Keyhole threshold and morphology in laser melting revealed by ultrahigh-speed x-ray imaging

Cunningham, Ross; Zhao, Cang; Parab, Niranjan D.; Kantzos, Christopher; Pauza, Joseph; Fezzaa, Kamel; Sun, Tao; Rollett, Anthony D.

doi:10.1126/science.aav4687

Cited by 733 publications

(328 citation statements)

References 26 publications

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“…The experimental schematic is shown in Fig. S1 of the Supplemental Material [6], with the details in Appendix A [7,23,24]. Figure 1 and Supplemental Material Fig.…”

Section: A Mhz Synchrotron-x-ray Imaging Of Laser-induced Spatteringmentioning

confidence: 99%

Bulk-Explosion-Induced Metal Spattering During Laser Processing

Zhao

Guo

et al. 2019

Phys. Rev. X

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Spattering has been a problem in metal processing involving high-power lasers, like laser welding, machining, and recently, additive manufacturing. Limited by the capabilities of in situ diagnostic techniques, typically imaging with visible light or laboratory x-ray sources, a comprehensive understanding of the laser-spattering phenomenon, particularly the extremely fast spatters, has not been achieved yet. Here, using MHz single-pulse synchrotron-x-ray imaging, we probe the spattering behavior of Ti-6Al-4V with micrometer spatial resolution and subnanosecond temporal resolution. Combining direct experimental observations, quantitative image analysis, as well as numerical simulations, our study unravels a novel mechanism of laser spattering: The bulk explosion of a tonguelike protrusion forming on the front keyhole wall leads to the ligamentation of molten metal at the keyhole rims and the subsequent spattering. Our study confirms the critical role of melt and vapor flow in the laser-spattering process and opens a door to manufacturing spatter-and defect-free metal parts via precise control of keyhole dynamics.

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“…The experimental schematic is shown in Fig. S1 of the Supplemental Material [6], with the details in Appendix A [7,23,24]. Figure 1 and Supplemental Material Fig.…”

Section: A Mhz Synchrotron-x-ray Imaging Of Laser-induced Spatteringmentioning

confidence: 99%

Bulk-Explosion-Induced Metal Spattering During Laser Processing

Zhao

Guo

et al. 2019

Phys. Rev. X

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“…Recent work has used synchrotron X‐ray imaging to probe how melt‐pool behavior relates to spatter and melt‐pool dynamics in conditions approximating overhang regions, which are particularly defect‐prone regions in bulk part builds . Cunningham et al imaged the melt pool and vapor depression on bare Ti64 using synchrotron‐based X‐rays to study how the power and velocity of the incident laser beam affected the heat‐transfer mode of the sample (i.e., conduction or keyhole mode) and its relation to laser drilling speed, vapor depression dynamics, and front keyhole wall angle . Herein, we utilize high‐speed X‐ray imaging to quantify subsurface‐defect formation and dynamics during the LPBF process in Ti64.…”

Section: Introductionmentioning

confidence: 99%

Laser‐Induced Keyhole Defect Dynamics during Metal Additive Manufacturing

et al. 2019

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Laser powder bed fusion (LPBF) metal additive manufacturing provides distinct advantages for aerospace and biomedical applications. However, widespread industrial adoption is limited by a lack of confidence in part properties driven by an incomplete understanding of how unique process parameters relate to defect formation and ultimately mechanical properties. To address that gap, high‐speed X‐ray imaging is used to probe subsurface melt pool dynamics and void‐formation mechanisms inaccessible to other monitoring approaches. This technique directly observes the depth and dynamic behavior of the vapor depression, also known as the keyhole depression, which is formed by recoil pressure from laser‐driven metal vaporization. Also, vapor bubble formation and motion due to melt pool currents is observed, including instances of bubbles splitting before solidification into clusters of smaller voids while the material rapidly cools. Other phenomena include bubbles being formed from and then recaptured by the vapor depression, leaving no voids in the final part. Such events complicate attempts to identify defect formation using surface‐sensitive process‐monitoring tools. Finally, once the void defects form, they cannot be repaired by simple laser scans, without introducing new defects, thus emphasizing the importance of understanding processing parameters to develop robust defect‐mitigation strategies based on experimentally validated models.

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“…In some AM technologies which use high-density powers, material might be deposited or melted in keyhole mode, which is a melt pool with a deep and narrow shape. Cunningham et al [8] showed the threshold, morphology, and different stages of keyhole formation through X-ray imaging, and suggested that different combinations of laser power density and scanning velocity might result in the formation of keyholes; 2.…”

Section: Introductionmentioning

confidence: 99%

Influences of Horizontal and Vertical Build Orientations and Post-Fabrication Processes on the Fatigue Behavior of Stainless Steel 316L Produced by Selective Laser Melting

et al. 2019

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In this paper, the influences of build orientation and post-fabrication processes, including stress-relief, machining, and shot-peening, on the fatigue behavior of stainless steel (SS) 316L manufactured using selective laser melting (SLM) are studied. It was found that horizontally-built (XY) and machined (M) test pieces, which had not been previously studied in the literature, in both stress-relieved (SR) or non-stress-relieved (NSR) conditions show superior fatigue behavior compared to vertically-built (ZX) and conventionally-manufactured SS 316L. The XY, M, and SR (XY-M-SR) test pieces displayed fatigue behavior similar to the XY-M-NSR test pieces, implying that SR does not have a considerable effect on the fatigue behavior of XY and M test pieces. ZX-M-SR test pieces, due to their considerably lower ductility, exhibited significantly larger scatter and a lower fatigue strength compared to ZX-M-NSR samples. Shot-peening (SP) displayed a positive effect on improving the fatigue behavior of the ZX-NSR test pieces due to a compressive stress of 58 MPa induced on the surface of the test pieces. Fractography of the tensile and fatigue test pieces revealed a deeper understanding of the relationships between the process parameters, microstructure, and mechanical properties for SS 316L produced by laser systems. For example, fish-eye fracture pattern or spherical stair features were not previously observed or explained for cyclically-loaded SLM-printed parts in the literature. This study provides comprehensive insight into the anisotropy of the static and fatigue properties of SLM-printed parts, as well as the pre- and post-fabrication parameters that can be employed to improve the fatigue behavior of steel alloys manufactured using laser systems.

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Keyhole threshold and morphology in laser melting revealed by ultrahigh-speed x-ray imaging

Cited by 733 publications

References 26 publications

Bulk-Explosion-Induced Metal Spattering During Laser Processing

Bulk-Explosion-Induced Metal Spattering During Laser Processing

Laser‐Induced Keyhole Defect Dynamics during Metal Additive Manufacturing

Influences of Horizontal and Vertical Build Orientations and Post-Fabrication Processes on the Fatigue Behavior of Stainless Steel 316L Produced by Selective Laser Melting

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