cladded CoCrFeNiMnTix HEA cladding coating with different Ti contents.Along the depth profile from top to bottom of cladding coating, the coating delineates into distinct zones: the cladding zone, the heat-affected zone, and the substrate.As can be seen from the Figure 5, with the increases in Ti content, the hardness of the cladding coating gradually increases, and the hardness of the cladding zone is greater than that of the substrate, the hardness of the heat-affected zone is greater than that of the  dispersion during the solidification process.As a result, as the molten pool solidifies, fewer pores are created and a tiny amount of gas is prevented from escaping [35].Figure 4 depicts the energy spectrum of the CoCrFeNiMnTi0.6 HEA cladding coating.Analysis of the graph reveals a relatively homogeneous distribution of each alloy element within the coating.Notably, the incorporation of Ti element exhibits uniform dispersion throughout the entirety of the cladding coating.It shows that the process of adding Ti element to the alloy system is feasible and can ensure the consistency of the whole performance of cladding coating.

Microhardness Analysis
Figure 5 illustrates the microhardness distribution curves of laser-cladded CoCrFeNiMnTix HEA cladding coating with different Ti contents.Along the depth profile from top to bottom of cladding coating, the coating delineates into distinct zones: the cladding zone, the heat-affected zone, and the substrate.As can be seen from the Figure 5, with the increases in Ti content, the hardness of the cladding coating gradually increases, and the hardness of the cladding zone is greater than that of the substrate, the hardness of the heat-affected zone is greater than that of the

Microhardness Analysis
Figure 5 illustrates the microhardness distribution curves of laser-cladded CoCrFeNiMnTi x HEA cladding coating with different Ti contents.Along the depth profile from top to bottom of cladding coating, the coating delineates into distinct zones: the cladding zone, the heat-affected zone, and the substrate.As can be seen from the Figure 5, with the increases in Ti content, the hardness of the cladding coating gradually increases, and the hardness of the cladding zone is greater than that of the substrate, the hardness of the heat-affected zone is greater than that of the cladding zone.With increasing Ti content, the average hardness value of the cladding zone ascends from 230.46 HV to 453.38 HV, reaching a peak of 823 HV in the heataffected zone.This phenomenon primarily arises from two factors according to the previous microstructure and energy spectrum analysis.On the one hand, the evident grain refinement effect within the cladding coating as Ti content rises, synergizing with the inherent benefits of laser cladding itself, thus accentuate the refinement strengthening effect.On the other hand, the atomic radii of Co, Cr, Fe, Ni, Mn, and Ti are 125 pm, 124 pm, 124 pm, 121 pm, 124 pm, and 145 pm, respectively, the largest atomic radius of Ti element in contrast to other alloy element constituents accentuates lattice effects, bolstering solid solution strengthening and consequently enhancing the microhardness of the cladding coating.This result is accordance with Liang et al. [36], who used laser cladding to manufacture the CoCrFeNiMoTi x coatings and discovered that when the Ti concentration hits 0.75 at.%, the coating's average hardness peaks at 950 HV.
cladding zone.With increasing Ti content, the average hardness value of the cladding zone ascends from 230.46 HV to 453.38 HV, reaching a peak of 823 HV in the heataffected zone.This phenomenon primarily arises from two factors according to the previous microstructure and energy spectrum analysis.On the one hand, the evident grain refinement effect within the cladding coating as Ti content rises, synergizing with the inherent benefits of laser cladding itself, thus accentuate the refinement strengthening effect.On the other hand, the atomic radii of Co, Cr, Fe, Ni, Mn, and Ti are 125 pm, 124 pm, 124 pm, 121 pm, 124 pm, and 145 pm, respectively, the largest atomic radius of Ti element in contrast to other alloy element constituents accentuates lattice effects, bolstering solid solution strengthening and consequently enhancing the microhardness of the cladding coating.This result is accordance with Liang et al. [36], who used laser cladding to manufacture the CoCrFeNiMoTix coatings and discovered that when the Ti concentration hits 0.75 at.%, the coating's average hardness peaks at 950HV.

Friction and Wear Performance
Figure 6 illustrates the friction coefficient curves of the CoCrFeNiMnTix HEA cladding coating.It is evident that during the initial phase of the friction test, all cladding coatings with different Ti contents undergo a running-in stage before gradually stabilizing over time.The friction coefficient curve of each sample has a sawtooth fluctuation, which is mainly due to the periodic accumulation and elimination of abrasive chips.After reaching the stable stage, the average friction coefficients of the coatings with Ti contents ranging from x = 0 to x = 0.8 are 1.123, 1.062, 0.941, 0.835, and 0.828, respectively.This shows that the average friction coefficient of the coating decreases with the increase in Ti content, and the wear resistance of the coating is significantly improved with the addition of Ti element.The friction coefficients at x = 0.6 and x = 0.8 are approximate, indicating that with the increase in Ti elements, the wear resistance of the coating approaches its limit.The friction coefficient curve of each sample has a sawtooth fluctuation, which is mainly due to the periodic accumulation and elimination of abrasive chips.After reaching the stable stage, the average friction coefficients of the coatings with Ti contents ranging from x = 0 to x = 0.8 are 1.123, 1.062, 0.941, 0.835, and 0.828, respectively.This shows that the average friction coefficient of the coating decreases with the increase in Ti content, and the wear resistance of the coating is significantly improved with the addition of Ti element.The friction coefficients at x = 0.6 and x = 0.8 are approximate, indicating that with the increase in Ti elements, the wear resistance of the coating approaches its limit.

Friction and Wear Performance
Figure 7 illustrates the wear morphology of laser-cladded CoCrFeNiMnTi x HEA coatings.From the diagram, it is clear that abrasive and adhesive wear are the two main wear processes.At x = 0, the worn surface of the coating exhibits plow-shaped wear accompanied by partial pits.Upon magnification, numerous plow furrows are observed within the pits [Figure 7a].This phenomenon primarily arises from the relatively low hardness of the coating, resulting in the formation of plow furrows on the surface due to the reciprocating sliding action of the steel balls against the coating.As the Ti content increases from x = 0.2 to x = 0.8, the number of pits and plow furrows on the worn surface of the coating decreases.However, abrasive wear and adhesive wear remain predominant, accompanied by the presence of peeled debris.Under the frictional heat generated during operation, these peeled debris adhere to the worn surface of the coating, acting as abrasives that continuously micro-cut the coating surface, leading to fluctuations in the friction coefficient.With the increases in Ti content, the hardness of the coating increases, resulting in smaller peeled debris and a weakening of the micro-cutting action of the abrasives on the coating surface, consequently reducing the friction coefficient.This result is in accordance with ref. [36] and ref. [37].The wear scars on CoCrFeNiMoTi x cladding samples consistently show an initial drop followed by a subsequent pattern as the Ti content increases.The wear scars on CoCrFeNiMoTi 0.75 coating, on the other hand, are the lowest and most wear resistant [36].Ti increases the FeCoCrNiAlTi x coating's resistance to wear; in the wear test, the grooves disappear and there is less debris on the coating surface as the concentration of Ti rises [37].Figure 7 illustrates the wear morphology of laser-cladded CoCrFeNiMnTix HEA coatings.From the diagram, it is clear that abrasive and adhesive wear are the two main wear processes.At x = 0, the worn surface of the coating exhibits plow-shaped wear accompanied by partial pits.Upon magnification, numerous plow furrows are observed within the pits [Figure 7a].This phenomenon primarily arises from the relatively low hardness of the coating, resulting in the formation of plow furrows on the surface due to the reciprocating sliding action of the steel balls against the coating.As the Ti content increases from x = 0.2 to x = 0.8, the number of pits and plow furrows on the worn surface of the coating decreases.However, abrasive wear and adhesive wear remain predominant, accompanied by the presence of peeled debris.Under the frictional heat generated during operation, these peeled debris adhere to the worn surface of the coating, acting as abrasives that continuously micro-cut the coating surface, leading to fluctuations in the friction coefficient.With the increases in Ti content, the hardness of the coating increases, resulting in smaller peeled debris and a weakening of the micro-cutting action of the abrasives on the coating surface, consequently reducing the friction coefficient.This result is in accordance with ref. [36] and ref. [37].The wear scars on CoCrFeNiMoTix cladding samples consistently show an initial drop followed by a subsequent pattern as the Ti content increases.The wear scars on CoCrFeNiMoTi0.75 coating, on the other hand, are the lowest and most wear resistant [36].Ti increases the FeCoCrNiAlTix coating's resistance to wear; in the wear test, the grooves disappear and there is less debris on the coating surface as the concentration of Ti rises [37].

Corrosion Resistance Analysis
Figure 8 presents the potentiodynamic polarization curves of the CoCrFeN-iMnTix HEA cladding coating in a 3.5 wt.% NaCl solution.By fitting the curves, the corrosion potentials and corrosion current densities of all cladding alloy samples are determined (Table 3).According to electrochemical corrosion principles, the corrosion potential and corrosion current density represent the corrosion tendency and corrosion rate of the alloy, respectively.Because the alloy material is less likely to experience corrosion reactions, a higher corrosion potential and a lower corrosion current density indicate greater resistance to corrosion.Titanium element exhibits excellent   3).According to electrochemical corrosion principles, the corrosion potential and corrosion current density represent the corrosion tendency and corrosion rate of the alloy, respectively.Because the alloy material is less likely to experience corrosion reactions, a higher corrosion potential and a lower corrosion current density indicate greater resistance to corrosion.Titanium element exhibits excellent corrosion resistance, as its generated oxide film can protect the material surface from further oxidation.Therefore, with an increase in Ti content in cladding coating, the corrosion potential of the CoCrFeNiMnTi x HEA cladding coatings gradually increases from −495 mV to −251 mV, and the corresponding corrosion current density gradually decreases.Particularly, at Ti molar ratios of x = 0.6 and 0.8, compared to x = 0, 0.2, and 0.4, the CoCrFeNiMnTi x HEA coatings exhibit a reduction in corrosion current density of an order of magnitude.This indicates that the addition of Ti element in the cladding coating significantly improves the corrosion resistance of the CoCrFeNiMn alloy system.

Corrosion Resistance Analysis
Figure 8 presents the potentiodynamic polarization curves of the CoCrFeN-iMnTix HEA cladding coating in a 3.5 wt.% NaCl solution.By fitting the curves, the corrosion potentials and corrosion current densities of all cladding alloy samples are determined (Table 3).According to electrochemical corrosion principles, the corrosion potential and corrosion current density represent the corrosion tendency and corrosion rate of the alloy, respectively.Because the alloy material is less likely to experience corrosion reactions, a higher corrosion potential and a lower corrosion current density indicate greater resistance to corrosion.Titanium element exhibits excellent corrosion resistance, as its generated oxide film can protect the material surface from further oxidation.Therefore, with an increase in Ti content in cladding coating, the corrosion potential of the CoCrFeNiMnTix HEA cladding coatings gradually increases from −495mV to −251mV, and the corresponding corrosion current density gradually decreases.Particularly, at Ti molar ratios of x = 0.6 and 0.8, compared to x = 0, 0.2, and 0.4, the CoCrFeNiMnTix HEA coatings exhibit a reduction in corrosion current density of an order of magnitude.This indicates that the addition of Ti element in the cladding coating significantly improves the corrosion resistance of the CoCrFeNiMn alloy system.Figures 9-11 show the corrosion morphology of the samples immersed in a 3.5 wt.% NaCl solution for 72 h.Chromium (Cr) elements in the high-entropy coating form an oxide layer to protect the substrate.This protective film is a thin and relatively porous metal oxide compound [38,39].During prolonged immersion, chloride ions (Cl − ) gradually permeate the alloy surface and interact with the metal, causing the alloy to dissolve.The Cl − migrates concurrently along the passive film's grain boundaries.During this migration, it locally dissolves and thins the surface passive film up until it reaches the passive film/metal interface.This can result in cracking or detachment of the passive film, leading to the dissolution of the metal.Consequently, after prolonged immersion, Figure 9 indicates that the surface of sample (x = 0) is completely corroded, with obvious corrosion products.It is hypothesized that iron oxide (Fe 2 O 3 ) and iron hydroxide (Fe(OH) 3 ) make up the corrosion products when combined with the EDS data displayed in Figure 9c.The presence of residual Cr elements in the EDS results is attributed to residual Cr-containing passive film, as indicated by Kocks U. F.'s research [40].Additionally, the presence of Cl elements in the EDS results, combined with analysis from ref. [41], suggests that as the passive film thins, Cl − migration through defects accelerates, with some Cl − preferentially reaching the alloy surface through membrane defects and reacting with the metal to form chlorine-containing particles.Stress is created between the metal and the passive film as these particles expand.Due to the weaker binding strength between passive film grains compared to the bond strength between metal atoms, passive film peels off.Many black corrosion pits are visible in Figure 9b, indicating pitting corrosion.It can be inferred that chloride ions (Cl − ) penetrate longitudinally into the corrosion after damaging the surface oxide protective layer.The EDS results in Figure 9d indicate that some areas are not covered by corrosion products, revealing exposed high-entropy coatings.Figure 10 illustrates that the corrosion products reduce with the addition of Ti content.This phenomenon arises due to the formation of compounds such as titanium dioxide (TiO2) facilitated by titanium, which subsequently yields a protective corrosion-resistant oxide layer.This oxide layer acts as a barrier to external media, slowing down the corrosion process.Notably, in samples with x = 0.2 and x = 0.4, the corrosion product layers appear relatively dense.The corrosion product layer is not uniform, indicating different corrosion rates across different regions, resulting in variations in the thickness of the corrosion product layers.It is inferred that during the corrosion process, corrosion products precipitate along grain boundaries, and the main corrosion mechanism is intergranular corrosion.Figure 10 illustrates that the corrosion products reduce with the addition of Ti content.This phenomenon arises due to the formation of compounds such as titanium dioxide (TiO2) facilitated by titanium, which subsequently yields a protective corrosion-resistant oxide layer.This oxide layer acts as a barrier to external media, slowing down the corrosion process.Notably, in samples with x = 0.2 and x = 0.4, the corrosion product layers appear relatively dense.The corrosion product layer is not uniform, indicating different corrosion rates across different regions, resulting in variations in the thickness of the corrosion product layers.It is inferred that during the corrosion process, corrosion products precipitate along grain boundaries, and the main corrosion mechanism is intergranular corrosion.with electrochemical data analysis, the heightened corrosion resistance of the coating due to the addition of Ti elements is evident.This results in a reduced corrosion rate, accompanied by diminished corrosion product formation and thinner, more porous corrosion product layers.During experimental procedures, such corrosion product layers are highly susceptible to detachment.Notably, there no apparent pitting corrosion signs in samples x = 0.6 and x = 0.8.

Conclusions
This study focuses on the preparation of novel CoCrFeNiMnTix HEA coatings on 45 steel using laser cladding.The investigation primarily examines the microstructure, hardness, wear resistance and corrosion resistance of these coating.
(1) The CoCrFeNiMnTix HEA cladding coating primarily consists of a face-centered cubic solid solution.Its microstructure mainly comprises dendritic and equiaxed grains, Figure 10 illustrates that the corrosion products reduce with the addition of Ti content.This phenomenon arises due to the formation of compounds such as titanium dioxide (TiO 2 ) facilitated by titanium, which subsequently yields a protective corrosion-resistant oxide layer.This oxide layer acts as a barrier to external media, slowing down the corrosion process.Notably, in samples with x = 0.2 and x = 0.4, the corrosion product layers appear relatively dense.The corrosion product layer is not uniform, indicating different corrosion rates across different regions, resulting in variations in the thickness of the corrosion product layers.It is inferred that during the corrosion process, corrosion products precipitate along grain boundaries, and the main corrosion mechanism is intergranular corrosion.
Figure 11a,b clearly depict that with further increases in Ti content, the corrosion layer in sample x = 0.6 appears incomplete, with some regions devoid of accumulated corrosion products.This phenomenon becomes more pronounced in sample x = 0.8.EDS analysis results indicate that Dot 1 represents the corrosion product layer, while Dot 2 represents the exposed high-entropy coating without apparent oxides.Combining this with electrochemical data analysis, the heightened corrosion resistance of the coating due to the addition of Ti elements is evident.This results in a reduced corrosion rate, accompanied by diminished corrosion product formation and thinner, more porous corrosion product layers.During experimental procedures, such corrosion product layers are highly susceptible to detachment.Notably, there are no apparent pitting corrosion signs in samples x = 0.6 and x = 0.8.

Conclusions
This study focuses on the preparation of novel CoCrFeNiMnTi x HEA coatings on 45 steel using laser cladding.The investigation primarily examines the microstructure, hardness, wear resistance and corrosion resistance of these coating.
(1) The CoCrFeNiMnTi x HEA cladding coating primarily consists of a face-centered cubic solid solution.Its microstructure mainly comprises dendritic and equiaxed grains, with an increase in equiaxed grain quantity as the Ti content rises.The added Ti element is uniformly distributed within the cladding coating.
(2) The hardness of the cladding coating gradually increases with the increase in Ti content, reaching a peak hardness of 823 HV at x = 0.8.The wear resistance of the cladding coating improves progressively with the Ti content increment, resulting in a gradual reduction in friction coefficient.Notably, at x = 0.6 and 0.8, the friction coefficients are relatively close, with the lowest average friction coefficient being 0.828.
(3) The predominant forms of wear in the cladding coating are adhesive and abrasive wear.When Ti content is low, the worn surface exhibits noticeable pits and furrows.However, as the Ti content increases, the occurrence of pits and furrows decreases, along with a reduction in the amount of detached debris.
(4) With an increase in Ti content in cladding coating, the corrosion potential of the CoCrFeNiMnTi x HEA cladding coating gradually rises, while the current density gradually decreases, indicating a progressive enhancement in corrosion resistance.

Figure 4
Figure 4 depicts the energy spectrum of the CoCrFeNiMnTi0.6 HEA cladding coating.Analysis of the graph reveals a relatively homogeneous distribution of each alloy element within the coating.Notably, the incorporation of Ti element exhibits uniform dispersion throughout the entirety of the cladding coating.It shows that the process of adding Ti element to the alloy system is feasible and can ensure the consistency of the whole performance of cladding coating.

Figure 4
Figure 4 depicts the energy spectrum of the CoCrFeNiMnTi 0.6 HEA cladding coating.Analysis of the graph reveals a relatively homogeneous distribution of each alloy element within the coating.Notably, the incorporation of Ti element exhibits uniform dispersion throughout the entirety of the cladding coating.It shows that the process of adding Ti element to the alloy system is feasible and can ensure the consistency of the whole performance of cladding coating.

Figure 6
Figure 6 illustrates the friction coefficient curves of the CoCrFeNiMnTi x HEA cladding coating.It is evident that during the initial phase of the friction test, all cladding coatings with different Ti contents undergo a running-in stage before gradually stabilizing over time.The friction coefficient curve of each sample has a sawtooth fluctuation, which is mainly due to the periodic accumulation and elimination of abrasive chips.After reaching the stable stage, the average friction coefficients of the coatings with Ti contents ranging from x = 0 to x = 0.8 are 1.123, 1.062, 0.941, 0.835, and 0.828, respectively.This shows that the average friction coefficient of the coating decreases with the increase in Ti content, and the wear resistance of the coating is significantly improved with the addition of Ti element.The friction coefficients at x = 0.6 and x = 0.8 are approximate, indicating that with the increase in Ti elements, the wear resistance of the coating approaches its limit.Figure7illustrates the wear morphology of laser-cladded CoCrFeNiMnTi x HEA coatings.From the diagram, it is clear that abrasive and adhesive wear are the two main wear processes.At x = 0, the worn surface of the coating exhibits plow-shaped wear accompanied by partial pits.Upon magnification, numerous plow furrows are observed within the pits [Figure7a].This phenomenon primarily arises from the relatively low hardness of the coating, resulting in the formation of plow furrows on the surface due to the reciprocating sliding action of the steel balls against the coating.As the Ti content increases from x = 0.2 to x = 0.8, the number of pits and plow furrows on the worn surface of the coating decreases.However, abrasive wear and adhesive wear remain predominant, accompanied by the presence of peeled debris.Under the frictional heat generated during

Figure 8
Figure 8 presents the potentiodynamic polarization curves of the CoCrFeNiMnTi x HEA cladding coating in a 3.5 wt.% NaCl solution.By fitting the curves, the corrosion potentials and corrosion current densities of all cladding alloy samples are determined

Table 2 .
Process parameters of laser cladding.

Table 2 .
Process parameters of laser cladding.

Table 3 .
Electrochemical parameters of CoCrFeNiMnTi x HEA cladding coating.