Online monitoring and control of laser keyholewelding is highly desirable, especially in safety-criticalaeronautic applications, in order to detect and preventpossible defects in the weld. In this article we describea closed-loop control system which ensures fullpenetration in keyhole welding by controlling the focusposition and power of a 4 kW Nd:YAG laser.We present a laser power control system whichdetermines the degree of penetration by analysis of animage of the welding keyhole. The focus positioncontrol system, meanwhile, is based on a previouslyreported system, exploiting the chromatic aberration ofthe focusing optics. We demonstrate performance inbead-on-plate 'welding' of Inconel 718 and Titanium.The focus control system maintained focal position ontilted and non-flat workpieces, and the penetrationmonitoring technique successfully controlled the laserpower to maintain the full penetration regime in thepresence of linear and step changes of thickness.Finally we discuss the performances and the limits ofthe systems when applied to a realistic complexaerospace component.
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