Video: For 3D Printed Aircraft Structure, Machining Aids Fatigue Strength

Big Metal Additive used its DED capabilities to produce experimental, topology-optimized aircraft structural components for the U.S. Air Force. The aluminum parts were designed to reduce both mass and the number of components needed relative to a more typical aircraft structure, but the result was a hollow form practical to produce only through additive manufacturing on a relatively large-travel deposition-style machine. The machine that produced the components was a hybrid, and testing revealed an important advantage of incorporating machining into the process. Compared to the as-printed versions, the parts that were machined inside and out as the forms were being built had thinner walls due to the machining, but nevertheless performed dramatically better in fatigue life. I learned about this during a visit to the company’s Denver, Colorado, facility.

Mentioned in the video: A projectile housing for the U.S. Navy is an application of DED to meet leadtime challenges
How scaling production capacity required Big Metal Additive to not only develop its DED capability, but also establish a quality system

Transcript

The Air Force is exploring lightweight, low part count aircraft structures designed to be made through additive manufacturing.

I’m at Big Metal Additive and their facility in Denver, Colorado. They made these sections, aluminum components, structural sections designed by the Air Force through topology optimization.

Big Metal Additive does additive manufacturing through wire arc directed energy deposition on a robot, in some cases on gantry machine tools, in other cases.

Five axis machine tools that do metal deposition and machining as parts like these are being built, and the machining proved significant here. One thing that Big Metal Additive discovered, these structural forms actually perform their structural duty better if material is taken away.

The as 3D printed form is beefier. The wall is about a quarter inch thick as printed. So heavier wall, but that printed form allows for stress concentration and that lowers fatigue life. By contrast, this form, smoother as you can see, while it was being 3D printed it was also five axis milled, not just on the outside, but also on the inside as it was being built. The result? The wall thickness is much thinner. It’s about a 10th of an inch versus a quarter of an inch here. But these smooth walls mean no stress concentration, and much longer fatigue life, significantly longer.

Under high frequency fatigue testing the structure that was only 3D printed, not machined, failed after 30,000 cycles, but the even lighter weight structure that was 3D printed and machined, that lasted multiple orders of magnitude longer.

One failed at 30,000 cycles, the machined one failed at 2 million cycles.