Zach Murphree Zach Murphree
VP of Technology Partnerships

The Challenges of 3D Printing Superalloys

May 17, 2021


Superalloys, as their name implies, have powerful material qualities that extend beyond those of more common alloys. For example, the traditional alloy bronze combines tin with copper, melts fairly easily and is relatively malleable—but has limited industrial use. A mix of nickel, chromium, iron and molybdenum, on the other hand, produces Hastelloy X, a superalloy (from Haynes) that is formable and weldable, yet provides an extremely high melting point, superior mechanical strength, low creep and long-lasting resistance to oxidation and/or corrosion.

Elwood Haynes created a range of nickel-molybdenum alloys starting in the early 1900s, including the Haynes Stellite Alloy that became the patented Hastelloy X trade name. The nascent aviation industry was thrilled with the robust properties of the new superalloy; Pratt & Whitney used it in the jet engines that powered the first Boeing 707 aircraft in the 1950s. Hastelloy X (HastX) has since become a material of choice for gas-turbine hot-section components in aerospace, as well as a variety of high-temperature/high-corrosion oil-and-gas and energy applications. Examples include transition ducts, combustor cans, spray bars and flame holders, as well as afterburners, tailpipes and cabin heaters. HastX also finds use in other settings, such as industrial furnaces, due to its unusual resistance to oxidizing, reducing and neutral atmospheres. In the chemical-processing industry, it is used for retorts, muffles, catalyst support grids, furnace baffles, tubing for pyrolysis operations and flash-drier components.

The long-wear characteristics of HastX extend product lifetimes. For example, jet engines frequently are driven to overhaul when combustor components wear out due to excess corrosion or cracking. HastX provides better corrosion and creep resistance at very high temperatures compared to some other superalloys that see such resistance diminish over time when exposed to extreme heat. Using HastX in jet-engine components can thus significantly prolong time between engine overhauls. 

 

 

Industry Looks to 3D Printing For Innovative Alternatives

Hastelloy-X-jet-engine-3d-printedWith such material considerations in mind, aviation, aerospace and heavy industries have begun looking at additive manufacturing (AM) of superalloys as potential game-changing technology. AM can deliver complex, previously unmanufacturable, innovative designs that boost product performance while reducing supply-chain delays and associated production costs. Particularly in high-temperature gas turbines—traditionally assemblies of tens-to-hundreds of parts including tubes, flow paths and sheet metal structures that must be formed and welded to other components—AM presents some highly attractive opportunities for design simplification and part consolidation.

Given the desirable qualities that superalloys can deliver, it’s perhaps not surprising that companies beginning to explore the potential of AM are asking for equipment—or access to contract manufacturers running advanced industrial AM systems—that can handle such material. 

Hastelloy-X-turbine-blade-3D-printedAM machine providers, in turn, are embracing the opportunity to use their technology to solve problems encountered by engineers when using traditional manufacturing to make high-performance, mission-critical components from superalloys. Response to this demand is met, however, with inherent challenges to perfecting the superalloy 3D-printing process.

Fine-Tuning the Process

Metal AM, essentially a micro-welding operation in which a geometry-directed laser beam melts ultra-thin layers of material sequentially to build up a three-dimensional part, is impacted by material purity. While many OEMs have their own recipes for metal powder, quality standards allow for a range of certain elements within an alloy, which means there can be more or less oxygen or carbon in a superalloy formula. With HastX, lower carbon content leads to better results; the most successful print runs occur when material choice controls for that.

Another issue: HastX can create base-metal soot or condensate during the laser-sintering process, fouling the surrounding powder. Soot landing directly on the powder bed can cause issues with energy absorption. Or, if the gas flow used to flush out the chamber during AM doesn’t completely evacuate the soot, the chamber volume can become loaded with airborne condensate that blocks the laser beam itself. Such occlusion changes the amount of power delivered to the powder bed and interferes with the welding process. 

To ensure clean gas flow around a part being sintered from HastX, next-gen AM systems operate in an argon-gas environment that keeps oxygen levels extremely low. While other systems use nitrogen, argon typically delivers better results throughout a build. 

Lastly, AM of HastX can lead to hot cracking—less of a problem than with some other superalloys, but still something that requires attention, particularly when building thin-walled, high-heat structures.  Here, precise directing of the thermal energy of the laser is key, due to how the active meltpool cools and solidifies during an AM build. Cooling and solidification depend greatly on what’s around the build—usually metal on one side and powder on the other. Powder is an insulator that does not conduct heat well; as a layer cools, that thermal energy predominantly transfers into the newly created thin metal walls. Too much thermal energy in the wrong place can weaken and warp the structure. Therefore, it’s critical that the laser power be applied at the precise location and strength every millisecond—an attribute of the in-process monitoring and feature-specific parameter control provided by some AM systems. 

Matching the Material to the Application

Given the variety of challenges inherent in matching materials to production processes, it’s interesting to note that many AM-machine makers now have portfolios of as many as 40 different materials. Yet, in many cases the cost of printing doesn’t align with the material chosen. The application space also can be limited. 

Suppliers of newer AM systems have prioritized customer and market considerations, and consciously limited the range of applications for which they’ve developed their exacting AM processes and accompanying software. Capabilities such as support-free manufacturing (at angles now down to fully horizontal overhangs) of extremely thin-walled parts align well with high-temperature applications involving fluid flow and heat exchange. This, in turn, dovetails well to the production of parts and components made from HastX and other superalloys. 3DMP

Industry-Related Terms: Additive manufacturing, Metal powder, Powder bed
View Glossary of 3D Metal Printing Terms

 

See also: Velo3D

Technologies: Applications, Metal Powders, Powder-Bed Systems

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