Will 3D Metal Printing Replace CNC Machining Rapid Prototyping Soon?

I have worked closely with SLM (Selective Laser Melting; the most common method for 3D printing of metals) technology for almost 4 years and my short answer is: No, not in the near future, there are still some fundamental problems to overcome in terms of the 3D printing of metals. These problems are generally not reported in the media.

To expand up upon that answer I’ll briefly list what are (in my own opinion) the biggest obstacles for SLM in terms of directly competing with CNC:
– Surface finish: This is a big one, and one of the things I get asked most about. The parts produced by SLM are very impressive, however in terms of surface finish for a part which may have to connect accurately or form part of a precise mechanical system it is simply not good enough. The surface of SLM formed material is inherently rough due to the attachment of partially melted metal particles, and the layer-by-layer way in which it is formed.

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– Support Structures: All overhanging surfaces must be attached to the build plate by support structures which (in general terms) hold the overhanging sections in place during building and provide a heat transfer route to avoid overmelting of the metal power. These structures must be removed and although they are generally designed to be broken off easily they leave a notably poorer underside surface finish to the SLM processed part where they have been attached.
 
– Deformation due to residual stress: This is dependant on the material, but in general SLM manufactured components are prone to high levels of residual stress (caused by the very localised heating and cooling inherant with the process) which can cause them to deform significantly upon removal from the build plate. This can be mitigated somewhat with heat treatments, but still poses a major problem.

– Inconsistency of material structure within the part: The thermal history of the SLM processed material varies hugely depending on the geometry being built. For example a thin walled section may cool much slower than a thick section. This has a huge impact on the microstructure and therefore the mechanical properties of the material. Research is being carried out on these effects, however they remain one of the great unknowns of the process.

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– Cost, build time, metal powder: SLM machines cost a round the £500,000 mark (depending on manufacturer etc. I’m not a salesman so i don’t have precise figures). This is an expensive machine considering it really doesn’t build that quickly. Deposition rates are tricky to generalise, as depend on the material and the geometry, however it is not uncommon for parts to be building for several days and well over a week for larger pieces. Additionally although the metal powder feedstock can be re-seived and reused it is not cheap to produce in the first place and although SLM is often pitched as a “green” technology, the energy required to atomise the power and the waste from this process are rarely taken into consideration.
 
I would however like to conclude by maybe adding that although I don’t think SLM will replace CNC in the near future, I would like to see it become a complementary technique, a further tool for engineers and designers to call upon or the right application where CNC is inappropriate.

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