A camera-based in situ monitoring system for the Nd:yttrium–aluminum–garnet laser keyhole welding process of aluminium has been developed. The use of an external illumination source for image formation decouples the performance of this system from many process parameters. A prototype of the monitoring system was used to investigate two different welding modes, which have been observed during the laser welding process of AA5182 alloy sheets. One welding mode results in a circular keyhole shape, while the second mode has a more elliptical shape. Welds resulting from both welding modes also have a different surface structure at the top and bottom weld surface. Mechanical tests showed a marginal difference in mechanical properties of both welding types. However, the welding mode with an elliptical keyhole shape is more susceptible to holes in the weld seam. Experiments showed that this welding process can be visualized well with the developed monitoring system and that both welding modes can be distinguished clearly.
A fuzzy logic controller (FLC) scheme has been developed for laser welding. Process light emissions are measured and combined to determine the current status of the welding process. If the process is not in a desired welding state, the FLC will adapt the laser power. The FLC has been demonstrated for the laser welding of zinc coated steel sheets in an overlap configuration. Experiments showed that the controller is capable of steering the process towards full penetration keyhole welding, avoiding both blowholes as well as lack of penetration. Under the presence of different process disturbances, like changes in focal position and material thickness, the controller proved to be able to maintain full penetration keyhole welding. The architecture of the controller is generic, thus facilitating an implementation for the laser welding process of other materials or configurations.
In this paper broad band spectroscopic measurements of the CW Nd:YAG laser welding process of AA5182-H111 are discussed. This specific alloy is e.g. used for the production of Tailor Welded Blanks in the automotive industry. For many optical sensor applications it is important to know the spectral content of the welding process, as such information is crucial for the selection of optical filters. The measurements were conducted in the range of 600 to 1600 nm. Measurements were conducted in two different configurations. In the first configuration the angle between the workpiece and the optical axis of the optical system used for the measurements was 15 o . In the second configuration this angle was 75 o . In both configurations 16 measurements were taken and the results were averaged. In this way the influence of the fluctuating overall intensity of the weld plume was reduced. However these fluctuations still remain the main source of noise in the measurement results. It was found that the region between 1000 and 1600 nm was dominated by temperature radiation. In the region of 700 to 1000 nm a broad band peak was seen that is quite distinct from temperature radiation. There were many peaks found in the measured spectra that correspond with wavelengths at which atomic emission lines are present of Ar, Al, Mg, N and O atoms. However no vibrational or rotational emission lines of AlO, MgO, AlH, N 2 or O 2 could be detected.
Monitoring systems for the laser keyhole welding process of aluminium Tailor Welded Blanks, are in many cases vital to ensure a certain weld quality. Especially process visualization with camera based systems gives a lot of insight. Although for steel it has already been demonstrated that images can be obtained in real-time from the laser welding process using these cameras based on silicon chips, for aluminium this turns out to be not so easy. The light emitted by the weld plume hides the weld pool from the coaxially mounted camera. In this paper recent experiments are discussed in which a monitoring system is used, that is composed of low cost standard components, to visualize the CW Nd:YAG laser keyhole welding process of AA5182. This monitoring system utilizes a diode laser to illuminate the welding process, combined with an optical interference filter and a CMOS camera. In this way the monitoring system is not overradiated by the optical emissions of the sample material. It proved to be possible to eliminate the influence of the light emitted by the weld plume on the image and to detect the melt pool. Future efforts will focus on visualizing the keyhole in these images.
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