- Email: [email protected]
Interaction between Electrical Arc and Nd: YAG Laser Radiation
Interaction between Electrical Arc and Nd: YAG Laser Radiation 1
1
1
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U. Stute (3); R. Kling ; J. Hermsdorf Laser Zentrum Hannover e.V., Dept. of Production and Systems – Submitted by Prof. H. Trumpold (1), Chemnitz, Germany
Abstract This paper describes the stabilization and guidance of plasma plumes through the interaction with low power laser radiation. The stabilization and guidance have been found to be governed by several physical effects. One effect is the localized rise of electrical conductivity. As the laser provides a channel of increased conductivity an alignment and a stabilisation of the electrical arc can be obtained. This feature can be exploited to achieve more efficient and flexible plasma processes. Investigations on plasma guidance with respect to different laser wavelengths and interaction modes will be presented by means of example in welding and EDM. Keywords: Laser beam machining (LBM), Welding, Electrical discharge machining (EDM)
1 INTRODUCTION In order to enhance conventional electric arc processes for joining and erosion, laser or electron beam assisted methods are currently under investigation, forming combined or hybrid processes. One main problem in efficiency and precision for arc processes is the column movement, especially when oxidized surfaces are concerned. For monopulse discharge in EDM it was found that a thin oxide film on the work piece increases the spot movement and the diameter of these spots [1]. Low intensity in welding of aluminium leads to an unstably jumping arc because the plasma plume is burning at the arc root point on oxidized surfaces. Mainly, due to a thermal stabilisation of the arc, the welding is not following the feed rate of the needle until the distance between the arc root point and the needle exceeds a critical limit and the arc jumps to a new root point. In hybrid welding, the positive features of both processes have been combined since the late seventies [2; 3]. The electric arc process increases the ability of gap bridging not only by its filler material but also by the wider range of process parameters [4]. The high energy density is the advantage of high power laser welding. The combination leads to high penetration depths with profile connection. Other improvement of this combination is the increased energy input of the electric arc into the work piece and the stabilization effect of the laser radiation on the electric arc [5]. Thereupon this hybrid process was developed to an industrial standard. Thereby the understanding of the interaction between arc plasma and laser radiation can be an improvement for the combined processes. While for stabilisation of EDM pulsed lasers have been applied, the stabilisation effect within hybrid welding was achieved with cw-lasers. This paper focuses on the interaction between an electric arc and a low power laser beam. The physical effects are discussed in order to distinguish the mechanisms for the interaction, being responsible for the stabilization and guidance of plasma plumes. Investigations on plasma guidance with respect to different laser wavelengths and interaction modes will be presented by means of example in welding. Opportunities to use the interaction for EDM are discussed.
Annals of the CIRP Vol. 56/1/2007
2
PRINCIPLE REALISATION AND EFFECT DEFINITION The raise of the stabilization of the electric arc by tungsten inert gas welding (TIG) process was researched in [6] and was proven with experimental results. During the welding process with a positive arc the laser radiation was switched on, leading to a drop of the discharge voltage and a simultaneous rise of the arc current. This change in the discharge characteristics can be deduced to an increased conductivity of the arc in the overlap region of laser beam and arc column [6]. The interaction between an electric arc and low power laser radiation were investigated at the Ohio State University. There a machine was patented for the region of North America to lead and initialise an electric arc with a laser output power of 7W. The low laser power is the main difference between the state of the art hybrid processes. The engage laser radiation is used exclusively for the stabilization and guidance of plasma plumes from the electric arc. The heat input from the laser is low because of the minor continuous radiation. The interaction between the laser and the electric arc follow up because of the ionisation effect from the laser beam into the volume (optogalvanic effect see section 3). This leads to a rising of the conductivity into the plasma plume. The increased conductivity implements a high energy input into in work piece. The definition for this laser effect of the stabilization and guidance of plasma plumes is as follows: •
A contribution from the laser to the total power of only 10-20%
•
Constriction of the electric arc by laser radiation
3 PHYSICAL EFFECTS Responsible for the interaction between the laser beam and the electric arc are the following 5 physical effects which will be briefly described: 1) Photoelectrical Effect 2) Laser induced Thermo Emission 3) Inverse Bremsstrahlung 4) Laser induced plasma 5) Optogalvanic Effect
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doi:10.1016/j.cirp.2007.05.048
All effects arise from the laser radiation and the effects are taking place in two zones. The effects 1), 2), 4) are taking place directly on the material surface, whereas 3) and 5) occur in the plasma plume above the surface. To study the effects special gas combinations and variations of laser power and laser wavelengths can be used.
(pw) radiation. For both types material is removed and a laser induced plasma (LIP) ignites, if the power density in the focus exceeds the limit for an optical breakdown. In contrast to hybrid welding, cw lasers can be excluded for the application of LIP-stabilization due to their requested low intensity according to the definition in section 2. For the ablation of aluminium, a power density 8 9 2 near 10 -10 W/cm is needed. This high density can be achieved with pulses in the nanosecond range and focus diameter minor 50 μm [9, 10, 11]. A priori, laser induced ablation has two effects on the conductivity and stabilization of the overall system. First, oxides on the surface are removed and second, the localized plasma induces a guidance of the electric arc. The contribution of the removal of oxides was investigated by generating a track on the surface of the work piece in an inert atmosphere. Subsequently, the TIG needle was positioned in line with the laser generated track, but no stabilization was observed during the welding experiments and hence the reduced resistance is neglected for the stabilization of electric arc welding. By laser ablation, a thermal ionisation occurs on the surface and in the gas volume, where an ionisation grade -4 -3 of 10 to 10 is reached. The influence from such a LIP on the stabilization and guidance of the electric arc will be described in section 4.
3.1 Photoelectrical Effect The photoelectrical effect appears when a photon hits a surface and sets free a bound electron. Then the free electron contributes to the discharge current and the conductivity increases. This is only possible, if the photon energy is higher than the working function W e of the material. As a single photon releases an electron, the effect is not determined by a high power density but by the total number of photons. A calculation with W e= 4.2 eV as the working function for aluminium leads as result to a wavelength below 295 nm. Experiments have been carried out with a pw laser with a wavelength of 266 nm, a beam diameter of 2 mm and a negative polarisation of the electrode. The result of the researches was that there was no interaction between the electric arc and the laser beam. Therefore the photoelectrical effect is less important for influencing the electric arc. 3.2 Thermo Emission The understanding of thermo emission is that electrons are discharging the atomic structure because of a heat input into the base material. In difference to the photoelectrical effect a high photon density is very important for the thermo emission effect. Due to this energy induction electrons are able to bridge the bond energy of the atom. By increasing the temperature statistically more electrons are leaving the bulk material. This fact is described in the Richardson-Equation.
j = AT ⋅ e 2
3.5 Optogalvanic effect (OGE) The optogalvanic effect (OGE) occurs, when a plasma is irradiated with an extended laser beam. The laser light changes the population density of the excited states of the atoms, which results in a change of the arc discharge current. This effect is well understood in low pressure glow discharges, and the OGE spectra can be used for wavelength calibration [12]. The basic mechanism of the OGE can briefly be described as follows: In an electric arc the current is determined by the rate of producing charged particles in particular electrons and ions. The basic effect is the excitation and ionization by inelastic electron-atom collisions. As the kinetic energy of the electrons is to low to directly ionize the atoms, a single collision produces an excited atom, which can then relax by spontaneous emission of light or can be transferred into higher excited stated by an other collision. If the excitation reaches energy levels slightly below the ionization limit, the atom is very likely to be ionized and then the charged particles contribute to the discharge current. As the emission of light depopulates the excited states the collision frequency must be high enough to sustain a continuous discharge. If now the discharge is irradiated with laser light, the atoms are pushed towards higher energy states and the ionization rate is increased. This results in higher arc currents or lower voltages depending on the power supply. In general the OGE leads to an increased conductivity of the electric arc, which directly stabilizes the discharge. An important fact about the OGE is the dependence of the laser wavelength. As the laser light is only absorbed if the photon energy matches the energy gap between to atomic energy levels, the OGE is highly selective in laser wavelength. Therefore various lasers have to be tested for optimizing the effect. In order to maximize the OGE the overlap region of plasma and laser beam needs to be as large as possible. The stabilisation and guidance effect from the OGE will be fully specified in the experimental results.
−We k BT
2
with j= current density (A/m ); A= Constant of Richardson 2 2 (A/m xK ) which is material compounding; T= temperature (K); W e= working function of the material -5 (eV); kB= Constant of Boltzmann = 8.6173x10 eV/K. Calculations with W e= 4.2 eV as the working function for 2 2 2 aluminium and A=10 A/(m xK ) for metal oxides are leading to a temperature of more then 2200°C for a prominent current density. Experiments were carried out with a cw laser by a wavelength of 1064 nm, a focus diameter of 300 μm and a laser power of 300 W. In the trials the laser was focused on the material surface. With a feed rate of 2m/min without shield gas and a feed rate of 0.5 m/min with argon as shield gas temperatures less then 1200°C were measured with a calibrated quotient pyrometer. As a result of these tests the thermo emission due to additional laser heating is not decisive for the guidance and stabilization of the electric arc. 3.3 Inverse Bremsstrahlung The plasma in arc welding is a low-temperature plasma in LTE with electron temperatures ranging from about 10000 to 25000 K [7]. Absorption of laser light in such plasmas is dominated by inverse Bremsstrahlung due to free-free transitions of electrons in the field of the ionic component of the plasma. When the laser radiation enters the plasma plumes after a distance of 1cm, about 40% of CO2 laser power but only about 0.3% of the Nd:YAG laser power have been reported to be absorbed by the arc plasma [8]. Hence the inverse Bremsstrahlung by Nd:YAG laser radiation can be neglected.
4 EXPERIMENTAL RESULTS As a conclusion of section 3, the LIP generation and the OGE are supposed to be able to guide and stabilize the
3.4 Laser induced plasma (LIP) An ablation can be induced with a high power continuous wave (cw) laser radiation or with high power pulse wave
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electric arc. Both effects were studied by welding of aluminium.
electric arc is unstable burning in a distance of 2.1 mm from the zero position. It can be clearly seen that the laser guides the electric arc to the point where it hits the surface of the material and welding process is stable burning. Records show the stabilisation and guidance of the electric arc for different speeds (up to 5 m/min) and positions of the laser beam to the electric arc. The zero location is defined vertical under the electrode and the distance of the laser focus on the surface is changed from 0 – 3 mm (c in Figure 3) in the moving direction. Furthermore the vertical position of the laser is varied from -3 mm (below the surface) up to 7 mm over the material surface (Figure 1).
4.1 Experimental realisation For a better observation of the interaction between the laser beam and the electric arc no filler wire is used. In order to ignite the electric arc, the TIG needle gets in contact with the work piece. Then the TIG needle with the burning arc is lifted up to a pre-defined distance above the work piece (Figure 1). For each constant working speed, in the first phase only the TIG welding process is active. 26°
TIG welding head
0
Laser head
Direction of laser radiation
b Material surface Unstable electric arc
Process area
Laser beam
a
a
0 c Electric arc Stabilized electric arc
Figure 1: Principle experimental set up
Figure 3: Guidance of the electric arc
As pictured in the principle experimental set up in Figure1, the laser radiation is stepping into the process with an angle of 26° to the perpendicular direction. If both sources were located in the process area on the work piece, the electric arc can be guided to the location of the laser beam and then burns in a stable mode (current 80120 A). The effect of guidance and stabilization of the electric arc is classified by stability and width of the welding seam on the probes (Figure 2). For low energies of the single TIG welding process, the electric arc is nomadic, unstable burning on the work piece surface, while a straight and stable seam can be distinguished in case of positive interaction of the TIG and the laser radiation.
The ablation experiments were carried out with a laser wavelength of 266 nm, a focus of 40 μm and a mean power in the range of 0.9 - 1.7 W. With a pulse power of 8 2 6.9 kW and the reachable fluence was 9.7x10 W/cm an ablation depth of 0.25 μm per pulse was reached.
focus position 0mm
4.2 Discussion As a result from the experimental trials the most intense interaction between laser and arc have been found due to the laser induced plasma and OGE. An enhanced conductivity of the electric arc was observed with the laser ablation. Photon density has an important influence on the plasma temperature, resulting in higher ionisation grad of the plasma and hence a raising conductivity due to the generation of a significant number of charged particles. As found in the experiments, laser focus position and pulse energy are the most important parameters for the optimisation of guidance and stabilization of an electric arc. Under the condition that the ablation threshold was exceeded and ablated material was available for stabilisation, the effect was observed over the whole parameter range. While the investigations to determine parameters for optimal ablation have been carried out with a focus position on or within the work piece, best results have been found with focus positions above the work piece using a combination of ablation and optogalvanic effect. The optogalvanic effect (OGE) is also increasing the conductivity of the electric arc. The investigations were carried out with low power Nd:YAG cw laser. Compared to the laser induced plasma the OGE can be activated 5 2 with a density of 4.2x10 W/cm . Furthermore the effect has his impact above the material surface within the volume of the electric arc. Figure 5 shows the stabilization of the electric arc. At the beginning of the voltage measurement the electric arc is instable burning. After 50 sec the laser radiation is stepping into the process. Simultaneously the electric arc is stabilized and the voltage goes to the adjusted value. Cutting of the laser beam again leads to an instable arc. The OGE
sample number 21
1mm 22 2mm 23 3mm 24 Figure 2: Aluminium probes welded with a current of 120A, Laser power 300W cw Nd:YAG, by a constant feed rate and a variation of the focus position. The stabilization and guidance effect from the laser radiation can even more drastically be observed in motion. Figure 3 shows an example of two pictures extracted out of one movie. The cw laser beam with a wavelength of 1064 nm and 300 W is focused in 300 μm spot onto the material surface. On the left side the
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opens a channel of better conductivity in direction of the laser beam and in form of the laser beam caustic.
6 ACKNOWLEDGMENTS This research and development is / was funded by the German Federal Ministry of Education and Research (BMBF) within the Framework Concept ”Research for Tomorrow`s Production” (fund number 02PB2051) and managed by the Project Management Agency Forschungszentrum Karlsruhe, Production and Manufacturing Technologies Division (PTKA-PFT).
100
Laser off
Laser off
Voltage (V)
75
Laser on 50
7 REFERENCES [1] Kunieda, M., Xia, H., Nishiwaki N., Kinoshita N., 1992, Observation of the arc column movement during monopulse discharge in EDM, CIRP Annals 41/1/1992, p. 227 [2] Haferkamp, H., Ostendorf, A., Bunte, J., Szinyur, J., Höfemann, M., Cordini, P., 2002, Increased Seam Quality for Laser-GMA Hybrid Welding of ZincCoated Steel, Proceedings ICALEO 2002, S. 129137 [3] Abe, N., Agano, Y., Tsukamoto, M., 1997, Effect of CO2 laser irradiation on arc welding, Trans JWRI 1997 Vol. 26 (1) p 69. [4] Seyffarth, P., Krivtsun, I.V., 2002, Laser-ArcProcesses and their Applications in Welding and Material Treatment, Welding and Allied Processes Volume I, Taylor & Francis. [5] Dilthey, U., Lueder, F., Wieschemann, A., 1999, Technical and economical advantages by synergies in Laser Arc Hybrid Welding, Proceedings IIW International Conference "The Human Factor and it is Environment”, Lisbon, 1999, S. 141 – 152. [6] Cui, H., 1991, Untersuchungen der Wechselwirkung zwischen Schweißlichtbogen und fokussiertem Laserstrahl und der Anwendungsmöglichkeit kombinierter Laser-Lichtbogentechnik, ISSN03449629. [7] Lowke, J., Kovitya, P., Schmidt, H. P., 1992, Theory of free-burning arc columns including the influence of the cathode, J. Phys. D: Appl. Phys. 25 1600-6 [8] Paulini, J., Simon, G., 1993, A theoretical lower limit for laser power in laser-enhanced arc welding, J. Phys. D: Appl. Phys. 26 (1993) 1523-1527 [9] Tönshoff, H.K., Ostendorf, A., Körber, K., Kulik, K., Kamlage, G., 2001, Micro Material Processing using UV Laser and Femtosecond Laser, Proceedings of 10th International Conference on Precision Engineering (ICPE), pp. 62-66 [10] Ostendorf, A., Kulik, C., Stute, U., 2002, Production of Innovative Geometries with UV Lasers, Proceedings of SPIE Vol. 4637, pp. 280 – 290 [11] Tönshoff, H.K, Butje, R., König, W., Trasser, FR.-J., 1988, Excimer Laser in Material Processing, CIRP Ann. Vol. 37, no. 2, pp. 681-684 [12] Zhu, X., Nur, A.H., Misra, P., 1994, Laser optogalvanic wavelength calibration with a commercial hollow cathode iron-neon discharge lamp, Spectroscopy radiation Transfer Vol. 52, No.2, pp.167-17 [13] Hoshi, Y., Yoshida, H., 1999, Application of laserguided discharge to processing, Appl. Phys. A 68, 93–98
25
0 0
30
60
90
120
time [sec]
Figure 5: Voltage signal with a laser-induced stabilization of the electric arc in TIG welding Figure 2 shows that by defocusing the laser beam still the guidance and stabilization is possible. In addition the penetration depth of the electrical arc is rising from 0.65 mm to 0.9 mm when the laser focus is 2 mm above the work piece (Figure 6). In consequence of the focus shifting the spot size is enlarged by a factor of 3.5 to a diameter of more than 1 mm. The deeper penetration can lead back to a greater overlap region of plasma plume and laser beam. As the effect is easily saturated due to the low density of atoms in the plasma plume, an extended beam provides better results than a focussed beam and was demonstrated by experimental studies.
0mm
2mm
21
23
Figure 6: Penetration depth of aluminium probes 5 CONCLUSION AND OUTLOOK The coincidence of a laser beam and an arc discharge results in stabilisation of the arc. Several physical effects contribute to this phenomenon, but from our investigations it is obvious that for pulsed lasers the plasma is the dominant effect and for cw lasers the optogalvanic effect is the main reason for this positive interaction. In further investigations the effect will be maximized by finding the optimal wavelength and the ideal beam shape. Not only arc welding applications can benefit from the laser aided plasma, in electric discharge machining (EDM) it can be exploited as well [13]. This process is very slow due to the small gap between the electrodes. The spark ignites between electrode positions where the resistance of the dielectric gap is minimal in other words where the gap is minimal. Therefore in order to control the position of the spark discharge the space must not exceed 50 μm. If now a channel of reduced resistance is provided by a laser the discharge will propagate along this channel and the electrode spacing can be increased. This will result in higher ablation rates per shot and thus faster processes with constant surface qualities.
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1
1
1
U. Stute (3); R. Kling ; J. Hermsdorf Laser Zentrum Hannover e.V., Dept. of Production and Systems – Submitted by Prof. H. Trumpold (1), Chemnitz, Germany
Abstract This paper describes the stabilization and guidance of plasma plumes through the interaction with low power laser radiation. The stabilization and guidance have been found to be governed by several physical effects. One effect is the localized rise of electrical conductivity. As the laser provides a channel of increased conductivity an alignment and a stabilisation of the electrical arc can be obtained. This feature can be exploited to achieve more efficient and flexible plasma processes. Investigations on plasma guidance with respect to different laser wavelengths and interaction modes will be presented by means of example in welding and EDM. Keywords: Laser beam machining (LBM), Welding, Electrical discharge machining (EDM)
1 INTRODUCTION In order to enhance conventional electric arc processes for joining and erosion, laser or electron beam assisted methods are currently under investigation, forming combined or hybrid processes. One main problem in efficiency and precision for arc processes is the column movement, especially when oxidized surfaces are concerned. For monopulse discharge in EDM it was found that a thin oxide film on the work piece increases the spot movement and the diameter of these spots [1]. Low intensity in welding of aluminium leads to an unstably jumping arc because the plasma plume is burning at the arc root point on oxidized surfaces. Mainly, due to a thermal stabilisation of the arc, the welding is not following the feed rate of the needle until the distance between the arc root point and the needle exceeds a critical limit and the arc jumps to a new root point. In hybrid welding, the positive features of both processes have been combined since the late seventies [2; 3]. The electric arc process increases the ability of gap bridging not only by its filler material but also by the wider range of process parameters [4]. The high energy density is the advantage of high power laser welding. The combination leads to high penetration depths with profile connection. Other improvement of this combination is the increased energy input of the electric arc into the work piece and the stabilization effect of the laser radiation on the electric arc [5]. Thereupon this hybrid process was developed to an industrial standard. Thereby the understanding of the interaction between arc plasma and laser radiation can be an improvement for the combined processes. While for stabilisation of EDM pulsed lasers have been applied, the stabilisation effect within hybrid welding was achieved with cw-lasers. This paper focuses on the interaction between an electric arc and a low power laser beam. The physical effects are discussed in order to distinguish the mechanisms for the interaction, being responsible for the stabilization and guidance of plasma plumes. Investigations on plasma guidance with respect to different laser wavelengths and interaction modes will be presented by means of example in welding. Opportunities to use the interaction for EDM are discussed.
Annals of the CIRP Vol. 56/1/2007
2
PRINCIPLE REALISATION AND EFFECT DEFINITION The raise of the stabilization of the electric arc by tungsten inert gas welding (TIG) process was researched in [6] and was proven with experimental results. During the welding process with a positive arc the laser radiation was switched on, leading to a drop of the discharge voltage and a simultaneous rise of the arc current. This change in the discharge characteristics can be deduced to an increased conductivity of the arc in the overlap region of laser beam and arc column [6]. The interaction between an electric arc and low power laser radiation were investigated at the Ohio State University. There a machine was patented for the region of North America to lead and initialise an electric arc with a laser output power of 7W. The low laser power is the main difference between the state of the art hybrid processes. The engage laser radiation is used exclusively for the stabilization and guidance of plasma plumes from the electric arc. The heat input from the laser is low because of the minor continuous radiation. The interaction between the laser and the electric arc follow up because of the ionisation effect from the laser beam into the volume (optogalvanic effect see section 3). This leads to a rising of the conductivity into the plasma plume. The increased conductivity implements a high energy input into in work piece. The definition for this laser effect of the stabilization and guidance of plasma plumes is as follows: •
A contribution from the laser to the total power of only 10-20%
•
Constriction of the electric arc by laser radiation
3 PHYSICAL EFFECTS Responsible for the interaction between the laser beam and the electric arc are the following 5 physical effects which will be briefly described: 1) Photoelectrical Effect 2) Laser induced Thermo Emission 3) Inverse Bremsstrahlung 4) Laser induced plasma 5) Optogalvanic Effect
-197-
doi:10.1016/j.cirp.2007.05.048
All effects arise from the laser radiation and the effects are taking place in two zones. The effects 1), 2), 4) are taking place directly on the material surface, whereas 3) and 5) occur in the plasma plume above the surface. To study the effects special gas combinations and variations of laser power and laser wavelengths can be used.
(pw) radiation. For both types material is removed and a laser induced plasma (LIP) ignites, if the power density in the focus exceeds the limit for an optical breakdown. In contrast to hybrid welding, cw lasers can be excluded for the application of LIP-stabilization due to their requested low intensity according to the definition in section 2. For the ablation of aluminium, a power density 8 9 2 near 10 -10 W/cm is needed. This high density can be achieved with pulses in the nanosecond range and focus diameter minor 50 μm [9, 10, 11]. A priori, laser induced ablation has two effects on the conductivity and stabilization of the overall system. First, oxides on the surface are removed and second, the localized plasma induces a guidance of the electric arc. The contribution of the removal of oxides was investigated by generating a track on the surface of the work piece in an inert atmosphere. Subsequently, the TIG needle was positioned in line with the laser generated track, but no stabilization was observed during the welding experiments and hence the reduced resistance is neglected for the stabilization of electric arc welding. By laser ablation, a thermal ionisation occurs on the surface and in the gas volume, where an ionisation grade -4 -3 of 10 to 10 is reached. The influence from such a LIP on the stabilization and guidance of the electric arc will be described in section 4.
3.1 Photoelectrical Effect The photoelectrical effect appears when a photon hits a surface and sets free a bound electron. Then the free electron contributes to the discharge current and the conductivity increases. This is only possible, if the photon energy is higher than the working function W e of the material. As a single photon releases an electron, the effect is not determined by a high power density but by the total number of photons. A calculation with W e= 4.2 eV as the working function for aluminium leads as result to a wavelength below 295 nm. Experiments have been carried out with a pw laser with a wavelength of 266 nm, a beam diameter of 2 mm and a negative polarisation of the electrode. The result of the researches was that there was no interaction between the electric arc and the laser beam. Therefore the photoelectrical effect is less important for influencing the electric arc. 3.2 Thermo Emission The understanding of thermo emission is that electrons are discharging the atomic structure because of a heat input into the base material. In difference to the photoelectrical effect a high photon density is very important for the thermo emission effect. Due to this energy induction electrons are able to bridge the bond energy of the atom. By increasing the temperature statistically more electrons are leaving the bulk material. This fact is described in the Richardson-Equation.
j = AT ⋅ e 2
3.5 Optogalvanic effect (OGE) The optogalvanic effect (OGE) occurs, when a plasma is irradiated with an extended laser beam. The laser light changes the population density of the excited states of the atoms, which results in a change of the arc discharge current. This effect is well understood in low pressure glow discharges, and the OGE spectra can be used for wavelength calibration [12]. The basic mechanism of the OGE can briefly be described as follows: In an electric arc the current is determined by the rate of producing charged particles in particular electrons and ions. The basic effect is the excitation and ionization by inelastic electron-atom collisions. As the kinetic energy of the electrons is to low to directly ionize the atoms, a single collision produces an excited atom, which can then relax by spontaneous emission of light or can be transferred into higher excited stated by an other collision. If the excitation reaches energy levels slightly below the ionization limit, the atom is very likely to be ionized and then the charged particles contribute to the discharge current. As the emission of light depopulates the excited states the collision frequency must be high enough to sustain a continuous discharge. If now the discharge is irradiated with laser light, the atoms are pushed towards higher energy states and the ionization rate is increased. This results in higher arc currents or lower voltages depending on the power supply. In general the OGE leads to an increased conductivity of the electric arc, which directly stabilizes the discharge. An important fact about the OGE is the dependence of the laser wavelength. As the laser light is only absorbed if the photon energy matches the energy gap between to atomic energy levels, the OGE is highly selective in laser wavelength. Therefore various lasers have to be tested for optimizing the effect. In order to maximize the OGE the overlap region of plasma and laser beam needs to be as large as possible. The stabilisation and guidance effect from the OGE will be fully specified in the experimental results.
−We k BT
2
with j= current density (A/m ); A= Constant of Richardson 2 2 (A/m xK ) which is material compounding; T= temperature (K); W e= working function of the material -5 (eV); kB= Constant of Boltzmann = 8.6173x10 eV/K. Calculations with W e= 4.2 eV as the working function for 2 2 2 aluminium and A=10 A/(m xK ) for metal oxides are leading to a temperature of more then 2200°C for a prominent current density. Experiments were carried out with a cw laser by a wavelength of 1064 nm, a focus diameter of 300 μm and a laser power of 300 W. In the trials the laser was focused on the material surface. With a feed rate of 2m/min without shield gas and a feed rate of 0.5 m/min with argon as shield gas temperatures less then 1200°C were measured with a calibrated quotient pyrometer. As a result of these tests the thermo emission due to additional laser heating is not decisive for the guidance and stabilization of the electric arc. 3.3 Inverse Bremsstrahlung The plasma in arc welding is a low-temperature plasma in LTE with electron temperatures ranging from about 10000 to 25000 K [7]. Absorption of laser light in such plasmas is dominated by inverse Bremsstrahlung due to free-free transitions of electrons in the field of the ionic component of the plasma. When the laser radiation enters the plasma plumes after a distance of 1cm, about 40% of CO2 laser power but only about 0.3% of the Nd:YAG laser power have been reported to be absorbed by the arc plasma [8]. Hence the inverse Bremsstrahlung by Nd:YAG laser radiation can be neglected.
4 EXPERIMENTAL RESULTS As a conclusion of section 3, the LIP generation and the OGE are supposed to be able to guide and stabilize the
3.4 Laser induced plasma (LIP) An ablation can be induced with a high power continuous wave (cw) laser radiation or with high power pulse wave
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electric arc. Both effects were studied by welding of aluminium.
electric arc is unstable burning in a distance of 2.1 mm from the zero position. It can be clearly seen that the laser guides the electric arc to the point where it hits the surface of the material and welding process is stable burning. Records show the stabilisation and guidance of the electric arc for different speeds (up to 5 m/min) and positions of the laser beam to the electric arc. The zero location is defined vertical under the electrode and the distance of the laser focus on the surface is changed from 0 – 3 mm (c in Figure 3) in the moving direction. Furthermore the vertical position of the laser is varied from -3 mm (below the surface) up to 7 mm over the material surface (Figure 1).
4.1 Experimental realisation For a better observation of the interaction between the laser beam and the electric arc no filler wire is used. In order to ignite the electric arc, the TIG needle gets in contact with the work piece. Then the TIG needle with the burning arc is lifted up to a pre-defined distance above the work piece (Figure 1). For each constant working speed, in the first phase only the TIG welding process is active. 26°
TIG welding head
0
Laser head
Direction of laser radiation
b Material surface Unstable electric arc
Process area
Laser beam
a
a
0 c Electric arc Stabilized electric arc
Figure 1: Principle experimental set up
Figure 3: Guidance of the electric arc
As pictured in the principle experimental set up in Figure1, the laser radiation is stepping into the process with an angle of 26° to the perpendicular direction. If both sources were located in the process area on the work piece, the electric arc can be guided to the location of the laser beam and then burns in a stable mode (current 80120 A). The effect of guidance and stabilization of the electric arc is classified by stability and width of the welding seam on the probes (Figure 2). For low energies of the single TIG welding process, the electric arc is nomadic, unstable burning on the work piece surface, while a straight and stable seam can be distinguished in case of positive interaction of the TIG and the laser radiation.
The ablation experiments were carried out with a laser wavelength of 266 nm, a focus of 40 μm and a mean power in the range of 0.9 - 1.7 W. With a pulse power of 8 2 6.9 kW and the reachable fluence was 9.7x10 W/cm an ablation depth of 0.25 μm per pulse was reached.
focus position 0mm
4.2 Discussion As a result from the experimental trials the most intense interaction between laser and arc have been found due to the laser induced plasma and OGE. An enhanced conductivity of the electric arc was observed with the laser ablation. Photon density has an important influence on the plasma temperature, resulting in higher ionisation grad of the plasma and hence a raising conductivity due to the generation of a significant number of charged particles. As found in the experiments, laser focus position and pulse energy are the most important parameters for the optimisation of guidance and stabilization of an electric arc. Under the condition that the ablation threshold was exceeded and ablated material was available for stabilisation, the effect was observed over the whole parameter range. While the investigations to determine parameters for optimal ablation have been carried out with a focus position on or within the work piece, best results have been found with focus positions above the work piece using a combination of ablation and optogalvanic effect. The optogalvanic effect (OGE) is also increasing the conductivity of the electric arc. The investigations were carried out with low power Nd:YAG cw laser. Compared to the laser induced plasma the OGE can be activated 5 2 with a density of 4.2x10 W/cm . Furthermore the effect has his impact above the material surface within the volume of the electric arc. Figure 5 shows the stabilization of the electric arc. At the beginning of the voltage measurement the electric arc is instable burning. After 50 sec the laser radiation is stepping into the process. Simultaneously the electric arc is stabilized and the voltage goes to the adjusted value. Cutting of the laser beam again leads to an instable arc. The OGE
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1mm 22 2mm 23 3mm 24 Figure 2: Aluminium probes welded with a current of 120A, Laser power 300W cw Nd:YAG, by a constant feed rate and a variation of the focus position. The stabilization and guidance effect from the laser radiation can even more drastically be observed in motion. Figure 3 shows an example of two pictures extracted out of one movie. The cw laser beam with a wavelength of 1064 nm and 300 W is focused in 300 μm spot onto the material surface. On the left side the
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opens a channel of better conductivity in direction of the laser beam and in form of the laser beam caustic.
6 ACKNOWLEDGMENTS This research and development is / was funded by the German Federal Ministry of Education and Research (BMBF) within the Framework Concept ”Research for Tomorrow`s Production” (fund number 02PB2051) and managed by the Project Management Agency Forschungszentrum Karlsruhe, Production and Manufacturing Technologies Division (PTKA-PFT).
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7 REFERENCES [1] Kunieda, M., Xia, H., Nishiwaki N., Kinoshita N., 1992, Observation of the arc column movement during monopulse discharge in EDM, CIRP Annals 41/1/1992, p. 227 [2] Haferkamp, H., Ostendorf, A., Bunte, J., Szinyur, J., Höfemann, M., Cordini, P., 2002, Increased Seam Quality for Laser-GMA Hybrid Welding of ZincCoated Steel, Proceedings ICALEO 2002, S. 129137 [3] Abe, N., Agano, Y., Tsukamoto, M., 1997, Effect of CO2 laser irradiation on arc welding, Trans JWRI 1997 Vol. 26 (1) p 69. [4] Seyffarth, P., Krivtsun, I.V., 2002, Laser-ArcProcesses and their Applications in Welding and Material Treatment, Welding and Allied Processes Volume I, Taylor & Francis. [5] Dilthey, U., Lueder, F., Wieschemann, A., 1999, Technical and economical advantages by synergies in Laser Arc Hybrid Welding, Proceedings IIW International Conference "The Human Factor and it is Environment”, Lisbon, 1999, S. 141 – 152. [6] Cui, H., 1991, Untersuchungen der Wechselwirkung zwischen Schweißlichtbogen und fokussiertem Laserstrahl und der Anwendungsmöglichkeit kombinierter Laser-Lichtbogentechnik, ISSN03449629. [7] Lowke, J., Kovitya, P., Schmidt, H. P., 1992, Theory of free-burning arc columns including the influence of the cathode, J. Phys. D: Appl. Phys. 25 1600-6 [8] Paulini, J., Simon, G., 1993, A theoretical lower limit for laser power in laser-enhanced arc welding, J. Phys. D: Appl. Phys. 26 (1993) 1523-1527 [9] Tönshoff, H.K., Ostendorf, A., Körber, K., Kulik, K., Kamlage, G., 2001, Micro Material Processing using UV Laser and Femtosecond Laser, Proceedings of 10th International Conference on Precision Engineering (ICPE), pp. 62-66 [10] Ostendorf, A., Kulik, C., Stute, U., 2002, Production of Innovative Geometries with UV Lasers, Proceedings of SPIE Vol. 4637, pp. 280 – 290 [11] Tönshoff, H.K, Butje, R., König, W., Trasser, FR.-J., 1988, Excimer Laser in Material Processing, CIRP Ann. Vol. 37, no. 2, pp. 681-684 [12] Zhu, X., Nur, A.H., Misra, P., 1994, Laser optogalvanic wavelength calibration with a commercial hollow cathode iron-neon discharge lamp, Spectroscopy radiation Transfer Vol. 52, No.2, pp.167-17 [13] Hoshi, Y., Yoshida, H., 1999, Application of laserguided discharge to processing, Appl. Phys. A 68, 93–98
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Figure 5: Voltage signal with a laser-induced stabilization of the electric arc in TIG welding Figure 2 shows that by defocusing the laser beam still the guidance and stabilization is possible. In addition the penetration depth of the electrical arc is rising from 0.65 mm to 0.9 mm when the laser focus is 2 mm above the work piece (Figure 6). In consequence of the focus shifting the spot size is enlarged by a factor of 3.5 to a diameter of more than 1 mm. The deeper penetration can lead back to a greater overlap region of plasma plume and laser beam. As the effect is easily saturated due to the low density of atoms in the plasma plume, an extended beam provides better results than a focussed beam and was demonstrated by experimental studies.
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Figure 6: Penetration depth of aluminium probes 5 CONCLUSION AND OUTLOOK The coincidence of a laser beam and an arc discharge results in stabilisation of the arc. Several physical effects contribute to this phenomenon, but from our investigations it is obvious that for pulsed lasers the plasma is the dominant effect and for cw lasers the optogalvanic effect is the main reason for this positive interaction. In further investigations the effect will be maximized by finding the optimal wavelength and the ideal beam shape. Not only arc welding applications can benefit from the laser aided plasma, in electric discharge machining (EDM) it can be exploited as well [13]. This process is very slow due to the small gap between the electrodes. The spark ignites between electrode positions where the resistance of the dielectric gap is minimal in other words where the gap is minimal. Therefore in order to control the position of the spark discharge the space must not exceed 50 μm. If now a channel of reduced resistance is provided by a laser the discharge will propagate along this channel and the electrode spacing can be increased. This will result in higher ablation rates per shot and thus faster processes with constant surface qualities.
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