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It can be an ugly truth—just ask anyone who’s been picked last on the playground—but it can also be a source of pride; just think of all the engineering projects that are described as the largest, longest or tallest in the world. In manufacturing, large part production has historically been costly and time-consuming, but the growth of metal 3D printing (if you’ll pardon the pun) is changing that paradigm.

While metal 3D printing can be broken down into various sub-types—powder bed fusion (PBF), selective laser melting (SLM), electron beam additive manufacturing (EBAM), etc.—not all of these are suitable for (or even capable of) making large parts. 

“Parts that are smaller than a laptop computer are typically ideal for powder bed,” explained John O’Hara, Global Sales Manager at Sciaky, Inc. “They’re capable of making closed cavities, internal cooling channels and surfaces that are going to be useful as-deposited, but the trade-off to get that is a much lower deposition rate.”

When it comes to 3D printing large metal parts, deposition rate and the process capability for printing within a large work envelope are key.

“For large part production, EBAM has a very high deposition rate,” O’Hara said. “We’ve reliably reached the 25-pounds-per-hour mark, but deposition is contingent upon the material and application.” While 25 lbs/hour represents the high-end of EBAM’s deposition rate, the process of 3D printing near-net shapes is constrained by part geometry and the natural trade-off between speed and precision.  

“Kind of like my handwriting teacher used to tell me: the faster you go, the sloppier it gets,” O’Hara commented.

This represents an interesting optimization challenge, in terms of balancing deposition rate with material cost and the amount of machining time necessary for post-processing. The faster you want to go, the more material you’ll need and the further your part will be from net shape. “Somewhere in there, there’s a point where the deposition rate and the balance of the cost of material and machining will equal out,” O’Hara said.

“For titanium, somewhere around 12-15 pounds per hour is optimal,” he continued. “Any metals that have a higher value by the pound—some of the metals we run are ten times the cost of titanium—and that certainly warrants a slower deposition rate and closer to net shape. Anything on the other side of titanium, like low-alloy steel, stainless steels, tool steels and even aluminum—you’ll find that going faster typically offers a better business case.”

Ultimately, as O’Hara noted, determining the optimal deposition rate for a large part depends not only on its geometry, but also material and production volume. The best way to make this determination is with the help of a supplier with a proven history of expertise. Sciaky’s expertise goes back to the 1930s.

“Going back to the 1930s, Sciaky originated as a welding equipment supplier,” O’Hara said. “In the ‘80s and ‘90s, we started to shift to machines that were doing weld repair to aircraft engines. This was using a wire feed with an electron beam to add a thin layer of metal over a worn or damaged surface on an aircraft engine—and we still do that today, quite commonly.”

“From that technology,” O’Hara continued, “we said ‘Well, if we can apply this thin layer of metal, perhaps we could put another layer on top of it and build a feature onto that part,’ and then you just take the next leap: Why don’t we build the whole part out of this layer-weld-layer approach? So, that’s where EBAM was invented. In fact, even today, every one of our printers is built upon that same electron beam welding platform that we have been evolving for five decades now.”

As a result, if you have experience with electron beam (EB) welding, you should feel right at home on an EBAM machine. The biggest differences, according to O’Hara, are the addition of a wire feed and computer control. Moreover, machines upgraded in this way are still fully capable of EB welding. “In fact, the changeover from printing to welding is about one hour,” O’Hara said. “Some of our customers are printing features and then actually welding those same parts together in a hybrid approach to printing and welding.”

This represents another important difference between EBAM and PBF: changeover for metal powders typically requires personal protective equipment (PPE) and can take hours, whereas switching the wire in an EBAM machine can be done quickly and relatively painlessly. “The wire represents no risk to a person other than dropping the spool on your toe,” O’Hara quipped.

Because the EBAM platform is built on CNC welding technology, Sciaky has been able to leverage CNC positioning to ensure that the melt pool stays horizontal. “The overhang capability from layer to layer is limited to maybe 10° or so in titanium—but it’s different for every alloy due to differences in viscosity and surface tension. But, because we can use the CNC system to manipulate parts, such that the melt pool remains horizontal, we’re able to build more complicated structures than you would imagine.”

As is the case with metal additive manufacturing more generally, the aerospace industry is the biggest user of EBAM, and it’s not hard to see why. “The better business cases are going to come from metals with a high cost-per-pound and a high cost to machine them,” said O’Hara. “That describes every aerospace alloy out there.” These include titanium and nickel alloys, as well as some stainless steels and high-end aluminum alloys.

“Aero structures, parts for spacecraft, rockets, satellites, helicopters—these are all vehicles that benefit from the high strength-to-weight ratio of titanium, but titanium is a little bit expensive and really hard to machine,” O’Hara said. For similar reasons, EBAM is also being used in the power generation and chemical processing industries, both of which require expensive, difficult-to-machine metals.

While this is also true of the medical device industry, O’Hara noted that the scale at which EBAM operates is generally not suitable for medical devices. “We don’t print anything that will fit inside a human body under most circumstances,” he said.

In addition to being able to work with materials that are more challenging for traditional manufacturing processes, EBAM can considerably reduce turnaround times. This is particularly well-illustrated in the case of a titanium ballast tank for an autonomous underwater vehicle (AUV).

“They came to use because they heard we could make a ballast tank for them in three weeks, which is what we did,” O’Hara explained. “A lot of people come to us saying ‘Hey, we’re doing an additive project; hop on board.’ This was not that. They said, ‘I heard you can make a tank. We don’t care how you make it as long as it’s done in three weeks.’ That was one of the parts that was a hybrid, where the two separate hemispheres were printed and then EB welded together on the same machine.”

Old-school manufacturers may balk at the suggestion that 3D printing large parts can be faster than producing them with conventional manufacturing methods, but O’Hara noted the importance of selecting the right process for a part.

“In general, if you have a milling process that can be done faster than an additive process, then you’re not doing additive right,” he said. “For a lot of parts, I send people away because a good CNC center would be done with the part before our machine has even achieved a vacuum. That’s not a failure of additive manufacturing, that’s a person learning what AM can do and what it can’t do.”

O’Hara explained that supply chains can often weigh in favor of using AM for large parts, in cases where finding a large enough billet for machining is difficult, or if the specification is for a forging that would take too long to produce. “Human beings are very good at making parts these days,” O’Hara said. “We’ve been working on it for a long time, but there’s that subset of parts that represent a true manufacturing or supply chain problem that additive has the answer for. Those are the parts you should be doing. Don’t just do it for the sake of additive manufacturing.”

According to O’Hara, most of Sciaky’s customers start with using EBAM for a specific part, but these parts were often originally designed for traditional manufacturing methods. “As the world starts to adopt additive, we’re going to see more and more next-generation designs being done with additive as the intended process, and that’s going to be a revolution in the world of additive manufacturing. Next-generation means next-generation aircraft, rockets and satellites, all taking advantage of these new manufacturing methods.”

Aluminum Precision Machining

Qualification remains a significant barrier to additive adoption, especially given that the traditional manufacturing methods with which metal AM is competing have a serious head-start. Nevertheless, the path forward is clear.

“In the short term, the early production parts are going to be smaller, lower-risk and non-critical, but they’re paving the way,” O’Hara said. “We get those parts qualified, which provides confidence to manufacturers that this technology can be trusted. Then, follow-up projects get bigger, more complex, and more mission-critical. It’s just a matter of time.”

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