AMS Spring 2023

DED Metal 3D Printing Study Reduces Errors in Functionally Graded Materials

6K SmarTech

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FGMs are used to make parts properties that change throughout their geometries by mixing different materials, changing how they crystallize or how they are arranged. This means that, within a single printed object, you could create an iPhone case which is rigid, soft, hard, and flexible at different points. We’re not really sure right now what FGMs can do for us or how widely they will be used in 3D printing going forward. However, we can be sure that this will lead to new applications, if all the kinks are ironed out.
One of these kinks is the lack of process control generally and errors in components. The Korean team has now looked into controlling the mixture of different materials at different ratios to see where errors occur. What’s really cool is that they did this for directed energy deposition (DED) which has seen a lot less attention than powder bed fusion (PBF) in research circles. In this case, they looked at the interface layers between different materials, 316 (L) stainless steel and Inconel 718. Between layers, cracks and defects can appear and, ideally, the right mix of materials relative to one another can reduce this.
Inconel 718 has excellent properties, but it is expensive. By mixing it with STS 316L to create a high-performance FGM, we have not only improved its technical and commercial advantages, but its economic feasibility as well,” Professor Do-Sik Shim stated.
The team used an Optomec LENS system for its experiment, building a series of parts and playing with the gradients, as well as laser power and feed rate. They created components whereby the steel was printed on the Inconel, with graded variants mixed in at 10% and 25%. In the sample without a gradient, cracks formed between the two materials. In contrast, the graded samples “had cracks only in specific regions due to ‘columnar-to-equitaxial transition’ (a transition in the microstructure of the FGM), precipitation, or the inclusion of titanium, aluminium or chromium impurities.” Meanwhile, the 25% graded sample “showed the highest tensile strength and elongation.”
“These findings will lead to improvements in the field, such as reduced costs, extended component lifespans in equipment, and enhanced functionality,” Professor Shim opined.
Now, the team will look at geometries and how they affect their results. DED is always seen as a very rough technology. I half jokingly call it metal Cheez Whiz. Lack of control over temperatures and airflows, as well as rough layers, mean the process often needs a lot of machining and cannot build the most complex parts. However, PBF is limited by chamber size, making large, meter-long parts very expensive. Meanwhile, large binder jet parts warp or collapse under their own weight in their green state. DED is really our best bet for creating very large structural parts of several meters or more. The kind of work performed by the Korean team could advance DED to the point where it becomes a more viable production technology.

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