Investigation of the feasibility to characterize the properties of critical brazing joints by means of non-destructive ultrasonic testing

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It is generally important for the brazing process to avoid the formation of brittle phases in a joining process. However, in order to identify potential hazards at an early stage, reliable methods for non-destructive quality assurance, especially for safety-related components, are needed. Although ultrasonic testing is sporadically used for a non-destructive analysis of soldered seams, their potential is not yet fully exploited. Currently, the detection of striking flaws such as cracks in the joint or porosity, which show a high display due to the transition from material/air, are in the focus when using ultrasonic testing. Our own preparations, however, have revealed other unused possibilities of ultrasonic testing that can be used to detect even small changes in material properties. Thus, the aim of this proposal is structured along two decisive scientific questions. The focus of this research project is to find answers to these questions:The detection of resistance-reducing brittle phases is especially important for industrial users, as these phases affect the lifetime of the joint and can lead to catastrophic damages, especially when they occur in safety components. Therefore, the research project will examine the possibility of reliably identifying such brittle phases in different solders. Additionally, the geometric influence in the form of prolonged acoustic delay lines in the base material until the soldered seam is reached, has to be considered from a technical point of view, as this can reduce the resolution of the test method. For a reliable detection of brittle phases, the acoustic properties of the brittle structural constituents in the material to be brazed have to be distinctly different from the remaining part to be brazed. This was confirmed preliminary studies. The influences of the geometry of the component as well as the change of the acoustic characteristic values by brittle phases are determined through fundamental investigations.Based on multi-frequency testing, ultrasonic testing offers the potential to determine the gap width of a solder non-destructively. Compared to a weld, the brazed seam is composed of two interfaces, which are sometimes only a few microns apart. This can lead to severe interferences during ultrasonic examinations, which depend on the test frequency. One the one hand, these interferences make a precise examination of the brazed joints difficult and request a heightened diligence when selecting the test frequencies. On the other hand, these interferences might provide an opportunity to control the solder gap in the component non-destructively after soldering.