Measurement of particle velocity and characterization of deposition in aluminum alloy kinetic...

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Applied Surface Science 252 (2005) 1368–1377

Measurement of particle velocity and characterization of

deposition in aluminum alloy kinetic spraying process

Jingwei Wu a, Hongyuan Fang a, Sanghoon Yoon b,HyungJun Kim c, Changhee Lee b,*

a State Key Lab of Advanced Welding Production Technology, Harbin Institute of Technology, 150001 Harbin, Heilongjiang, PR ChinabDivision of Materials Science and Engineering, Hanyang University, 17 Haengdang-dong,

Seongdong-ku, Seoul 133-791, South KoreacWelding Research Center, Research Institute of Industrial Science and Technology, Pohang, South Korea

Received 9 November 2004; received in revised form 17 February 2005; accepted 17 February 2005

Available online 2 April 2005

Abstract

Particle velocity is a very important parameter in kinetic spraying (or cold gas dynamic spraying). It is difficult to measure the

velocity of a particle with supersonic speed at low temperature (lower than 500 8C). Thus, in many investigations only estimated

values are used for evaluating coating processes. In this paper, the modeling of particle acceleration was reviewed, and the

measurement of in-flight particle velocity in a kinetic spraying process was performed. Particle velocity and flux distributions

from different process gas temperatures and pressures were investigated. The influences of process gas temperature and pressure

on particle velocity were discussed. Characteristic of Al–Si feedstock deposition onto a mild steel substrate was described by

comparing coatings structures with the in-flight particle conditions. The deposition behavior showed two critical particle

velocities for Al–Si powder deposition onto a substrate and for particle–particle bonding.

# 2005 Elsevier B.V. All rights reserved.

PACS: 104570

Keywords: Kinetic spraying; Cold gas dynamic spraying; Particle velocity; Deposition; Critical velocity

1. Introduction

During kinetic spraying processes, small solid

particles are accelerated to supersonic velocity and

impinged onto a substrate. The kinetic energy of high

* Corresponding author. Tel.: +82 2 2290 0388.

E-mail address: [email protected] (C. Lee).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2005.02.108

velocity particles is a major factor in impinging and

deformation behaviors. Critical particle velocity is

accepted for characterizing this spray technique [1–3].

Numerous experimental studies have shown that

particles require a minimum critical velocity in order

to deposit onto a substrate. The data suggested that

higher impact velocities yield better coating bond

strength, and lower porosity in the coatings [4,5].

.

J. Wu et al. / Applied Surface Science 252 (2005) 1368–1377 1369

For system optimization and nozzle design, many

studies have focused on flying particle behaviors. In

the early work of Dykhuizen et al. [6], the effect of

Mach number (speed of sound) on particle velocity

was investigated based on an isentropic gas flow

model. In another study by Dykhuizen [4], particle

acceleration and deceleration models were discussed.

Van Steenkiste et al. [5,7] built a simple one-

dimensional model for particle velocity and tempera-

ture computations. Another numerical simulation of

gas and particles flow of the kinetic spraying process

was done by Voyer et al. [8].

During the kinetic spraying process, high-pressure

and pre-heated gas obtains a supersonic velocity by

passing through the throat region of de Laval type

nozzle. In the isentropic gas flow model, sonic

conditions of flow gas were only obtained for

sufficient stagnation gas pressures [6]. After the sonic

condition is obtained, increasing the pressure does not

result in an increase in gas velocity upstream of the

throat [7]. In this model, the gas velocity ng can be

expressed as:

ng ¼ Mns ¼ M

ffiffiffiffiffiffiffiffiffigRT

Mw

r(1)

where M is local Mach number, ns is the speed of

sound, g is the ratio of specific heats. For monatomic

gases g is 1.66, and for diatomic gases g is typically

1.4. R is the gas constant (8314 J/kmol K). T is gas

temperature andMw is the molecular weight of the gas.

Local Mach numberM only depends on the inner form

of the nozzle, the calculation of which is presented by

Dykhuizen [4,6] in detail.

In one-dimensional numerical models about par-

ticle acceleration the by high speed gas [4,8], particle

velocity can be calculated by a drag force on a single

particle in a fluid flow. When nitrogen is used as the

process gas, Alkhimov et al. [9] obtained an empirical

equation which includes the particle velocity, the gas

velocity and the gas pressure:

np ¼ng

1þ 0:85ffiffiffiDx

q ffiffiffiffiffiffiffirpn

2g

P0

q (2)

Here np is the particle velocity, P0 is the nitrogen

supply pressure measured at entrance of the nozzle, rpis the particle density, D is the particle diameter, and x

is the axial position. When we replace the gas velocity

with Eq. (1), the simple empirical correlation of the

particle velocity, the gas temperature and pressure can

be rewritten as:

np ¼1

1M

ffiffiffiffiffiffiffiMN2

gRT

qþ 0:85

ffiffiffiDx

q ffiffiffiffirpP0

q (3)

whereMN2is the molecular weight of the nitrogen gas.

Particle acceleration and particle velocity can be

obtained from the isentropic gas flow model and one-

dimensional numerical particle acceleration model.

However, there has never been detailed investigation

about the velocity distribution of in-flight particles.

Even more, there is little experimental data available

about the velocity distribution of in-flight particles. In

current study, a professional velocity measuring

device is used, and experimental velocity measure-

ment results are shown. Aluminum alloy feedstock

deposition behaviors are observed, and an analysis of

deposition characteristic to in-flight particle condition

is conducted.

2. Experimental procedures

2.1. Spraying system

In this study, a commercially available CGT kinetic

spraying system was used. The system and the coating

process have been described in detail in other studies

[10–12]. A de Laval type nozzle with a round shape

exit was used. Nitrogen gas was used for both the

process gas and the carrier gas. The process gas

pressures used were 21, 25 and 29 MPa. The process

gas temperatures used were 300, 400 and 500 8C. Theflow quantity of carrier gas was set at 10% of the

process gas.

Al–12Si powder with a mean particle size of 25 mm

and a density of 2.66 g/cm3 was used as feedstock. A

micrograph and the size distribution of the feedstock

are shown in Fig. 1. The feeding rate was 8.0 g/min.

Mild steel was used as the substrate. Two kinds of

surface conditions were prepared by polishing and

grit-blasting. The roughness of as-polished surface

and grit-blasted surface were Ra 1.52 and 17.98,

which were measured with a laser scan microscope.

Optical microscopy, X-ray diffraction (XRD) and

scanning electron microscope (SEM) were used for

normal analysis. The deposition efficiency (DE) of the

J. Wu et al. / Applied Surface Science 252 (2005) 1368–13771370

Fig. 1. Feedstock: Al–Si (78:12, w/w), spherical, +5 mm �45 mm (mean size 25 mm): (a) SEM micrograph of Al–Si powder morphology; (b)

Laser scatter value of Al–Si powder size distribution (volume fraction).

Fig. 2. Mounting of the SprayWatch system (copies from Spray-

Watch-2i user’s manual).

coating was measured from the weight change of the

specimen. The surface and cross section morphologies

of coatings were observed.

2.2. Particle velocity measurement

A high speed camera (SprayWatch camera unit)

was used to monitor the spraying processes. A laser

device (HiWatch laser unit) was used for particles

illumination. These two units were connected to a

personal computer, which was used for system

controlling, calculation and display. The spray watch

system mounting is shown in Fig. 2. The size of the

analyzed region was 20 mm � 20 mm � 1 mm. The

center of the measured region was 30 mm from the

nozzle exit in the axes direction. A multi-pulse

imaging technique was used in this system. During

one exposure of the camera, three or more laser pluses

were emitted (Fig. 3a). Then particle’s stretch image


Fig. 3. Measurement of in-flight particle velocity; (a) corresponding

action of camera and laser; (b) sketch image of flying particles; (c)

calculation of particle velocity.

Fig. 4. Measured particle velocity at the gas condition of 2.5 MPa

and 400 8C.

was obtained, as shown in Fig. 3b. The particle

velocity was calculated from the interval time of laser

emission and the particle flying distance measured

from image, as shown in Fig. 3c.

Fig. 5. Particle velocity distribution curves grouped by the process

gas temperature.

3. Results and discussion

3.1. Particle velocity measurement

After passing thought the nozzle, the high velocity

flying particles formed a distribution rod with a

diameter about 10 mm in the measured region. The

cross section images of the spraying beam on a vertical

plane in the axial direction were obtained from the

spraywatch camera. From these images, particle

velocity could be measured. In Fig. 4, one point

shows one particle’s velocity, and the abscissa is the

particle’s position in vertical direction. Fig. 4 shows

that the particle velocity distribution at 2.5 MPa and

400 8C is symmetrical with the axis of the spray beam.

The particles at the center of the beam have the highest

velocities. For simplification, a Gauss fit curve was

fitted to the data.

Fig. 5 shows particle velocity curves for all

experimental process gas conditions used in this

study. The velocities of the center particles ranged

from 640 to 820 m/s (Table 1). The curves are divided

into three groups according to the gas temperature.

Particle velocity increased with increasing process gas

pressure. At higher temperature, the gas pressure has a

greater effect on particle velocities. When the gas

pressure was increased from 2.1 to 2.9 MPa, the

velocity of the center particles increased from 675 to

820 m/s at 500 8C, while the velocity increased from


Fig. 7. Comparisons of the experimental results to Alkhimov’s

correlation.

Table 1

The velocity of center particles at all process gas conditions

Particle velocity (m/s) Process gas temperature

300 (8C) 400 (8C) 500 (8C)

Process gas pressure (MPa) 2.1 640 655 675

2.5 680 715 760

2.9 705 760 820

640 to 705 m/s at 300 8C. For all gas conditions, thevelocity distributions are symmetrical and the

particles in center have the highest velocity. A very

similar result is found in Fig. 6, where the curves are

grouped according to the gas pressure. Fig. 6 shows

that increasing the gas temperature causes faster rising

of particle velocities at high pressure.

During the spraying process, gas pressure and

temperature are used to control the particle velocity.

Fig. 7 shows the comparisons of the experimental

results to Alkhimov’s correlation (Eq. (3)), where the

local Mach number at the nozzle exit for our experi-

mental nozzle was calculated as 3.91. The numerical

particle velocity ofAlkhimov’s correlation is calculated

considering the particle size distribution. That is say, in

each gas condition the velocities of all particles with

different sizes are computed, andmean particle velocity

is the sum of each velocity times its volume fraction in

Fig. 1b. The results show that the calculated values well

match with the experimental results.

Further more, the flux distribution of flying

particles can also be obtained from the spraying

images. The measured region is divided into many

sections in the radial direction (Fig. 4). The particles

Fig. 6. Particle velocity distribution curves grouped by the process

gas pressure.

number through each section is counted, and then the

particle flux distributions can be determined. Fig. 8

shows the relative flux at all gas conditions. There are

small differences among these curves. The gas

temperature and pressure have a little influence on

flux distribution of flying particles. Most particles are

aligned at a cylinder around nozzle centre with a

diameter of about 10 mm. These curves can be

regarded having a normal distributions, which is

affected by the nozzle type, powder feeding rate and

the distance from the nozzle exit.

3.2. Spraying trace deposition characteristics

The distribution rod of Al–Si particles with a

diameter about 10 mm spraying onto the mild steel

substrate were investigated by single, double and multi

Fig. 8. Particle flux distribution.


pass spraying. An understanding how the coating

builds up can be obtained by dividing the spraying

process into four basic phenomena [13]:

Stage 1: Substrate cratering and first layer build up of

particles (initial cratering, deformation of substrate,

trace formation, fracturing of surface oxides on metal

particles and substrate, and first particle layer

formation).

Stage 2: Particle deformation and realignment (multi-

layer coating build-up).

Stage 3: Metallic bond formation between particles

(particle–particle bonding) and void reduction.

State 4: Further densification and compaction of the

coating.

The deposition and bonding mechanisms of the

coating layer build up were not well known until now.

A critical particle velocity was accepted for char-

acterizing particle deposition. A function and some

data about critical velocity were given by Assadi et al.

[1]. In their investigation, the critical velocity was

determined by computer simulation, and it was proven

that the critical velocity is a function of the feedstock

materials properties and experimental conditions. This

function is very simple because many parameters are

not involved, such as substrate material properties,

impact surface conditions, and so on.

Characterization of Ni-base alloy deposition by

kinetic spraying was discussed in a previous work

[14], where the particle impact angle of a single or

multi pass spraying trace was considered, as shown in

Fig. 9. From the trace width and spraying distance of

present research, the particle impacting angle of this

Fig. 9. Particle impacting angle within a cross section of particle

flux.

research was calculated to be between 878 and 908 forsingle and double pass spraying traces. In Li et al.’s

investigation about the effect of spray angle on

deposition characteristics in cold spraying [15], they

found that the deposition efficiency increased with

increasing the spray angle. Three segments of

impacting angle were suggested in their study,

including no deposition region, transient region and

maximum deposition region. The range from 878 to

908 is located in the maximum deposition region. This

indicates that the effect of particle impacting angle in

this experiment has little effect on particle deposition,

especially for the single pass spraying trace. Para-

meters that have greater impact on particle deposition

would be the in-flight particle velocity, temperature,

flux, and their distributions.

Fig. 10 shows optical micrographs of the surface

after single pass spraying, visualizing the shot-peened

zone and the zone of deposition. All deposited

particles form the trace, whose shape depends on

the process gas pressure and temperature. When the

gas pressure is lower than 2.1 MPa or the gas

temperature is lower than 150 8C, it was difficult to

obtain deposition of Al–Si coating on mild steel

substrate. After increasing the gas pressure to 2.5 MPa

or gas temperature to 300 8C, the first layer coating

builds up, but its thickness could not increase any

more even after multi pass spraying, as shown in

Fig. 11a. By further increasing the process gas

pressure and temperature, spraying traces can be

obtained by single or multi passes. In Fig. 11b, the

SEM micrograph shows that an approximately

200 mm thick coating (in the center position of trace)

can be built up at 2.9 MPa and 500 8C by a single pass.

Fig. 12 shows the thickness and width of spraying

traces from single and double passes at different

process gas pressures and temperatures, from which

the deposition characteristics can be estimated. At

2.5 MPa and 150 8C, no Al–Si powder was deposited

on the substrate. By increasing gas temperature to

300 8C or gas pressure to 2.9 MPa, a thin layer with a

thickness of about 40 mm can be obtained, but the

thickness does not increase with the repeated passes.

Sufficient deposition was obtained at high gas

temperatures (500 8C) and pressures (2.5 and

2.9 MPa). Prominent overlapping deposition also

occurred at these two process gas conditions. This

suggests that first layer build-up (stage1 of deposition


Fig. 10. Optical graphs of spraying traces; (a) single pass spraying

trace at 2.9 MPa, 300 8C; (b) single pass spraying trace at 2.9 MPa,

500 8C.

Fig. 11. SEM micrograph of cross section of spraying traces; (a)

multi pass spraying trace at 2.9 MPa, 300 8C; (b) single pass

spraying trace at 2.9 MPa, 500 8C.

process) and particle–particle bonding (stage 3 of

deposition process) require different particle velo-

cities. The velocity for Al–Si particle deposition onto

the mild steel substrate is lower than that for Al–Si

particle bonding with Al–Si.

Fig. 13 compares the spraying trace form to in-

flight particle velocity and flux distribution at same gas

pressure and temperature. In this figure, it can be

observed that only when the particle velocity exceeds

a certain value deposition is obtained and that a critical

velocity is needed for overlapping. Here two critical

velocities are indicated: one is for first layer build-up

and another is for overlapping deposition. In order to

evaluate the deposition characteristics by thickness of

trace, the threshold for overlapping deposition is set to

36 mm, which comes from Fig. 12 with considering

the particle deformation during spraying process. Note

that in the case of monolayer coatings the deposited

layer is compacted by the particles which can not

overlapping deposit but rebound. So the threshold for

first layer building up is difficult to be set as a certain

value. Here zero is used for this threshold. After

marking the velocities on these two thresholds of

single pass trace thickness in Fig. 13, we can know that

the critical velocity for first layer Al–Si feedstock

deposition onto mild steel substrate is about 580 and

700 m/s is the critical velocity for Al–Si feedstock

bonding with its coatings. The coating of Al or Al


Fig. 12. Forms of spraying traces; (a) single pass at 2.5 MPa; (b) Double pass at 2.5 MPa; (c) single pass at 2.9 MPa; (d) double pass at 2.9 MPa.

alloys needs high particle velocity. The results of

Assadi et al.’s research [1] showed that the critical

velocity of pure Al lied within a region between 650

and 700 m/s. Considering similar physical and

mechanical properties of pure Al and Al–Si powder,

the function of Assadi was used for estimating the

velocity for particle–particle bonding.

The deposition efficiency (DE) of the coating was

measured from the weight change of the specimen. In

Fig. 14, the DE value increases with the mean particle

velocity and the maximum DE is about 37% at the

mean particle velocity of 800 m/s. Compared to the

DE of other metal feedstock such as Cu, Ni, and so on

[6,7,13,14], the DE of Al or Al alloys is very low. To

increase the DE of Al alloy coating, high gas

temperature and pressure are needed. The available

operating pressure and temperature for most com-

mercial equipment using N2 gas is under 3.0 MPa and

650 8C. However, at severe gas conditions, other

problems do occur, such as nozzle clogging. Actually,

the Al–Si plugged the nozzle after long time (20 min)

spraying at 500 8C. The other methods for increasing

particle velocity are using He as the process gas or

some special spraying nozzle. The DE-velocity curves

also show two marked increases of the DE with

increasing particle velocity that comes from the first

layer build-up and overlapping. It is noticed that the

DE didn’t change observably with the substrate

surface conditions. The effect of roughness on DE

did not show any crucial importance, because after

first layer built up, the substrate surface condition can

not affect the deposition of overlapping.


Fig. 13. Comparison of spraying trace form to in-flight particle

condition at 2.5 MPa, 500 8C.

Fig. 14. Deposition efficiency of single and double pass traces on

mean particle velocity.

4. Conclusions

The kinetic spraying process in this study was

monitored by a professional device. From in-flight

particles images, particle velocity and flux distribu-

tions of different process gas pressures and tempera-

tures are measured. Particle velocity increases with

increasing the process gas pressure and temperature.

At higher temperature (or pressure), gas pressure (or

temperature) do more effect particle velocities. The

maximum particle velocity is obtained from optimum

gas conditions of 2.9 MPa and 500 8C. The particle

velocity distribution was symmetrical with the axis of

the spray beam. The particles in the center of the spray

beam had the highest velocities. A flux with normal

distribution is measured for all gas conditions. It is

believed that gas pressure and temperature have little

effect on the flux distribution of flying particles.

Particle deposition behaviors depend on process

gas pressure and temperature. The width and thickness

of spraying traces increase with process gas pressure

and temperature. With increasing particle velocity,

two critical velocities are observed: one is for particle

deposition onto the substrate and the other for

particle–particle bonding. For Al–Si feedstock coating

onto the mild steel substrate, the particles more easily

bond to the substrate than to the coatings. 580 and

700 m/s are measured as these two critical velocities

when the mean particle size is 25 mm. The deposition

efficiency (DE) value increases with the mean particle

velocity and the maximum DE is about 37% at a mean

particle velocity of 800 m/s. The substrate surface

roughness has not great effect on DE.

In this research, parameters such as particle and

substrate temperatures effect were not studied. Further

investigation is needed for a better understanding of

the deposition and bonding mechanisms of kinetic

spraying.

Acknowledgements

This work was financially supported by Ministry of

Commerce Industry and Energy (MOCIE) under the

project: Development of Advanced Surface Modifica-

tion Technologies Based on Welding.

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