LEC 6

• Definition:

  • AM process in which thermal energy (caused by radiation) selectively and layerwise fuses regions of powder bed

• Process:

  • Apply powder layer (recoater)

  • exposure with layser/ electron beam

  • Lowering building platform & recoat new powder

• Subcategories:

  • PBF-EB/M: Electron Beam Melting

  • PBF-LB/M: Laser Beam Melting

+high dimensional accuracy/ resolution

+complex geometries possible

+wide range of material possible

-high cost

-harmful powder

-post processing is needed (support structure removal, geat treatment)

• Material: Metalls

• feedstock: Powder

• state of fusion: Melted State

• AM principle: selective fusion of material in powder bed

• material distribution mechanism: Powder Bed (Recoater)

• Source of energy: Laser Beam, Electron Beam

• CAD-Data & Material Requirements (Density, Surface Quality, residual Stresses)

• PBF-Process:

  • Energy & Material & Information combined

  • Beam source:

    • Melt Pool generation

    • Heat dissipation/ Microstructure formation

  • Powder Layer Generation

• Post-Processing

  -> Final PBF Part

1. External energy (light or electricity) pumps ground-state electrons up to a higher energy level

2. Incoming photon hits an excited electron

   • Electron drops back to lower Energy level

   • Releases a second identical photon

3. Photons bounce back and forth between to mirrors creating a chain reaction, that increases identical photons

4. Once photon intensity is high enough, photons can pass the partial mirror, building a high intense Laser

• Pump source: supply of ecternal energy to the system

• Gain medium: atoms/ material that stores the energy and emittatd photons once hitted

• Optical Resonater: high reflective and partial mirror

• Beam source: typically fiber laser

  • Gain medium: doped fiber (rare earth ions Nd, Yb)

  • good beam qualities at high power (up to 1kW)

  • Wavelength near infrared (λ=1064nm-1080nm)

• Beam Defelchtion: Galvanometer Scanners

  • highly dynmaic electro optical component

  • fast rotable mirrors

  • high precise & repeatable position of laser beam

• Cushion Distortion:

  • For deflection of the Laser two mirrors are used, which are not sharing a rotation axis, causing image distortion across the flat build

  • Soltution: software using correction table

• Rotating the mirrors increases the traveling distance to the flat powder with fixed distance

  • Solution:

    • Prefocused System

    • Flat Field Objectives

• Powder tank:

  • Powder conveyed from above and distributed from (re)coater after

• Dosing Platform:

  • Powder conveyed from bottom to the top and distributed from (re)coater after

• established:

  • blade knife

  • Brush

  • (upper) Lip

• conduction welding (mainly surface absorbtion)

• keyhole welding (deep depenetration of the material)

• additive Manufacturing (sweetspot between keyhole welding and conduction welding)

• changes absorption/ coupling of process radiation

• for stable process:

  • unifrom

  • reproducible

  • complete

  powder layers

• Prevents oxidation during melting/solidification.

• Removes spatter and smoke from the melt pool. (Flows in opposite printing direction)

• Reduces contamination and porosity.

• Incorrect gas flow conditions can cause melt pool instability

• Ar-Shielding

• He-Shielding:

  • reduction of spatters

  • increase of vapour blume speed

• 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

• Rayleigh-Plateau limit: cylindric shape collapses into small droplets (l=2pi*r)

• decrease scan speed

• decrease hatch distance

• increase laser power

  
  

• Lack of fusion:

  • non circular/ irregular shaped cavities, filled with unmolten material

• keyhole formation:

  • high laser power & low scanning speed form vapor capillaries

  • deep penetration welding

  • overheating ang gas pores as consequence

• Gas pores:

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

  • gas bubbles in melt pool , caused by overheating, distributed by melt pool dynamics

• 
  

• Maragoni Effect:

  • heat gradient causes heat flow leading to a gradient of surface tension

  • Liquid metal flows along the surface interface from regions of low surface tension (hot center) to high surface tension (colder)

• Recoil pressure:

  • caused by vaporization at the surface of the melt pool

  • escaping metal vapor pushes back onto the liquid melt pool, creating a downward pressure

  • leads to keyhole generation

• scan vector length should be short (less heat flows in workpiece)

• Scan strategies within one Layer:

  • Area exposure

  • Stripes exposure

  • Chessboard exposure

• Scan strategies within the build:

  • alternating exposure

  • rotating exposure

• Spiral Chessboard exposure

  • reduction of thermal distortion

  • increase in density

  • homogenization

• Geometry and dimension:

  • Geometric inancuraccy

  • dimensional inacuraccy:

    • depends feature/ element size/ process parameters

    • highst accuracy of AM Techniques

• Surface Quality:

  • Surface Roughness:

    • due use of powder & layerwise build

    • downskin surfaces

  • Balling

• Microstructure:

  • Anisotropy

  • Porosity

• Mechanical Properties:

  • Cracks:

    • resulting from residual stress

    • causes component failure under load & process instability

  • Delamination:

    • loose layers caused by residual stresses or lack of fusion

• grain growth:

  • atlong the temeprature gradient (z-direction), causing anisotropical mechanical behaviour

• grain size:

  • high heating/ cooling rate leadts to fine grains

  • high strengths

• Measurement to reduce anisotropy:

  • HIP-Heattreatment: leads in larger grains, reduces anisotropy, but reduces hardness/ strength but improves ductility

  • 
    

Definition

• directional dependency of part properties

• depends on orientation during build process

• caused from Microstrutural grain growth atlong the temperature gradient (z-direcion)

Effect:

• In AM best highest mechnaical properties are in x-y-direction

• Support overhanging structures.

• Increase heat dissipation from the part to the build plate.

• Reduce residual stresses, distortion, and overheating.

• lowering temperature gradients to reduce

  • residual stresses

  • crack formation

• preheating temperature: 80-1000°C

• energy source: LB vs EB

• process atmosphere: Ar/He vs Vacuum

• preheating of the powder bed:

  • EB needs higher temperatures

• feedstock material: powder differs