Gas Environments and CO2 / Nd:YAG Laser Welding Efficiencies

* Engineering Department, University of Cambridge. E-mail: jg278@eng.cam.ac.uk
** TWI
*** Material Science Department, University of Cambridge

Paper presented at International Congress on Laser Advanced Materials Processing (LAMP 2002), Osaka University, 27 - 31 May 2002

When laser welding mild steel under high power densities, vaporised material is ejected from the keyhole and forms a plume/plasma above the weld pool. In previous studies on plume formation and extent of ionisation, the influence of the laser wavelength and the gas environment has been observed. In this study a comparison between CO₂ and Nd:YAG laser welding has been performed using the same energy density (~ 1.24MW/cm², produced using 3.5kW of power and a focal spot size of 0.6mm) in He, Ar and N₂ gas environments and in vacuum. Plume/plasma evolution was recorded with high-speed video at 9000 frames/second and these images have been correlated with the characteristics of the weld cross-section. The fusion and heat-affected zone profiles have been measured to analyse the melting efficiency at different processing speeds. The temperatures and electron densities in the plume/plasma have also been calculated by spectroscopic methods to estimate the losses caused by the plume/plasma development. By analysing the differences in the weld shape profiles and the plume/plasma behaviour, the temporal evolution of the laser welding process efficiency was also obtained.

1. Introduction

Welding experience in the mid-70s with CO₂ lasers led to the use of side jets of high ionisation potential gas to suppress or control the plasma formation. Helium is the most efficient gas for plasma control although due to its high price, argon and nitrogen have also been used for this purpose. ^[1] Based on the experience of high power CO₂ laser welding, it was logical to apply the same approach for the control of plasma to the high power Nd:YAG laser welding process. However, Matsunawa ^[2] and Lacroix ^[3] indicated that the vapour ejected from the keyhole under high energy densities in Nd:YAG laser welding is a high-temperature thermally excited gas rather than a partially ionised plasma. Greses et al ^[4] discussed the differences between CO₂ and Nd:YAG plasma/plume formation and temperatures during keyhole welding, confirming that the ionisation potential of the gas does not play an important role in Nd:YAG laser welding. However, the different plasma/plume formation has important effects on the weld geometry (i.e. penetration and width), therefore affecting the process efficiency.

The pioneering analytical studies by Swift-Hook and Gick ^[5] on the heat transfer analysis (Rosenthal equations) showed that, in high-speed keyhole welding, a theoretical maximum of 48.39% of the absorbed energy could be used for melting. Fuerschbach ^[6] further developed the analytical model, but using the fusion area rather than just the penetration and width values of the weld cross section. In both cases only CO₂ lasers were used, and the effect of the shielding gas was not considered.

In this paper a comparative experiment was devised, keeping as many parameters as possible the same for both sets of experiments performed, except the wavelength of laser light used (10.6µm for the CO ₂ laser and 1.06µm for the Nd:YAG laser). In that way the efficiency of the welding process produced by both lasers could be compared in equal terms, allowing effects of a gas side jet to be characterised.

2. Experimental Configuration

A CO₂ LASER ECOSSE AF5 fast axial flow laser, giving 3.5kW cw power at the workpiece with a multimode beam, was focused by a Zn-Se lens with a focal length of about 300mm, to a focal spot of 0.62mm in diameter. This relatively large focal spot was deliberately chosen for comparability with the Nd:YAG laser welding experiments. The Nd:YAG laser used was a GSI LUMONICS ASM series laser. The laser delivered 3.5kW cw to the workpiece, through an optical fibre and the beam was focused using 1:1 imaging to produce a 0.6mm diameter focal spot with a lens to workpiece distance of about 190mm. The power of both lasers remained unchanged throughout the experiments.

A modified glove-box was used to form a controlled atmosphere around the weld. The size of the chamber, with an octagonal section (length 120cm, sides 30cm) and a total volume capacity of around 2m³, in conjunction with the short length of welds produced, minimised the effects of fume build up. A flat Zn-Se window positioned in one of the ports in the top of the chamber provided access for the CO₂ laser light. It proved convenient to seal the Nd:YAG focusing head to the same port, for the second set of experiments ( Fig.1).

The sealable chamber allowed experiments to be performed in atmospheres of air, helium, argon or nitrogen. In addition trailing helium, argon and nitrogen side jets could be applied at different flow rates (10-50 l/min), at a range of positions ±2mm from the laser beam-material interaction point (i.e. impingement point). Nozzle diameters of 2 and 4mm were used, and the side jet was positioned at 0° and 45° to the horizontal, in line with the welding direction.

C-Mn mild steel was used throughout the experiments, for bead-on-plate welding. Test plates had dimensions 300x120x12mm, and before welding the surface was ground to remove any scale and de-grease. The material composition is shown in Table 1.

Table 1 Chemical Composition of the 12mm thickness C-Mn mild steel.

All bead-on-plate experiments were undertaken in a vertically down, 12 o'clock, orientation. The weld bead profile cross-sections of bean-on-plate welds were examined after grinding, polishing, and etching in 2% Nital solution. The bead-on-plate weld bead profiles, penetration and top width (P and W respectively in the tables), were measured using a microscope. Weld discontinuities such as porosity and undercut were not quantified. The microscope images were also digitised and analysed using image-processing software, enabling the fusion and heat affected zone (HAZ) areas to be measured.

3. Energy Balance and Process Efficiency Analysis

The energy balance equation for the laser weld can be expressed as

E _Total = E _Losses + E _Fusion + E _HAZ + E _Evap + E _Cond     [1]

where E _{Losses (before reaching the keyhole)} = E _Reflection + E _Plasma/Plume

The energy lost in the reflection ( E _Reflection ), once the process has been stabilised, can be estimated to be about 5% (or ~200W if the laser output is 3.5kW, ^[7] ). Although at room temperature the absorption (for carbon steel under CO ₂ laser welding) is around 4%, at the melting temperature this could be more than 30% and reaches 90% at vaporisation temperature. The absorption depends on the metal properties, temperature, surface finish and the wavelength of the laser beam. When the vapour pressure creates a keyhole, this behaves as a blackbody absorbing up to 95% of the incident beam power. ^[7] For Nd:YAG laser welding the absorption figures for mild steel are similar.

The losses arising from the plasma or the plume, have been estimated by considering the radiant emittance of the vapour outside the keyhole given by the Stefan-Boltzmann law. The radiant emittance of a blackbody (i.e. the radiant energy emitted per unit time and unit area) is proportional to the fourth power of the temperature ^[8] :

where E is the total radiant power at all wavelengths (W); A the area of the radiating surface (m ²); T the temperature (K), and σ the Stefan-Boltzmann constant (Wm ^-2K ^-4).

In Nd:YAG welding the vapour emerging from the keyhole can be modelled as a cone, which can be approximated to a trapezoid in the vertical section. If the plume of vapour has a height of 50mm, a base of 0.6mm (laser spot size) and an aperture of 20° (approximated from the high speed images of the plume), the trapezoidal area is ~9.4x10 ^-4m ². For a plume temperature of ~2000K ^[4] the radiant power is just over 1000W. In CO ₂ laser welding the radiant surface is better approximate by an ellipsoid of ~2x10 ^-4m ². With an average plasma temperature of ~6000K ^[4] , the resultant radiant power (~1500W) is slightly higher than the Nd:YAG result. The radiant power is very sensitive to the temperature and an important part of it (probably less than 50%) will still be delivered to the workpiece.

The energy lost in by the plasma absorption (only for CO ₂ laser welding ^[9] ) can be estimated from

where z is distance through the plasma (cm); β is absorption coefficient (cm ^-1); N _e is the electron density (cm ^-3) and T the electron temperature (K).

The average reported values in argon gas for the electron density and temperature are 2x10 ¹⁷cm ^-3 and 13000K, respectively. ^{[4, 10]} Therefore the energy absorbed by the plasma is around ~25% (or ~900W for 3.5kW). With helium gas, the electron density is 5x10 ¹⁶cm ^-3 and the temperature is 6000K. The energy absorbed by the plasma in this environment is less than 5% (or less than 200W in this study).

The energy required to melt a volume of metal ( E _Fusion ) is

where ρ is the density (7800 kg/m ³); A is the area of the fusion zone (m ²); v the velocity (m/s); C _p the specific heat (680 J/(kgK)); T _m the melting temperature (1800 K), T _room (300 K) and ΔH _f the heat of fusion (2.5x10 ⁵ J/kg). The average values for mild steel are presented in brackets. ^[11,12]

The energy deposited in the HAZ area is given by

where A _HAZ is the area of the HAZ zone (m ²) and T _Phase the phase transformation temperature (1200 K).

The energy lost in the evaporation process ( E _Evaporation ) can be defined by

where m _EV is the evaporated mass (kg) and v _EV the vaporisation speed (m/s).

This energy is negligible since although the speed is very high (the metal vapour flow is assumed to be supersonic with a speed of around 377m/s [13]

), the evaporated mass is very small. ^[14] Assuming that the maximum mass lost in evaporation is 5% of the melted mass (~0.35g) and the density of steel is 7800 kg/m ³, then E _EV is below 3W.

The melting and transfer efficiencies were calculated following the model established by Fuerschbach, ^[6] Fig.2. The melting or fusion efficiency ( η _melt ) is defined as the ratio of the energy used for melting the metal (eq. ^[5] ) and the heat input. The heat input is equal to the output laser energy (3.5kW for all the experiments) multiplied by the transfer efficiency ( η _transfer ). Using Fuerschbach's material-independent model, ^[6] the energy transfer efficiency can be calculated given the molten (or fusion) area, the total incident power and the welding speed. This model assumes a maximum melting efficiency of 48%.

4. Results and Discussion

In Tables 2, 3, 4 and 5 are presented the experimental results. The energy loss is a combination of the reflection loss plus the plasma/plume loss. The reflection loss can be considered constant with a value of ~200W for both CO ₂ and Nd:YAG laser welding of mild steel, as indicated in the previous section. The energy loss due to conduction also includes the energy of evaporation, although this was found to be practically negligible, and any other energy that has not been estimated in the analysis, such as sputter energy.

As a general trend the melting efficiency approaches the maximum theoretical value of 48% for most of the results, Fig.2. This is an expected result since the melting efficiency, as defined by Fuerschbach ^[6] , is dependent on a dimension-less parameter (R _y) given by the welding speed, laser power/irradiance and material properties. Only when the speed is decreased to 0.25m/min (in reduced pressure conditions) does the melting efficiency decrease to less than 40%.

Table 2 shows that the transfer efficiency ( η _Transf) increases with speed when Nd:YAG laser welding for all the gases employed, at 1.5m/min the only energy lost is that reflected (~200W). As the speed reduces the laser beam has more time to interact with the plume, and therefore be attenuated, i.e. absorbed and/or scattered, by it. The values for the energy plume loss are around 1000W, which is in line with the calculated value in the previous section (eq. (2)). The energy plume loss shows dramatic differences depending on the gas side jet and the gas environment. The use of argon gas in the side jet reduces the energy plume losses with respect to helium. Although the melting efficiency is very similar for both, lower plume losses results in a higher weld penetration for the argon side jet. Nitrogen shows higher energy losses than argon and helium, both with and without a gas side jet, and therefore produces lower penetration.

There are no significant changes in transfer efficiency, penetration and top bead width values when no side jet is used with argon, helium or nitrogen as the gas environment. In these cases the corresponding energy losses are ~1000W. This could be due to the difficulty of displacing the plume from the laser beam path without a side jet, and confirming that the ionisation potential of the gas does not play an important role in Nd:YAG laser welding, as it was discussed previously by Greses et al. ^[4] At reduced pressure (down to 60mbar absolute, Table 3), the energy losses and the HAZ energy are higher than at atmospheric conditions for the penetration and top bead width measured.

In Table 4 the different energies and efficiencies arising from a series of experiments to optimise the gas side jet for Nd:YAG welding are presented, for nozzles of 2 and 4mm in diameter. Lower energy losses are obtained with the 2mm nozzle, and this results in deeper penetration. Using a horizontal nozzle, the energy losses were in the medium range (~500W), also with good penetration. The HAZ energy, using the horizontal nozzle, was higher than with the standard 40° angle, possibly because the lack of cooling effect from the jet over the weld pool. This effect was also noticed to a lesser extent with the +2mm impingement position.

The best results in terms of penetration for Nd:YAG laser welding were obtained with the use of argon side jet and on air gas environment. The use of a 2mm diameter angled nozzle and high gas flow were also beneficial for the penetration, although some precision is needed in positioning the gas jet.

The CO ₂ laser welding results are presented in Table 5. The differences in penetration, top bead width, and fusion and HAZ areas with respect to the gas side jet and gas environment are more significant than in Nd:YAG laser welding. In the CO ₂ case the best results for penetration are obtained with helium gas, but there is little difference between using a helium side jet and in a helium environment, and a simple helium environment. It can also be noticed that with the optimised conditions Fuerschbach's equation for determining the transfer efficiency can not be applied since the melting energy is higher than the theoretical 48%, Fig.2. This may be explained because the plasma area under these circumstances is very small and, despite the high temperature in the plasma, the radiant energy is also small. The plasma could also act as an efficient mechanism for coupling the laser beam energy depending on the plasma absorption coefficient, and therefore attenuating the reflection loss. ^[15] The argon environment on its own is not enough to prevent the plasma from detaching from the weld pool, therefore absorbing and/or defocusing nearly all the laser energy. With the argon gas, the fusion and HAZ areas of the welds produced have a higher standard deviation due to instabilities in the plasma, therefore affecting the efficiency results.

5. Conclusions

The melting, HAZ, loss and conduction energies and the transfer and melting efficiencies have been measured and/or calculated for different gas environments in a comparison between CO ₂ and Nd:YAG high power laser welding.

For CO ₂ laser welding, the major influence on penetration is due to plasma loss, although in this case the Fuerschbach model to calculate the transfer efficiency could not be applied. The melting efficiency values obtained for CO ₂ welding were higher than those obtained with the Nd:YAG laser. This may be due to efficient coupling between the laser beam and the plasma in the case of CO ₂ welding.

For Nd:YAG laser welding, the major influence on penetration arises due to transfer efficiency (i.e the plume loss). Penetration optimisation was found to be significantly dependent on the transfer efficiency, and was achieved using a side jet of argon gas delivered at a high flow-rate and using a small diameter, precisely positioned nozzle.

References

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Appendix : Data Tables

Table 2 Efficiencies for different gas side jets in different gas environments for Nd:YAG laser welding.

Table 3 Reduced pressure results for CO₂ and Nd:YAG laser welding.

Table 4 Efficiencies for optimisation of the gas side jet for Nd:YAG laser welding.

Table 5 Efficiencies for He and Ar side jet conditions in Ar, He and Air gas environments for CO₂ laser welding.