3D Metal Printing--What's Next?

BY BRAD F. KUVIN, EDITORIAL DIRECTOR

Recognized additive-manufacturing industry leaders like General Electric, Airbus and Rolls Royce, stalwarts of the aircraft/aerospace industry, have evolved to where they now can start to manipulate the 3D metal-printing process to enable them to repeatably build sound, quality parts. But they recognize that there is still so much to learn, and they lack the resources and knowledge base to explore some of the very basic issues that will allow them to get to the next level, and beyond.

Such is the foundation of the industry-university collaborative research and development efforts that have risen in recent years, none more significant, innovative and industry-driven than the NextManufacturing Center. Born early in 2015 on the campus of Carnegie Mellon University (CMU) in Pittsburgh, PA, the NextManufacturing Center is a collaboration of faculty and students from across the college. And, in July 2016 the center launched a consortium that includes a diverse portfolio of member organizations, all interested in building the future of additive manufacturing.

During our recent visit to the NextManufacturing Center, we learned of its commitment to building the knowledge base needed to expand the use of additive processes throughout manufacturing. And, 3D metal printing is at its center of attention.

In an interview with 3D Metal Printing, NextManufacturing Center director Jack Beuth, a CMU professor of mechanical engineering, notes: “Our mindset is performing the research needed to directly impact additive manufacturing, and we’ve identified several focus areas that promise to expand the use of 3D metal printing within the next 5 yr., in the aerospace industry and in other industries.”

The NextManufacturing Center 3D metal-printing roadmap includes several points of emphasis, including microstructure control and porosity modeling; machine learning and part qualification; topology optimization; and powder spreading and melt-pool geometry.

Beuth highlights the center’s focus areas in broad terms:

• Process design concurrent with part design
• Monitoring and feedback-based control systems
• Controlling build microstructure and material properties by location within a part
• Expanding the variable sets used to manufacture acceptable parts and eliminate or control internal porosity
• Expanding the acceptable range of powder properties, while developing new alloys to take advantage of the rapid solidification rates present during 3D metal printing.

NextManufacturing Center executive director Sandra DeVincent Wolf, flanked by associate director Tony Rollett (right) and director Jack Beuth. “Driven by the needs and demand of the aerospace industry,” says Wolf, “we are focused on understanding where the processes need to be in the future…we want to be able to expand the parameter set so manufacturers can more readily make qualified parts.”

Closed-Loop Process Control

“Closed-loop process control is coming,” Beuth says. “The ability to adapt for variability in the process is a big concern particularly in the aerospace industry. We have projects underway focused on how to manage the large amount of data emanating from the machines, and we have shown some positive results. For example, one of our Ph.D. students, Luke Scime, has developed a machine-learning algorithm that, for each image of the powder bed, analyzes the image to look for spreading failures. His algorithm can identify several types of spreading failures; the next step is to engineer the in-process adjustments needed to overcome those failures.”

Other process-control research underway, explains NextManufacturing Center associate director Tony Rollett (a CMU professor of materials science and engineering), addresses the microstructural effects of varying melt-pool cooling rates during the electron-beam powder-bed process, because part temperature can vary during a build.

“Heat is generated as the part builds,” Rollett says, “which causes lower cooling rates at the top of the part than at the bottom, and a corresponding difference in material grain size and properties.”

Another Ph.D. student, Brian Fisher, collaborating with the University of Texas El Paso, has demonstrated that with a dependable thermal-feedback control system, build parameters can be adjusted to control the cooling rates throughout the build and maintain consistent grain size.

Making Magic in the Lab

The NextManufacturing Center team also is hard at work developing process maps to allow aerospace manufacturers, and other adopters of metal-additive manufacturing, some leeway in how they operate their machines. As explained by the center’s executive director Sandra DeVincent Wolf, the machines—laser and electron-beam—can do much more than they currently are asked to do.

“The process-variable sets being used are very narrow,” Wolf explains. “We’re showing that it’s possible to work with a wider range of parameters—beam power and travel speed—and manipulate the process to achieve very specific outcomes. Within 5 yr., we expect that this will become an important skill set in industry.”

Two important aspects of bringing this work to life: controlling porosity during the build; and allowing manufacturers to work with a wider variety of powders—varying in particle size and shape—and still yield acceptable parts.

“We’ve identified the parameter windows for beam power and diameter, speed, layer thickness, path spacing and local part temperature that users need to stay within in order to control porosity,” says Rollett. “Research on porosity control has the potential to eliminate concerns with porosity across powder systems and alloys.”

We saw evidence of this development work during our tour of the NextManufacturing Center, home to two Arcam S12 electron-beam metal-printing machines, an EOS M 290 laser-sintering metal machine and an ExOne Innovent binder-jetting machine. Also in the lab are polymer printers (Objet and Stratasys models) and finishing equipment, including a GF Machining Solutions wire-EDM machine. In particular, CMU mechanical-engineering Ph.D.-candidate student Zack Francis explained to us the impact of beam-spot size, power and speed on the melt pool and on part quality. Also noted: how high-speed imaging can be used to better understand what goes on during the build.

Most recently, CMU mechanical-engineering Ph.D. student Sneha Narra explained, researchers have begun to understand how to vary process parameters at different locations in the build to customize material characteristics—more ductile in one area of a part, for example, and harder or more impact-resistant in another area. And, fellow mechanical-engineering Ph.D. student Colt Montgomery is looking closely at a fairly common concern regarding 3D metal printing: the challenges of printing overhanging features. A key variable here: being able to change beam-spot size during the build, which gives the user more flexibility with power and velocity. (The students note that not every 3D metal-printing machine offers this capability.)

The NextManufacturing Center facility is home to two Arcam S12 electron-beam metal-printing machines, an EOS M290 laser-sintering metal machine and an ExOne Innovent binder-jetting machine. Also in the lab are polymer printers (Objet and Stratasys models) and finishing equipment, including a GF Machining Solutions wire-EDM machine.

New Powder Alloys on the Horizon

It’s also important to understand and prepare for the coming increase in the types of alloys available for 3D metal printing. For the most part, explains Wolf, manufacturers are printing with the same alloys that have been developed for other processes, such as casting and thermal-spray coating. However, that soon may change, she says, thanks in part to a new project, led by CMU assistant professor of materials science and engineering Bryan Webler, to develop new powder alloys designed specifically to take advantage of the high cooling rates of additive manufacturing.

“These new alloys will have unique material properties,” Wolf says, “combining strength, corrosion resistance, creep resistance, etc. Aerospace companies and others surely will recognize huge opportunities to improve part performance using these new alloys, which should become available within the next 10 yr.”

Computed Tomography Paints Porosity Portraits

A large area of focus for the NextManufacturing Center is powder characterization and porosity, and defect analysis. Among the goals: understanding better how process variables impact part quality, and developing practical guidelines for setting limits on defect size and population so that users can then optimize their processes to work within those boundaries.

Walking me through the process of part-microstructure imaging and analysis in the lab were CMU materials science and engineering Ph.D. students Ross Cunningham and Samikshya Subedi. They’re using the latest in supercomputing tools to analyze data collected at the national synchrotron-radiation light-source research facility at Argonne National Laboratory in Chicago, IL.

“Part of our work here centers on powder characterization,” Cunningham explains. “We’re learning how the raw materials used impact part properties and defects. From the 3D images and models, we’re learning how powder porosity transfers to the parts, and have been able to share that knowledge with the powder suppliers. This work also supports the process-mapping projects underway at the NextManufacturing Center—looking at porosity results as the process changes.

“So, you might optimize the process for build rate or part tolerances,” Cunningham continues, “but the map might not be optimized for defect population. By better understanding this variable we can help users of the technology better balance their processes accounting for all of the build properties.”

Computer Vision Comes to Additive Manufacturing

Lastly, we sat with Elizabeth Holm, CMU professor of materials Science and engineering, to discuss work she’s leading for the NextManufacturing Center on automating the process of analyzing microstructural images. Capitalizing on concepts of computer vision, which enables computers to interpret visual images, this technology allows researchers to quickly and accurately analyze data-laden images and organize them by specific features—much more efficiently than can humans.

“The scans become fingerprints that represent the visual content of a material,” Holm explains. “Then, we can use that content and do many things that impact additive manufacturing. We can, for example, classify images by the material being processed, and what the processing conditions were that yielded the images. Another application involves characterizing the metal powders that we’re printing with. We then can ask, ‘Does this batch of powder match the previous batch?’ or ‘How has recycling changed this powder?’ In this way, we can train the system to recognize the powders used most often, quickly evaluate and classify a new batch of powder, and use that data to allow for timely adjustments to be made to the machine setup and processing parameters.”

Another application, of particular interest to the Federal Aviation Administration (FAA), is the ability to use microstructure images—taken, for example, of 3D-printed metal parts—to classify the success of a build.

“The FAA, Holm says, “would like to be able to scan 3D-printed structures and quickly prove-out a process based on this imagery and analysis. The industry needs to be able to efficiently classify successful build microstructures. Today this is done subjectively, by manually examining properties such as grain size and orientation. Our work in computer vision will allow this process to be automated, allowing the FAA and others to efficiently qualify builds, write specifications and establish visual tolerances.

“All of this,” concludes Holm, “opens up new avenues for technology development, and material scientists and designers will be working hand in hand to help 3D metal-printing processes evolve.”

Seen here with CMU Ph.D. students Ross Cunningham and Tugce Ozturk, NextManufacturing Center associate director Tony Rollett (top) oversees research and development efforts focused on powder characterization and properties. At their disposal: a state-of-the-art computer tomography lab, where NextManufacturing Center projects have addressed, among other issues, how powder porosity transfers to parts and how to alter process maps to optimize defect population in 3D-printed metal parts.