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Laser-vapour interaction in high-power cw CO2 laser welding (October 2003)

   

J Greses, P A Hilton, TWI, Granta Park, Great Abington, Cambridge, UK; C Y Barlow, Engineering Department, University of Cambridge, Cambridge, UK; W M Steen, Material Science Department, University of Cambridge, Cambridge, UK and Laser Group, University of Liverpool, Liverpool, UK

Paper P546 presented at ICALEO 2003 conference, Jacksonville, Florida, USA, 13-16 October 2003

Abstract

During high-power cw CO 2 laser welding vapour is ejected from the keyhole. Spectroscopic studies of the vapour emission have demonstrated that the vapour can be considered as a partially ionised plasma. In this paper a review of the electron temperature and density in the plasma is presented. Despite many previous studies, the accuracy of measurement of the electron temperature and density in the plasma required to calculate the plasma absorption coefficient, still remains open to interpretation. The difficulties and problems faced in analysing the spectroscopic results have been reviewed and explained. Nevertheless, it seems clear that laser energy absorbed by the plasma is negligible if shielded with a gas of high ionisation potential. However, the gradient of temperature and density in the vapour plasma leads to a gradient in the refractive index across it, defocusing the laser beam and changing its energy density. Methods for mitigating the effects of the interaction between the laser and the plasma vapour are presented together with their advantages and disadvantages. A flow of high ionisation potential gas has been established as a satisfactory method for controlling, stabilising and reducing the effects of the plasma formation when CO 2 laser welding.

1. Introduction

Many types of lasers are used in industry for laser material processing, although CO 2 gas and Nd:YAG solid state lasers are the ones mainly used for high-power welding. The CO 2 laser delivers a beam with a wavelength of 10.6µm and powers up to 20kW are commercially available. Nd:YAG lasers produce a beam with a wavelength of 1.06µm and powers up to 6kW are commercially available. In recent years it has also been possible to combine several Nd:YAG beams into a single beam achieving a high-power Nd:YAG wavelength output, with the state-of-the-art being a three-beam combiner of 3 and 4kW lasers giving around 10kW on the workpiece. [1]

The use of such high-power Nd:YAG laser light for welding has presented new issues and problems when compared to high-power CO 2 laser welding. In early work [1,2] it was clear that the formation of a large and difficult to control plume, energetic in the orange-yellow part of the spectrum, when high-power continuous wave (cw) Nd:YAG laser welding, was very different to the intensely bright bluish colour of the plasma formed when CO 2 laser welding. [3,4] The mechanism responsible for this is the difference in interaction between the laser beam (of different wavelengths) and the vapour ejected from the keyhole. Since a detailed description of these physical processes was unclear, a research project was established to develop a better understanding of these mechanisms. The first part of the outcome of this project, dealing with high-power CO 2 laser welding, is presented in this paper. A second paper [5] , also included in these proceedings, deals with high-power cw Nd:YAG laser welding.

2. Optical emission from plasma vapour

Thermal sources, such as the vapour generated in laser welding, can be divided into two classes, blackbody radiators and line sources. [6] Blackbody radiators are opaque bodies or hot, dense gases, that radiate at virtually all wavelengths. Line sources radiate mainly at discrete wavelengths. It is possible to establish the temperature of the both vapour emission types by analysing its optical properties.

The vapour emissions from a keyhole when CO 2 laser welding have been described in several studies as line sources. [7-9] These studies agree on the theoretical process of plasma vapour formation, but the experimentally derived electron temperature and density resulting from the vapour show a great variance (more than 5000K depending on many parameters). Nevertheless, from the results available, made with and without plasma control gases, the level of ionisation in the vapour is low, and the vapour is described as partially ionised plasma. [4] Plasma ( (1) Greek word for jelly) is defined as 'a highly ionised gas which contains (approximately) equal number of ions and electrons in sufficient density so that the Debye shielding radius is much smaller than the dimensions of the gas.' [10] In addition to these characteristics, there should be many particles within a Debye sphere, with the consequence that the statistical treatment of Debye shielding is valid. Exactly when the transition between a high temperature gas and ionised plasma occurs is largely a matter of nomenclature. [11] The important point is that a partially or fully ionised gas has unique properties, such as among others a gradient of electron temperature and density along its volume, and an electromagnetic field arising from the charge separation between ions and electrons.

spjgoct2003ae1a.gif
(1)

An explanation of fully and partially ionised laser-plasmas is out of the scope of this paper and it can be found elsewhere. [11,12] Nevertheless, there are four main types of sources for emission of electrons from a plasma, multiphoton ionisation, natural presence of free electrons, thermionic effect and thermal ionisation, or photoionisation, of the vapour [13] , although only the multi-photon ionisation is relevant to CO 2 laser welding. Multi-photon ionisation applies when an atom absorbs enough photons to reach its ionisation potential threshold, thus establishing seed electrons that initiate plasma heating via the inverse Bremsstrahlung effect. Inverse Bremsstrahlung can be defined as absorption of electromagnetic radiation (photons) by an electron making a Coulomb collision in the proximity of an ion [11] , increasing the kinetic energy of the electron and thus, its temperature. [11]

Under the assumption of local thermodynamic equilibrium (LTE), that is that all particles have a Maxwellian velocity distribution and that collisional processes dominate the rate equations such that Boltzmann statistics apply, the Boltzmann distribution of the ionised atoms in the plasma vapour is governed by the Saha equation. [14,15] The formulae that describe the behaviour and characteristics of a plasma are available in the literature. [4,12,16] The main methods to determine the electron temperature in the plasma, such as the Boltzmann-plot method, and the electron density are also available in the literature. An extensive review of electron density measurement methods was given by Ready. [17]

3. Review of the plasma electron temperature and density

Many researchers have tried to investigate, both theoretically and experimentally, the electron temperature and density in the plasma vapour when CO 2 laser welding. Most of the studies were carried out focusing on the plasma outside the keyhole, where direct access to the plasma emission is relatively easy. Nevertheless, some direct observations of the plasma vapour inside the keyhole in deep penetration welding have been performed based on the original idea of Verigin [18] , of determining the composition of the gas phase through a lateral observation channel linked to the keyhole.

Sokolowski [19] used a spectral diagnostic during the keyhole formation at the beginning of the metal sheet as well as the end edge of the workpiece. Although this method suffered from a poor signal-to-noise ratio due to the near-collapse conditions of the keyhole, it was possible to estimate a range of electron temperatures in the plasma vapour from 9630K to 12060K, and of electron densities from 1x10 17cm -3 to 3.1x10 [18] cm -3. No reference to changes in temperature along the depth of the keyhole were made. Fabbro [20] conducted spectroscopic studies of the keyhole plasma with the same method as Verigin [18] concluding that microscopic plasma parameters inside the keyhole were similar to those just outside the entrance of the keyhole. Funk [21] also used a similar experimental method to that of Verigin and Fabbro with a 15kW CO 2 laser at different processing speeds. Electron temperatures, between 6960K and 10440K, depending on the welding speed, and densities, were measured inside the keyhole, these being slightly higher than those measured outside the keyhole. Maiwa et al [22] showed theoretically and experimentally, with an array of photodiodes, that a hot spot of plasma could be detected inside the keyhole, very close to its opening. Tu et al [23] expanded Miawa's work, providing a distribution of the electron temperature and density of the plasma along the keyhole length when CO 2 laser welding at 20kW. The electron temperature, around 8100K, was measured to be fairly constant over the keyhole length. Conversely, the electron density varied from a maximum of 1.5x10 17cm -3 very close to the top of the keyhole, to 0.29x10 17cm -3 at the lower part of the keyhole.

Many researchers have performed spectroscopic studies on plasma formed outside the keyhole when CO 2 laser welding of steel. Herziger [24] summarised some research results up to the mid 1980s, while Duley [4] presented a compilation, from many references, listing the electron temperatures and densities of the plasma outside the keyhole when CO 2 laser welding. An expanded version of these results is shown in Table 1, where P is the laser power, Ø the diameter of the focal spot at the focal position, h w the height above the surface of the workpiece, I the energy density of the laser beam, Gas the plasma control gas, N e the electron density, T e the electron temperature, α ib the inverse Bremsstrahlung absorption coefficient (calculated from equations [1-3] below) and Tr the transmittance (calculated from equation [4]), through a 10mm length of plasma, based on the available data when cw CO 2 laser welding. A basic review of electron temperatures and electron densities obtained with theoretical models was presented by Chen and Wang. [25] In general, the theoretical results show higher electron temperatures (~15000) and densities compared to values in Table 1.

Table 1 Electron density and temperature, inverse Bremsstrahlung and transmittance when cw CO 2 laser welding (expanded from Duley [4] ).

MetalP
(kW)
Ø
(mm)
I
(W/cm 2)
Gash w
(mm)
N e
(cm -3)
T e
(K)
α ib
(cm -1)
Tr
(%)
Reference
Steel 2.0 0.30 ~
2.8x10 6
Ar 0.5 25x10 16 8000-
12000
1.78-
1.00
16.85
36.92
[26]
Fe 1.0 - ~
1x10 6
He 0.0 6.7x10 16 6250 0.18 83.44 [8]
1.0 3x10 16 5200 0.05 95.43
15.0 ~0.80 ~3x10 6 He 0.0 8.2x10 16 6700 0.25 7819
2.0 4.4x10 16 6250 0.08 92.49
5.0 3x10 16 5500 0.04 95.76
Stainless Steel (S/S) 2.5 0.20 ~6.4x10 6 He 0.0 4.4x10 16 8000 0.06 94.63 [27]
2.0 1.8x10 16 6200 0.01 98.69
Ar 0.0 8.5x10 16 8600 0.19 83.05
2.0 3.5x10 16 7200 0.04 96.03
Steel 22 - - Ar 0.0 11x10 16 12900 0.17 84.05 [28]
60.0 7.0x10 16 12150 0.08 92.61
Fe 10.5 - - Ar - 5x10 16 6200 0.10 90.31 [29]
Steel 3.0 0.30 ~4.2x10 6 Ar 0.5 2x10 16 9500 0.01 99.11 [22]
-0.5 20x10 16 13000 0.57 56.67
Steel 2.5 - - He 1.0 5-15x10 16 - - - [30]
20.0 - - He 1.0 11-15x10 16 - - -
S/S 2.0 0.26 ~3.7x10 6 Ar 0.0 11x10 16 10800 0.22 79.89 [9]
(~15% accuracy)
1.0 10x10 16 10000 0.21 81.28
Steel 5.44 - - Ar Core 18x10 16 20000 0.25 78.23 [31]
Edge 6x10 16 12000 0.06 94.42
Steel 2.0 0.20 ~
6.4x10 6
He - - 9050 0.10 90.41 [32]
Ar - - 10000 - -
1.5 0.20 ~
4.8x10 6
Ar - - 9750 - -
S/S 2.0 0.20 ~
6.4x10 6
Ar - - 9400 - - Fe I [33]
10900 - - Cr I
11700 - - Mn I
Steel 20.0 0.50 10x10 6 He -0.1 6x10 16 8100 0.10 90.41 [23]

4. Plasma absorption coefficient and transmission

Based on plasma keyhole electron temperature and density measurements, Sokolowski [19] deduced that three types of absorption mechanism are operating inside the keyhole when CO 2 laser welding: Fresnel, micro-plasma and plasma absorption. Direct and multiple reflection Fresnel absorption is the main mechanism by which the laser beam is absorbed into the keyhole walls, [24] however, micro-plasma absorption can be considered negligible. [19] Plasma absorption can occur when the plasma volume inside the keyhole absorbs the directly incoming or multiple reflected laser radiation via inverse Bremsstrahlung. [35] From Mitchner and Kruger, [36] the inverse Bremsstrahlung absorption coefficient ( α ib ) can be defined as the sum of the electron-neutral atom (for Fe, α e-Fe ) and the electron-ion ( α e-i ) absorption coefficients:

spjgoct2003ae1.gif
[1]

where γ is the collision frequency between different species, ν e-Fe the electron-neutral atom (of Fe) collision frequency, ν e-i the electron-ion collision frequency, c is the speed of light, w the frequency of light ( w = 2 πc/ λ, where λ is the laser wavelength), and w p the plasma frequency.

The formulae for w p , v e-Fe and v e-i can easily be found in the plasma literature. However, assuming LTE conditions [17] , the electron-ion absorption coefficient ( α e-i ) is

spjgoct2003ae2.gif
[2]

where N e,i are the density of electrons and ions respectively, Z the average charge, e the electron charge, h the Planck constant, m e the electron mass, T e the electron temperature and k the Boltzmann constant.

From equation [2], it is clear that the electron density has a high influence on the electron-ion absorption coefficient. For the electron temperature and density range given when CO 2 laser welding ( Table 1), the electron-ion absorption coefficient completely dominates over the electron-neutral atom one, and the latter could be considered negligible for most calculations. Thus,

spjgoct2003ae3.gif
[3]

Considering only the effect of inverse Bremsstrahlung (IB) absorption in the plasma, the laser intensity I(z) transmitted through the plasma-laser interaction zone along a path length z can be described by Beer's law, [37] where I 0 is the incident intensity of the laser beam,

spjgoct2003ae4.gif
[4]

Thus, the percentage of the laser beam intensity transmitted through a plasma length of 10mm (Tr or transmittance, based on equation [4]), for the given IB absorption coefficient ( α ib ) is shown in Table 1.

In Table 1, it can be noticed that if the appropriate shielding gas is used (i.e. helium rather than argon), the IB absorption is low and the transmittance of the laser beam intensity over a length of 10mm is, in most cases, well over 90%. The transmittance values are obtained assuming that the IB absorption coefficient is constant over the 10mm length. In reality the coefficient is not constant, as can be seen in Table 1 and varies with a varying h w (the height above the surface of the workpiece). The IB absorption coefficient decreases both in the direction of the incoming laser beam from the workpiece surface and in the radial direction away from the laser beam axis. Thus, in real terms, the transmittance will be even higher than that calculated in Table 1.

5. Difficulties and issues in plasma spectroscopy

Table 1 clearly shows the great variance in electron temperature and electron density measurements. For example, the maximum estimated temperature was 20000K [31] and the minimum 5500K. [8] Direct comparison of electron temperatures and densities is, however, extremely difficult since the plasma temperature is affected by a great number of parameters. These include; material composition, power density of the laser, beam quality, type of shielding gas, gas flow, gas delivery system (coaxial or side jet nozzle), nozzle geometry, position of collection area of the plasma emission, etc. Also the characteristics (mainly, but not only, the quantum efficiency) of the spectrometer, photon detector, data acquisition system, and light receptor optics will affect the electron density and temperature estimation.

Furthermore, since the total number of atomic (2383) and ionic (1207) lines of iron that can be identified over the whole spectrum is vast, different researchers use different lines to calculate the electron temperature. This usually depends on which lines are easily identified with a given spectrometer. The uncertainty of the spectral line data (a minimum of 3%, between 10 and 25% for the most commonly used lines and up to 50% or more for certain other lines) also plays a critical role in the accuracy of the temperature measurements.

Most of the results in Table 1 have used atomic Fe lines to determine the plasma temperature with only a few of them using the Fe I lines in combination with ionic (Fe II) lines. Szymanski [9] argued that low values of plasma temperature reported in the literature (mainly those below 8000K) are due to the use of only Fe I lines. They argued that for more exact calculations the ratio between ionic and atomic lines should be used because the emission coefficients for each line (atomic or ionic) reach a maximum at a predetermined temperature, which is usually higher for the ionic lines. Szymanski [9] also showed that by using the intensities of the atomic lines, only the edge or peripheral temperature of the plasma vapour could be calculated, and not the plasma core temperature. Unfortunately, it is very difficult to identify the ionic Fe lines without expensive laboratory high-resolution spectrometers. In addition, since most spectroscopic studies have been performed in order to evaluate the possibility of plasma emission as a quality control mechanism for the laser welding process, only the stronger Fe I lines that a robust and relatively inexpensive spectrometer can identify, tend to be used. Furthermore, the use of the Abel inversion to determine the axial temperature of the plume could also produce a higher than measured plasma temperature.

Since most of the methods used to calculate the electron density of the plasma generated when laser welding are dependent on the electron temperature, an analysis of the electron density faces similar problems to the analysis of the electron temperature described in the previous paragraphs. Additionally, as the electron density ( N e ) varies exponentially with respect to the electron temperature ( T e ), a variation of ~7% in T e leads to a variation of ~50% in N e . It is also possible to determine the electron density of plasma with the Stark broadening method, [9,38] although this method is not exempt from problems either. Both researchers, [9,38] together with Tu, [23] determined the Stark broadening of the 5383.37Å atomic iron line because this is one of the few lines for which relatively accurate data is available and, at the same time, this line is isolated from the influence of other surrounding lines. However, a systematic use of the 5383.37Å atomic iron line and the Stark broadening method, could lead to the same problems as those reported by Szymanski, [9] in the previous paragraph, with respect to the electron temperature measurement with the Fe I and Fe II lines. Nevertheless, the electron density values obtained using Stark broadening are in line with those of other methods as shown in Table 1.

These uncertainties give rise to some concerns about the temperatures published in the literature and the degree of accuracy that can be achieved through spectroscopic measurements. Poueyo-Verwaerde [8] calculated a plasma temperature of ~6100K using the Fe I lines 4427.31 and 4422.57Å. Using the same Fe I lines, but different welding and shielding conditions, Greses [16] also calculated a temperature of ~6000K. However, during the same experiments by Greses, [16] a much higher electron temperature (8000 to 10000K) was obtained [16] when different Fe I lines were used to calculate the plasma temperature.

6. Methods of plasma control

In high-power laser welding, the process gases play two important roles. Firstly, gases are used to shield the molten or hot weld pool and the keyhole, and are referred to as shielding gases. These gases mainly prevent oxidation which would otherwise occur due to direct contact between oxygen in the atmosphere and the hot metal, improving the quality of the weld. Practically any commercially available inert welding gas can achieve shielding of the hot metal. Secondly, gases can be used for plasma/plume control, and will be referred to as control gases. This plasma control function is much more difficult to achieve than simply shielding the weld, particularly at high laser powers and low travel speeds.

Besides these two main roles the composition and delivery of the gas also influence other aspects ( Table 2) of the laser welding process. [39]

Several methods of controlling, by mainly suppressing or restricting the plasma formation above the keyhole when laser welding with CO 2 lasers, have been investigated. [39] These are summarised in Table 3. However, it should be noted that almost all high-power CO 2 laser welding systems used in production, utilise a flow of high ionisation potential gas for plasma control.

Table 2 Secondary roles of the use of process gas in laser welding (expanded from [39] ).

Gas RoleComments
Protection Protects the focusing lens and mirrors from projections, spatter and condensed particles in the vapour emerging from the keyhole.
Weld Porosity Incorrect process gas conditions may cause gas to be trapped inside the weld, resulting in porosity. [40]
Fluid Flow Patterns Addition of surface-active elements (e.g. oxygen and sulphur) may influence the convective energy transport in the weld. The molten metal surface tension is modified, thereby influencing the magnitude and direction of the melt or Marangoni flow. [41,42] Thus, the weld shape can be modified.
Weld Composition High levels of weld metal oxygen and nitrogen can increase significantly as a result of the gas composition used or due to the pollution of the gas shield by air. Changes to the weld metal composition have direct implications for the quality of the weld.
Energy Coupling Reactive gases may result in the formation of a compound on the weld pool surface. With oxygen, for example, oxidation of the weld pool surface may occur. As the absorption coefficient of the oxide is generally higher than that of the metal, this generally increases the energy coupling of the laser beam into the material.

Table 3 Main mechanisms for plasma control when welding with CO 2 lasers (expanded from [39] ).

Plasma Control MechanismsDescription and Comments
Reduced Ambient Pressure At reduced ambient pressures (below 10 -1Torr) [43,44] the mean free path is about 1 mm and the energy contained in the metallic vapour rapidly disperses, as soon as it emerges from the keyhole. Therefore, the ionisation of the metal vapour is very low and inverse Bremsstrahlung absorption does not occur.
The increased penetration may also be due to the smaller differences between the boiling and melting points, leading to thinner melt walls and hence a more stable keyhole. [3]
Electric and Magnetic Fields Tse [45] and Peng [46] applied magnetic fields reducing the radial extent of plasma formation and driving away the charged particles in the plasma, respectively. Increases up to 7% in penetration were measured.
Magnetically supported laser beam welding (MSLBW) process [47] - The CO 2 laser welding process can be stabilised, increasing penetration and reducing porosity, by applying a magneto-fluid dynamic mechanism that is able to change the flow conditions in the weld pool.
Mechanical (beam oscillation) or Laser Power Pulsing Beam walking - Periodically moving the laser beam ahead to dwell on a new focal area ahead of the original position of the keyhole. [3]
Beam spinning - The beam is rotated, creating a larger keyhole and weld pool. [3]
Laser pulsing - The timing of the release of the laser pulse with the reduction in plasma size, after formation and expansion. [48,49] Although it is difficult to match an adequate laser pulse with the new plasma cycle for every change in parameters.
Flow of Gas Delivered to the laser-material interaction zone via a coaxial nozzle, trailing/leading angled side jet or full shielding shoe. Dawes [50] reviewed the basic nozzle designs. The blowing velocity of the control gas may affect the shape and behaviour of the keyhole making the process and quality control more difficult. [51]

The physical properties of the gases which strongly influence plasma formation are described in Table 4 (expanded from [52,39] ). Relevant properties of the gases He, Ar, O 2, N 2 and CO 2 are compared to those of pure iron (Fe) in Table 5.

Table 4 Gas properties affecting plasma formation and characteristics (expanded from [39,52] ).

Gas PropertiesDescription
Ionisation potential The plasma control gas should have a higher ionisation potential than that of the iron (at 7.87eV) for the laser welding of steel. A low ionisation potential gas will be easily turned into ions and electrons and thereby contribute to the further build-up of plasma above the keyhole. However, even in an argon atmosphere, argon atoms (ionisation potential of 15.8eV) were able to dissociate by colliding with rising metal vapour. [44]
Thermal conductivity Increasing the recombination rate by an increase in electron collisions with ions and neutral atoms (of Fe), i.e. cooling the plasma, can lead to diminishing the absorption of the laser beam. The cooling efficiency is higher for gases that have a high thermal conductivity (which is inversely related to atomic weight) and is further affected by the gas velocity. Helium, being a light gas, is more effective at cooling the electrons due to collisions, increasing the recombination rate. [53] A high kinetic speed of the gas atoms results in effective heat removal and the associated reduction in temperature. As explained earlier, an increase in the diffusion losses out of the interaction volume of the laser beam and plasma can be best achieved by blowing a gas side jet across the interaction zone.

The control gas thermal conductivity cannot be calculated over the temperature range that occurs when CO 2 laser welding. [54]
Dissociation properties The dissociation of a molecule by absorption of energy can remove heat reducing further the size of the plasma. The dissociation potential of the gas should be below the ionisation potential of iron (7.87eV). Among the process gases generally considered for laser welding, H 2, N 2 and CO 2, with a dissociation potential of 5.5eV producing CO and O, fit into this category, although the free oxygen may be chemically undesirable.
Electro-negativity Some gases such as F 2, Cl 2, O 2, SF 6, CF 4 and H 2 have high electronegativity. Consequently, they can reduce the concentration of free electrons in the plasma by catching and binding them thereby reducing the absorption of the laser beam and thus the plasma electron temperature and density. Fluorine, F, added as SF 6 or CF 4 increases the welding depth. CF 4 is highly recommended since high quantities of SF 6 (> 4%) can produce a thermal blooming effect.
Density Heavier gases are more effective in shielding when downhand welding.

Table 5 Physical properties of most widely used laser welding control gases. [39,55]

GasAtomic MassThermal Conductivity
(W/cmK)
Density @ 237K & 1atmElectro-negativy
(Pauling)
1 st Ionisation
Potential (eV)
He 4.0026 0.00152 0.1785 - 24.587
Ar 39.948 0.0001772 1.7824 - 15.759
N 14.00674 0.0002598 1.2506 3.04 14.534
O 15.9994 0.0002674 1.429 3.44 13.618
CO 2 - 0.080 W/mK at 1200 K - - 13.800
           
Fe 55.847 0.802 7.874g/cc @ 300K 1.83 7.870

6. Discussion and conclusions

Spectroscopic studies of the vapour generated when CO 2 laser welding have been performed since the mid 1980s based on the methods developed in the 1970s for estimating the temperatures in electrical arcs (Lucas 1979). More than 25 years later, and despite the advances in CCD spectrometers and software analysis the subject still remains one of extreme difficulty. Despite all the investigations presented in Table 2, measurement of the electron temperature and density in the plasma when cw CO 2 laser welding is open to much debate. This is not only for pure scientific knowledge, but one of practical importance, since the electron temperature and density determine the physical mechanisms of power loss in the vapour outside the keyhole and power absorption inside it. Furthermore, this information is also fundamental for creating a consistent mathematical model of the laser welding process.

It is reasonable to state that the temperature of the vapour plasma when CO 2 laser welding mild steels lies between 6500 and 12000K, depending on the plasma control conditions and other parameters. It has been proved that estimates of the plasma temperature when CO 2 laser welding are very sensitive to the chosen atomic/ionic lines, and depending on the chosen spectral lines a range of temperatures, varying by ~3000K or more, could exist. If the uncertainty in spectral line data (between 10 and 25%) is taken into account, then the variance in calculated plasma temperature values is even greater. As an example Szymanski [9] determined the accuracy of their electron temperature results to be around 12%, i.e. nearly 1500K for a temperature of 10000K. The electron density uncertainty was even higher at ~17%.

It seems clear that the use of different control gas conditions does not affect the core temperature of the plasma, which is related to the metallic vapour, but does affect the size and extent of the plasma vapour. Furthermore, direct absorption of the laser beam in the plasma is minimal if shielding with a high ionisation potential. However, a small plasma absorption is enough to cause a gradient of temperature and density in the vapour plasma leading to a gradient in the refractive index across it, defocusing the laser beam and changing the energy density within it. The larger the size of the plasma, the larger the gradient of refractive index across the plasma, resulting in a greater beam defocusing. A defocusing of the laser beam will reduce the maximum penetration that can be achieved at a certain energy density and speed. Defocusing is also responsible for changes in the weld shape, mainly the wine-glass cross section, and porosity formation due to the plasma instabilities causing keyhole collapse.

When CO 2 laser welding thick section steel, the combination of high ionisation potential and high thermal conductivity of helium gas has made it the most efficient and common gas to be used for plasma control in both production and research environments. Argon, being a cheaper gas, is mostly employed as shielding gas delivered via coaxial nozzle, where higher flows are necessary. Argon gas is also used via a side jet as direct plasma control when the powers employed are low or welding speeds are high. For economic reasons, in addition to helium and argon, nitrogen and CO 2 gases are very much in use in Japan, with their dissociation properties partially compensating for their low ionisation potential.

7. Acknowledgements

TWI equipment and the dedication of the staff of the Laser Department made this work possible. Part of this research was carried under EPSRC grant No. 99313089.

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Meet the authors:

José Greses started to work at the Laser and Sheet Processes Group of TWI (UK) in May 2003, after completion of his PhD on laser welding at the Engineering Department of the University of Cambridge (UK). Paul Hilton is Technology Manager at the Laser and Sheet Processes Group at TWI (UK), Claire Barlow is Senior Lecturer at the Engineering Department of the University of Cambridge (UK) and Bill Steen is Emeritus Professor at the University of Liverpool and Distinguished Research Fellow at the Univ. of Cambridge (UK).

For more information please email:


contactus@twi.co.uk