A stochastic model of the deep penetration laser welding of metals Available to Purchase

The deep penetration laser welding of metals involves the action of a laser beam usually incident normally on a translating work piece. This generates a keyhole in the metal surrounded by a molten region which secures the weld when the metal freezes. The relevant process parameters involved are the power of the laser beam, the translation speed of the metal and the depth of penetration of the laser beam into the metal, together with appropriate physical parameters characterising the metal. It is the aim of mathematical models of these processes to produce a relationship between these quantities with a view to providing an effective simulation of the process as well as a guide to optimal operating conditions both for CW and pulsed laser systems. It is usual when pursuing such an analysis of the process to assume a suitably averaged keyhole structure to exist. The surface of the keyhole is the region of the metal whose coupling of the laser beam to the material takes place and energy is transferred to the metal, usually by the process of Fresnel absorption and by inverse bremsstrahlung and conduction processes in the vapour in the keyhole, particularly in the case of a CO₂ laser. Several such models have been produced and mathematically a strictly deterministic relation between the relevant quantities is usually assumed to exist. The assumption of a steady-state keyhole with suitably averaged properties, however, constitutes a considerable idealisation of the process. Actually, the keyhole manifests itself as a writhing, twisting entity whose behaviour as such has been photographed by several investigators. Such behaviour is clearly seen even for the CW laser case. It arises from a number of different instabilities that can and do inevitably come into play in the laser welding process. Such effects represent in essence the reaction of the material being welded to the laser beam. No mathematical model currently is known which attempts to take account of this behaviour which nevertheless has real significant practical effects which both influence the welding process and can lead to its breakdown. It can become even more necessary to consider such reactions of the material to the laser beam when pulsed lasers or CW lasers with pulsed modulation are considered. The details concerning the particular characteristics of specific instabilities are not considered here. Instead, it is the objective of this paper to provide the first stochastic description of some aspects of the laser welding process when the laser operates in the continuous wave case. It is thus intended to attempt to extend the applicability of current mathematical models and provide increased insight into the nature of the laser welding process.