In situ investigation of the pore development during reactive air brazing of aluminium oxide ceramics

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The aim of the research project is the fundamental investigation of RAB brazing processes, which lead to porous defects in the brazing joints. With the gained knowledge, a reduction of the porosity of RAB brazed components is aimed. For this, the high speed X-ray radiation technique is used for the first time as an examination method for in situ analyses of the material flow in high temperature brazing applications. The existing X-ray radiation system is adapted for this purpose to the requirements of high temperature brazing. This new examination method enables to identify the occurring temperature-dependent processes of the movements in the brazing gap with non-destructive in situ observation methods without the limitations of optical light examination processes. The high speed X-ray radiation further allows to analyze the boundary layers that were wetted at different times during the brazing process. The different areas that were identified are selectively analyzed metallographically and strength examinations are conducted, whereby the relations between different local pore structures, reaction layers, structures, and resulting strengths can be correlated with the material dynamics during the brazing process. In addition to the in situ examination of the material movements in the brazing gap, high-speed X-ray radiation offers the possibility to analyze the wetting attempts in an unprecedented quality. The temporal high-resolution observation of open wetting samples inside view allows an exact determination of the wetting angle at any temperature and dwell time. Problems of the depth of field, as known from optical hot-stage-microscopy, do not occur. A combined examination of the propagation velocity in open wetting attempts, in brazing gaps with different width, and at different temperatures will determine the influence of the wettability, the brazing temperature, and the gap width on the propagation velocity, pore size, and pore density. These cognitions lead to an improved understanding of the development of porous defects in RAB brazed ceramic composites.By in situ observation of the pore formation fundamental knowledge about their behavior in dependence of the influencing factors is expected. So far, it was technically not possible to conduct in situ analyzes during the brazing process. A transfer of the gained knowledge even on other brazing processes and material groups can contribute to an increase in the tightness and service life of RAB brazed components.