Method and apparatus for monitoring the size variation and the focus shift of a weld pool in laser welding

Abstract

In a laser welding, a laser beam is focused on a workpiece by a focusing lens or lenses. The focusing lens or lenses image an aperture liming the size of the laser beam on the workpiece and the size of focused laser beam is the image size of the aperture on the workpiece at the wavelength of the laser. A weld pool is generated by the interaction of the focused laser beam and the workpiece. Due to the thermal conduction of the workpiece, the size of the weld pool is generally not the same as the size of the focused laser beam and varies with the power of the laser or with the focus shift of the focusing lens or lenses. The weld pool radiates a thermal radiation. An apparatus and method is disclosed wherein the thermal radiation is measured back through the focusing lens or lenses and through the aperture limiting the size of the laser beam or any other aperture limiting a size of the thermal radiation to be measured in at least three spectral bands with single element detectors. Due to the chromatic aberration of the focusing lens or lenses, the transmittance of each spectral band of the thermal radiation varies with the size variation and with the focus position of a weld pool and the spectral band signals measured with single-element detectors vary if the size and/or the focus position of a weld pool varies. Algorithm to monitor the size variation and/or the focus position of a weld pool is disclosed wherein the size variation of a weld pool is monitored independently from the focus shift of the focusing lens or lenses and the focus position of a weld pool is monitored independently from the power variation of the laser.