Inert versus Open Atmosphere for Laser Metal Deposition
Optomec's Jim Cann explains why and when to choose one over the other.
We know that laser metal deposition is dangerous if exposed to oxygen. And we know that there are two ways to prevent exposure to oxygen: additive manufacturing in an inert atmosphere, or additive manufacturing using a shielding gas. But what do these strategies do, and why would you choose one over the other?
Optomec's Jim Cann has some answers for these questions (delivered in a presentation at the 2016 Additive Manufacturing Conference). Cann is the sales manager for Optomec's laser engineered net shape (LENS) technology. The company offers this technology both as stand-alone, inert atmosphere systems and in its LENS print engine, which uses shielding gas.
In an inert atmosphere, the machine is completely sealed and filled with argon to keep oxygen levels below 5 or 10 ppm. Parts have to pass through an airlock to move in or out of the system without greatly affecting the atmosphere; if the door is opened, the operator will have to wait for the atmosphere to reach safe levels again before running the machine, which can add up to a matter of hours.
An open atmosphere system avoids these difficulties by pumping a shielding gas—typically argon—around the laser deposition head to protect the meltpool as parts are made. This is a more flexible method that can be used in a greater number of situations—on retrofitted hybrid machines, on robot arms, for large parts and any other situations where a completely closed system is not possible or practical. However, the shield gas only works when the nozzle is over the meltpool; when it moves away, the material remains at a high temperature and has a tendency to oxidize.
To determine whether to use an inert atmosphere, Cann says manufacturers should look first at the application, especially the material. Alloys such as aluminum, super alloys and magnesium are particularly prone to oxidation and do best when additively manufactured in an inert atmosphere. For high-value, low volumes of parts made from these materials, the extra expense and time of an inert atmosphere is likely worth it. For other components made of materials such as stainless steel, nickel- and cobalt-based alloys, bronze/copper alloys, and tungsten carbide matrix, an open atmosphere system with shielding gas may be suitable.
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