Real-time temperature measurement for process monitoring of laser conduction welding

Conduction welding offers an alternative to the keyhole welding process. Compared with keyhole welding it is intrinsically a very stable process since vaporization is minimal. However, as with keyhole welding, an on-line process monitoring system is part of the necessary quality assurance process in order to maintain the required penetration depth, which in conduction welding is more sensitive to changes in heat sinking. The maximal penetration is obtained when the surface temperature is just below the boiling point, and so we normally wish to maintain the temperature at this level. We describe a two-color optical system that we have developed for real-time temperature profile measurement of the conduction weld pool. The key feature of the system is the use of a CMOS standard color camera leading to a simplified low-cost optical set-up. We present and discuss the real-time temperature measurement performance of the system when using a defocused beam from a high power Nd:YAG laser on 13 mm aluminium workpieces.