Multiples passes welding of very high thickness parts (up to 60 mm) with a Nd:Yag laser requires at first the welding of a root pass, located at the bottom of the groove. Its geometry has also to be defined in order to minimise the total number of passes. So, a narrow gap groove is necessary, and consequently this narrow gap geometry can only be welded with a long focal length optic, giving a narrow beam. But this constraint produces a large focal spot, and the welding process is not really optimised. These limitations need an adaptation of the groove geometry in order to keep a full penetration of the bottom area of these parts with enough laser power transmission through it.
Moreover, the minimisation of overall deformation in order to keep the gap opened, leads to the use of high welding speed and specific clamping devices.
This study presents the results we have obtained for welding such configurations. Specific instrumentation using on-axial visualisation has been installed on focussing head in order to have a control of the full penetration, which was also correlated with laser transmission measurements.