Although Additive Manufacturing has been widely known and used in the industry for many years now, the long-awaited breakthrough still remains elusive.
Why? Well, there are a few reasons for that. In this article we will touch on one aspect: post-processing. Let’s consider AM metal printing with LPBF (Laser Powder Bed Fusion) type of manufacturing method. Let’s further consider a geometry with inner channels < 10 mm diameter.
1st Challenge: Support structures:
- Even with geometries without inner channels it can become difficult, especially with mass production in mind. Many AM printing services are still removing them manually. Yet, there are some solutions that can accelerate the process (of removing support structures from the outer geometry) by a leap.
- But what to do with inner channel support structures for small diameters? While claims can be found that promise a technical solution, it remains one big issue, even more so if mass production is the focus. From the technical point of view: Electro-chemical processes allow for such removal (in theory), if one can precisely adjust its parameters to “attack” the support structures only (usually printed less dense than the main structure). But this is very fragile: even if the support structures are removed completely, you might and up with some nasty craters beneath your support structure attachment points. This is due to the fact, that the attachment points of support structures to the main geometry are denser than the rest of the support structures or even somewhat denser than the main material.
2nd Challenge: Surface Roughness
- Let’s say you have managed to print your metal part without support structures in those inner channels (which should be very possible especially with those diameters). How do you improve the surface roughness of your inner channels if this is required?
- The majority of surface roughness improvement methods for additively manufactured metal parts are designed for the outer surfaces in mind: electro-chemical, chemical, chemical+mechanical and so on. Of course, results are depending on many parameters, e.g. the material used for the parts to say the least. Other post-processing methods which are actually designed to improve inner channels’ surface roughness do not preserve surface structure/pattern very well and lack homogeneity over 3D inner channel geometries.
- One important indicator for any surface roughness improvement method is how much material for a given change in surface roughness Ra (Ra is just one possible way to describe surface roughness) is removed. Why is this important? A) Of course, you want to remove as little material as possible. Considering tens, hundreds or even thousands (hopefully one day) of parts, one would need to use more material for the printing (deviation from nominal geometry). B) As critical as the efficiency of material usage is the fact that the more material is removed the higher the likelihood of deteriorating the Ra value (after initial improvement). This can be seen in the schematic drawing below. The cause for this behavior are imperfections in the print, which will then be exposed by the material removal. So, in conclusion it becomes clear that the lower the gradient MaterialRemoved/deltaRa the better.
By the way: Speaking of surface roughness values such as Ra. What actually is the “as-sintered surface roughness” of a metal powder based print? Is it the same all over the print? For example, particle size distribution (or the “span” as the width of the distribution = (D90 – D10) / D50) of the metal powder in use or the layer thickness or even the direction of printing plays an important role in distribution of values such as density/porosity in the print and also on the surface pattern distribution. The usage of Ra values to describe surface roughness is widely spread in the industry, but how well suited is it especially when considering the as-printed surface of a metal powder based print? Looking at the pictures below, one can see that the surface imperfections of a LPBF and EBM metal print are looking much different than let’s say a classically machined metal part, where the tracks of the tool leave a pattern that is very much in line with the idea of the Ra (or Rz) value definition.
3rd Challenge: Speed
- The challenge that we find in Additive Manufacturing is equivalently found in post-processing parts printed by AM: speed.
- It goes without saying, that this might be one of the biggest drawbacks for industrialising additively manufactured metal parts in a broad manner
4th Challenge: Material
- And what about the material of the metal part? It is at least as important as the other aspects mentioned. Why? Because…
- The compatibility of the post-processing method with the material will dictate the outcome of the treatment. This is especially the case with methods where chemicals are involved.
- This means the composition of the metal is key, thus materials cannot be chosen solely and freely by considerations of e.g. thermo-mechanical characteristics with the application in mind
- The heat treatment can have a big impact on the results, too. This is a whole chapter on its own!
The above clearly shows that using Additive Manufacturing in production is a system of interdependent processes and has to be approached holistically. Thus, if post-processing is wanted, it actually determines the material used as well as the manufacturing process itself.
As Nuvaya Technologies, we are not only providing supply of AM metal parts. With our holistic AM service we are also supporting our customers in finding the most suitable methods for post-processing AM metal parts according to their requirements.
At the moment we are in the kick-off phase of a very interesting investigation project on tool steel applications in the context of additive manufacturing and especially post-processing of these special steels printed with LPBF with one of our customers. Remember that compatibility of the material with the post-processing chemicals is key? Here we go into detailed investigation of steel-type-post-processing matching as well as the heat treatment and its eventual effect on the tool steel. So stay tuned!…
If you have similar questions and challenges, contact us! Nuvaya Hendese Technologies GmbH
P.S.: A nice introductioin and overview to post-processing AM metal parts can be found in this video by Desktop Metal.
