Nanoparticles exhibit a decrease in sintering and melting temperature with decreasing particle size in comparison to the corresponding bulk material. After melting or sintering of the nanoparticles, the material behaves like the bulk material. Therefore, high-strength and temperatureresistant joints can be produced at low temperatures, which is of big interest for various joining tasks. Joints (substrate: Cu) were prepared with an Ag nanoparticle-containing paste. The influence of the adjustable process parameters joining pressure, joining temperature, holding time, heating rate, thickness of paste application, surface treatment, pre-drying process, and subsequent heat treatment on the strength behavior of the joints was investigated. It is shown that in particular, the joining pressure exerts an essential influence on the achievable strengths. In addition, temperature, holding time, and thickness of paste application have a significant effect on strength behavior. In contrast, the pre-drying process, heating rate, surface pre-treatment, and subsequent heat treatment possess hardly any influence on joint strength.

Brazing has a long tradition at the Institute of Material Science and Engineering of the University of Chemnitz, Germany. During the last years, comprehensive and innovative knowledge in brazing and soldering technologies were generated. Originating from high-temperature brazing, topics like metal-ceramic and light metal brazing, ultrasound assisted joining processes through to brazing of metal matrix composites were examined. In addition, new topics like joining by nanoparticles or corrosion behavior of brazed heat exchangers are in the focus of research. Prof. Bernhard Wielage managed the institute for 22 years. Today, Prof. Guntram Wagner introduces new topics like friction stir welding and continues the activities in brazing.Keywords: brazing; soldering; heat exchangers; brazing of metal-ceramic joints; soldering with diamond particles StreszczenieTradycje związane z tematyką lutowania twardego w Instytucie Materiałoznawstwa i Inżynierii Uniwersytetu w Chemnitz (Niemcy) są długie. W ciągu ostatnich lat wygenerowano kompleksową i innowacyjną wiedzę dotyczącą technologii lutowania miękkiego i twardego. Zajmowano się problematyką badawczą wywodzącą się od lutowania wysokotemperaturowego, taką m.in. jak: lutowanie twarde metali lekkich z ceramiką i wspomaganie procesu spajania ultradźwiękami w lutowaniu kompozytów metalowych. Obecnie przedmiotem badań są nowe zagadnienia, takie jak: spajanie nanocząsteczkami oraz zachowanie odporności korozyjnej wymienników ciepła lutowanych na twardo. Profesor Bernhard Wielage zarządzał Instytutem przez ostatnie 22 lata. Obecnie, nowy Dyrektor Instytutu Profesor Guntram Wagner zajmuje się takimi zagadnieniami, tjak np. zgrzewaniem tarciowym z wymieszaniem materiału zgrzeiny (FSW) i kontynuuje prace badawcze związane z lutowaniem twardym.Słowa kluczowe: lutowanie twarde; lutowanie miękkie; wymienniki ciepła; lutowanie materiałów różnoimiennych metal-ceramika; lutowanie miękkie z cząstkami diamentu

Quenching and partitioning (Q&P) steels show a good balance between strength and ductility due to a special heat treatment that allows to adjust a microstructure of martensite with a fraction of stabilized retained austenite. The final heat treatment step is performed at low temperatures. Therefore, joining of Q&P steels is a big challenge. On the one hand, a low joining temperature is necessary in order not to influence the adjusted microstructure; on the other hand, high joint strengths are required. In this study, joining of Q&P steels with Ag nanoparticles is investigated. Due to the nano-effect, high-strength and temperature-resistant joints can be produced at low temperatures with nanoparticles, which meets the contradictory requirements for joining of Q&P steels. In addition to the Ag nanoparticles, activating materials (SnAg and Sn) are used at the interface to achieve an improved bonding to the steel substrate. The results show that the activating materials play an important role in the successful formation of joints. Only with the activating materials, can joints be produced. Due to the low joining temperature (max. 237 °C), the microstructure of the Q&P steel is hardly influenced.

Nanoparticles exhibit a decrease in sintering and melting temperature with decreasing particle size. The utilization of this effect is of great interest for joining at low temperatures. First, the paper identifies possible applications for joining and their significance. Furthermore, a commercially available Ag nanopaste is characterized with respect to its thermal properties and strength behavior. Subsequently, it is qualified for joining at low temperatures. The shape, distribution and size of the nanoparticles are determined using transmission electron microscopy (TEM). The thermal behavior is characterized by differential scanning calorimetry (DSC) and thermogravimetry (TG). Furthermore, first examinations of strength properties were executed for the base material copper in order to characterize the influence of different process parameters on joining strength. The analyses show that the nanopaste has a great potential for joining at low temperatures. It is verified that the organic shell of the nanoparticles oxidatively decomposes at temperatures of about 410 °C followed by a sintering process. The sintered microstructure exhibits the thermal properties of bulk silver. It is also shown, that it is possible to produce joints at even lower temperatures of about 300 °C, whose thermal resistance and joint strength is significantly superior to conventional soldered joints.

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