Holger Braun

Camera-based laser beam welding sensor in the near infrared spectral range

We demonstrate a novel process sensor system for use in laser beam welding that uses image processing of near infrared (NIR) images. Additionally to 2-dimen-sional process monitoring in the visible (VIS) spectral range, an NIR camera detects thermal radiation from the welding spot and its proximity in the spectral range above 1 µm. Coaxial observation through the welding optics delivers information from the laser spot, the weld pool and the heat-affected zone. The evaluation of the various areas allows a fast online characterization of the welding process and its results, which may serve as the input for active process control.We present examples of laser beam welding of process parameters and the respective process sensor evaluation, using a multi-kW Yb:YAG thin disk laser.We demonstrate a novel process sensor system for use in laser beam welding that uses image processing of near infrared (NIR) images. Additionally to 2-dimen-sional process monitoring in the visible (VIS) spectral range, an NIR camera detects thermal radiation from the welding spot and its proximity in the spectral range above 1 µm. Coaxial observation through the welding optics delivers information from the laser spot, the weld pool and the heat-affected zone. The evaluation of the various areas allows a fast online characterization of the welding process and its results, which may serve as the input for active process control.We present examples of laser beam welding of process parameters and the respective process sensor evaluation, using a multi-kW Yb:YAG thin disk laser.

Online characterization of laser beam welds by NIR-camera observation

We have investigated process monitoring of laser beam welding with a TruDisk disk laser to detect process faults. Additionally to monitoring laser beam welding processes by a conventional VIS camera an NIR (near-infrared) camera reveals new information. Our sensor detects thermal radiation between 1100 and 1700 nm from the weld zone, which represents surface temperatures above 1000 K. Using the thermal radiation from the process we can observe all major weld defects without auxiliary illumination. The camera is integrated in a standard TRUMPF welding optics for on-axis observation. A real-time image processing system analyzes the camera images regarding welding irregularities and delivers information to characterize the weld process and its result. Actually, we perform an online passive heat-flow thermography that uses the process itself as the heat source and that probes the thermal attributes of the seam. By this means we can detect regions of bad fusion (“false friends”) virtually during the welding process. In addition to conventional thermography we have investigated the use of ratio pyrometry by using to NIR-cameras that observe the process in two different spectral bands. By considering the pixel-per-pixel ratio the influence of surface effects it greatly reduces and we obtain images of the weld zone with an absolute temperature scale. We have compared ratio pyrometry measurements with conventional thermography.

Online NIR diagnostic of laser welding processes and its potential for quality assuring sensor systems

We have integrated an imaging thermographic sensor into commercial welding optics for observation of the weld zone. Key element of the sensor is an InGaAs-camera that detects the thermal radiation of the welding process in the wavelength range of 1,200 to 1,700 nm. This is well suited to record images of the keyhole, the melt pool and the thermal trace. The sensor was integrated to the welding heads for on-axis observation to minimize the interfering contour to ensure easy adaption to industrial processes. The welding heads used were established industrial-grade TRUMPF optics: a BEO fixed optics with 280 mm focal length, or a TRUMPF PFO-3D scanner optics with 450 mm focal length. We used a TRUMPF TruDisk 16002 16kW-thin disk laser that transmits its power through a 200 μm core diameter light cable. The images were recorded and features of the various process zones were evaluated by image processing. It turns out that almost all weld faults can be clearly detected in the NIR images. Quantitative features like the dimension of the melt pool and the thermal trace can be derived from the captured images. They are correlated to process input parameters as well as to process results. In contrast to observation in the visible spectrum the NIR camera records the melt pool without auxiliary illumination. As an unrivaled attribute of the NIR sensor it supports an online heat flow thermography of the seam and allows identifying missing fusion (“false friends”) of lap joints virtually during the welding process. Automated weld fault detection and documentation is possible by online image processing which sets the basis for comprehensive data documentation for quality assurance and traceability.

Detection of faults in laser beam welds by near-infrared camera observation

We monitor thin disk laser beam welding processes coaxially through the welding optics with a camera that is sensitive in the near-infrared (NIR) spectrum (1200…1700 nm). With this sensor we detect the thermal radiation from the process zone and we can analyze it in real-time during the welding process. From the observed features we monitor the laser weld in respect to various welding faults.In particular, we analyze the thermal trace of the weld seam and its vicinity. For lap joints the analysis of the thermal trace enables us to detect missing fusion immediately and in real-time. This in-situ online heat flow thermography is a unique method to detect “false friends” during the welding process itself.We monitor thin disk laser beam welding processes coaxially through the welding optics with a camera that is sensitive in the near-infrared (NIR) spectrum (1200…1700 nm). With this sensor we detect the thermal radiation from the process zone and we can analyze it in real-time during the welding process. From the observed features we monitor the laser weld in respect to various welding faults.In particular, we analyze the thermal trace of the weld seam and its vicinity. For lap joints the analysis of the thermal trace enables us to detect missing fusion immediately and in real-time. This in-situ online heat flow thermography is a unique method to detect “false friends” during the welding process itself.

Formation mechanism of process instabilities and strategies to improve welding quality

The deep penetration welding process with CO2 lasers has been employed successfully for many years in industry. It generates little spatter on the work piece surface and therefore produces excellent seam quality at high process speed with good process reliability over a wide range of parameters. With a wavelength around 1 µm, solid-state lasers are being increasingly used in industrial production thanks to simple beam guidance by means of laser light cable and their high electrical efficiency. Disk and fiber lasers have advanced into the domain of CO2 lasers by way of power and beam parameter product. However the seam quality is highly dependent on the focusing conditions used, whereby the mechanisms that cause process instabilities are still not properly understood. Also, at high intensities with high feed rates, considerable spatter is generated on the work piece surface, reducing productivity in applications where the demands on surface quality are high. In this publication, based on online X-ray observation and high-speed imaging, the suitability for welding and the formation mechanism of process instabilities like spattering and humping were compared at different feed rates. Based on a better process understanding strategies which can improve welding quality are presented.The deep penetration welding process with CO2 lasers has been employed successfully for many years in industry. It generates little spatter on the work piece surface and therefore produces excellent seam quality at high process speed with good process reliability over a wide range of parameters. With a wavelength around 1 µm, solid-state lasers are being increasingly used in industrial production thanks to simple beam guidance by means of laser light cable and their high electrical efficiency. Disk and fiber lasers have advanced into the domain of CO2 lasers by way of power and beam parameter product. However the seam quality is highly dependent on the focusing conditions used, whereby the mechanisms that cause process instabilities are still not properly understood. Also, at high intensities with high feed rates, considerable spatter is generated on the work piece surface, reducing productivity in applications where the demands on surface quality are high. In this publication, based on online X-ray obser...