DBT 301 POWDER METALLURGY CHAPTER 2 POWDER CHARACTERIZATION

DBT 301 POWDER METALLURGY CHAPTER 2: POWDER CHARACTERIZATION Semester 1 2016/2017

2. 0 Outline Particle Shape. Surface Area Analysis. Inter-particle friction. Compressibility. Powder structure.

2. 1 CONCEPT & PRINCIPAL OF PARTICLE SIZE A powder can be defined as a finely divided particulate solid Engineering powders include metals and ceramics Geometric features of engineering powders: � Particle size and distribution � Particle shape and internal structure � Surface area

2. 1 CONCEPT & PRINCIPAL OF PARTICLE SIZE Production of metal powders; In general, producers of metallic powders are not the same companies as those that make PM parts The methods of powder production are: 1. Atomization • Produces liquid-metal stream by injecting molten metal. 2. Reduction • Removal of oxygen involves gases such as reducing agents. 3. Electrolytic deposition • Uses either aqueous solutions or fused salts. 4. Carbonyls 5. Comminution

Gas Atomization Method High velocity gas stream flows through expansion nozzle, siphoning molten metal from below and spraying it into container

comminution

IRON POWDERS FOR P/M

IRON POWDERS FOR P/M Iron powders produced by decomposition of iron pentacarbonyl (photo courtesy of GAF Chemical Corp); particle sizes range from about 0. 25 ‑ 3. 0 microns (10 to 125 -in).

PARTICLE SIZE, DISTRIBUTION & SHAPE Particle size is measured and controlled by screening. Other methods are also used for particle size analysis: 1. Sedimentation 2. Microscopic analysis 3. Light scattering 4. Optical means 5. Suspension of particles Size distribution of particles affects the processing characteristics of the powder. SEM is one of the best tools to observe powder characteristic.

PARTICLE SIZE, DISTRIBUTION & SHAPE Particle size is important in powder characteristics. More irregular the particle shape, more difficult in getting the particle size. Particle size analysis should convey info on: 1. Distribution. 2. Shape. 3. The basis of measurement. Particle shape influences on processing characteristics. Shape is described in terms of aspect ratio or shape index the ratio of the largest dimension to the smallest dimension of the particle. Shape index, or shape factor (SF), is a measure of surface area to the volume of a particle.

2. 1 CONCEPT & PRINCIPAL OF PARTICLE SIZE

PARTICLE SHAPE

2. 2. MEASURING PARTICLE SIZE TECHNIQUE Most common method uses screens of different mesh sizes Mesh count - refers to the number of openings per linear inch of screen �A mesh count of 200 means there are 200 openings per linear inch � Since the mesh is square, the count is equal in both directions, and the total number of openings per square inch is 2002 = 40, 000 � Higher mesh count = smaller particle size

2. 2 MEASUREMENT TECHNIQUE 1. Microscopy q Use the ability of eye. q Image is generated by optical, scanning electron or transmission electron microscopes. q A frequency distribution can be generated by microscopic counting of diameter, length, height, or area. q The distribution record the relative frequency of various particle dimensions.

2. 2 MEASUREMENT TECHNIQUE q q q Major problem in obtaining a dispersed sample for image analysis. Agglomeration occur making it difficult to distinguish the actual particle size & shape. Intelligent discrimination between single & multiple particles is a common problem.

2. 2 MEASUREMENT TECHNIQUE 2. Screening q Most common technique is based on screening. q Square grid of even space wire create mesh. The mesh size is determined by number of wires per unit length. q Opening size varies inversely with mesh size. q Large mesh sizes mean small opening sizes and vice versa. q Mesh sizes cannot very small, only for particles larger than 38 micron.

Screen Mesh Screen mesh for sorting particle sizes.

2. 2 MEASUREMENT TECHNIQUE q q q Analysis begins with a stack of screens of increasing mesh sizes, with smallest opening size at bottom. Powder is loaded onto the top screen and the screen stack is vibrated for a period. The amount of powder in each size interval is weighed and the interval

2. 2 MEASUREMENT TECHNIQUE q q Common problem is overloading where it inhibit powder from reaching the limiting screen size. More problem when amount of powder per unit screen area increases, which content of fine particles with smaller screen openings. Defects in screens allow oversized particles to pass. Too long, particle break into fine pieces. Too short,

2. 2 MEASUREMENT TECHNIQUE 3. Sedimentation q Most applicable to finer particle sizes (0. 05 to 60 microns). q Particles settle in a fluid will reach terminal velocity, depend on particle size & fluid viscosity. q Size can be estimated from settling velocity. q Powder is placed at the top of tube; amount of powder settling vs. settling time were calculated for particle size distribution. q Fastest = largest; longest = smallest.

2. 2 MEASUREMENT TECHNIQUE q q q Limited to a narrow size range. Particles below 1 micron have problems with slow settling & turbulence. Internal porosity, irregular particles, container walls & particle interactions can affect the analysis. Concentration must be keep below 1%. Fluid & powder must not react chemically.

2. 2 MEASUREMENT TECHNIQUE 4. 5. 6. 7. Other techniques: Light scattering. Electrical Conductivity. Light blocking. X-Ray techniques.

2. 3 MEASUREMENT TECHNIQUE 1. Problems: q Limited size range that can be accurately resolved analyzed. a) Sieves: >38 micron. b) Optical microscopy: >1 micron. q More irregular shape, more parameters needed to size a particle. q When more than one particle is in the detection zone, they are sized as one large particle. q Agglomeration of powder.

Particle Shape Influence packing, flow & compressibility. Also provide info on powder fabrication and help explains character of processing. The shape are difficult to quantify; use qualitative descriptor instead. It varies with size & manufacturing technique. For size analysis, usually shape are assumed constant, but this will give large error. A simple quantitative shape descriptor is adequate.

Particle Shape Quantify shape descriptor with aspect ratio. � Maximum particle dimension divided by the minimum particle dimension. Microscopy technique is suitable to quantify shape. � easy to understand. � Easy to obtain the shape from projected images. �Some example of particle shapes descriptors: Spherical, angular, rounded, cubic, sponge, acicular, cylindrical, irregular, flake, polygonal, dendritic, aggregate, fibrous, ligamental, tear drop.

Particle Shape

2. 3 SURFACE AREA ANALYSIS Measure (average) the external condition of a large number of particles. Use to correlate it with kinetic & geometric characteristics. Provides idea in powder behavior during chemical activity, catalysis, friction, adsorption, contamination, pressing & sintering. But not the powder distribution, texture or internal structure.

2. 3 SURFACE AREA ANALYSIS Two main analysis techniques to measure surface area: 1. Gas Adsorption 2. Gas Permeability Molecular adsorption on a surface

2. 4 INTERPARTICLE FRICTION Why? –two main concerns which are powder flow and powder packing. When surface area increases, � amount of friction increases. � the flow & packing become less efficient. These affected the process of auto die filling, packaging, transportation, blending & mixing. Resistance to flow is main feature of friction.

2. 4 INTERPARTICLE FRICTION A powder is feed under gravity through small opening � flow rate can be measured then. Mostly, small & sub sieve powder will not flow through due to its high interparticle friction. They are called as non-free flowing powder. Short flow times indicate free flowing powders while longer times indicate high interparticle friction.

2. 4 INTERPARTICLE FRICTION Small particles will have more interparticle friction, giving a lower number of nearest neighbor. Particle shape & friction characterization can be assess through bulk & tap densities. Ratio of tap to bulk = Hausner ratio; will be use to categorized relative interparticle friction.

2. 5 COMPRESSIBILITY Compressibility/compactability measures the ability to densify a powder under an applied load. Die is loaded with powder, then measure density after compaction (green density). Powder fill depth can be calculated: (height) x (CR) CR=ρg / ρa CR= Compression ratio ρg= green density ρa= apparent/bulk density

2. 6 POWDER STRUCTURE Cross section view magnified through a powder shows condition of fabrication & problems in processing. From cross section, internal pores and segregation can be analyzed to detect inclusions & oxides.

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