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1. A Numerical Study on the Keyhole Formation During Laser Powder Bed Fusion Process

   https://doi.org/10.1115/1.4044100  

   Shrestha, Subin; Kevin Chou, Y. (October 2019, Journal of Manufacturing Science and Engineering)

   The dynamic phenomenon of a melt pool during the laser powder bed fusion (LPBF) process is complex and sensitive to process parameters. As the energy density input exceeds a certain threshold, a huge vapor depression may form, known as the keyhole. This study focuses on understanding the keyhole behavior and related pore formation during the LPBF process through numerical analysis. For this purpose, a thermo-fluid model with discrete powder particles is developed. The powder distribution, obtained from a discrete element method (DEM), is incorporated into the computational domain to develop a 3D process physics model using flow-3d. The melt pool formation during the conduction mode and the keyhole mode of melting has been discerned and explained. The high energy density leads to the formation of a vapor column and consequently pores under the laser scan track. Further, the keyhole shape resulted from different laser powers and scan speeds is investigated. The numerical results indicated that the keyhole size increases with the increase in the laser power even with the same energy density. The keyhole becomes stable at a higher power, which may reduce the occurrence of pores during laser scanning. 

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2. Quantitative analysis of in-situ synchrotron X-ray imaging for crack propagation in laser powder bed fusion of aluminum alloy

   Roney, Md; Ali, M; Quan, P; Thampy, V; Tassone, C; Liu, S (July 2026, Manufacturing letters)

   Laser powder bed fusion (LPBF) is a metal additive manufacturing technique that enables the production of high-performance, geometrically complex parts by selectively melting and solidifying fine metal powders under a laser source. However, the LPBF process is often accompanied by the formation of internal defects such as lack-of-fusion pores, keyhole pores, gas porosity nucleation, and vapor-induced porosity, and cracks, which can significantly deteriorate the mechanical integrity and reliability of the fabricated components. Among these defects, cracks remain one of the least investigated due to the challenges associated with capturing their rapid formation and propagation during the LPBF process. To observe crack initiation and propagation in real time, in-situ synchrotron X-ray imaging was employed to capture high-speed videos of the LPBF process of aluminum (Al) alloy. The experiment was conducted at the Stanford Synchrotron Radiation Lightsource (SSRL) to observe Al-6061 alloy LPBF process melt pool and crack formation dynamics with in-situ synchrotron X-ray imaging at 20 kfps with 2 µm spatial resolution and capture the transient melt pool and crack dynamics. The captured X-ray videos were analyzed through quantitative grayscale pixel-intensity mapping to identify crack initiation sites and analyze their propagation behavior. This robust analysis visualizes (i) the timing of crack initiation, (ii) the direction of crack propagation, and (iii) the relationship between process parameters and crack formation. Two key observations were made: (i) cracks predominantly initiate within the melt pool length and propagate vertically upward along the solidification direction, and (ii) at higher laser powers, the upward-propagating cracks develop secondary side branches inclined at distinct angles to the main crack path. These findings provide critical insight into the mechanisms of crack initiation and propagation and provide foundation for developing predictive models and scan strategies to mitigate defect formation in LPBF Al alloy printing. 

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3. Critical instability at moving keyhole tip generates porosity in laser melting

   https://doi.org/10.1126/science.abd1587  

   Zhao, Cang; Parab, Niranjan D.; Li, Xuxiao; Fezzaa, Kamel; Tan, Wenda; Rollett, Anthony D.; Sun, Tao (November 2020, Science)

   null (Ed.)

   Laser powder bed fusion is a dominant metal 3D printing technology. However, porosity defects remain a challenge for fatigue-sensitive applications. Some porosity is associated with deep and narrow vapor depressions called keyholes, which occur under high-power, low–scan speed laser melting conditions. High-speed x-ray imaging enables operando observation of the detailed formation process of pores in Ti-6Al-4V caused by a critical instability at the keyhole tip. We found that the boundary of the keyhole porosity regime in power-velocity space is sharp and smooth, varying only slightly between the bare plate and powder bed. The critical keyhole instability generates acoustic waves in the melt pool that provide additional yet vital driving force for the pores near the keyhole tip to move away from the keyhole and become trapped as defects. 

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4. Process-microstructure relationship of laser processed thermoelectric material Bi2Te3

   https://doi.org/10.3389/femat.2022.1046694  

   Oztan, Cagri; Şişik, Bengisu; Welch, Ryan; LeBlanc, Saniya (December 2022, Frontiers in Electronic Materials)

   Additive manufacturing allows fabrication of custom-shaped thermoelectric materials while minimizing waste, reducing processing steps, and maximizing integration compared to conventional methods. Establishing the process-structure-property relationship of laser additive manufactured thermoelectric materials facilitates enhanced process control and thermoelectric performance. This research focuses on laser processing of bismuth telluride (Bi 2 Te 3 ), a well-established thermoelectric material for low temperature applications. Single melt tracks under various parameters (laser power, scan speed and number of scans) were processed on Bi 2 Te 3 powder compacts. A detailed analysis of the transition in the melting mode, grain growth, balling formation, and elemental composition is provided. Rapid melting and solidification of Bi 2 Te 3 resulted in fine-grained microstructure with preferential grain growth along the direction of the temperature gradient. Experimental results were corroborated with simulations for melt pool dimensions as well as grain morphology transitions resulting from the relationship between temperature gradient and solidification rate. Samples processed at 25 W, 350 mm/s with 5 scans resulted in minimized balling and porosity, along with columnar grains having a high density of dislocations. 

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5. High frequency beam oscillation keyhole dynamics in laser melting revealed by in-situ x-ray imaging

   https://doi.org/10.1038/s43246-023-00332-z  

   Wu, Ziheng; Tang, Guannan; Clark, Samuel J.; Meshkov, Andrey; Roychowdhury, Subhrajit; Gould, Benjamin; Ostroverkhov, Victor; Adcock, Thomas; Duclos, Steven J.; Fezzaa, Kamel; et al (December 2023, Communications Materials)

   Abstract The metal additive manufacturing industry is actively developing instruments and strategies to enable higher productivity, optimal build quality, and controllable as-built microstructure. A beam controlling technique, laser oscillation has shown potential in all these aspects in laser welding; however, few attempts have been made to understand the underlying physics of the oscillating keyholes/melt pools which are the prerequisites for these strategies to become a useful tool for laser-based additive manufacturing processes. Here, to address this gap, we utilized a synchrotron-based X-ray operando technique to image the dynamic keyhole oscillation in Ti-6Al-4V using a miniature powder bed fusion setup. We found good agreement between the experimental observations and simulations performed with a validated Lattice Boltzmann multiphysics model. The study revealed the continuous and periodic fluctuations in the characteristic keyhole parameters that are unique to the oscillating laser beam processing and responsible for the chevron pattern formation at solidification. In particular, despite the intrinsic longer-range fluctuation, the oscillating technique displayed potential for reducing keyhole instability, mitigating porosity formation, and altering surface topology. These insights on the oscillating keyhole dynamics can be useful for the future development and application of this technique. 

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