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.

The emission spectra from the solid-liquid interface irradiated by a pulsed laser were studied. The solid target used in this study was graphite and boron nitride, and the liquid in which the target was immersed was water, benzene, n-hexane, and carbon tetrachloride. The results showed strong continuous spectrum immediately after a pulse shot, whereas after Ϸ100 ns later from the irradiation it was greatly reduced, and instead, the emission from small molecules dominated the spectra. The line spectra of small molecules observed in the later time range indicate the chemical reaction between the ablated species and the species originated from the liquid molecules. The intensity of the continuous spectrum was very prominent compared to what has been observed for solid-gas interfaces. This is due to rapid electron ion recombination or bremsstrahlung due to highly confined interface plasma.

In laser and electron-beam welding, a deep cavity called a keyhole or beam hole is formed in the weld pool due to the intense recoil pressure of evaporation. The formation of the keyhole leads to a deep penetration weld with a high aspect ratio and this is the most advantageous feature of welding by high-energy-density beams. However, a hole drilled in a liquid is primarily unstable by its nature and the instability of the keyhole also causes the formation of porosity or cavities in the weld metal. In particular, the porosity formation is one of the serious problems in very high-power laser welding, but its mechanism has not been well understood. The authors have conducted systematic studies on observation of keyhole as well as weld pool dynamics and their related phenomena to reveal the mechanism of porosity formation and its suppression methods. The article will describe the real-time observation of keyhole and plume behaviors in the pulsed and continuous-wave laser welding by high-speed optical and x-ray transmission methods, the cavity formation process and its suppression measures.

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.

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