Direct Metal Printing

Design constrains for direct metal printing

INDEX:


Manufacturing process explanation

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

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

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

 

Overhang > 60°

                                    

Holes

 

 

 

Down facings

 

 

 

Use material friendly

 

 

 

 

 

 

 

 

3D print design constraints

 

 


Surface

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

  • Staircase effect

 

 

  • Roughness:

    • Layer thickness
    • Inclination
    • Material

 

 

 

 

 


Build orientation

 

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

  • High melting temperatures

    • Ti: 1650°C
    • Stainless steel: 1200°C
    • W: 3500°C
  • Fast cooling rates (< 1ms à 100°C)

  • Stresses accumulate throughout layers

  • Deformation behavior = material specific

 

 

 

 

 

 


Do’s and don’t


Information provided by:
www.3dsystems.com
Companies applying this technology