AM Applications Explode with Multimaterial Printing

Additive manufacturing (AM) creates a multitude of opportunities across industries thanks to ongoing improvements in machines, methods, and materials. And, as with multimaterial designs built via conventional manufacturing processes, the small but growing field of multimaterial 3D printing is unleashing even more of AM's power.

Multimaterial systems enable such variations as functional gradient builds, composites, novel alloys, and new ways of making electric and electronic components. The method is used to make devices as diverse as joint implants, robotic end-of-arm grippers, and circuit boards.

It also can bring out the scientist in practitioners. "With multimaterial 3D printing, what if I selectively deposit a little bit of stuff A here and a little bit of stuff B there?" posited Greg Paulsen, director of applications engineering at the digital manufacturing marketplace Xometry Inc., North Bethesda, Md.

"You can create what we call digital materials, which are where you dope a rigid material with a little bit of rubber-like material to make that material act softer and more pliable," Paulsen said. "Or you can go the other way around, have a really soft rubber and make it a little bit firmer by adding a little bit of rigid material. And all of a sudden, in a single print or run, you can create an object that can have either a simulated overmold or it can have different properties built in the same print."

There is a caveat, however. Due to its newness, most multimaterial applications to date have been relegated to research and academia, Paulsen noted, characterizing the practice as a "relatively new field."

He isn't alone. Aconity3D GmbH, Herzogenrath, Germany, which makes laser-powder-bed-fusion (LPBF) printers for metals, described its multimaterial customers as "early adopters" that are mostly in academia. In fact, the technology transfer from idea to reality thus far has been more of a trickle than a torrent as multimaterial printing establishes itself as an industrial process.

Printing and post processing of two different materials in AM requires a careful evaluation of their properties. "It's certainly not unlimited," said Shawn Allan, vice president, Lithoz America LLC, Troy, N.Y., which makes lithography-based ceramic 3D printers.

When asked whether two materials are compatible throughout the high-temperature sintering process and subsequent cool down, Allan gave the question some thought. "Things that we have to look out for are: Do these materials bond to each other? Because some materials when you fire them do not want to stay connected to each other very well," he said. "One of the biggest factors we have to look at is thermal expansion mismatches."

If the mismatch between the two materials is acute, internal residual stress can build up in the part as it's processed. According to Allan, this will either cause the materials to delaminate or "pop apart energetically"—aka explode!

Dilatometry measures both thermal expansion and contraction in sintering. As a result, Allan said, users can evaluate how two materials behave on their own and identify how they overlap with each other to get "a feel for if this potentially is a good combination to put together."

In addition to combining two ceramics, Lithoz is tinkering with adding metals to ceramics. "Then you’ve got the ability to put conductivity paths through materials, where normally that might have to be done as two things that are made separately or screen printing," Allan said. "But we can print conductive traces through an insulator. It opens up the design for electrical components that way."

Practical applications for printing metal inside of a ceramic might include industrial reactors for chemical processes and devices for cautery during surgery. However, ceramics require much higher temperatures during firing than some metals can withstand without melting.

"If somebody wants to put a ceramic together with aluminum, it's not going to work ... because aluminum will melt at 600°, maybe 660° C, but we’re finding that the very lowest temperature ceramics will fire is around 1,000° C," Allan said. "And those are usually some special glass ceramics that are designed to work well with metals."

For metals such as copper, silver, and gold, Lithoz uses low-temperature, co-fireable ceramics (LTCC). Similar to glass ceramics, LTCCs are designed to sinter at 800-1,000° C, which is a very low temperature for ceramics, and is below the temperature that copper, silver, and gold melt.

"If you want to use alumina and copper, it's not going to work because the alumina needs to go to 1,600° C to sinter," Allan said. "But we could, let's say, (print) alumina and platinum together or alumina and molybdenum. They really like each other, they work well together."

Printing with metal powder, as is done with Aconity3D's printers, creates the possibility of making "quasi alloys," something not possible with metals formed into ingots.

"You can take powder A, let's say chromium, and powder B, copper," explained Aconity3D CEO Yves Hagedorn. "If mixed at 50/50, these materials don't form an alloy used for creating ingots. As a result, with conventional casting there is no way that you would ever have in your hands a part made of half chromium and half copper."

However, possibilities open up with a dry-mixed combination of 50% each chromium and copper powder. "You shoot at it with a laser, and then you get what's called a quasi alloy," Hagedorn said. "So, not a real alloy, but you get the advantages and, disappointingly enough, the disadvantages of both materials."

Linares, Spain,-based Meltio's multimaterial approach focuses on the use of commodity welding wire made of various metals—stainless steels, titanium, carbon steels, and Inconel—to exploit their advantageous properties.

"This could be, for example, having a more wear-resistant material in areas that are subjected to wear due to friction or other factors during operation while having a less expensive and maybe more ductile material for the main part," said Giorgio Olivieri, Meltio's application engineering manager.

The technique may also include a corrosion-resistant material on the surface, while a common material such as stainless steel is used in the interior. Meltio's strategy maximizes economic benefits by using expensive materials (e.g., Inconel), super alloys, and nickel-based alloys only where needed. At the same time, hard-to-machine materials are deposited where necessary to reduce post processing.

"There are cases where you have to use a much more valuable material just because of the conditions on the external surfaces, while there are no mechanical properties or advantages given by this material on the core," Olivieri noted. "So being able to deposit the material just where it is needed is a huge savings in terms of the actual cost of the part."

For example, if Meltio is making an impeller for a pump, an external layer of Inconel would give it corrosion resistance without having to make the entire part out of this expensive material. Or, as an alternative to stainless steel and Inconel, a part may be made of tool steel with the addition of Stellite, a corrosion-resistant cobalt alloy, in areas subject to wear.

Igus GmbH, a Cologne-based manufacturer and distributor of technical products made of high-performance plastics, sees potential for so-called smart bearings made with multimaterial 3D printing.

The round bearings are printed in two layers. The outer skin is a thin (0.25-0.3-mm) self-lubricating plastic made from a tribologically optimized polymer blend. Underneath the top covering is a layer of an electrically conductive plastic, while a controller continuously measures the electrical resistance of the part.

"Through the lifetime of the bearing the resistance changes," said Tom Krause, head of igus’ 3D-printing department. "As soon as the (self-lubricating plastic) surface is worn down, there's electricity flowing from one circuit through the shaft to the other circuit. And that's information for us that the bearing is broken."

Several types of user interfaces are available, the simplest being an LED light with green and red lamps. "It's really only on or off," Krause pointed out. "There is only flowing electricity or not. There's nothing in between."

Another option is to use igus’ i.Sense controller, which can display a dashboard alert, trigger a controller to turn the machine off, or both. Although engineers are interested in this," Krause doesn't think they know where to start "because you need to design the part, and you need to have an idea where to use it."

Conversely, he said, igus engineers know exactly how to use a smart bearing. The company makes intelligent cable carriers for its energy chains using the same concept applied to its bearings—a self-lubricating layer over a conductive layer of plastic. As the self-lubricating layer wears away, the electrical conductivity changes.

The biggest difference is in the shape of the part. In this case, it's more of a rectangular block, rather than rounded, with four layers, according to Krause.

"When each layer is burned down, we get information if it's worn down or not."

This enables predictive maintenance. When an operator knows how long it took to wear down the first layer, she can calculate how much longer it will take for the second, third, and fourth layers to wear down. Because the energy chains are built in sections, each part can be equipped with a smart cable carrier that can both detect and identify the location of a fault.

"If there's a breakage in just one area, it's very easy for us to repair that or to replace those individual components," said Mike Rielly, who heads igus' AM business unit in Rumford, R.I. "So if there's some breakage due to stress or whatever it was, when we get that notification, instead of the whole system going down, we can avoid downtime by just quickly repairing that area, that section."

Igus’ engineers are tinkering with adding force sensing capability to its intelligent bearing. In this case, the inner material is not only conductive, it's elastic as well. "And this elastic conductive material deflects more if you have outer forces," Krause said. "And through this deflection, we can measure differences in the resistance again, so that we can also give information before something's broken."

Once the sensor reaches a certain threshold signaling a problem is arising, operators can turn off a machine before it breaks down. "But this is still really very experimental," Krause said. "We don't have any customer applications yet."

Aconity3D also sees great potential for sensors, but its ideas are about embedded or integral detectors. "Basically what you play with is the difference in electrical resistivity," said Hagedorn.

He described a part made out of aluminum with a line of copper printed within it. The copper line is connected to two cables. "In the end, you just connect the cables and you would have your sensor already implemented in your part," Hagedorn said. "Under a mechanical load, the resistivity of the copper line would increase. And now if you’re experiencing load, you could measure it."

Beyond sensors, there's potential for multimaterial 3D printing of electronics. At least one company, Nano Dimension Ltd., Waltham, Mass., is promoting industrial production of additively manufactured electronics with its DragonFly printer.

Once material compatibility has been defined, there's a big potential for using ceramics with multimaterial printing, according to Allan. In particular, Lithoz has worked with functionally graded ceramics.

"Maybe we have two different slurries of the same material, but one has a sacrificial additive to it that will give it a higher porosity," he said. "That lets you tailor things like thermal and even acoustic properties of the material from one end to another. Another common thing, again, might be with more or less the same material, but with different dopants added for color."

Colorants can control opacity, which can be necessary in electronics packaging. For dental restorations, such as veneers and crowns, color gradients can provide a natural appearance. Colors may also be incorporated for design or branding.

Functionally graded porosity can act as a gatekeeper for heat conduction. Similarly, it may allow or impede gases or fluids flowing through a part. "We might want a section that's impervious and maybe another section of the part that acts like a filter or a pathway for fluid or gas to travel from one place to another," Allan said.

Multimaterial 3D printing also could act as an aid or substitute for brazing ceramics components together—or for creating a gas-tight seal. "Now maybe it's possible to design that joint differently, even more three-dimensionally, so that maybe we can get better interlocking of the braze in the material because usually those things are just layered up and fired together or pressed together as separate components," Allan said. "But maybe we can actually integrate the braze into the ceramic a little better."

Cesar Terrazas-Najera, president of AconityUS Inc., El Paso, Texas, sees potential for industrial security measures with multimaterial 3D printing. "So one of the ideas is to actually encapsulate QR codes within the part itself, so it couldn't be counterfeited," he said. "That's very powerful for industry, because now I can guarantee that no one is going to counterfeit a part shipped to a customer. The customer can scan it and make sure that that's the part that was intended."

Terrazas-Najera explained that the QR code can be printed in a metal that contrasts with a part's material as imaged on a CT scan. There's even further potential for printing other information, such as instructions for making the part or details to trace the part, he said.

Meltio views multimaterial 3D printing as an alternative to laser or welded cladding. "When we talk about cladding, it's on external surfaces of relatively simple parts usually," Olivieri said. "By building a part layer by layer with dual materials, our system can deposit the "cladding" material in areas that could never be reached by (traditional) cladding."

Multimaterial 3D printing not only inspires new ideas, it also requires a different way of thinking for designers and engineers, according to Paulsen. The traditional way of thinking is to add features to a part that has the same properties throughout, but that's not necessarily the case anymore.

"It flips it on its head," Paulsen said. "And I think that's something that will require both a different way of thinking, but also intuitive software. We may have CAD (computer-aided design) designers and engineers that have interest in it, but may not have the ability to really or easily create a file that can benefit strongly from that."

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