An aerospace engineering professor at the University of Cincinnati has come up with a cool new approach for inspecting metal 3D printed parts and identifying defects in their structure. The technique involves a few key ingredients, including one part 3D printing, a dash of ultrasonic waves and ice.
Francesco Simonetti, the aerospace engineer behind the novel approach, is an expert in sound waves and recently turned his research focus to ice. As part of this research, the engineering professor realized the way that sound waves interact with ice could offer a new way to inspect metal 3D printed components.
In short, the approach consists of dipping a 3D printed part in water and freezing it inside a cylinder of ice. The iced part is then exposed to ultrasonic waves. The ice acts as a coupling medium while the ultrasound waves enter and reflect against the part’s defects if there are any.
A song of ice and sound waves
The innovative field of research, pioneered by Simonetti, has been dubbed cryoutrasonics by the professor, who believes the grouping of ultrasound and ice will have an impact on the additive manufacturing industry because of its ability to inspect safety-critical parts, such as jet engines or power plant components.
As the researcher explains, with subtractive manufacturing processes, ultrasound testing is effective for identifying defects. Because additive manufacturing relies on the layer-by-layer construction of parts, however, ultrasonic waves tend to bounce off the angles and curves of the parts instead of the cracks or defects, making it more challenging to use.
“Sound needs a coupling medium to propagate from a source transducer into the volume of a part,” said Simonetti. “When the contrast in mechanical properties between the coupling medium and the part is large, very little energy enters, and it doesn’t work.”
Water was initially explored as a coupling medium, but because of the difference between water’s mechanical properties and those of metals, it did not prove to be very effective. With water as a coupling medium, very little ultrasonic energy makes it to the part. Frozen water, however, turned out to be a more interesting option.
“Living in Cincinnati, you’re always removing ice from the drive way. I got curious to see what the ice properties were,” commented Simonetti. “We tried all the conventional techniques and nothing would work. At that point, we looked for desperate measures, and I just thought, ‘Why don’t we try?’”
Despite some initial challenges (Simonetti says the first attempts were “disastrous”), ice eventually proved to be an effective coupling medium for inspecting parts with ultrasonic waves. This is because ice’s physical properties are similar to those of metals, enabling the ultrasound waves to pass through the ice and metal part to pick up any defects.
As one might expect, the process isn’t quite as simple as filling up an ice cube tray. To get the best results, the ice has to be crystal clear—without any cracks or bubbles—so that the ultrasound waves don’t bounce around. The solution? A custom-built machine capable of freezing water without causing bubbles, cracks or cloudiness.
The custom machine was built by Simonetti and consists of a cylinder with a metal base and plastic sides. When making the ice, the researcher places the 3D printed part inside the cylinder and then fills it with water. Then, the metal base of the ice maker is chilled which causes the water to freeze from bottom to top (rather than from the outside in). As the water freezes around the part, it expands to the open top of the cylinder instead of outwards, reducing the risk of cracks.
To prevent bubbles from occurring, Simonetti needed to minimize the amount of excess air in the water as it froze. “In order to prevent this phenomenon, you need to simply reduce the concentration of air on top of the freeze front. To do that, we stir the water to have constant flow,” he said. This constant flow is achieved with the use of a spindle, which keeps the water it motion as it freezes.
A brand new approach
Currently, Simonetti is working on integrating nanoparticles into the ice to make it resemble the physical properties of a given metal part even more. This continuation of the research involves freezing suspensions of nanoparticles in water to make the ice denser, heavier and mechanically stronger.
“It’s entirely new,” he said. “Whenever you have something that is so novel, there are a lot of skeptics from the academic community. When you freeze water, it looks terrible. You think, ‘This is not going to work.’”
Though the approach is brand new, it shows real promise and has grabbed the attention of many, including engineering firms, car manufacturers and the military. Simonetti’s research was recently published in NDT&E International, a journal focused on research in nondestructive testing and evaluation methods.