Faster sinteringand lower costswith ultra-fineMIM powders Recent advances made in Japan in the production of ultra-fine stainless steel powders mean improvements in MIM processing can be realised…

Metal injection moulding or

MIM is a segment of the

broader field of powder

injection moulding (PIM).

It is a relatively new technology that uses

the shaping advantages of plastic injection

moulding but expands the applications to

numerous high-performance metals and

alloys, as well as metal matrix composites

and ceramics [1, 2].

The MIM process consists of mix-

ing a small amount of organic material

– the binder phase – with the desired

inorganic powder (metals or alloys) to

create a feedstock that can flow like

plastic under temperature and pressure.

This feedstock can be injection moulded

into a “green” shape that is an oversized

replica of the final part. Generally the

organic binder is removed during a step

known as debinding, though in some

applications the as moulded part is the

final component.

After debinding, the part is consolidated

to high densities which are typically great-

er than 96 per cent of theoretical density

of the metal or alloy. This can be achieved

by pressureless sintering (high temperature

treatment) or pressure assisted sintering.

In this way the MIM process provides

designers and engineers with a powerful

material shaping technique that can form

metals and alloys into extremely complex

shapes without any metal removal steps

such as machining, milling or drilling.

Numerous variations of the MIM proc-

ess are practised by different companies

which reflect the different combinations

of metal or alloy powders, multi-compo-

nent organic binders, different moulding

techniques and widely diverse debinding

processes. The final consolidation step

of sintering is generally similar for most

MIM applications, with variations being

primarily dictated by the material and

powder characteristics.

The MIM process can be divided

into four main steps: feedstock prepara-

tion, injection moulding, debinding, and

consolidation. The major differences in

MIM processing techniques are dictat-

ed by the initial choice of the organic

binder systems, which in turn dictates

the debinding process used to remove

the organic binder. The binder systems

that are in currently in commercial use

are based primarily on wax-polymers,

oil-wax-polymer, water-gel, polyacetal

and water-polymer. Debinding tech-

niques are usually tailored to ensure

clean removal of organic binders, and

this has been responsible for the myriad

variations of debinding processes – cata-

lytic debinding, pure thermal debinding,

wicking, drying, supercritical extrac-

tion, organic solvent extraction, water-

based solvent extraction and freeze

drying among them. The choice of the

debinding equipment is dictated by the

choice of the debinding technique used,

and it eventually impacts the cost of pro-

ducing the final part.

Once the injection moulded part has

been debound (and generally presintered)

to ensure that all the organic binder has

been removed, the consolidation of the

parts is typically carried out in conventional

furnaces. Typically, consolidation of most

ferrous materials (Fe-Ni alloys, stainless

steels, low alloy steels, etc.) and several

non-ferrous alloys (nickel and cobalt-based

alloys, tungsten alloys, etc.) are sintered

in furnaces using some form of reduc-

ing atmosphere – typically hydrogen or

a mixture of hydrogen with other gases.

There are significant differences in the fur-

naces that are used for consolidation. They

include batch furnaces used only for sinter-

ing, batch furnaces capable of debinding

and sintering, continuous furnaces, and,

more latterly, microwave sintering furnaces.

Though processing variations are nec-

essary due to the initial choice of the

organic binder system, the choice of the

metal or alloy powder dictates the final

properties of the consolidated part. It is

quite obvious that the use of different met-

als or alloys will result in widely different

properties in the final component. For

example, the use of a 2Ni-98Fe alloy will

not have the same corrosion resistance as a

stainless steel alloy, while a stainless steel

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26 MPR May 2008 metal-powder.net

Figure 1. a) SEM of SUS316L PF-15; b) SEM of SUS316L PF-5.

alloy will not have the same strength as a

tungsten carbide based material.

The improvements brought about by ultra-high pressure water atomisation have recently led to the availability of very fine powders for the MIM industry at a reasonable cost.

However, what is not so obvious is

the fact that significant property vari-

ations can be achieved with the same

metal or alloy processed under the same

conditions (especially the consolidation

conditions), simply by using starting

powders with different characteristics.

Also, when using different powders, it

should be possible to attain similar prop-

erties in the same metal or alloy system

even when using consolidation condi-

tions that are different. In general, a

finer powder when sintered under similar

conditions (same temperature, heating

rate, and time) will result in a part with

higher sintered density, better mechani-

cal properties, and smoother surface

finish as compared to a coarse powder

of the same alloy. These characteristics

of finer powders can be exploited by

metal injection moulding to open up new

applications and improve the properties

of existing applications.

Finessing the issue of yields

In the past, achieving high superfine

powder yields was a major issue which

impacted the cost of the finer powders by

making it cost-prohibitive. The improve-

ments brought about by ultra-high pres-

sure water atomisation have recently led to

the availability of very fine powders for the

MIM industry at a reasonable cost. This

could provide a major breakthrough in

the area of powders for the MIM industry,

and it will be one that will have a positive

impact on the overall industry.

Among the PIM materials that are

currently in commercial production,

stainless steel is perhaps the most

important. Though, Fe-Ni-based alloys

were also quite popular in the early days

of the PIM industry, with the availabil-

ity of fine MIM stainless steel powders

the volume of stainless steel powders

used by the industry increased sub-

stantially. Fe-Ni-based alloy used to be

based primarily on elemental powder

mixes. These required homogenisation

along with densification of the material.

Incomplete homogenisation resulted in

property variations. This problem has

been overcome through the development

of fine prealloyed Fe-Ni-based alloys [3].

Within the stainless steel alloy family,

the 316L and 17-4 precipitation hardened

are the two most popular alloys. The

furnaces used for sintering these stain-

less steels are either batch or continuous

with the preferred sintering atmosphere

being typically a reducing one. Over

the years, there has been little change

in the sintering method for MIM stain-

less steels, although several different

atmospheres have been used to sinter

316L steel [4, 5]. The introduction of

very fine stainless steel powders could

bring about change in his area. A pre-

liminary discussion of the sintered den-

sity attained with ultrafine powders was

recently reported [6].

This study looked at the processing

and properties of two 316L stainless steel

powders. The first powder was conven-

tional with a mean particle size of around

10 µm, while the second was ultrafine

powder with a mean powder particle size

of around 5 µm.

The powders used were ultra-high

pressure water atomised stainless steel

powders from Atmix, Japan. The first

powder designated as SUS316L PF-15 was

a conventional material with a mean

particle size in the range of 7 to 9 µm.

The second powder was a superfine pow-

der that had a mean particle size in the

range of 3 to 5 µm and was designated as

SUS316L PF-5.

Particle size measurements on the two

powders were performed using the laser

diffraction method (Microtrac, Inc., HRA

9320-X100). The tap density of the powder

was measured using two different methods.

In the first, the height of 100gm of powder

taken in a 100 ml cylinder and tapped

around 300 times was recorded, but due to

surface unevenness it was impossible to get

a correct reading. This was then modified

to include an attachment that flattened out

the surface of the powder after 300 taps.

After flattening, it was followed by another

100 taps. Even with the modification, there

was still significant variation. An alternate

method was devised to eliminate the effect.

This divided the cylinder into two sections

and set a slide in the upper section after

Table 1: Particle size analysis, tap density, and specific surface area of the two

powders.

Sample

Designation

D10

(µm)

D50

(µm)

D90

(µm)

Tap

Density,

g/cc

Pycnometer

Density, g/cc

Specific

Surface Area,

m2/g

Atmix SUS316L PF-15

3.2 8.5 19.1 4.4 7.89 0.27

Atmix SUS316L PF-5

2.1 4.0 7.3 3.9 7.85 0.47

Table 2: Chemical composition of the two powders.

Sample

Designation

Cr

wt%

Ni

wt%

C

wt%

Fe

wt%

O

wt%

N

wt%

Atmix SUS316L PF-15

16.5 12.53 0.026 Bal. 0.39 0.07

Atmix SUS316L PF-5

16.66 12.49 0.021 Bal. 0.40 0.04

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metal-powder.net MPR May 2008 27

tapping to smooth the powder surface.

This resulted in repeatable results with

excellent batch to batch consistency. The

Tap Density results reported here were

obtained from this method.

The specific surface area was measured

by the BET method (Mountech Co. Ltd.,

Macsorb HM model-1201). The theoretical

density of the powder was also measured

using a gas pycnometer (Micromeritics'

AccuPyc Pycnometer). It should be realised

that the pycnometer density provides a

measurement of the powder density which

is generally lower than the density of the

metal itself due to the adsorbed moisture

and dissolved gases (oxygen and nitrogen).

The finer powder is expected to have more

gases in solution as well as adsorbed gases

on the surface due to the higher surface

area. Though the powder density was

measured to be in the range of 7.85 to

7.89 g/cc, the measured density from the

ladle was around 7.95 g/cc. This latter

density was assumed to be the theoretical

density of the 316L composition used.

The result of the particle size analysis,

tap density, pycnometer density, and spe-

cific surface area of the two powders are

shown in Table 1. The detailed chemical

compositions of the powders are given in

Table 2. The Scanning Electron photomi-

crographs of the two powders are shown in

Figure 1.

Each of the two powders was mixed

with a proprietary organic binder to

produce the desired feedstock. The mix-

ing was carried out in a kneader for one

hour to produce the feedstocks. The

moulding of the tensile bars was carried

Figure 2. Relationship between sintering temperature and sintered density of the ultra fine (SUS316L PF-5) and conventional (SUS316L PF-15) powder MIM parts.

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out in an injection moulding machine

(Nissei Plastic Industrial). The debind-

ing was carried out in a nitrogen atmos-

phere at 475°C. The total debinding

time was around 20 hours. After debind-

ing, the tensile samples were removed for

presintering. A pre-sintering step was

used to ensure that there was absolutely

no binder remaining in the samples. The

presintering was carried out in nitrogen

using a ramp rate of 5°C/min and a one-

hour hold. The sintering was carried out

at several different sintering tempera-

tures ranging from 900 – 1350°C, using

an Argon partial pressure in the range

of 100-500 Pa (1-5Torr). The 900°C and

950°C sintering temperatures were used

only for sintering the superfine powder,

while both the powders were sintered

at all the other temperatures of 1000,

1050°C, 1100°C, 1200°C, 1300°C, and

1350°C. A constant hold time of two

hours at the maximum sintering tem-

perature was used for all the sintering

runs. The sintered densities of the parts

were measured by water immersion. The

surface roughness of the parts sintered

at 1000°C, 1050°C, 1100°C, 1200°C,

and 1300°C were measured using a con-

tact type surface roughness measuring

instrument (Taylor Hobson).

The injection moulded and sintered ten-

sile bars were subjected to tensile testing.

For each sintering condition, five tensile

bars were pulled to failure using a rate of

3mm/min. The ultimate tensile strengths

and tensile elongations of the sintered ten-

sile bars were determined for several sinter-

ing conditions. Some of the as-sintered bars

were sectioned for microstructural studies.

The sectioned samples were mounted, pol-

ished, etched with Aqua Regia and observed

in an optical microscope.

As the powder size becomes finer, the

internal friction of the powder particles

is increased. This in turn typically trans-

lates to a lower tap density and apparent

density (not measured in this case) for

the finer powder compared to a coarser

powder of similar shape. Also, as the

powder shape remains the same but the

powder particle size is decreased, the

specific surface area of the powder will

increase. All of the expected trends were

followed by the two powders used in

this study as shown by the data in Table

1. As a result of the increased surface

area and lower tap density of the finer

powder, the viscosity of the material was

expected to be higher with the finer pow-

ders. However, the near-spherical shape

of the powder particles formed by the

ultra-high pressure water atomisation,

results in a lowered viscosity compared

to conventional irregular shaped water-

atomised powder.

One of the key advantages of using

finer powder is the attainment of higher

density at a particular sintering tem-

perature. Figure 2 shows the sintered

Figure 3. Relationship between sintering temperature and tensile strength of the ultra fine (SUS316L PF-5) and conventional (SUS316L PF-15) powder MIM parts.

Figure 4. Relationship between sintering temperature and tensile elongation of the ultra fine (SUS316L PF-5) and conventional (SUS316L PF-15) powder MIM parts.

the use of the ultra-fine powder will result in significantly better surface finish of the final part.

28 MPR May 2008 metal-powder.net

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density of the metal injection

moulded parts sintered at different

sintering temperatures for the two

different powders. It can be seen

that even at extremely low sinter-

ing temperatures (for 316L sinter-

ing) of 1100°C, the parts using

the ultra-fine powder (SUS316L

PF-5) has already attained a sin-

tered density that is greater than

7.7 g/cc which is almost 97 per

cent of theoretical. In contrast,

the coarser powder (SUS316L PF-

15) sintered at the same tempera-

ture exhibited a sintered density

of around 7.2 g/cc, which is only

around 91 per cent of theoreti-

cal. The finer particle size of the

ultra fine powder provides a sig-

nificantly higher sintering poten-

tial, which in turn translates into

higher sintered density under the

same sintering conditions. It can

also be seen that in order to attain

a density of around 7.7 g/cc with

the conventional powder (SUS316L

PF-15), the sintering temperature

has to be over 1300°C.

Many of the metal injection

moulded parts had a sintered den-

sity requirement of around 7.7 g/cc.

The ability of the finer powders to

achieve that density at a lower tem-

perature would be a major advan-

tage to part producers, especially

for parts that do not have very high

strength requirement.

Figure 3 shows the relationship

between sintering temperature and

the ultimate tensile strength of

the metal injection moulded parts

fabricated from the two powders.

The strength initially shows a sharp

increase with sintering tempera-

ture for the ultra fine powder. A

substantial increase in strength is

observed when the sintering tem-

perature is increased from 900°C

to 1000°C. In fact, the peak tensile

strength of the parts made from

the ultra fine powder is seen at a

sintering temperature of 1050°C,

Figure 5a. SEM of SUS316L PF-5 sintered at 1000oC.

Figure 5b. SEM of SUS316L PF-5 sintered at 1300oC.

Figure 5d. SEM of SUS316L PF-15 sintered at 1300oC.

Figure 5c. SEM of SUS316L PF-15 sintered at 1000oC.

References1. Randall M German and Animesh Bose, Injection Molding of

Metals and Ceramics, 1997, Metal Powder Industries Federation,

Princeton, NJ.

2. Beebhas C Mutsuddy and Renee G Ford, Ceramic Injection Molding,

1995, Chapman & Hall, London, UK.

3. Hisataka Toyoshima, Tokihiro Shimura, Atsushi Watanabe and

Hidenori Otsu, “Sintered Compact Properties of Pre-alloyed 2%Ni-

Fe Water Atomized Powder” Journal Japan Society of Powder

Metallurgy, 2005, vol. 52, no. 6, pp. 437-441.

4. J C Rawers, F Croydon, E A Krabbe, and N W Duttlinger,

“Tensile Characteristics of Nitrogen Enhanced PIM 316L

Stainless Steel,” Advances in Powder Metallurgy and Particulate

Materials, Compiled by M Phillips and J Porter, Metal Powder

Industries Federation, Princeton, NJ, 1995, vol. 6, part 6, pp.

229-242.

5. G R White and R M German, “Effect of Process Conditions

on the Dimensional Control of Powder Injection Molded

316L Stainless Steel,” Advances in Powder Metallurgy and

Particulate Materials, Compiled by C Lall and A J Neupaver,

Metal Powder Industries Federation, Princeton, NJ, 1994, vol. 4,

pp. 185-196.

6. Hisataka Toyoshima, Minoru Kusunoki, and Isamu Otsuka,

“Sintering properties of high-pressure water atomized

SUS 316L ultra fine powder,” Proceedings of the PM

World Congress, Pusan, Korea, 2007.

7. Materials Standards for Metal Injection Molded Parts, 2007

Edition, MPIF Standards 35, Publisher, MPIF, Princeton, NJ, p.19,

2007.

metal-powder.net MPR May 2008 29

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30 MPR May 2008 metal-powder.net

after which the tensile strength shows a

slight decrease.

In contrast, the coarse powder (SUS316L

PF-15) does not show a peak for the tensile

strength in the sintering temperature range

used for this investigation. The strength

curve for the coarse powder samples almost

flattens out after a sintering temperature

of 1200°C is reached. If one considers the

sintering temperature of 1300°C, the dif-

ference in the tensile strengths between the

ultra fine and the coarse powder samples

is almost 50 MPa. However, the difference

between the maximum strength achieved

by samples made from the two powders is

more than 100 MPa [7].

The variations in the tensile elonga-

tion with sintering temperature for the

two powders are shown in Figure 4.

The tensile elongations, however, did

not follow the same trend as the tensile

strength properties as shown in Figure 3.

The tensile elongation for both the pow-

ders is seen to increase with increasing

sintering temperature, though there is

a general flattening out of the curves at

the higher temperatures. The elongation

of the parts made from ultra fine powder

increases rapidly when the sintering tem-

perature was increased from 950°C to

1000°C. However, even though the tensile

strength for the ultra fine powder

decreases after 1050°C, the ten-

sile elongation continues to increase

up to a temperature of 1200°C after

which is flattens out. It can be seen

that around 50 per cent of elongation

in the ultra-fine samples is attained

at a sintering temperature of 1100°C.

This is in the typical range of MIM

properties reported in the MPIF

standard [7].

Both the sintered density and the ten-

sile strength of the two powders show

a similar variation with sintering tem-

perature. The initial increase in the den-

sity and strength is quite rapid at the

early stage. The ultra fine powder, due

to its associated surface energy, sinters

quite rapidly and tends to achieve high

density at a much lower temperature.

The mass transport and elimination of

porosity will be quite rapid due to the

association of the porosity with the

grain boundaries. In practice, once the

sintering potential was used up and the

porosity isolated within the grains, the

densification rate quickly levelled off.

For the coarser powder, however, the

densification rate did not flatten out as

dramatically as the ultra fine powder.

The drop in the tensile strength in the

ultra fine powder MIM parts is likely

associated with a rapid grain growth

that starts taking place at elevated tem-

peratures. Figure 5a and 5b shows the

microstructures of the ultra fine powder

(SUS316L PF-5) MIM parts sintered at

1000°C and 1300°C, respectively. The

large difference in the grain size is easily

observed from the two microstructures.

This grain growth is primarily respon-

sible for the lowering of the tensile

strength of the samples at the elevated

temperatures. Figure 5c and 5d shows

the microstructures of the conventional

powder (SUS316L PF-15) MIM parts

sintered at 1000°C and 1300°C, respec-

tively. The lower sintering temperature

shows the presence of high amount of

porosity and prior particle boundaries.

The prior particle boundaries were not

discernable in case of the ultra fine pow-

der MIM part sintered at 1000°C. At the

higher sintering temperature of 1300°C

the coarse powder also showed very large

grain size.

The surface roughness of the parts

sintered under the same sintering

conditions decreased as the

powder particle size became finer. This

was an expected trend as the prior parti-

cle boundaries will depend on the initial

powder particle size and was expected

to influence the surface roughness of

the part. It can be seen that an increase

in the sintering temperature causes a

slight increase in the surface roughness

of the materials. Figure 6 shows the sur-

face roughness variation with sintering

temperature for the parts fabricated

from the conventional and ultra fine

powders. It can be observed that the

use of the ultra-fine powder will result

in significantly better surface finish of

the final part. It can also be concluded

that the use of the ultra-fine powder will

result in not only better surface finish of

the part but will also result in more com-

plete fill in parts that have significantly

finer details.

The Authors

THIS article is based on Metal injec-tion molding of ultra-fine 316l stainless steel powders, a paper by

Animesh Bose¹, Isamu.Otsuka²,

Takafumi Yoshida² and Hisataka

Toyoshima².

¹ Materials Processing Inc, 5069 MLK

Freeway, Fort Worth, TX – 76109,

USA

² Epson Atmix Corporation, 4-44

Kaigan, Kawaragi, Hachinohe-shi

Aomori-ken, 039-1161 Japan

Figure 6. Relationship between sintering temperature and surface roughness of the ultra fine (SUS316L PF-5) and conventional (SUS316L PF-15) powder MIM parts.

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