Development of an ultrasonic-assisted brazing process for fluxless joining of cemented carbides to steel

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When cohesively joining carbide to steel by means of brazing technology, the use of fluxes is currently, inevitable. The fluxes that are used are not only harmful, but also cause a not negligible and usually not completely predictable porosity in the joining, which significantly affects the strength of the bond, depending on their sizes and distribution. A substitution of the flux by generating cavitation in the liquid solder by means of ultrasonic waves seems a promising solution.The aim of the project is to develop an ultrasound-assisted soldering process for a flux-free joining of carbide to steel. Preliminary works with other joints already showed an increase of the strength by 1.5 times. It was revealed that constituents of the cemented carbide can dissolve and get into the liquid solder. It is therefore necessary to examine the process control concerning how far the mechanical properties of brittle carbides and the resulting joint are affected by the penetration of the ultrasound. Furthermore, a suitable process control should be developed that minimizes the impact of the hard metal during brazing, yet results in a non-porous material. Materials that were previously considered either had a strong resistance to oxidation (sapphire or SiC) or, due to a low pilling Bedworth ratio, formed a very thin passivation layer of (Al, Ti alloys), which are broken up by cavitation. In this regard, the oxidation behaviour or scale behavior of steel must be taken into account. Especially at high process temperatures, steel tends to a strong, non-decaying oxide formation. Compared to the current state of technology, this is one of the biggest challenges of this research project. It is necessary to analyze the oxidation behavior of various types of steel during heat treatments and brazing. Furthermore, it needs to be clarified as in how far the application of ultrasound alongside the resulting mechanical penetration and the cavitations can activate the surfaces and rid it from existing oxides.