DMLS Wind Tunnel Models

Additive manufacturing, sometimes called direct digital fabrication or rapid prototyping, has been in the news quite a bit lately. I wrote a post recently for Dayton Diode about the many additive manufacturing options available for fabricating functional parts or tooling in response to comments on a piece in the Economist, and commented recently on Armed and Dangerous in a discussion about 3D printed handguns. There are just so many exciting processes and materials available for direct digital parts production today. Some of the work I've been doing recently to qualify one particular additive process for fabricating high-speed wind-tunnel models (abstract) was accepted for presentation at next year's Aerospace Sciences Meeting.

We used Direct Metal Laser Sintering (DMLS) to fabricate some proof-of-concept models in 17-4 stainless steel. DMLS is a trade name for the selective laser melting process developed by EOS. The neat thing about DMLS (and additive processes in general) is that complicated internal features like pressure tap lines can be printed in a single-piece model. Being able to reduce the parts count on a model to one while incorporating 20 or so instrumentation lines (limited only by the base area of our particular model) is really great, because one part is much faster and less expensive to design and fabricate than a multi-component model with complicated internal plumbing. The folks down at AEDC are also exploring the use of DMLS to fabricate tunnel force and moment balances for much the same reason we like it for models and others like it for injection mold tooling: intricate internal passages, in their case, for instrumentation cooling and wiring.

I got the idea for doing DMLS models one evening after watching Paul Breed talk about 3D printing rocket motors.

Doing the proof-of-concept model is great to exercise the benefits of the technology, but there's also a lot of supporting work required to verify the material properties through tensile, fatigue and shear specimen testing. Greg Morris of Morris Technologies describes that process in an interview about his company's new Arcam machine,

D2P: By “validate the technology,” do you mean that you’re working directly with the aerospace and medical companies to try to come up with new technologies and types of parts for them?

GM: Yes, we work directly with the OEMs. The validation process means that if an OEM comes to us to make their products, especially if it’s a critical application like an airplane engine or body part, they need to make sure that there isn’t going to be a failure. So a way to do that with a new technology is to understand all of the little levers and buttons needed to lock in the process. We really have to understand all of the variables in the process that could affect the quality of the product or parts.

This is one element; the other element is ‘what are the mechanical properties of the parts coming off of the machine?’ Usually, this is a pretty expensive endeavor to qualify a particular material for a particular technology. So, it’s not unheard of to say you’ll be over a million hours to qualify a particular material off of a particular technology, like EBM or DMLS.

The reason is because we have to make hundreds of test bars, which is expensive. And then you have to machine those, and then we have to pull those test bars and cycle them if we’re doing low-cycle or high-cycle fatigue testing. Therefore, it takes a lot of time and a lot of money to qualify the material and understand the properties of the material. This is the dual-edged sword of an exciting, new technology. The one side of the sword is it’s enabling and disruptive, and it changes the game; the other side of it is you have to have some patience and staying power in order to make it through all of the various toll gates that customers are going to have to go through.

Our strategy for qualifying this process/material for model fabrication had two independent components. Proof-of-concept model entries in blow-down facilities, and a parallel material properties characterization effort. Blow-down tunnel operators are less risk-averse than continuous-flow tunnel operators because they do not have expensive rotating components downstream of their test sections. If a model does break the consequences are less severe. So we could test models designed using only the supplier's material properties claims in blow-down tunnels prior to collecting our own verification data.

One of the goals of our qualification project was to use modern design of experiments to get the "hundreds of test bars" that Mr Morris mentions down to something we could execute more quickly and affordably than legacy approaches, or approaches intended to estimate statistics that are sensitive to the tails of the material property distributions. The factors of safety for tunnel models are usually 4 or higher (one of our tests had a factor of safety of 30 for a simplified analysis of the wing-root) as opposed to the factors of safety around 1.2 to 1.4 for flight applications which also rely on higher-fidelity analysis. For our application we're happy to measure the mean response reliably, and quantify any effect of build direction. This means fewer samples are required. We used the uncertainties that EOS publishes to do a prospective power analysis to help us select the sample size, and we found ourselves in the happy position of having smaller noise than expected in the final results (a good indication that the supplier has a well controlled production process). Per usual YMMV, or as they say in the additive manufacturing world, "it's geometry dependent."