The arc spraying technology uses the arc generated by the intersection of two metal wires as electrodes at the port of the spray gun as the heat source to melt the metal wire, and then atomize the molten metal into micro-droplets with compressed air, and accelerate the spray of the micro-droplets A process in which a coating is formed on the surface of the workpiece, followed by deposition and cooling. In the past few decades, arc spraying technology has been widely used for its high efficiency, energy saving, material saving and other advantages, and has gradually become one of the most important thermal spraying technologies. However, the structure and properties of traditional arc spraying coatings are still far from plasma spraying and high-velocity flame spraying (HVOF), which limits the application range of arc spraying technology to a certain extent. High-speed arc spraying technology (HVAS) is an advanced thermal spraying technology developed in the 1990s. Because of its substantial increase in the flight speed of the droplets (up to 350m/s) and further improvement of the atomization effect, it can be Similar high bonding strength, low porosity and high quality coatings greatly broaden the application field of arc spraying.
The properties of arc sprayed coatings are affected by many factors, and their rapid solidification structure is closely related to the kinetics and heat transfer of the droplets during the atomization process. The analysis of the kinetics and heat transfer behavior of the atomization process is not only an important basis for the selection of spraying process parameters, but also helps to correctly understand the formation and evolution mechanism of the high-speed arc sprayed coating. However, due to the limitation of experimental technology, it is difficult to obtain the heat transfer parameters such as droplet temperature and cooling rate by actual measurement. The theoretical model is usually used for numerical simulation calculation, and the determination of atomizing airflow velocity and droplet velocity is the precondition for simulation calculation.
In this paper, based on the theory of gas dynamics and two-phase flow hydrodynamics, a mathematical model of the atomizing gas flow and droplet velocity of high-speed arc spraying will be established, and numerical simulation will be carried out. Average velocity at different spray distances to validate the mathematical model.
1 Mathematical model and numerical simulation
Since high-speed arc spraying is affected by many process parameters, and its atomization kinetics is quite complicated, the following assumptions must be made to simplify the problem to establish the mathematical model of atomizing air flow and droplet velocity.
(1) The motion of the fluid (including the atomized airflow and the molten droplet) is a one-dimensional steady flow.
(2) The molten droplet is formed at the initial moment of atomization, and once formed, it is spherical due to the action of surface tension.
(3) The flight of the atomized droplets is only the result of the drag force of the gas, the influence of its own gravity is ignored, and the collision and adhesion between the droplets are not considered.
1.1 Atomizing Air Velocity
The velocity of the droplets in high-speed arc spraying is determined by the velocity of the atomizing gas stream. The supersonic airflow ejected from the spray gun can be regarded as a single-phase free jet, and its radial velocity is very small, so the axial velocity can be used to approximate the velocity distribution of the atomized airflow. On the basis of theoretical analysis and a large number of experimental results, the literature [4] gives the expression of the axial airflow velocity as:
where u is the axial airflow velocity; u0 is the initial velocity of the axial airflow at the outlet of the spray gun; x is the axial distance; λ is a constant related to the attenuation of airflow velocity, and
In the formula: α is an empirical constant related to aerodynamic viscosity, take α=10.5; Ae=πR20 is the nozzle outlet area; R0 is the nozzle outlet radius. The initial velocity u0 is given bywhere: Jg is the gas flow rate; R is the gas constant; T0 is the gas temperature at the outlet; γ is the gas specific heat capacity; P0 is the gas pressure; ρg is the gas density; At=πR2t is the nozzle throat area ; Rt is the nozzle outlet radius. Find the distribution of the atomizing gas velocity along the axial direction.
1.2 Droplet velocity
There is a speed difference between the droplet formed at the intersection of the two wires and the high-speed atomizing airflow, so the droplet is accelerated by the drag force of the airflow. The motion of a spherical droplet with a diameter of d under a one-dimensional steady airflow can be given in the form of Newton's second law
where: v is the droplet velocity with diameter d; ρg is the gas density; ρd is the droplet density; CD is the drag coefficient. Equation (6) is the motion equation of the alloy droplet. This equation ignores the effects of time-varying and additional mass, that is, the droplet motion is only determined by the drag force of the airflow. The drag coefficient CD is mainly related to the Reynolds number, which is a dimensionless coefficient that characterizes the effect of the gas on the droplet.
In the formula: Re is the Reynolds number of the droplet; μg is the dynamic viscosity of the gas. After differential transformation, we have
The relationship between the axial velocity of the droplet and the spraying distance can be obtained.
1.3 Numerical simulation
Using the mathematical calculation software Matlab5.3, the numerical simulation calculation was carried out on the axial velocity of atomizing air flow and the velocity of Al and 3Cr13 droplets with different diameters. The parameters used in the calculation are shown in Table 1.
2 Test process
2.1 Determination of Atomizing Air Velocity
The velocity of the atomized airflow at the nozzle outlet was measured by the pitot tube total pressure method [7] . If the air flow at the exit of the nozzle is supersonic, a normal shock wave will be generated at the nozzle of the Pitot test probe. The stagnation pressure of the airflow after the normal shock wave is denoted as P1, and the local air pressure is Pa. The Mach number can be determined from the ratio of Pa to P1, and the velocity of the airflow can be calculated. The test principle is shown in Figure 1.
2.2 Determination of droplet velocity
The average flying speed of the high-speed arc spraying droplets at different spraying distances was measured by the double turntable method. The spraying materials and process parameters are shown in Table 2, and the test principle is shown in Figure 2. That is, a narrow slit is opened on the first disk. When the disk does not rotate, the particle beam emitted by the spray gun can pass through this slit to leave a mark on the second disk, and then rotate the coaxial double turntable at high speed to continue spraying. Another mark is left on the second disc. The formula for calculating the average speed of particle flight is
where: L is the distance between the two discs; s is the arc length between the two marks; R is the radius of the turntable; n is the rotational speed of the turntable, and vm is the average velocity of the particles.
3 Results and Analysis
3.1 Axial airflow velocity distribution
The numerical simulation and test results of the axial airflow velocity distribution show that the two are basically consistent. It shows that the mathematical model used can describe the distribution of the axial airflow velocity in the process of high-speed arc spraying well.
The calculated initial velocity of the airflow is 653m/s, and this velocity is maintained within 0.08m from the nozzle outlet, after which it begins to decay with increasing distance. The measured airflow velocity also reflects the same law, except that the airflow velocity fluctuates in the range of 600~700m/s at the initial stage, and then begins to decrease. Since the new high-speed arc spray gun adopts a retracted Laval nozzle, when the high-pressure air of 0.65MPa passes through the Laval nozzle, an under-expanded supersonic airflow is formed, and the airflow continues to expand and accelerate after exiting the tube until the jet boundary (free boundary). The expansion wave is reflected on the free boundary as a compression wave, and the two intersect to form a Mach cone, which causes the axial velocity of the supersonic airflow to fluctuate within a certain distance from the nozzle outlet. As the jet distance increases, the interaction with the ambient atmosphere also increases, the resulting compression wave becomes stronger and stronger, and the axial velocity of the airflow enters the decay stage. It can be seen that the use of the high-speed arc spray gun expands the range of the supersonic region of the airflow, which will not only improve the flying speed of the droplets, but also improve the atomization effect of the droplets.
3.2 Droplet velocity distribution
Figure 4 shows the numerical simulation results of the droplet velocity distribution. 3Cr13 is selected for the calculation, and the diameter of the droplet is 5~100 μm. It can be seen that the droplet is accelerated to a maximum speed by the atomizing airflow and then decelerates. At the maximum speed of the droplet, the airflow velocity is equal to the droplet velocity, and the Reynolds number of the droplet is zero at this time. That is, the droplet has experienced a process of acceleration and deceleration.
The smaller the diameter of the droplet, the smaller its Reynolds number. The smaller the Reynolds number, the larger the drag coefficient CD. Therefore, small droplets can be accelerated to their maximum velocity in a shorter time and distance than large droplets. After reaching the maximum speed, the droplet loses the acceleration force and starts to decelerate because the airflow velocity is lower than the droplet velocity; and the Reynolds number of the droplet with a diameter less than 20μm becomes very low, and the inertial force is very small, and the obstruction effect of the airflow is obvious at this time. . Therefore, the droplet velocity drops rapidly, showing a maximum velocity peak on the curve. The drag coefficient of droplets larger than 30 μm is approximately 0.7 after reaching the maximum velocity, so the large droplets still have a large inertial force without significant deceleration. From the curve, except for the initial stage of atomization, the velocity of the large droplets does not change much with the spraying distance.
3.3 Average velocity distribution of Al and 3Cr13 droplets
Since the double turntable method is used to measure the average velocity of all diameter droplets at a certain distance from the spray gun, in the numerical simulation, the velocity of the average diameter droplets is calculated and compared with the measured value. After the solidified particles sprayed into the water were separated, cleaned and dried, the particle size distribution was counted on the Q500MC image analyzer, and the average diameters of the Al and 3Cr13 particles were calculated to be 20.4 μm and 48.9 μm, respectively.
The results of numerical simulation and actual measurement of the average velocity of Al and 3Cr13 droplets can be considered that the numerical simulation basically reflects the movement law of the droplets. The results show that the high-speed arc spraying atomized droplets can reach a high flying speed driven by the supersonic atomizing airflow. Within the spraying distance of 0.3m, the maximum speed of the Al droplet is about 342m/s, and the maximum speed of the 3Cr13 droplet is about 388m/s, both exceeding the speed of sound (the local speed of sound is about 340m/s). In the traditional arc spraying, the maximum speed of the droplet is about 250m/s [2] . Since the thermal spray coating is mainly mechanically bonded to the substrate and the flat particles, the significant increase in the droplet velocity is beneficial to improve the bonding strength of the interface between the coating and the substrate and the cohesive bonding strength between the flat particles. At the same time, due to the increase of the droplet velocity, the atomization flight time of the droplet is shortened, the temperature when the droplet hits the matrix is also increased, and the spreading ability of the droplet at the moment of impact is increased, which can not only reduce the content of oxides, but also It can reduce the porosity, thereby improving the overall performance of the arc sprayed coating.
4 Conclusion
(1) Numerical simulation is carried out with the established mathematical model of high-speed arc spraying atomization airflow and droplet velocity distribution, and the calculated results are basically consistent with the experimental data.
(2) The speed of the atomizing air flow will maintain the initial supersonic speed within a certain distance from the nozzle outlet, and then attenuate with the increase of the spraying distance. This is related to the interaction of expansion and compression waves generated by supersonic airflow through the Laval nozzle.
(3) The droplets experience a process of first acceleration and then deceleration during the atomization flight. Small droplets can be accelerated to the maximum speed in a short distance; after reaching the maximum speed, the small droplets decelerate rapidly due to the small inertial force, while the large droplets are not significantly decelerated due to the large inertial force. The change in droplet velocity is determined by the Reynolds number of the droplet.
(4) The maximum speed of Al and 3Cr13 droplets exceeds the speed of sound within the spraying distance of 0.3m. (end)
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