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Sparking Change? Advances in Direct Metal Printing

The 3-D printing of metal components remains a developing manufacturing process, but it has the potential to significantly impact the metalcasting industry.

Nicholas Leider, Associate Editor

(Click here to see the story as it appears in the May issue of Modern Casting.)

3-D printing, as just about every business and tech publication has boasted in recent months, is a disruptive technology that has experienced remarkable growth in the last five years. Experts estimate additive manufacturing has been and will continue to expand by as much as 30% annually. Metalcasters have already seen the impact of 3-D printing in the production of patterns for investment castings and sand cores and molds—a process that can shave weeks off lead times and reduce costs related to product development.

3-D printing metal has lagged behind methods of printing other materials, but recent advancements have led to the technology being used for prototyping and small-run production parts. With firms like GE, Airbus, Boeing and Ford Motor Co. pouring millions of dollars into direct metal printing, the technology appears to be primed for an increased presence among other traditional manufacturing capabilities, including metalcasting, especially as printing speeds improve and more materials are made available.

“The industry is moving from a prototyping past to production future,” said Tim Caffrey, senior consultant for Wohlers Associates, a leading consulting group in additive manufacturing. “It’s in the process of growing up.”

3-D printing of metal, as it matures and grows out of its infancy, is a technology that should be monitored by the metalcasting industry. Advancements could impact metalcasters in a variety of ways. For one, the processes may develop into potential sources for tooling and dies. Direct metal printing also may become another viable option for rapid prototyping and small run components, becoming a new resource for metalcasters.

Manufacturing experts will continue to disagree on the potential impact of additive manufacturing and direct metal printing—some call it the next revolutionary technology, some dismiss it as a gimmick—but these processes appear en route to becoming feasible options for certain components.

“Additive manufacturing used to be used primarily for prototyping, whether it be functional prototyping, visual aids and for patterns for metal castings or polyurethane plastic parts,” Caffrey said. “But in regards to metals, we’re seeing a real increase in uses for end-use parts, for direct part production. That application category has been growing very quickly.”

Metal Powder Bed Fusion    

By far the most common method for printing metal parts involves directing a heat source onto a bed of powdered metal to fuse granules together. With so-called “metal powder bed fusion” techniques—also called laser melting, selective laser melting or direct metal laser sintering (DMLS)— a laser or other heat source binds the materials together in a micro-welding procedure according to a 3-D model that is sliced into thin layers.

As the part is printed, it will be supported by lattice structures of the same material. Once printing is complete, the supports are removed by hand and/or CNC machining. Research has shown objects created by powder bed fusion, with proper heat treatment, can exhibit physical and mechanical properties similar to cast parts, though further studies are ongoing.

“The main difference is the grain structure,” said Bry Ewan, product manager of metals, Stratasys Direct Manufacturing, Valencia, Calif., a service provider of additive and advanced manufacturing including 3-D printing. “If you cut a DMLS part in half, the internal grain structure will look different when compared to a cast part. But if you’re using an alloy that can be cast, the part will behave just like its cast or forged cousin.”

Similar to binder-based metal printing methods, powder bed fusion has made significant inroads in the aerospace, medical and oil/gas industries. (See the sidebar for a case study involving a hinge for the Airbus A320.) GE Aviation recently announced plans to invest $50 million to introduce high volume additive manufacturing to its facility in Auburn, Ala. This facility will produce the 19 fuel nozzles in its LEAP jet engine via powder bed fusion. The facility expects to increase annual production from 1,000 to 40,000 by 2020.

“We spent years proving out this technology for a critical component in the heart of the engine,” said Greg Morris, general manager, GE Aviation, of the nozzles, which will reduce weight by 35% and parts from 18 to one. “Now we are well positioned to apply this technology to other components in the same harsh environment which could prove to be game-changing for future engine programs and designs.”

Alternative Approaches

Though powder bed fusion is the most utilized process, a variety of 3-D-printing methods using metal are on the market. One alternative is directed energy deposition, which deposits wire or powder into thin layers that are then melted using an energy source. Unlike powder bed fusion, though, directed energy deposition often requires additional processing to improve surface finish.

Binder-based jetting systems is another 3-D-printing process, which is similar to techniques already familiar to metalcasters in the production of cores and molds. The process features a print head selectively spraying a binder solution over a metal powder bed. The layer is dried with a heating lamp and then a new layer of powder is spread on top. This process repeats until the object is fully formed. The object is carefully removed from the non-bonded metal powder and placed in an oven to fully cure the binder. The fragile part then is placed in a kiln where it’s infused with bronze powder to create a highly solid metal matrix component.

“Really what we print is a matrix, instead of an alloy,” said Bernie Potts, sales manager, ExOne, Troy, Michigan. “For example, it could be 60% stainless steel with 40% bronze infiltrated into it. Using this process, we’re able to produce components with 97 or 98% density.”

The capabilities of a binder jetting system can be seen in a case study featuring a medical prosthetics manufacturer looking to improve production of a component used in a prosthetic hand. It was looking for alternatives for what had been parts produced via investment casting. The components printed in a stainless steel/bronze matrix reduced weight and integrated multiple pieces into a single assembly. Direct metal printing more than halved lead times, from as long as eight weeks to two or three, while the cost per part dropped from $250-$1,500 to $25-$150.

Laser powder forming is an additive manufacturing technology that uses a metal powder injected into a molten pool created by a high power laser beam.  

The process can go from metal and metal oxide powder to metal parts, in many cases without any secondary operations. Metal powder is applied only where material is being added to the part at that moment and has primary applications in repair and overhaul, rapid prototyping, rapid manufacturing, and limited-run manufacturing for aerospace, defense, and medical markets.  

The technology covers several alloys, including titanium, stainless steel, aluminum, and other specialty materials. Mechanical testing reveals outstanding as-fabricated mechanical properties This process holds opportunities for die and tooling repair.

Competing Considerations

The design freedoms associated with building something layer by layer is the biggest advantage of additive manufacturing. Cavities, internal passages and other complex features can be designed directly into the component, without as much consideration for the manufacturing method and/or secondary machining.

“Additive manufacturing can resolve a lot of constraints in traditional manufacturing,” said Andrew Snow, senior VP, EOS of North America Inc. “You can reduce part numbers through design, you can reduce weight by getting rid of unnecessary material, you can produce fully customized parts for on-demand-type applications.”

Additionally, 3-D-printed metal components do not require gating or risers and can be produced without upfront investments in tooling. If a small number of parts are needed quickly, they can be printed and shipped in a matter of days, thanks to a reduction in up-front work necessary to manufacture the part.

“In direct metal printing, you don’t have to worry about designing gating and risering. You are just printing the part,” Potts said. “If you’re going to pour metal into a mold, you have to go through quite a bit of engineering to make sure you’ll get a sound casting. But with metal printing, you don’t really have that early work.”

While engineers are afforded design freedoms not seen in other manufacturing processes, including metalcasting, production speeds have hampered the technology’s ability to produce large amounts of components in a relatively short time. The sheer time needed to build a metal part layer-by-layer is the biggest driver of cost.

“The two major cost drivers solely in printing parts—not post machining, heat treating, etc.—are materials and machine costs. Materials are a factor, but a relatively small one. What the cost really comes from is the time to run the machine.”

A component’s design can improve efficiency; by decreasing a build’s height, the 3-D printer can complete the layering process more quickly. But the technology is not at a point where it can produce production parts in the hundreds or thousands. It has been a commercially viable short-run production method for a half-decade, but larger volumes will require faster printers with larger build boxes.

For the powder bed fusion printing process, the build box offers another constraint. The laser is fixed and then steered to the powder bed with mirrors. The beam can experience problems when it is redirected farther and farther from the center of the build box. These complications can lead to components that are out of tolerance, variances in the laser’s energy and other complications. Currently, powder bed fusion is restricted to build boxes with axes around 10 in. Binder jetting systems can produce larger parts, but are still constrained by the size of the build box.

3-D printing methods are somewhat limited by the materials available for use. The number of alloys that can be used for powder bed fusion is only a few dozen, including stainless steel 316, Inconel 625 and 718, titanium Ti64 and a cobalt-chrome alloy. These offers should increase as the technology develops and the industry continues to invest in R&D.

“DMLS is a microwelding process. In principle, if you can weld an alloy, it’s a candidate for DMLS,” said Snow. “The products that have been brought to market and commercialized are the low hanging fruit, developing applications for titanium, stainless steels and nickel-based alloys. These are materials that the marketplace is telling us they want first and we’ve delivered those.”

Similarly, for binder jetting, only a handful of materials, including 316 and 17-4 stainless steels, an iron-chrome-aluminum alloy and a cobalt chrome alloy, can be infiltrated with bronze to produce metal matrices that are 98% solid.

In addition to diversifying materials, direct metal printing faces a number of challenges associated with a technology’s natural maturation process.  

Traceability, supply chain, repeatability, reliability, real time control, software simulation—all these things have been worked out with other industries such as metalcasting,” Caffrey said. “These are things that have yet to be solved for our industry.”

But the future is still bright. Considering 3-D metal printing’s ability to penetrate the medical and aerospace industries, two markets notoriously demanding and risk averse, the obstacles of meeting requirements of other industries in other materials do not appear insurmountable.

“We’ve used 3-D printing for plastic tooling, so it only makes sense that metal is the next step,” said Brandon Lamoncha, sales manager, Humtown Products, Columbiana, Ohio. “The industry should embrace this technology when it makes sense, and we’ve seen that start to happen.”

Humtown Products, the Youngstown Business Initiative and American Foundry Society helped launch the American Makes initiative, an effort to facilitate collaboration among leaders from business, academia, non-profit organizations and government agencies to make the U.S. 3-D printing industry more globally competitive.

Finding Space on the Factory Floor

The potential for direct metal printing is real, but the technology should be seen as complementary to traditional metal component manufacturing and doesn’t pose a major threat to metalcasting facilities in the near future.

“I think all comes back to scalability of the process. I firmly believe we need to get to a point where the economies of scale makes sense, like in many traditional methods of manufacturing,” Ewan said. “If that’s possible, I think [direct metal printing] will be one more tool along with metal casting, subtractive methods like CNC machining and other manufacturing methods. They all kind of dovetail—where one is strong, another is weak.”

Even if 3-D metal printing becomes more widespread in manufacturing, the technology is a potential partner for a hybridized production method. Just as 3-D-printed molds allow investment casting facilities to have rapid prototyping capabilities, direct metal printing may provide the industry with improved capabilities.

“A lot of people have this misunderstanding of additive manufacturing that it’s going to be a technology that will displace many of the traditional manufacturing processes like milling and grinding,” said Snow. “But it’s the exact opposite. We need to coexist and depend on each other for secondary finishing. We don’t see this as a threatening technology to the current structure of manufacturing. It’s a complementary piece of equipment that’s another tool on the factory floor.”

3-D printing, including the additive manufacturing of metal parts, will continue to be a hot topic in the years to come. The developing technology is definitely worth following, even if its significance in manufacturing appears to be gradually increasing in the coming years and decades.  

“People hint about the third industrial revolution that’s going to change everything,” Caffrey said. “But a more realistic scenario is a sophisticated contract manufacturer will have a factory floor that’s going to have all kinds of machine tools. The more high tech operations may have castings capabilities, and they may have additive metal technology.”

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