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AM is the best thing since sliced bread, they said. But has the potential of AM for compressor manufacturing been more promise than reality?
For the last 15-20 years, additive manufacturing (AM) or 3D printing has been widely hailed as the best thing since sliced bread. This optimism has turned to irrational exuberance in the turbomachinery industry, where AM is supposed to solve complex geometry fabrication problems, lower manufacturing costs, and reduce lead times of critical components. Unfortunately, to date many of the promises of AM have yet to be realized. There appears no clear path forward to address some of the apparent limitations of this manufacturing technology.
Additive manufacturing (AM) is basically a niche manufacturing technology between casting and subtractive manufacturing. So far, the potential of AM for compressor manufacturing has been much more promise than actual reality.
Industrial centrifugal compressors use fairly standardized casing, sealing, and rotor support elements, but their internal flow path can be performance optimized for a specific application. Thus, their impellers appear at first glance to be ideal candidates for AM technology, which favors one-off customized fabrication.
This is especially true for modern, complex-geometry 3D impellers. Significant aerodynamic design improvements due to advanced computational fluid dynamics (CFD) and finite element analysis (FEA) tools have changed the shape of impellers from traditional, simple 2D to highly flow-shape, curved 3D geometries. The move from 2D to 3D compressor impeller geometries happened in parallel with a slow industry transition from two-or three-piece construction to single-piece construction, using either welding or brazing to combine the compressor hub and shroud. Single-piece impellers can usually handle higher-stress loads than two-piece impellers and also reduce fabrication quality flaws.
For most modern impellers that have complex internal geometries and are not cast, fabrication techniques typically require five-axis machining for large impellers or electrical discharge machining (EDM) for smaller geometries. AM has shown promise for the manufacturing of smaller impellers, but most commercially available AM machines are limited in size and cannot handle impellers larger than about 20 inches in diameter. Closed impellers add complexity to AM methods as most metal additive processes do not build in free space. The bridging of the cover between impeller blades requires temporary internal supports to provide a base to build upon. These supports have to be removed during post-processing. This is difficult due to the limited access resulting from the small size and complex geometry of the impeller.
The most common, commercially available AM method for printing metal compressor impellers is laser metal sintering using a metal powder bed (also called selective laser melting, direct laser melting or laser powder bed fusion). A laser acts as a localized heat source to melt and fuse metal powder into a desired 3D shape, using sequentially layered metal powder. This technology has rapidly advanced over the last 10 years and can be utilized with many of the standard metals, such as steels and advanced nickel alloys typically used for compressor impellers
The industry focus on additive manufacturing has been largely focused on nickel-based and titanium alloys that cost more than $50 per pound and are utilized for specialty applications. Most components within turbomachinery are manufactured from carbon steel, low-alloy steel, or martensitic stainless steel. While it is possible to additively manufacture components from these alloys, it is typically more cost-effective to machine them from wrought material. Additive manufacturers have very limited experience with carbon steels and stainless steels, and powder is not readily available for these alloys.
The printing process of a typical small impeller will usually take several days, sometimes weeks, depending on the speed of the AM printer, the shape, and the desired printing quality. Internal support structures will usually have to be added to the impeller model to maintain shape integrity during the sintering process. The programming of AM printers is not an automated process and requires special operator skills and experience.
After completion of the printing process, there is usually significant post processing and testing required. Removal of internal support structures can be complex and labor-intensive, particularly due to the fact that printed components are often manufactured from titanium or nickel-based alloys. The removal of the internal supports often results in ridges and internal passage imperfections that require post-machining, EDM, acid bath, or abrasive slurry extrusion for removal. Surface finish is usually grainy and rough, and the quality depends on the speed of the printing process and metal powder grain size. External surfaces require machining and grinding to improve surface finish. This all adds to lead times and costs. Current estimates are that for a typical AM manufactured impeller, about 60% of the cost comes from post-processing and testing, rather than from the printing itself.
Due to the nature of the printing process there is an increased chance of surface cracks or internal voids with AM. Surface testing such as dye penetrant and internal crack/void detection using CT scans or x-rays is usually required. Also, on a typical compressor impeller, geometric tolerances are about 3-5 times higher than similar impellers fabricated using EDM or five-axis machining. These increased manufacturing tolerances increase proportionally with the size of the AM impeller.
The original promise of AM was that “anyone” can print a piece without any special training required as long as they have the appropriate metal printer. This led to the idea that one could print compressor impellers and other spare parts at distributed local repair shops, avoiding transport logistics, long shipping delays, and reducing the overall lead time. However, the reality is that for current AM machines, specialized operators are required; and the post processing steps such as support material removal and surface machining still require qualified machinist and a well-equipped machine shop.
Clearly, AM has the potential to solve many problems in the future including making it possible to provide aerodynamically and structurally optimized geometries for higher head and higher efficiency, or allow for faster and local “as-needed” manufacturing, reduce manufacturing times. New methods are beginning to address some of the shortfalls, and AM’s capabilities are likely to expand in the years ahead. But the current state-of-the-art still limits AM to small and specialty geometries in the compressor industry. AM is basically a niche manufacturing technology between casting and subtractive manufacturing. So far, the potential of AM for compressor manufacturing has been much more promise than reality. ■
Derrick Bauer contributed to this article.
Klaus Brun is the Director of R&D at Elliott Group. He is also the past Chair of the Board of Directors of the ASME International Gas Turbine Institute and the IGTI Oil & Gas applications committee.
Rainer Kurz is the Manager for Systems Analysis at Solar Turbines Incorporated in San Diego, CA. He is an ASME Fellow since 2003 and the past chair of the IGTI Oil and Gas Applications Committee.
Any views or opinions presented in this article are solely those of the authors and do not necessarily represent those of Solar Turbines Incorporated, Elliott Group, or any of their affiliates.