Brazing & Soldering Today

Brazed components play an essential role in consistent joining techniques. The brazing process is characterized by low process temperatures and fast processing times, compared to welding techniques. Beside the resulting low manufacturing costs, brazing can join dissimilar materials, e.g. metals and ceramics. Consequently, brazing provides potential for many industrial applications like the production of power electronic devices, in automotive engineering, and for power generation. Simultaneously, brazing provides potentials for seminal applications, such as for the production of structures for particle accelerators and in space technology, as well as for heat exchanging systems for hydrogen energy production. With the use of advanced furnace brazing methods (Ref.1), brazing of steel structures becomes more economical and efficient. High-temperature (HT) furnace brazing is performed at temperatures > 900°C in vacuum or with a shielding gas. These brazed components (e.g., compressor impellers or turbine parts) are subjected to complex loading conditions in service, comprising mechanical, thermal, or thermomechanical loads (Ref. 2). Brazed joints form a heterogeneous anisotropic system consisting of the base metals, the diffusion zone, and the filler metal. Under mechanical loading, the properties of the system vary significantly from those of the individual joining partners. The different elastic-plastic properties of the filler metal and the base metal lead to a triaxial stress state in the brazing zone, caused by constraining effects of the base metal on the filler metal alloy. As a result, large hydrostatic stresses occur in the braze layer and the ultimate tensile strength of the joint can be several times higher than the strength of the unconstrained layer material (Refs. 3, 4). Furthermore, defects such as pores or incomplete joint filling may arise during brazing and act as stress concentration sites, leading to crack initiation, propagation, and spontaneous failure. Consequently, it is very important to estimate the influence of defects on the quasistatic properties and on the fatigue behavior of brazed components. To estimate the influence of defects on bulk materials and on welded structures, defect assessDefect Assessment of Brazed Steel Components