Technical development of the laser welding process

Abstract: This thesis describes the technical development of the laser welding process and presents the results from a number of experimental investigations. The seven papers that make up the thesis investigate various aspects of the technical development of the laser welding process mechanism as well as industrial applications. The thesis provides an introduction to the subject of CO2 laser welding. The mechanism and techniques of the process are analysed and the future commercial growth of the subject is discussed. An investigation of laser welding of construction steels used in the heavy mechanical industries is presented. The results include welding parameters for high power lasers up to 17 kW, weld metallurgy and static and dynamic weld strength of test specimens. Also components designed for laser welding were tested. An investigation of the effect of plasma control on porosity levels when welding zinc-coated steel is is presented including the reduction of plasma production by using a pulsed laser beam. Laser welding of deep drawing sheet steel has been performed with simultaneous feeding of iron powder into a butt joint. A new method using a magnetic field to feed the powder into the joint gap has been developed. A continuous wave Nd:YAG-laser was used to weld sheet metal parts with very high demands on the weld geometry and quality. Examples include applications from the aircraft and aerospace industry. To achieve improved weld geometry and to reduce the probability of porosity formation, a dual-focusing optic was designed and successfully tested. Fatigue tests were carried out on thin section stainless steel lap welds produced by Nd:YAG-laser. Analysis of the fatigue tests results and the fracture surfaces identified sheet thickness and inter sheet gaps as having considerable effect on fatigue life. Weld width has a more moderate influence on fatigue life and weld porosity has a negligible effect at low concentrations. Three metals with different thermal properties were welded under similar conditions by Nd:YAG and CO2 lasers. The Nd:YAG laser was found to give superior welding performance. The welding process efficiency was found to be affected by four material properties: the specific heat, the density, the melting point and the keyhole absorption.