Chiang Mai J. Sci. 2010; 37(1)
55
Chiang Mai J. Sci. 2010; 37(1) : 55-63
www.science.cmu.ac.th/journal-science/josci.html
Contributed Paper
Gas Atomization of Low Melting-Point Metal Powders
Monnapas Morakotjinda* [a], Kittichai Fakpan [b], Thanyaporn Yotkaew [a],
Nattaya Tosangthum [a], Rungthip Krataithong [a], Anan Daraphan [a], Pisarn Siriphol [a],
Pongsak Wila [a], Bhanu Vetayanugul [a], and Ruangdaj Tongsri [a]
[a] Powder Metallurgy Research and Development Unit (PM_RDU) National Metal and Materials Technology
Center (MTEC), Thailand Science Park, Klong Luang, Pathum Thani 12120, Thailand.
[b] Department of Production Technology, Faculty of Engineering, King Mongkut’s Institute of Technology
North Bangkok, Bangkok 10800, Thailand.
*Author for correspondence; e-mail: [email protected]
Received: 1 March 2009
Accepted: 27 April 2009
ABSTRACT
A pilot-scale gas atomizer, with capacity of 35 kilograms of metal charge/batch,
was developed for producing low melting-point metal powders. Its components such as
structure, melting furnace, nozzle system, atomizing chamber, powder collector and cyclone,
were designed and assembled. The gas atomizer had been tested experimentally by productions
of tin and tin alloy (Babbitt metal) powders. Tin and tin alloy (Babbitt metal) powders had
been produced experimentally with varied processing factors such as nozzle design, melt flow,
atomizing gas pressure and melt superheat. Yield, particle size and microstructure of the
powders were analyzed. Particle size distribution of the powders was compared to a mean
particle size calculated by using Lubanska’s equation. It was found that the calculated mean
particle size was closer to the experimental one when a constant K of Lubanska’s equation was
varied. Microstructures of tin and tin alloy powders indicated that solidification phenomena
occurred via nucleation and growth.
Keywords: Gas atomizer, atomization, low-melting metal powders.
1. INTRODUCTION
Growth of powder metallurgy (P/M)
industry in Asia has been being increased
contentiously due to its superior benefits
compared to conventional ingot metallurgy
[1]. The benefits include high productivity, raw
materials and energy saving and near net-shape
character. In Thailand, most P/M parts have
been consumed by electronics (54%), followed
by automobile (44%) and other (2%) industries
[2]. However, there are a few powder
production industries in the Kingdom.
Shortages of human resources and metal
powder manufacturing technology in this
country are big burdens for Thai P/M
industry development and competitiveness
improvement.
Atomization means disintegration of
liquid metal into fine droplets, which are
56
simultaneously cooled down and solidified to
form metal powders. To break up the liquid
metal into droplets, there must be a force
exerting to a liquid metal. The force can be in
the forms of atomizing media impingement,
centrifugal force or liquid metal explosion.
Atomization may be classified into 3 methods
including two-fluid atomization, centrifugal
atomization and vacuum atomization. In the
two-fluid atomization, a liquid metal stream
(the first fluid) is impinged with high-velocity/
high-pressure gas or liquid (the second fluid)
to form droplets, which are then cooled to
be powders [3]. In centrifugal atomization,
the liquid metal is disintegrated by rotating
a melting metal electrode or crashing the
metal melt with the rotating disk [4, 5]. The
pressurized liquid metal is exploded when it
is released to a vacuum chamber. This is
known as vacuum atomization [6]. The
additional details of atomization have been
reviewed by some authors [7, 8].
Powder production, using a gas atomization
process, has been being widely investigated
and applied in industry, due to its advantages
including high capacity, high flexibility for both
elemental and prealloyed powder production
and capability for rapidly solidified metal
powder production [9]. The rapidly solidified
metal powders usually exhibit superior
properties caused by fine microstructure,
chemical homogeneity, extended solid solution
and metastable phase formation. Therefore,
metal parts produced from the rapidly
solidified metal powders show superior
mechanical properties.
In principle, when the metal melt is caused
unstable by any forces it will be broken into
forms of smaller pieces or droplets. Melt
disintegration mechanism in a gas atomization
process includes five steps as follows;3
(i) impingement of atomizing gas on the
melt causes unstable wavy melt stream
(ii)ligament formation occurs at the end
Chiang Mai J. Sci. 2010; 37(1)
of melt stream wave
(iii) ligament further discomposes into
droplets (primary atomization)
(iv)melt droplets are further disintegrated
(secondary atomization)
(v) satellite formation by crashing
between melt droplets.
In a gas atomization process, there are
important factors controlling particle size and
size distribution of the powders. The factors
include nozzle design, atomizing gas flow rate,
metal melt flow rate, type of metal melt and
melt superheat. Investigation of processing
parameters on powder particle size is very
useful because information, correlation and
prediction models are needed by powder
production industry [10].
Investigation on atomization of water
and oil yielded an equation which could be
used for metal melt atomization [11]. The
equation is as follows;
(1)
where U g = (γPg / ρ g ) 0.5
d m = averaged size of metal powder
particles
δ = diameter of melt feed tube
ρ g = gas density
ν m = kinematic viscosity of the melt
M = melt flow rate
A = atomizing gas velocity
P g = gas pressure.
One of the famous studies on processing
parameter-powder particle size correlation
was carried out by Lubanska [11]. Investigation
on production of metal powders (iron, aluminium and tin) resulted in Lubanska’s equation
as follows;
(2)
Chiang Mai J. Sci. 2010; 37(1)
57
temperature of the atomizer melting system
was designed to be 1,000oC. In this research,
nitrogen gas was used as an atomizing
medium. The design also allowed atomizing
media to be a compressed air or other gases.
Flexibility of the atomizer components, such
as nozzle system and metal melt delivery tube,
was given in order that those components
could be adjusted or changed. On the
atomizing chamber front view, a transparent
window was attached, so liquid metal
disintegration could be observed from
outside. The structure of the gas atomizer is
shown in Figure 1.
where We =
K = constant
ν g = kinematic viscosity of a gas
We = Weber number.
In this investigation, results obtained
from tin and tin alloy (Babbitt) powder
production have been analyzed compared to
the Lubanska’s equation.
2. MATERIALS AND METHODS
2.1 Gas Atomizer Specification
Sketches of gas atomizer parts were
drawn, fabricated and assembled at the Pilot
Plant Building of the National Metal and
Materials Technology Center (MTEC) in
Thailand Science Park. The gas atomizer
design was similar to that given in [3, 8]. Its
capacity was designed for producing metal
powder of 35 kg/batch. This gas atomizer
was expected to be suitable for research works
on metal powder production. Maximum
2.2 Materials and Equipment
Materials employed for this investigation
were tin and tin alloy (Babbitt). Their chemical
compositions, analyzed by X-Ray fluorescence
(XRF), are shown in Table 1. A gas atomizer
used for metal powder production is shown
in Figure 1. Two types of nozzles namely
confined and free-fall nozzles (Figure 2) were
Table 1. Chemical compositions of tin and Babbitt.
Element
Material
Sn (%)
Sb (%)
Al (%)
P (%)
Cu (%)
Others (%)
Tin
99.6
-
0.0157
0.0158
0.274
0.0945
Babbitt
83.04
10.19
-
-
5.50
1.27
Figure 1. Sketch of the pilot gas atomizer.
58
Chiang Mai J. Sci. 2010; 37(1)
used. Effect of nozzle design was determined.
(2.3) Atomization procedure. Tin and Babbitt
were melted in a furnace (Figure 3). Superheat
temperature of both metal melts were 68oC.
The melt was released through a melt feed
tube. When it emerged from the tube, it was
crashed with high-velocity nitrogen gas.
Impingement between gas and melt resulted
in formation of flakes, ligaments and spherical
powder particles. All the atomized products
were sieved. The quantity of powders with
particle size less than 180 mm was used for
weight fraction calculation and powder particle
characterization. Experimental procedure is
illustrated in Figure 4.
(a)
(b)
Figure 2. Sketches of confined (a) and free-fall (b) nozzles.
Figure 3. Metal melting equipment.
3. RESULTS AND DISCUSSION
3.1 Gas Stomization Unit
The structure was designed to have 3
working floors (Figure 1). The upper (3rd)
floor was used for installing of control and
melting units. Just below the melting unit, a
nozzle equipped with a heating element was
attached. On the middle floor, observation
of melt disintegration was possible through a
transparent window. This floor was also
designed for facilitating of nozzle installation.
Figure 4. A flowchart of experimental
procedure.
The lower floor was where the whole structure
and gas supply system stood. On this floor
workers could collect metal powders. The
dimensions of the atomizer pilot plant were
5.14 m (width) 3.00 m (length) 3.20 m
(height).
In this investigation two designs of
nozzle, namely confined and free-fall nozzles
(Figure 2), were chosen for producing metal
powders. The confined nozzle (Figure 2(a))
was designed in order that a high-velocity
Chiang Mai J. Sci. 2010; 37(1)
atomizing gas impinged on a melt stream at
the end of the melt feed tube. Due to a short
distance between the gas releasing and the
impingement points, lose of kinetic energy
of the atomizing gas was minimized. The freefall nozzle (Figure 2(b)) allowed the melt to
flow with a certain distance before being
disintegrated by the atomizing gas. For both
nozzle designs, the melt feed tube had a
declined end in order to minimize lose of
kinetic energy of the atomizing gas. The
declined-end feed tube was previously
studied by Anderson et. al. [12]. It was found
that a high-velocity atomizing gas showed
higher powder production efficiency when the
declined-end feed tube was used.
The atomizing chamber had height of 3
meter and diameter of 1 meter. The chamber
was constructed using stainless steel grade 304.
On the front view of the chamber, a
transparent window was attached. It was
linked with a cyclone, made of stainless steel
grade 304, by a tube. Flow of the atomizing
gas and some fine metal powders would pass
through the tube and enter the upper end of
the cyclone. To ensure complete powder
collection, the second cyclone was sequentially
linked to the first one. At the bottom of each
cyclone, a metal powder collector was
attached.
A melting furnace was designed as given
in Figure 3. Inside the steel shell, a refractory
ceramic was lined in order to keep heat inside
(insulation). Next to the lining, heating element
was wound around a ceramic tube. Numbers
and location of heating element sets were
designed in order that the generated heat was
used efficiently. Temperature fluctuation was
limited in the range of 20oC. The space in
side the furnace was able to fit a crucible with
a capacity of 35 kilograms of iron. At the
bottom of the crucible, a melt feed tube was
attached. The end of the tube was extended
into an atomizing nozzle. Due to this simple
59
design, the melt flow, caused by gravitational
force, could be varied by changing the tube
diameter.
Infrastructure system, including atomizing
gas and power supplies, was designed and
installed. The atomizing gas was pure nitrogen
(99.9%) compressed in two packs of cylinders
(16 cylinders/pack) under the pressure of 200
bar. The pressure, the factor controlling of
gas velocity, of the gas could be adjusted by
using a regulator. The power supply used in
this project was not complicated because only
the melting unit was necessary controlled. A
simple control box for power and current
regulation was designed and assembled.
3.2 Atomization of tin and Babbitt
materials
Experimental values of tin and Babbitt
powder particle sizes were obtained by using
a powder particle size analyzer. Calculation of
the powder particle sizes was carried out by
using Lubanska’s equation (Equation (2)).
Materials property (Table 2) and nozzle
constant (Table 3) were taken for calculating
of the powder particle size. The constant K
in Lubanska’s equation was recommended to
be in the range of 40-50 [11]. The gas flow
rate (A) and melt flow rate (M) were calculated
according to equations (3) and (4) [10],
respectively.
(3)
where a = gas exit area
kT = ratio between specific heat capacity
of an atomizing gas at a constant pressure
and specific heat capacity of the atomizing
gas at a constant temperature (Cp/Cv)
g = gravitational acceleration
R = gas constant
T = gas temperature.
60
Chiang Mai J. Sci. 2010; 37(1)
Table 2. Materials property.
Materials property
Density (kg/m3)
Tin (300oC)
Nitrogen (25oC)
6958.316
1.15
-
28.013
2.4458712 10-7
1.55 10-5
0.53925
-
-
1.41
Molecular weight (kg/kmol)
Kinematic viscosity (m2/s)
Surface tension (N/m)
kT (Cp/Cv)
Table 3. Nozzle constant.
Parameter
Value
Gas exit area of the confined nozzle
1.79x10-5 m2
Gas exit area of the free-fall nozzle
4.0x10-5 m2
Melt exit area (3 mm)
7.07x10-6 m2
(4)
where am = melt exit area
ρ m = melt density
h = height of melt in a crucible
ΔP = pressure difference between a
crucible and an atomizing chamber.
Plots of experimental and calculated
values of powder particle size against gas
pressure for confined and free-fall nozzles are
presented in Figures 5(a) and 5(b), respectively.
When the confined nozzle was employed
the plots of experimental powder particle
sizes of tin and Babbitt (Figure 5(a)) were
similar to the plots between the calculated
powder particle size against gas pressure.
When the constant K = 40 was used, the plot
of the calculated data moved closer to the
plot of experimental ones. When the free-fall
nozzle was employed, the plot of tin powder
particle size (Figure 5(b)) was close to the plot
of the calculated one, particularly when the
constant K = 40 was used.
Figure 5 indicates that the experimental
powder particle sizes are smaller than the
calculated ones. The powder particle size
difference may be attributed to some reasons.
The first is an error arisen from Weber number
miscalculation. The second comes from the
nature of the nozzle. A distance between the
gas exit and impingement points is short so
kinetic energy loss is low. Low lost kinetic
energy means high energy impingement of
gas molecules on metal melts. This causes
smaller powder particle formation. The last
reason is attributed to the input data (materials
property) for calculation. In this study, only
property of tin was used due to lack of
information about Babbitt. Tin property
cannot represent Babbitt one.
The gas-atomized tin and Babbitt
powders particles (Figures 6(a) and 6(b)) were
typically spherical. Some particles showed
evidences of satellite formation, which was
Chiang Mai J. Sci. 2010; 37(1)
61
Figure 5. Plots of powder particle size against atomizing gas pressure for (a)
the confined and (b) the free fall nozzles.
(a) Morphology of gas-atomized tin powders
(b) Morphology of gas-atomized Babbitt powders
(c) Microstructure of a gas-atomized tin powder
particle
(d) Microstructure of gas-atomized Babbitt
powder particles
Figure 6. Morphology and microstructure of the gas-atomized powders.
62
Chiang Mai J. Sci. 2010; 37(1)
caused by attachment of smaller solid powder
particles onto larger melt droplets. The attachment was in turn attributed to turbulent flow
of particles and gas in the atomizing chamber.
Cross section of a gas-atomized tin
powder particle exhibited polycrystalline
structure. In a coarse tin powder particle, there
were some fine equi-axed grains (grain size <
20 μm) (Figure 6(c)). Formation of this
microstructural type is resulted from
nucleation sites on the melt droplet surface.
After nucleation, competitive growth of
several nuclei occurs. When the solid/liquid
interfaces of each nucleus meet, grain
boundaries are formed.
Cross section of a gas-atomized Babbitt
powder particle exhibited very fine precipitates
of intermetallic Sn-Sb and Sn-Cu compounds
homogeneously distributed in the matrix
(Figure 6(d)). The presence of intermetallic
compounds causes difficulty for grain and
grain boundary observation.
addition [14]. Admixing of Ni powder was
found to improve some mechanical properties
of sintered 316L stainless steel. However, Ni
powder addition is not economically feasible
due to its high cost. A metal powder, designated
as PMTEC1 which is gas-atomized tin powder,
was employed as an alternative admixing
powder for performance improvement of
the sintered 316L stainless steel. It was found
that the factors, such as N2 content in the
atmosphere, Ni and PMTEC1, showed some
effects on some properties. Particularly,
addition of the PMTEC1 significantly
increased sintered density, yield strength and
hardness with severely sacrificed elongation.
The PMTEC1 powder caused grain growth,
no matter the sintering atmosphere
compositions employed. Nitrogen content in
the atmosphere caused formation of nitride.
The presence of PMTEC1 not only accelerated
grain growth, but also activated nitride
eutectoid formation.
3.3 Application of Tin and Babbitt Powders
as Sintering Activators for Densification
316L Powder Compacts
Recently, the gas-atomized tin and Babbitt
powders, with powder particle size of less
than 32 mm, were admixed with 316L powders
[13]. The mixed powders were processed via
a ‘press and sinter’ technique. Density of the
sintered 316L material was increased significantly when added tin and Babbitt amounts
were higher than a specific content. Addition
of tin and Babbitt also affected microstructure
of sintered materials. Grain growth was an
obvious evidence of microstructural change.
Most mechanical properties, such as ultimate
tensile strength, yield strength and hardness,
excluding elongation, were improved when
4 and 6 wt. % of tin and Babbitt were added.
For densification improvement of the
316L powder compacts, effect of tin powder
addition was comparable to that of Ni powder
4. CONCLUSIONS
The pilot scale gas atomizer, which was
designed and constructed, showed effective
capability on producing of low-melting metal
powders. Tin and tin alloy (Babbitt) were
experimentally gas-atomized with various
factors such as nozzle design, melt flow,
atomizing gas pressure and melt superheat
affected yield and particle size of the powders.
The powders showed typical spherical shape
of gas-atomized powders. Design of the
pilot-scale powder production plant was
flexible for further development of medium
to high melting-point metal powders.
For experimental gas-atomization of
low-melting point metal powders, its conclusive
remarks include;
(1) Lubanska’s equation is useful for prediction
of tin atomization when the constant K = 40
is employed.
(2) Gas-atomized tin and Babbitt powders
exhibited spherical shape.
Chiang Mai J. Sci. 2010; 37(1)
(3) Microstructure of a gas-atomized tin
powders clearly indicates that solidification
occurs via nucleation and growth. In a gasatomized Babbitt, precipitation of intermetallic
compounds occurs in addition to matrix
solidification.
The gas-atomized tin and Babbitt powders
could be used as a sintering activator for
densification of 316L powder compacts.
ACKNOWLEDGEMENTS
The authors would like to express their
sincere gratitude to National Metal and
Materials Technology Center (MTEC) for
financial support for this investigation.
REFERENCES
[1] Ito S., Global general session Japan, Asia
and Oceania, Advances in Powder Metallurgy
& Particulate Materials, 1966, (A-17)–(A24).
[2] Sundaram S.R., Sintered component
production in Asia-Oceania. Mater. Chem.
Phys., 2001; 67, 307-310.
[3] Klar E. and Fesko J.W., Gas and water
atomization, ASM Handbook (Vol. 7
Powder metallurgy), ASM International,
USA, 1984: 25-39.
[4] Roberts P.R., Rotating electrode process.
ASM Handbook (Vol. 7 Powder metallurgy),
ASM International, USA, 1984: 39-42.
[5] Patterson II R.J, Rotating disk
atomization. ASM Handbook (Vol. 7
Powder metallurgy), ASM International,
USA, 1984: 45-47.
[6] Fox C.W., Vacuum atomization. ASM
Handbook (Vol. 7 Powder metallurgy), ASM
International, USA, 1984: 43-45.
[7] Lawley A., Atomization: The production of
metal powders, Metal Powder Industries
Federation, New Jersey 1992: 1-159.
63
[8] Yule A.J. and Dunkley J.J., Atomization of
melts for powder production and spray deposition,
Clarendon Press, Oxford, 1994: pp. 1397.
[9] Dunkly J.J. Gas atomization a review of
the current state of the art. Advances in
Powder Metallurgy & Particulate Materials,
1999, 55-66.
[10] Fakpan K., Morakotjinda S., Tosangthum
N., Coovattanachai O., Krataitong R.,
Mata S., Daraphan A., Vetayanugul B.,
Srisukhumbowornchai N. and Tongsri
R., Production of Tin Powder by a Gas
Atomisation. NSTDA Annual Conference
2005, Pathum Thani, Thailand, March 2830, 2005.
[11] Lubanska H., Correlation of Spray Ring
Data for Gas Atomization of Liquid
Metals, J. Met., 1970; 22: 45-49.
[12] Anderson I.E., Figliola R.S. and Morton
H., Flow mechanisms in high pressure
gas atomization, Mater. Sci. Eng., 1991;
A148: 101-114.
[13] Coovattanachai O., Tosangthum N.,
Morakotjinda M., Yotkaew T., Daraphan
A., Krataitong R., Vetayanugul B. and
Tongsri R., Performance improvement
of P/M 316L by addition of liquid phase
forming powder, Mater. Sci. Eng., 2007;
A445-446: 440-445.
[14] Tosangthum N., Coovattanachai O.,
Krataitong R., Morakotjinda M.,
Daraphan A., Vetayanugul B. and Tongsri
R., Density and Strength Improvement
of Sintered 316L Stainless Steel, Chiang
Mai J. Sci., 2006; 33(1): 53-66.