For lightweight constructions, joining dissimilar metals is often indispensable to achieve exceptional properties. A common challenge is the bonding of steel and aluminum parts. The use of cold-sprayed coatings as a bonding agent is an innovative approach for high pressure die casting (HPDC) aluminum-steel hybrid components in order to achieve a metallurgical bonding, although it comes with high requirements in terms of coating adhesive and cohesive strength. Therefore, the main aim of this study is the optimization of a post-processing treatment of cold-sprayed coatings in order to improve the cohesive strength to help the introduced coatings withstand the mechanical and thermal stresses during HPDC. The effect of the heat treatment on the mechanical properties of the cold-sprayed Al99.0 and AA7075 coatings was investigated. Freestanding coatings were heat-treated at a temperature of T = 400 °C for different dwell times in order to analyze the recrystallization kinetics through hardness measurements. Two different heat treatment states along with an as-sprayed condition were chosen to investigate the evolution of the mechanical properties of the coatings by means of 3-point bending tests. Besides the softening of the coatings during the heat treatment, sintering effects at splat boundaries and their impact on fracture mechanisms were investigated using electron microscopy.

Since the emergence of the first Ni‐based filler metals in the end of the 1940s, the formation of brittle phases due to the metalloids Si, B, and P has been a major challenge. A new approach to selectively manipulate the microstructure of brazing joints with Ni‐based brazing alloys is to inoculate the brazing alloy with Ti. The aim of this study is to characterize the influence of Ti on the melting behavior and brazeability in brazed joints consisting of hot‐work tool steel X38CrMoV5‐1 and inoculated Ni‐base brazing alloy Ni 620. For this purpose, the melting behavior of the brazing alloy in the inoculated and noninoculated state is investigated by means of combined differential scanning calorimetry (DSC)/thermogravimetric (TG) analysis. In addition, the flow behavior of the filler metal as well as the resulting microstructure are evaluated by means of acoustic microscopy and scanning electron microscopy on the basis of a special specimen shape. The results show that Ti inoculation has a significant influence on the melting behavior as well as on the flow behavior and the microstructure in this brazing joint.

Induction brazing is widespread in many industries due to its ability for local contactless heating as well as its high heating rates and the associated short cycle times. As a result of these advantages, this brazing process is particularly widespread in the tool industry for mass products made of cemented carbide/steel, for example, in saw blades or milling tools. The state of the art is the use of one or more pyrometers for measuring the surface temperature of the component. However, the measurement can be significantly falsified due to oxidation of the component surface and flux residues or due to the evaporation of the flux. Due to the resulting control deviations and the lack of information about the temperature inside the brazing gap, critical residual stresses can be induced in the joint, which can lead to premature failure of the component. An approach is presented that uses distance and force measurements to monitor the condition of the filler metal in the brazing gap. By using these sensors, the point in time at which the entire filler metal has liquefied can be determined. In this way, overheating of the component can be avoided and the holding time and thus also the residual stresses can be reduced to a minimum. Furthermore, numerical calculations are presented, which compare the resulting residual stresses with conventional brazing foils as well as with brazing foils with a pure copper intermediate layer. In addition, the influence of the thickness of the filler metal was examined. The results show that the choice of the right brazing foil architecture and the thickness of the brazing foil have a significant influence on the residual stresses.

Aluminum alloys have a strong tendency to form alumina layers on their surfaces when exposed to atmospheric air, even at room temperature. This is a severe challenge for brazing aluminum alloys as the alumina layer acts as a diffusion barrier and hinders the interactions between the filler metal and the base material. In order to achieve a good metallurgical bond between the filler metal and the aluminum alloy, it is of crucial importance to remove the alumina layer as well as to simultaneously prevent further oxidation of the aluminum alloy. The current investigation focuses on the detailed micro-structural changes that occur during in-situ brazing of liquid filler metal, 95Sn-5Cu (wt.%) on an aluminum alloy, Al-7Si-0.3Mg. These in-situ studies were performed in a large chamber scanning electron microscope in order to monitor the interactions of the filler metal and the base material, particularly the role of Cu on alumina detachment. After the in-situ experiments, the local surface and cross-sectional regions were analyzed by scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy to understand the variation in chemistry across the wetted region, which includes the interfacial region between filler metal and the base material. As the alumina scale present on the aluminum alloy is very thin (<50 nm), nanoscale characterization techniques such as transmission electron microscopy in scanning mode, including selected area electron diffraction for crystal structure determination, were performed. From this investigation, it was found that the Cu in liquid filler metal diffuses into the base material via the oxide layer, resulting in the formation of Al2Cu intermetallic precipitates.

The wetting behavior of liquid metals is of great importance for many processes. For brazing, however, a targeted modification beyond the adjustment of conventional process parameters or the actual set-up was not possible in the past. Therefore, the effect of direct electric current along the surface of a steel substrate on the wetting behavior and the formation of the spreading pattern of an industrial nickel-based filler metal was investigated at a temperature above T = 1000 °C in a vacuum brazing furnace. By applying direct current up to I = 60 A the wetted surface area could be increased and the spreading of the molten filler metal could be controlled in dependence of the polarity of the electric current. The electric component of the Lorentz force is supposed to be feasible reasons for the observed dependence of the electrical polarity on the filler metal spreading direction. To evaluate the influence of the electric current on the phase formation subsequent selective electron microscope analyses of the spreading pattern were carried out.