A single-pass Ni60 alloy coating was prepared on the surface of TC4 by synchronous powder feeding laser cladding technology, and the effects of laser power and scanning speed on the structure and hardness of the cladding layer were studied. The microstructure of the cladding layer was observed by optical microscope, and the microhardness of the cladding layer was measured by microhardness tester. The results showed that when the scanning speed was constant, the height and depth of the cladding layer increased with the increase of power. When the cladding power was constant, the faster the scanning speed, the lower the height and depth of the cladding layer. The height and depth of the cladding layer showed an overall increasing trend with the increase of line energy EL. Compared with the height of the cladding layer, the line energy had a more obvious effect on the depth of the cladding layer. Among them, the cladding layer prepared under the conditions of scanning speed 3mm/s power 1050W; scanning speed 4mm/s power 1050W; scanning speed 5mm/s power 1350W, the interface between the cladding layer and the substrate was well bonded, without cracks and pores. When the scanning speed is 4 mm/s and the power is 1050 W, the maximum hardness of the cladding layer is 1314 HV, which is three times that of the substrate.
Titanium alloy has the characteristics of low density, high specific strength, good corrosion resistance, good thermal stability and non-magnetic, making it the preferred material in aerospace, petroleum, military industry and biomedicine [1]. However, titanium alloy has poor friction performance, is prone to adhesion and micro-wear, and is easily oxidized under high temperature conditions, which limits the application of titanium alloy as a key moving part [2]. Therefore, improving the surface properties of titanium alloy is of great significance to improving the service life of titanium alloy machines and expanding the application range of titanium alloy.
At present, the surface modification technologies that have been applied to titanium alloys include laser cladding, chemical heat treatment, ion implantation, vapor deposition technology, thermal spraying, etc. [3-4]. However, the coating thickness prepared by chemical heat treatment, ion implantation and vapor deposition technology is only 50 nanometers to several microns, which can only provide moderate wear protection; the thick coating prepared by thermal spraying contains a large number of cracks and micropores, and has poor bonding with the substrate titanium alloy, and cannot withstand severe mechanical wear [5]. In contrast, laser cladding technology has the advantages of controllable coating thickness, metallurgical bonding with the substrate, design of cladding materials according to specific needs, high processing accuracy, and local repair [6-8].
Cladding materials directly affect the quality and performance of the cladding layer [9]. Among them, self-fluxing alloy powders are the most widely used and studied in metal powder material systems. Nickel-based alloy coatings have the advantages of excellent wettability, self-fluxing, wear resistance and low price. Ni60 powder is a nickel-based self-fluxing alloy powder. The Si and B elements it contains have strong deoxidation and self-fluxing effects and form low-melting eutectics [10]. Shen Zehui [11] et al. used laser cladding to prepare nickel-based alloy coatings on the surface of titanium alloys. They studied the addition of different contents of WS2 in Ni60 alloy powder and found that the microhardness of the cladding layer with added WS2 was lower than that of the Ni60 cladding layer. Qin Xin [12] et al. used laser cladding technology to clad NiCrCoAlY Cr3C2 composite coatings on the surface of TC4, and the hardness was 3.8 times that of the substrate. Fan Ming [13] et al. laser clad Ni60/Ni/MoS2 coatings on the surface of TC4 and found that the microhardness of the cladding layer increased with the increase of Ni/MoS2 content. At present, domestic and foreign research mainly focuses on enriching the cladding coating powder system, but there is little research on the effect of laser cladding coating process parameters on the coating structure and mechanical properties.
This experiment uses synchronous powder feeding laser cladding technology, with argon as the protective gas and powder feeding gas. The cladding process parameters are optimized by adjusting the scanning speed and laser power, and finally the process parameters are obtained, which are dense and crack-free, well bonded with the substrate, and significantly improve the surface mechanical properties.
1 Experimental materials and methods
The experimental substrate is a 200mm×200mm×8mm TC4 (Ti-6Al-4V) alloy plate. TC4 alloy is an α+β type titanium alloy. Since an oxide layer of about 10μm thick can be observed on the surface of the untreated titanium alloy under a high-power microscope, the oxide layer absorbs N and O elements in the air, and pores and cracks will appear here after laser cladding [14]. Therefore, before laser cladding, the surface of the TC4 alloy is polished with sandpaper, and after removing the oxide layer, it is cleaned with anhydrous ethanol and dried. The coating cladding material uses Ni60 self-fluxing alloy powder with a powder particle size of 44μm. Its chemical composition is shown in Table 1. The cladding powder was heated and dried in a vacuum drying oven at 200℃ for 2h to remove moisture and avoid defects such as pores in the cladding layer.
The laser cladding experimental equipment uses an IHN-1GX-3000P fiber laser and a 10L carrier gas synchronous powder feeding system. In order to prevent Ni60 powder and TC4 substrate from being oxidized and burned at high temperature, high-purity argon is used as a protective gas and powder feeding gas. The powder feeding gas pressure is set to 0.5MPa, the flow rate is 9L/min, and the powder feeding rate is 50g/min. The laser spot diameter is 4mm, the defocus is 20mm, and the laser cladding process parameters are orthogonal experiments with scanning speeds of 3mm/s, 4mm/s, 5mm/s, 6mm/s and laser powers of 450W, 600W, 750W, 900W, 1050W, 1200W, and 1350W. The cross-sectional metallographic samples were prepared and etched with 4ml nitric acid and 96ml ethanol for 3-5min. The cross-sectional macroscopic morphology of the samples was observed using a Neophot-21 optical microscope. The microhardness from the top center of the cladding layer to the TC4 substrate was measured using an HRD-1000TMC/LCD microhardness tester. The measuring load was 1Kg and the holding time was 15s.
2 Experimental results and analysis
Laser cladding process parameters determine the forming quality of the cladding layer. Inappropriate process parameters will lead to irregular cladding layer forming, weak bonding with the substrate, and excessive dilution rate. Therefore, the selection of laser cladding process parameters is crucial to the quality of the cladding layer.
2.1 Effect of laser process parameters on the macroscopic morphology and surface quality of the cladding layer
Figure 1 shows the macroscopic morphology of some samples of Ni60 cladding layer clad on the surface of TC4. It can be observed from the figure that when the laser output power remains unchanged and the scanning speed increases, the cladding layer width does not change much. When the laser power is 600W and the scanning speed increases, the cladding layer height decreases; when the laser scanning speed remains unchanged and the output power increases, the cladding layer width increases significantly. It can be seen that the laser output power mainly affects the cladding layer width, and the cladding layer width increases with the increase of output power; the laser scanning speed mainly affects the cladding layer height. When other conditions remain unchanged, the cladding layer height decreases with the increase of scanning speed. When the output power is too large and the scanning speed is too small, the molten pool may be overburned; when the output power is too small and the scanning speed is too large, the powder may not be clad on the substrate.
2.2 Effect of laser process parameters on the micromorphology of the cladding layer
As can be seen from Figure 2, at the same scanning speed, the depth of the cladding layer increases with the increase of power. When the laser power is 450W, the laser power is too small, most of the laser energy is used to melt the alloy powder, the laser heat transferred to the substrate is low, and only the surface of the substrate material is melted, but the bonding between the cladding material and the substrate is not strong. When the laser power is 900W, the laser power increases, the laser energy can melt the cladding material, and at the same time, the substrate material is melted to a certain extent. The cladding layer forms a strong metallurgical bond with the substrate and the bonding is strong, and there are fewer defects such as cracks and pores in the cladding layer. When the laser power is 1200W, the cladding layer is irregularly formed. Due to the excessive laser power, obvious cracks appear at the junction of the cladding layer and the substrate. The cladding layer is prone to peeling under the action of external force.
As can be seen from Figure 3, when the laser power remains unchanged and the scanning speed increases, the height of the cladding layer decreases, and the width of the cladding layer also decreases. When the scanning speed is 3mm/s, due to the low scanning speed, the laser stays in one position for a long time, and the tendency of pores and cracks in the cladding layer increases. When the scanning speed is 4mm/s, the laser power and speed match well, the melting height and melting depth are suitable, and no cracks and pores are found. When the scanning speed is 5mm/s, the scanning speed is too fast, the coaxial powder feeding stays in one position for a short time, the height of the cladding layer is small, and large pores appear near the surface of the cladding layer. When the scanning speed is 6mm/s, the scanning speed of the cladding layer is the fastest, the height of the cladding layer is also small, pores appear on the surface of the cladding layer, and cracks appear at the junction of the cladding layer and the base material. In summary, when the process parameters are 4mm/s and 1050W, the cladding layer is well formed.
2.3 Effect of laser process parameters on the height and depth of the cladding layer
Laser cladding is to use a heat source to melt alloy powder with special properties on the surface of the substrate to achieve metallurgical bonding. Figure 4 is a cross-sectional schematic diagram of the cladding layer. The depth and height of the cladding layer are two important data for judging the formation of the cladding layer. Nano Measurer software is used to measure the height and depth of the cladding layer.
Figure 5 (a) is a trend diagram of the cladding layer height changing with laser power under different scanning speed conditions. It can be seen from the figure that when the scanning speed is the same, the cladding layer height generally increases with the increase of power. When the cladding power is the same, the faster the scanning speed, the smaller the cladding layer height. When the laser power is 450W and the scanning speed is greater than or equal to 4mm/s, the heat input is too small due to the excessive scanning speed, and the alloy powder is not melted, and the cladding layer height is almost 0. When the scanning speed is 3mm/s, the cladding layer height is significantly higher than other scanning speeds, but when the scanning speed is 3mm/s and the power is too small, the bonding with the substrate is poor. When the power is too large, pores and cracks are easily generated. Figure 5 (b) is a trend diagram of the cladding layer depth changing with laser power under different scanning speed conditions. It can be seen from the figure that when the scanning speed is the same, the cladding layer height generally increases with the increase of power. When the cladding power is the same, the faster the scanning speed, the smaller the cladding layer height. The influence of laser power and scanning speed on the depth of cladding layer is more significant than that on the height of cladding layer.
When the laser power remains unchanged and the scanning speed changes, the residence time of the laser at a fixed position varies greatly, and the effect on the molten pool varies significantly. The laser power and scanning speed have a comprehensive influence on the formation of the cladding layer, so it is more objective to use the concept of laser cladding line energy. Laser cladding line energy EL is defined as the amount of heat input per unit cladding length, that is, the ratio of laser power P to laser scanning speed Vs, and its expression is: El=P/Vs (1)
The size of laser cladding line energy EL under different process parameters is calculated by formula 1. Figure 6 is a curve showing the relationship between line energy and the height and depth of the cladding layer. It can be seen from the figure that with the increase of line energy EL, the height and depth of the cladding layer show an overall increasing trend. Compared with the height of the cladding layer, the influence of line energy on the depth of the cladding layer is more obvious.
2.4 Effect of output power and scanning speed on the hardness of the cladding layer
Figure 7 is the isovalue distribution diagram of the microhardness of the cladding layer under different process parameters. It can be seen that when the scanning speed is 3m/s and 4m/s, the laser power increases, and the hardness of the cladding layer increases first and then decreases. When the scanning speed is 5m/s and 6m/s, the laser power increases, and the hardness of the laser cladding layer fluctuates greatly. Among them, the microhardness fluctuation of the laser cladding layer with a scanning speed of 4mm/s is small, and the distribution is more uniform, indicating that the matching of the scanning speed and the laser power is reasonable. When the scanning speed is 4mm/s and the power is 1050W, the maximum hardness of the cladding layer is 1314HV, which is equivalent to three times that of the substrate. The hardness of the Ni60A cladding layer is significantly improved relative to the substrate. From the figure, we can also see that under fixed process parameters, the hardness increases with the distance from the surface of the cladding layer, showing a trend of increasing first and then decreasing, and a turning point appears at the cross section of the cladding layer and the substrate material.
2.5 Optimizing the process parameters of the cladding layer
The process parameters are optimized mainly by the cladding layer height, whether cracks and pores are generated, hardness, etc. By analyzing the cladding layer height, depth and hardness under different process parameters, it can be seen from Table 2 that when the power is too small (≤600W), the cladding layer mainly has the problem of too small cladding layer height, and when the power is too large (≥1200W), the cladding layer mainly has the problem of pores and cracks. Only when the laser power and scanning speed match can a cladding layer with basically no defects be obtained. According to Figure 6 and Table 2, combined with the concept of line energy EL, it can be concluded that when the line energy EL is in the range of 260~350, a cladding layer with excellent performance can be obtained.
According to Table 2, when the scanning speed is 3mm/s, power is 1050W; scanning speed is 4mm/s, power is 1050W or scanning speed is 5mm/s, power is 1350W, the cladding layer has good height and almost no cracks and pores. The hardness of the cladding layer is the highest when the parameters are 4mm/s and power is 1050W.
3 Conclusion
(1) When the scanning speed is the same, the depth and height of the cladding layer increase with the increase of power; when the laser power is unchanged, the depth and height of the cladding layer decrease with the increase of scanning speed. The influence of laser power and scanning speed on the depth of the cladding layer is more significant than that on the height of the cladding layer.
(2) When the scanning speed is 4mm/s, the microhardness of the cladding layer fluctuates less and is more evenly distributed, indicating that the matching of scanning speed and laser power is reasonable. When the scanning speed is 4mm/s and the power is 1050W, the maximum hardness of the cladding layer is 1314HV, which is equivalent to three times that of the substrate.
(3) The process parameters were optimized by three aspects: cladding layer height, depth and hardness. The best set of process parameters for laser cladding Ni60 coating on titanium alloy surface were: scanning speed 4 mm/s, power 1050 W.
