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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.
Table 1 shows Particle Shape and the Method of Powder Production ONE DIMENSIONAL Acicular Irregular Rod-like 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 Ø ASTM Committee B-9 Ø MPIF Standard Committee ASTM Standards B 215 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)(b) Good sampling technique. (c)(c) Sampling procedure to be adopted for high mass flow rate
• 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: v 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 Si. O 2, Al 2 O 3, Ca. O, Ti. O 2 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 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 o. C or higher. • The oxygen is released as CO and measured by infrared absorption or alternatively converted to CO 2 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 HNO 3 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 B 214 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 ----- 100 g or 50 g • Mechanical sieve shaker ---- shaken for 15 minutes
ASTM sieves size, Tyler sieves size, and U. S. standard Tyler sieve series sieve 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 * x 2 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: Ø Elutriation is a process of sizing particles by means of an upward current of fluid, usually water or air. Ø The process is the reverse of gravity sedimentation, and Stokes' law applies. Ø All elutriators consist of one or more "sorting columns“ in which the fluid is rising at a constant velocity. Ø Feed particles introduced into the sorting column will be separated into two fractions, according to their terminal velocities, calculated from Stokes' law.
ØThose 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. Ø 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/cm 3 can be calculated from Stokes law in the form ---- v = 29. 9 x 10 -4 ρx 2
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 cm 3 /sec must be ------- F = 47. 1 x v x D 2 • 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: • • • 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 Xray beam is transmitted through a settling suspension of metal powder.
Coulter Counter: • Suspension is passed through a sensing zone • Dilute suspension --- particles pass through the zone 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 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 B 212 and MPIF standard 4 for “apparent density” • ASTM standard B 213 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