Pascal Aubry

Optical techniques for real-time penetration monitoring for laser welding

Optical techniques for real-time full-penetration monitoring for Nd:YAG laser welding have been investigated. Coaxial light emission from the keyhole is imaged onto three photodiodes and a camera. We describe the spectral and statistical analyses from photodiode signals, which indicate the presence of a full penetration. Two image processing techniques based on the keyhole shape recognition and the keyhole image intensity profile along the welding path are presented. An intensity ratio parameter is used to determine the extent of opening at the rear of a fully opened keyhole. We show that this parameter clearly interprets a hole in formation or a lack of penetration when welding is performed on workpieces with variable thicknesses at constant laser power.

Closed-loop power and focus control of laser welding for full-penetration monitoring

We describe a closed-loop control system ensuring full penetration in welding by controlling the focus position and power of a 4-kW Nd:YAG laser. A focus position monitoring system was developed based on the chromatic aberration of the focusing optics. With the laser power control system we can determine the degree of penetration by analyzing the keyhole image intensity profile. We demonstrate performance in bead-on-plate welding of Inconel 718 and titanium. The focus control system maintained a focal position on tilted and nonflat 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.

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.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 perf...

Evaluation of coaxial process control systems for Nd: YAG laser welding in aeronautics application

A sensor system for process monitoring of laser welding has been developed, based on co-axial detection of light produced in the welding keyhole. Two detection techniques are used in parallel, the first uses an optical fibre to collect light and guide it to discrete photodiodes, whilst the second uses a high speed camera to image the welding keyhole. This system has been developed for Nd:YAG welding of aeroengine components. Detection of focal errors and state of penetration are demonstrated for Inconel 718 and Titanium Ti-Al-6V.A sensor system for process monitoring of laser welding has been developed, based on co-axial detection of light produced in the welding keyhole. Two detection techniques are used in parallel, the first uses an optical fibre to collect light and guide it to discrete photodiodes, whilst the second uses a high speed camera to image the welding keyhole. This system has been developed for Nd:YAG welding of aeroengine components. Detection of focal errors and state of penetration are demonstrated for Inconel 718 and Titanium Ti-Al-6V.

Real-time focus controller for laser welding with fibre optic noninvasive capture of light

Laser welding is being introduced in the aerospace industry due to its many advantages over traditional techniques. However, welding of parts with complex shapes requires precise control of the focal point of the laser in order to achieve full penetration over the entire seam. In this paper, we present an improved control system for real-time adjustment of the correct focal position, which is based on the monitoring of the light emitted by the process in two different spectral bands. The reported system has been optimized for use in a real environment: it is robust, compact, easy to operate, able to adjust itself to different welding conditions, materials and laser setups, and includes a direct connection to an external PC. Results from recent field trials on complex aerospace structures are provided.