Ultrasonics to monitor liquid metal melt pool dynamics for improving metal 3D printing

Christopher Kube – kube@psu.edu
Twitter: @_chriskube

Penn State University, 212 Earth and Engineering Sciences Bldg, University Park, PA, 16802, United States

Tao Sun, University of Virginia
Samuel Clark, Advanced Photon Source, Twitter: @advancedphoton

Find the authors on LinkedIn:
www.linkedin.com/in/chriskube
www.linkedin.com/in/suntao

Popular version of 3pID2-Acoustics for in-process melt pool monitoring during metal additive manufacturing, presented at the 183rd ASA Meeting.

3D printed or additively manufactured (AM) metal parts are disrupting the status quo in a variety of industries including defense, transportation, energy, and space exploration. Engineers now design and produce customizable parts unimaginable only a decade ago. New geometrical or part shape freedom inherent to AM has already led to part performance often beyond traditionally manufactured counterparts. In the years to come, another revolutionary performance jump is expected by enabling the AM process to control the grain layout and structural features on the microscopic scale. Grains are the building blocks of metal parts that dictate many of the performance metrics associated with the descriptors of bigger, faster, and stronger.

The second performance revolution of AM metal parts requires uncovering new knowledge in the complicated physics present during the AM process. 3D printed metals are born from an energy source such as a laser or electron beam to selectively melt feedstock material at microscopic locations dictated by the computerized part drawing. Melted locations temporarily form liquid metal melt pools that solidify after the energy source moves to another location. Resulting grain structure and pore/defect formation strongly depends on how the melt pool cools and solidifies.

Over the past five years, high-energy X-rays only available at particle accelerators are used for direct real-time visualization of AM melt pool dynamics and solidification. Figure 1 shows an example X-ray frame, which captured a laser-generated melt pool moving in a single direction with a speed of 800 mm/ms.

This situation mimics the laser and melt pool movement found during 3D printing metal parts. Being able to directly observe melt pool behavior has led to new and improved understanding of the underlying physics. Unfortunately, experiments at such X-ray sources is difficult to ascertain because of extremely high demand across the sciences. Additionally, the measurement technique relegated to high-energy X-ray sources is not transferrable to metal 3D printers that exist in normal industrial settings. For these reasons, ultrasonics are being explored as a melt pool monitoring technology that can be deployed within real 3D printers.

Ultrasound is commonly used for imaging and detecting features inside of solid materials. For example, ultrasound is applied in medical settings during pregnancy or for diagnostics. Application of ultrasound for melt pool monitoring is made possible because of the tendency of ultrasound to scatter from the melt pool’s solid/liquid boundary. The development of the technique is being supported alongside X-ray imaging at the Advanced Photon Source at Argonne National Laboratory. X-ray imaging is providing the extremely important ground truth melt pool behavior allowing for easy interpretation of the ultrasonic response. In Figure 1, the ultrasonic response from the exact same melt pool given in the X-ray video is being shown for two different sensors. As the melt pool enters the field of view of the ultrasonic sensors (see online video), features in the ultrasound response confirms their sensitivity to the melt pool.

In this research, high-energy X-rays are being used to develop the ultrasonic technique and technology. In the coming year, the knowledge developed will be leveraged such that ultrasound can be applied on its own for melt pool monitoring in real metal 3D printers. Currently, no existing technology can capture the highly dynamic melt pool behavior through the depth of the part or substrate.

Practical benefits and value of melt pool monitoring within 3D printers are significant. Ultrasound can provide a quick check to determine the optimal laser power and speed combinations toward accelerated determination of process parameters. Currently, determination of the optimal process parameters requires destructive postmortem microscopy techniques that are extremely costly, time-consuming (sometimes more than a year), and wasteful. Ultrasound has the potential to reduce these factors by an order of magnitude. Furthermore, metal 3D printing processes are highly variable over many months, across different machines, and even when using feedstock powder from different suppliers. Ultrasonic melt pool monitoring can provide period checks to assure variability is minimized.

A moth’s ear inspires directional passive acoustic structures

Lara Díaz-García – lara.diaz-garcia@strath.ac.uk
Twitter: @laradigar23
Instagram: @laradigar

Centre for Ultrasonic Engineering, University of Strathclyde, Glasgow, Lanarkshire, G1 1RD, United Kingdom

Popular version of 2aSA1-Directional passive acoustic structures inspired by the ear of Achroia grisella, presented at the 183rd ASA Meeting.

When most people think of microphones, they think of the ones singers use or you would find in a karaoke machine, but they might not realize that much smaller microphones are all around us. Current smartphones have about three or four microphones that are small. The miniaturization of microphones is therefore a desire in technological development. These microphones are strategically placed to achieve directionality. Directionality means that the microphone’s goal is to discard undesirable noise coming from directions other than the speaker’s as well as to detect and transmit the sound signal. For hearing implant users this functionality is also desirable. Ideally, you want to be able to tell what direction a sound is coming from, as people with unimpaired hearing do.

But dealing with small size and directionality presents problems. People with unimpaired hearing can tell where sound is coming from by comparing the input received by each of our ears, conveniently sitting on opposite sides of our heads and therefore receiving sounds at slightly different times and with different intensities. The brain can do the math and compute what direction sound must be coming from. The problem is that, to use this trick, you need two microphones that are separated so the time of arrival and difference in intensity are not negligible, and that goes against microphone miniaturization. What to do if you want a small but directional microphone, then?

When looking for inspiration for novel solutions, scientists often look to nature, where energy efficiency and simple designs are prioritized in evolution. Insects are one such example that faces the challenge of directional hearing at small scales. The researchers have chosen to look at the lesser wax moth (fig 1), observed to have directional hearing in the 1980s. The males produce a mating call that the females can track even when one of their ears is pierced. This implies that, instead of using both ears as humans do, these moths’ directional hearing is achieved with just one ear.

Lesser wax moth specimen with scale bar. Image courtesy of Birgit E. Rhode (CC BY 4.0).

The working hypothesis is that directionality must be achieved by the asymmetrical shape and characteristics of the moth’s ear itself. To test this hypothesis, the researchers designed a model that resembles the moth’s ear and checked how it behaved when exposed to sound. The model consists of a thin elliptical membrane with two halves of different thicknesses. For it, they used a readily available commercial 3D printer that allows customization of the design and fabrication of samples in just a few hours. The samples were then placed on a turning surface and the behavior of the membrane in response to sound coming from different directions was investigated (fig 2). It was found that the membrane moves more when sound comes from one direction rather than all the others (fig 3), meaning the structure is therefore passively directional. This means it could inspire a single small directional microphone in the future.

Laboratory setup to turn the sample (in orange, center of the picture) and expose it to sound from the speaker (left of the picture). Researcher’s own picture.
Image adapted from Lara Díaz-García’s original paper. Sounds coming from 0º direction elicit a stronger movement in the membrane than other directions.