Hybrid manufacturing, sometimes call hybrid machining, combines additive and subtractive processes in a single machine. (Images courtesy of DMG MORI.)
Where’s my 3D-printed car?
For that matter, where’s my 3D-printed house or clothing or smartphone? 3D printing was supposed to be the biggest technological game-changer since the PC. But what have we gotten for the past three decades? Trinkets, art projects, prototypes and the occasional medical or aerospace pièce de résistance.
When are we going to see honest-to-goodness additive manufacturing (AM)?
Perhaps I’m being unfair.
If you’ve attended the last few International Manufacturing Technology Shows (IMTS), then you’ve been able to witness the growth of industrial 3D printing firsthand. In a relatively short time, we’ve gone from making PLA prototypes with cobbled-together desktop units to making production-grade parts on industrial-grade 3D printers.
If you think we’re anywhere close to the limits of what 3D printing can do for mass production, imagine someone who might have been inclined to say the same thing about computers in 1974. Technologies take time to mature and—like human teenagers—they tend to go through phases that aren’t always well understood.
Consider hybrid manufacturing.
What is Hybrid Manufacturing?
The simplest way to understand hybrid manufacturing is as a combination of additive processes—3D printing, known in the context of production as additive manufacturing (AM)—and subtractive processes, such as milling. While there are plenty of parts being made through some combination of these processes—and more are being introduced all the time—the crucial qualifier for hybrid manufacturing is that both processes occur on the same machine.
A part that is printed on a metal 3D printer, surface machined to improve its finish and separated from its build plate using a wire EDM would be an impressive example of modern manufacturing techniques, but it wouldn’t count as an example of hybrid manufacturing. Consequently, the number of parts produced via hybrid manufacturing may be relatively small. The technology is still relatively new, even for an industry as young as 3D printing.
And yet, much like 3D printing, the potential benefits of hybrid manufacturing have made some early adopters very optimistic about the technology’s future prospects. Michael Sealy, assistant professor of mechanical and materials engineering at the University of Nebraska-Lincoln is one of them.
“Additive really opens up the doors in terms of being able to print your own mechanical properties layer-by-layer or zone-by-zone,” he said. “That’s one of the big advantages, so I see hybrid AM exploding in the next few years just because of all that potential.”
Hybrid Machining Formats & Processes
Although the overall number of available hybrid machines is still relatively small, it’s helpful to divide them into several types. The most basic distinction to draw is between off-the-shelf hybrid machines and additive modifications for conventional machine tools.
As of this writing, major players in the machine tool industry with hybrid offerings include DMG MORI, ELB-Schliff, Matsuura, Mazak, Mitsui Seiki and Okuma. Lesser known companies with hybrid machines include Diversified Machine Systems, Fabrisonic and Optomec.
The options for additive modifications to machine tools are more limited, represented by Hybrid Manufacturing Technologies (HMT) and 3D-Hybrid Solutions, Inc. We’ve covered the former company before, while the latter is a relative newcomer. In both cases, the core technology involves one or more metal 3D printing tools that are designed to operate alongside the standard set of subtractive tools which populate machine tool magazines.
AMBITs alongside conventional subtractive tools in a tool magazine. (Image courtesy of Hybrid Manufacturing Technologies.)
Although hybrid add-ons are designed to be purchased and installed independently, some machine tool builders are starting to offer them as standard options, including ELB-Schliff, Mazak and Mitsui Seiki, in HMT’s case. 3D-Hybrid Solutions’ founder, Karl Hranka, confirmed that his company is on the same path: “We’re working with some of our early customers to progress their applications and we’re starting to partner with machine tool builders as a dedicated additive manufacturing tool developer.”
Beyond this basic distinction, the available options for hybrid manufacturing can also be divided in terms of their underlying additive technologies. These include directed energy deposition (DED), wire-arc additive manufacturing (WAAM), cold spray (CS) and Fabrisonic’s ultrasonic additive manufacturing (UAM). There are important differences between these technologies, and the manufacturers of hybrid machine tools have each placed their bets, so to speak, so it’s worth looking at these technologies in more detail.
Directed Energy Deposition (DED)
Directed energy deposition involves feeding powder into a melt pool generated on the surface of a part using a laser or electron beam. The process is essentially the same as selective laser sintering (SLS), except the powder is applied only where material is being added to the part at that moment.
(Image courtesy of DMG MORI.)
Titanium, stainless steel, aluminum and other difficult-to-machine metals are among the materials supported by DED. Another advantage of this approach—at least for some of Okuma’s LASER EX series of super multitasking machines and some options from 3D-Hybrid Solutions—is the ability to perform hardening operations with the machine’s laser.
Depending on the material being used, DED often requires the build chamber to be filled with inert gas. However, for some hybrid machine tools—such as DMG MORI’s LASERTEC 65 3D hybrid and LASERTEC 4300 3D hybrid—a local inert shroud gas can be sufficient to shield the melt pool for better control of material properties.
HMT’s AMBIT heads use laser metal deposition (LMD), a subtype of DED. Consequently, ELB-Schliff, Mazak and Mitsui Seiki’s offerings are all DED-based. Optomec’s laser engineered net shaping (LENS) technology, which is featured on its LENS 500 and LENS 860 hybrid machine tools, also falls under DED.
Wire-Arc Additive Manufacturing (WAAM)
While DED is best for parts that require higher levels of precision or accuracy—powder bed fusion (PBF) is even more accurate and precise but it is not, as of this writing, an option for hybrid machine tools—WAAM wins out on deposition rates.
WAAM using a laser. (Image courtesy of 3D-Hybrid Solutions.)
“With our wire-arc solution, we’re around two to five pounds per hour, depending on the alloy,” Hranka said. “We believe we can go faster, but we’re still optimizing.”
The DMS Huron Peak hybrid system is based on wire-arc technology, with deposition rates of three to five pounds per hour. It’s also worth noting that wire-arc systems do not require an inert environment, though they do need to be shielded for safety like any arc welding process. Peter Gratschmayr, senior sales engineer at Midwest Engineered Systems (MES), further explained what distinguishes WAAM from other additive systems:
“This really doesn’t compete with other laser additive manufacturing technologies,” he said, “because those are meant for a higher definition, smaller component. It ends up costing somewhere between 12 and 25 dollars an ounce for the powder to be able to make the part, because there’s usually a 20 percent scrap rate that comes out of it, so not all the powder gets used.”
“The other thing to keep in mind is that we’re putting down material at 15 to 20 times the weight of powder,” Gratschmayr continued. We can make parts that are up to 42 meters long by six meters wide, two meters high and keep it at repeatability somewhere around 20 to 30 thousandths.”
Cold Spray (CS)
Cold spray is a coating deposition method that was originally developed for shaft coating applications, but which is now being used in hybrid manufacturing. 3D-Hybrid Solutions offers two cold spray tool heads, one designed for processing harder alloys and one which is laser-assisted for higher speed deposition.
The cold spray process grew out of the thermal spray market, but unlike thermal spray processes, which generally melt metal powders, cold spray processes keep the metal powders in a solid state.
“We might get up to 80 percent of the melting point,” explained Tom Woods, Director of Business Development for VRC Metal Systems. “It’s a softened powder, so instead of liquidizing the metal and spraying it—which doesn’t give you a very strong bond—we spray the powder through a supersonic nozzle that accelerates it up to about Mach 2 or Mach 3.”
“That deforms the metal particles on impact,” he continued, “and they shear into whatever metal substrate you’re spraying them on. Because of that, you get a metallurgical bond, not just a mechanical one. We end up with generally greater than 8,000 psi bond strength, less than one percent porosity and tensile strength—for a material like titanium—we’ve gotten over 80,000 psi.”
Ultrasonic Additive Manufacturing (UAM)
Developed by Fabrisonic, UAM is based on a technology that’s been around since the 1950s: ultrasonic welding. “We have a proprietary patented design of a roller which rolls over the foil back and forth and vibrates as it’s rolling, giving us the scrubbing action we need to make the bond,” explained Mark Norfolk, president and CEO of Fabrisonic.
“The great thing about ultrasound is its low temperature,” he continued. “A part doesn’t need to go above 200° F. So, the material properties going in are the same material properties coming out. You can also combine dissimilar metals in the same part without forming intermetallics or having metallurgical consequences you don’t want.”
Fabrisonic takes off-the-shelf CNC mills and adds the company’s weld-head to it. “Because we have a CNC mill, we’re using standard G-code to drive the machines motion and we’ll print thin foils side by side and then on top of each other in a brick laying pattern to build up a three-dimensional shape.” Using this technique, the hybrid machines can print parts in near-net shape with the weld-held and then deploy cutting tools for the subtractive work.
Hybrid Manufacturing Applications
All talk of technology aside, the eternal question in manufacturing persists: What’s the application?
“Like machining, the applications are diverse: aerospace, medical, mold and die, lots of different things,” Hranka said. “Metal 3D printing is a new technology, and everyone using it is facing the challenge of material qualification. We’re focused on printing very fast and utilizing the strengths of the CNC machine.”
(Image courtesy of Hybrid Manufacturing Technologies.)
This raises an important point: right now, the two biggest industries for metal additive manufacturing of any kind are aerospace and medical. Working in these industries requires adherence to strict regulations, and when it comes to additive, that can mean qualifying not just a part, but the process, material and machine as well. The ability to layer metals and develop new alloys is unquestionably exciting, but that excitement needs to be tempered with a reminder about the onerousness of industry regulation.
Pessimism notwithstanding, the potential applications for hybrid manufacturing are tantalizing indeed. Dr. Sealy has been working on a particularly intriguing application for the medical implant industry.
“Whenever you break a bone, you get either a titanium, stainless steel or cobalt chromium implant,” he explained. “We’re talking about plates, screws and rods. The problem is that having these inside your body can create long-term complications. In my case, I have two screws in my elbow and it’s starting to hurt whenever I carry a gallon of milk or unload the washing machine. That’s why orthopedic surgeons will often recommend that you take the implant out after six to eight weeks.
“We thought, ‘Instead of having this whole second surgery, let’s make an implant that degrades,’ and we use hybrid additive manufacturing to control how fast that happens. So, we can tailor the degradation rate to be very fast for someone who’s younger and still growing or much slower for someone who’s older and doesn’t regenerate bone tissue very quickly. Hybrid manufacturing enables us to adjust how fast an implant will degrade, and we achieve that just by changing the way we manufacture it.”
Michael Sealy, an assistant professor of engineering at the University of Nebraska-Lincoln is using an Optomec hybrid machine to produce biodegradable medical implants. (Image courtesy of Optomec.)
Even the comparatively mundane applications for hybrid manufacturing are impressive, as Hranka explained. “We’re showing off our system with Takumi USA, and leveraging the strengths of their machine, which in this case is in the mold and die industry. So, think of repairing molds, printing conformal cooling channels or even hardfacing molds so they last longer.”
Jason Jones, CEO and co-founder of Hybrid Manufacturing Technologies agrees. “DED is really ideal for repair and remanufacturing, and it’s really mature for those type of applications,” he said. “In our experience, the halfway point between those is re-manufacturing, just adding a little bit of material.”
Tom Cobbs, LENS Product Manager at Optomec, also emphasized the advantages of using hybrid machining for repair applications, as well as re-manufacturing. “We can scan a part,” he said, “compare it to a CAD drawing, and then repair the coating—whether that’s a wear-resistant coating on the exterior surface or a corrosion coating on an inner bore for a valve or pipe. Let’s say you have an existing part and you want to add a feature to it. You can machine down the original and then add something on to it—for example, we took a stock bar, machined it down to a rod, and then printed a fitting on the end of it.”
Hybrid Machines vs Standalone 3D Printers
When it comes to hybrid machine tools, the obvious question to ask is whether merging additive and subtractive processes in a single machine is really necessary. Given that we already have plenty of standalone subtractive options, more standalone metal additive options cropping up all the time, and pallet-changing systems galore, what’s the benefit of putting it all in one machine (aside from obvious added floorspace)?
(Image courtesy of Hybrid Manufacturing Solutions.)
“Here is the crux of the challenge that I pose to folks who want to go hybrid,” said Dhruv Bate, associate professor at Arizona State University. “Unless it is able to do everything for me in one step—that means support removal and finishing—then I do not see the advantage in investing in a hybrid machine, because it’s very likely that I will still need all these downstream operations to truly get my part production-ready.”
Dr. Sealy offered a different perspective. “To do hybrid manufacturing on a reactive material—for example, if you need to machine a magnesium part—you have to worry about the chips and powder becoming combustible,” he said. “So, you need the ability to have an inert environment through all the processing steps, as opposed to printing, pulling the part out, and then going back to the printer.”
Hranka also emphasized the benefits of being able to switch between additive and subtractive operations without having to move the workpiece. “With powder bed printing, you can’t do any machining internally,” he said. “You can only print your part complete and you can’t go back into it. With hybrid, you can stop printing, machine, and then print further. I always think of it like a ship in a bottle: being able to print the bottle, print the ship, machine with precision to get the surface finish and the ship’s sails, and then print the whole thing shut. There’s no way to get a cutting tool into a part that’s completely closed.”
Jones pointed to the benefits of consolidating your equipment needs, particularly when capital expenditure is a pressing concern. “A company I visited in South America has historically been making a product which requires a traditional pre-heat prior to adding metal by manual welding,” he said. “With our technology, they’re going to skip that entirely. They’re going to almost half of the capital equipment requirements to produce their parts by going to a single-setup approach. They’re working on castings coming from a foundry, which will go onto a hybrid all-in-one machine and what has historically been three or four different setups will now be merged into one.”
Additive & Subtractive – Together at Last
As much as I might hope, hybrid manufacturing isn’t likely to give me a 3D-printed car, house, clothing or smartphone in the near future. What it is likely to do is change the way we design, manufacture, repair and re-manufacture crucial components in the aerospace, medical and tool and die industries. As with metal additive manufacturing more generally, adoption by the automotive industry hasn’t taken off quite yet, but the simplicity and relatively low cost of hybrid manufacturing, combined with the ease of access that comes with—in many cases—using familiar systems and software, suggests that it could be the best bet for bringing metal additive manufacturing to mass production.
Considering metal AM?
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