Optimisation of plasma/plume control for high power Nd:YAG laser welding of 15mm thickness C-Mn steels

C A Olivier
TWI Ltd, Cambridge, United Kingdom (currently with British Energy, Gloucester, United Kingdom)

Paper presented at 6th International Conference on Trends in Welding Research, 15 - 19 April 2002, Callaway Gardens Resort, Pine Mountain, Georgia, USA

Abstract

1. Introduction

The availability of such high power laser sources generates an increasing interest in single-pass welding of thicker sections (i.e. greater than 10mm thickness in steel), for example in the areas of shipbuilding, structural steel work, off-road vehicles, power generation, containment plant, offshore structures and line pipe. The advantages of laser welding for these applications result from the deep penetration, reducing the number of weld passes and the thermal distortion and increasing the joint completion rate.

Deep penetration welds are made via the so-called keyhole welding mechanism. A keyhole is generated if the power density in the focused laser spot is high enough (~10 ⁴W/mm ^{2 [1]} ) to cause melting and vaporisation of the metal before significant quantities of heat are removed from the processing zone via thermal conduction. The keyhole essentially is a cylindrical hole with molten walls that are kept from collapsing mainly by the vapour pressure inside the keyhole. Via inverse Bremsstrahlung ^[2] , the vapour inside the keyhole is ionised and forms a plasma of ions and free electrons, which dramatically improves the energy coupling between the laser beam and the workpiece.

To achieve deep penetration laser welds, however, it is usually necessary to weld at travels speeds of less than 1m/min, at which speeds the process can be adversely affected by vapour and plasma periodically escaping from the keyhole and forming a cloud above it. In the cloud, the vapour and plasma absorb and diffusely re-radiate some of the laser beam's energy. This can generally be seen from the weld by a decreased penetration depth and widened top bead (which may lead to the so-called 'nail-head' or 'wine-glass' weld profile).

Whereas with CO ₂ laser welding the cloud is thought to be a partly-ionised gas plasma consisting of ions, electrons and neutral atoms ^[2-4] , in Nd:YAG laser welding it is thought to be merely a 'hot gas' or vapour plume consisting of neutral atoms only ^[4-6] . This arises because the energy absorption coefficient for inverse Bremsstrahlung is proportional to the square of the wavelength, which makes the cloud more transparent to Nd:YAG (wavelength λ = 1.064µm) than CO ₂ laser light (wavelength λ = 10.6µm). For that reason, it may be expected that there are differences in the most effective plume control measures for CO ₂ and Nd:YAG laser welding as well.

Considerable research effort has been spent over the years to investigate this plasma/plume formation and to develop techniques for suppressing or at least reducing it. ^{[e.g. 2,3,7-10]} So far, this research has mostly concentrated on high power CO ₂ laser welding, which has been more commonly used because of the higher powers available and the lower cost per kilowatt of laser power. Nonetheless, high power Nd:YAG lasers (typically 4kW) are now available, and of greater application due to the flexibility that the optical fibre beam delivery of Nd:YAG lasers can give. Furthermore, with new techniques such as diode pumping and disc lasers, the available Nd:YAG laser power is set to rise.

This paper describes research specifically into plume control for deep penetration Nd:YAG laser welding.

2. Objectives

• To establish the effectivity of different process gases and gas mixtures for plume control during high power Nd:YAG laser welding;
• To establish an effective gas delivery system for plume control during high power Nd:YAG laser welding.

3. Experimental approach

3.1 Materials

Table 1: Chemical compositions (in weight%) of the steels used.

3.2 Equipment

3.3 Experimental approach

Firstly, to establish the most suitable plume control set-up, different gas delivery systems were investigated. A schematic of each delivery system tested and the parameters under investigation can be found in Figure 1. Tested were the angled jet, which is the preferred system for CO ₂ laser welding, plus modifications to attempt and improve the performance and tolerance to set-up. As helium is generally considered to be the best plasma control gas when CO ₂ laser welding ^{[e.g. 11]} , it was used for these experiments. The laser power used was 7.5kW at a travel speed of 0.5m/min; the steel used was 14mm thickness C-Mn steel. The commonly used co-axial shielding nozzle was also tested (at 9kW), but this was already reported earlier ^[12] .

Fig.1a) Angled jet

Fig.1b) Horizontal jet

Secondly, using the best performing delivery system, the influence of the process gas itself was tested. The common process gases helium, nitrogen, argon and carbon dioxide as well as mixtures (helium-nitrogen, helium-carbon dioxide, helium-oxygen, argon-oxygen) were studied. An indication of the different properties of the unmixed gases can be found in Table 2

Table 2: Properties of the used process gases (as indication only).

Lastly, the performance of the optimised set-up was further evaluated when attempting full penetration melt runs in 14mm thickness at 7.5kW and in 15mm thickness steel at 9kW of laser power.

3.4 Weld quality assessment

Transverse cross-sections were prepared at a location thought to be indicative of that melt run. The sections were ground, polished, and etched in a 2% solution of nital to reveal the weld bead and heat affected zone. The cross-sections were assessed in terms of penetration, bead shape, and incidence of solidification cracking and porosity.

4. Results

4.1 Evaluation of gas delivery systems

• The deepest penetration depth observed was 12mm. This was achieved with as plume control parameters:-

  - Nozzle diameter 2mm, impingement position +1mm, helium flow rate 30 l/min;
  - Nozzle diameter 2mm, impingement position +2mm, helium flow rate 30 or 40 l/min;

  
  

• Deep penetration melt runs were associated with a small plume and a smooth top bead. Radiographic examinations of these melt runs showed low levels of porosity. However, the occurrence of short solidification cracks, although rare,was noted.

  
  

• When optimum parameters were not used, the penetration depth decreased to 8mm and the plume was seen to significantly increase in size. The top bead of all low penetration melt runs showed a 'pulsing effect', which was also linkedto extensive cracking. An example of a top bead showing this pulsing effect can be seen in Figure 2.

  
  

• No melt runs of satisfactory penetration depth and weld quality were produced with the 1.2mm or 4mm diameter nozzles.

Rectangular cross-section angled jet

Rectangular cross-section horizontal jet
A rectangular cross-section nozzle with its axis parallel to the workpiece did not enable the production of a melt run of acceptable quality over the range of parameters investigated. Most melt runs exhibited a 'pulsing' top bead and relatively low penetration depths (8-10mm). In these experiments, the position of the nozzle with respect to the workpiece and the laser beam-material interaction point proved to be of little influence on the penetration depth. In addition, it was noticed that the weld shape and appearance were not directly linked to the visible plume size, as seen previously.

Circular cross-section angled jet with shaped nozzle
Only two experiments were performed with an angled circular cross-section jet with shaped exit nozzle as indicated in Figure 1c. Neither of the experiments produced a melt run of acceptable quality. Even the use of very high helium flow rates and gas bottle pressures did not allow control of the plume. Both melt runs exhibited pulsing top beads and small penetration depths.

Summary
From the experiments, it was concluded that of the gas delivery systems tested with helium, the optimum conditions, enabling 12mm penetration in C-Mn steel with a Nd:YAG workpiece power of 7.5kW and at a travel speed of 0.5m/min, were as follows:-

• Circular gas jet of diameter 2mm;
• Jet oriented in line with the welding direction and trailing the beam at a 35° angle to the workpiece;
• 10mm nozzle-to-workpiece stand-off distance;
• Impingement point 2mm ahead of the beam;
• Helium flow rate of 40 l/min.

4.2 Evaluation of different process gases

The maximum penetration depth achieved was 12.5mm with argon, nitrogen, carbon dioxide or a 90% helium-10% oxygen mixture. The melt runs produced with these process gases, apart from those made with carbon dioxide which were not radiographed, exhibited low or acceptable porosity levels (to Class B (stringent) of BS-EN-ISO 13919-1:1997 ^[13] ).

Very short cracks (1mm) were occasionally detected in some of the melt runs made with nitrogen. However, although not further investigated, this was not thought to be related to the process gas. It is interesting to note, that the use of nitrogen did allow the plume to be controlled, whereas with high power CO ₂ laser welding, it tends to give a very hot and fiery plume.

Argon was found to give the best overall results because it was most tolerant to set-up variations, is inert and does not have an alloying effect. It was therefore concluded that the optimum set of welding conditions, enabling a 12.5mm penetration depth in C-Mn steel with a Nd:YAG laser power of 7.5kW at a travel speed of 0.5m/min, was as follows:-

• Circular gas jet of diameter 2mm;
• Jet oriented in line with the welding direction and trailing the beam at a 35° angle to the workpiece;
• 10mm nozzle-to-workpiece stand-off distance;
• Impingement point 2mm ahead of the beam;
• Argon flow rate of 20 l/min.

4.3 Verification of optimised set-up

Full penetration was achieved at travel speeds of 0.30 and 0.35m/min ( Figure 3) using the following plume control set-up:-

• Circular gas jet of diameter 2mm at a 35° angle;
• 10mm nozzle-to-workpiece stand-off distance;
• Impingement point 3mm ahead of the laser beam at the workpiece surface;
• Argon gas flow 30 l/min.

The melt run made at 0.30m/min showed little porosity, acceptable under Class B according to BS-EN-ISO 13919-1:1997 [13]

Experiments at 9kW
As full penetration melt runs were attempted in the 15mm thickness steel as well, the welding position was changed to the PC position, to prevent weld metal sagging.

Full penetration melt runs could indeed be achieved using the optimised angled jet plasma control at a laser power of 9kW and a travel speed of 0.30m/min at zero focus ( Figure 4) and at 0.35m/min at -3mm focus ( Figure 5). No significant change in plume control set-up was required to cope with the change of welding orientation, although plume control was still necessary. The plume control set-up used was:-

• Circular gas jet of diameter 2mm at a 35° angle;
• 10mm nozzle-to-workpiece stand-off distance;
• Impingement point 2mm ahead of the laser beam at the workpiece surface;
• Argon gas flow 20 l/min.

The melt runs exhibited smooth top beads and consistent under-beads. All melt runs contained very low porosity levels and in that sense qualified as Class B according to BS-EN-ISO 13919-1:1997. ^[13]

5. Discussion

5.1 Introduction

5.2 Observations

• As was to be expected, plume control becomes increasingly difficult as the laser power increases and/or the travel speed decreases, because the increased input of laser energy tends to promote plume formation.
• For a constant laser power and travel speed, the diameter of the angled jet, its position (i.e. impingement point) and the process gas and flow rate all have a significant effect on the weld profile and penetration.
• The ionisation potential of the process gas has no obvious importance in the control of the Nd:YAG 'hot gas' plume. In fact, all gases and gas mixtures could be used to make deep penetration melt runs, providing the set-up of thegas jet was correct for that specific gas. This observation differs from what is commonly known for high power CO ₂ laser welding (at travel speeds of less than 1m/min) where helium is, by far, the best process gas for plasma control due to its high ionisation potential and thermal conductivity. With Nd:YAG, even nitrogencould be used, which generally gives a very fierce plasma when used in high power CO ₂ laser welding.
• The optimum gas flow rate is gas specific. Lighter gases (He) and gas mixtures (He-CO ₂, He-O ₂) require high flow rates (40-60 l/min). All heavier gases (Ar, N ₂, CO ₂) and gas mixtures (Ar-O ₂), despite significantly lower ionisation potentials and thermal conductivity, require lower flow rates (10-30 l/min) to maintain melt runs of similar or even higher penetration.
• When attempting to achieve full penetration melt runs, the position of the gas jet and consequently the gas flow has to be re-optimised if either or both the laser power and the travel speed vary significantly.

The parameters which critically affect the success of the welding operation therefore include:-

• Nozzle size and shape;
• Positioning of the plume control jet;
• Gas type for plume control and shielding of the weld;
• Gas flow rates;
• Material composition and thickness;
• Laser power;
• Travel speed.

Most of these parameters mutually influence one another.

5.2 Mechanism

The actual mechanism by which the gas jet influences the formation or displacement of the plume is not readily understood. It could be that the gas jet prevents its escape and forces it into the keyhole and/or pushes the plume that has escaped from the keyhole away from the interaction zone.

Whatever its exact role, accurate positioning of the gas jet was found to be critical for plume control. Furthermore, because the angled jet is non-axisymmetric with regard to the laser beam, it will be very difficult to apply to non-linear joints, unless a robotic device is used which can keep the angled jet aligned with the joint line and keyhole. But even then, the influence of the change of direction on the keyhole may affect the efficiency of the plume control device.

6. Conclusion

• Circular cross-section, 2mm in diameter;
• Jet following the laser beam in line with the seam and at an angle of 35° to the workpiece surface;
• 10mm nozzle-to-workpiece stand-off distance;
• Gas flow impingement point about 2mm ahead of laser beam-material interaction point for argon;
• Argon at a flow rate of 10-40 l/min.

Using this set-up as a basis, it may need to be further optimised for each particular application, material and set of welding parameters.

Acknowledgements

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