Effect of Powder Particle Size and Shape on Appearance and Performance of Titanium Coatings Prepared on Mild Steel by Plasma Cladding

Abstract

The preparation of Ti coatings on mild steel can both effectively improve the corrosion resistance of the substrate and reduce the application cost of Ti, which is an effective measure to improve the service performance of mild steel in the marine environment. Plasma cladding technology is an efficient method for preparing metal coatings, and the type of powder is a key process parameter for coating preparation. In this work, high-performance Ti coatings are prepared on the surface of mild steel by plasma cladding technology, and the effects of different particle sizes and shapes of Ti powders on the surface morphology, microstructure and properties of the coatings are studied. The results show that powder particle size and sphericity are the key factors affecting the morphology, structure and service performance of Ti coatings. After 1000 h of salt spray test, the spherical powder cladding coatings only suffer slight corrosion, while the irregular shape powder coating is more severely corroded. Powder cladding with moderate powder particle size and good sphericity have a smoother coating and fewer defects. Ti powders with different particle sizes and shapes all have the diffusion of Fe element during the cladding process. The surface of Ti coating prepared by spherical powder are dominated by α-Ti and Fe_0.2Ti_0.8 phases, while the surface of Ti coating prepared by irregular shape powder is dominated by FeTi and Ti₂Fe. The interface between the coating and the substrate shows metallurgical bonding, and the increase in Ti-Fe brittle phase will deteriorate the mechanical properties and corrosion resistance of the coating. The shear strength of coatings prepared from spherical Ti powders of 75–150 μm can reach 105.18 MPa, the corrosion potential is the most positive (−0.2206 V), and the self-corrosion current density is the lowest (6.220 × 10⁻⁸ A/cm²).

1. Introduction

Steel is the most widely used material in marine engineering, and the strong marine corrosive environment poses a serious threat to steel and other marine engineering materials, which causes huge economic losses every year [1,2,3]. Ti and Ti alloys have excellent corrosion resistance in the marine environment, so they are known as “marine metals” and have the potential for large-scale applications in marine engineering [4]. However, compared with steel, its high price limits its competitiveness in industrial applications [5].

Based on this, researchers deposit Ti or Ti alloy coating on the surface of steel as a substitute for Ti and Ti alloy. The Ti alloy coating can prevent the corrosion of steel matrix, and steel matrix can reduce the cost of the whole composite material. At present, the cold spray method is the most widely used for preparing Ti alloy coating on steel surface. However, the low bond strength and low density are the reasons that limit the cold spray coating, which restricts the wide application of this method [6]. Plasma cladding technology is a metal surface treatment technology improved and developed on the basis of plasma surfacing technology [7]. Compared with traditional spraying, surfacing, coating and other surface treatment technologies, it has the characteristics of high cladding efficiency, low cost, environmental protection and no pollution. Plasma coating technology can be used to prepare high-quality coatings, and can also regulate the composition according to the requirements, which is a potential material surface treatment technology [8,9]. The plasma cladding process is a localized rapid heating and rapid cooling process. In this process, the cladding material is one of the key factors affecting the coating morphology and structure, and determines the overall performance of the coating [10]. Alloy powders are usually used as raw materials for cladding due to their ease of feeding and controlled melting. The quality of the coating is significantly affected by the properties of the raw material [11,12] including the shape, particle size distribution, composition and flowability of the powder. Drahomír Dvorska et al. [13]. prepared alloys by spark plasma sintering using different powders, revealing the effect of powder size on the mechanical, corrosion and ignition properties of the prepared alloys. Rodolpho F. Vaz et al. [14] reported the effect of powder properties on 316L stainless steel coatings, and found that the coatings prepared from quasi-spherical powders were thick and dense, and had better mechanical properties. Therefore, the particle size and shape of the powder are important factors affecting the morphology and properties of the coating [15].

Previous studies have shown that Ti can easily react with Fe to form highly brittle intermetallic compounds due to the large differences in the physical and chemical properties of Fe and Ti [16,17,18]. Moreover, the regulation mechanism of Ti-Fe interdiffusion behavior and the evolution mechanism of microstructure are still unclear, so there are few reports on the preparation of Ti coatings on steel substrates by plasma cladding process. Therefore, it is of great significance to study the formation mechanism of intermetallic compounds at the Ti-Fe interface and develop practical plasma coating technology for Ti alloy coating on steel surface.

In this paper, Ti alloy coatings were prepared by plasma cladding technology. The effects of pure Ti powders with different particle sizes and shapes on the morphology, structure and service performance of Ti coatings were studied. Moreover, the structure–activity relationship between the particle size and shape of pure Ti powders and the service performance of Ti coatings were discussed. The results are expected to provide theoretical basis and research ideas for the preparation of Ti coatings with excellent service performance on steel substrates by plasma cladding process.

2. Materials and Methods

2.1. Materials and Coating Preparation

Q235 mild steel blocks (C: 0.12–0.2 wt.%, Si: 0.3 wt.%, Mn: 0.3–0.7 wt.%, S: ≤ 0.045 wt.%, Fe: bal.) with dimensions of 100 mm × 80 mm × 10 mm are used as the base material. The surface of the mild steel blocks was polished with 60-grit sandpaper until it had a metallic luster, and was cleaned. Four types of pure Ti powder (purity >99.9%) with different particle sizes and shapes were used as the cladding material, which were 53–75 μm spherical Ti powder, 75–150 μm spherical Ti powder, 150–200 μm spherical Ti powder and 50–150 μm irregular shaped Ti powder. All powders were purchased from Changsha Tianjiu Metal Materials Co., Ltd. The Ti coating was prepared by a plasma cladding machine (Duomu DML-03AD), the schematic diagram of the equipment is shown in Figure 1a. The synchronous powder feeding method was adopted, the moving speed was 300 mm/min, the cladding time was 4.4 min, the current was 90 A, the rotational speed was 8 r/min, the rate of powder feeding gas was 1.5 L/min, the rate of ion gas (the formation of ion arc by high voltage excitation) was 0.8 L/min, and the rate of protective gas was 10 L/min. Ar was used for powder feeding gas, ion gas and protective gas. The schematic diagram of the cladding process is shown in Figure 1b. The cladding process was carried out without breaks.

The particle size and shape of the cladding powder affect the coating appearance and structure. The microscopic morphology of the cladding powder used in this work is shown in Figure 2. Figure 2a shows spherical Ti powder with a particle size of 53–75 μm, and Figure 2b shows spherical Ti powder with a particle size of 75–150 μm. Figure 2c shows spherical Ti powder with a particle size of 150–200 μm, and Figure 3d is an irregular-shaped Ti powder with a particle size of 50–150 μm. It can be seen from the figures that the spherical powders have good sphericity (Figure 3a–c), but the irregular powders in Figure 2d have different shapes. The flow rate of different types of Ti powders are shown in

Table 1. Flow rate of different types of titanium powder.

Sample	Flow Rate (s/50 g)
53–75 μm spherical Ti powder	27.3
75–150 μm spherical Ti powder	26.5
150–200 μm spherical Ti powder	28.5
50–150 μm irregular shape Ti powder	43.7

. The flow rate of the powders increases first and then decrease with the particle size. The flow rate of spherical powders is faster than that of irregular powder. The flow rate of spherical Ti powder with a particle size of 75–150 μm is the fastest, at 26.5 s, while the irregular-shaped Ti powder with a particle size of 50–150 μm has the slowest flow rate, at 43.7 s. Different powder fluidities have an impact on the cladding process. During the cladding process, the spherical Ti powders are fed smoothly and uniformly, while the irregular shape Ti powders have poor fluidity and discontinuous feeding.

These four powders were used to prepare Ti coatings by plasma cladding coating on the surface of Q235 mild steel blocks. Ti coatings were obtained by cladding the samples with 53–75 μm spherical Ti powder, 75-150 μm spherical Ti powder, 150–200 μm spherical Ti powder and 50–150 μm irregular Ti powder, named Ti-S, Ti-M, Ti-L and Ti-I, respectively. It can be seen from Figure 3 that all the four powder-coated Ti coatings have a metallic luster after polishing, but the macroscopic morphology of the coatings differs due to the different particle sizes and shapes of the powders. The surfaces of the Ti-S and Ti-M coatings are relatively flat, the weld bead is well overlapped, a few holes appear at the initial position of the coating cladding, and there are no obvious macroscopic cracks on the surface of the coating. Ti-L coating did not form uniform lap at the initial stage of cladding, which may be due to the large particle size of the powder and the great difference in physical properties between Ti powder and Fe matrix, resulting in insufficient mixing of the molten pool. After the formation of a well-lapped coating, the surface became flat and there was no obvious macroscopic crack. The surface of the Ti-I coating is uneven, the depth of the weld bead lap varies greatly, and there are obvious macroscopic cracks through the coating. This is due to the fact that the spherical powder was fed uniformly, and a continuous melt pool formed during the melting process. Meanwhile, the irregular powder has poor stability during the powder feeding process, which cannot guarantee continuous and uniform powder feeding. In addition, the powders collide with each other during the cladding process. Then, the molten pool is stirred violently and irregularly, resulting in the uneven surface height of the formed coating.

2.2. Test and Characterization

The powder flow performance was tested by Hall flowmeter (Granuflow, Granutools), and the flow time of each 50 g sample was recorded. The phase composition was analyzed by X-ray diffractometer (XRD, Empyrean, PANalytical) with Cu-Kα radiation source. Scanning electron microscope (SEM, EVO MA10, Carl Zeiss) was used to observe the microstructure of the coating cross-section (perpendicular to the coating surface), and the element distribution on the coating surface was analyzed with energy dispersive spectrometer (EDS). The surface element content of the coating was measured by hand-held X-ray fluorescence analyzer (XRF, vantaC, Olympus). Coating hardness was measured using a Vickers hardness tester (HVS-1000, Veiyee) with a load of 100 gf, and three valid results were tested at different positions and the average value was taken as the hardness value of the specimen. The polarization curves of the coatings were tested based on an electrochemical workstation (Interface 1010e, Gamry) with a three-electrode system: working electrode (sample), counter electrode (Pt) and reference electrode (Ag/AgCl), and the electrolyte solution was 3.5 wt.% NaCl. Before the polarization curve test, the sample surfaces were polished to 2000 mesh using a series of SiC papers, and then polished with a 2.5 μm diamond suspension. After ultrasonic cleaning and drying, the sample was stabilized in 3.5 wt.% NaCl solution for 30 min, and then the polarization curve was tested. The shear strength and tensile strength of the coatings were tested by a universal testing machine (DNS50, Sinotest Equipment). The shear strength test process was carried out in accordance with the China National Standard GB/T6396-2008. The coating sample was cut into the shape shown in Figure 4a. The total length of the shear sample was 65 mm and the width was 25 mm. The length, thickness and width of the coating retained on the sample were 3 mm, 3 mm and 25 mm. The shear strength was measured by the shear fixture, and the loading rate of 100 N/s was used during the test. The tensile strength of the coating was tested according to ASTM C633. The sample to be tested was bonded to the mold using E-7 epoxy resin glue (as shown in Figure 4b), held at 180 °C for 3 h, cooled to room temperature and then conducted a tensile test. The loading rate was 100 N/s. In accordance with the China National Standard GB/T 10125-2012, the salt spray experiment was carried out in the salt spray test chamber (LRHS-270-RY) for 1000 h.

3. Results and Discussion

3.1. Appearance and Composition

The cross-section SEM images of different Ti coatings are presented in Figure 5. The results show that there is a fusion line at the interface between the four Ti coatings and the substrate, which is due to the growth of the grains in the form of planar crystals in the melting zone on the surface of the substrate. It marks the formation of a metallurgical bond between the coating and the substrate [19]. The Ti-S coating forms a two-layer transition structure at the interface between the coating and the substrate (Figure 5a–c). A small amount of pores and microcracks are generated at the Ti-Fe junction, and the interface junction is relatively flat. As the powder particle size is smaller, the powder melts faster, the temperature gradient (G) at the bottom of the melt pool is larger, and the solidification rate (R) is lower. With the decrease in the molten pool temperature, the eutectic reaction L → α-Fe + TiFe₂ occurs in the molten pool, and the α-Fe and TiFe₂ phases are formed, thus the Ti-Fe interface reaction layer is formed. As the progress of solidification process, the ratio of G to R gradually decreases, forming a layer of dendritic grains structure, which is perpendicular to the solidification interface. Ti-M and Ti-L coatings are similar (Figure 5d–i). The Ti coatings are flat and uniform, and there are a few unmelted Ti powder particles in the coatings. Compared with Ti-S coating, the dendritic grain’s structure was directly formed at the interface of Ti-M and Ti-L coating after the planar crystal growth. This is due to the fact that the larger the particle size of the powder, the more heat it absorbs in melting, and the ratio of G and R decreases. Compared with the coatings prepared by spherical powders, the Ti-I coating is quite different (Figure 5j–l). Due to the discontinuity of irregular powder feeding, the penetration depth of Ti-I coating is uneven. The powder produces violent and random agitation of the molten pool, resulting in uneven diffusion between the substrate and the coating. During the solidification process, the change in the molten pool leads to a larger ratio of G and R at some positions, forming a Ti-Fe interface reaction layer, and then forming a unit cell along the front of the interface where the components are supercooled, and columnar crystals are formed with the increase in the solidification rate. The plasma beam has concentrated heat and high energy density. The huge heat input causes the powder and the substrate to melt rapidly to form a molten pool. The liquid metal in the molten pool directly generates non-spontaneous nucleation on the substrate surface, and the molten pool rapidly solidifies and crystallizes, which makes the coating present good metallurgical bonding [20]. There are obvious differences in the morphology of coatings prepared from spherical powders and coatings prepared from irregular shape powders. Coatings prepared from spherical powders showed equiaxed dendrites from the outside of the cladding layer near the fusion line. The microstructure is smaller than that of the coatings prepared by irregular shape powder. This is related to the fact that the temperature gradient is relatively reduced due to the faster heat dissipation during the spherical powder cladding process, and the power of grain epitaxial growth is insufficient. When the sphericity of the powder is fine, the powder particles are arranged more regularly. Energy beam irradiating on spherical powder has a higher probability of repeated reflection and more times, so the absorption rate is also higher. The absorption rate of the powder to the plasma arc is higher, the heat dissipation during the cladding process is faster [21].

The distribution of elements at the interface between the coating and the substrate is shown in Figure 6. Many studies have shown that the reaction between Ti and Fe easily generates intermetallic compounds [22,23]. In Figure 6a, The Fe element in the Ti-S coating forms two diffusion layers with significantly different contents, corresponding to the Ti-Fe interface reaction layer and the dendritic grains layer, respectively, and the Fe content in the layers gradually decreases when the different diffusion layers are far from the substrate. Ti-M and Ti-L have a distinct Fe and Ti interface at the coating junction (Figure 6b,c). However, the Fe element of the Ti-I coating diffuses violently to the Ti layer (Figure 6d), and a large number of Ti-Fe compounds with high Fe content are formed at the interface. Some Fe elements are also diffused in the Ti coating far from the substrate.

Figure 7 shows the EDS line scan results from the substrate to the edge of the coating. It can be seen from the figure that although the Fe content in the coating at the Ti-Fe interface decreases significantly, Fe diffusion occurs in the four Ti coatings prepared far from the substrate. This is mainly due to the melting point of Ti is higher than that of Q235, so the agitation of the molten pool drives the long-distance diffusion of Fe under the high energy beam. Different from the diffusion behavior of Fe, the diffusion distance of the Ti element to the matrix at the interface is very short, because Fe always maintains a face-centered cubic structure below the melting point. When the temperature reaches a certain level (885 °C), Ti will undergo a phase transformation from the hexagonal close-packed structure (α-Ti) to the body-centered cubic structure (β-Ti). During the plasma cladding process, the diffusion of body-centered cubic Ti to face-centered cubic Fe will be strongly hindered, so the diffusion distance of Ti in the matrix is very short [24].

Although Fe diffusion occurs during the cladding process of the four Ti coatings, there are differences in the Fe content of each Ti coating. Combined with the results of XRF composition analysis of the coating surface in

Table 2. XRF composition analysis of coatings surface.

Sample	Ti (%)	Fe (%)
Ti-S	84.03	15.97
Ti-M	82.34	17.66
Ti-L	79.07	20.93
Ti-I	55.49	44.51

, it can be found that the Fe content of the coatings prepared by spherical Ti powder are much lower than that of the coating prepared by irregular shape Ti powder. The particles impact the molten pool during powder feeding, causing melting, interfacial mixing and diffusion [25]. Irregular particles have a greater impact on the molten pool and more spatter than spherical particles [26]. This indicates that the irregularly shaped Ti powder has a greater impact stirring on the bottom of the melt pool during the cladding process, which in turn causes a large amount of molten state Fe to diffuse into the Ti coating. The larger Fe content means the higher probability of forming Ti-Fe brittle intermetallic compounds. The formation and aggregation of a large number of brittle and hard phases are the main reasons for the occurrence of macroscopic cracks on the surface of Ti-I coatings.

Figure 8 shows the XRD patterns of the surfaces of different types of powder cladding coatings. Due to the diffusion of Fe, the characteristic peaks of Ti-Fe compounds (including Fe_0.2Ti_0.8, FeTi and Ti₂Fe) appear in the XRD pattern of the coating. In the coatings Ti-S, Ti-M and Ti-L prepared from spherical powder, the phases are dominated by α-Ti and Fe_0.2Ti_0.8, while in coating Ti-I prepared from irregular shape powder, the main phases are FeTi and Ti₂Fe, and a small amount of Fe is contained. This is consistent with the EDS line scan and XRF results of the coatings. It should be noted that due to the poor fluidity of irregular powder, the total amount of melted coating powder of irregular powder in the cladding process is less than that of spherical powder. Due to the less absorbed energy of the powder, the matrix dissolution increases under the same energy input, which aggravates the diffusion of Fe, and the temperature gradient increases at this time. Therefore, compared with other coatings, the thermal interaction between Ti and Fe in Ti-I is stronger, forming a large number of Fe rich phases. The impact and stirring of irregular powder on the bottom of the molten pool are more intense, which will also make the coating form a large number of brittle intermetallic compounds with high Fe content. These brittle metal compounds will affect the service performance of the coatings.

3.2. Mechanical and Corrosion Performance

In order to evaluate the adhesion of the coating, the strength of the coatings was tested. The average shear strengths of each coating are listed in Figure 9. the average shear strengths of Ti-S, Ti-M, Ti-L and Ti-I are 75.44 MPa, 105.18 MPa, 76.27 MPa and 47.77 MPa, respectively. Ti-M has the highest shear strength, while Ti-I has the lowest shear strength, which is related to the generation of brittle intermetallic compounds at the coating interface. Under the same process parameters, the Ti-M molten pool is stable, the cooling rate is fast, and less intermetallic compounds are generated at the coating interface. However, the stirring of Ti-I molten pool is intense, and the diffusion of Fe is serious, resulting in more brittle intermetallic compounds at the coating interface. The tensile strength of the coating was also tested. Since the tensile strength of the coating is tested by the test method shown in Figure 4b, the theoretical maximum adhesive force of the glue used is 60 MPa. In the actual test, the strength of the glue will be affected by the state of the glue and the surface of the coating. If the bonding force of the coating is greater than the adhesion force of the glue, the sample will break at the glue. Conversely, it will break at the coating. During the actual test, the fracture positions of the prepared coatings were all at the glue adhesive position. The average tensile strengths of Ti-S, Ti-M, Ti-L and Ti-I were 50.61 MPa, 45.11 MPa, 48.36 MPa and 47.55 MPa, respectively, as shown in Figure 9b. It can be seen that the tensile strengths of the tensile specimens prepared by the four powder coatings are greater than 50.61 MPa, 45.11 MPa, 48.36 MPa and 47.55 MPa. The huge heat input of the plasma beam makes the coating present a good metallurgical bond, which makes the coating have a good bonding strength.

In addition, taking the fusion line as the initial position, the Vickers microhardness test was carried out on the coating and the substrate on both sides. Figure 10 shows the microhardness test results of the cross section of the coatings. As shown in Figure 10, the microhardness of each coating is maximum at the interface, which is mainly due to the formation of more intermetallic compounds near the coating interface. The hardness decreases rapidly from the interface to both sides, the hardness of the matrix has little change, while the hardness of the coating shows a fluctuating decreasing trend with the diffusion of Fe. Due to the formation of a large number of intermetallic compounds in the Ti-I coating, Ti-I has the highest hardness among all coatings, while Ti-S, Ti-M and Ti-L generate the softer phase Fe_0.2Ti_0.8 and less hard phase Ti₂Fe, so their hardnesses are lower. The diffusion of Fe element in Ti-S presents a delamination phenomenon, and the Fe content decreases gradually along the direction from the substrate to the coating surface, and the hardness also decreases gradually. Comparing the mixing enthalpy of Ti and Fe [27], it can be seen that Ti and Fe have strong atomic bonds. Therefore, when Fe diffuses into Ti coating and forms Ti-Fe intermetallic phase, the hardness of Ti coating can be greatly improved. It can be seen that after titanium is cladding on the surface of mild steel to form a coating, due to the diffusion of Fe in the coating, the coating becomes a titanium alloy composite coating containing Ti-Fe intermetallic compounds, and its hardness is slightly higher than that of pure titanium [28], which is expected to have a positive impact on the wear resistance of the coating.

In order to evaluate the corrosion resistance of the coatings, the polarization curves of pure titanium, four kinds of Ti coatings and Q235 were tested, respectively. The results are shown in Figure 11. The corrosion potential (E_corr) and corrosion current density (I_corr) obtained from the polarization curves are shown in

Table 3. The fitting data of polarization curve of the Ti coatings, Q235 steel and Ti.

Sample	Ecorr (V)	Icorr (A/cm2)
Ti-S	−0.2401	2.597 × 10−7
Ti-M	−0.2206	6.220 × 10−8
Ti-L	−0.2356	7.057 × 10−8
Ti-I	−0.3951	4.363 × 10−6
Q235	−0.5797	1.472 × 10−6
Ti	−0.2465	1.351 × 10−8

. The corrosion resistance of the four Ti coatings obtained by plasma cladding is significantly improved compared with that of Q235. The Ti-S, Ti-M and Ti-L coatings clad with spherical Ti powders show better corrosion resistance than Ti-I coatings clad with irregular shape Ti powder. Ti-I coating has the minimum E_corr of −0.3951 V and the maximum Icorr of 4.363 × 10⁻⁶ A/cm², while Ti-S, Ti-M and Ti-L have similar corrosion resistance, and Ti-M has the highest E_corr (−0.2206 V) and minimum I_corr (6.220 × 10⁻⁸ A/cm²). Among the four Ti coatings, the Ti-I coating has the worst corrosion resistance due to the existence of macro cracks and high Fe content. The electrolyte solution can enter the inside of the coating through the macroscopic cracks and even reach the surface of the substrate, which leads to the deterioration of the corrosion resistance of the coating. This result shows that reducing coating defects is crucial for improving the corrosion resistance of Ti coatings. Compared with pure Ti, the corrosion tendency of the coatings prepared from spherical powders is smaller than that of pure Ti in terms of corrosion potential, but the corrosion potential is only a concept of thermodynamics, and corrosion current density should also be considered when evaluating corrosion resistance [29]. Pure Ti has a lower I_corr (1.351 × 10⁻⁸ A/cm²), and the coating prepared by spherical powder has a slightly higher Icorr than pure titanium due to a small amount of Fe on the surface. While the Ti-I surface contains more Fe and has a worse performance than pure Ti. The overall corrosion performance of coatings prepared from spherical powders is close to that of pure Ti.

In addition, 1000 h salt spray experiments were carried out on Ti-S, Ti-M, Ti-L and Ti-I coatings to simulate the corrosion of Ti coatings in the actual working marine environment. The images of the samples before and after the experiments are shown in Figure 12. The results show that after the 1000 h salt spray test, Ti-S, Ti-M and Ti-L only appear to have slight corrosion, which occurs at the weld bead lap. However, the surface corrosion of Ti-I is more serious, which mainly occurs in the coating cracks and the uneven lap of the coating. This is because the thickness of the coating at the lap defect is thinner, and the solution is more likely to penetrate into the coating from here, and even to the surface of the substrate. Therefore, the location of uneven laps and macroscopic cracks in the coating is more likely to produce corrosion.

The SEM and EDS mapping scan were carried out on the coatings after the 1000 h salt spray test, and the results are shown in Figure 13 and Figure 14. It can be seen from Figure 13 that corrosion occurs at the surface defects of Ti-S, Ti-M and Ti-L, while there are no obvious corrosion traces elsewhere. However, Ti-I showed obvious corrosion at and around the cracks. Comparing the EDS mapping scans, it can be seen that Ti-S, Ti-M and Ti-L show obvious Fe-rich, O-rich and Ti-poor phenomena at the defects, indicating that Fe oxides are produced here due to corrosion. And other Ti-rich sites in the coatings did not undergo significant oxidation. In the Ti-I coating, the Fe content is more and O is rich in many places on the surface, indicating that corrosion occurs in many parts of the coating. Ti-S, Ti-M and Ti-L exhibited better corrosion resistance than Ti-I after 1000 h salt spray test due to less surface defects. Therefore, the denser coating without cracks is the key to ensure the coating has excellent corrosion resistance.

All coatings were prepared under the experimental conditions in Section 2.1. As for the powder shape, the sphericity of the powder affects the flowability of the powder. Under the huge energy input of the plasma arc, the flowability of the powder will affect the deposition efficiency and the coating characteristics. The better the fluidity is, the more the cladding powders are. Then the absorption rate of the powder to the plasma arc increases, and the temperature gradient of the molten pool decreases [14]. Therefore, the outer side of the coatings prepared from spherical powders present fine equiaxed dendrites (as shown in Figure 5a–i). The irregular powder has poor fluidity. When the powder is less, the absorption of the plasma arc is less. This increases the dissolution of the matrix and aggravates the diffusion of Fe. and affects the purity and thickness of the coating (as shown in Figure 5, Figure 6 and Figure 7). Moreover, the powder shape also makes a difference to the stability of the molten pool. Gao et al. [30] simulated the melt pool dynamics in the cladding process between spherical powders and irregular-shaped powders, and found that the molten pool shape is relatively continuous for spherical powder. For powders containing non-spherical particles, the slender gaps formed between the particles lead to severe fragmentation of the liquid metal, making the molten pool prone to instability. In addition, the average flight speed of irregular angular powders is higher than that of spherical powders; they may be more likely to bounce and break on the deposited particles, and have a greater impact on the molten pool [28]. This further aggravates the diffusion of Fe in the Ti-I coating. Therefore, the thermal interaction between Ti and Fe in the Ti-I coating is stronger, and a large amount of Fe-rich FeTi and Ti₂Fe brittle hard phases are more easily formed. The diffusion of Fe in the coating prepared by spherical powder is less than that of Ti-I coating, so the Ti-rich phase dominated by α-Ti and Fe_0.2Ti_0.8 is generated (Figure 8). The difference of Ti-Fe compounds in the coating affects the performance of the coating. For the particle size of the powder, the particle size directly affects the heating condition of the powder in the high energy beam. Large particle size requires large heat, and the powder is prone to unmelted or semi-melted state. When it collides with the surface of the substrate, the degree of spreading is limited, and the contact area of the molten pool is reduced. Too small powers will cause some difficulties in powder feeding, so that the deposition rate of the coating decreases. Although the small particle size is easy to melt and accelerate in high energy flow, the collision of the substrate is intensified [31], which increases the depth of the molten pool after particle deformation and spreading. It will also aggravate the diffusion of Fe element in the molten pool. These will have an impact on the mechanical properties of the coating. In general, the particle size and shape of the cladding powder are the key factors affecting the microstructure and composition of the Ti coating, and the powder shape has a greater impact on the structure and performance of the coating.

4. Conclusions

The Ti coatings were successfully prepared on the surface of Q235 mild steel by plasma cladding technology. The effects of pure Ti powders with different particle sizes and shapes on the appearance, composition and service performance of the coating were studied. The conclusions can be summarized as following:

(1)

The flowability will affect the coating quality. The powder with moderate particle size and good sphericity can obtain a dense Ti coating. The coating obtained by spherical powder cladding is relatively flat and does not appear macroscopic cracks, and has good formability. In contrast, the coating with irregular shape powder cladding has poor formability and more cracks on the surface. Among the coatings prepared in this work, the Ti coatings prepared with 75–150 μm spherical titanium powder as the cladding material have better comprehensive service performance.

(2)

The interface between the coating and the substrate prepared by Ti powders with different particle sizes and shapes showed metallurgical bonding, and Fe diffused in the Ti coating during the cladding process. The surface phases of Ti coatings prepared by spherical powders are dominated by α-Ti and Fe_0.2Ti_0.8, while the surface phases of Ti coatings prepared by irregular shape powders are dominated by FeTi and Ti₂Fe.

(3)

The Ti-Fe intermetallic compounds have an effect on the service performance of Ti coatings. The prepared Ti coatings show good mechanical properties. The coating prepared from 75–150 μm spherical powders has higher mechanical properties, and its shear strengths can reach 105.18 MPa and tensile strengths exceed 45.11 MPa. Affected by the diffusion of Fe, the average hardness values of the coatings are all greater than 250 HV. The corrosion resistance of the prepared Ti coatings is greatly improved than that of traditional mild steel. The coatings prepared from spherical powders have similar corrosion resistance and are close to pure titanium. The coatings prepared from 75–150 μm spherical titanium powder have the highest E_corr of −0.2206 V and the smallest I_corr of 6.220 × 10⁻⁸ A/cm². Due to the formation of more Fe-Ti intermetallic compounds, the coatings prepared from irregular powders have more defects and poor relative corrosion resistance.