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LASER SINTERING?!?

September 3, 2016

 

As I introduce people to metal additive manufacturing, I have to disabuse them of the idea of laser sintering.  The metal additive process that most people are thinking of is melting, not sintering.  Sintering is the process of compacting and forming a solid mass of material by heat and/or pressure without melting it to the point of liquefaction.  But Powder Bed Fusion (PBF as ASTM calls it) involves full-on melting of the metal particles and substrate several layers down.  The picture above shows both sintered and welded parts.  Note that for sintering, the particles are simply touching real well and there are large between them throughout the body.  Contrast that against the melted image on the right.  The particles seen are partially melted balls on the exterior surface straight out of the machine that can be worn off through light abrasion.  Behind the spheres you can see that the metal is a solid mass with no visible voids.  In fact, you can’t even distinguish layers from the additive process because with every layer of melting, the laser re-melted a couple layers beneath the current exposure layer.

 

But people continue to call this process DMLS (Direct Metal Laser Sintering).  25 years ago it was indeed sintering, as the founder of eos invented it.  The laser power wasn’t very high so the particles merely sintered together and then you had to run the parts through a Hot Isostatic Press (HIP) secondary process to try and reduce the voids.  eos trademarked their process DMLS and since they are the oldest players in town, the name stuck.  Then about 20 years ago the founder of Concept Laser used a new laser and began fully melting the powder.  (Regretfully their tradename is LaserCUSING which isn’t quite as catchy).  Today, most metal 3D printing manufacturers use this same melting process although they claim slight differences among them.  Arcam is a little unique in that they use an electron beam instead of a laser but they are still melting the powder.  The one exception is ExOne which uses “binder jetting”.  This process truly relies on sintering through a secondary HIP process.

 

Who cares what they call it?  This technology would gain acceptance more quickly than it does if people didn’t continue to envision large voids in the parts.  People who are new to the process tend to learn the term “DMLS” first and from then on they associate it with sintering in their heads and they question how strong the parts are.  While there are microscopic voids here and there (depending on the parameters) it is nothing remotely close to the voids you get with sintering even after HIPing.  The image below shows Concept Laser’s Hot-Work Maraging Steel microstructure.  Note that it is practically a solid chunk of metal.  The strength is really close to the wrought properties of the material.  I tell people to back off their strength values by 10% to account for variability in manufacturer parameters but that’s about it (the exception is fatigue in the Z-direction which can vary greatly from supplier to supplier).

 

 

So what do I call it?  None of the terms out there really satisfy my taste.  I guess they don’t satisfy anyone else either because none of the terms have really stuck other than the traditional DMLS.  If I had my way I’d call it “Precision Micro Welding” but that’s not very catchy either.  Below is a table showing the trade names of the various manufacturers for your reference.

 

 

 

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