While the structures produced through 3D printing can be fascinating, tangible, and extremely inspiring—not to mention actually useful—you may still find the operational processes behind the technology to be a little intimidating as it often seems the impossible is being fulfilled with a bit of filament, software, and hardware that may not even be that expensive. You may also find it surprising to know that even researchers extremely accustomed with 3D printing may still be in the dark about what happens during some of the internal processes too, especially in using laser/powder combinations.
To take a closer look, a research team combining forces from both Northwestern University and Argonne National Laboratory began examining exactly what happens during 3D printing rather than after, publishing their findings in ‘In-situ high-speed X-ray imaging of piezo-driven directed energy deposition additive manufacturing.’ Authors Sarah J. Wolff, Hao Wu, Niranjan Parab, Cang Zhao, Kornel F. Ehmann, Tao Sun, and Jian Cao point out that historically it has been challenging to monitor the internal processes in minute detail because any interruption of the process can be detrimental to the end product. And while we have followed many different instances of researchers analyzing 3D printing and unique materials, along with working to find improved strategies in AM, this study definitely has a different focus:
“Monitoring of advanced manufacturing processes to evaluate changes in thermal history, structure and properties is crucial for an understanding of the physical phenomena that occur during the process and for closed-loop control of built properties in AM,” state the researchers.
Directed energy deposition relies on heated powder particles, melted to create the layers of a structure. This method of fabrication has been growing in popularity, especially in rapid prototyping and parts maintenance. Benefits include faster and better solidification of printed objects, along with flexibility in types and use of materials.
“Due to the complexity of deposition of powders and their interactions in DED, however, monitoring the influence of individual particles on the melt pool and the resulting build is challenging,” state the researchers. “In this study, a low-cost piezo-driven powder delivery system is used to deposit individual particles as they interact with a moving laser beam as means to capture the underlying physics of laser-matter interaction during DED using high-speed X-ray imaging.”
Although piezo-electrics may be a term new to many, it involves the release of a charge when under pressure or other exertion. We followed a study regarding piezoelectric materials earlier this year, in regards to their potential in 3D printing for developing a variety of different sensors and smart materials. In this more recent study, the researchers are interested in studying piezo-directed energy with X-rays to understand more about how lasers and powder interact during 3D printing.
For this experiment, the team designed a sealed chamber, encompassing the piezo-driven system and required argon gas.
“The triggering sequence begins with the actuation of the piezo element in the powder delivery system. The signal from the piezo element switches the laser on. The “on” signal from the laser begins the laser scan by moving the galvo scanner mirrors,” state the researchers regarding the observation system. “When the galvo mirrors are at a certain position that aligns the laser beam with the piezo-driven powder delivery system, the X-ray shutter open to allow the X-ray beam traverse into the chamber.”
Piezo vibration causes the powder to flow into the area where both the laser and X-ray beams line up. Specific parameters were set up for controlling the flow rate by controlling the frequency and power of piezo vibration. Any porosity was tracked and evaluated, along with spattering and associated particle ejection. The ultimate hope in performing such imaging experiments was to understand more about DED processes and the materials involved.
“High-speed X-ray images that reveal the laser-matter interactions in various modes of DED processing can aid in the validation of thermal, thermo-fluid dynamic and thermo-mechanical models,” state the researchers. “In the case of gravity-fed, low powder mass flow, the laser-induced vapor plume scatters particles away from the melt pool with velocities of up to 10 m/s.”
The researchers point out that most sensors are not capable of providing enough data, and do not possess the resolution necessary to show DED rapid cooling processes. In this study, they were able to pinpoint some of the reasons behind porosity but point out that more research is needed to understand particle entrainment.
“Controlling individual particle trajectories relative to a laser beam can lead to more conclusive observations about how particles enter the melt pool. This work reveals the necessity of an inert carrier gas to aid particle flow. Without carrier gas, most particles scatter away from the melt pool, whereas carrier gas allows particles to penetrate the laser-induced vapor-plasma plume,” conclude the researchers.
“Future work that investigates the influence of carrier gas pressure and velocity is required to capture the phenomena in more representative DED processing. Coupling high-resolution thermal monitoring can also aid in further understanding of cooling and more specifically, solidification behavior of the melt pool.”
Find out more about the materials and custom system used for testing here.
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