Characterisation of cw Nd : YAG laser keyhole dynamics

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Abstract

The paper concerns laser–matter interaction characterisation. In this work, we use a rapid CCD camera located coaxially to the laser beam and we compare recorded images with those obtained by numerical modelling. Because images are difficult to understand, we compute thermal radiation emitted by a keyhole of fixed geometry and we adjust it trying to approach the camera record. The modelling treats radiative heat transfer within the keyhole and determines the sensor illumination map. By adjusting the geometrical characteristics of the hole, we seek to obtain the image that corresponds as well as possible to the realised experiment. Results are compared with other experimental methods simultaneously performed plume characterisation with an electric probe and spectrometric analysis. They show the existence of two distinct behaviours of the keyhole: a pseudo-steady state associated with regular and pseudo-constant keyhole shapes, low frequencies of electric current in the plume, and generally good welding results, and a highly dynamic mode associated with irregular and rapidly varying keyhole shapes, high frequencies in the plume current and generally poor welding results.

Introduction

Today laser welding presents an interesting alternative to processes classically used in industry. However, difficulties remain in certain cases such as aluminium alloys welding. They come from complex interaction phenomena occurring between the beam and the matter. Therefore, their comprehension is necessary for the development of this technology. During a welding operation, the intense evaporation (induced by the laser beam) pushes away by reaction the molten metal: a keyhole full of vapour and micro-droplets is created, which traps the incident radiation. The knowledge of its geometry and its temporal evolution is thus necessary to understand the energy transfer from the incident beam to the metal. In this work, we chose to observe the irradiated surface using a rapid CCD camera. These 2D images do not make it possible to find directly the geometry of the keyhole. This is why we developed a numerical model. It allows, starting from 3D geometry, to calculate the emitted energy density of each point of the keyhole in the direction of the sensor. The walls of the keyhole are supposed to be isothermal at the substrate boiling temperature and starting from the radiative equilibrium conditions between the various points of the keyhole surface, the radiated energy of each of these points towards the sensor is calculated.

The interaction plume has been characterised and its influence on CCD camera results is considered. Further, recordings are correlated with electric probe results.

Section snippets

Welding and keyhole

The very strong density of power delivered by the laser beam causes the metal to boil a few milliseconds after the beginning of irradiation [1]. The pressure created by the intense vaporisation tends to dig the molten metal zone. Then, the laser beam penetrates deeper and deeper in the matter, and the supplied zone becomes a thin hole, named keyhole. This phenomenon allows deep welding.

During Nd : YAG continuous wave welding process and when no major problems occur, the keyhole shape is close

Investigation means

The experimental set-up is composed of an Nd : YAG laser for the welding treatment and an analysis set (electric probe, rapid CCD camera, spectrometer, etc.) for the laser–matter characterisation.

We used two types of Nd : YAG lasers for the welding process. The first one is a pulsed Haas laser (maximum 50 J/pulse). The beam is carried to the target via a 400μm optical fibre core and via a focusing head (magnification: 0.75). The second one is a cw Haas laser able to deliver 3kW. The beam is

Keyhole radiation study

The radiosity method is used here to compute radiation exchanges within the keyhole and to determine incident radiation flux striking the sensor.

Conclusion

The laser–matter interaction modelling is of great importance for the user. It will allow to use the laser as an interesting tool for industry especially for the welding process. In the meantime, this theoretical approach needs some experimental works for interaction characterisation. Because of the high complexity of occurring phenomena, many methods have to be implemented together. One of them is the “visual” observation of the irradiation area. If we observe this area coaxially to the laser

Acknowledgments

The camera pictures of the welding process were recorded with the coaxial process control system of the Fraunhofer Institute for Laser Technology ILT (Steinbachstraße 15, 52074 Aachen, Germany). We thank Mr Abels (E-mail: [email protected]) and Mr Kratzsch (E-mail: [email protected]) from the Fraunhofer ILT for their support during the experiments with the camera system. They are responsible for the development of the coaxial process control system at the ILT.

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