Direct Metal Printing
Design constrains for direct metal printing
INDEX:
- Manufacturing process explanation
- Process physics
- Supports
- Design tips
- Surface
- Build orientation
- Dimensional accuracy
- Do’s and don’t
Manufacturing process explanation
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With selective laser melting thin layers of atomized fine metal powder are evenly distributed using a coating mechanism onto a substrate plate, usually metal, that is fastened to an indexing table that moves in the vertical (Z) axis. This takes place inside a chamber containing a tightly controlled atmosphere of inert gas, either argon or nitrogen at oxygen levels below 500 parts per million. Once each layer has been distributed each 2D slice of the part geometry is fused by selectively applying the laser energy to the powder surface, by directing the focused laser beam using two high frequency scanning mirrors in the X and Y axes. The laser energy is intense enough to permit full melting (welding) of the particles to form solid metal. The process is repeated layer after layer until the part is complete.
Process physics
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Powder bed
- All particles have same composition
- No binders, no additives
- Powder = porous = isolator
High intensity laser focus
- Complete melting of the powder bed
- Powder > heating > melt pool > solidification > solid
- Solidification & cooling induces thermal stresses
Extremely rapid cool down
- Unique super fine microstructure
- Excellent mechanical properties
Supports
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Down Facings
- Steel, stainless steel, Inconel, …:
- Big surface a > 60 °
- Medium surface a > 50-55 °
- Small surface a > 45 °
- Titanium, aluminum:
- Big surface a > 50 °
- Medium surface a > 40-45 °
- Small surface a > 35 °
- Holes
- No support needed if Ø < 8 mm
Up Facings
- Stair stepping effect on up-facings
- Less prominent than down-facing
Freeform surfaces
- Better part quality
- Fewer supports
- Improved visual appearance
- Improved accuracy
Design tips
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Overhang > 60°
Holes
Down facings
Use material friendly
3D print design constraints
Surface
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Surface quality
- Basic Ra 4 – 7 µm
Definition of surface types
Middle surface
- α = angle between horizontal build platform and tangent line of the object surface
Up-facing surfaces
- Upfacing surfaces are characterized by the normal of the object pointing away from the build platform
Downfacing surfaces
- Downfacing surfaces are characterized by the normal of the object pointing towards the build platform
- Below a certain α-value downfacing areas can not be built without support.
Supports
- Below a certain α-value downfacing areas can not be built without support
Downfacing not requiring supports
Horizontal circular holes
- Support less Øin < 8mm
- Support needed Øin > 10mm
Horizontal bridges
- Support less L < 1,2mm
- Support needed L > 1,5mm
Horizontal downfacing flange
- Support less L < 0,75mm
- Support needed L > 0,75mm
Surface quality
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Staircase effect
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Roughness:
- Layer thickness
- Inclination
- Material
Build orientation
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CASE: how to build a square tube
Option 1
- Lots of support, α=0° surfaces
Option 2
- Little support needed, but poor connection to baseplate, difference in roughness for UF/DF
Option 3
- No support needed, overall best result with regards to accuracy, roughness
- Simple example à orientation is unambiguous
- Actual applications:
- Compromise in orientation
- Optimization of part geometry
Dimensional accuracy
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High melting temperatures
- Ti: 1650°C
- Stainless steel: 1200°C
- W: 3500°C
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Fast cooling rates (< 1ms à 100°C)
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Stresses accumulate throughout layers
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Deformation behavior = material specific