High-power laser welding of contaminated steel sheets

A deep penetration laser welding process differs from a heat conduction laser welding process in the generation of a steam capillary through the material. The so called keyhole is formed if high energy densities of the laser beam vaporize the base material, leading to a vapor-filled channel which is surrounded by a liquid weld pool. The shape and the oscillations of the keyhole are affected by various physical parameters of the welding process. Under constant process conditions a balance of pressure keeps the capillary open. If the material is contaminated by a layer with physical properties differing from the bulk material (e.g. oil), the equilibrium is disturbed in a negative way. The coating evaporates during the melting of the base material, resulting in vapor locks at the capillary and the weld pool, leading to pores and failures along the weld seam. In order to understand and avoid this effect, it is the aim to analyze and describe the failure generating mechanisms. Additionally, a control method of the laser power is investigated which influences the keyhole's oscillations and could suppress an unwanted collapse.