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

Abstract

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)

Use of a Secondary Current Sensor in Plasma during Electron-Beam Welding with Focus Scanning for Process Control

“…A comparative analysis between the model's amplitudes for 1 (21) and empirical values (24) confirmed the model's adequacy. The Fisher criterion [34] was 151, the value was 10 −10 , and the correlation coefficient was 0.84, which indicates A comparison of experimental data with the modeled results confirms the assumption that given high-frequency focus scanning the behavior of the change in the amplitude of the first harmonic 1 was caused by the extreme dependence of the magnitude of the flux density of the energy added by the electron beam on the focus current. It is also significant that a statistical treatment confirmed the modeled results, which indicate the weak influence of the scanning frequency in the range under consideration on the amplitude of the first harmonic.…”

“…The recommended distance from the collector to the welding zone is 20-50 mm, which lowers the level of ion-acoustic instability in plasma Figure 11: Functional diagram of the modified focus control system: 1, electron gun; 2, focus coil; 3, amplifier for the focus coil current; 4, electron collector; 5, modulation block; 6, bias voltage source; 7, load resistor; 8, high-frequency filter; 9, synchronous detection block for harmonic 1 ; 10, control block; 11, synchronous detection block for harmonic 1 ; 12, division block. [28,34], which arises when exciting a non-self-sustained discharge. The synchronous detection block, whose operation is explained by the block diagram in Figure 10, processes the secondary waveform in accordance with (24).…”

The Studies of Plasma Torch Processes by Laser Beam Welding

“…In the majority of works [2,3,4], dedicated to the studies of how plasma streams are created by laser welding, we can find the description of plasma formation above the welding bath. The series of laser beam welding experiments with through pro-melting of metal [which were carried out in order to work out an on-line control testing method] demonstrated that plasma is also emitted from "keyhole" of a welded seam [10,11]. Such emission of plasma happens only by through promelting of metal.…”