Quality criteria can be specified by the user in terms of target vs. actual divergence thresholds of the welding position or focusing, keyhole depth limits, and seam profile tolerances together with the allowable process parameters ranges.  As a result of automated QA analysis performed online by Lessmüller Lasertechnik software, prompt pass/fail signal is generated permitting either to correct in real time beam position, focus, and process  parameters,  or  reject  immediately any parts that do fail as soon as the weld is complete.

To assure the reliable production of laser-processed workpieces, the industry certainly requires repeatable and consistent laser process monitoriing techniques. To satisfy this need, Lessmüller Lasertechnik proposes several solutions:

  • OCT optical  coherence  tomography
  • WELDEYE camera-monitoring with  external  illumination
  • WELDCHECK photodiode-based process sensor

 

Prozess Monitoring with OCT

Laser processing heads equipped with a Lessmüller Lasertechnik OCT system provides robust and fast online quality inspection. High accuracy detection of the welds or faults with extraordinary high or small aspect ratio is achieved.

Together with the industrialization of OCT for laser welding, the development of industrial solutions for sequential online pre-, in- and post-process control with OCT are ongoing. One of the most important factors related directly to the weld quality and revealed in-process is the depth of the vapor capillary (keyhole) in deep penetration welding. OCT is a proven technology enabling welding of the workpiece with simultaneous reliable measurement of the keyhole depth coaxially to the processing laser in a nondestructive manner. An essential development attempt is a use the OCT keyhole depth data for closed-loop laser power control to keep the weld depth constant.

Welding setup including BEO D70 welding optics using defocus gradient at laser power 4 kW and OCT control system; real-time keyhole depth measurements with OCT at welding velocity of 80 mm/s (green dots), the blue line is the automatically determined statistical depth value.

 

It is not trivial to look inside a thin keyhole with a high aspect ratio (depth to width). It can be measured, however, with OCT, because of the decoupling of lateral and axial resolution of OCT. Moreover, OCT has a long and variable working distance while maintaining high resolution.  It enables automated real-time (in-process) monitoring of the laser penetration depth. Destructive  sample inspection can therefore be reduced.

 

 

To correlate the OCT-measured and the cross-section depth, the transverse cross-sections were made. Comparing the OCT results with metallographic transversal cross-sections shows a good agreement between the measurements. The average difference between the OCT in-process and microscopic measurements is 0.04 mm. Systematic studies show that the keyhole depth measured by OCT is in good agreement with actual weld depths within an average error of 9%.

 

 

The OCT keyhole depth measured at various laser powers and welding velocities were compared with transversal and longitudinal cross-section images. (a) The  laser  penetration  depth  measured by OCT was overlaid on the longitudinal  cross-section.  Good  correlation of the laser power variation with the real welding depth was found.

 

OCT offers the repeatability and reproducibility of the joint profile measurements at various angles of incidence and high welding velocity. OCT is a stable and reliable technique for remote monitoring applications during industrial laser processing.

Micrometer resolution of OCT can be utilized to inspect surface porosity during industrial laser welding. For example, by recording the topography of the circular weld with open pores, it is possible to assess the weld quality and obtain quantitative measures of the pores.

OCT acquires 3D surface images to map the weld bead topography with high axial and lateral measuring accuracy. Open pores and weld bead humps are detected and evaluated. The number and size of the pores are crucial for the “weld failed” decision.

 

Lessmülle Lasertechnick is working on the OCT application for process monitoring and control in additive manufacturing (AM).

All the above mentioned potential merits of OCT and the three-dimensional nature of the OCT data makes OCT in many respects superior to the traditional technologies. It empowers high productive and flexible production line layouts with increased volume of produced components. OCT makes welding in series production faster, more accurate, and thus more cost-effective than currently achievable with today’s conventional sensor technology.

 

Process Monitoring with WELDEYE

During the active welding process with laser optics equipped with WELDEYE system, the process light is optimally suppressed by a narrowband filter. The images are recorded aternately with and without external illumination. Illuminated images are recorded to measure the seam joint position, the intensity and position of the illuminated light and reflections from the cooled seam. The light intensity distributions are compared to those of the reference welds. These images are used to visualize the regions around the process beam as well as for measuring the weld joint gap. In unilluminated images, the intensity of the reflected process light is examined and compared to reference values. The parametrisation is classified either according to absolute limits or according to limits obtained from reference welds.

 

Process Monitoring with WELDCHECK

WELDCHECK, integrated to the processing optics or inside the laser source, enables real-time monitoring of the laser process light intensity at different wavelength ranges. By means of variations of the emitted process light, different faults could be recognized.

 

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