Unlocking the Potential of Additive Manufacturing in Aerospace Engineering
In the ever-evolving landscape of aerospace engineering, innovation has been the driving force behind advancements that have pushed the boundaries of what was once thought possible. From the propeller that powered Amelia Earhart’s historic non-stop flight across the Atlantic to the space suits worn by generations of U.S. astronauts, United Technologies Corp (UTC) and its subsidiaries have been at the forefront of defining the aerospace and defense industries for decades.
Today, with a global presence of over 110,000 aerospace employees stationed at more than three hundred sites worldwide, UTC continues to drive innovation in commercial and business aviation, intelligent defense solutions, helicopter systems, and spacecraft design and production. The company’s portfolio also powers critical communications, navigation, and surveillance technology for the world’s leading aircraft manufacturers.
As part of the UTC enterprise, Pratt & Whitney and Collins Aerospace Systems represent a deep commitment to aerospace innovation. Both businesses have a long history of manufacturing ingenuity, and each has played a crucial role in building the modern air transportation network. Now, as new technologies fuel rapid advances in the aerospace industry, UTC is building on its legacy of innovation with cutting-edge Additive Manufacturing (AM) initiatives.
Pioneering Additive Manufacturing at UTC’s AMCoE
For over three decades, UTC and its business units have been using AM to prototype polymer and non-metallic parts. But more recently, recognizing the vast potential of metal AM, the company has ramped up its efforts through the creation of UTC’s Additive Manufacturing Center of Expertise (AMCoE) in East Hartford, Connecticut, USA.
“UTC companies helped build the second industrial revolution in the early decades of the 20th century,” said John-Paul Clarke, Vice President of strategic technology at UTC. “As we build the next industrial revolution, additive could be a game changer in manufacturing. Our AMCoE is a mechanism for driving a critical mass capability across the enterprise.”
While each business unit under the UTC umbrella has its own path on the UTC AM roadmap, the AMCoE is a cross-divisional organization that aims to bring together the creative minds in materials science, mechanical design, diagnostics, and machine learning to solve the toughest challenges. Launched two years ago, the AMCoE is responsible for accelerating the adoption of AM across UTC’s engineering and manufacturing organizations, focusing on three key goals:
- Achieving process certification for key AM processes
- Deploying a design-for-additive toolchain
- Training UTC’s engineering and operations teams on both AM design and manufacturing
Together with ongoing efforts at individual UTC business units, these initiatives will demonstrate new engineering and operations capabilities to produce cost-competitive additively manufactured parts across a range of materials and functions, including nickel-based superalloys, titanium, aluminum, and engineered plastics for brackets, fuel nozzles, complex manifolds, and other components.
Pioneering a Conformal Heat Exchanger through Additive Manufacturing
Most recently, UTC broke new ground in its AM efforts through the successful design, full-scale build, and testing certification of a conformal heat exchanger, which this article will explore in greater detail.
“Until now, manufacturing has been constrained by what’s possible,” stated Venkat Vedula, Executive Director of UTC’s AMCoE. “But with additive, if you can imagine it, you can build it. In partnership with America Makes and the Collins Aerospace additive team, the conformal heat exchanger demonstrated that Additive Manufacturing opens the door to novel designs that allow us to truly optimize for performance.”
While the AMCoE is a catalyst for scaling AM operations across the enterprise, it’s hardly UTC’s first or only foray into the field. Since the 1980s, Pratt & Whitney has additively manufactured more than 100,000 prototypes, hundreds of which supported the development of its PurePower® Geared Turbofan™ (GTF) engine range. In 2017, the company took the major step of entering additively manufactured parts into production for its PW1500G Geared Turbofan engines, which were used exclusively by Bombardier for its CSeries aircraft family, now branded as the Airbus A220.
“Additive Manufacturing isn’t just driving improvements in performance and quality, it’s changing the way we design and even think about manufacturing challenges,” explained Jesse Boyer, Technical Fellow at Pratt & Whitney. “Each additive project we undertake generates insights and creates opportunities for the ones that come next.”
Overcoming the Limitations of Traditional Heat Exchanger Manufacturing
As the next generation of aircraft technology becomes more sophisticated, it’s clear that getting the most out of Additive Manufacturing, with new designs and novel concepts, will become more important than ever. Inspired by that challenge, United Technologies recently partnered with industry accelerator America Makes to develop a heat exchanger produced using Laser Powder Bed Fusion (L-PBF) for the Department of Defense and Air Force Research Laboratory (AFRL).
“Heat exchangers play a critical role in the thermal management systems that remove heat generated from advanced electronics and more efficient jet engines,” stated Brian St Rock, Project Manager, America Makes / AFRL project, UTRC. “But even though they must handle greater demands, performance improvements have been hindered by the limitations of traditional manufacturing. This project gave us an opportunity to experiment with conformal shapes that let us put performance first.”
Traditional heat exchangers are rectilinear, box-like forms created with metal roll sheets that are brazed and welded together in a multi-step process often associated with low yield. Because the heat exchanger must fit into constrained, non-rectilinear spaces on aircraft, sometimes wrapped around the engine core, these linear segments are connected using transition headers to make a curved conformal shape. But the addition of these bridge-like components adds volume and weight and can create efficiency losses.
Conformal heat exchangers with curved sides and without the transition headers will improve thermal performance and flow distribution while reducing volume by around 10%. However, manufacturing such heat exchangers through traditional methods is economically unfeasible and operationally arduous.
Harnessing the Design Freedom of Additive Manufacturing
Additively manufactured heat exchangers, on the other hand, promise to more effectively use overall volume by eliminating welding lines and distribution headers, thereby unitizing the structure and improving yield, as well as reducing lead time for new products. They also improve thermal performance and flow distribution through complex fin topologies and spatially varied geometry. Localized geometry modifications, such as fillets, can also reduce stress concentrations and improve fatigue life.
A preliminary optimization and benefit assessment conducted by UTC suggested that with an advanced additively manufactured heat exchanger design, a 40% improvement in precooler UA per unit volume (the product of the overall heat transfer coefficient and the heat transfer area) could be achieved. That would correspond to a 29% improvement in cooling capacity or an 11% decrease in the amount of engine bleed needed to drive the thermal management system.
However, existing AM technology is not without its limitations. When it comes to producing a heat exchanger, merely switching from traditional manufacturing to out-of-the-box AM actually more than doubles the volume of the part, largely because default AM scan strategies generate fins that are 0.25 to 0.4 mm thick with a surface roughness of 7.6 to 25.5 microns, depending on build orientation. That is significantly greater than traditionally manufactured fins, which are 0.1 to 0.15 mm, with a surface roughness of 0.1 to 0.4 microns.
When the fins are too thick and rough, the pressure drop exceeds the requirement, and can be two-to-ten times higher than with smooth conventional fins. While roughness can help increase heat transfer, depending on the fluid, its impact on the pressure drop is more significant. This is a highly critical trade-off, and if the roughness issue is not properly addressed, the resulting AM heat exchanger will not perform as needed.
Developing Innovative Build Strategies and Process Models
To achieve the aggressive goals of the project, which touched various aspects of the AM value-chain, UTC assembled a multi-disciplinary team of experts with backgrounds in product engineering, design, AM, process modeling, in-process sensing, and post-processing. UTC also embraced an approach that was systematic, model-guided, performance-driven, and feature-based.
“This project provided freedom and required creativity, which made it a fascinating undertaking,” commented Vijay Jagdale, Principal Engineer at the UTC AMCoE and lead investigator on the project. “We had to ask, ‘What are the critical features of the heat exchanger that are driving performance? And what is the response needed at the feature level?’ That thought process led us to focus on the fin thickness, its surface roughness, and leak-free parting sheets.”
Recognizing the constraints of existing AM technology, UTC internally developed a product performance prediction tool to conduct the trade-off analysis between surface roughness and pressure drop and provide the necessary manufacturing targets.
“The industry is still developing post-processing techniques to achieve the desired surface finish reliably and repeatably, as well as thermal post-processing to control defects,” Jagdale continued. “Many a time, this can add up to be a significant portion of the additive product cost. We set out to minimize post-processing required to achieve surface roughness targets in the as-built configuration, as well as avoid the Hot Isostatic Pressing (HIP) process. This required taking calculated risks and pushing the limits of the technology through novel build strategies.”
One of the team’s critical innovations was to divide the geometry of the desired heat exchanger into various sections, including fins, parting sheets, and headers. From there, they worked together, sample by sample, to use process models, in-situ process monitoring, and process-mapping approaches to identify the optimal build strategies and parameters for each specific section.
Given that existing commercial AM toolchains and build set-up programs do not allow for the level of customization required, UTC developed many new tools and approaches to enable the AM machines to, ultimately, create the desired end product.
“We took on more risk than some of our team members were accustomed to,” Jagdale stated, “but we were comfortable, because our hypothesis was supported by physics-based models and a rigorous experimentally-validated scale-up approach.”
UTC’s physics-based model takes important process inputs into account, including key powder feedstock characteristics, machine and laser level characteristics, thermo-physical material properties, and key scan strategies, and then predicts a feasible process space in terms of laser power (P) and scan speed (V). It can also help classify process zones according to how defect-prone they may be, from predicting lack-of-fusion defects and balling to identifying conduction-dominated ‘good’ zones and keyhole regions of higher energy intensity.
Once the model identifies ‘good’ zones, it is possible to then superimpose other variables, such as feature resolution, residual stress, distortion, microstructure, and surface roughness, to narrow down the process parameter selection as a function of a particular feature and its performance needs. This physics-based model enabled the UTC-led team to rapidly develop a feasible process space, use select experiments to validate the process map, and use these feature-based parameters for heat exchanger development.
This approach to modeling also helped empower the team to abandon the traditional linear waterfall approach to manufacturing, in which product engineers hand over directions to designers, who in turn deliver digital CAD files of 3D models to manufacturing operators.
“Usually designers, product leads, and manufacturing teams work in siloes, barely speaking to each other. For this project, we were collaborating and iterating continuously,” Jagdale explained.
Unlocking New Possibilities with Conformal Heat Exchangers
The team’s willingness to test new boundaries and ways of working ultimately paid off. When the program came to a close, they had not only developed a high-performance conformal heat exchanger, but a systematic, repeatable methodology for manufacturing it.
Compared to one produced conventionally, UTC’s heat exchanger achieves about a 20% increase in heat exchanger effectiveness for the same volume. As the team continues to refine its proprietary parameters and scan strategies, as well as incorporate advanced heat exchanger topologies, it expects to move even closer to the over 30% reduction in volume suggested by its initial analysis.
“The fact that we were able to do things conformally is a huge piece of the puzzle,” stated Paula Hay, Executive Director of Additive Design and Manufacturing at Collins Aerospace. “That really creates new possibilities for us.”
For example, now that the team has demonstrated that it can additively manufacture parts robustly and utilize feature-based scan strategies, UTC can advance novel concepts in designing flow passages that improve efficiency in thermal management systems. And further, Hay said it brings UTC closer to unlocking the true value of Additive Manufacturing.
“Replacing existing, traditionally manufactured parts with AM parts can deliver some advantages. But the real potential is leveraging the design freedom offered by AM,” Hay emphasized.
Driving Adoption through Collaboration and Standardization
The motivating factor in all of UTC’s work in AM is the enterprise’s planned shift from development into production. The company has built up a strong base of materials knowledge and process-related best practices through years of prototypes and proof-of-concept projects. Now, it is focused on working through the certification and regulatory agencies that will help scale production.
Part of that focus requires enhancing internal knowledge of its AM processes to enable it to overcome machine-to-machine variations and truly standardize part production. Another part of that means working with external partners so that the industry is aligned with itself and the regulatory bodies responsible for certification and approvals.
Hay noted that the heat exchanger project, conducted with America Makes and other partners across the industry, is a good example of the kind of collaboration the industry needs in order to move forward. Given that so many groups, from peer companies to academic institutions to AM associations, are working toward similar goals, she said it only makes sense for everyone to work together.
“We’re driving toward the adoption of additive,” said Hay. “But we can’t do it alone. Additive is ready for takeoff. There’s no stopping what it can do.”
In the coming years, UTC’s AM initiatives, including the AMCoE and the United Technologies Research Center, will continue to explore new developments in materials, design, and data analytics. Building end-to-end design tools for AM that capture the entire value-chain, from mechanical design to analysis to post-processing, is another key priority for the enterprise.
“One of our key challenges is that we don’t have design tools that offer end-to-end toolchains,” commented Vedula. “The industry needs tools that can optimize the process from concept and design all the way through to production.”
Applying cutting-edge data analytics insights to Additive Manufacturing is also a critical target. For example, UTC is developing an automated, robust data-harvesting process that will establish a data path from initial part material feedstock to final characterization. The goal is to assimilate unstructured and pre-aggregated data into a database environment that facilitates trend generation and reporting. It is also using machine learning methods to enhance the build quality of additively manufactured parts and develop a deeper understanding of the AM process.
Furthermore, the team is currently looking at the development of ‘smart’ parts, developed by embedding sensors in additively manufactured components.
Carving a Unique Approach to Accelerating AM Adoption
While industry peers are also pursuing Additive Manufacturing solutions, UTC believes that it is carving its own approach to accelerating AM development and deployment. With more than ninety years of manufacturing experience and over 430 unique patent families in AM technologies, the company is uniquely qualified to integrate AM technology into the design process.
Its combination of physics-based models and in-process monitoring is unlocking an in-depth understanding of the process’s capabilities and limitations, which will advance adoption through the certification of material and machine combinations. In short, this heat exchanger project demonstrates how UTC’s holistic approach to AM balances the drive to get AM parts to production quickly, while achieving or exceeding aerospace quality standards.
As the team at https://www.aircooledheatexchangers.net/ continues to push the boundaries of what’s possible with air-cooled heat exchangers, the pioneering work of UTC’s AMCoE serves as a testament to the transformative potential of Additive Manufacturing in the aerospace industry and beyond. By embracing creativity, collaboration, and a relentless pursuit of innovation, these experts are paving the way for a future where thermal management challenges are solved with unprecedented efficiency and agility.
