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.

Copper is widely used in high heat flux and electrical applications because of its excellent electrical and thermal conductivity properties. Alloying elements such as chromium or nickel are added to strengthen the material, especially for higher temperatures. Cu4Cr2Nb, also known as GRCop-42, is a dispersion-strengthened copper-chromium-niobium alloy developed by NASA for high-temperature applications with high thermal and mechanical stresses such as rocket engines. Additive manufacturing (AM) enables applications with complex functionalized geometries and is particularly promising in the aerospace industry. In this contribution, a parametric study was performed for GRCop-42 and the AM process laser powder bed fusion (PBF-LB/M) using a green laser source for two-layer thicknesses of 30 and 60 μm. Density, electrical conductivity, hardness, microstructure, and static mechanical properties were analyzed. Various heat treatments ranging from 400 to 1000 °C and 30 min to 4 h were tested to increase the electrical conductivity and hardness. For both layer thicknesses, dense parameter sets could be obtained with resulting relative densities above 99.8%. Hardness and electrical conductivity could be tailored in the range of 103–219 HV2 and 24%–88% International Annealed Copper Standard (IACS) depending on the heat treatment. The highest ultimate tensile strength (UTS) obtained was 493 MPa. An aging temperature of 700 °C for 30 min showed the best combination of room temperature properties such as electrical conductivity of 83.76%IACS, UTS of 481 MPa, elongation at break (A) at 24%, and hardness of 125 HV2.

The epitaxial growth of coarse and columnar grain structures along the build direction of additive manufactured metals is a usual phenomenon. As a result, as-built components often exhibit pronounced anisotropic mechanical properties, reduced ductility, and, hence, a high cracking susceptibility. To enhance the mechanical properties and processability of additive manufactured parts, the formation of equiaxed and fine grained structures is thought to be most beneficial. In this study, the potential of grain refinement by ultrasonic excitation of the melt pool during laser wire additive manufacturing has been investigated. An ultrasound system was developed and integrated in a laser wire deposition machine. AISI 316L steel was used as a substrate and feedstock material. A conversion of coarse, columnar grains (dm = 284.5 μm) into fine, equiaxed grains (dm = 130.4 μm) and a weakening of typical -fiber texture with increasing amplitude were verified by means of light microscopy, scanning electron microscopy, and electron backscatter diffraction analysis. It was demonstrated that the degree of grain refinement could be controlled by the regulation of ultrasound amplitude. No significant changes in the dendritic structure have been observed. The combination of sonotrode/melt pool direct coupling and the laser wire deposition process represents a pioneering approach and promising strategy to investigate the influence of ultrasound on grain refinement and microstructural tailoring.

A current driving force of research is lightweight design. One of the approaches to reduce the weight of a component without causing an overall stiffness decrease is the use of multi-material components. One of the main challenges of this approach is the low bonding strength between different materials. Focusing on steel-aluminum multi-material components, thermally sprayed copper coatings can come into use as a bonding agent between steel sheets and high pressure die cast aluminum to improve the bonding strength. This paper presents a combination of cold gas spraying of copper coatings and their subsequent structuring by rolling as surface pretreatment method of the steel inserts. Therefore, flat rolling experiments are performed with samples in "as sprayed" and heat treated conditions to determine the influence of the rolling process on the bond strength and the formability of the coating. Furthermore, the influence of the rolling on the roughness and the hardness of the coating was examined. In the next step, the coated surface was structured, to create a surface topology suited for a form closure connection in a subsequent high-pressure die casting process. No cracks were observed after the cold rolling process with a thickness reduction of up to ε = 14 % for heat treated samples. Structuring of heat treated samples could be realized without delamination and cracking.

There are only a few cost-effective solutions for coating applications in combined mechanical loading and corrosive environments. Stainless steel AISI 304 has the potential to fill this niche, showing excellent corrosion resistance while utilizing the deformation-induced phase transformation from γ-austenite to α’-martensite, which results in an increase in strength. However, it is not known whether this can occur in laser cladded material. Therefore, laser cladded AISI 304 coatings in as-cladded condition and after heat treatment at 1100 °C for 60 min were investigated before and after bending deformation, by means of light microscopy, energy-dispersive X-ray spectroscopy and electron backscatter diffraction. It was shown that due to the dendritic microstructure accompanied by an inhomogeneous distribution of the main alloying elements (Cr and Ni), no deformation-induced phase transformation occurred in the as-cladded coating. The applied approach with subsequent solution heat treatment at 1100 °C for 60 min resulted in a homogeneous γ-austenite microstructure, so that a deformation-induced martensitic transformation (DIMT) could occur in the coatings. However, the volume fraction of martensite that had been formed locally at individual shear bands was rather low, which can be possibly attributed to the high Ni content of the feedstock, stabilizing the γ-austenite microstructure. This study shows the possibility of exploiting the DIMT mechanism in heat-treated laser-cladded AISI 304 coatings.