LEC 3

• expectation skyrocket

• inflated expectations disappoint

• gradual work in the field leads to realistic success

• Mechanical crushing:

  • Milling

• Atomization: (most used)

  • gas/ water jet

  • centrifugal

  • plasma

  • ultra sonic

• Electrolysis

• Chemical Reactions:

  • vaporizing + recritallization

  • chemical reduction

• Ball Mill:

  • Impact: collision of powder particles and grinding ball

  • Abrasion (Abrieb): via friction between particles

  • Shearing: energy input leads to decreasing size of particles

  • Compression: particles are compressed until they break

  • Size distribution: 5µm - 10mm

• Planetary Ball Mill

  • Grinding pots sit on a rotating disc and turn in the opposite direction, creating high centrifugal forces

  • Particle Transformation steps:

    1. Elastic Deformation

    2. Plastic Deformation

    3. Shear Deformation

    4. breaking/ chemical reaction

• most common method producing metal powders

• high velocity gas (air, N2, Ar, He) stream hits outflowing metal melt, breaking (atomizing) it into small droplets

• Size distribution: 1-500 µm

1. Melt is heated and poured into the furnace

2. Liquid Metal stream flows into the atomization tank

3. High-pressure water jets hits melt stream, breaking (atomizing) it into small dropplets

4. Particles are rapidly solidified by the water

higher pressure -> smaller particles

• Instead of high-speed gas, ultrasonic vibrations are used for the atomization process

• Wire feedstock is fed into a chamber

• melted and atomized by high-temperature plasma torches

• creating: highly spherical metal powders with excellent flowability

water jet:

• irregular particle shape

• for inert materials only (Materials that do not react chemically)

gas jet:

• mostly spherical powder particles

Ultrasonic:

• best spherical powder particles

• minimal Satellite formation

• attachment of tiny, fine powder droplets to the surface of larger, primary spherical particles

Milling:

• Production volume: high

• particle size: 5 µm - 10mm

• particle shape: irregular, shrap edge

• powder characteristic: impurities from grinding balls

Atomizing:

• production volume: high

• particle size: 1-500µm

• particle shape: spherical

• powder characteristic: high purity

Electrolysis:

• production volume: small

• particle size: <1µm

• particle shape: irregular/ dendritic

• powder characteristic: high purity

Chemical Reaction:

• production volume: small

• particle size: <1µm

• particle shape: irregular, clotted (verklumpt)

• Rotary Riffler

• All-Layer-Collector

smaller particles -> higher surface energy -> higher affinity to agglomerate

• morphology

• particle size distribution

• flowability (static, dynamic)

• powder density

• chemical composition

• measurement of particle dimensions

• dependent on method of measurement

• possible measurable size parameters:

  • measures for for sieving

  • equivalent spherical diameter

  • sphere with equivalent sedimentation (Sinkgeschwindigkeit) velocity

• measurement that shows what percentage of a powder is made up of different particle sizes (small, medium, or large)

  (cumulative values)

• apparent density:

  • Powder flows freely through a defined funnel (Trichter) until cup is filled

  • = powder mass/ cup volume

• tap density:

  • fill the powder into a cylinder

  • tapping apparatus tapps powder with a specified number of taps (until no more volume changes)

    =powder mass/ powder volume after tabbing

• apparent density < tapped density

  
  

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• direct influenced by: morphology & particle size distribution

• determine flowability:

  • Flow rate:

    • timing how fast powder drains through a gravity funnel

  • Powder Rheometer:

    • recording the energy required for a rotating blade to move through the powder bed

    • closer to real conditions

  • Dynamic Avalanching Behavior:

    • Uses a rotating drum and a camera to measure the Avalanche Angle (Lawine)

1. Extrusion molding:

   • Melt metal gets poured into a large block (ingot)

2. Forging:

   • rollers squeeze and hammer the block to turn a thick rod

3. Rolling:

   • decrease the rod diameter

4. Coil:

   • rolled material is wound into intermediate coils (Spulen)

5. Wire drawing:

   • Pulled through progressively smaller dies to achieve the target diameter

6. Annealing:

   • Heat treatment applied in between drawing passes to relieve strain hardening, restore ductility, and prevent breaking.

7. Final heat treatment

• wire is pulled through a series of drawing dies with progressively smaller hole diameters to shrink its cross-section

  (Querschnitt)

• D=9,5mm->D=1,6mm in 15-20 steps

Wire types:

• solid wire

  • uniform, solid cross-section made entirely of one alloy

• cored wire

  • hollow metal outer sheath filled with powdered metals or flux in the core

Welding wire storage:

• wire roll

• wire coil

• cost:

  • wire/ filament: cheaper

• Materials:

  • wire filament: several, but min. tensile strength necessary due to wire feeding

• Material Usage:

  • wire filament: >99,5%

  • powder: >80%

    
    

• Porosity:

  • small holes/ gas pockets left in final part

• Balling

  • Surface tension forces melt to break into discrete spheres rather than forming a smooth, continuous melt track

  • insufficient Volumetric Energy Density E=P/(vhd)

  • P = Laserpower

  • v= scan velocity

  • h= hatch distance

  • d=layer thickness

• Delamination:

  • high internal residual stresses from rapid heating and cooling cause printer layers to seperate or lift from building platform

• Cracks

  • high internal residual stresses from rapid heating and cooling cause the solidifying material to fracture

• Lack of fusion porosity

  • low enegery density

• Keyhole Porosity:

  • high energy density

• Gas Porosity:

  • caused hydrogen (moisture on surface of powder, dissolved in melting)