Objective To design a powder feeding nozzle for ultra-high-speed line spot laser cladding, prepare stainless steel cladding coatings under extremely high cladding efficiency and extremely low overlap rate, and compare the microstructure and performance of the cladding coatings under circular spot and line spot. Methods Based on the simulation study of the flow field of the powder feeding nozzle and the motion trajectory of the powder particles, a powder feeding nozzle dedicated to ultra-high-speed line spot laser cladding was designed. On this basis, with 27SiMn as the substrate, a 1 mm x10 mm line spot was used to clad a FeCr alloy thin coating with an ultra-high-speed line spot laser at a 10% overlap rate and a cladding efficiency of 4.5 m’2/h; as a comparison, an ultra-high-speed circular spot (2 mm) laser was used to clad a FeCr alloy coating at a cladding efficiency of 0.2 m’2/h. SEM and XRD were used to compare and analyze the microstructure and microhardness of the line spot/circular spot coatings. Results The single-channel powder delivery nozzle with a convergence angle of 25°~27° can obtain a powder beam with uniform distribution and moderate flight speed. Comparative study of ultra-high-speed line spot and circular spot laser cladding coatings shows that the coatings prepared by the two types of spots at the same scanning speed are relatively dense, without cracks and pores, and show a trend of plane crystal-columnar crystal-equiaxed crystal from the bottom of the cladding layer to the surface of the cladding layer. The hardness of the line spot and circular spot coatings is 700~800HV. The hardness distribution of the cladding layer under the line spot is more uniform, the surface roughness Ra can be as low as <4 μm, the overlap rate can be as low as 10%, and the cladding efficiency can reach 4.5 m’2/h, which is much higher than the cladding efficiency under the circular spot laser. Conclusion The microstructure, phase composition and hardness of the coatings under the two spot modes are comparable, but the ultra-high-speed line spot laser cladding layer has a higher surface finish, lower surface roughness, and a cladding efficiency of 20 times that of the circular spot.

Laser cladding is a new generation of surface treatment technology. It uses high-energy laser beam radiation to melt the cladding powder and the surface of the substrate material at the same time, and then quickly solidify to form a cladding coating. Laser cladding technology has concentrated heat input, a small heat-affected zone of the substrate, fine grains of the cladding layer, and dense structure. In addition, the technology is simple and easy to automate. It can prepare functional coatings such as wear-resistant, high-temperature-resistant, and corrosion-resistant. It has broad development prospects and is one of the ideal processes to replace electroplating technology [1-3]. Existing data [4] show that the production cost of laser cladding is 6 times that of electroplating chromium. High cost has become one of the important factors restricting the development and application of this technology. As an effective means to reduce costs, improving cladding efficiency has become an urgent problem to be solved in laser cladding technology.

In 2017, the Fraunhofer Laser Institute in Germany and the German Aachen Joint Technology Company proposed ultra-high-speed laser cladding technology [5]. This technology greatly improves the powder utilization rate and achieves a significant improvement in cladding efficiency by optimizing the position relationship between the laser focus and the powder and substrate. At the same time, during the ultra-high-speed laser cladding process, the laser energy mainly acts on the powder, effectively controlling the thermal impact on the substrate[6]. Li Liqun et al.[7] from Harbin Institute of Technology compared the 431 stainless steel cladding layer clad on 27SiMn at cladding speeds of 1.5 m/min and 50 m/min and found that ultra-high-speed laser cladding has less heat input to the substrate, finer structure, more stable cladding layer performance, and a cladding efficiency of 0.35 m’2/h compared to conventional laser cladding. Tan Tai Fanliang et al.[8] from Shanneng Group prepared a uniform and smooth iron-based cladding layer on the surface of a 27SiMn hydraulic support column at a speed of 50 m/min and a 70% overlap rate. The comprehensive performance exceeded that of conventional laser cladding, and the cladding efficiency reached 0.45 m’2/h.

In order to prepare dense coatings with high surface finish, ultra-high-speed laser cladding based on circular spot usually adopts a high overlap rate of 70%~90%. However, Ren Chao et al. [9] found that in multi-layer and multi-pass cladding, there is often softening in the overlap area of ​​the cladding layer, which will cause periodic fluctuations in the lateral hardness. The research of Li Liqun et al. also showed that due to the high overlap rate, the organization of ultra-high-speed laser cladding often presents a “domino” stacking distribution, and its performance will be affected. However, the research of Lian et al. [10] showed that in the process of multi-pass laser cladding, appropriately increasing the overlap rate and reducing the scanning speed can reduce the pore area of ​​the cladding layer. Therefore, in order to ensure high efficiency and obtain high-quality coatings, the overlap rate should be controlled within a reasonable range.

At present, the commonly used spot shapes for laser cladding are circular, annular, and rectangular [11-12]. Among them, the energy of circular spots is mostly Gaussian distribution, which is unevenly distributed, while line spots are easier to obtain uniformly distributed spot energy. At the same time, due to the increase in spot size, the use of line spots is expected to greatly improve processing efficiency [13-17]. Therefore, changing the laser spot and its coupled powder feeding system and developing ultra-high-speed line spot laser cladding technology is the future development trend of laser cladding. Among them, the ultra-high-speed line spot laser cladding head is a key component. Based on numerical simulation of the interaction between powder and laser, it can provide a convenient method for optimizing cladding conditions. However, there are few reports on the design of coaxial powder feeding nozzles for simulating ultra-high-speed laser cladding. The traditional line spot nozzle design and numerical simulation research methods can provide some reference. Guo Xiangyu et al. from Wuhan University of Technology [18] focused on powder and used the Fluent discrete model to study the effects of different convergence angles and different exit gaps on the powder beam state. They designed a coaxial powder feeding nozzle suitable for high-power broadband laser cladding. Chen Ru et al. from the University of Chinese Academy of Sciences [19] designed a powder feeding nozzle structure suitable for broadband laser cladding and additive manufacturing. The structure uses a multi-channel outlet to effectively reduce the divergence angle of the powder in the cladding direction. Hu Xiaodong et al. from Zhejiang University of Technology [20] designed an adjustable size powder feeding system for linear spot. The powder feeding width can be adjusted between 8 and 30 mm to adapt to the change of spot width. The powder utilization rate can reach 50%. Lei Dingzhong et al. from Soochow University [21] designed an internal powder feeding laser cladding nozzle based on external broadband laser cladding to improve powder utilization efficiency. The cladding speed is usually less than 20 mm/s. Yang Jiaoxi et al. from Beijing University of Technology [22] used broadband laser beam to prepare WC/cobalt-based alloy layer on Crl2 substrate, but the cladding efficiency was only 0.16 m’2/h. Gao et al. [23] prepared Al-Si alloy coating on AZ91HP magnesium alloy substrate, but the cladding efficiency could not be further improved. Zheng Min et al. from Guizhou University [24] used broadband laser cladding process to prepare bioactive β-TCP+HA bioceramic composite coating on Ti alloy surface, and a good metallurgical bond was formed between the substrate and the coating. Sun et al. [25] prepared Fe-based coating with ultra-wide laser spot (14 mm), and increased the cladding efficiency to 0.45 m’2/h at the low scanning speed (10 mm/s) of traditional laser cladding.

In summary, ultra-high-speed line spot laser cladding is one of the important development directions of ultra-high-speed laser cladding in the future, and the powder feeding nozzle dedicated to ultra-high-speed line spot laser cladding is the key to this technology. Based on the independently developed line spot optical shaping system and 1 mm×10 mm line spot output, this study carried out a simulation study on the powder flow field and designed an ultra-high-speed line spot laser cladding powder feeding nozzle. On this basis, an ultra-thin FeCr coating with good performance was prepared on the 27SiMn substrate with extremely high cladding efficiency and extremely low overlap rate. The differences in the microstructure and performance of the circular spot and line spot cladding coatings at the same cladding speed were compared and analyzed.

1 Experiment

In this experiment, the base material was 27SiMn steel, and the cladding was performed on the surface of a finely ground solid round rod with a diameter of 30 mm. Before cladding, the surface layer was removed by grinding with fine sandpaper, and the oil was removed by cleaning with alcohol. The cladding material used was spherical FeCr alloy powder with a particle size of 25~50 μm, and its composition is shown in Table 1. Before the experiment, the powder was baked at 90 ℃ for 20 min.

Ultra-high-speed laser cladding was carried out using an MFM-6000W multi-mode continuous fiber laser with a maximum output power of 6 kW and a line spot synchronous coaxial powder feeding cladding head designed by the laboratory. The laser spot was a linear spot of 1 mm×10 mm.

According to the previous exploration process, the coating thickness can be controlled by changing the overlap rate and the number of cladding layers to achieve the preparation of coatings with varying thicknesses of 30~200 μm. At the same time, a 2 mm diameter circular spot was selected to prepare the cladding coating at an overlap rate of 80% for comparative tests. Since the line spot enlarges the spot area, a higher laser power than the circular spot is required to ensure energy input. Therefore, a laser power 3 times that of the circular spot was selected to ensure that the energy density of the line/circular spot was 600~636 W/mm2. Both the protective gas and the powder feeding gas used high-purity Ar gas. The specific cladding parameters are shown in Table 2.

The cladding coating samples prepared by ultra-high-speed line spot/circular spot laser cladding were ground and polished, and cleaned with acetone. The samples were etched with Kohler reagent (95% H2O+2.5% concentrated HNO3+1.5% concentrated HCl+ 1.0% HF, volume fraction) for 10 min, and the coating macromorphology and microstructure were observed using an S-3400 scanning electron microscope, and the coating element distribution was analyzed using its matching energy dispersive spectrometer (EDS). The hardness was tested by HV-1000 microhardness tester, and the test position was from the surface of the cladding layer to the bonding area with the base. Before testing the hardness, the sample was polished and ultrasonically cleaned. The test load force was 0.496 N, and the load time was 15 s.

2 Results and analysis

2.1 Line spot flow field optimization and geometric structure optimization

The line spot powder feeding nozzle needs to match the output rectangular spot. In order to fully melt the powder, it is necessary to ensure the overall airflow is stable, reduce the boundary effect, and make the powder as evenly distributed as possible at the deposition point. In this study, multi-channel and single-channel powder feeding nozzles were designed respectively. Based on Fluent (v16.1) software, the internal flow field of the powder feeding nozzle and the powder distribution based on the discrete phase were studied, and the geometric structure of the line spot powder feeding nozzle was clarified. The k-ε turbulence model that conforms to the internal airflow of the line spot laser cladding is adopted. The flow model uses the Reynolds number time-averaged control equation. The quadratic term will produce additional terms containing pulsation values ​​after time-averaged processing.

The flow field of the multi-channel line spot powder feeding nozzle is shown in Figure 1a. Through calculation, it can be seen that when the multi-channel powder feeding gas is connected to the multi-channel line spot powder feeding head, the airflow is uneven in the initial stage of the flow channel. The more the number of flow channels, the longer the distance for the airflow to converge to form a uniform airflow, which limits the length of the multi-channel line spot nozzle. Figure 1b is a gas velocity cloud diagram at the cross section of the multi-channel nozzle outlet. Due to the mutual influence of the airflow between the flow channels, the airflow on the same plane produces different velocity distributions. The gas velocity at the junction of the flow channels is low. The macroscopic manifestation is that the airflow accelerates the particles unevenly, resulting in non-uniform powder velocity.

Since the airflow in the multi-channel powder feeding head has a greater impact on the particles, the interaction between the particles can be ignored. The discrete phase model is further used to calculate the velocity distribution of the particles in the multi-channel after being accelerated by the airflow. Among them, the force balance equation of the particles is shown in formula (1), where D p ( ) F u u  is the unit mass drag force of the particle, see formula (2).

Figure 2 shows the distribution of particles at different heights from the nozzle outlet when the number of flow channels is 3, based on the discrete phase model. It can be found that at different heights L, the particle distribution is different, and the particle acceleration performance is poor at the overlap of the flow channels. As L gradually increases, the particle distribution gradually tends to be uniform. However, the simulation results show that when a multi-flow channel design is adopted, as the number of flow channels increases, the particle velocity at the overlap can gradually approach the velocity of the particles in the middle of the flow channel, and the particle velocity becomes more uniform as the number of flow channels increases. But at the same time, the increase in the number of flow channels will lead to an increase in the size of the line spot nozzle, resulting in a mismatch between the nozzle and the laser. Therefore, it is difficult for a multi-flow channel powder feeding nozzle to meet the technical requirements, so a small-sized single-flow channel line spot powder feeding nozzle is used.

The airflow velocity of the ultra-high-speed line spot laser cladding powder feeding is much higher than the actual velocity of the particles sent out from the powder feeding nozzle. The particles need to be decelerated inside the nozzle through particle-particle and particle-nozzle collisions. When a single-flow channel powder feeding nozzle is used, the size and convergence angle of the nozzle will affect the deceleration effect when particles collide with each other and when particles collide with the nozzle. This paper studies the collision behavior and trajectory distribution of particles after acceleration by using EDEM discrete element simulation software. A deceleration platform is required inside the nozzle to reduce the particle speed from the high speed state in the powder feeding tube to a speed suitable for cladding. Although it will reduce the particle speed, it will cause the particles to be blocked inside the nozzle. In the actual cladding process, as the powder feeding nozzle is heated, it is easy to cause powder blockage, affecting the stability of the cladding process. Further adjusting the nozzle convergence angle to 25°~27°, a powder feeding nozzle structure with uniform particle distribution can be obtained.

Based on the Hertz-Mindlin non-slip basic collision theory and the Linear Spring basic contact model, the particle velocity distribution after particle-particle and particle-nozzle collision is further obtained. As shown in Figure 3, the particle speed has different reduction degrees under different nozzle structures. The nozzle angle and length affect the particle deceleration degree. When the nozzle angle is small, the particle collision deceleration degree is small, but the small angle corresponds to a longer nozzle, which increases the number of particle-nozzle collisions; correspondingly, when the nozzle angle is large, the particle collision deceleration degree is large, but the corresponding shorter nozzle length reduces the number of collisions. The simulation shows that when the convergence angle is between 25° and 27°, the particle velocity can be reduced by 55% to 65%.

2.2 Macroscopic morphology of coating

Figure 4 shows the surface morphology, roughness and overlap of the coating layer with different spot shapes. The roughness of the circular spot and the line spot are 17.30 µm and 2.06 µm, respectively. It can be seen that a large amount of powder sticking occurs in the circular spot at 2 000 W and 80% overlap rate. This is because the circular laser is limited by the Gaussian distribution of the laser energy. The energy is high in the middle and low around the circular range. Therefore, at a high overlap rate, some unmelted powder in the low energy area will be deposited on the surface of the already formed cladding layer. The overall energy distribution of the line spot is more uniform, and the amount of unmelted powder is less.

Due to the arc-shaped surface limitation of the single-pass cladding layer, the circular spot generally requires a very high overlap rate during the laser cladding process to ensure the smoothness of the cladding layer surface, while the shape of the single-pass line spot cladding layer is more like a trapezoid, so the cladding layer can be operated at a very low overlap rate, ensuring low roughness while greatly increasing the cladding efficiency. Figure 5 is a comparison of the cladding efficiency of the two types of spots. As the cladding speed increases, the cladding efficiency of the line spot becomes dozens of times that of the circular spot. The efficiency of the existing ultra-high-speed circular spot laser cladding and the traditional low-speed line spot laser cladding is not much different, between 0.2 and 0.5 m2/h. The cladding efficiency calculation formula is: See formula (3) in the figure
Where: η represents the cladding efficiency; W represents the spot width; V represents the cladding speed; R represents the overlap rate.

Figure 6 shows the cross-sectional SEM photos of the circular spot and line spot laser cladding layers. It can be seen that the cladding layers of different spots have the advantages of dense structure, no pores, and no cracks. There are obvious differences in thickness of different cladding layers. The thickness of a single layer of a circular spot is about 180 µm, while the thickness of a single layer of a line spot is only 30~40 µm. Therefore, the line spot ultra-high-speed laser cladding coating provides a new way to prepare ultra-thin coatings. At the same time, due to the high overlap rate of the circular spot ultra-high-speed laser cladding, obvious and dense overlap marks can be seen in the cross section of the cladding. The marks are so dense that a similar “multi-layer” structure appears in the cross section of the single-layer cladding layer. This is also mentioned in the literature [7], while the line spot is rarely observed.

2.3 Microstructure characteristics and element distribution of coating

Figure 7 shows the microstructure of the line spot cladding layer. It can be seen that due to the secondary heat input of the laser at the overlap position, the lower structure is slightly coarsened, while the upper part close to the surface cools faster, mostly small, uniform equiaxed crystals, including a small amount of dendrites; due to the decrease in cooling rate in the middle and the increase in temperature gradient, it is mainly columnar crystals along the temperature gradient direction. The current research on the circular spot ultra-high-speed laser cladding coating shows that the surface structure of the coating under the line spot is not much different from that of the circular spot. It can be seen that the spot shape has no effect on the solidification structure.

The line scanning results of the element distribution of the cladding layer are shown in Figure 8 and Table 3. It can be seen that in the cladding coatings under different spots, Fe and Cr elements have obvious transition zones at the interface between the substrate and the cladding layer. For circular spot ultra-high-speed laser cladding, the Fe and Cr elements in the cladding layer and the substrate diffuse within 8 µm, while for the linear spot ultra-high-speed laser cladding coating, the interdiffusion area of ​​Fe and Cr elements is only 4 µm.

The calculation of dilution rate is generally divided into geometric method and composition method. Since the ultra-high-speed laser cladding substrate molten layer is very thin and difficult to measure accurately, the composition method is used for calculation. The calculation formula [26] is: See formula (4) in the figure
Where: ρc is the density of the cladding powder; ρs is the density of the substrate material; xc is the mass fraction of element X in the cladding powder; xc+s is the mass fraction of element X in the cladding layer; xs is the mass fraction of element X in the substrate material. After calculation, the dilution rate of traditional circular spot ultra-high-speed laser cladding is only 4%, while the dilution rate of linear spot coating is about 10%, but it is still much lower than the dilution rate of conventional laser cladding.

During the laser cladding process, the size of the heat input is the key factor that determines the size of the dilution rate. In ultra-high-speed laser cladding, most of the energy acts on the powder, causing the powder to melt during flight. Therefore, in order to ensure that the powder is fully melted, the energy density of the spot cannot be too low. Compared with the circular spot, the spot area of ​​the line spot is larger, so a larger laser power is required during the cladding process to increase the laser energy density. Under the test parameters of this experiment, the heat input of the line spot to the substrate will be slightly larger. However, under the action of greater heat input, the transition zone of the line spot ultra-high-speed laser cladding coating is smaller, which means that the heat input of the substrate to the ultra-high-speed line spot laser cladding is effectively controlled under a significant increase in the cladding speed.

Figure 9 shows the scanning results of the cladding layer. Existing research [26] shows that compared with traditional laser cladding based on circular spots and ultra-high-speed laser cladding, due to the different coupling positions of the laser focus with the substrate and powder, the substrate in the traditional cladding layer will be greatly affected by heat, and elements will diffuse into the coating. For the iron-based stainless steel substrate on 27SiMn steel, the iron content in the coating increases. However, because the laser focus of ultra-high-speed laser cladding is above the substrate, most of the energy is used to melt the powder, and the heat input to the substrate is less, so fewer elements in the substrate penetrate into the coating. The cladding head design of ultra-high-speed line spot laser cladding is still focused above the substrate. Therefore, it is also found on the line spot cladding layer that as the scanning speed increases, the amount of matrix elements penetrating into the cladding layer becomes less and less. It also shows that under the action of large heat input, ultra-high-speed line spot laser cladding effectively controls the element diffusion phenomenon caused by substrate melting under high-speed cladding. Line spot laser cladding can produce coatings with low dilution rate and small thermal impact on the substrate at high scanning speed.

2.4 XRD analysis of coatings

Figure 10 shows the XRD diffraction results of ultra-high-speed circular spot and linear spot laser cladding coatings. It can be seen that the cladding layers prepared by the two types of spots are mainly body-centered cubic α-Fe, and no other new phases are generated. Therefore, even if the change in the spot shape may cause an increase in heat input due to the increase in laser power, it will not affect the formation of the phase.

2.5 Hardness analysis of coatings

Compared with conventional laser cladding, the overlap rate of ultra-high-speed circular spot laser cladding is generally 80%~90%. Even for single-layer cladding, a similar “multi-layer” cladding effect will appear [7]. As shown in Figure 11, there is almost no difference in the effect of ultra-high-speed spots of different shapes on the hardness of the substrate. Regardless of the linear spot or the circular spot, the hardness of the coating prepared by ultra-high-speed laser cladding is higher than the hardness of the substrate. However, the uniformity of the microstructure and performance of the circular spot laser cladding coating prepared under high overlap rate is affected by multiple heating in the overlap area. The coating will have a certain scale of intermittent microstructure coarsening, which has a certain impact on the hardness. Therefore, the overall fluctuation of the hardness of the circular spot coating is relatively large, while the hardness distribution curve of the line spot is smoother.

3 Conclusions

1) The traditional multi-channel line spot cladding powder feeding nozzle is not suitable for the ultra-high-speed line spot laser cladding needs. The single-channel powder feeding nozzle with a convergence angle of 25°~27° can obtain powder with uniform distribution and appropriate speed, which is conducive to improving the cladding quality.

2) The ultra-high-speed line spot laser cladding layer has a dense microstructure, without defects such as pores and cracks. The thickness of the cladding layer can be reduced to 30~40 µm, and the surface roughness Ra is 2.06 µm, which provides a new method for the preparation of ultra-thin coatings with smooth surfaces. At the same scanning speed, the ultra-high-speed laser cladding efficiency based on 1 mm× 10 mm line spot in this study reached 4.5 m‘2/h, which is dozens of times that of circular spot.

3) The microstructure distribution trend of the ultra-high-speed circular spot and line spot coating from the bottom to the surface is plane crystal-columnar crystal-equiaxed crystal, with no obvious difference. The phase composition of the cladding layer is not affected by the spot structure, but the element transition zone of the line spot cladding coating is smaller, and the thermal effect on the substrate is smaller.

4) There is no obvious difference in the average hardness of the ultra-high-speed circular spot and line spot coating, which is between 750~850HV, which is much higher than the substrate. However, the line spot has a smoother microstructure transition and a more uniform hardness distribution.