The presence of tiny gas pockets in the end product, which can result in cracks and other failures, is a prevalent issue that inhibits the potential of additive manufacturing to transform the industry.
New study reported in Science on February 27th, 2019, led by scientists from Carnegie Mellon University and Argonne National Laboratory, has identified how and when these gas pockets develop and also created a technology to predict their formation—a key finding that could radically enhance the 3D printing process.
The research in this paper will translate into better quality control and better control of working with the machines. For additive manufacturing to really take off for the majority of companies, we need to improve the consistency of the finished products. This research is a major step in that direction.
Anthony Rollett, Professor, Materials Science and Engineering, Carnegie Mellon University.
The researchers used the very bright high-energy X-rays at Argonne’s Advanced Photon Source (APS), a Department of Energy Office of Science User Facility, to capture super-fast video and images of a process known as Laser Power Bed Fusion (LPBF), which involves using lasers to melt and fuse material powder together.
The lasers—which scan across each layer of powder to fuse metal where it is required—actually produce the finished product from the bottom to top. Defects are formed when pockets of gas are entrapped into these layers, leading to flaws that could result in cracks or other breakdowns in the end product.
So far, manufacturers and scientists were not sure about how the laser penetrates into the metal, creating cavities known as “vapor depressions”; however, they supposed that the strength of laser or the kind of metal powder was responsible for such cavities. Consequently, manufacturers have been employing a trial-and-error method with various kinds of metals and lasers to find a way to mitigate the defects.
Actually, the study demonstrates that these vapor depressions are present under almost all conditions in the process, regardless of the metal or laser. What is increasingly important is that the study illustrates how to predict when a small depression will develop into a large and unstable one that can potentially cause a defect.
“We’re drawing back the veil and revealing what’s really going on,” stated Rollett, who is a co-director of the NextManufacturing Center at Carnegie Mellon. “Most people think you shine a laser light on the surface of a metal powder, the light is absorbed by the material, and it melts the metal into a melt pool. In actuality, you’re really drilling a hole into the metal.”
Scientists used highly specialized equipment at Argonne’s APS—one of the most powerful synchrotron facilities in the world—to observe what happens when the laser moves over the metal powder bed to form each layer of the product.
Under ideal conditions, the shape of the melt pool is semicircular and shallow—known as the “conduction mode.” However, at the time of the actual printing process, the high-power laser, usually moving at a low speed, can modify the melt pool shape to something resembling a keyhole in a warded lock: big and round on top, with a narrow spike at the bottom. Such “keyhole mode” melting can potentially cause defects in the end product.
Based on this research, we now know that the keyhole phenomenon is more important, in many ways, than the powder being used in additive manufacturing. Our research shows that you can predict the factors that lead to a keyhole—which means you can also isolate those factors for better results.
Ross Cunningham, Graduate, Carnegie Mellon University.
Cunningham is also one of the co-first authors of this study.
The study reveals that keyholes are created on reaching a particular laser power density that is sufficient to boil the metal. This consecutively shows the vital significance of the laser focus in the additive manufacturing process, an element that has not received much attention to date, say the researchers.
The keyhole phenomenon was able to be viewed for the first time with such details because of the scale and specialized capability developed at Argonne. The intense high-energy X-ray beam at the APS is key to discoveries like this.
Tao Sun, Author and Physicist, Argonne National Laboratory.
The experiment platform that aids the study of additive manufacturing consists of a laser apparatus, devoted beamline instruments, and specialized detectors.
In 2016, the Argonne team, along with their research associates, for the first time took the X-ray video of laser additive manufacturing at micrometer and microsecond scales. That research augmented curiosity in the effect of Argonne’s APS on manufacturing methods and challenges.
We are really studying a very basic science problem, which is what happens to metal when you heat it up with a high-power laser. Because of our unique experimental capability, we are able to work with our collaborators on experiments that are really valuable to manufacturers.
Cang Zhao, Postdoc, Argonne National Laboratory.
Zhao is also the co-first author of the study.
The researchers hope that this research could inspire manufacturers of additive manufacturing machines to provide more flexibility when controlling the machines and that the improved use of the machines could result in a considerable enhancement in the end product. Furthermore, if these understanding are acted upon, the process for 3D printing could be made more rapid.
“It’s important because 3D printing, in general, is rather slow,” Rollett said. “It takes hours to print a part that is a few inches high. That’s OK if you can afford to pay for the technique, but we need to do better.”
Co-lead authors are Ross Cunningham from Carnegie Mellon University and Cang Zhao from Argonne National Laboratory; other authors from Carnegie Mellon are Christopher Kantzos and Joseph Pauza; other authors from Argonne are Niranjan Parab and Kamel Fezzaa.
(Video credit: Carnegie Mellon University College of Engineering)