Fabrice Bardin

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.

Laser Bonding of Glass to Silicon Using Polymer for Microsystems Packaging

Laser joining is a promising technique for wafer-level bonding. It avoids subjecting the complete microelectromechanical system (MEMS) package to a high temperature and/or the high electric field associated with conventional wafer-level bonding processes, using the laser to provide only localized heating. We demonstrate that a benzocyclobutene (BCB) polymer, used as an intermediate bonding layer in the packaging of MEMS devices, can be satisfactorily cured by using laser heating with a substantial reduction of curing time compared with an oven-based process. A glass-on-silicon (Si) cavity bonded with a BCB ring can be produced in a few seconds at a typical laser intensity of 1 W/mm2 resulting in a local temperature of ~300degC. Hermeticity and bond strength tests show that such cavities have similar or better performance than cavities sealed by commercial substrate bonders. The influence of exposure time, laser power, and applied pressure on the degree of cure, bond strength, and hermeticity is investigated. The concept of using a large area uniform laser beam together with a simple mirror mask is tested, demonstrating that such a mask is capable of protecting the center of the cavity from the laser beam; however, to prevent lateral heating via conduction through the Si, a high-conductivity heat sink is required to be in good thermal contact with the rear of the Si.

Modelling and calibration of bending strains for iterative laser forming

We describe a simplified model for laser forming that identifies key features in bending behaviour and use approximations resulting from the model to characterize bending by the thermal gradient mechanism (TGM). The model predicts local curvature change in terms of area energy, interaction time and material thickness and from this derives the bending angle in terms of laser power, spot diameter and feed rate. It provides reasonable agreement between theory and measurement without the need for finite element (FE) analysis and identifies the areas where FE is needed to improve accuracy. In particular, it gives accurate predictions for threshold and saturation area energies, and so can be used in conjunction with the calibration tests to identify the optimum operating conditions for TGM laser forming.

Process control of laser conduction welding by thermal imaging measurement with a color camera

Conduction welding offers an alternative to keyhole welding. Compared with keyhole welding, it is an intrinsically stable process because vaporization phenomena are minimal. However, as with keyhole welding, an on-line process-monitoring system is advantageous for quality assurance to maintain the required penetration depth, which in conduction welding is more sensitive to changes in heat sinking. The maximum penetration is obtained when the surface temperature is just below the boiling point, and so we normally wish to maintain the temperature at this level. We describe a two-color optical system that we have developed for real-time temperature profile measurement of the conduction weld pool. The key feature of the system is the use of a complementary metal-oxide semiconductor standard color camera leading to a simplified low-cost optical setup. We present and discuss the real-time temperature measurement and control performance of the system when a defocused beam from a high power Nd:YAG laser is used on 5 mm thick stainless steel workpieces.

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.

Optoelectronic device for non-invasive focal point measurement and control of the laser welding process

Although the laser welding process has many advantages over other techniques, especially for the welding of complex parts, it requires a precise focusing of the laser beam onto the workpiece to achieve the proper penetration over the entire weld seam. However, the unavoidable set-up tolerances and thermal distortion of the workpiece can result in focus errors that should be minimized. In this paper, an optoelectronic device for real-time measurement and control of the focal position is presented. It is based on the non-intrusive capture of the light emitted by the welding process by means of an optical fibre inside the laser head, and the estimation and correction of the focal error from the analysis of the light at two different spectral bands. The reported system has been optimized for use in a real environment: it is robust, compact, easy to operate and able to adjust itself to different welding conditions. Details of the design, figures of performance obtained from lab testing and results from recent field trials on complex aerospace parts are provided.

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...

Real-time temperature measurement for process monitoring of laser conduction welding

Conduction welding offers an alternative to the keyhole welding process. Compared with keyhole welding it is intrinsically a very stable process since vaporization is minimal. However, as with keyhole welding, an on-line process monitoring system is part of the necessary quality assurance process in order to maintain the required penetration depth, which in conduction welding is more sensitive to changes in heat sinking. The maximal penetration is obtained when the surface temperature is just below the boiling point, and so we normally wish to maintain the temperature at this level. We describe a two-color optical system that we have developed for real-time temperature profile measurement of the conduction weld pool. The key feature of the system is the use of a CMOS standard color camera leading to a simplified low-cost optical set-up. We present and discuss the real-time temperature measurement performance of the system when using a defocused beam from a high power Nd:YAG laser on 13 mm aluminium workpieces.Conduction welding offers an alternative to the keyhole welding process. Compared with keyhole welding it is intrinsically a very stable process since vaporization is minimal. However, as with keyhole welding, an on-line process monitoring system is part of the necessary quality assurance process in order to maintain the required penetration depth, which in conduction welding is more sensitive to changes in heat sinking. The maximal penetration is obtained when the surface temperature is just below the boiling point, and so we normally wish to maintain the temperature at this level. We describe a two-color optical system that we have developed for real-time temperature profile measurement of the conduction weld pool. The key feature of the system is the use of a CMOS standard color camera leading to a simplified low-cost optical set-up. We present and discuss the real-time temperature measurement performance of the system when using a defocused beam from a high power Nd:YAG laser on 13 mm aluminium workpi...

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.

Laser joining of glass to silicon using adhesive for MEMS packaging applications

Laser joining is a promising technique for wafer-level bonding. It avoids subjecting the complete MEMS package to a high temperature and/or the high electric field associated with conventional wafer-level bonding processes, using the laser to provide only localized heating. We demonstrate that a benzo-cyclo-butene (BCB) polymer, used as an intermediate bonding layer in packaging of MEMS devices, can be satisfactorily cured with a substantial reduction of curing time compared with an oven-based process by using laser heating. A glass-on-silicon cavity bonded with a BCB ring can be produced in few seconds at typical laser intensity of 1 W/mm2 resulting in a local temperature of ~ 300°C. Hermeticity and bond strength tests show that such cavities have similar or better performance than cavities sealed by a commercial substrate bonders which require a minimum curing time of 10 minutes. The influence of exposure time, laser power and pressure on degree of cure, bond strength and hermeticity is investigated. The concept of using a large area, uniform laser beam together with a simple mirror mask is tested, demonstrating that such a mask is capable of protecting the centre of the cavity from the laser beam; however to prevent lateral heating via conduction through the silicon a high conductivity heat sink is required to be in good thermal contact with the rear of the silicon.