Friction Stir Additive Manufacturing Used for Functionally Graded Components

Metal AM Markets
AMR Military

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Over the past years we’ve seen a number of companies such as Sciaky, Optomec and Trumpf take traditional welding processes and turn them into 3D printing processes. Aeroprobe and the Edison Welding Institute are now working on doing the same for Friction Stir Welding with Friction Stir Additive Manufacturing, also called Additive Friction Stir. New research has shown that using this process, one could make functionally gradient metal components. These could be used to make new types of materials and have applications in defense, part repair and aviation.

What is Friction Stir Additive Manufacturing?

A diagram of Friction Stir Welding

Friction Stir Welding is a solid state welding process. In Friction Stir Welding a spinning toolhead with a pin is pushed down onto a material and creates friction while it passes over the material. The heat from the friction, downward force and spinning toolhead creates pressure which welds the material together. This material is concurrently stirred by the pin which mixes the material. In Friction Stir Additive Manufacturing, stacks of weld plates are lap welded and then Friction Stir Welded to form an object layer by layer. This process is the one that the Edison Welding Institute uses. Aeroprobe’s process uses a friction stir welding head which also deposits powder from the center of the head. Alternatively one could use a metal wire.

The Friction Stir Additive Manufacturing Process.

Friction Stir Additive Manufacturing (FSAM?) is a relatively new process for most of the market. The applications for it are in aerospace and defense. Along with Powder Bed Fusion, Electron Beam Welding, Directed Energy Deposition and other technologies, this could make structural parts for aircraft or smaller technical parts. With EBAM and FSAM post machining is often required to smooth down a part to the required dimension. Compared to Powder Bed Fusion, these processes can build much larger parts and may be suitable for making the entire structures of spacecraft or satellites.

The Aeroprobe nozzle

Magnesium, aluminum, aluminum silicon carbide, copper, copper matrix materials and steels have been successfully made with FSAM. The Friction Stir Welding technology itself is comparatively new, invented by TWI, and there are now over 1800 Friction Stir-related patents. Hitachi holds 214 related patents and Boeing 67. Additionally there are around 172 license holders for the TWI patents. This means that several industrial groups may be able to commercialize FSAM or FSAM-related 3D printing technologies. Indeed groups such as UTC have patents on FSAM systems. The interest in FSAM is partially motivated by the fact that these organizations have access to the technology. FSAM or FSAM-like processes could also perhaps have better mechanical properties than other processes. Tensile strength and creep strength of parts could be increased. Metals that cannot be processed well through processes that require a liquid to solid state transformation may possibly be processed well in a solid state process. Tantalizingly FSAM-like processes could potentially create custom materials on the fly by controlling the mixing of materials and even the microstructure of parts. Through these means FSAM processes are also a path to gradient materials or functionally gradient components.

The Friction Stir Welding Process by the Edison Welding Institute.

Aeroprobe’s FSAM process.

What are Functionally Graded Components and Gradient Materials? 

Functionally Graded Components or Functionally Graded Materials or Gradient Materials are materials or parts whereby the structure of the material is changed to give different areas of the part different properties. If a microstructure of a metal 3D printed part is changed, then one printer with one process and one material can make a part that behaves differently at different points or areas. Or a part could have one microstructure that is changed across a gradient to in effect become a new material or to be a combination of several materials with the useful qualities of all of them. Gradient materials are so new and interesting that we cannot presently quite see just how large their application areas can be. Generally, however, people expect to be able to design new materials that can outperform those that now exist. What I’m most excited about is that by designing ‘materials on the fly,’ a 3D printer or other manufacturing system could determine the gradient of materials and alter this during the print to improve overall processing or to dynamically change the performance of the part. Instead of a wing with struts we could alter the composition of the wing so that particular areas are stronger in one direction to redirect forces in that direction, at the same time we can make another area more flexible to absorb those forces. A wing wouldn’t be made out of one grade of aluminum but rather of many different compositions of aluminum, all optimized to have the highest degree of functionality for that precise three dimensional point in a part. If this were possible at scale it would fundamentally alter engineering for high-end applications.

A new paper 

A new paper proposes exactly this for FSAM. In the paper the team looks at making composite materials on the fly, resulting in the “manufacture of tailor-made functionally graded composites.” The team made an Aluminum + Titanium Carbide graded composite over a specified length.

“A specific process condition vis-à-vis the number of passes, volume faction, and particle size combination may promote one or more phenomena such as continuous dynamic recrystallization, particle fragmentation, and breaking of initial matrix grains, which eventually affect particle mixing and matrix grain size and thus cause property gradients,” the paper states. “The findings are expected to enable the manufacture of functionally graded composites products of larger size.”

Another paper two years ago looked at the strength of magnesium friction welded parts and their microstructure, and showed improved mechanical properties. A different paper looked at the properties of Inconel parts made with FSAM. Inconel is of particular interest to aerospace companies looking for high-heat applications for the material.

What does this all mean? 

There are three broad implications from these developments. One is that Powder Bed Fusion as a technology could see increased competitive pressure from traditional welding companies. Powder Bed Fusion parts generally have more design freedom, however, and so far are more accurate, especially small parts. FSAM could along with the Sciaky and Optomec processes (and competing technologies such as UAM) be a path to 3D printing large structural components for aerospace. The third and most exciting implication is that we will at one point be able to create gradient parts that may be several meters in length. This would bring a completely different path to designing parts, materials and structures to manufacturing. We can’t yet know what the complete implications of this would be. Higher performing aerospace parts and new ways to make large structures such as missiles and satellites could have major implications for those industries and applications. What can gradients of Inconel do to increase the performance of a jet or rocket engine? Frankly, I don’t think anyone (publicly) has any real idea of the potential, but the prospect of this is exciting enough to fuel many research teams. Share your thoughts on this technology in the FSAM forum at 3DPB.com.

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