DEVELOPMENT OF ATOMIZED POWDERS FOR ADDITIVE
MANUFACTURING
Christopher T. Schade and Thomas F. Murphy
Hoeganaes Corporation, Cinnaminson, NJ
Chris Walton
Hoeganaes Specialty Products, Cinnaminson, NJ
ABSTRACT
Powders for additive manufacturing require characteristics not typically found in
atomized powders for press and sinter applications. In general, these powders require
high apparent densities to achieve maximum density when deposited in packed beds. In
addition, flow of the powders is critical when these powders are dispensed from
equipment that builds the prototype part. This paper reviews both water and gas
atomized powders produced specifically for additive manufacturing. Powder
characteristics such as particle shape, particle size distribution, surface roughness and
chemistry are studied to determine the effectiveness of these powders for use in additive
manufacturing. The performance of water atomized and gas atomized powders were
studied in relation to their performance for use in additive manufacturing.
INTRODUCTION
Additive manufacturing methods such as three dimensional printing (3DP) or binder
jetting and selective laser melting (SLM) are becoming more common as there is a
demand for rapid prototyping and production of parts that require design features that
cannot be manufactured by the conventional press and sinter process normally used in
powder metallurgy[1-6]. 3DP is an additive manufacturing process which builds a part
layer by layer out of particulate material, a chemical binder and a digital design. Parts
manufactured with the 3DP may not have the high level of mechanical properties of other
additive manufacturing techniques because of the lower density and the poor adherence
between particles. To improve the mechanical properties, the part is sintered similar to a
conventional PM process. If further improvements in mechanical properties are required,
the part can be infiltrated with a second metal, such as bronze or copper.
SLM is a digitally driven additive manufacturing process that uses focused laser energy
to fuse metallic powders in to 3D objects. By varying the power of the laser, the process
can be full melting, partial melting, or in combination with the proper material, liquidphase sintering. The finished density of the part depends on the power of the laser and the
material being processed, but in many cases can reach 100%. The high density allows
the material properties on parts produced by SLM to approach that of conventional
manufacturing processes such as casting and machined parts.
Common to both 3DP and SLM is the fact that the build takes place on a bed of powder
(Figure 1). During production, a thin layer of metal powder is spread evenly across the
build chamber by a roller. In 3DP, a printer head deposits a binder to adhere the metal
powders in the shape of the part to be produced. In the case of SLM, the laser moves
across the powder and, as it moves, sinters a cross section of the object . In both cases,
after the binder or laser fuses the powder in the shape of the part being produced, a new
layer of powder is then spread over the top of the previous layer and the next cross
section of the part is produced using a roller or wiper. During and after the building, the
part is surrounded by un-agglomerated powder. This excess powder encases the object in
the printing process, providing support for complex geometries.
Figure 1: Schematic of laser sintering process [7].
In both processes just described, the physical characteristics of the powder play an
important role in transporting the powder to the bed and the particle packing within the
bed. A number of characteristics of the powder, such as particle size, shape, surface area,
and packing density play a role in the formation of the powder bed. The packing density
of the powder bed influences the sintered density, mechanical and physical properties of
the final part. In this work, physical properties of powder are compared for both gas and
water atomized grades of powder manufactured for use in additive manufacturing.
EXPERIMENTAL PROCEDURE
Flow Rate and Apparent Density were measured using a Hall apparatus and were tested
according to MPIF Standard Test Methods, Standards 3 and 4 respectively [8].
Determination of Sieve Analysis was performed following MPIF Standard 5.
Nitrogen and Oxygen contents were measured using an inert gas fusion Leco EF-400
nitrogen/oxygen determinator while carbon and sulfur measurements were performed on
a LECO CS 200 analyzer.
Laser particle size analysis was performed using a Sympatec Helos BF laser particle size
analyzer.
Loose powder specimens for microstructural characterization using the SEM were
secured to an aluminum substrate using electrically conductive carbon adhesive tape.
The density of the particles on the substrate was kept low to ensure that the small
particles were visible when surrounded by the larger ones. The mounts were then
examined in the uncoated and unetched condition.
Evaluation of powders using light optical microscopy was made on cross-sections of
individual particles. These were prepared by mixing the loose metal powders with either
fine phenolic mounting powder or a two-part liquid epoxy and processing the mixture as
a normal metallographic mount. The cured mount was then ground with only fine grit
media and polished using a standard grit/polishing sequence. The iron powder samples
were etched with a combination of 2 v/o nital and 4 w/o picral where required.
Automated image analysis was used to measure the dimensions of the loose powder
cross-sections. The procedure used a 20x objective with calibration of 0.34 m per pixel.
Five hundred fields were viewed on each surface with >1000 particles measured from
each sample. Provision was made in the program to separate particles that were touching
to ensure the data was generated for only single particle cross-sections.
RESULTS AND DISCUSSION
x50 = 38.70 µm
x90 = 53.17 µm
SMD = 36.32 µm
VMD = 39.74 µm
x16 = 29.59 µm
x84 = 49.48 µm
x99 = 71.13 µm
SV
Sm
= 0.17 m²/cm³
= 587.86 cm²/g
100
4.0
90
80
3.5
70
60
2.5
3.0
50
40
2.0
1.5
30
20
10
0
0.10
1.0
0.5
0.5
1
5
10
parti cle size / µm
50
100
(a)
(b)
Figure 2: (a) SEM image of 316L Gas Atomized Powder and (b) typical particle size
distribution of 316L powder for additive manufacturing.
Figure 2 shows the typical gas atomized 316L stainless steel powder used for additive
manufacturing. The powder is characterized by its spherical nature with a particle size
distribution of 15 to 45 micron. This powder would be considered of excellent quality
0
500
densi ty distri bution q3l g(x)
cumulative distrib ution Q3(x) / %
x10 = 26.92 µm
due to lack of agglomerated powder and few satellites. The apparent density, a measure
of the packing density of the powder, is 4.41 g/cm3, which is an indication of its spherical
nature. The majority of the powder is under 400 mesh (37 microns), which is a fairly fine
powder, however because the powder smaller than 10 microns is removed the powder
still has excellent flow rate 21.5 secs/50 grams.
Table I: Chemical and Physical Properties of Gas Atomized 316L
Sieve Size (mesh)
Material
Carbon
(w/o)
Sulfur
(w/o)
316L
0.03
.005
Oxygen Nitrogen
AD
(w/o)
(w/o)
(g/cm3)
.05
0.11
4.41
Flow
(secs)
+270
+325
+400
-400
(%)
(%)
(%)
(%)
21.5
0.2
0.6
2.7
96.5
GAS ATOMIZED VERSUS WATER ATOMIZED
Generally gas atomized powders are preferred for additive manufacturing because of the
spherical nature of the powder. Water atomization is the most common and economical
technique to produce metal powders. Generally water atomizing, due to the rapid cooling
rate, produces powders that are irregular in shape. In addition, the high water pressures
impact more energy into the molten metal stream leading to the rough shape of the
powder particles. This irregular shape is less desirable because it increases the flow time
and possibly reduces the packing density. However, if a low water to metal ratio is used
in water atomizing along with a high pressure, a spherical powder with a particle size
distribution optimized for additive manufacturing can be produced. An example of this is
shown in Figure 3.
x50 = 32.14 µm
x90 = 51.17 µm
SMD = 27.31 µm
VMD = 33.69 µm
x16 = 19.53 µm
x84 = 47.15 µm
x99 = 79.47 µm
SV
Sm
= 0.22 m²/cm³
= 781.84 cm²/g
100
2.25
90
80
2.00
70
60
1.50
50
40
1.00
1.75
1.25
0.75
30
20
10
0
0.10
0.50
0.25
0.5
1
5
10
parti cle size / µm
50
100
0
500
(a)
(b)
Figure 3: (a) SEM image of water atomized iron powder and (b) typical particle size
distribution of iron powder for additive manufacturing.
There are other processing conditions during atomizing that effect the shape and particle
size distribution of water atomized powders. The jet distance, jet angle, impingement of
partially solidified particles on the atomizing chamber walls and collisions among
powders that are not fully solidified leading to coalescence of smaller particles onto large
ones (satellites) are all factors that must be optimized to produce the optimum material.
densi ty distri bution q3l g(x)
cumulative distrib ution Q3(x) / %
x10 = 16.61 µm
Table II: Chemical and Physical Properties of Water Atomized Iron
Sieve Size (mesh)
Material
Carbon
(w/o)
Sulfur
(w/o)
Iron
.01
.005
Oxygen Nitrogen
AD
(w/o)
(w/o)
(g/cm3)
.12
.001
3.80
Flow
(secs)
+270
+325
+400
(%)
(%)
(%)
(%)
29.7
0.0
0.1
0.4
99.5
-400
In general, the powder characteristics most desirable for additive manufacturing are
spherical powders with a fine particle size. Having a distribution of particles that flows
and forms dense or well packed beds is important in both powders for 3DP and SLM.
One of the major problems for powder production is that powder flow and particle size
have an inverse relationship. Figure 4 shows the relationship between the powder
apparent density and particle size. Apparent density is measure of the packing
characteristics of powders [9]. Loose packing of powders gives low apparent density and
efficient packing yields a high apparent density. The apparent density can be affected by
particle size, shape and size distribution. As seen in Figure 4, the finer the powder the
lower the apparent density. The Hall Flow test measures the inter-particle friction in the
powder. This can be influenced by the surface roughness of the powder and generally
increases as the surface area increases (finer powders have higher flow times). So in
order to produce an optimum powder for additive manufacturing the two properties (flow
and apparent density) need to be balanced.
4.6
120
4.4
100
Hall Flow (secs)
3
Apparent Density (g/cm )
No Flow
4.2
4
3.8
60
40
20
3.6
3.4
20
80
40
60
80
100 120 140 160 180
Screen Size in Microns
0
20
40
60
80 100 120 140 160 180
Screen Size in Microns
Figure 4: Apparent Density and Hall Flow of Gas Atomized Iron Powder as a function of
the particle size.
Shape Analysis
Theoretically the best flow of powder can be achieve if the powder is perfectly spherical
and has a very narrow particle size range. The best packing can be achieved if there are
smaller particles which can fill the voids in between the larger particles in the bed. Both
of these traits must be balanced in order to achieve an adequate yield of powder to make
the material cost effective. If water atomized powder can be produced in this manner the
cost savings should be significant when compared with gas atomizing. One of the
advantages of high pressure water atomizing versus gas atomizing is the higher yield of
finer particle sizes. The disadvantage is that the higher momentum delivered to the
molten metal and the high cooling rate leads to a greater percentage of irregularly shaped
powder particles, especially compared with gas atomized powders. However, there exist
several technologies for separating out these irregularly shaped particles, thus improving
the performance of high pressure water atomized powders for additive manufacturing.
Sample
AD
Flow
Sample
AD
Flow
A
3.51
20.1
B
3.81
17.5
(a)
(c)
(b)
Sample
AD
Flow
Sample
AD
Flow
C
3.99
16.5
D
4.20
15.1
(d)
Figure 5: Micrographs of water atomized iron powder shown at various stages of powder
separation progressing in time from a through d. Light Optical Microscopy (LOM,
unetched).
Several of these processes are classifying, dry and wet spiral separation, magnetic and
frictional separation. The use of this separation technology on high pressure water
atomized powder is shown in Figure 5. As more and more of the irregular particles are
removed from the powder (Sequence a through d) the apparent density of the water
atomized powder increases and the flow rate of the powder decreases. The final powder
(Figure 5d) has apparent density and flow rate that is equal to or better than gas atomized
powders.
Perhaps the easiest way to quantify the shape of a powder is to measure the length to
width ratio. If the particle is perfectly spherical it will only have one characteristic
dimension, the diameter. In this case, the ratio of the two dimensions (length to width)
will equal unity. The more irregular the powder shape (deviation from spherical) the
aspect ratio will increase (> 1.0). Automated image analysis was used to evaluate the
different powders shown in Figure 5. Figure 6 shows the percentage of powders
exhibiting different aspect ratios at various stages of separation. It is evident from Figure
6 that the spherical nature of the powder was improved as the distribution of the aspect
ratio narrowed and approached unity as more of the irregular shaped powder was
removed. This improvement should lead to better performance of the powder in additive
manufacturing processes such as 3DP and SLM.
Figure 6: Aspect ratio (length to width ratio) of the powders shown in Figure 5.
Figure 7 shows powder characteristics for iron powder with a mean particle size of 60
microns that was both gas atomized and water atomized. The shape of the water
atomized powder is spherical and the apparent density and flow rate are comparable to
the gas atomized powder. The oxygen content of the water atomized iron powder was
0.08 w/o. The water atomized powder has been successfully used in a 3DP process to
produce an impeller that was later infiltrated with bronze.
Sample
AD
Flow
Sample
AD
Flow
Gas
4.39
15.1
Water
4.20
15.1
(a)
(b)
Figure 7: Optical Micrographs of atomized iron powder: (a) Gas Atomized and (b) Water
Atomized. (LOM-unetched)
Alloy Development
The growth of additive manufactuing is tied to the materials available. Material types
such as stainless steel, nickel alloys, superalloys, tool steels and cobalt alloys are being
used in a range of applications from medical to aerospace. To ensure uniform and
consistent part builds all of the powders must have consistent flow and high packing
density. One alloy that has seen considerable interest is a maraging steel (Table III).
Table III. Chemistry Specification for Maraging Steel (DIN 1.2709).
1.2709
Specification
C
Si
Max. .03%
Max. .10%
Mn
P
Cr
Mo
Max. .15% Max. .010% Max. .025% 4.50-5.20%
Ni
Ti
Co
17-19%
.80-1.20%
8.50-10.0%
This steel, which uses nickel as the primary strengthening element rather than carbon, is
known for its’ superior strength and toughness. Despite its high strength the material can
be easily machined or formed and after these treatments it can undergo an aging (heat
treatment) step that forms intermetallic precipitates involving cobalt, molybdenum and
titanium which aid in increasing the tensile strength. Due to the high nickel content the
alloy has high hardenability and has wear resistance that is suitable for many tooling
applications. The material can be heat treated in air at low temperatures and because of
the low thermal coefficient of expansion, has excellent dimensional stability. The low
carbon content also helps when used in SLM since the material is not susceptible to
thermal stress cracks during cooling. A comparison of the maraging tool steel made by
water atomization and gas atomization is shown in Figure 8.
x50 = 31.84 µm
x90 = 45.48 µm
SMD = 26.54 µm
VMD = 31.78 µm
x16 = 21.62 µm
x84 = 42.07 µm
x99 = 57.38 µm
SV
Sm
= 0.23 m²/cm³
= 804.49 cm²/g
100
3.0
90
80
2.5
70
60
2.0
50
40
1.5
1.0
30
20
0.5
10
0
0.10
0.5
1
5
10
parti cle size / µm
0
500
100
(c)
x50 = 28.13 µm
x90 = 46.00 µm
SMD = 22.98 µm
VMD = 29.58 µm
x16 = 16.50 µm
x84 = 41.55 µm
x99 = 70.82 µm
SV
Sm
cumulative distrib ution Q3(x) / %
x10 = 13.80 µm
= 0.26 m²/cm³
= 929.14 cm²/g
100
2.25
90
80
2.00
70
60
1.50
50
40
1.00
1.75
1.25
0.75
30
20
0.50
0.25
10
0
0.10
0.5
1
5
10
parti cle size / µm
(b)
50
100
0
500
(d)
Figure 8: SEM image of (a) gas atomized maraging steel; (b) water atomized maraging
steel and corresponding particle size distributions: (c) gas atomized maraging steel; (d)
water atomized maraging steel.
The water atomized powder has a slightly finer particle size and higher apparent density
(Table IV) than the gas atomized powders but a higher oxygen content. Development
work is ongoing to lower the oxygen content.
Table IV: Chemical and Physical Properties of maraging steel, Gas versus Water
Atomized.
Sieve Size (mesh)
Samples
C
S
O
N
AD
Flow
+270
+325
+400
-400
Gas Atomized
0.007
.002
.04
.003
3.77
NF
0.0
0.5
10.6
88.9
Water Atomized
0.010
.002
.36
.001
3.84
NF
0.0
0.1
2.4
97.5
Another example material is 420 stainless steel. This alloy is a martensitic chromium
stainless steel capable of being heat treated to a maximum hardness of 50 Rockwell C.
Furthermore it has good corrosion resistance in the heat treated and tempered condition.
It has higher strength, better hardness and greater wear resistance than 410L and can also
be forged. This stainless steel is used in a variety of applications such as ball bearings,
cutlery, plastic mold cavities, and surgical and dental instruments.
densi ty distri bution q3l g(x)
(a)
50
densi ty distri bution q3l g(x)
cumulative distrib ution Q3(x) / %
x10 = 18.38 µm
Table V: Chemistry Specification for 420 Stainless Steel
Si
Mn
Min. 0.15%
Max. 1.0%
P
Cr
Max. 1.0% Max. .040%
12-14%
x10 = 17.87 µm
x50 = 31.32 µm
x90 = 43.70 µm
SMD = 26.79 µm
VMD = 31.63 µm
x16 = 20.90 µm
x84 = 41.21 µm
x99 = 58.84 µm
SV
Sm
= 0.22 m²/cm³
= 797.15 cm²/g
100
3.5
90
80
3.0
2.5
70
60
2.0
50
40
1.5
30
20
1.0
0.5
10
0
0.10
0.5
1
5
10
parti cle size / µm
0
500
100
(c)
x50 = 26.30 µm
x90 = 49.28 µm
SMD = 19.51 µm
VMD = 28.94 µm
x16 = 13.43 µm
cm²/g
x84 = 43.29 µm
x99 = 82.80 µm
SV
Sm
cumulative distrib ution Q3(x) / %
x10 = 10.79 µm
= 0.31 m²/cm³
= 1094.63
100
1.8
90
80
1.6
70
60
1.2
50
40
0.8
1.4
1.0
0.6
30
20
0.4
0.2
10
0
0.10
0.5
1
5
10
parti cle size / µm
(b)
50
0
500
100
(d)
Figure 9: SEM image of (a) gas atomized 420 stainless steel; (b) water atomized 420
stainless steel and corresponding particle size distribution: (c) gas atomized 420 stainless
steel; (d) water atomized 420 stainless steel.
The particle size of the water atomized powder is slightly finer than the gas atomized
powder accounting for the slightly lower apparent density (Table IV). The oxygen
content of the water atomized powder is also higher than the gas atomized powder but is
typical of oxygen contents used for PM applications. When used in an additive
manufacturing process that is followed by a sintering step this is not a great concern.
Table VI: Chemical and Physical Properties of 420 Stainless Steel, Gas versus Water
Atomized
Sieve Size (mesh)
Samples
C
S
O
N
AD
Flow
+270
+325
+400
-400
Gas Atomized
0.30
.007
.04
.076
3.89
15.8
0.0
0.5
10.6
88.9
Water Atomized
0.28
.008
.23
.037
3.75
22.0
0.0
0.1
0.4
99.5
density distribution q3lg(x)
(a)
50
densi ty distri bution q3l g(x)
C
cumulative distrib ution Q3(x) / %
420SS
Specification
CONCLUSIONS




Both water and gas atomized powders can be produced for use in additive
manufacturing.
In general, water atomized powders can be produced that are spherical (high
apparent density) and with Hall Flow rates approaching those of gas atomized
powders.
The oxygen content of water atomized powders is higher than gas atomized
powders.
Alloy development is on-going for many grades of water atomized spherical
powder including those presented and also for additional grades such as 17-4PH
and 316L stainless steel, Cast Iron, Inconel and low alloy steels.
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