Fenomen i laserbaserad materialdeponering

This thesis is regarding the use of a laser beam to deposit material. Phenomena in two processes, laser beam welding with filler wire and blown powder directed energy deposition (DED) also known as laser metal deposition (LMD)1, are discussed. High-speed imaging is used as a central tool, supported by cross-sectional macrographs, surface images, X-ray images, computed tomography scans and quantitative analysis of the acquired results to observe many phenomena. Several results generated could be used in the manufacturing industry. A novel concept of feeding the filler wire off-axis to the joint in laser beam welding is presented. The formation of defects called undercuts depended mainly on the stability of the wire feed and irregular melting of its tip. Process parameters played a key role in the robustness of the process, with higher welding speeds and laser powers increasing the chance for formation of defects. Powder catchment in DED, and the various influencing factors are discussed. The position of initial interaction between powder grains and the melt pool plays an important role in defining incorporation behaviour. Powder grains can float on the surface of melt pool and travel along the direction of surface tension driven melt flows before fully incorporating. In high-deposition rate DED, an island of unmelted powder can form in the melt pool, depending on the laser beam shape and powder feeding configuration used. This island could lead to formation of spatter from the melt pool and porosity in resulting clads. Solid oxide skins present on the melt pool in low temperature areas can act like a barrier preventing complete incorporation of powder grains or possibly causing localised boiling, forming spatter. For the first time, near-unprocessed material was used as feedstock in the DED process. A single large melt pool is formed in the relatively calm process, and phenomena like cloud formation while feeding of material and spatter were observed. Single and multi-layered deposition resulted in porous tracks and delamination from the substrate. While the process is not industrially useable in its current state, it is a step towards processing cheap unprocessed material with a laser beam to manufacture low cost parts or for in-situ reduction. The roles of material composition and surface conditions of the substrate in DED are also presented. Both, the composition and surface condition affect the absorption of the laser radiation. Material composition influences the time taken for incorporation of powder grains. The size of the melt pool and dilution depends on the thermal conductivity of the substrate material. Surfaces that are rough or coated with (several sorts of) paint produce wider tracks, with better wetting angles as compared to milled or ground surfaces. Coatings like paints or cold-galvanising primers do not negatively affect the process. Deposition directly on rough or painted surfaces could significantly reduce processing time and the resources needed for cleaning before cladding or repair processes.