Zr coating was prepared on the surface of SP-700 titanium alloy by laser cladding technology, and the influence of scanning speed and powder feeder speed on the morphology of laser cladding layer was studied to obtain the best process. The characteristic parameters such as overlap width, melt thickness and crack rate were characterized by optical microscopy; the microstructure, melt thickness and bonding interface of the infiltration layer under different laser cladding processes were analyzed by scanning electron microscopy (SEM); the microhardness, friction factor, wear morphology and wear resistance of the cladding layer were tested by micro Vickers hardness tester, reciprocating friction and wear tester and laser confocal microscope. The results show that when the scanning speed is 6mm/s and the powder feeder speed is 4r/min, the surface forming quality is better, the melt thickness and overlap width are in the middle, and the crack rate of the cladding layer is low; the microstructure distribution in the cladding layer is uniform, the crystals are clear, and the dispersion is low; the microhardness is high, and the overall hardness is 1.8 times higher than that of the substrate; the friction and wear morphology is good, there are no obvious defects, and the wear resistance is good.

In the future, steam turbine units will inevitably develop towards the goal of high parameters, high reliability and long life. The realization of this goal not only depends on the research and development of high-performance materials, but also depends on the progress of performance prediction and life extension technology of its key components. The use of titanium alloys with good performance to manufacture blades is the development trend of steam turbine units and one of the important indicators to measure the level of development of steam turbine technology. SP-700 titanium alloy has a high strength-to-weight ratio and good corrosion resistance, high heat treatment strengthening ability, good ductility and cold working properties, and has great application prospects in the manufacture of turbine last-stage blades. However, blades are prone to surface damage due to high-speed impact of droplets. Surface strengthening technology is an effective technical approach to improve the service performance of titanium alloy components [2]. At present, the anti-corrosion technology of turbine last-stage blades mainly includes inlaying Stellite alloy, electric spark strengthening, high-frequency quenching, flame quenching and thermal spraying [3]. These protective layers can effectively improve the corrosion resistance of blades, but most of them are accompanied by sacrificing the fatigue strength performance of the blade part. Recent studies have shown that laser cladding technology has outstanding advantages in surface modification of high-melting-point metals. Chakraborty et al. [4] prepared a titanium-hydroxyapatite composite coating on the titanium surface and found that the coating helps to improve its overall corrosion resistance. Majumdar et al. [5] clad TiC on the surface of Ti45A15Nbo.5Si alloy and found that TiC can improve the microhardness. Xu Zhengjun et al. [6] prepared a TiB/Ti composite coating on the surface of Ti-6AI-4V. The microhardness of the cladding layer is 1 times higher than that of the Ti-6Al-4V matrix. Su et al. [7] successfully prepared a TC4 coating on the surface of carbon fiber reinforced thermoplastic resin (CFRP) and obtained a reliable bond between the TC4 coating and the CFRP matrix. Wang et al. [8] mixed Ti, Al, and C powders and laser clad the Ti-6AI-4V alloy. When the Al content was 20% or 40%, the coating had a good metallurgical bond with the matrix and the microhardness of the cladding layer increased by 2 times. Laser cladding technology uses a high-energy laser beam to melt the cladding material onto the alloy surface, forming an alloy layer with special physical and chemical properties that is metallurgically bonded to the substrate[9]. Compared with traditional surface modification technology, laser metallization technology can obtain a denser, smoother and well-bonded layer with the substrate. More importantly, it can maintain the toughness of the material while improving the surface performance.

Various coatings have their own advantages and disadvantages. For different protection needs, the most suitable material needs to be selected as the coating. Zirconium has high corrosion resistance, high hardness and good chemical stability. Zirconium is a neutral element. In α-Ti and β-Ti, the solubility of zirconium is relatively high, and it plays a greater role in supplementing and strengthening the substrate material. In terms of plasticity, zirconium is stronger than aluminum and has less adverse effects. Zirconium has excellent mechanical properties, good weldability, and is easy to process. In addition, zirconium can even inhibit the formation of phases. In summary, zirconium has a high research value as a protective material.

The author uses laser cladding technology to prepare coatings on the surface of SP-700 titanium alloy materials, selects zirconium with excellent corrosion resistance as the source material, and performs metal infiltration process; explores the influence of scanning speed and powder feeder speed on the macroscopic morphology, microstructure and performance of the cladding layer, and determines the optimal process.

1 Experimental materials and methods

1.1 Experimental materials

The base material selected in this paper is SP-700 titanium alloy plate. The material is first smelted and then forged into a plate and then tempered. Its composition is shown in Table 1. The cladding material is zirconium powder with a purity of 99.9% and a particle size of 30μm. The zirconium powder is dried to remove the moisture of the powder and improve its fluidity. At the same time, it prevents the cladding layer from having pores or loosening of the whole piece due to excessive powder humidity and powder detachment.

1.2 Laser cladding process and parameters

The laser cladding machine is mainly composed of a laser, a coaxial working head, a powder feeder, a thermometer and a CNC system. The sample is placed below the coaxial working head, and the distance between the laser emission port and the substrate surface to be clad is adjusted by the laser numerical control system. The substrate moves at a uniform speed toward the laser nozzle, and the laser emitter switch is turned on. Under the protection of the protective gas argon, the surface of the sample begins to be clad. The laser parameters used in the experiment are shown in Table 2.

1.3 Measuring equipment and parameters

The main parameters of the cladding layer are the overlap width L, the melt thickness H and the crack rate Ψ. The crack rate is mainly measured based on the number of cracks A, and the calculation formula is: Ψ=A/L.

The JSM-6400F high-resolution scanning electron microscope was used for tissue observation, and the MVT-1000A micro-Vickers hardness tester was used to measure the microhardness of the cladding layer to the substrate. The loading load selected in this experiment is
200g, the holding time is 10s, 10 points per row, 100μm interval, 5 rows horizontally, 300μm interval. Before testing the coating hardness, the surface needs to be screened to avoid defective areas such as cracks and pores to prevent these defects from affecting the hardness value of the coating.

The friction and wear test uses a reciprocating friction and wear tester (model UMT-2). A 440 carbon steel ball is used to make linear reciprocating motion on the surface of the sample, and the load is set to 10N, the friction rate is 20mm/s, the friction stroke is 10mm, and the wear time is 60min. The friction data between the coating and the substrate are obtained, and the friction factor curve is drawn. The three-dimensional wear morphology of the wear scar is obtained by testing the Olympus 4000 laser confocal microscope, and the wear amount (Vm=S cross-section·Lm), wear rate (K=Vm/T), wear resistance (e=1/K) of each sample and the relative wear resistance of the cladding layer and the substrate (e,=E coating/ε substrate) are calculated, where S cross-section is the cross-sectional area of ​​the wear scar, L㎡ is the length of the wear scar, T is the time, e coating and e substrate are the wear resistance of the cladding layer and the substrate respectively.

1.4 Process parameter design

In actual production, metal surface strengthening is generally carried out over a large area, and multi-pass laser cladding is more suitable for actual production. Therefore, this paper mainly studies the multi-pass laser cladding process. The formation of the multi-pass cladding layer is not only affected by the scanning speed and the speed of the powder feeder, but also by the temperature rise of the substrate under laser irradiation and the flow of the molten pool during the cladding process, which may cause the cladding powder to deviate and affect the cladding forming. In addition, the overlap rate is also an important influencing factor. If it is too small, the cladding powder input will be insufficient, the overlap area cannot be filled, and the forming effect is poor; if it is too large, it will cause material waste, coarse molten layer structure, and deformation of the cladding layer. According to the previous exploration work [11, the selected laser power is 700W and the multi-pass overlap rate is 40%. In order to study the influence of different scanning speeds and powder feeder speeds on the surface and cross-sectional morphology of the multi-pass cladding layer, the experimental process parameter design is shown in Table 3.

2 Results and analysis

2.1 Macromorphology

Table 4 shows the macromorphology of the surface and cross-section of the cladding layer. It can be seen that when the scanning speed is constant, as the powder feeder speed increases, the coating surface changes significantly. When the powder feeder speed is 2r/min, the sample surface is dark yellow and relatively smooth, which is the result of oxidation of the titanium alloy during cladding. As the powder feeder speed increases, when the powder feeder speed is 4r/min, the surface is dark black, the roughness increases, there are obvious fish-like patterns, and the continuity decreases. The higher the scanning speed, the higher the roughness. When the powder feeder speed is 6r/min, the cross-sectional view shows obvious transverse expansion cracks. The higher the powder feeder speed, the more obvious the broken groove defect, and the uneven edge of the coating. At the same time, the higher the scanning speed, the more obvious the defect. This is because under the high energy input of the laser, the heat transfer method is mainly heat conduction, so that the coating and part of the substrate melt together to form a molten pool, and the coating is impacted by the laser thermal load. The cooling and shrinkage process of the coating and the substrate is restrained by the cold substrate and produces large stress, and cracks appear in the cladding layer [12]. However, if the powder feeder speed is too high and the scanning speed is too high, the coating liquid cannot be quickly replenished during heating and cooling, resulting in holes, and the strength of the inclusions is low, resulting in stress concentration, resulting in defects such as grooves and transverse cracks, causing uneven edges on the upper part of the coating.

2.2 Overlap width and melt thickness

The overlap width and melt thickness of the cladding layer were measured, and the influence of different powder feeder speeds on the overlap width and melt thickness at two scanning speeds was analyzed based on the measurement data. Figure 1 (a) shows the influence of different powder feeder speeds on the overlap width and melt thickness at a scanning speed of 6 mm/s. It can be seen that as the powder feeder speed increases, the overlap width does not change significantly and is basically horizontal. The average overlap width is 6858.475 μm; when the powder feeder speed is less than 4 r/min, the melt thickness does not change much and is generally horizontal. After the powder feeder speed is greater than 4 r/min, it increases roughly in proportion. Figure 1(b) shows the effect of different powder feeder speeds on the overlap width and melt thickness at a scanning speed of 8mm/s. It can be seen that as the powder feeder speed increases, the overlap width is also basically horizontal and does not change much. The average overlap width is 6830.665μm; while the melt thickness shows a slow upward trend overall. This is because the zirconium powder is fed too fast and accumulates to a certain extent, which increases the melt thickness and the roughness accordingly. By comparing the results of different scanning speeds, it can be seen that the overlap width at a scanning speed of 6mm/s is wider, but the overall change is gentle; the melt thickness is also thicker and the change is more obvious.

2.3 Crack rate

The number of cracks in the cladding layer is measured, and the crack rate is obtained according to the formula Ψ=A/L. The results are shown in Figure 1(c) and Figure 1(d). As can be seen from Figure 1(c), as the powder feeder speed increases, the crack rate generally shows an upward trend, and the crack rate is the largest when the powder feeder speed is 8r/min. This may be because the powder feeder speed is too fast, and the powder cannot be completely clad, resulting in cracks and pores. As can be seen from Figure 1(d), as the powder feeder speed increases, the crack rate tends to increase, but when the powder feeder speed is less than 4r/min, the crack rate changes slowly; when the powder feeder speed is greater than 4r/min, the crack increase rate increases, and the crack rate reaches the maximum when the powder feeder speed is 8r/min. Compared with the scanning speed of 6mm/s, the crack rate of the scanning speed of 8mm/s at the same powder feeder speed is higher. The reason is that the scanning speed is too fast, the molten pool has too much liquid phase, the molten pool is too large, and the cladding produces pores. These pores often lead to the initiation of microcracks, thereby increasing the crack rate.

2.4 Microstructure

Figures 2 and 3 show the cross-sectional microscopic scanning electron microscope morphology of the laser cladding layer at different powder feeder speeds at 6mm/s and 8mm/s scanning speeds, respectively. As can be seen from Figures 2 and 3, during the laser cladding process, the high-energy laser beam causes the zirconium powder to melt rapidly. When the beam is removed, the molten pool begins to solidify from the bottom. At this time, the temperature gradient of the solidification interface is large, the solidification speed is fast, and the solute is enriched in the solidification interface at the front of the matrix, forming a certain degree of component supercooling [16]. The heat of the molten pool is mainly transferred through the matrix material. The heat can be transferred more effectively perpendicular to the matrix direction. Therefore, a large number of cellular dendrites growing perpendicular to the interface appear at the junction. Overall, the boundary between the coating and the matrix area is obvious.

When the powder feeder speed is 2r/min and 4r/min, a large number of needle-shaped crystals and columnar crystals in the middle cross section can be observed in its microstructure. The overall structure is relatively clear and uniform, without obvious cracks and segregation. Through high-power microscope observation, it is found that the dendrites are clearly separated and the equiaxed structure is relatively dense. When the powder feeder speed is increased to 6r/min and 8r/min, the organizational structure is significantly different from the cladding layer when the powder feeder speed is 2r/min and 4r/min. Pores appear in the cross section, the overall crystal dispersion is obvious, and the organizational morphology has changed significantly. A large number of ridge-shaped crystals, needle-shaped crystals and granular crystals appear in the sample at the powder feeder speed of 6r/min. The cladding layer of the sample at the powder feeder speed of 8r/min has obvious cracks, poor surface and unevenness. Too fast a powder feeding speed will cause a large amount of powder to accumulate, and the substrate cannot be completely clad, resulting in unevenness and poor organization. Comparing the results of different scanning speeds, it can be seen that the cross-section of the sample at the scanning speed of 6mm/s has less segregation and fewer defects; while the coating organization at the scanning speed of 8mm/s is relatively dispersed and the grain size is relatively small. This is because the increase in laser scanning speed increases the solidification rate of the laser molten pool, resulting in a decrease in dendrite size and an increase in dispersion.

2.5 Hardness

Figure 4 shows the micro-Vickers hardness of the cladding layer at different scanning speeds (load is 200g). Figure 4 (a) shows the longitudinal microhardness distribution of the cross section of the SP-700 titanium alloy laser cladding layer prepared at different powder feeder speeds at a scanning speed of 6mm/s. As can be seen from Figure 4 (a), the microhardness of the cladding layer of each sample is significantly higher than the hardness of the substrate. As the longitudinal distance of the cladding layer increases, the hardness decreases significantly. Under the dilution effect of the substrate on the coating, the hardness of the heat-affected zone decreases rapidly. The hardness curve changes in different powder feeder speeds are consistent, and the hardness is highest at a powder feeder speed of 4r/min, followed by the hardness at a powder feeder speed of 6r/min. Under these conditions, the zirconium powder fed in is evenly clad under the action of the laser, resulting in better uniformity of the structure, fewer cracks, and higher hardness. At a powder feeder speed of 8r/min, the powder is fed in the fastest, which will cause the powder to be incompletely melted, so the hardness is the lowest. Figure 4(b) shows the average hardness of the cladding layers of four samples prepared at different powder feeder speeds at a scanning speed of 6 mm/s. As can be seen from Figure 4(b), the hardness of the cladding layer shows a trend of first increasing and then decreasing with the increase of the powder feeder speed. The average hardness of the sample at the powder feeder speed of 4 r/min is the highest, reaching 697.8, and the average hardness at the powder feeder speed of 6 r/min is the second, which is 672.3. The coating area is 1.8 times harder than the substrate, and the average hardness at the powder feeder speed of 8 r/min is the lowest, which is 607.4. Figure 4(c) shows the longitudinal microhardness distribution of the cross-section of the SP-700 titanium alloy laser cladding layer prepared at different powder feeder speeds at a scanning speed of 8 mm/s. As can be seen from Figure 4(c), the microhardness of the cladding layers of each sample is significantly higher than the substrate hardness, and the hardness decreases significantly with the increase of the longitudinal distance of the molten layer. Under the dilution effect of the substrate on the coating, the hardness of the heat-affected zone decreases rapidly, which is very similar to the situation at a scanning speed of 6 mm/s. Comparing the results at different powder feeder speeds, it can be seen that the hardness distribution trends of all cladding layers are similar, but the hardness is highest at a powder feeder speed of 4r/min and lowest at a powder feeder speed of 2r/min. Figure 4(d) shows the average hardness of the cladding layer of the sample prepared at a scanning speed of 8mm/s. As can be seen from Figure 4(d), the hardness of the cladding layer shows a trend of first rising, then falling, and then rising again with the increase of the powder feeder speed. The average hardness of the sample at a powder feeder speed of 4r/min is the highest, reaching 657.4. The hardness of the coating area is 1.7 times higher than that of the substrate. The average hardness is the lowest at a powder feeder speed of 2r/min, which is 577.3.

2.6 Friction and wear performance

2.6.1. Friction factor

The curve of the change of friction factor over time can be divided into two stages: the front running stage and the stable stage. Figures 5 (a) to 5 (d) show the curve of the change of friction factor with the powder feeder speed at a scanning speed of 6mm/s. In the running-in stage, the friction time is short and the friction coefficient is low; then the friction coefficient changes slowly, and the fluctuation is relatively stable. At this time, it is in the stable stage [17]. The friction coefficient of the cladding layer is lower than the friction coefficient of the substrate in the running-in stage, while the friction coefficient at the 2r/min powder feeder speed rises slowly. This is because as the friction and wear process goes deeper, the depth of the groove on the substrate surface increases accordingly, the surface gradually becomes rough, and the friction coefficient gradually increases. The friction coefficient at the 2r/min powder feeder speed is lower than the friction coefficient at the other three powder feeder speeds. In the stable stage, the friction coefficient at the 6r/min and 8r/min powder feeder speeds exceeds the coefficient of the substrate, but the friction coefficient at the 2r/min powder feeder speed fluctuates greatly. The 4r/min powder feeder speed performs best under the four processes. Due to the gradual wear of the cladding layer, the zirconium powder formed continuously adheres during the wear process, which plays a certain hindering role, thereby increasing the friction coefficient, resulting in the overall friction coefficient being higher than the friction coefficient of the substrate. Zirconium powder will also fall off during the wear process, which in turn reduces the friction coefficient, causing the friction coefficient to fluctuate in the stable stage at a powder feeder speed of 2 r/min. The fluctuation of the friction coefficient will affect the stability of the cladding layer. The greater the fluctuation, the worse the stability. Figure 5(e) to Figure 5(h) show the variation curves of the friction coefficient with the powder feeder speed at a scanning speed of 8 mm/s. It can be seen that the overall friction coefficient is similar to that at a scanning speed of 6 mm/s, but the overall friction coefficient in the stable stage is higher than that at a scanning speed of 6 mm/s. At the same time, the friction coefficient at a powder feeder speed of 8 r/min varies greatly. This is mainly due to the fact that when the scanning speed is relatively high, the grain structure in the cladding layer is small and the uneven dispersion is large. The grain size and dispersion have a great influence on the strength and hardness of the cladding layer. The smaller the metal grain size and the greater the dispersion, the higher the friction coefficient and the worse the wear resistance [18]. The dense structure of the cladding layer can improve the ability of the cladding layer to resist external particle cutting and plastic deformation. The reduction of cutting stress can reduce the friction coefficient and improve the wear resistance of the protective layer.

In order to comprehensively compare the friction coefficients of the substrate and the cladding layer, the average values ​​of the above friction coefficients are calculated, as shown in Figure 6. As can be seen from Figure 6, the average friction coefficient of the cladding layer at 2r/min and 4r/min powder feeder speeds is smaller than that of the substrate. With the increase of the powder feeder speed, the average friction coefficient of the sample basically increases gradually. On the one hand, the amount of residual zirconium powder in the cladding layer increases, which plays a certain hindering role and increases the friction coefficient of the cladding layer; on the other hand, the sample with a high powder feeder speed has more surface cracks, which will block friction and increase the friction coefficient [19]

2.6.2 Wear morphology

Figures 7 and 8 are the three-dimensional wear morphologies of the wear marks obtained by laser confocal microscopy at scanning speeds of 6mm/s and 8mm/s, respectively, where the I and Z axes are the coordinate systems. Through overall analysis, it can be seen that obvious furrows appear on the surface of the samples. This is because the grinding process is subject to tangential force, which causes the surface to be plowed and cut, and finally forms typical furrows on the grinding surface that are consistent with the friction direction. The wear morphology of the samples under each process is significantly different. The wear marks of the substrate are deeper than those of other cladding layers, while the friction and wear surface morphology of the cladding layer is relatively smooth at low powder feeder speed. Powdery debris caused by the shedding of loose cladding layers can be seen on the material surface at a powder feeder speed of 8r/min, and the substrates on both sides are torn. The same phenomenon also occurs in a small number of areas at a powder feeder speed of 6r/min. On the one hand, due to the too fast speed of the powder feeder, the bond between the cladding zirconium layer and the substrate is not strong enough and the texture is loose, so it is easy to fall off during the entire friction and wear process, and the grinding ball and the substrate have a slight adhesion phenomenon, which leads to increased resistance and increased friction factor; on the other hand, it is affected by the cracks on the surface or shallow surface layer, which effectively hinders the continued development of the furrow. During the wear process, the cracks expand step by step, and the substrate is torn and damaged. Studies have shown that these powder adhesions will increase the friction factor between the coating and the friction pair. For samples with relatively low scanning speeds, the wear morphology of the cladding layer is smoother and the friction width is relatively small.

2.6.3 Wear resistance

The cross-sectional area is obtained by measuring the samples after friction and wear, and then the wear amount, wear rate and wear resistance are calculated. The results are shown in Table 5.

The wear resistance of the substrate is 5.677. After the zirconium layer is clad, the wear resistance at a powder feeder speed of 2 to 4 r/min is improved. The wear resistance at a scanning speed of 6 mm/s and a powder feeder speed of 4 r/min is increased by about 4 times. The wear resistance of other processes is slightly improved. This is due to the uniform crystal structure. These structures can reduce the lattice distortion of the cladding layer during the wear process, which can effectively improve the toughness of the cladding layer, prevent the generation and expansion of cracks, and improve the wear resistance of the cladding layer. However, the wear resistance at a powder feeder speed of 6 to 8 r/min becomes worse. This is because the uneven defects of the organizational structure itself and the cracks that have already occurred in the cladding gradually expand under grinding, causing the molten layer to be easily damaged and fall off, thereby affecting the wear resistance. At the same powder feeder speed, the wear resistance of the high scanning speed is poor.

3 Conclusions

(1) After laser cladding, the boundary between the titanium alloy cladding layer and the substrate is obvious. As the powder feeder speed increases, the macroscopic surface quality of the cladding layer decreases; the molten thickness increases rapidly when the powder feeder speed is greater than 4r/min, and the crack rate shows an upward trend. The crack rate is the highest when the powder feeder speed is 8r/min. As the scanning speed increases, the surface roughness increases, the boundary flatness decreases, and the crack rate increases.

(2) The hardness of the cladding area is the highest. The farther away from the molten layer, the lower the hardness. The hardness is the highest under the process of 6mm/s scanning speed and 4r/min powder feeder speed; the hardness is lower under the process of 8r/min powder feeder speed. As the powder feeder speed increases, the friction coefficient of the sample increases; as the scanning speed increases, the wear resistance deteriorates.

(3) The wear resistance is improved by about 4 times under the process of 6mm/s scanning speed and 4r/min powder feeder speed. The wear resistance is the best, which is the optimal process parameter.