A theoretical analysis of the energy balance in the laser - metal interaction zone is carried out. The heat transfer due to the recoil-pressure-induced melt flow is taken into consideration. It is shown that, for the absorbed laser intensities typical in welding and cutting, the recoil pressure induces high-velocity melt-flow ejection from the interaction zone. This melt flow carries away from the interaction zone a significant portion of the absorbed laser intensity (about 70 - 90% at low laser intensities); thus, convection-related terms can be ignored neither in calculations of the energy balance in the interaction zone nor in calculations of the thermal field in the vicinity of the weld pool or cutting front.

A physical model of keyhole support and propagation during high-translation-speed laser welding is described. A numerical code for the simulation of the front keyhole wall behaviour is developed on the basis of a `hydrodynamic' physical model assuming that: (i) only the front part of the keyhole wall is exposed to the high-intensity laser beam; and (ii) recoil pressure exceeds surface tension and propagation of the keyhole wall inside the sample is due to melt expulsion similar to that in laser drilling. The front keyhole wall profile, distribution of absorbed laser intensity and phase velocity of the solid/liquid (liquid/vapour) boundary are calculated for various processing parameters. The calculations show that, depending on the processing conditions, the absolute value of the keyhole wall velocity component parallel to the translation velocity vector can be higher than, smaller than or equal to the beam translation speed. When the component of the keyhole velocity vector parallel to the sample surface was higher than the beam translation speed, the formation of the humps on the keyhole wall was observed numerically.

The dynamics of the melt pool and keyhole was investigated during CO2 laser welding using high-speed video photography and the laser reflectometer technique. A low-power argon laser beam, focused on the weld pool, provided illumination to obtain a direct image of the weld pool surface. The near-surface plasma emission background was decreased by using a narrow-bandwidth interference filter centred at the argon laser wavelength (514 nm). A variation in the shape of the keyhole opening with a characteristic frequency higher than 1 kHz was observed both during spot welding and during welding with a moving beam. For the case of spot welding with a 20 ms laser pulse, long-wavelength (about 1 mm) oscillations of the weld pool were observed with a frequency during the laser pulse and the first 5 ms after the laser pulse in the range 200-500 Hz. In the time interval starting at 25 ms and ending at approximately 40 ms from the beginning of the laser pulse, the long-wave oscillation frequency increased up to 1.3 kHz. The solidification time was determined to be approximately equal to the pulse duration for the spot welding. Surface deformation during cooling was also observed. This information is used to develop a model illustrating the dynamics of the post-pulse weld pool.

A novel approach to simulating the energy beam welding of metals is presented.dominant dynamic processes present during concentrated A model for transient behavior of the front keyhole wall is developed.It is as;umed that keyhole propagation is dominated by evaporation recoil-driven melt expulsion from the beam interaction zone. Results from the model show keyhole instabilities consistent with experimental observations of metal welding, metal cutting and ice welding.Evaporation from a surface irradiated with a laser beam is an important process affecting energy transfer, melt hydrodynamics, and chemical composition of the processed work piece. A model of laser induced evaporation for surface temperatures below critical was developed by Anisimov [1]. In particular, this model provides the value of evaporation recoil pressure, which is the dominant factor determining melt motion [2] at elevated surface temperatures.Although high surface temperature and high recoil pressure can be achieved in keyhole laser welding, Anisimov's model has not been applied in welding simulation except for a few notable recent attempts [3,4]. Previous models of laser and e-beam welding considered a steady state keyhole and disregarded the effects of melt motion, related convective heat transfer, and hydrodynamic instabilities. These over-simplified, unphysical assumptions produced models valid only for conduction limited welding and lacking the accuracy required for industrial application.A transient model of laser welding incorporating evaporation recoil pressure was recently suggested by Semak [5]. The proposed concept is based on a set of experimentally verified [6][7][8][9][10][11][12] assumptions:(1) the front part of the keyhole wall is directly exposed to the laser beam; (2) the propagation of the front keyhole wall is due to drilling-like melt expulsion, generated by the evaporation recoil pressure; and (3) the back of the keyhole stays mostly outside the laser beam. This physical scheme was additionally confirmed by the results of high-speed photography of ice welding with a low power C02 laser (Fig. 1).Using the new physical scheme, the keyhole laser welding model can be divided into three major interdependent parts: the front part of the keyhole, the back and side walls of the keyhole, and the bulk of the weld pool. Here the dynamics of the front wall

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