Mechanisms of forming intergranular microcracks and microscopic surface discontinuities in welds
Douglas, Stephanie Anne
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Surface microstructures of various materials and weld types were observed using confocal laser microscopy to determine their effects on the formation of microscopic surface formations and microcracks in an attempt to classify the possible sensitivity to fracture in service. The general appearance of the weld, HAZ, and base metal were observed through the use of the confocal scanning laser microscope in order to categorize the presence of surface discontinuities and microcracks according to weld process and material type. The effects of welding conditions such as heat input and welding speed were considered along with the effects of specimen thickness. The materials investigated were mild steel, aluminum, 304 stainless steel, titanium, and high strength steels, including HY-100, HY-130, and HSLA 100. The welding processes considered were electron beam (EB) welding, gas metal arc (GMA) welding, gas tungsten arc (GTA) welding, and laser beam (LB) welding. The specific heat input and welding speed was varied for the electron beam welding and several of the thin plate samples. The surface microstructure was examined with a Nikon Optiphot confocal He-Ne scanning laser microscope. This microscope allows surface visualization for magnifications with a resolution limit of 0.25 microns. The image monitoring equipment associated with the microscope provides for the slow scanning of specimens for measuring surface contours, superimposing layers of images, and producing a single clear image in memory of a highly textured surface.
CIVINS (Civilian Institutions) Thesis document
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