Rethink 3D metal printing of turbomachinery parts

Recent breakthroughs in industrial metal additive manufacturing (AM) are lifting restrictions on pump and compressor designs and how they are manufactured. These restrictions apply to limits current-generation AM systems face in creating passageways, blades and channels. The breakthroughs occur in AM optics, powder bed quality, chamber environment, consumables and real-time process monitoring and control for part validation in individual or series production.

For pump and compressor performance, a recently introduced advanced AM system can produce a shrouded impeller up to a 12-inch diameter, with shroud angles down to 5 degrees and no support structures are required to hold the blades and other features in place during manufacture in the 3D printer. This represents a highly significant improvement over all other currently available AM solutions. No internal supports are needed. Externally, just the extruding lip on the outer rim and the bottom edge are anchored. It is a good practice to extrude a wall around the diameter of the impeller to support the shroud; this material can be easily removed in a single operation in post-processing.

The system can produce most shrouded impeller geometries with a surface roughness lower than 10-micron SA on all surfaces, internal and external. In addition, the closed-loop melt pool control and metrology systems provide real-time control and monitoring throughout the build, ensuring the structural integrity of each finished piece and consistency from build to build. This accuracy, quality and ability to execute on extreme design complexity can be applied successfully to related fluid-and-air pump and heat-exchange equipment (see Figure 2).

Pump and compressor complexities

Pumps and compressors are some of the most common pieces of industrial equipment found in nearly every major market. One class of pumps is the centrifugal pump, which has many uses in very demanding applications. The impeller in these pumps can either be open or can have a shroud or covering depending on the application.

Shrouded or closed, impellers are used in high-performance pumps where efficiency is of concern. They also find significant use in pumping flammable or explosive fluids. The shroud on the impeller makes the pump less sensitive to impeller/volute tolerancing and drift, which increases the pump performance and eliminates the potential for sparks that might result from contact with the volute. Examples of applications include turbopumps (particularly for aerospace and rocket propulsion), electric submersible pumps for oil and gas and industrial compressors.

Because of the rise in commercial space exploration, impellers are widely used in rocket engines. Shrouded impellers are used in the turbopumps that feed the engine, often with Inconel impellers on the liquid oxygen (LOX) side and Titanium impellers on the fuel side. In some applications (e.g., oxygen-rich staged combustion with high-pressure LOX), even tried-and-true Inconel 718 can be found to be lacking — or at worst, fuel for a catastrophic event. In this case, many people have turned to Monel or another proprietary (e.g., Mondaloy) alloys. With so many considerations to their use, shrouded impellers often are customized for each job, designed to suit the demands of a given operating environment.

The complexities in creating shrouded impellers make them a good fit for the benefits of recent additive manufacturing advances.

Design issues addressed via additive manufacturing have included the parts consolidation and internal passage creation. Conversely, the traditional impeller manufacturing process involves 5-axis machining, with the bottom impeller and shroud created as separate pieces that are brazed or EBM welded. This process can be expensive, time-consuming and relatively low yield. Producing such parts in an additive workflow has long been an attractive proposition, but one that has remained challenging because of the restrictions inherent in support-dependent solutions.

The angle of overhangs and the need to remove supports after manufacturing while maintaining acceptable surface finish have been among the primary limitations of employing additive manufacturing for applications like shrouded impellers. Furthermore, post-processing requirements have been extensive, consisting of electrochemical polishing, extrude honing and/or other processes, each with its own set of advantages and disadvantages. Electrochemical polishing, for example, is expensive and can require complicated tooling, while extrude honing preferentially removes/polishes certain surfaces in the fluid channels.

Looking ahead

The unique needs and challenges of shrouded impellers are a strong fit for the new technology discussed in this article.

When using costly metals and creating critical-use parts, it’s important that any manufacturing process produce and maintain reliable results — not only meeting but exceeding existing capabilities. Unlike complex machining from a block of metal, new AM technology for impeller creation uses only the material necessary for each part, with the elimination of supports reducing extraneous material usage even further. Cutting costs as well as final part weight through optimized design, the new process for support-free 3D printing shrouded impellers offers a compelling alternative to the manufacturing status quo because traditional manufacturing via machining or use of AM processes depends on extensive supports and anchors.

The latest advances in support-free additive manufacturing open new possibilities for industry to rethink the design and fabrication of existing pump and compressor products — expanding their performance, depth of customization and cost benefits.

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