Arguments for an AM Alternative: What’s Old Is New Again

In 2018, aerospace giant Lockheed Martin Corp. made news with the announcement that it had completed a 46" (1.16 m) diameter, high-pressure fuel tank comprised of two 3D-printed, dome-shaped end caps welded to a traditionally manufactured tubular vessel, all made of titanium. The decision to print the domes reportedly cut delivery time from two years to just three months.

Lockheed is far from alone. Launcher Inc. routinely prints tanks, combustion chambers and other rocket componentry, some from a proprietary copper, chromium and zirconium alloy. SpaceX 3D prints parts from similarly challenging materials for its Raptor engine, while NASA, Rocket Lab, Orbex and Ursa Major use metal additive manufacturing (AM) to produce everything from nose cones to engine nozzles. This March, Relativity Space one-upped all of them by sending the first entirely 3D-printed rocket into space.

All cite lower manufacturing costs, decreased part count and the ability to rapidly iterate as the primary drivers for their decision to embrace AM. The technology didn’t exist when the United States launched its first space shuttle, and it was just becoming mainstream on the last mission.

It seems that the aerospace industry (among others) is going gaga over metal AM. And for good reason. With minimal tooling investment and virtually unlimited design freedom, this relative newcomer to the manufacturing industry is proving to be a formidable and flexible alternative to traditional processes such as machining, casting and metal-injection molding.

But it’s not the only manufacturing game in town. Far from it. In fact, I’ll argue that if Lockheed Martin had used a different, far more mature process, the components would have boasted better metallurgical properties than those obtained with 3D printing, and the final assembly would have needed one less welded joint. While there would have been some tooling expense and a lengthier lead time for process development with this alternative approach, both would be minimal.

What is this miracle process? Some might know it as “spinning,” a metalworking technology employed thousands of years ago by the ancient Egyptians. In those times, a metal disc was placed over a rotating wooden mandrel and pressure was applied using a “paddle” or similarly blunt tool, gradually forcing the workpiece to take the mandrel’s shape. The result, at least in those bygone days, was strong, consistently formed bowls, vases and ornamental objects.

That basic process is still in use, although, like everything else in manufacturing, advanced spinning is now performed on computer-controlled equipment. And while mandrels are still used in many cases, they’re now made of steel machined on a CNC lathe. It’s also possible to eliminate the mandrel by instead tracing the inner and outer surfaces of even very complex geometries with a series of hardened steel rollers—not unlike using your fingers and a potter’s wheel to form a lump of clay into a beautiful vase.

When coupled with precise heat application, electronic force monitoring and advanced CAM software, the more accurate term for what is now a very mature, highly predictable metalworking technology is simply this: flow-forming, aka spinning or spin-forming.

Many flavors exist. Cone flow-forming, shear-forming, shear-spinning, tube flow-forming, forge flow-forming—these are some of the terms used to differentiate the techniques used to “flow” a sheet metal disc, flame cut plate or forged blank into a cylindrical, hollow, typically thin-walled object ranging in size from an inch or so in diameter up to several yards in each direction, one with high dimensional accuracy, excellent surface quality and robust mechanical properties.

There are many techniques and many names, but for now we’ll lump them all under the umbrella term “advanced metal forming.” It is a chip-less and fast process used to produce all manner of tubes, cones, liners, cylinders and much more—many of them closed on one end—from a variety of materials, each boasting the attributes described, and delivered quickly and cost-effectively.

What are those materials? The ancient Egyptians stuck with copper for its great malleability, and while that remains a common workpiece material today, it has since been joined by aluminum, 300-series and PH stainless steels, nickel-based superalloys, maraging steels and even hardenable metals like tool steels. The titanium fuel tanks mentioned at the outset are very flow-formable, as are the various rocket components made of exotic copper-chromium alloys currently sitting on a launchpad somewhere.

Metal AM is an excellent choice for many space-bound components. Yes, the powders and wire feedstocks are relatively expensive, but this unfortunate detail is offset by the fact that there’s little waste with any printing process. Machining is typically required, limiting some of AM’s “unlimited” design freedom, although far less than if parts were made of billet or cast material. And by eliminating most of the fixtures, cutting tools and multiple operations, metal AM is both faster and more cost-effective than traditional manufacturing processes.

Which brings us back to the “forged blank” comment. The metallurgical properties of any part made via laser-powder-bed fusion or DED are similar to that of a welded or investment cast part. AM is also subject to voids, warpage and layering effects, which is why costly CT scanning is typically required for certification, especially on flight-critical components.

This is why aerospace manufacturers should take a close look at advanced metal forming. Compared to cast and welded parts, it produces superior grain alignment and density, especially when the process begins with a workpiece blank that has been rotary forged. Without getting into the details here, we'll say that the resulting parts are stronger and more durable than the alternatives—3D-printed and otherwise—and are ideal for domes, nosecones, fuel tanks, combustion chambers and similar axially symmetric rocket componentry.

Maybe it’s time to revisit a well-known friend?

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