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The optical coherence tomography (OCT) is a measuring technology which is well-established in medical engineering since the early 1990s. More recently, the technology found its way into laser materials processing where it is used for seam tracking and inspection and also for monitoring and control of deep-penetration laser welding. In this work, deep-penetration laser welding of aluminum and steel using an OCT-system for in-process monitoring of the weld depth was investigated. It is shown that statistical data processing is mandatory to extract the actual keyhole depth. Therefore, two different measures, percentile filtering and considering the frequency distribution of the OCT-data, were considered. Thereby, it is demonstrated that the frequency distribution of the OCT-data has a specific pattern with a local maximum which correlates with the keyhole depth. Moreover, this feature is more significant for welding aluminum and therefore enables one to detect the weld depth more independently and accurately compared to currently applied measures.

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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.