Keyhole Depth is just a Distance

Abstract

Talking about laser welding predominately means talking about the generation of a keyhole, the physics behind and invariably the depth of this steam capillary. Having the ability to measure this depth would undoubtedly raise the confidence in laser welding and also raise the quality of the processed part on a higher level.With the IDM (In-Process Depth Meter), Precitec developed a sensor system that is able to measure the depth of the keyhole in-process. On the basis of low-coherence interferometry, with a high robustness of the measured values against process emissions, the system is perfectly qualified to provide the measurement that the industry has been asking about for decades.As a leading manufacturer of processing heads, Precitec is able to provide a solution that is easy to integrate into existing optics. As a leading manufacturer of non-contact measurement systems, the company is also able to reduce the hardware to the most compact size. Both have proven their industrial suitability in hundreds of applications.A substantial reason for the increasing use of the laser in numerous fields of industrial production is the increased efficiency in comparison with competing techniques. Another reason would be the unique features of the laser beam as a tool itself. To really gain a profit by the use of this tool, a highly automated quality control of the production process is needed.The laser welding process offers several possibilities for process monitoring systems or process control but the complexity of the laser process itself, meaning the dependence of the processing result on several process input parameters, does not facilitate their use.As only continuous supervision of the manufacturing process can guarantee the high demands on the quality of the produced parts, process monitoring systems have become more and more standardized devices in laser applications. There is no doubt that the basis for reliable on-line process monitoring systems is the possibility to measure significant indicators, which demonstrates the instantaneous condition of the interaction zone and/or neighbouring areas.One of the most significant pieces of information that needs to be measured in order to qualify the strength of the weld with respect to mechanical load and stress is the depth of the keyhole. There have been numerous approaches to find a sensor technology to be best placed to discover a correlation between the keyhole depth and the measured signal. These attempts have been discussed extensively in R&D and some have found their way to industrial applications. The common feature of these solutions is that they need basic understanding of the beam-materialinteraction to correlate the signal with the quality criteria. These systems pro-vide an estimate and not the actual keyhole depth.The IDM system is able to really measure the depth of the keyhole. The technology and the comparison between existing sensor systems and application results will be content of the following article.

Meeting the challenges in e-mobility with laser welding and sophisticated system technology

“…In this context we talk about monitoring systems based on photo diode technology [3], evaluating quality based on the emissions emerging from the interaction zone. We also talk about the adaptation of OCT sensor technology [4] [5][6] [7][8] [9] to the processing tools being able to perform real measurements in physical units deriving a go/no go message. Within laser material processing, this sensor technology came into focus about 10 years ago.…”

Interferometric surface inspection with movable measurement spot: high-speed OCT imaging for thickness and topography in industry

“…If the sample is within the measuring range of the measuring head, both surfaces of the sample vibrate synchronously, and the thickness measurement is not negatively affected. Interferometric sensors are also used for quality assurance and process control in industrial laser material processing [1], [2], [3], [4], [5], [6], [7]. The emissions from the interaction zone are not coherent with the light generated by the sensor, so there is no effect on the measurement.…”

Towards process control by detecting acoustic emissions during ultrashort pulsed laser ablation of multilayer materials

“…For example, a commonly used and sophisticated system for monitoring welding processes is optical coherence tomography (OCT). [1][2][3] Yet, the use of OCT requires a sufficiently large key hole, which precludes its application from drilling micro-holes, e.g. for processing modern printed circuit boards [4][5][6][7] , using USP lasers.…”