Root sided monitoring system for laser welding

The newly developed method improves the monitoring and control process for beam welding. In contrast to most commonly used methods, the new monitoring process is root sided, i. e. it takes place on the back part of the workpiece. Furthermore, the invention describes a monitoring device optimized for beam welding. Beam welding using the invention avoids incomplete fusion at the weld interface and increases the stability of the weld joint.

Challenge

Macro material processing via beam welding puts high demands on the alignment and positioning accuracy of the system. Typically the welding process is monitored at the upper side of the workpiece where the optical analysis of the keyhole provides a measurement for the quality of the weld. However, incomplete fusion cannot be excluded by only monitoring the upside of the workpiece. It is, for example, not possible to register a tilt between the beam and the joining gap. Already small angles lead to incomplete fusion if the material is thick. But not only a tilted beam can lead to incomplete fusion - also the geometry of the workpiece might introduce an angle between the beam axis and the joining gap. Such an angle results in incomplete fusion at the root area even if the process is monitored on the upper side of the workpiece. The error can only be detected by monitoring the welding from the root side. Some workpieces do not allow for a perpendicular alignment between workpiece and beam axis, in this case monitoring the upside of the workpiece is equally insufficient.
Finally, besides the need to adjust the alignment of the beam it is also often necessary to adjust the power of the laser beam in order to maintain the necessary welding depth.

The process monitoring reveals an increasing distance between the weld seam and the joining gap resulting in an elongation of the weld pool. (Image Source: Stefan Kaierle, LZH)

Our Solution

It is possible to detect a misalignment by monitoring the root sided weld pool and the position of the joining gap on the backside of the workpiece simultaneously. Here, the relative position between the joining gap and the main axis of the weld pool is determined instead of monitoring the small keyhole. The measured values can be used to correct the misalignment of the beam axis via the positioning system.

The geometry of the weld pool provides additional information about the reached welding depth and can thus be used to control the beam intensity.

The optical monitoring device on the root side has to be protected from all kinds of emissions caused by the welding. Thus, the newly developed apparatus is installed into a compact and robust housing. It is, however, not possible to monitor the process directly beneath the welding zone since the detector would eventually be destroyed by the welding beam. Instead, the device is shifted relative to the welding zone and the registration takes place under a small angle. To avoid negative effects on the imaging quality, a tilt-shift objective is used.
To image both, the weld pool and the joining gap, simultaneously, a high dynamic range of the detector is necessary. To reduce the required dynamic range, an additional external illumination is added into the housing.
At least one window has to be included into the housing which can reduce the image quality by causing reflexions. Anti-reflex coatings are available but expensive and vulnerable to process emissions. Thus, the newly developed system uses windows which are aligned under the brewster angle. In combination with a p-polarised illumination, reflections can be avoided.

Advantages

Prevention of incomplete fusion even with high material thickness and in the root area.
Adjustment of beam intensity to the optimal value for the currently processed position.
Root-sided, compact and robust monitoring device.
High image quality due to tilt-shift optic.
Resilient windows – no anti-reflex coating necessary.

Applications

Processing of thick-walled components, e.g.in the construction of:

Ships
Pipes
Windtowers
Cranes
Commercial vehicles.

Development Status

Construction model (left) and realization (right) of the root sided process monitoring system. (Image Source: Stefan Kaierle, LZH)

A first realization of the methods was successful. It was possible to obtain results which demonstrated the advantage of process monitoring and controlling. Furthermore a construction model and its realization for the monitoring system exist including the tilt-shift optic as well as the windows aligned according to the brewster angle.

This video shows the root sided monitoring system for laser welding in progress.

Patent Status

German patent application: DE102015115270A1

Patent applicant: Laser Zentrum-Hannover e.V. (LZH)

Contact

Dr. Ireneusz Iwanowski
Patent Manager (Physics, Technology and Software)
E-Mail: iiwanowski(at)sciencebridge.de
Tel.: +49 (0) 551 30 724 153
Referenz: CPA-1809-LZH
www.sciencebridge.de