Control of transient processes during CO 2 laser beam welding

Observing molten pool surface oscillations during keyhole processing in laser powder bed fusion as a novel method to estimate the penetration depth

“…The use of transmitted electromagnetic waves through the large powder bed in an industrial system still requires fundamental scaling up and safety issues to be considered. An indirect approach for detecting the melt pool depth can be developed by analysing the light emitted from the melt pool [38][39][40][41][42][43]. Alternatively, 7 surface wave oscillation detection by means of probe light reflections can be employed as an indirect penetration depth measurement method and has been conducted in both Gas Tungsten Arc welding processes and laser based processes [44][45][46][47][48][49].…”

Searching for chaos in fluctuations of a plasma induced during cw- laser welding

“…The stability of the welding process can be disturbed by a number of transient processes which not only are governed by the specific shape of the workpiece (ends, edges and so on) but also may be caused by changes in various process parameters such as the finishing of the surface and fluctuations of the laser output power. The other source of the observed irregularities may be for instance nonlinear coupling between the laser and the fluctuating plasma that expands from a keyhole or the interaction of the keyhole's surface with flows of shielding gas and melt in a weld pool [6]. The first group of reasons can be regarded as random effects which result in noisy signals whereas those of the second group may indicate that the nature of the welding process, in particular of the keyhole oscillations, is chaotic in the sense of deterministic chaos.…”

“…There are several papers reporting and analysing plasma, melting pool and keyhole fluctuations observed during welding with the use of cw CO 2 lasers [5,6,[13][14][15][16][17][18][19][20][21][22]. There were attempts to look for characteristic modes and eigenfrequencies of these fluctuations.…”

“…Thus, ordering chaos could be the way to make the laser welding more regular. For instance, the response of the process to modulation of the laser power is suggested as a method of 'suppressing' the process fluctuations by converting them into more regular oscillations, which would result in better weld seam quality [6]. The methods of this kind can be understood as passive control of chaos [19].…”

Forced oscillations of the keyhole in penetration laser beam welding

“…Large oscillation amplitudes may lead to a collapse of the keyhole and result in splash generation. On the other hand, intentional modulations of the keyhole plasma or gas flow out of the keyhole affect the shielding efficiency, a mechanism that can be used for penetration depth stabilization [2]. Also, frequency analysis of the ultraviolet or near-infrared process light is being used for on-line process diagnostics and control [3][4][5][6][7].…”