Excercises

• Sag Effect:

  • Horizontal features sag due to gravity

    -> Avoid long horizontal part sections

    -> use self supporting geometrie,

• Drag Effect:

  • deformation from friction between part and support plate

  -> minimize contact with the support plate

  -> avoid large flat bottom surface

  -> thicker & shorter walls

• Material Preparation of the powder

• Printing (Poweder Coating, Binder printing, heating)

• Curing (Drying, better greenpart strenght)

Steps done only once

• Debinding (burn out mainbinder)

• Sintering (Densification, Shrinkage, material particles joined)

• Curing

• Depowdering

• Debinding

• Sintering

• Machining to achieve required dimension

Satuation = Fraction of binder in available volume of voids between powders

Scaling factor:

• calculate the shrinkage factor

shrinkageV= density final part/ density cured part

shrinkageL=(shrinkageV)^(1/3)

• dimensionate part length with this factor

Shrinkage prediction models

use uniform wall thickness

include post processing allowance

• add more material then needed

• remove material in an addtional machining step

• avoid long horizontal part sections (Sag Effect)

• Use Self supporting Geometries

• Avoid thin walls -> prone to fail during depowdering

• uniform cross-sections/ wall thickness for homogenious shrinkage

• minimize contact area to baseplate (Drag Effect)

Defintion:

• How well a Laser can be focused

Effects:

• growth of grains in direction of temperature gradient (z-direction)

• growth over multiple layers

Reason:

• high cooling rates (1-40*10^6 K/s

• temperature gradient (1-50 K/micrometer)

Directional dependence of a property

• minimizing support structures & residual material

  • support structures -> rough contact surface

• Apply Powder Layer (Coating)

• Exposure with laser beam

• Lowering the build platform and dose new powder

• density of the powder is lower then of the bulk material

• powder layer height is not the real layer height

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

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

  • good absorption for metallic materials

• mainly given by Scanning speed and Laserpower are defects which limitate the process window:

  • occuring errors are:

    • to high scan speed & to low laser power

      • Lack of fusion:

      • Balling:

      • no metling

      • sintering

    • to high laser power combined with low scan speed:

      • keyhole formation

      • Gas pores

    
    

• conduction mode I~10^5W/cm^2:

  • mostly gets absorped at surface

• keyhole mode I>10^6W/cm^2

  • deep penetration of the material

  • meltpool is deep (multiple layers)

  • high dynamic meltpool

• fine grains due to high cooling rates

• elongated grains in z-direction, because groth is in direction of temperature gradient

Metallographic Test Method:

• process of preparing a material sample so that its microstructure can be observed under a microscope

• Cutting:

  • cut material specimen in representative area

• Embedding:

  • embedding of specimen in resin (Harz) to provide

    • stable orientation

    • standardized size

    for automatic handling

• Grinding & Polishing:

  • provide of smooth surface to observe the structure

• Etching (Ätzen):

  • reveals microstructure by using difference between crystals (orientation, compositon, chemical resistance)

PBF-LB/M:

• Track, hatch and layer pattern clearly visible in the microstructure

BJM:

• particles and their bounds clearly visible in the microstructure

MEX:

• particles anf their bounds clearly visible in the microstructure

DED:

• drastically varying microstructure between different DED-Processes

• Prepared specimens are photographed using a light microscope.

• Images are getting binarized:

  Each pixel is assigned to either material or pore

• Ratio of material pixel and all pixel = density

• Ratio of pore pixel and all pixel = porosity

Advantage: Information about size, shape and pore distribution

Limitation: Other defects can be mistaken for pores

1. Part is weighted in air

2. Part is weighted in liquid

3. Calculated part density

Advantage: Simple, entire volume range of the sample

Limitation: No information about type, distribution and shape of strutural porosities

PBF-LB/M:

• with suitable process parameters high densities >99% can be achieved

DED:

• with suitable process parameters high densities ~99% can be achieved

MEX:

• Debinding step leads to highly porous part (density 50-60%)

• Sintering decreases porosity significant ~97%

BJT:

• Debinding step leads to highly porous part (density> 50-60%)

• Sintering/ Infiltration decreases porosity significant ~96%

  • portion of pores remains

• comparabiltiy only given with same measurement technique

Methods are:

• optical areal surface measurement

  • Confocal Laser Scanning Microscopy

  • Focus Variation

  • Fringe Projection

• Pofilometer

• Profilometer

  + well established, standardized

  - surface can be changed by the touch of the probe tip

• Optical Areal surface measurement:

  +contacless

  - optical disturbance of the environment

• specimen out of massiv cylinder d=10mm, length=62mm

• spcimen is loaded in tension until fracture to determine mechanical properties

• tests need to be performed under realistic conditions to their future deployment for valuable data

• PBF-LB/M:

  • highest dimensional accuracy due to scanner system, no sintering

• Binder Jetting:

  • largest tolerance influence from sintering shrinkage

• Material Extrusion:

  • Tolerances influeced by Nozzle diameter and sintering

• Directed Energy Deposition:

  • Tolerance mainly limited by robot/ positioning accuracy & large meltpool, no sintering

• fine grain size due to high cooling rate

• elongated grains in z-direction, grown over multiple layers, cause of growing in temperature gradient direction

• heat treatment enlarges grain structure and delivers adapted mechanical behaviour

• fine grains lead to Brittle fractures (Sprödbruch)

• heat treatment leads to more ductility

• Post heat treatment

• decreasing surface roughness (machining)

• optimized build direction

• better ductility

• higher elongation (Bruchdehnung)

• no longe brittle fractures

PBF:

• product with smallest form, dimensional, positional tolerance

  • Scanner system provides high accuracy

  • Small spot size & layer height produces minimal stairstepping effect

  • no sintering (shrinkage)

• Process Parameters

• Variables of the feed material