Transcript Document

METAL POWDER TESTING
Characterization of Metal Powders:
• Quality monitoring
• The following steps are involved in P/M prior to
processing into compact shape:
a) Powder characterization and testing
b) Powder handling and mixing
a) Powder Characterization and Testing:
1. Powder sampling
2. Chemical Testing
i) Oxygen content of the powder
ii) Acid insoluble content of powders
3. Particle-related vs mass-related properties
4. Particle size and particle size distribution
i) Sieving
ii) Microscopic sizing
iii) Sedimentation methods
iv) Coulter Counter and particle analysis by
light observation
v) Laser light scattering
5. Particle shape and structure
6. Specific surface area
7. Characteristics determining the processing
behavior of metal powder:
i) Flow rate and apparent density
ii) Compactibility
iii) Dimensional changes of powders due to
sintering
b) Powder mixing and handling
1. Special precautions in handling and storage of
metal powders
2. Powder Mixing
i) Mixing and demixing
ii) Mixing apparatus
PARTICLE SHAPE:
• Figure shows various possible particle shapes of
powders.
• Spherical powders show excellent flow properties
but give poor green strength as compared to
irregular powders.
• Water atomization ------ ranging from near
spherical to highly irregular.
• Gas atomization --------- spherical powder
particles.
• The reactivity of the metal or alloy essentially
determines the particle shape. If the alloy-water
reaction produces a strongly adherent film then
irregular particles are formed. Spherical shapes are
produced when the oxide formed are highly fusible
at the melting point of the alloy as they have no
strength to over-come the forces of surface tension.
• High melting metals/alloys have tendency to form
spherical particles because of long freezing times.
• Very short freezing times for low melting
metals/alloys tend to form highly irregular particles.
Table1 shows Particle Shape and the Method of Powder Production
ONE DIMENSIONAL
Acicular
Irregular Rod-like
Chemical decomposition
Chemical decomposition
Mechanical comminution
TWO DIMENSIONAL
Dentritic
Electrolytic
THREE DIMENSIONAL
Spherical
Atomization
Carbonyl Fe
Precipitation from a liquid
Flake
Mechanical comminution
Rounded
Atomization
Chemical decomposition
Irregular
Atomization
Chemical decomposition
Angular
Mechanical disintegration
Carbonyl Ni
Porous
Reduction of oxides
Powder Properties:
• Processing conditions and final sintered properties
are determined to a very large extent by the
characteristics of the powder:
Such as;
chemical composition
particle size and size distribution
particle shape
structure
surface condition
Sampling of Powders:
• Standard methods
 ASTM Committee B-9
 MPIF Standard Committee
ASTM Standards B215
MPIF Standard 1
• Method A is for powders in the process of being
packaged from blenders or storage tanks.
• Method B is for powders already packaged in
containers.
A representative sample of the whole lot
• Samples from the entire cross section of the stream
of powder as it flows from the blender.
• The first when the first shipping container is half
full, the second when half of the burden of the
blender has been discharged and the third when the
last shipping container is half full.
• Portions of these samples are blended ------- sample
splitter.
Figure: Sampling from falling
streams.
(a)Bad sampling technique.
(b) Good sampling technique.
(c) Sampling procedure to be
adopted for high mass flow rate
Sample Spliter
Sampling spears
• Representative sample from a shipment consisting
of several drums.
• Thieve sampling
• “Thieves” are devices to take samples from
different layers (top, center, bottom) of drums filled
with powder.
CHEMICAL TESTS
a) Hydrogen Loss Test:
 ASTM standard E 159, ------- MPIF standard 2
for the so-called hydrogen loss of Cu, W and Fe powder
• A sample of powder is heated in a stream of hydrogen
for a given length of time and at a given temperature.
• Loss of weight ---- an approximate measure of the
oxygen content of the powder.
• Hydrogen loss values may be lower than the actual
oxygen content -------- Oxides not reduced by hydrogen
under the test conditions such as SiO2, Al2O3, CaO,
TiO2 etc
• The hydrogen loss value may be higher than the actual
oxygen content in the presence of elements forming
volatile compounds with hydrogen, i.e. S or C.
• Some metals volatile at the test temperature, i.e., Zn, Cd and
Pb.
• To avoid measuring the content of C, S, or volatile metals in
the in the metal powder, a modified hydrogen loss test is
used.
• The amount of water vapor produced by heating in a stream
of dry hydrogen is determined by titration.
• Total amount of oxygen in a metal powder including oxygen
in refractory oxides, fuse a sample in a small single-use
graphite crucible under a flowing inert atmosphere at a
temperature of 2000 oC or higher.
• The oxygen is released as CO and measured by infrared
absorption or alternatively converted to CO2 and measured
by a thermal conductivity difference.
Acid Insoluble Content of Cu and Fe Powder:
• Samples of Fe powder are dissolved in HCl and those of
Cu in HNO3 under specified conditions.
• The insoluble matter is filtered out, ignited in a furnace
and weighed.
• Silica, insoluble silicates, alumina, clays and other
refractory materials
• In Fe powder, the acid insoluble may also include
insoluble carbides.
Particle Size and Particle Size Distribution:
Methods:
(a) Sieving
(b) Microscopic sizing
(c) Methods based on Stokes’ Law
i) the Roller air analyzer
ii) the Micromerograph
iii) Light and X-ray (Sedigraph) turbidimetry
(d) Coulter Counter and Particle Analysis by Light
Obscuration
(e) Laser Light scattering; the Microtrac particle analyzer
** 44 microns
Sieving:
ASTM Standard B214
MPIF Standard 5
• A set of sieves is assembled from the finest to the
coarsest in ascending order with a collecting pan at
the bottom under the finest sieve.
• Sample ----- 100g or 50g
• Mechanical sieve shaker ---- shaken for 15 minutes
ASTM sieves size,
Tyler sieves size, and
and U. S. standard
Tyler sieve series
sieve designation, μm designation, μm
180 (No 80)
175 (80 mesh)
150 (No 100)
149 (100 mesh)
106 (No 140)
104 (150 mesh)
75 (No 200)
74 (200 mesh)
45 (No 325)
44 (325 mesh)
Figure: Schematic of sieve series
stacked in order of size
Figure: Stacked sieves on a shaker with rotary
and tapping action
Figure: Sieved size of an irregularly shaped particle
Quantitative Microscopy
• 0.5 – 1000 μm
• Optical and electron microscopy are used to directly
observe and measure individual particle size and shape.
• Reproducible, direct measurement, inexpensive
• Can be automated
• Time consuming if done manually
• However, automatic counting and image analysis techniques
have advanced significantly with computer technology, and
has made possible the rapid sizing of fine particles using
small laboratory samples.
Figure: Histogram and size frequency curve for
log-normal size distribution
Figure: Cumulative plot used in determining
median particle size
Methods Based on Stokes’ Law:
• Sedimentation and Elutriation
• Stokes’ Law gives the settling velocity ν of spherical
particles with a diameter x and a density ρ in a fluid
medium with density ρF and viscosity η
ν = g (ρ - ρF ) / 18 η * x2
Where g is the gravitational constant.
* Particles which are not spherical will also settle.
Their “Stokesian” size is defined by the diameter of
a sphere of the material which has the same settling
velocity as the irregular powder particle.
• Convection currents in the suspending fluid must be
avoided.
• The relative rate of motion between the fluid and the
powder particles must be slow enough to guarantee
laminar flow, which means the Rynolds number should be
less than 0.2
x ν ρF /η
Where x is the particle size, ν is the settling velocity, ρF
the density of the fluid and η its viscosity.
• The particles in the suspension must be perfectly
dispersed and the suspension must be dilute enough to
guarantee independent motion, which means maximum
concentration of about 1 % by volume of particles in the
suspending medium.
Elutriation Techniques:
 Elutriation is a process of sizing particles by means
of an upward current of fluid, usually water or air.
 The process is the reverse of gravity sedimentation,
and Stokes' law applies.
 All elutriators consist of one or more "sorting
columns“ in which the fluid is rising at a constant
velocity.
 Feed particles introduced into the sorting column
will be separated into two fractions, according to their
terminal velocities, calculated from Stokes' law.
Those particles having a terminal velocity less than
that of the velocity of the fluid will report to the
overflow, while those particles having a greater
terminal velocity than the fluid velocity will sink to
the underflow.
 Elutriation is carried out until there are no visible
signs of further classification taking place or the
rate of change in weights of the products is
negligible.
The Roller Air Analyzer:
• Elutriation in a stream of air ---- air classification.
• Particle size fractions in the range between 5 and
40 μm.
• ASTM Standard B 283 and MPIF Standard 10.
• Cylindrical settling chamber.
• The velocity v of air stream through the chamber in
cm/sec which just balances the settling velocity of
particles with diameter x in μm and a density ρ in
g/cm3 can be calculated from Stokes law in the
form -------- v = 29.9 x 10-4 ρx2
Roller Air Analyzer
• To obtain this velocity v in cm/sec of the air stream
through a cylindrical settling chamber of diameter D
in cm, the volume rate of air flow F in cm3 /sec must
be ------- F = 47.1 x v x D2
• With this volume rate of flow, particles with a size
smaller than x will be carried through the settling
chamber into the collecting system which consists of
an extraction thimble. Large particles will fall back
into U-tube.
• By using a series of vertical settling chambers with
diameters in the ratio 1:2:4:8 and a constant
volumetric rate of flow, the powder may be
classified into particle size fractions with the
maximum sizes in the ratio of 1:2:4:8 (e.g. 5, 10, 20
and 40 μm).
The Micromerograph:
• Sedimentation balance
• Sub-sieve metal powders.
• The powder is suspended in air by projecting the sample
with a burst of nitrogen.
• Settling chamber --- a thermally insulated vertical
aluminum tube 10 cm inside diameter and 2.5 m high.
• An automatic balance at the bottom of the chamber.
• A recorder records the cumulative weight of powder
settled as a function of time, from which the particle size
distribution is calculated on the basis of Stokes’ law.
• Range 2 to 100 μm.
• Tendency of the powder to cling the walls of the column.
Light and X-Ray Turbidimetry:
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•
•
•
•
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Sedimentation method
Refractory metal powders, W and Mo
Refractory metal compound powders, WC
ASTM -------- B 430
Change in the intensity of the light beam.
The intensity of light beam is determined by the current
generated in a photocell.
• Low cost
• X-rays can be used instead of white light.
• An instrument called Sedigraph is based on measuring the
variation of intensity with time when a finely collimated
X-ray beam is transmitted through a settling suspension of
metal powder.
Turbidimetry
Coulter Counter:
• Suspension is passed through a sensing zone
• Dilute suspension --- particles pass through the zone
one by one
• Suspending liquid must be electrically conducting
• The particles pass through an orifice which has
immersed electrodes on either side.
• Change in resistance
• The passage of the particle changes the resistance of
an electric circuit through the liquid between the
electrodes causing voltage pulses proportional to the
particle volume which are counted.
Coulter Counter
Coulter Counter Particle Size Analyzer
Coulter Counter Particle Size Analyzer
Flow Rate and Apparent Density:
• Die filling
• A uniform and reproducible amount of powder
should fill the die cavity from stroke to stroke.
• Apparent density of the powder must be controlled.
• Hall flowmeter: both flow rate and apparent density
• ASTM standard B212 and MPIF standard 4 for
“apparent density”
• ASTM standard B213 and MPIF standard for
“flow rate”
• Sample ------ 50 g
Hall Flow Meter
Powder Conditioning:
• The metal powder directly after production may not have
the necessarily required physical and/or chemical
characteristics for immediate use.
• The required characteristics may be attained by mechanical,
thermal or chemical treatment or by alloying.
• The impure, wet (defective) powder may be washed, dried
or softened and purified by a reducing anneal in a hydrogen
gas atmosphere.
• The required shape, size and size distribution may be
achieved by sieving, mixing or milling.
• Mixing or milling may give the required uniformity of
physical and chemical characteristics.
• The mixing may involve various powders to give the
required chemical composition and other additions such as
binders, or lubricants to assist the processing with required
green strength and ultimate controlled porosity.
• The lubricants affect the flow and apparent density and
mixing of the powders.
• The lubricants are burned off during the early stages of
sintering.
• Mixing and milling ----- in air or under controlled
atmosphere or under a suitable liquid medium to minimize
oxidation or segregation.
Various problems of powder mixing are:
i) Filling the powder into the mixer
ii) Determination of the optimum amount of the powder
iii) Grinding action and agglomeration during mixing
iv) Oxidation
v) Addition of impurities by abrasive action
vi) Determination of optimum mixing time
vii) Extraction of the mix
viii) Sampling difficulties
ix) Evaluation of mixedness