Process control of laser keyhole welding

Online monitoring and control of laser keyhole welding is highly desirable, especially in safety-critical aeronautic applications, in order to detect and prevent possible defects in the weld. In this article we describe a closed-loop control system which ensures full penetration in keyhole welding by controlling the focus position and power of a 4 kW Nd:YAG laser. We present a laser power control system which determines the degree of penetration by analysis of an image of the welding keyhole. The focus position control system, meanwhile, is based on a previously reported system, exploiting the chromatic aberration of the focusing optics. We demonstrate performance in bead-on-plate 'welding' of Inconel 718 and Titanium. The focus control system maintained focal position on tilted and non-flat workpieces, and the penetration monitoring technique successfully controlled the laser power to maintain the full penetration regime in the presence of linear and step changes of thickness. Finally we discuss the performances and the limits of the systems when applied to a realistic complex aerospace component.