Feature Of The Week 07/07/2017: Tomographical process monitoring of laser transmission welding with OCT
Laser transmission welding as a contactless and locally limited joining technique is well-established in different production areas. It is used in a variety of demanding applications like for example parts which require a particle-free joining process. Sensitive parts demand a joining technique which does not affect the inner components. The welding process itself faces higher requirements concerning for example tightness of the weld joint or cleanliness of the produced part (no material must impurify the part during the process).
During the laser transmission welding process the laser radiation is locally focused on an absorbing surface. The joining is performed between a transmissive part (regarding the process laser wavelength) and an absorbing part. After solidification the weld seam is created at the intersection of both parts within the material. The weld seam is hermetically enclosed within the material and cannot be visually inspected.
A very narrow process parameter window during the laser process or process deviations have led to an increase on the requirements for the laser transmission welding process and on monitoring devices. A qualitative feedback for process adaptation is not given by today´s state of the art measurement systems. Pyrometers detect the process emission but return no tomographical information. Quality assurance of the fabricated product is done via destructive testing resulting in production of scrap parts.
Inline integrated optical measurement systems capable of providing non-invasive tomographical images of the transparent material, the weld seam and its surrounding areas with micron resolution would improve the overall process by providing more detailed information about the actual forming process. Hereby obtained measurement data enable qualitative feedback into the system to adapt process parameters to obtain a more robust laser transmission welding process. Non-destructive testing during the laser welding process would reduce the process cost as well.
Within the European-funded research project Manunet “Weldable” an inline monitoring device based on Fourier-domain optical coherence tomography was developed at the Fraunhofer IPT. This device, after adaptation to the laser transmission welding process is optically integrated into the existing laser machine. Pre-process analysis of the work piece can as easily be performed as well as post-process quality assurance inspection. The main target lies within the inline process control destined to extract tomographical geometrical measurement data from the weld seam forming process. Hereby process feedback is given to adapt prior set laser parameters as well as high resolution images are made available for quality inspection. In-process obtained data make obsolete the need for further testing and reduce the handling of the product. Usage of this non-destructive and non-invasive technology makes offline destructive testing of produced parts unnecessary as any defects occurring during the process can be identified and its information feedback for laser process adaptation, delivering hereby a reliable weld process and a safe product.
For more information see recent Article. Courtesy Robert Schmitt from RWTH Aachen University and Philippe Ackermann from Fraunhofer-Institut für Produktionstechnologie.