Is additive manufacturing finally ready for lift off?

Additive manufacturing is popular amongst its users, but its practical application can be harder to find. However, as Nasa develop and test fire its next generation space rocket using 3D printed parts we ask, is the technology ready for lift off?

Someone tweeted me recently to ask if 3D printing would bring about a revolution? Is it the catalyst that will bring about a transition to a more localised manufacturing model? It is a nice thought as the status quo of shipping products halfway around the world does seem a rather wasteful and inefficient way of doing things, even if it is deemed the most cost effective.

The reality, from all corners however, is a resounding 'no'. And that is likely to remain the case for some years yet. Additive manufacturing (AM), more commonly known as 3D printing makes up a tiny proportion of manufactured parts and products. And despite the impressive market growth of machines, the process is more commonly used in addition to, and not as a replacement of.

A major barrier is throughput and corresponding unit cost. Most simplistic plastic parts made using an injection moulding machine can be produced in their hundreds or even thousands in the same time as it takes to print one part on even the fastest of AM machines. While some improvement can be made by designing parts for 3D printing, the reality is the volumes are just not viable for anything other than single or very short run production.

Another shortfall is the materials themselves. Again while improvements have been impressive, hands up those that have handled a 'printed' component and thought it was flimsy, or have winced at the surface finish? People seem to accept and look past these shortfalls, but engineers indulging in geek value is one thing, production parts are quite another.

Anyone that has a 3D printer will no doubt have the 20 or so 'experiments' lying around the machine that were essentially done for fun. There is also no doubt engineers love using them but finding examples of practical application, i.e. using them to somehow make your company more money, is somewhat pottered.

However, maybe this is slightly unfair. One term originally coined for the technology was Rapid Prototyping and that perhaps best describes its suitability for engineers; the ability to check the geometric form and fit of products during the design phase.

Moving beyond printing plastic
Although polymer based machines are dominating the market, recent developments are allowing metallic materials to be used in the same way. The technique known as selective laser melting (SLM) is still in its infancy and has been used to date to produce near neat shape metal components that reduce machining time. However, advances in the technology are allowing increasingly accurate shapes to be produced.

SLM uses a high powered laser beam to create 3D metal parts by fusing fine metallic powders together layer upon layer in much the same way as most other types of AM machines. This is allowing printed parts to be made of a representative metallic material that can be tested, and in some cases actually used. The result has seen some really interesting applications come to the fore, an example being US space agency Nasa, which has been experimenting to see how it compares with traditional manufacturing routes.

With the retirement of the Space Shuttle in 2011, Nasa has been busy developing its replacement; the Space Launch System, known more commonly simply as SLS.

The next generation
Nasa engineers began a project earlier this year to qualify its prediction that producing rocket parts from SLM saves money and has the potential to open up new affordable design possibilities.

Nasa engineers set about making comparisons during subscale acoustic tests of the RS-25 rocket engines being developed for the SLS and wanted to build two subscale injectors using SLM. In little more than a month the subscale injectors completed 11 main stage hot-fire tests, accumulating 46s of total firing time at temperatures nearing 3300°C while burning liquid oxygen and hydrogen.

The traditional subscale rocket injectors for early SLS acoustic tests took six months to fabricate, were made from four parts, had five welds, required detailed machining and cost more than £6,000 each. In comparison engineers built the same injector in a single piece by sintering Inconel steel powder with a state of the art 3D printer. After minimal machining and then inspection it took three weeks for the part to reach the test stand and cost just over £3,000 to manufacture.

"It took about 40 hours from start to finish to make each injector using SLM and another couple of weeks to polish and inspect the parts," says Ken Cooper, a materials engineer whose team made the part. "This allowed engineers to take advantage of an existing SLS [rocket development] test to examine how 3D printed parts performed against traditionally made parts of similar design."

Post test inspections showed the injectors remained in excellent condition giving the team enough confidence to continue developing rocket parts using SLM. As a result, a more complex assembly of the 3D printed injector and thrust chamber liner has been produced to accumulate further hot-fire test data and assess durability.

Chris Singer, a director of Nasa's Marshall Space Flight Center, says: "The additive manufacturing process has the potential to reduce the time and cost of making complex parts by an order of magnitude."

There are limitations however. Parts are restricted by the size of an SLM enclosure, likely to be no bigger than 500mm3. In addition SLM?parts reportedly had weaker physical properties than those forged and milled, though does it avoid the potential weakness of welds.

Indications point to the process offering the most advantage where complex geometry is involved. This is of no great surprise as 3D printing is generally selling itself on this ability. Items that have hidden voids or thin walls such as complex heat sinks continue to offer exciting possibilities as do custom implants for orthopaedics.

The aerospace and medical sectors continue to actively evaluate the technology as a future manufacturing process. However, for any kind of mainstream use, certainly one that is able to compete with conventional manufacturing techniques, additive manufacturing has a long way to go. In the mean time, however, expect some very impressive - albeit niche - applications.

Justin Cunningham

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