Laser-welded seams are state of the art in car body construction. Strength is the most important criterion for a weld seam. This is why quality monitoring of the commonly used materials steel and aluminum must focus on the reliable connection of the joining partners. But how can exemplary welded seams with a perfect appearance but without this connection be recognized?

Most test methods work indirectly by observing the process emissions. Their behavior is used to draw conclusions about later characteristics of the weld seam. However, with this method the subsequent effects are only predicted, not verified - with corresponding uncertainty. The reasons for this are thermomechanical influences, the localized and temperature-dependent expansion and shrinkage in the component as well as local cooling conditions that determine the behaviour of the material in the weld zone. These influences cause unpredictable effects independent of the process emission. As a result, the best results are achieved by those test methods that take place when the molten material has come to rest again.

However, measuring the surface of a weld bead and assessing its uniformity says nothing about the welding depth or internal pores. It is therefore not possible to make a statement about the strength of the weld seam.

This question could be solved by methods using ultrasound or X-rays that shine through the weld. However, they are only possible after the welding process when the material has largely cooled down. In addition, they require a separate measuring station and take a long time, which only qualifies them for spot checks.

Measuring the annealing

A different approach is taken by the 'SeelectorICAM Laser' quality assurance system from Hema electronic in conjunction with 'Intelliweld', a 3D scanning unit for remote laser welding applications from Blackbird. This system detects the so-called 'false friends' by measuring the annealing, which is clearly visible in steel for several milliseconds after the laser has finished applying energy to the weld seam. The intensity and spread of the annealing of the seam form the basis for the evaluation of the weld seam. This provides access to the primary characteristics of the weld, which actually provide information about the strength and quality.

Principle sketch of the 'SeelectorICAM Laser' system with robot and camera.

© Hema electronic

Extensive test series of the test procedure realized by Hema Electronic and Daimler in cooperation have shown that, in addition to detecting 'false friends', superficial seam defects can also be reliably assessed.

But how does the test procedure work in detail? The laser welds its seam to join the sheets. After welding, the weld itself continues to glow slightly. This visible annealing can be further improved by briefly reheating the seam, which is done by a second pass with a defocused laser and reduced power in the opposite direction. The resulting uniformity of the post-annealing increases the reliability of the evaluation.

The laser power is then lowered and the deflection mirror is directed at the seam. After a short waiting time of approx. 10 ms, during which the solidification process of the melt has progressed sufficiently, the image is captured. The system's camera focuses on the welded seam through the laser optics and takes an image. The laser then moves to the next welding point. The evaluation of the images obtained begins immediately and takes place in parallel with the subsequent welds and images. As soon as the last seam has been welded, the entire evaluation is available. The result of the evaluation is used to eject the component as IO or NOK. The evaluation data is available for further use in the production system and for reworking; the system transmits the data to the process computer and releases the system for the next component.

The image analysis

During image evaluation, 'SeelectorICAM Laser' calculates measured values such as brightness values, geometric data of the image features and evaluation results for assessing the weld and stores these in Excel tables, where they are available for process monitoring.

Thermal image of a defective weld seam with fault markings.

© Hema electronic

The rework station displays the rejected welds - both their position on the component and the defects found - so that quick and targeted reworking is possible. In addition, the system recognizes any malfunctions of the welding system or unsuitable settings from the images.

During set-up operation, the user works interactively with the images and measurement results. The operator can set appropriate parameters and has control over the evaluation process at all times. Data and images can be called up on the system PC. The user can choose to save them in data records, with assignment to serial numbers of the manufactured components and other production information.

In order to integrate the quality assurance system into an existing production system, a laser welding system with a scanner head is required. Laser welding is usually done on-the-fly. Suitable lasers include scanners such as 'Intelliweld' from Blackbird.

The quality assurance system itself consists of the camera including software, a PC for parameterization, display and storage of details, a database interface for inspection results (RDS), a PLC connection for controlling the inspection and the material flow as well as a connection on the laser for triggering the 'seam photos'.

'SeelectorICAM Laser' is an intelligent camera or embedded system with an on-board computer. This allows the raw data in the camera to be evaluated in real time. The camera is located in the beam path of the laser and has a brightness dynamic of 170 dB for strong image contrasts. Its software consists of the image acquisition and evaluation software on board the camera and the operator software on the PC. The camera is operated via a user interface on the system PC. When changing components, the operator simply selects the new associated program. Teaching-in or adaptation to a component for which a program already exists is generally not necessary.

Author:
Charlotte Helzle is Managing Director at Hema electronic in Aalen.