Volume 62, Issue 12

Nickel-based solders are commonly used to join high-alloy steels, as well as cobalt and nickel alloys. Developed in the late 1940’s for soldering engine parts in aircraft construction, these solder materials have a wide range of applications. Due to their high and low temperature resistance, strength, oxidation and corrosion resistance, they have a wide range of applications. They are typically used in the form of foils or powders for brazing in vacuum or inert gas furnaces for heat exchangers for power plant construction, as well as in aviation and medical technology. They are characterized by good gap-filling properties and low porosity. To ensure that the soldering temperatures are well below the melting temperatures of stainless steels, solder alloys are alloyed with elements that significantly lower the melting point. Boron, silicon, and phosphorus are particularly suitable for this purpose; however, they form brittle intermetallic phases in larger solder gaps [1]. Working temperature, material wetting, and solder gap filling are important factors in selecting the solder material. However, microstructure formation in the solder and the heat-affected zone of the base material, as well as the resulting properties such as tensile strength, elongation, toughness, and corrosion resistance, are also important considerations. Diffusion processes during soldering are necessary for bonding the solder to the base material. The longer the duration, the stronger the diffusion between the solder and base material and the more the microstructure changes. This has been proven by diffusion annealing tests, which improve the toughness of soldered joints [2]. Typically, the diffusion rate of atoms from the base material into the liquid solder exceeds the rate of diffusion in the opposite direction. For instance, when joining stainless steels with nickel-based solders, chromium diffuses from the steel into the solder. This promotes the formation of brittle phases, such as borides, silicides, and phosphides, which form preferentially in the center of the solder joint. These brittle phase bands can reduce the strength of the soldered joint. A notable characteristic is the significant impact of solder gap width on solder joint properties. The formation of brittle phase bands is favored by wide gaps [1]. With very small gap widths, no eutectic forms in some solders. Gap widths greater than 50 μm result in the formation of various binary and ternary eutectics from intermetallic phases, depending on the alloy composition. These dependencies were confirmed in the examined samples. When boron-containing solders are used, chromium-iron borides can form on the grain boundaries of the steel in the heat-affected zone as boron diffuses from the solder. This can lead to chromium depletion and increased susceptibility to intergranular corrosion. This study aimed to investigate how the solder gap affects the formation of complex microstructures and identify the phases in three solders in comparison with each other.

Additive manufacturing (AM) processes, such as laser powder bed fusion (LPBF), enable tool-free, layer-by-layer production directly from digital models, allowing for a wide range of design freedoms. However, support structures or fine, thin-walled lattice structures are necessary for producing overhangs or cooling channels. These structures can be used as functional geometries or must be removed later as temporary supports. Since these structures may have different properties than the main component, they can influence the manufacturing process. This study investigated the microstructure, hardness distribution, and geometric deviations of LPBF-manufactured support and lattice structures made of Inconel 718 (IN718). LPBF structures with layer-by-layer melt lines typically exhibit inhomogeneities both locally and within a series of samples. Geometric deviations, particularly reduced cell height in the build direction, must be considered when designing components such as heat exchangers. The findings of this study improve our understanding of the interactions between microstructure, mechanical properties, and geometric deviations in the LPBF process. These findings could contribute to the optimization of additive manufacturing technologies.

High service stresses in investment cast rotating turbine blades of large gas turbine engines require high-quality castings, free of defects that could propagate into cracks and fractures during engine service. If such defects are detected in fabrication, affected castings will usually have to be scrapped, lowering the yield rate and driving production costs. To eliminate such defects from further production parts, it is imperative to understand the nature of these casting flaws, and to learn the mechanisms and process problems that caused them in the first place. In this context, it is important to follow a precise terminology and to differentiate between individual casting defects by designating them correctly. In this contribution, two casting defects in investment cast turbine blades, made of two different nickel-base superalloys, are described that have basically the same formation mechanism, but exhibit different defect morphologies. It is important to discern one from the other, since both have different remedial measures that are needed to prevent defect formation in the future. They almost have the exact same name, except for one small character: cold shuts and cold shots .