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Laser-metal interaction process scrutinized with X-ray movies

18 Jun 2019

US-based researchers find microscopic manufacturing flaws and suggest routes to improvement of welding and treatments.

Microscopic defects that occur in laser-based manufacturing of metal parts can lead to significant problems if undetected. The process of fixing these flaws can increase the time and cost of high-tech manufacturing. But now, new research into the cause of these flaws could lead to a solution.

Researchers from Missouri S&T, Argonne National Laboratory, Lemont, IL, and the University of Utah, Salt Lake City, UT, created high-speed X-ray movies of a manufacturing phenomenon known as laser spattering.

Laser spattering refers to the ejection of molten metal from a pool heated by a high-power laser during laser-based manufacturing processes, such as laser welding and laser-additive manufacturing.

Such laser manufacturing technologies are used to fabricate parts aerospace and automotive industries, healthcare and in construction. The researchers describe their findings in the journal Physical Review X.

Using X-ray imaging, the researchers captured the spattering behavior of titanium alloy Ti-6Al-4V during fabrication. The microscopic movies reveal a novel mechanism of laser spattering – the bulk explosion of a tongue-like protrusion that forms in one region of the metal, the researchers say in their paper, titled “Bulk explosion induced metal spattering during laser processing.”

The paper explains this observation thus, “We attribute this bulk explosion phenomenon to the large thermal fluctuation inside the tonguelike protrusion. Superficially, it possesses some of the key characteristics of the well-defined phase explosion (aka explosive boiling) and vapor explosion (aka steam explosion and fuel coolant interactions) processes.”

Dr. Lianyi Chen, assistant professor of mechanical and aerospace engineering at Missouri S&T and one of the paper’s corresponding authors, commented, “The newly discovered mechanism will guide the development of approaches to mitigate defect formation in welds and additively manufactured parts.”

Chen collaborated with Dr. Tao Sun’s team at Argonne and Dr. Wenda Tan’s team at Utah on the research. The group created the images through the use of a high-energy synchrotron X-ray at Argonne along with image analysis and numerical simulations.

“The high penetration power of hard X-rays and high resolutions of the imaging technique enable us, for the first time ever, to connect the spattering behavior above the surface with dynamics below the surface and inside the titanium sample,” said Chen.

Working with Chen on the research is Qilin Guo, a Ph.D. student in mechanical engineering at Missouri S&T.

About spattering

Spattering has been a problem in metal processing involving high-power lasers, like laser welding, machining, and recently, additive manufacturing. Conventionally, understanding of this problem has been limited by the capabilities of in situ diagnostic techniques, typically imaging with visible light or laboratory x-ray sources, so prior to this development comprehensive understanding of the laser-spattering phenomenon had not been achieved.

The Physical Review X paper concludes, “We have discovered a novel mechanism for metal spattering under high-power-laser processing: The bulk explosion of a tonguelike protrusion forming on the front keyhole wall drives the melt ligamentation around the keyhole rims and the subsequent spattering. Using MHz single-pulse synchrotron-x-ray imaging, the complete physical processes involved in a spattering activity are captured with unprecedented detail.”

Considering the possible routes to improvement of laser-metal interactions, the paper concludes, “Unambiguously, spattering tends to happen when strong melt flow and intense vapor exist. Therefore, an effective approach to mitigate (if not exterminate) spattering is to suppress the melt flows around the keyhole. In practice, this could be achieved through innovations in laser-beam profiles and in-process feed-forward control systems.”

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