Experimental Investigation on High-Cycle Fatigue of Inconel 625 Superalloy Brazed Joints

Experimental Investigation on High-Cycle Fatigue of Inconel 625 Superalloy Brazed Joints

The high-cycle fatigue performance and crack growth pattern of transient liquid phase-brazed joints in a nickel-based superalloy Inconel 625 were studied. Assemblies with different geometries and types of overlaps were vacuum-brazed using the brazing paste Palnicro-36M in conditions such as to generate eutectic-free joints. This optimal microstructure provides the brazed assemblies with static mechanical strength corresponding to that of the base metal. However, eutectic micro-constituents were observed in the fillet region of the brazed assembly due to an incomplete isothermal solidification within this large volume of filler metal. The fatigue performance increased significantly with the overlap distance for single-lap joints, and the best performance was found for double-lap joints. It was demonstrated that these apparent changes in fatigue properties according to the specimen geometry can be rationalized when looking at the fatigue data as a function of the local stress state at the fillet radii. Fatigue cracks were nucleated from brittle eutectic phases located at the surface of the fillet region. Their propagation occurred through the bimodal microstructure of fillet and the diffusion region to reach the base metal. High levels of crack path tortuosity were observed, suggesting that the ductile phases found in the microstructure may act as a potential crack stopper. The fillet region must be considered as the critical region of a brazed assembly for fatigue applications.

研究了镍基高温合金Inconel 625瞬态液相钎焊接头的高周疲劳性能和裂纹扩展模式。使用Palnicro - 36M钎料膏在无共晶接头的条件下对具有不同几何形状和搭接类型的组件进行真空钎焊。这种最佳的微观结构使钎焊组件具有与母材相当的静态机械强度。然而，由于在大量填充金属内等温凝固不完全，在钎焊组件的角焊缝区域观察到共晶微观成分。对于单搭接接头，疲劳性能随着搭接距离的增加而显著提高，双搭接接头的性能最佳。研究表明，当将疲劳数据视为角焊缝半径处局部应力状态的函数时，根据试样几何形状出现的这些疲劳性能的明显变化是合理的。疲劳裂纹从位于角焊缝区域表面的脆性共晶相形核。它们通过角焊缝的双峰微观结构和扩散区域扩展，直至到达母材。观察到较高程度的裂纹路径曲折，这表明微观结构中的韧性相可能起到潜在的裂纹阻止作用。对于疲劳应用，必须将角焊缝区域视为钎焊组件的关键区域。