Alloying- and microstructure-based fatigue life characterisation and prediction of vacuum brazed AISI 304L/NiFeCrSiB joints in corrosive environments

研究焊接接头在叠加腐蚀载荷作用下的疲劳寿命，对于汽轮机部件或废气再循环冷却器等工业应用具有重要意义，并有助于提高这些部件的可靠性和经济性。因此，从化学成分和温度的角度了解不同腐蚀环境下的合金-工艺-组织-性能关系是非常有意义的。因此，该项目旨在对镍基钎焊接头的合金相关腐蚀疲劳机制进行广泛的表征。将应用人工神经网络焊接接头建模方法，并结合破坏性测试结果、微观结构研究和无损测量进行培训，以实现考虑环境影响参数复杂叠加的寿命预测。该方法将进一步用于计算有关铁和钼含量的填充金属的最佳合金化成分。在此计算修改的基础上，将产生优化的样品，并反复测试以进行进一步的表征。因此，将评估是否可以通过良好调整钼含量来改善腐蚀疲劳性能。此外，还将了解铁在镍基合金中的影响。由于显微组织分析是了解腐蚀疲劳性能的关键因素，因此这两种元素对脆性相形成的影响将通过EDX和EBSD扫描来确定。此外，还将在高分辨率下研究合金元素对残余应力的影响。这使得能够分析不同的钎焊区与疲劳性能相关的裂纹萌生和扩展的相关性。此外，还将应用涡流和直流电位降测量，以非破坏性地得出有关铜焊缺陷的结论和关于接头质量的一般声明。这些选定的分析和测试方法的结合、适应和进一步发展旨在使人们能够详细了解钎焊接头的合金-工艺-组织-性能关系，甚至超越腐蚀疲劳行为，新的铜焊方法应适用于其他合金系统和应用条件。

The characterisation of the fatigue life of brazed joints under superimposed corrosive loads is of major importance for many industrial applications, such as turbine components or exhaust gas recirculation coolers, and will help to improve the reliable and economical operation of these. Therefore, the knowledge of alloy-process-structure-property relationships for different corrosive environments in terms of chemical composition and temperature is of great interest. Thus, the project aims to generate an extensive characterisation of the alloying-dependent corrosion fatigue mechanisms of nickel-based brazed joints. An artificial neuronal network modelling approach for brazed joints will be applied and trained with destructive testing results, microstructure investigations, and non-destructive measurements to enable lifetime predictions considering the complex superimposition of environmental influencing parameters. This approach will further be used to calculate optimized alloying compositions of the filler metals regarding the iron and molybdenum content. Based on this calculated modifications, optimised specimens will be produced and tested iteratively for further characterisation. Thus, it will be evaluated whether an improvement of corrosion fatigue properties through a well-adjusted molybdenum content can be reached. Further, an understanding of the influence of iron in nickel-based alloys will be gained. Since microstructure analysis is the key element for an understanding of corrosion fatigue properties, the influence of both elements on the formation of brittle phases will be determined by EDX and EBSD scans. In addition, the effect of alloying elements on residual stresses will be investigated at high resolution. This allows the analysis of different brazed zones regarding correlations to crack initiation and propagation in relation of fatigue properties. Eddy current and DC potential drop measurements will be applied additionally to allow non-destructive conclusions about brazing defects and a general statement about the joint quality. The combination, adaption and further development of these selected analysis and testing methods aims to enable a detailed understanding of alloy-process-structure-property relationships for brazed joints even beyond the corrosion fatigue behaviour, and new approaches for brazing shall be transferable to other alloy systems and application conditions.