Application of chromatic filtering to process monitoring of CW laser welding

Abstract

The chromatic filtering of thermal radiation has been applied to the process monitoring and the auto-focus control in pulsed laser welding. The transmittance of thermal radiation from a weld pool depends on a position of radiation source in a weld pool and on the distance of the weld pool from a focusing lens. Its dependence has been analyzed to monitor the variations in weld pool size and to control the focus shift in a pulsed laser welding. The chromatic filtering has many advantages if the thermal radiation is measured when the weld pool size is reduced to the laser spot size. This requirement is easily satisfied in a pulsed laser welding by measuring the thermal radiation between the laser pulses during the cooling stage. However, it can not be satisfied for CW laser welding where the weld pool size is always larger than the laser spot size except at the beginning of laser welding.To apply the chromatic filtering to a CW laser welding, the algorithm measuring the size of a weld pool is analyzed and the requirements to obtain a reference size in measuring the increment or the decrement of weld pool size from that reference size in absolute value are shown. This weld pool size variation is used in monitoring the power variation at a workpiece or the weld depth in a CW laser welding. However, this weld pool size monitoring depends on the focus shift. Therefore, to make it useful in industry, it is essential to keep the focus position in some range required in the weld pool size monitoring. The optimization of optical parameters to minimize the induced error from a power variation in the focus shift monitoring is shown. It turns out that the focus shift can be controlled in the required range at the weld pool size of up to 1.5 times the laser spot size. In conclusion, the weld pool size variation can be monitored and the focus shift can be controlled in the required range in a CW laser welding as well as in a pulsed laser welding.The chromatic filtering of thermal radiation has been applied to the process monitoring and the auto-focus control in pulsed laser welding. The transmittance of thermal radiation from a weld pool depends on a position of radiation source in a weld pool and on the distance of the weld pool from a focusing lens. Its dependence has been analyzed to monitor the variations in weld pool size and to control the focus shift in a pulsed laser welding. The chromatic filtering has many advantages if the thermal radiation is measured when the weld pool size is reduced to the laser spot size. This requirement is easily satisfied in a pulsed laser welding by measuring the thermal radiation between the laser pulses during the cooling stage. However, it can not be satisfied for CW laser welding where the weld pool size is always larger than the laser spot size except at the beginning of laser welding.To apply the chromatic filtering to a CW laser welding, the algorithm measuring the size of a weld pool is analyzed and th...