LEC 3 - Questions

• process to create powder

• During the process:

  • Impact: Collision of grinding balls and powder particles

  • Abrasion (Abrieb) via friction between particles

  • Shearing: Energy Input leads to decreasing size of particles

  • Compression: particles are compressed until they break

• Size distribution: 5 µm - 10mm

• Expected particle shape:

  • irregular, sharp edge particles

• Risk:

  • possibility of impurities through grinding balls

Reasons for Atomization:

• spherical particles (except for water jet atomization)

  • increases flowability & packing density

• Controlled particle size distribution of 1 – 500 µm

• high purity

• high production rate

• particle size distribution:

  • small particles diameters, have higher surface energy, leading to a higher affinity to agglomerate

  • (agglomerate = fine powder particles stick together to form larger particles)

  • influeces powder density, flowability

• apparent denisty:

  • layer homogeneity during recoating

• moisture content on the surface of powder, gets dissolved in the melting, leads to gas porosity

  
  

• Costs: it is cheaper then powder

• Material Usage: >99,5%

  (Powder only has: >80%)

• Safer Handling (Eliminates health risks associated with inhaling fine metal powders)

• Balling:

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

  • caused by high scan speed

• Cracks:

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

• Delamination:

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

• Porosity:

  • small holes/ gas pockets left in final part

    • keyhole porosity: caused by high energy density

    • lack of fusion porosity: caused by low energy density

    • gas porosity:

      • caused by hydrogen

      • moisture on powder particles, dissolved in melting