NASA 3D prints rocket parts — with steel, not plastic
Share This article
When steel is stronger than steel
When something more robust than plastic is needed, you probably think of steel as a suitable choice. It may therefore sound strange to say that steel, at least in its tempered form, is in fact relatively soft. Fortunately, as ancient cave man discovered, it is readily hardened by firing and rapidly quenching in water, into something that is truly hard. But if this “heat treatment” is overdone, the metal becomes brittle like glass. Heat treatment is the reason that it is possible to use a hacksaw or file made of steel to cut through non-hardened steel without dulling the tool. It is true that diamond is polished with diamond in the form of dust or paste, but this is economically achieved only by converting a small volume of the polishing agent into a large surface area of sharp abrasion that can be sacrificially dulled.
Tools, cutlery, and weapons that have been around since long before the industrial revolution are actually examples of what we might today call “smart materials.” They are smart in that their microscopic or grain structure is tightly controlled to yield specifically desired properties in different locations. A hammer or screwdriver is first heat treated in such a way as to optimize tensile strength so that it can be pried upon without brittle fracture. Then the working surfaces are hardened to resist the wear of repeated use by “case hardening” to a desired depth through the introduction of extra carbon with a quick heating and chemical treatment. There are literally hundreds of different kinds of steels available, each one a unique alloy optimized for a particular use. If 3D printed materials are to become competitive with traditionally manufactured materials, they will have to be capable of functioning under the wide range of operating conditions that traditional materials routinely do.
The future of 3D printing
An attractive feature of 3D printing is that, as an additive manufacturing process, it can be more efficient in many ways than traditional machining. There is little doubt that it will come to replace machining for many types of products. It should be realized however, that only a small portion of our everyday products are made from subtractive techniques like CNC milling or turning. Most common articles are made by mass production processes involving various combinations of casting, forging, extruding, stamping, bending, spinning, drawing, or molding. In order to compete with these processes, 3D printing will need to gain efficiency by multiplicity. In other words, pipeline several machine heads in parallel such that one control and one set of motive elements drives the simultaneous creation of many parts. The process would be reminiscent of a pantograph copier but instead of a single copy, many would be made at once.
The recent Maker Faire in New York showcased many new and novel machine geometries including flexible multi-axis or hexapod machines with non-Cartesian geometries. The geometry of the machine can be optimized to the parts it is destined to make. Peculiarly absent from the Maker Faire was the 3D printing equivalent of the basic and universal machine tool, the lathe. Lathes, while in some sense the most primitive machine tool, can be used to easily make threads in metal. Threads are only robust in the hardest of plastics, yet perhaps they will become more dispensable in a world where complex parts can be printed whole rather than pieced together from simpler components. The side effect is that parts will not be easily disassembled, and when they fail, a complete part will need replacement.
Dentistry is one area where 3D printing of hard materials has successfully demonstrated its advantages over milling. Replacement teeth are small parts, yet to produce their complex geometry with subtractive machining, a large 5-axis machine is required. 3D printing will allow the manufacturing technology to be physically closer to the point of end use; in this case, the dentist’s chair.
One new concept ideally suited to 3D printing is brought to us thanks to Omote 3D, in Japan. With its new system, patrons are scanned in a booth and a colored replica of them is soon printed in any one of 3 sizes — at a cost of $500. Productions by standard multi-axis mills producing foam props for the entertainment industry come nowhere close to this performance or price point.
There are plans already, to make 3D printing workable in the zero gravity conditions of space. This would present unique challenges and opportunities, and hints at aspirations of forward thinkers at NASA and elsewhere. The structural integrity of 3D printed parts like the J-2x will still need to be fully vetted in the materials testing lab. They will be crushed, stretched, diced, and X-rayed to reveal grain microstructure. Provided the parts hold up to the standards expected, the J-2x will be just the beginning of 3D laser printed parts at NASA and beyond.
- 2 of 2