Microstructural manipulation by laser irradiation of prepared samples : The ’Snapshot Method’

Sammanfattning: Various metallurgical microstructures and their formation are studied in this thesis by using a laser beam to melt a variety of materials with different chemical compositions over a range of thermal cycles. The laser beam was used conventionally in a narrow gap multi-layer weld, used for welding large depths with filler wire additions, as well as a non-traditional simulated welding approach labelled here as the Snapshot method.In laser beam welding, materials go through rapid heating and cooling cycles that are difficult to mimic by other techniques. In welding, any microstructural development depends on complex combinations of chemistry and thermal cycles but is also influenced by melt flow behavior. In turn, microstructural morphologies influence the mechanical behavior which can suffer due to inappropriate microstructural constituents. The Snapshot method, through control of thermal cycling and material composition, can achieve the same rates while guiding microstructural development to form tailored properties.The tunable laser beam properties can be exploited to develop an experimental welding simulation (Snapshot method), which enables the complex interlinked chemical and thermal events which take place during welding to be studied in a controlled manner. Exploring the microstructural relationships to their thermal history provides a greater knowledge into tailoring microstructural compositions to obtain various required mechanical properties for laser welding, additive manufacturing and also non-laser welding techniques.The feasibility of the Snapshot method is investigated in the three appended journal publications. High speed imaging and thermal recording have proved to be essential tools in this work, with analysis from optical microscopy and EDX/EDS to provide additional support. The Snapshot method is introduced as a concept in Papers I and II, demonstrating successfully guided thermal histories after obtaining molten material. Application of a second and third heating cycle, reheating the structure without melting, yielding altered microstructures. Reaching the austenitisation temperature range allowed for the simulation of complex multi-layer welding thermal histories. Geometrically non-uniform material additions are utilized in Paper III, which investigated the formation of microstructures through the chemical composition route. New chemical compositions were obtained by different degrees of dilution of the weld filler wire by the base material.