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Before I explain this misconception, let me offer some background for those relatively new to metal additive manufacturing. The melting of metal powder by a laser needs to happen in an inert atmosphere (usually argon or nitrogen). This is because even small amounts of oxygen and carbon dioxide can impair the mechanical or chemical properties of the final part. It can lead to discoloration and even lower fatigue resistance. This is especially so for reactive materials like titanium and aluminium alloys which are very sensitive to oxygen. In fact, these alloys are also affected by nitrogen. So for them, you absolutely need to use argon.
I’ve heard people say stuff like “all printers print in an inert environment” and “a little oxygen is no big deal”. This is the misconception I would like to talk about in this part of my blog series.
The most common way to achieve an inert atmosphere in the build chamber of a metal 3D printer is to push out the air and by flushing in argon or nitrogen. This is done before the printing starts till the oxygen reaches a level good enough to achieve satisfactory printing results, usually 500 ppm (parts per million). But there is a problem. With each successive build the metal powder picks up oxygen as well as humidity. As a result, the material properties of the powder change and after a few build cycles the powder becomes unusable to an extent that it becomes unfit for additive manufacturing and you need to throw it away. In speaking to customers, I have noticed that manufacturers tend to hide this crucial fact from them. I can understand why. Telling a customer that they need to throw away 200 kgs of expensive titanium powder 10 times a year is not going to go down well and will completely wreck the running cost calculations of the printer.
But there is an even bigger problem what is often ignored and is something I want to highlight here. Remember the first article in this series where I spoke about certification and the four components that are involved – printer, material, parameters and post-processing? When there is powder degradation, one of the four components, the material, becomes a variable and your certified workflow goes out of the window. You can no longer build parts with high quality and repeatability. A part built using fresh powder will be quite different from the part built using powder reused a number of times. This is a very serious problem.
At 3D Systems, we recognized the problem or powder degradation a long time ago and took care of it by what is now famously known as the “vacuum concept”. After we close the door of our DMP Flex 350, we start a high-power vacuum pump and suck out all the air from the build chamber. Then we fill the chamber with argon or nitrogen depending on the material being used for printing. This completes the first vacuum cycle. This procedure is repeated three times. Officially we state that we achieve an oxygen content of 25 ppm. But I’ve seen DMP Flex 350’s where the oxygen sensor shows a value of 5 ppm. In fact, we even suck out the air trapped between the powder particles. The powder bed literally bubbles as if its boiling as the air trapped between the powder particles is sucked out. Here is a video which shows the process.
Above: DMP Flex 350 – “Boiling” effect of powder during the vacuum cycle/Video Credit: 3D Systems/YouTube
After the three vacuum cycles are completed, we add a little more argon into the chamber to increase the pressure above atmospheric pressure. This is so that in case there is a leak, the argon will flow from inside the chamber to the room outside. Air containing 20% oxygen won’t flow from the room into the inert chamber.
Due to this vacuum concept, printing happens in the presence of a minuscule amount of oxygen. 5 ppm is a hundred times lesser than 500 ppm. As a result, the powder picks up such a small amount of oxygen that it never degrades to a point that we need to throw it away. While this significantly brings down the running cost of the printer low, it also solves the problem of consistent repeatability.
Going back to the four components of certification – printer, material, parameters and post-processing, when you use a metal 3D printer which employs the vacuum concept to print in a highly inert environment, there is very little powder degradation. Material doesn’t become a variable in the equation. It remains a constant and your certification stays intact. You can be assured of high part quality and superior repeatability. The part you build with fresh powder will be almost exactly same as the part you build with powder that has been multiple times.
Having said so much about oxygen content in powder, I need to clarify one more thing. I’ve been talking about throwing powder away. I want to clarify that you will need to do this only if you are printing to a standard that demands a certain maximum level of oxygen content in the powder. This will be true for highly demanding applications like healthcare and aerospace. For example, when you are printing titanium implants using grade 23 powder, the oxygen absolutely cannot exceed 0.13%. But if you are printing parts to be used in prototyping or any other non-critical applications you can still continue to use degraded powder. Of course, the results you get will be different than if you used powder that has not degraded. But if that’s acceptable to you then you are good to go.
In fact, you can do three things with powder that has been degraded:
Throw it away and use new powder. This way you will be able keep to track of batches of powder as well as how many times the powder is used. This will be mandatory for applications like aerospace and healthcare, whether you like it or not.
Blend the degraded powder with new powder of the same batch until there is no powder left in the batch. Using this method, you will be able to keep track of batches of powder but not the number of times the powder is used.
Blend the degraded powder with new powder of any batch. Using this method, you won’t be able to keep track of batches of powder nor the number of times the powder was used.
Depending on your application, you will need to adopt the right strategy. But if you use a printer which maintains very low levels of oxygen content in the chamber and in the powder bed then you won’t need worry about throwing away powder or blending it in a way that you can continue using it.
This is a huge advantage which the healthcare industry recognized a long time ago and which the aerospace industry is now beginning to realize. I say this because our DMP Flex 350 and its predecessors using the vacuum concept have printed more than a million titanium implants so far. This means that are a million people walking the face of this planet with 3D printed implants that have been printed using the most stringent of specifications. Since we have never thrown away titanium powder, these million implants are a testament to the fact that our vacuum concept works and does an excellent job keeping the oxygen content under control.
(Note: The views expressed in this article are of the author alone and do not represent those of 3D Systems or Manufactur3D. Original blog can be viewed here.)