Microstructure and Flight Behaviors of Droplet and its Solidification in Twin-Wire Arc Sprayed NiAl Composite Coatings

Droplet flight and solidification behaviors during twin-wire arc sprayed (TWAS) composite coatings were systematically investigated. Both theoretical model and numerical method were established for calculating the droplet deformation, breakup and solidification process in air flow based on the volume of fluid (VOF) dual-phase flow model jointed with the standard k-ε model. The experimental simulation results indicate that TWAS droplet is broken through explosion or two steps breaking process. The calculation of TWAS gas flight dynamics demonstrates that the TWAS particles are accelerated at first and then slowed down. Microstructure of the TWAS prepared Ni-5wt.%Al and Ni-20wt.%Al composite coating was accordingly characterized by XRD, SEM and TEM, so the phase compositions of the Ni-Al composite coatings were obtained. TEM analysis also showed that an amorphous phase was formed according to the characteristic of diffraction ring in Ni matrix solid solution at an original state.


Introduction
The wire tip is melted and sprayed on a fine droplet through dry compressed air in the twin-wire arc spraying technique (TWAS) for preparing composite coatings.The state of the droplet largely influences the coating structure and properties.Generally, droplet particles with higher velocity would generate coating of higher quality in TWAS.The droplet deformation and breakup are very complicate at high velocity region 1,2 .The research for the droplet breakup through coating microstructure analysis and evaluation is necessary for optimizing the TWAS thus enhancing the coating performance [3][4][5] .
Droplets flight in low velocity region, such as natural air flow, has been investigated by Hu J 6 and Blackwell BC 7 .According to the difference of Weber number, the droplet goes through the varied stages successively with shock breakup, bag breakup, multi-pattern breakup, shear breakup and explosive breakup.Milind K 8 revealed the first and the second dispersed droplets were formed in the atomization gas.The size distribution of the dispersed droplets obeys a simple normal distribution.The pressure of atomized gas mainly determines the final Reynolds number of the flying droplet.The deformation and solidification behaviors of the molten droplet influence the porosity, bonding strength and surface roughness of coatings [9][10][11] .With the increase of both velocity and temperature for the molten particles, surface roughness and porosity of the coating decrease.The flat deformation process occurs during the process of TWAS.Higher impact velocity induces higher coating density and stronger bonding force of the flat particles.Therefore, it is more significant to investigate their deformation and solidification of droplets with a high impact velocity [12][13][14] .In this paper, the volume of fluid method is adopted to track the free surface and the interface of the collision between the droplet and the substrate.Theoretical models were established and the processes of deformation and solidification were analyzed.Through characterization of microstructures of a TWAS Ni-Al coatings, the droplet breakup and solidification behaviors were experimentally studied.

TWAS flight dynamics
In order to analyze and simplify the evolution of droplets during the flight, the interaction among droplets is neglected.The conveyed characteristics are introduced to describe spray droplet particles in the air by single droplet particle.Assuming the high-velocity air flow is isokinetic and isothermal, and droplet particle is spherical.The key point of this analysis is the instability of the droplet particles and the change of the droplet velocity with the time or the flight distance.
The heat transfer of the spray droplets in high temperature and high velocity air flow mainly includes the convection transfer and the radiation between the droplet particles and the air flow.The convection mode would be dominant in consideration of that the droplet particles are heated in the air flow.Physical parameters for numerical analysis are listed in Table 1 [15][16][17][18] .
According to the Lagrangian motion, equation of particles can be obtained as follows: (1) Formula ( 1) is applied for calculating the flight time of the spray droplet particles.where p -particle, g -gas, u p -particle velocity, u g0 -gas velocity, ρ g -mass density of gas, ρ p -density of particles, d p -particle diameter, C 1 -0.33 ρ g /d p •ρ p , t -particle flight time.
In addition, the relationship between the flight distance and the flight velocity of the droplets can be obtained by the integral operation: (2)   where L p -flight distance of droplet particles.Relationship between the temperature and the flight time of droplet particles as follows: (3) where T p -particle temperature, T 0 -room temperature (300 K), T g0 -air temperature, α-heat transfer coefficient, C p -specific heat of particles, d p -particle diameter, ρ pdensity of particles, t -particle flight time.
Fig. 1 illustrates the relationships among the velocity, time and flight distance of Ni-Al droplet.The acceleration of droplet is relatively slow.Assuming the spraying distance is around 200 mm, the larger molten droplets with 50 µm diameter reaching the surface of the work piece can get a velocity up to 15% of the gas velocity.The larger the diameter of droplet is, the smaller the flying velocity is.This also indicates that the size of the droplet is an important factor impacting the final droplet velocity.Fig. 1c indicates that the Table 1.Related processing parameters for numerical analysis.

Materials
Melting point (K) gas velocity is the most vital factor to determine the velocity of droplet.The velocity of the particle with 25 µm diameter can get only ~80 m/s when the gas velocity is 200 m/s, and its velocity reaches ~260 m/s when the gas velocity is 1000 m/s.That is to say, the velocity of the particles increases as the gas velocity increases.In addition, the gas pressure also has a greater impaction on the velocity of particles (as shown in Fig. 1d).Fig. 2 is the relationships between the particle temperature and the flight time.The droplet particles temperature first is increased to a stable magnitude (Fig. 2a) and then slowly decreased during the flight.The cooling rate of the large droplets is lower than that of small droplets during the TWAS process.With the increase of the flight time, the temperature of droplet particles in atomization gas is quickly reduced until reaching room temperature.The wider the droplet particle velocity spans, the higher the droplet particle velocity in the air is and the shorter oxidation exposure time is, thus the coating possesses a relatively higher bonding strength and density.

Atomization droplet breakup behavior
Arc spraying process is a complicated process and the dynamics of droplet formation is modulated by both the atomization gas and the arc.The detailed simulation process is displayed about atomization droplets.Thermo-physical properties of flame gas and particle are listed in Table 2 [15][16][17][18] .
Fig. 3 shows the deformation and breakup process at different time for a droplet with initial Ø1.6 mm and at atomization gas pressure 0.3 MPa.All the color bar units are temperature K in the paper.The droplet particles become crescent at ~10 µs, and the center of the droplet begins to break at ~20 µs.Between 20 µs and 30 µs, droplet particles undergo a transition from the first breakup to the second breakup process.The second breakup is completed at 50 µs, and the particles of the region become fewer at 80 µs.Subsequently, the breakup particles quickly fly out of the computational region.During this process, the droplet particles are firstly deformed, and then the droplet particles proceeded the first breakup, followed by the second breakup.
In the above process, the droplet particle breakup is explosive type.The calculation region of flight distance is ~45 mm, and the time is 80 µs.The droplet particle velocity reaches ~562.5 m/s when the atomization pressure A larger pressure can result in a faster cooling of the broken particles, but the bonding strength will be decreased after final solidification.Fig. 4 shows the deformation and the breakup process at 10 µs with the droplet diameter of 1.6 mm and the pressure range of 0.2 MPa ~ 0.7 MPa.At 0.5 MPa, the crescent center suffers the first breakup, and can cause the second breakup between 0.5 MPa and 0.6 MPa.With the pressure increase, the droplet particles firstly are deformed and then the first broken, followed by the second breakup, and finally formed a stable phase.
Fig. 5 indicates the deformation and breakup process at 30 µs with the droplet diameter of 0.8~3.2mm and pressure of ~0.5 MPa.The particles are completely broken at 30 µs when the diameter of droplet is between 0.8 mm and 1.6 mm.Droplet particles with a diameter of 2.4 mm are broken at 30 µs at the beginning of the second breakup, but the particles with diameter of 3.2 mm are still at the deformation stage at 30 µs, no breakup.Higher pressure is requested to break the molten droplet particles.Fig. 6 shows the dependence of gas velocity on the atomized pressure.With the increase of atomizing gas pressure, the gas velocity obviously increases.The practical spraying particle velocity can about reach Through the following formula of Weber number: We= ρ g d 0 U 0 2 /σ, where ρ g is the atomization gas density, d 0 is the droplet diameter, U 0 is the atomization gas velocity, σ is the surface tension.The relationships between the atomization gas pressure and Weber number for four-kind diameters are obtained through both the gas pressure and the velocity as shown in Fig. 6.With the increase of pressure and the droplet diameter, the Weber number increases.Between 0.2 MPa and 0.8 MPa, the Weber number of the 1.6 mm diameter droplet is in the range of 359.2~1436.7.

Atomization droplet impact and solidification behavior
Fig. 7 shows the deformation and solidification process of a droplet.With the time extension, the droplet spreads quickly and the spreading diameter decreases to 234.4 µm at 0.2 µs with spreading thickness of 6.28 µm; the spreading diameter reaches 637.6 µm at 1 µs.
Fig. 8 shows the deformation of the droplet on the substrate.With the velocity increasing, the degree of particles flatten becomes aggravated.With the increase of the droplet size, both spreading diameter and thickness of the corresponding width are increased.Generally, larger size droplet under high velocity impact will form larger diameter flattened particles.Fig. 9 shows the relationship between the diameter and the thickness of droplet spreading and solidification time.With the extension of the deformation time, the droplet spreading diameter shows an increasing trend.Meanwhile, the larger droplet diameter and the higher velocity are corresponding to the larger spreading diameter.The spreading diameter tends to be stable after 1 µs.As shown in Fig. 9b, the thickness of the spreading droplet is reduced with the extension of deformation time.This spreading thickness is enhanced with the increase of the droplet volume and the decrease of the velocity.Fig. 10 shows the time dependence of droplet solidification temperature field of a droplet with diameter of 50 µm and velocity of 200 m/s.With the time increasing, the area of droplet temperature field becomes larger.The flying edge phenomenon can be easily generated for small droplet particles at higher velocity.With the increase of droplet spreading time on the substrate, flat particle radius increases, and the heat diffusion region extends to the surrounding region along Y axis from the center.Due to the high velocity at the axis center, the temperature decreases rapidly along the Y axis.
Fig. 11 shows the TWAS melt droplet solidification temperature field of a 50 µm diameter droplet at the moment of 1 µs with the particle velocity.For the droplet particles of 50 µm diameter, the temperature field becomes larger with the increase of velocity.The small size droplet particle under the high impact velocity has fast solidification.The high velocity can keep a certain temperature and form some semi-solid flattened particles.Fig. 12 reveals the relationships between temperature and time, and the maximum temperature value is extracted at different time for a 50 µm diameter droplet.It can be seen that the cooling of the droplet is quite fast at the range of 0~10 µs.The temperature decreasing procedure is extended roughly to 50 µs.After 50 µs, all molten droplets tend to near room temperature, and these can be calculated from the rate of solidification of the molten droplet range of 3.1~7.6(×10 7 K/s).Therefore, twin-wire arc sprayed coating has a characteristic of rapid solidification.

Characteristics of powder and microstructure of Ni-Al coatings
Generally, the Ni-Al powder under different technological parameters is mostly collected by using water.Processing parameters for the Ni-Al powders are listed in Table 3 19,20 .The obtained Ni-Al powder is precipitated for final collection, closed and under 373 K drying for 2 h.The surface and cross-section morphology of Ni-Al powder is observed by SEM.With the increase of atomization pressure, the Ni-Al particle size reduces, thus the porosity of the coating becomes smaller and the bonding strength between coating and substrate increases.On the contrary, with the decrease of atomization pressure, the Ni-Al particle size becomes larger, correspondingly the surface roughness becomes larger, and the anti-slip performance and the wear resistance of the coating are therefore improved.By analyzing the morphology of particles in the atomization process, it will play a positive role in guiding the design of spraying processes.
The spraying system used in this experiment is a Praxair/ TAFA controller and model 9935 twin-wire arc spraying gun which is clamped by the manipulator to be sprayed.Before spraying, the surface of specimens should be removed the rust and oil, and roughened treatment.The specimens are cleaned by alcohol or acetone after sand blasting and blown dry by compressed air.The specimens of sand blasting should not be placed more than 4 h before spraying.Related sandblast parameters before arc spraying are listed in Table 4 19,20 .Processing parameters of twin-wire arc spraying are listed in Table 5 19,20 .Process of twin-wire arc sprayed Ni-Al coating is shown in Fig. 14a, and a sprayed specimen is shown in Fig. 14b.The thickness of the coating is controlled by the gun speed.The overlap between the coating and the coating should be appropriate.The temperature of the substrate should not exceed 473 K during the spraying process.The Ni-Al coating was successfully prepared by twin-wire arc spraying with the Ni-5wt.%Alalloy as the bonding layer and Ni-20wt.%Al as the surface layer.
Microstructure and surface morphology of the coating were observed by SEM, as shown in Fig. 15a and Fig. 16a.The coating surface shows relatively homogeneous morphology.

Conclusions
The droplet of twin-wire arc spraying is accelerated at the initial stage, then tends to be stable during the gas flow.The temperature of droplet reaches the highest firstly and then decreases rapidly under the air flow actions.
The theoretical model of metal droplets is established based on VOF dual-phase flow model.The droplet shows a breakup model possessing both two stages breakup in the lower velocity and explosive breakup at high velocity.
The main phase of Ni-5wt.%Alcoating obtains Ni solid solution, NiAl and Al 2 O 3 .The main composition of Ni-20wt.%Alcoating is Ni solid solution, NiAl, Ni 3 Al, Al and Al 2 O 3 .Notably, there also exists an amorphous phase in the original coatings.

Figure 1 .Figure 2 .Figure 3 .
Figure 1.Relationships between the particle velocity and spraying parameters for flight time (a), flight distance (b), gas velocity (c), and gas pressure (d)

Figure 6 .
Figure 6.Relationships between atomizing gas pressure and Weber number

Figure 7 .
Figure 7. Changes of a droplet deformation and solidification with time for Ø50 µm and 200 m/s

Figure 8 .
Figure 8. Changes of droplet deformation and solidification with diameter and velocity at 1 µs

Figure 10 .Figure 11 .
Figure 10.Changes of a droplet temperature field with time for Ø50 µm and 200 m/s

Figure 9 .Table 3 . 4 Figure 12 .
Figure 9. Relationships between droplet spreading diameter and solidification time for (a), and between spreading thickness and solidification time for (b)

Figure 13 .
Figure 13.SEM morphology of Ni-Al powder for surface morphology (a) and inner cross-section morphology (b)

Table 4 .
Related sand blast parameters before arc spraying process.

Table 5 .
Processing parameters of twin-wire arc spraying technique.