C Peters

Focus control system for laser welding.

We describe a focus control system for Nd:YAG laser welding based on an optical sensor incorporated into the fiber delivery system to detect light generated by the process. This broadband light is separated into two wavelength bands, and simple electronic processing gives a signal proportional to focal error as a result of chromatic aberrations in the optical delivery system. Focus control is demonstrated for bead-on-plate welds in different thicknesses of titanium alloy, aluminum alloy, mild steel, and stainless steel. The control system works for both pulsed and continuous laser radiation.

Real-time focus control in laser welding

We describe a focus control system for Nd:YAG laser welding based on a non-intrusive optical sensor incorporated into the fibre delivery system to detect light generated by the process. This broadband light is separated into two wavelength bands. Simple electronic processing gives an error signal proportional to focal error as a result of chromatic aberrations in the optical delivery system. Focus control is demonstrated for bead-on-plate welds in different materials of varying thicknesses and at various welding speeds. The system was demonstrated with both pulsed and CW laser welding.

Optical focus control system for laser welding and direct casting

Abstract A non-intrusive optical sensor system has been developed for focus control of laser welding. This detects the light generated by the process through the laser delivery optics, and exploits the chromatic aberrations of these optics to detect any laser focal error at the workpiece. This system works for a wide range of materials and welding parameters, and example results are presented. The sensor has also been applied to laser ‘direct casting’, a process in which 3-D structures are built by flowing metal powder into a focused laser beam. In this case, melt pool temperature is also important, and so additional optics are incorporated into the sensor to provide a pyrometric temperature measurement which is used to control the laser power.

Optical signal oscillations in laser keyhole welding and potential application to lap welding

We describe an optical sensor for process monitoring of Nd:YAG laser welding. This sensor detects the broadband radiation produced by the welding process, dividing it into broad spectral bands (designated UV/visible and IR). Fourier analysis is used to investigate an oscillatory intensity modulation of the optical signals, which is believed to arise from a combination of keyhole and weld-pool oscillations. The spectral content of the oscillations may be used to detect a fully open welding keyhole and to determine the work-piece thickness under this welding regime. These oscillations have also been utilized in the development of a tracking technique which detects the misalignment from an overlap welding seam and excessive gaps in the lap joint.

Applications of optical sensing for laser cutting and drilling

Any reliable automated production system must include process control and monitoring techniques. Two laser processing techniques potentially lending themselves to automation are percussion drilling and cutting. For drilling we investigate the performance of a modification of a nonintrusive optical focus control system we previously developed for laser welding, which exploits the chromatic aberrations of the processing optics to determine focal error. We further developed this focus control system for closed-loop control of laser cutting. We show that an extension of the technique can detect deterioration in cut quality, and we describe practical trials carried out on different materials using both oxygen and nitrogen assist gas. We base our techniques on monitoring the light generated by the process, captured nonintrusively by the effector optics and processed remotely from the workpiece. We describe the relationship between the temporal and the chromatic modulation of the detected light and process quality and show how the information can be used as the basis of a process control system.

Nd: YAG laser welding process monitoring by non-intrusive optical detection in the fibre optic delivery system

This paper describes a non-intrusive optical sensing technique for Nd:YAG laser welding monitoring. The cladding power monitor (CPM) detects the light returned through the cladding of the delivery fibre optic, and appropriate spectral filtering isolates the light radiated from the plume of hot gas which forms above the weld. The successful detection of welding faults using this optical signal is described, including laser focus errors and shield gas interruption.

In-situ laser material process monitoring using a cladding power detection technique

Abstract Progress in laser material processing may require real-time monitoring and process control for consistent quality and productivity. We report a method of in-situ monitoring of laser metal cutting and drilling using cladding power monitoring of an optical fibre beam delivery system—a technique which detects the light reflected or scattered from the workpiece. The light signal carries information about the quality of the process. Experiments involving drilling and cutting of two samples, a thin aluminum foil and a 2-mm thick stainless steel plate, confirmed the effectiveness of this method.

A fibre-optic-based sensor for optimization and evaluation of the laser percussion drilling process

A non-intrusive inter-process measurement technique to determine the diameter of laser-drilled holes is demonstrated. This allows holes to be analysed immediately after processing, before drilling additional holes in the same part. Remedial action can therefore be taken if necessary. The sensor also measures the position of the workpiece relative to the focal point of the drilling laser to ensure optimal processing.

Origin of oscillations in optical signals for Nd:YAG laser welding

Intense radiation is generated by the laser welding process over a broad range of wavelengths, from UV to IR. The core of the delivery optical fibre has previously been used to collect this light from which it is possible to detect various weld faults including focal errors and shield gas interruptions. We have designated this arrangement for the detection of process generated light as the core power monitor (PM). The detected light has a characteristic intensity modulation, postulated to be due to oscillations of the welding keyhole. In this paper, we describe imaging the welding process using a high speed camera, confirming the existence of keyhole oscillations and demonstrating that the light collected with the core PM is predominantly generated within the keyhole.Intense radiation is generated by the laser welding process over a broad range of wavelengths, from UV to IR. The core of the delivery optical fibre has previously been used to collect this light from which it is possible to detect various weld faults including focal errors and shield gas interruptions. We have designated this arrangement for the detection of process generated light as the core power monitor (PM). The detected light has a characteristic intensity modulation, postulated to be due to oscillations of the welding keyhole. In this paper, we describe imaging the welding process using a high speed camera, confirming the existence of keyhole oscillations and demonstrating that the light collected with the core PM is predominantly generated within the keyhole.

Welding with a 3kW Nd:YAG laser

An Nd:YAG laser facility with a maximum average output power of 3kW has been established with the aim of determining the material processing benefits offered at power levels significantly above those available from current commercial Nd:YAG lasers.Welding trials have been conducted in a wide variety of materials representative of those used in the automotive, aerospace and power generation industries.Encouraging results have been obtained in overlap and spot welding of zinc coated automotive steels, overlap and butt welding of aluminium alloys and welding of aerospace and structural alloys in thicknesses up to 9.6mm.An Nd:YAG laser facility with a maximum average output power of 3kW has been established with the aim of determining the material processing benefits offered at power levels significantly above those available from current commercial Nd:YAG lasers.Welding trials have been conducted in a wide variety of materials representative of those used in the automotive, aerospace and power generation industries.Encouraging results have been obtained in overlap and spot welding of zinc coated automotive steels, overlap and butt welding of aluminium alloys and welding of aerospace and structural alloys in thicknesses up to 9.6mm.