Past work to determine the cause for the formation of pores has focused on the keyhole mode of melting.
“A laser acts as a high-intensity drill by focusing light into a metal,” Rollett explains. “The result can be drilling slightly more than 100 microns into the metal powder, which creates a deep and narrow cavity known as a keyhole.”
Rollett points out that while keyholes do not contribute to the formation of metal fatigue cracks, they can lead to the production of pores. Fear not, however—work is underway to better understand how the generation of keyholes can lead to the production of pores.
Operando High-Speed X-Ray Imaging
In their research, Rollett and his colleagues applied a laser beam along a straight line to a sample layer of Ti-6Al-4V powder sandwiched between two glassy carbon plates. By varying the power of the laser and the scan speed across the metal surface, they developed a power-velocity (PV) map to determine the regions where pore formation was occurring.
“The two most important parameters that need to be evaluated in LPBF are power and scan speed,” Rollett says. “These two characteristics provide a good view as to how the 3D printing process is progressing.” Rollett selected Ti-6Al-4V powder for the study due to its prevalence in aerospace and medical applications.
The researchers used a technique known as operando high-speed X-ray imaging to better understand how pore formation occurs, based on instabilities found in the keyholes.
“Operando high-speed X-ray imaging generates extremely bright high-energy X-rays that can be used to produce movies at high speed, showing pore formation,” says Rollett. “This technique enables us to follow the process under the microscope at a microsecond time scale.”
The PV map generated during the study illustrated a very defined boundary between an area of stable melting and a region where pores are formed from keyholes. “Scanning at slower speeds,” Rollett explains, “can produce pores regardless of the power generated by the laser.”
The accompanying figure displays a series of megahertz X-ray images showing the formation of a keyhole. As shown in the first image (at 0.0 μsec), as the keyhole fluctuates the tip forms a J-shape that pinches off to form a pore (the image at 2.76 μsec). The bottom row of images illustrates continued pore formation.
“The keyhole is suspended by the liquid metal in an unstable environment that leads to oscillations of a magnitude of 100 kHz and the formation of acoustic waves,” says Rollett. “This results in the pores being pushed away from the keyhole into the body of the metal where they become trapped after solidification and can initiate fatigue cracks, causing premature failure.”
This research provides a foundation for establishing specific laser-power and scan-speed conditions to minimize the formation of pores from the keyhole mode of melting.
“We have developed a qualification procedure that can be used with specific metals to minimize defect formation through the use of specific qualification protocols,” Rollett concludes. 3DMP
Neil Canter heads Chemical Solutions, Willow Grove, PA. This article was edited and reprinted with permission from the February 2021 issue of Tribology and Lubrication Technology (TLT), the monthly magazine of the Society of Tribologists and Lubrication Engineers (STLE), Park Ridge, IL.
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