This doctoral dissertation presents the process innovation of Bifocal Hybrid Laser Welding (BHLW) and its application for highest quality seams in welding of aluminium, zinc-coated steels, and its potential for the welding of other materials. Over the last decade a plethora of process models has been conceived for fusion welding of aluminium. The respective empirical process models claim one of the following mechanisms as the cause for instabilities: intensity distribution fluctuations, deformation of the geometry of the keyhole, resonant vibrations within the melt pool, or detrimental physical properties of the melt. Although these empirical models are hardly senseless, this dissertation shows that none fully accounts for the formation of instabilities. go to the book