In the current aviation industry, laser cladding is an ideal repair and surface treatment technology for TC4 alloy parts, which has advantages over traditional metal repair technology in terms of process. In this work, the surface of the alloy specimen was repaired by laser cladding at a power of 2 kW and different laser scanning speeds were used to detect and analyze the changes in the metallographic structure, electrochemical corrosion performance and mechanical properties of the surface after repair. The results show that significant microscopic morphology changes occurred during the laser repair process; the repair surface with a laser scanning speed of 150 mm/min had the best corrosion resistance; and the repair surface with a laser scanning speed of 200 mm/min had the best microhardness and wear resistance.

TC4 titanium alloy (Ti6Al4V) has many advantages such as low density, light weight, high specific strength, high temperature resistance, corrosion resistance, non-magnetic, and good biocompatibility. It has gained a wide range of applications and is also the first type of titanium alloy to be used in my country’s aviation field [1]. However, the disadvantages of titanium alloy materials, such as high friction coefficient and low hardness, have always affected the performance and service life of its parts. Some mechanical components are prone to fatigue and slight damage[2], and replacing a large number of slightly damaged parts will bring extremely high operation and maintenance costs. Therefore, it is a very valuable research topic to find a low-cost and convenient method to repair the surface of titanium alloy materials to extend the service life of mechanical components and reduce the maintenance cost of enterprise production.

Laser cladding technology is a new type of material surface modification technology[3], which has the advantages of strong applicability, high processing efficiency, good compatibility between the cladding layer and the component substrate, economy and environmental protection, and has been widely used in the field of surface modification of various alloys[4]. Many scholars have made fruitful research in the field of metal material repair and performance enhancement through laser cladding technology. Xia Sihai et al. [5] used laser cladding technology to prepare Ni60A composite cladding layers containing different mass fractions of TiC on the surface of TC4 titanium alloy substrate, which effectively improved the average hardness of the cladding layer and reduced the friction coefficient of the surface. Qi et al. [6] used powder pre-setting laser cladding technology to prepare mixed tungsten carbide particle reinforced metal matrix composite coating on the surface of TC4 alloy substrate. It was found that the mixed coating with TC4 powder added had a more obvious strengthening effect than the pure tungsten carbide coating. The reason is related to the uniformity of tungsten carbide distribution in the coating. Liu Yanan et al. [7] prepared a Ni-based rare earth cladding coating on the surface of Ti811 alloy, enhanced the coating performance by coupling the added rare earth elements, and analyzed the effect of laser scanning speed on the coating structure and performance; Zhang et al. [8] prepared a Ni60-hexagonal boron nitride coating on the surface of TC4 substrate by laser cladding. The results showed that the coating had high microhardness and exhibited excellent wear resistance and friction reduction properties under high temperature environment. Tan Jinhua et al. [9] prepared a titanium-based Ni60+BN composite coating on the surface of TC4 alloy by multi-pass laser cladding, and analyzed the changes in the coating structure and performance at different scanning speeds; Rashid et al. [10] repaired the surface of 300M steel by laser cladding, and studied the changes in tensile properties in multiple directions after repair. Compared with the samples only ground, the samples repaired by cladding had better tensile strength and elastic modulus.

At present, the research on the performance enhancement of TC4 titanium alloy mainly focuses on the use of other elements to enhance the performance of the cladding layer. There are few studies on repair and performance enhancement by cladding the same composition powder [11-12]. This method is simple in process and can also achieve the effect of strengthening the repair coating by controlling the processing parameters. In this work, the repair coating is prepared on the surface of TC4 alloy by laser cladding technology, and the repair effect at different scanning speeds under a certain power is analyzed and compared, and the feasibility and optimal parameter configuration of using high-power and large-spot industrial-grade lasers for industrial repair of damaged surfaces are explored.

1 Experimental materials and methods

1.1 Experimental materials

TC4 alloy is used as the base material, which is wire-cut into 50 mm×15 mm×5 mm specimens. The specimens are surface-polished with sandpaper of different roughness and cleaned with anhydrous ethanol to remove the surface oxide layer and dirt. The cladding material uses TC4 powder of the same material. In order to reduce the influence of oxidation and splashing of the powder on the repair effect during the cladding process, the powder is mixed with ethyl cellulose and anhydrous ethanol under heating conditions to make a prefabricated coating colloid, which is evenly covered on one of the 50 mm×15 mm surfaces of the TC4 specimen to be processed to make a prefabricated coating, and then laser cladding processing is performed. The composition of the substrate and cladding layer materials is shown in Table 1.

1.2 Experimental method

The pre-coated specimens were subjected to single-pass cladding processing using a COHERENT HighLight8000D laser system. The laser cladding processing power P was set to 2 kW, the argon protection flow rate was 4.5 L/min, the spot diameter was 15 mm, and the scanning speed V was 100, 150, 200,
300 mm/min. After the specimens were processed, the macroscopic morphology was observed, and after cutting and embedding, 80#, 240#, 600#, 1000#, and 1500# sandpapers were used for grinding and 5, 1, and 0.05 μ diamond suspensions were used for polishing. The cross-section and cladding surface samples were prepared. The cross-section metallographic samples were etched using a Kroll etching solution with a volume ratio of hydrofluoric acid, nitric acid, and water of 1:4:20. The LEICA MEF4 The metallographic structure of the specimen cross section was observed by a metallographic stereo microscope; the microscopic morphology of the specimen cross section was observed by a S-3400 scanning electron microscope (SEM); the element spectrum of the specimen cross section was scanned by an EDS energy spectrum analyzer; the microhardness of the specimen cross section was tested by an HVS-1000Z micro-Vickers hardness tester; the surface sample of the cladding layer was analyzed by X-ray diffraction (XRD); the electrochemical corrosion experiment was carried out by a Metrohm Autolab electrochemical workstation, and the friction and wear performance of the cladding layer surface was tested by a pin-disc friction and wear tester, with the experimental pressure set to 49 N; the turntable speed was 100 r/min; the rotational friction was carried out for 15 min, the sliding friction force was measured and the friction factor was calculated, and the wear scar depth was detected by a probe-type wear scar measuring instrument after the friction experiment, and the wear volume was calculated. The friction ring has a median diameter of about 4 mm.

2 Results and discussion

2.1 Macromorphology analysis

Figure 1 shows the macromorphology of the cladding surface of the specimen at different scanning speeds. As can be seen from Figure 1, when the specimen is clad at a speed of 100 mm/min, the cladding material and the specimen surface receive sufficient energy, the processed surface is subjected to strong heat, melts rapidly, presents a more obvious molten state, and flows due to the thermal shock of the laser, and cools to form a wavy shape from left to right. The surface oxide film is reddish brown and relatively rough (Figure 1 (a)); when the scanning speed increases to 150 mm/min, the reddish brown area of ​​the surface oxide layer becomes lighter, and the color of some areas turns darker, the surface flow behavior weakens, the melting marks become lighter, and the roughness decreases (Figure 1 (b)); when the scanning speed increases to 200 mm/min, the morphology of the entire cladding surface changes significantly, the reddish brown area gradually degenerates and turns into gray-black, the surface roughness further decreases, and the melting marks are lighter (Figure 1 (c)); when the scanning speed increases to 300 mm/min When the cladding surface is dark gray-black to navy blue, some areas have a certain gloss, there are fine particles attached to the surface, there are almost no traces of surface melting flow, and the overall surface is smoother and flatter (Figure 1 (d)).

Figure 2 is the macroscopic morphology of the cross-sectional metallographic structure of the specimen observed by a stereo microscope. As shown in Figure 2, each specimen has a relatively obvious zoning and stratification phenomenon after cladding. According to different macroscopic morphological characteristics, it can be divided into cladding layer (CL), heat affected zone (HAZ) and unaffected zone (UZ), but its morphology shows different states under different processing parameters.

When scanning at a speed of 100 mm/min, the cross section has obvious traces of heating. The cladding material and the surface of the specimen are fully melted and combined under laser thermal shock to form a thick cladding layer area. The heat-affected zone is inside the specimen matrix. Due to the high heat input, the heating range is expanded to the entire cross section, showing a typical blue marble-like metallographic morphology and a significant phase change. β grains of different shades can be seen in the figure (Figure 2 (a)); when scanning at a speed of 150 mm/min, the colors reflected by different grains show that the cladding layer and the heat-affected zone have consistent color reflections, and the grains are larger than the latter. The position difference between the cladding layer and the heat-affected zone of the substrate can be distinguished, and the two have a close metallurgical bond (Figure 2 (b)); when the scanning speed is increased to 200 mm/min, the three-layer metallographic stratification reappears, the cladding layer and the heat-affected zone become thinner, the grains become finer, and the regional boundaries are obvious (Figure 2 (c)); when the scanning speed is increased to 300 mm/min When the laser energy acts on the surface, a downward temperature gradient is generated, the heat is transferred downward, the surface cladding material melts, and combines with the substrate surface. When the laser energy leaves, an upward temperature gradient is generated, cooling and crystallizing, and after solidification, a cladding repair layer with different thicknesses and physical and chemical properties is formed.

2.2 Micromorphology Analysis

Figure 3 shows the microstructure morphology of the cross section of the cladding repair layer under various scanning speed parameters. When the scanning speed is 100 mm/min, the top is heated and then cooled to form equiaxed crystals distributed in bands and columnar crystals aggregated in parallel. The former constitutes the β grain boundary and separates the latter of different sizes, namely α grains (Figure 3 (a-1)). The top area is heated more, the α grain volume is larger, and the grain shape at the β grain boundary is quite different from that inside. Its direction is shown by the white line. The inside of the β grain is formed by cooling columnar α grains, which show different sizes, different arrangements and growth directions in different β grains (Fig. 3 (a-1)). The cooling rate in the middle of the cladding layer is slower, and the heat continues to transfer downward. Relatively smaller and shorter α columnar grains are formed inside the β grain boundary, which are vertically interwoven with each other. The white circle is the β grain boundary, which is composed of longer paramecium-like grains connected end to end, with a small amount of smaller cellular crystals interspersed in it (Fig. 3 (a-2)) [13]. The heat-affected zone is heated relatively less, the cooling rate is slow, and the microscopic morphology is quite different. The β grain boundary is finer, and the inside grows in different directions to form a shallow needle-shaped α martensite phase transformation. The larger β crystals formed by heating still retain a clear 3 (a-3))[14-15].

When the scanning speed is 150 mm/min, a large number of regular needle-shaped α martensite phase transformations grow along different directions on the top of the cladding layer, stacking and interweaving with each other (Fig. 3 (b-1)). There is an obvious contrast between the different forms of phase transformation zones inside different β grains. Due to the complete metallurgical combination of the cladding layer and the heat-affected zone under this parameter, the metallographic morphology of the two areas tends to be consistent. The needle-shaped martensite inside the β grain is longer, more complete and clear, and symmetrically distributed along the β grain boundary. There are some secondary crystal dendrites growing laterally between the branches (Fig. 3 (b-2))[16]. When this part is enlarged (Fig. 3 (b-3)), it can be seen that the needle-shaped martensite bundles parallel to each other inside the β grain are clustered. When the scanning speed is 200 mm/min, the input heat is significantly reduced, the grains at the top of the cladding layer are refined, and fine α grains are formed. The cladding materials are tightly combined with each other under the action of the laser input heat to form a layer of densely structured coating (Figure 3 (c-1)). There is a partial β transformation structure in the middle and bottom of the cladding layer, surrounded by the α grains that have been transformed (Figure 3 (c-2),
(c-3)); when the scanning speed is 300 mm/min, the input heat continues to decrease, the structure of the top of the cladding layer is further refined, and it is completely cooled to form α grains (Figure 3 (d-1)). There is also a β transformation structure in the middle of the cladding layer, forming a mixed structure with the transformed α grains
(Figure 3 (d-2)). A large number of β-transformed structures and a small amount of α grains appeared at the bottom of the cladding layer. Fine needle-shaped martensite was observed to be precipitating in the β-transformed structure. A cross-intersecting network of shorter needle-shaped α-martensite was formed in the heat-affected zone. The two structures were interconnected at the junction to form a clear and fine interwoven structure (Fig. 3 (d-3)) [17].

2.3 XRD and EDS energy spectrum analysis

The surface of the laser cladding layer was analyzed by X-ray diffractometer, with the initial scanning angle 2θ of 10°, the end angle of 100°, and the scanning step length of 0.02°. Figure 4 shows the XRD spectrum of the specimen in the range of 30° to 85° at different scanning speeds.

As shown in Figure 4, there is a large amount of α-Ti in the laser cladding layer at different scanning speeds, and there are certain differences in the composition of other phases. When the scanning speed is 100 mm/min, α-Ti and β-Ti coexist in the cladding layer, which makes the diffraction peaks overlap and the intensity increases, and a small amount of AlTi2C intermetallic compounds are produced, and the corresponding unique diffraction peaks appear; when the scanning speed is 150 mm/min, the content of β-Ti at the top decreases and gradually transforms into α-Ti, the height of the diffraction peak decreases, and the phase change state of the cladding layer and the heat affected zone of the substrate are close, and the diffraction peak shows α-Ti; when the scanning speed is 200 mm/min, the surface of the cladding layer is almost composed of α-Ti in the melting process, and the diffraction peak shows an obvious α-Ti style, but Ti(CNH)x solid solution appears in the cladding layer, and its diffraction peak intensity is low; when the scanning speed is 300 mm/min, the melting degree of the cladding material continues to decrease, and the diffraction peak morphology is similar to that when the scanning speed is 200 mm/min, and Ti(CNH)x also exists. Solid solution, the reason for its appearance is that the prefabricated coating colloid and some components in the air, such as C and H, are mixed into the cladding layer to form a solid solution.

The EDS energy spectrum scanner is used to perform a line scan of the bottom of the cladding layer and the junction of the heat affected zone. Figure 5 shows the change of the relative content of Ti/Al at the junction of the bottom of the cladding layer and the heat affected zone along the scanning distance. It can be found from Figure 5 that the relative content of the two elements Ti and Al has relatively obvious characteristics. From the horizontal comparison, it can be seen that at a scanning speed of 300 mm/min, the relative content of Ti/Al has a large fluctuation along the measurement line, and the Ti content at some positions is significantly higher than that at other positions, and element segregation occurs. The element content at a certain position on the cladding layer at a scanning speed of 300 mm/min is detected. Figure 6 (a) shows the distribution of Ti elements (green area), and Figure 6 (b) shows the distribution of Al elements at the corresponding position (red area). According to the results, there is an obvious uneven metallographic distribution of Al in the β transformation structure. The Al content inside the transformed α grains is low, while the content at the edge is similar to that of the surrounding grains. This may be due to the high scanning speed, small laser heat input, rapid cooling and solidification of the cladding layer, and the formation of the α-Ti lattice when the nucleation growth occurs in the β grain boundary. The Al element in the molten pool has no time to diffuse inward. Al atoms only enter the α-Ti edge lattice and solidify, resulting in dislocation, forming a solid solution, and Al segregation. A more prominent equiaxed crystal morphology appears in the metallographic structure, and a small amount of Ti(CNH)x solid solution should be formed inside due to the lack of Al element. At the same time, part of the α-Ti/Al solid solution lattice structure that has been formed around is dissolved by the metallographic etching solution, and the rest of the lattice structure except the newly formed equiaxed crystal still maintains the original Al content level.

Combined with the scanning electron microscope test results in Figure 3, it can be further learned that during the laser irradiation of TC4 powder pre-coating, the solubility of the α-Ti solid solution changes with the increase of the scanning speed, which is a non-equilibrium rapid melting and solidification process. In the process of further cooling of the cladding material and the substrate surface from the β phase, the smaller the scanning speed, the more heat input of the laser to the cladding surface, the higher the temperature reached by the surface, the smaller the average cooling rate, and the longer the solidification time. When the molten pool reaches the supercooling degree, the β crystal is formed first, and the α phase nucleates at multiple points in the original β crystal grain boundary and grows inside the β crystal along a certain direction. The relatively abundant Al atoms move at high speed in the molten pool with the heat shock convection. In the process of the growth and formation of the α phase, they are “captured” by the lattice, thus forming the α-Ti solid solution and needle-shaped martensite [18]. When the scanning speed is 200 mm/min, the input heat causes the cladding layer to form more Ti(CNH)x solid solution. The more C elements affect the determination of the Ti content, resulting in a lower Ti content in the determination result. At this time, the Al element is still fully diffused and has not segregated, resulting in a lower relative content of Ti/Al. In contrast, when the scanning speed is low, the cladding layer mainly forms α-Ti solid solution and needle-shaped martensite, and the influence of the C element is small; when the speed is high, the Al element segregates, and the segregation is serious in some positions, resulting in a higher relative content of Ti/Al and greater fluctuations.

2.4 Analysis of corrosion resistance of cladding layer

An electrochemical corrosion test was conducted on the cladding layer on the surface of the specimen. A sodium chloride solution with a mass fraction of 3.5% was used as the electrolyte. The surface of the specimen cladding layer was connected as the working electrode, the platinum electrode as the auxiliary electrode, and the Ag/AgCl electrode as the reference electrode to form a three-electrode electrochemical measurement circuit. After the open circuit potential was stabilized, the electrochemical impedance spectroscopy (EIS) and scanning voltammetric polarization curves were measured. Figure 7 is the Nyquist spectrum of the electrochemical impedance of the cladding layer. As can be seen from Figure 7, with the increase of scanning speed, the radius of the capacitive reactance arc shows a trend of gradually increasing. Except for the specimen with a scanning speed of 150 mm/min, the other three specimens all show a 1/4 arc shape and gradually tend to be horizontal, proving that the protective performance of the cladding layer is gradually improving; while the surface of the 150 mm/min specimen is close to a 1/2 arc, and the two-phase film impedance of the cladding layer is in a stable range.

Figure 8 is a Bode diagram of laser cladding layers at different scanning speeds. As shown in Figure 8 (a), the resistance values ​​of the specimens in the medium and high frequency regions are relatively close. At the beginning of the low frequency region, the surface resistance value of the 300 mm/min specimen is relatively high. As the frequency increases, the surface resistance value of the 150 mm/min specimen gradually approaches it, and then it is basically at the same level. The resistance property of the cladding layer gradually improves, the capacitance property weakens, and the protection performance improves; as shown in Figure 8 (b), the images of the 150 mm/min and 300 mm/min specimens in the low frequency region obviously move toward the large angle direction. At about 1 Hz, the angle of the 150 mm/min specimen exceeds that of the 300 mm/min specimen, and the resistance difference reaches the maximum value at this time. In addition, lg Z-lg f is close to a linear relationship in the medium frequency region. The reason is that when the corrosive medium diffuses on the surface of the cladding layer, it encounters the obstruction of fine particles on the surface of the cladding layer and can only penetrate inward along the gaps between particles.

Figure 9 shows the voltammetric polarization curves of the laser cladding layer surface at different scanning speeds. The electrochemical properties of the cladding layer surface can be obtained according to the polarization curve. The self-corrosion potential and corrosion current density of the cladding layer surface can be calculated by the Tafel zone. The annual corrosion rate and polarization resistance can be calculated by software[19]. The measurement results are shown in Table 2.

From Table 2, it can be seen that when the scanning speed is 150 mm/min, the self-corrosion potential of the cladding layer is the highest, the self-corrosion current is the lowest, the self-corrosion rate is the lowest, and the corrosion resistance is the best. The reason is that the cladding material and the substrate surface are fully melted under the heat transfer of laser irradiation, with a high degree of metallurgical bonding, completing the β crystal transformation and needle-shaped α martensite phase transformation. The needle-shaped martensite formed on the cladding surface is arranged into a dense structure, restoring the excellent corrosion resistance of TC4 alloy. Compared with the surface processed at 100 mm/min, the grain size is larger; secondly, the scanning speed is 300 mm/min. Due to the small amount of heat absorbed, the cladding material is not completely melted, and the grains on the surface of the cladding layer are small and dense, arranged compactly, and closely combined with the substrate through the network martensite structure, which greatly reduces the reaction contact area, reduces the self-corrosion rate of the surface, and increases the polarization resistance while reducing the self-corrosion current. The cladding layer under these two parameters shows good corrosion resistance.

2.5 Microhardness analysis of cladding layer

Figure 10 shows the Vickers microhardness of laser cladding layers at different scanning speeds and different surface distances. Generally, the Vickers microhardness of the TC4 alloy substrate is about 300 HV[20], and the microhardness of the cladding layer is higher than that of the substrate. When the scanning speed is 200 mm/min, the hardness of the top of the cladding layer is relatively high; as the distance from the surface gradually increases, the microhardness of the specimens with scanning speeds of 150, 200, and 300 mm/min decreases rapidly. At 1 mm, the microhardness of the 150 and 300 mm/min specimens is close to the unheated area of ​​the substrate, and the 200 mm/min specimen has an obvious three-level stepped hardness distribution, divided at 0.75 mm and 2 mm. The surface hardness above 0.75 mm is relatively high but decreases rapidly, and below 2 mm is close to the hardness level of the substrate. The reason may be that the grain refinement of the cladding layer surface increases the hardness of the surface layer, but the refined layer is thin, and the rapid decline interval corresponds to the heat affected zone and unheated area of ​​the substrate.

The hardness of the interface between the cladding layer and the substrate of the 100 mm/min specimen increased slightly, and the hardness decreased relatively little in the entire measurement range, with a relatively high overall hardness. The reason may be that more laser energy was absorbed, and some air components were mixed into the cladding layer, forming AlTi2C dendrites, and the cladding material and the substrate were completely metallurgically bonded. During cooling, the temperature gradient at the interface was large and the solidification rate was fast, which resulted in grain refinement at this location and enhanced surface microhardness[21]. This performance enhancement and higher overall hardness can enable parts repaired by laser cladding to maintain certain mechanical properties after slight wear, reducing performance degradation caused by surface loss.

2.6 Analysis of wear resistance of cladding layer

Figure 11 shows the friction factor and wear scar profile in the friction and wear experiment.
In general, adding high-hardness reinforcing phase to the surface to improve wear resistance or adding lubricating phase to reduce friction factor is an effective means to improve the friction and wear performance of titanium alloy surface. As shown in Figure 11, with the passage of friction time, the friction factor of the substrate fluctuates slightly, and the friction factor of the repaired surface with a scanning speed of 100 mm/min fluctuates the most. There is obvious accumulation of friction debris on both sides of the wear scar of each surface. Table 3 gives the wear scar measurement results of the surface of laser cladding layer with different scanning speeds.

As shown in Table 3, the wear scar depth of the cladding layer is lower than that of the substrate, and the wear scar width is smaller than that of the substrate. The wear scar depth and width of the cladding layer with a scanning speed of 200 mm/min are the smallest, and the wear weight loss is the lowest. The wear resistance of a material is positively correlated with its surface hardness, that is, the higher the surface hardness of the material, the better the wear resistance[22]. The surface of the specimen with a scanning speed of V3 has finer grains, higher microhardness, and is fully bonded to the substrate surface, resulting in a large number of grain boundaries. Grain boundaries have an inhibitory effect on the movement of dislocations. The more grain boundaries there are per unit area, the more significant this inhibitory effect is, thus reflecting better wear resistance. However, due to insufficient melting, the repaired surface with a scanning speed of 300 mm/min has a relatively low bonding degree of the cladding material, which causes greater damage during friction and a relatively deeper and wider wear mark.

3 Conclusion

(1) The TC4 alloy surface was repaired by laser cladding at different scanning speeds. When the scanning speed was 150 mm/min, the cladding repaired surface had better corrosion resistance. The repaired surface generated a stable crystal structure, had a good metallurgical bond with the substrate surface, had the highest self-corrosion potential and the lowest corrosion rate.

(2) When the scanning speed is 200 mm/min, the microhardness of the cladding repair surface can be improved, and the wear amount is small. The large number of fine α grains generated by the transformation hinder the movement of lattice dislocations caused by external impact, which is manifested as higher microhardness and smaller wear amount on a macro scale.

(3) During the cladding process, the cladding material and the surface of the substrate are heated, melted, convected, and then cooled and solidified. Some elements in the alloy will inevitably dissolve in the structure reinforcement phase or substrate during lattice reorganization, resulting in lattice distortion, achieving a certain strengthening coating effect, which is manifested as comprehensive performance that is strengthened or maintained in different dimensions on a macro scale.