Oscillations of the keyhole in penetration laser beam welding

Many papers have sought correlations between the parameters of secondary particles generated above the beam/work piece interaction zone, dynamics of processes in the keyhole, and technological processes. Low-and high-frequency oscillations of the current, collected by plasma have been observed above the welding zone during electron beam welding. Low-frequency oscillations of secondary signals are related to capillary instabilities of the keyhole, however; the physical mechanisms responsible for the high-frequency oscillations (>10 kHz) of the collected current are not fully understood. This paper shows that peak frequencies in the spectra of the collected high-frequency signal are dependent on the reciprocal distance between the welding zone and collector electrode. From the relationship between current harmonics frequency and distance of the collector/welding zone, it can be estimated that the draft velocity of electrons or phase velocity of excited waves is about 1600 m/s. The dispersion relation with the properties of ion-acoustic waves is related to electron temperature 10 000 K, ion temperature 2 400 K and plasma density 10 16 m −3 , which is analogues to the parameters of potential-relaxation instabilities, observed in similar conditions. The estimated critical density of the transported current for creating the anomalous resistance state of plasma is of the order of 3 A · m −2 , i.e. 8 mA for a 3-10 cm 2 collector electrode. Thus, it is assumed that the observed high-frequency oscillations of the current collected by the positive collector electrode are caused by relaxation processes in the plasma plume above the welding zone, and not a direct demonstration of oscillations in the keyhole. C 2014 Author(s)

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“…[22][23][24][25][26] These auto-oscillations are of stochastic nature and are determined by the internal properties of the system beam/keyhole.…”

Section: Introductionmentioning

confidence: 99%

Current-driven ion-acoustic and potential-relaxation instabilities excited in plasma plume during electron beam welding

et al. 2014

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“…The high stability of the output radiation is one of the important advantages of high power fiber lasers in comparison with CO 2 -lasers, as it is well known that even small variations of the laser output power in the kHz-range of the frequency can lead to resonance generation of the melt pool macro oscillations and form defects in the weld seam [29,30]. This problem can be especially important in case of thick metal welding with a laser power of 10 kW and more, when the window of the technological parameters becomes narrow.…”

Section: Plume Attenuation Of Fiber Laser Radiationmentioning

confidence: 99%

Plume attenuation of laser radiation during high power fiber laser welding

et al. 2011

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Abstract:The results of an in-situ plume-laser interaction measurement during welding of mild steel with a 5 kW ytterbium fiber laser are reported. A measurement of the attenuation of probe laser beam passing through the plume has allowed to estimate the plume characteristics like the size of the extinction area and the spatial distribution of the extinction coefficient. The power loss of the fiber laser radiation propagating through the whole plume length was calculated. Together with a measured temporal characteristics of extinction the result indicates a significant decreasing of the laser radiation stability, which can lead to the formation of the macroscopic welding defects. Verification of the extinction coefficient distribution model with experimental data of the extinction signal depending on the plume height

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“…The physical phenomena taking place in the pool of molten material, such as heat conduction, melting, evaporation, and solidi"cation of material, #uid #ow, generation of plasma, etc., determine the dynamics of laser-beam welding [8,9]. By varying the power density supplied to the workpiece by the laser beam, various welding regimes can be achieved.…”

Section: Laser-beam Weldingmentioning

confidence: 99%

“…High power densities are required for deep-penetration welding. The energy supplied to the workpiece causes intense vaporization of the material, such that a capillary known as a keyhole is created in the molten pool [8,9]. The keyhole enables deeper penetration of the laser beam into the material, which deepens the molten pool.…”

Section: Laser-beam Weldingmentioning

confidence: 99%