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

Current driven ion acoustic and potential relaxation instabilities excited in plasma plume during electron beam welding D N Trushnikov, , G M Mladenov, , V Ya Belenkiy, , E G Koleva, , and S V Varushk[.]

plume during electron beam welding

D N Trushnikov, G M Mladenov, V Ya Belenkiy, E G Koleva, , and S V Varushkin,

Citation: AIP Advances 4, 047105 (2014); doi: 10.1063/1.4870944

View online: http://dx.doi.org/10.1063/1.4870944

View Table of Contents: http://aip.scitation.org/toc/adv/4/4

Published by the American Institute of Physics

AIP ADVANCES 4, 047105 (2014)

Current-driven ion-acoustic and potential-relaxation

instabilities excited in plasma plume during electron

beam welding

D N Trushnikov,1, aG M Mladenov,2,3, bV Ya Belenkiy,4, c

E G Koleva,2,3, dand S V Varushkin4, e

1The department for Applied Physics, Perm National Research Polytechnic University, Perm,

614990, Russian Federation

2Institute of Electronics, Bulgarian Academy of Sciences, 72 Tzarigradsko Shose, 1784,

Sofia, Bulgaria

3Technology Centre of Electron Beam and Plasma Technologies and Techniques, 68-70

Vrania, ap.10, Banishora,1309, Sofia, Bulgaria

4The department for Welding Production and Technology of Constructional Materials, Perm

National Research Polytechnic University, Perm, 614990, Russian Federation

(Received 29 November 2013; accepted 31 March 2014; published online 8 April 2014)

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 elec-tron 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 weld-ing 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 1016m−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 cm2collector 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) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4870944]

I INTRODUCTION

The study and control of the process of electron beam welding (EBW) using collected flows

of charged particles or emitted electromagnetic radiation from the interaction beam/sample region

a Author to whom correspondence should be addressed Electronic mail: trdimitr@yandex.ru

b Electronic mail: gmmladenov@abv.bg

c Electronic mail: mtf@pstu.ru

d Electronic mail: eligeorg@abv.bg

e Electronic mail: stepan.varushkin@mail.ru

are the subjects of many research papers.1 15Similar investigations have been executed in the field

of laser welding.16 – 18 This research has been directed towards determining correlations between the parameters of secondary signals, dynamics of the processes in the keyhole, and the results

of the technological processes However, in industrial EBW applications, only the registration of back-scattered electrons is currently used for creating joint images or coordinates.19 – 21The practical applications of technological process control, based on secondary signals registration practically is missing

Previously, the authors of this paper have studied signals-collected from plasma, which have been generated from the beam above the welding zone.7 , 8 , 13 – 15Spectra collected by plasma currents include some peaks with frequencies (of the order of tens of kHz) These values are higher than frequencies, created by mechanical instabilities due to the changes of the shape of the keyhole in the liquid welding pool.14,15It has been shown that these peaks are not the result of noise in the invertor power source or of control blocks of the electron beam plant, but are related to processes within the “keyhole-plasma-collector” system The records of oscillations in the current exhibit a series of narrow impulses, which follow each other and have random amplitudes The time between the series

of the impulses is also random The amplitude of the impulses is significant, reaching 0.5 A The frequencies of the spectra maxima do not depend on the beam power or the welding velocity

The study of the characteristics of the collected current was motivated by wishing to obtain information regarding the dynamic processes in the keyhole in the welding pool through which the beam penetrates in the workpieces It is generally accepted that fluctuations in the vapor-plasma plume above the welding zone reflect the processes of beam penetration and instabilities of the liquid metal in the keyhole and usually no attention is paid to the oscillations created in the plasma

In some papers on beam welding in which the penetration of the concentrated energy beam in the work piece is discussed keyhole have longitudinal, transverse, and azimuthal oscillations in the range of up to a few kHz.22 – 26 These auto-oscillations are of stochastic nature and are determined

by the internal properties of the system beam/keyhole

Another reason for the theoretical predictions of the oscillations was discussed in Refs 27

and28 In these papers, a model of plasma expansion during EBW was created The steady-state equations of continuity and motion, combined with Poisson’s equation, were utilized to analyze the plasma distribution The available experimental data were also used to validate this model Assuming that EBW generated plasma is a function of the radial direction, a cylindrical plasma column around the beam region is the source of the free plasma expansion to the grounded metallic walls of the vacuum chamber, which is where the zero electrostatic potential holds The motion through the vacuum chamber in the radial to beam direction of the electrons and ions is without collisions The plasma column is positively charged (about 1–6 V) This self-defined potential is created to save the plasma electrons from reaching the grounded keyhole near the contact of the plasma and workpiece The model permits the estimation of radial distributions of plasma potential and particles densities, and the solutions are dependent on the ratio of electron thermal energy to the ions initial energy:

E e /i = k b T e /(Mv2

here k b is the Boltzmann constant, Te is the electron temperature, and M is the ion mass Usually

E e/i is greater 1 The plasma’s positive potential and the density of the charged particles decrease following a logarithmic law with r (quickly near the cylindrical plasma column around the beam and more slowly further from the beam region) In Ref.28, the numerical solution for the distribution

of eU/kTe (where U is potential and e iselectron charge) from r oscillates in comparison with the

approximated solution.27 Because the electron temperature is higher than the ion temperature, the lighter and faster electrons oscillate around the slowly moving and heavier ions, and electron density

is a function of the potential only In Ref.28, small oscillations of the ratio eU/kT ewere found in the radial direction The transfer of the electron energy to the ions via collective interaction increases the ion energy by about three times and the numerical solution for the dependence of ion energy on the radial distance also oscillates The mean values of ion energy increase quickly at small distances from the beam region and more slowly - at greater distances

047105-3 Trushnikov et al. AIP Advances 4, 047105 (2014)

FIG 1 Schematic diagram of the experimental system.

In the case of EBW with an oscillating beam in the longitudinal direction of the joint the probability of excitation high-frequency fluctuation collected by the positive polarized collector is connected with the beam positions on the front and trailing walls of the keyhole Some features of phases and amplitudes of coherently accumulated current, collected by plasma in comparison with the deflection system current permit the new methods of control of the position of the beam focus during EBW

The explanation for the excitation of the oscillations of current, collected from the plasma above the welding pool, is that in the collector circuits is a measured current of non-independent discharge, created by the thermo-emission of electrons from super-heated spots on the keyhole walls and on ablation products, and that the plasma works as an electro-conductive media

In areas with local roughness on the front keyhole wall and due to ion focusing of parts of the beam, some spots become superheated These- emit electrons and they are ablated because of the high energy input and subsequent energy dissipation through thermal conductivity As a result vapor and blowout ablation material shield the irradiated areas from the beam and the beam power density in those areas decreases During the evacuation of the dense vapor and ablation products the critical overpower energy density is reached again and the next cycle of local superheating continues The agreement of the theoretical prognosis of the frequency of “micro-explosions” with the observed current oscillations is accepted as confirmation of this mechanism The disadvantage

of this explanation is the lack of understanding regarding the independence of current frequencies from the position of the beam focus

Note that all investigations were executed using a standard collector electrode fixed on the down end of the welding gun.13 – 15 In this paper the dependence of the collected current on the position

of the collector electrode is investigated The aim of this investigation is to provide new informa-tion on the physical mechanisms responsible for creating the high-frequency oscillainforma-tions observed during the frequency analysis of the spectra from the collected plasma current over the welding pool during EBW

II EXPERIMENTAL METHOD

The experiments were executed using a standard EBW machine (Fig.1)

FIG 2 Scheme of experimental electron beam equipment: 1- electron gun, 2-sample 3- coordinate axis of movement of the collector, 4 - the collector of electrons, 5- registration of collector current, 6-load resistor in collector circuits 50,7-

d-c-power source.

The specimens (workpieces) were made from chromium-nickel steel with the following com-position: up to 0.12 % carbon, 18 % chrome, and 0.8 % titanium The accelerating voltage was

60 kV, and the welding speed was 5 mm/s The weld depth was about 10 mm

During the experiments, the focusing current was changed around the value providing the maximal weld depth The beam power was 1.8 kW The pressure of residual atmosphere in the welding vacuum chamber was 10−2 Pa The working distance of the gun to the work pieces was usually 100 mm Parts of the experiments were executed with variable working distance of 150 to

350 mm

The electron collector was a disc electrode 20 mm in diameter, located above the welding zone,

connected through a load resistor R k = 50  with a DC power source providing an adjustable electromotive force of E = 0 79 V The voltage of the collector was registered for computer

treatment Current from the plasma was determined from the measured voltage using the formula I k

= (E − U k )/R k

The technique for measuring the plasma charge current in electron beam welding has been described in previous work.7 , 10 , 13 – 15 The charged particles are emitted from the region of the electron beam’s interaction with the welded material Most of the particles are low-energy (1–2 eV) thermoelectrons, backscattered electrons, and positive ions The value of the current from the plasma

is equal to the difference between the currents from electrons and from ions (I k = I e –I i) The value

of the measured current in the collector circuit can reach 0.1 1 A.10,15If the collector potential is sufficiently large (∼50 V), then the ion current is practically negligible, and the small current from the backscattered electrons may also be disregarded

The current in the collector circuit was registered for computer treatment The frequency of sampling was 400 kHz During the experiments, the collector position was changed on two axes: in the r and z directions (Fig.2)

III EXPERIMENTAL RESULTS

In the experiments with variable positions of the collector electrode it was observed that the frequency and amplitude of the high-frequency harmonics depended on the distance between the collector electrode and the welding zone Fig.3(a)–3(c)shows the spectra of the collected current for three positions of the collector measuring the plasma electron current under the same technological

regime The distance of the collector-beam axis (coordinate r) was 15 mm The collector position was

047105-5 Trushnikov et al. AIP Advances 4, 047105 (2014)

FIG 3 Spectra collected current at various welding zone-collector electrode distances: a) z = 25 mm; b) z = 50 mm; c) z= 70 mm U of the collector is 48 V and r = 15 mm.

changed along the z coordinate with respect to the workpiece It was observed that the frequency of the high-frequency oscillations of the collected current increased when distances z and respectively

R decreased For the case of z= 25 mm the spectra show harmonics at frequencies of about 55

and 110 kHz At elevated height z= 50 mm, the frequency of the first harmonic is moved to

35 kHz For height z= 70 mm the first harmonic is at 27 kHz, which multiplied by 2 and 3 provides

the frequencies of the following harmonics The impulse at f= 35 kHz is noise, generated by the invertor power source of the electron beam equipment

Dependence of the first harmonic on the reciprocal value of the distance from the welding zone

to the collector - 1/R = 1/√r2+ z2at variable height z of the collector is shown in Fig.4(a) An analogous relationship is shown for the change of position of the collector in the direction of the

r-axis at a constant distance between the collector electrode and the work piece i.e z= 32 mm (Fig.4(b)) It can be seen that the frequencies of the high-frequency oscillation peaks of the current

in the collector circuits is decreased following the increase of the distance between the collector and the welding zone

FIG 4 Dependencies of the oscillation’s frequencies on the welding zone-collector electrode distance: a) distance from

welding zone increases; b) distance from the beam axis is changed U of the collector is 48 V, z= 32 mm.

Additional measurements of the ion current were made to estimate the plasma density A negative electromagnetic potential of −50 V was applied to the collector in order to estimate the ion current To prevent high-energy electrons reflected from the interaction zone from hitting the electrode, a protective ground shield was used between the collector and the welding zone

In the case of long runs, the magnitude of the ion current does not depend on the temperature of the ions and is related to the ion density by the following expression29

I i = aen i (2kT e /M i)1/2 S, (2)

where a is a geometric constant (a = 0.8 for a surface probe), n i is the ion density, S is the area of the probe, T e is the temperature of the electrons, e is the charge of an electron, and M iis the mass of the ions

The temperature of the electrons was taken to be the value determined experimentally during EBW in work.30 4000 K The inaccuracy of this value hardly affects the order of magnitude of

the estimate of I i, because in expression(2)the temperature of the electrons is raised to the power

of 1/2

Based on the measurements of the ion current at various distances between the measuring electrode and the welding area, the concentration of ions (plasma density) was estimated using expression(2) Figure5shows the measurements when the beam is sharp focused It is clear that the plasma density decreases with distance from the welding zone: from∼1016m−3at a distance of

35 mm to∼1015m−3at distances of 50–100 mm

047105-7 Trushnikov et al. AIP Advances 4, 047105 (2014)

FIG 5 Relationship between ion density and distance from the welding zone.

IV DISCUSSION OF THE RESULTS

With a positively polarized collector above the welding bath, the electrons near the collector are extracted from the created electric field and the plasma potential becomes similar to the collector potential, creating quasi-electro-neutrality The increased plasma potential extracted the emitted from the electrons of the hot spots on the keyhole front wall balances the losses in the electrons in the collector region

The current collected from plasma at small collector potentials is small, due to the current depression by the potential barrier in front of the collector When the positive potential is applied to the collector, the plasma potential near the collector starts increasing This is followed by an increase

in the collector current, which in turn causes a decrease in the current of plasma electrons following the total current conservation This current suppression in the keyhole orifice means physically there

is an increase of plasma potential in the keyhole in front of the thermionic emitting hot spots on the current walls, which conversely enhances the electron current passing towards the collector When the collector potential increases considerably (in comparison with the space potential of the plasma), almost all plasma electrons can be subtracted and reach the collector The discharge “hot-spot on the keyhole wall plasma collector” becomes unstable and forms a potential dip at the keyhole orifice, and then the current depression occurs The enhanced diffusion leads to a reduction of the plasma density and conducted current After the plasma density is restored, the instability is excited again and the cycle repeats This situation has a very close relationship with the generation of potential relaxation oscillations and of current-driven ion-acoustic waves.31 , 32 The critical current density j

at which the anomalous resistance state of the plasma is observable,33 – 36 could be estimated by the critical value of the electron draft velocity, calculated as:

V d = j

The minimum speed V d, when the ion-acoustic instability occurs, is significantly less than the electron thermal velocity and practically the same as the ion acoustic wave velocity in plasma.35The

phase velocity V s of a plane ion acoustic wave, propagating along the z axis, can be written as:37

V s =(γ e k b T e + γ i k b T i)/M, (4) whereγ eandγ i are the electron and ion compressional coefficients, respectively, and T iis the ion temperature

The frequency of both kinds of fluctuation varies almost inverse proportionally with L, i.e.:

Therefore, the distance between and the boundaries at the welding zone and collector seem to be very important for wave excitation and propagation It is reasonable to consider that the wavelength

FIG 6 Frequency spectra of the current collected by plasma for a distance from the welding zone to collector of 180 mm.

of the fundamental mode is L (or 2L, if processes such as standing waves occur) Similarly the wavelength of the n-th harmonic waves is given by L/n (or 2L/n for a standing wave).

Using equation(4), the observed peaks (Fig.3) in the spectra of the fluctuations of the electron current collected from plasma give the instability phase velocity as equal to 1600 m/s atγe= 1; γ i

= 3; T e = 10 000 K; T i = 2 400 K The value of Te is assumed higher than that observed by the

Langmuir probe Te = 4 000 K (Krinberg et al.)27because in the plasma of the positively polarized +48 V collector, the electrical fields in the extraction zone of the thermion-emission electrons is higher

The evaluated by eq.(4)and(5)critical current density is 2.6 A/m2, and the impulse current on the collector and the average collector current are about 8 mA, respectively, if the collector area is

of the order of 3–10 cm2and n e≈ 1016m−3 Observed in experiments the impulses of current are about 0.2–0.5 A, which indicates a high probability of current-driven ion-acoustic instabilities Of

course the values of the plasma density n e≈ 1016m−3, as well as T e = 10 000 K, are only suitable

to evaluate the probability of the current-driven ion-acoustic instability The plasma density is not constant over the volume of the vacuum chamber and decreases with increasing distance from the

weld zone But smaller values of plasma density, as well as the value of T e, further increase the probability of the indicated phenomenon

The presence in the plasma plume of negatively charged blowout droplets, ablation products, and negative ions extracted from the welded metal oxygen, has strong influence of the characteristics

of the plasma wave mode by lowering the electron temperature to an effective value.37This could be reason for the variations of the instability phase velocity and could excite, together with oscillations

of the keyhole plasma parameters, the ion-acoustic instability in the plasma plume

The results showing that the evaluated wave velocities of the ion-acoustic instabilities are similar

to the observed spectra of the collected current fluctuations and that the amplitudes of these current fluctuations are bigger than the critical current value estimated by eq (3), suggest that the ion-acoustic instabilities and anomalous resistance state of the plasma are the reasons for the observed high-frequency oscillations of the current

It can be noted that at higher distances, the fundamental long wave harmonics of the plasma oscillations overlap the oscillating frequencies generated by the keyhole shape fluctuations, and that the interactions between these oscillations and the instabilities of the thermion emission give complex instabilities

Fig.6 shows many harmonics of the collected current for a distance from the welding zone

to the collector of R= 180 mm For the current instabilities spectra at this distance, the computed

values of the harmonic frequencies are f≈ (4.6, 9.2, 13.8, 18, 23, 27.6, 32.2, 36.8, 41.4, 46, 50.6 ) kHz The amplitude of the harmonics is distributed in a more complex way

It has been shown that the high-frequency component of the current collected by plasma obeys the impulse character.15 , 38For deep penetration of the beam in the keyhole, due to collisions between the charged particles and neutral atoms, the plasma fluctuations are depressed.37For the elevation of the position of the beam-wall interaction zone in the keyhole, the amplitude of the high-frequency

047105-9 Trushnikov et al. AIP Advances 4, 047105 (2014)

component of the collected current increases Upon reaching a critical current, the plasma changes

to an anomalous resistance state and instability occurs, transforming the current of the discharge within high-frequency oscillations

It can be noted that due to the above mentioned collisional decay of the plasma fluctuations, such high-frequency instabilities could not be expected in atmospheric pressure EBW or in usual laser beam welding

V CONCLUSIONS

1 It is shown that the frequencies of peaks in the spectra of the collected current are dependent

on the reciprocal distance between the welding zone and collector electrode Assuming the

wavelength of the n-th harmonic wave to be equal to L/n, where L is the distance between

the welding bath and the collector, the draft velocity of the electrons or phase velocity of the ion-acoustic waves can be estimated at 1600 m/s On the other hand, the observed instability

is similar to the potential-relaxation instability, characterized by a large-amplitude oscillation for a positively biased electrode in plasma Both instabilities have similar properties regarding excitation conditions and wave propagation

2 The obtained dispersion relation with the properties of the ion-acoustic waves is related to

electron temperature 10 000 К, ion temperature 2 400 К and plasma density 1016 m−3 The estimated critical density of the transported current for creating the anomalous resistance state

of the plasma is of the order of 3 A· m−2, i.e 8 mA for a 3–10 cm2collector electrode

3 The electron current in plasma caused the keyhole processes are modulated by the high-frequency plasma oscillations It can be assumed that these observed high-high-frequency oscilla-tions are the result of relaxation processes in the plasma plume above the welding zone It can be assumed that these observed high-frequency oscillations are the result of relaxation processes in the plasma plume above the welding zone

4 Based on experimental results, it is possible to draw the conclusion that the collector can be placed near the welding zone Thus, the high-frequency instabilities of the collected current will have peaks at frequency of more than 50 kHz and all fluctuations less than 20 kHz will be

a reflection of the processes of keyhole instabilities The amplitude of the plasma instabilities will be smaller; this also supports the study of the low-frequency component

5 The signal propagates with the ion-acoustic wave velocity During the creation of the current impulse in the discharge keyhole walls - plasma collector, the delay-time to registration on the collector could be evaluated asτ = L/Vs, which can be estimated to be of the order of

60μsec This additional delay time must be accounted for the study and control of EBW using

the method of synchronous accumulation of the high-frequency components of the current collected by plasma.38

ACKNOWLEDGMENTS

This research paper is sponsored by a grants from the Russian Foundation for Basic Research RFBR 13-08-00397 and 14-08-96008 and with financial support from the Russian Ministry of Education and Science within the base part of the state task (Project 1647)

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