Development of an optical monitoring device for remote laser welding of zinc coated steels

This dissertation was born in the partnership between the Warwick Manufacturing Group (University of Warwick) and the SITEC- Laboratory for laser application ( Politecnico di Milano) in the occasion of the "Remote LaserWelding System for Eco & Resilient Automotive Factories" project, a three year research funded by the European Commission, whose aims are to configure, integrate, test and validate application of remote laser welding in automotive assembly. In this work the study of remote laser welding technology with IPG 4 KW laser source and COMAU Smartlaser and its monitoring are presented. Zinc coated steel sheets of a classical automotive thicknesses are welded in overlap configuration. The zinc layers creates problem in the welding process generating defects such as explosions, spatters and porosity due to the facts that the zinc vaporizes at a temperature lower than the steel melts; this creates an overpressure in the molten pool projecting material outside the weld region. To avoid these problems, from an industrial point of view the most used technique is the laser dimpling that is the creation of metal protrusions to introduce a space between the two overlapped sheets in order to depressurize the zinc vapors. Main objectives of this work are the designing and realization of an on-line monitoring device, based on the optical radiation emitted during the welding process, able to describe the main Key Performance Indicators (KPIs) related to the welding. In addition due to the high sensitivity of the process on the gap variation, the identification of the real gap between the two metal sheets is considered crucial as well. For these reasons an off-axis monitoring device based on three different photodiodes has been designed and tested. Afterward the process parameter window has been drawn, characterizing the types of defects that occur outside the process window. In the end with different One Factor at a Time (OFaT) experimentation it has been possible to create relationships that link the laser back reflected signals to the main welding KPIs such as penetration and resistant section of the weld and to describe the gap variation.