Chapter 3: Powders Production and Characterization

Transcription

1 Chapter 3: Powders Production and Characterization Course Objective... To introduce selective powder production processes and characterization methods. This course will help you : To understand properties of powders such as morphology, composition, flow ability,.

2 Particle Characteristics Concerns beyond particle appearance require following quantitative data: Particle size and distribution Particle shape Flowing and packing Chemical and phase composition Density

3 Particle Size Sizes and shapes are important in blending and compaction. Particle size is a determination of the dimensions of a particle Particle size depends on: Measurement technique and particle shape Most analyzers make the assumption of a spherical particle shape

4 Powder Size Parameters Six possible measures of the particle size are shown: I. Three based on projected dimensions II. Three based on equivalent spherical diameters

5 Measurement Techniques Microscopy followed by image analyzers Optical, SEM, TEM Larger depth of field on the SEM is a distinct advantage Quantitative can be determined: d Diameter, Length, Height, Area and Frequency distribution

6 Measurement Techniques Powder Sieving is a common technique for rapidly analyzing particle size. A square grid of evenly spaced wires creates mesh. The mesh size is determined by the number of wires per unit length (inch). So the opening size varies inversely with the mesh size. Disadvantages: 3-7% permissible variation in average opening size Blocking the mesh opening Irregular shape particles

7 MESH SIZES -100/+200 mesh: negative means particle goes through, positive means particle does not go through. Thus, this mesh means particle sizes between 75 and 150 microns.

8 SEDIMENTATION TECHNIQUE The force balance leading to a constant settling velocity for a spherical particle in a viscous fluid. Stokes Law: V= gd 2 (ρ m ρ f ) /(18ŋ) Where: g = gravitational constant D = particle size ρ m = powder density ρ f = liquid density ŋ = liquid viscosity

9 Example: SEDIMENTATION A spherical nickel powder is to be analyzed for particle size using sedimentation. It is suspected that the particle size is near 8 µm. If the powder is dispersed d in water at the top of a settling column 100 mm high, then what is the expected settling time?

10 Measurement Techniques Laser diffraction A representative cloud of particles passes through a broadened beam of laser light which scatters the incident light onto a lens. This lens focuses the scattered light onto a detector array and a particle size distribution is inferred from the collected diffracted light data. Sizing particles using this technique depends upon accurate, reproducible, high resolution light scatter measurements to ensure full characterization of the sample. Limitations: Inter-instrument reproducibility Resolution Submicrometre characterization

11 Laser Light Scattering

12 PARTICLE SIZE DISTRIBUTION The most heavily populated size is the Mode Median is the "middle" value in the particle size, The 50% value

13 DIFFERENT PARTICLE SIZE DISTRIBUTIONS

14 Particle Shape Particle shape influences packing, flow, oxidation and decomposition during fabrication processes

15 Particle Shape

16 Chemical and Phase Composition Powder can be used in the following three groups: 1.High purity and single materials like Al, Cu 2.Pre-mixed powders like WC/8%Co 3.Pre-alloyed powders like Stainless steel, Ti-6Al-4V Chemical analyses: 1. Surface analyses (Oxide, adsorbed organic films, contamination) 2. Bulk analyses (Impurities, composition)

17 Chemical and Phase Composition

18 Powder flow ability and packing Interparticle friction can affect powder flow and packing. Friction between particles is dominated by the surface area, roughness and chemistry. As the surface area increases, the amount of friction in a powder mass increases. Contamination may increase adhesion between individual particles Roughness increases friction and inter lucking between particles

19 Powder Density Apparent density (bulk density): Mass/Volume when the powder is in the loose state without agitation. Tap density: The highest density that can be achieved by vibration of the powder without using any external pressure Theoretical density: The density when there is no porosity present

20 Powder Production Definition: Delivery of energy to material to create new surface areas Fabrication Techniques: 1. Mechanical 2. Chemical reaction 3. Electrolytic deposition 4. Atomization

21 Mechanical fabrication Processes: 1. Impacting: Rapid delivery of blow to a material causing cracks 2. Attrition: Using rubbing motion 3. Shearing: Cleavage type of fracture by cutting 4. Compression: Break a material by compression force if it is sufficiently brittle

22 Milling Process Mechanical impacting using hard balls for brittle materials Required Stress for Fracture: σ = ( 2 Er / D ) 1/ 2 Required Energy: W = a g( D f D a i ) σ is the required stress E is the elastic modulus r is the defect or existing crack tip radius D is the particle size Disadvantages: Irregular shaped particles Contamination ti Only brittle materials g is a constant depends on the material, balls, mill design and operation condition a is between 1 and 2. Time of milling depends on the available powder, particle size change, milling media size, and milling operation

23 ATTRITION MILLING The input materials goes through a sequence of cold welding and fracture steps.

24 MECHANICALLY MILLED POWDER SHAPE

25 Electrolytic Fabrication A powder can be precipitated at the surface of a cathode of an electrolytic cell under certain operating conditions The porous cathode deposit is removed, washed, dried, ground into fine powder and annealed to remove any strain hardening. Metals with very high h purity can be formed by this method

26 DETAILS OF ELECTROLYTIC Cu

27 Chemical Fabrication Decomposition of solid particles by gas (reduction of magnetite) Vapor decomposition and condensation Precipitation from a liquid Characteristics: High purity powder (99.5%) Small crystallite size Strong agglomeration tendency Irregular shape Poor flow and packing properties

28 OXIDE REDUCTION PROCESS FOR METAL POWDER FABRICATION

29 CARBONYL PROCESS

30 SHAPE OF CARBONYL POWDERS Purity ~ 99.5%

31 Atomization Formation of powder from molten material using a spray of gas or water forming molten material droplets Gas Atomization Air, Nitrogen, Helium, Argon Water Atomization Benefits: Homogeneity (rapid solidification) Fine scale microstructure Spherical powder Good packing and flow properties

32 HORIZONTAL GAS ATOMIZATION

33 VERTICAL GAS ATOMIZER

34 DETAILS OF DROPLET FORMATION

35 PARTICLE SHAPES FROM GAS ATOMIZATION

36 EFFECTS OF ATOMIZATION VARIABLES

37 EFFECTS OF ATOMIZATION VARIABLES Finer particles size has finer dendrite and precipitate size and gives better mechanical properties. p However, handling and compaction more difficult for finer sizes.

38 Water Atomization Water Atomization High pressure water jets are directed against the melt stream, forcing disintegration, and rapid solidification. Characteristics: Some oxidation Rough and irregular powder Rapid solidification No segregation Not suitable for reactive materials

39 Water Atomization

40 Water Atomization Particle size If d m in µm Then m=

41 CENTRIFUGAL ATOMIZATION