The paper presents a new method for on-line acquisition of weld-piece surface topography during pulsed laser welding. The method is based on illumination of the weld-piece surface with multi-line laser projector and imaging of the illuminated surface with a digital camera. The acquired image is processed by a personal computer to get the three-dimensional (3D) weld-piece surface topography. The set-up was applied to minimize weld-piece distortion during laser butt-welding of low-carbon thin steel sheets. Methodology based on Taguchi methods was developed enabling optimization of processing parameters. Root-mean-square (RMS) of surface distortion was used as the scalar parameter quantifying the amount of distortion. The results show that laser pulse duration and welding speed have a major influence on RMS distortion.
This contribution presents a new optical system, which allows real-time distortion measurement of weld-pieces on a laser welding machine. The system is based on simultaneous displacement measurement of several thousand points on a weld-piece surface employing rapid (non-scanning) laser profilometry. Weld-piece is illuminated by laser light, structured into multiple light planes and imaged by a digital camera. The position of the optical measuring system is fixed relative to the measured weld-piece. Acquired image is fed into a personal computer where it is processed to obtain the three-dimensional (3D) shape of imaged weld-piece surface. The maximum real-time measurement rate of the presented system is up to 20 surface measurements per second. We have applied the set-up to study the distortion of low-carbon thin steel sheet samples during Nd:YAG laser welding. In this paper we present characteristic evolution of surface distortion and distortion of the welding edges as a function of time. The produced weld-pieces exhibit characteristic V-shaped angular distortion mixed with longitudinal distortion, which bends down the welding edges at the end of weld. In welding edge distortion study we found out that the weld tends to annihilate vertical misalignment. The results show that the developed optical system allows fast and accurate temporally and spatially resolved evaluation of various types of weld-piece distortion. The presented system is scalable - the size of the measurement area can be adapted to the size of weld-piece.