In situ investigation of the pore development during reactive air brazing of aluminium oxide ceramics

本研究项目的目的是对RAB钎焊工艺进行基础性研究，这种工艺会导致钎焊接头出现孔洞缺陷。利用所获得的知识，目标是减少RAB钎焊件的气孔率。为此，首次将高速X射线辐射技术用作高温钎焊应用中材料流动的现场分析方法。现有的X射线辐射系统是为此目的而改装的，以适应高温钎焊的要求。这种新的检测方法可以用非破坏性的现场观察方法来识别焊缝处运动的随温度变化的过程，而不受光学检测过程的限制。高速X射线辐射进一步允许分析在铜焊过程中不同时间被润湿的边界层。对识别出的不同区域进行了选择性的金相分析和强度检验，从而可以将不同局部孔结构、反应层、结构和产生的强度之间的关系与钎焊过程中的材料动力学相关联。除了对焊接缝隙中的材料运动进行现场检查外，高速X射线辐射还提供了以前所未有的质量分析润湿尝试的可能性。通过在视野内观察开放润湿样品的时间高分辨率，可以准确确定在任何温度和停留时间下的润湿角。光学热台显微镜中已知的景深问题不会发生。对开放润湿尝试、不同宽度的钎焊间隙和不同温度下的传播速度进行联合检查，将确定润湿性、焊接温度和间隙宽度对传播速度、孔尺寸和孔密度的影响。这些认识有助于更好地理解RAB钎焊陶瓷复合材料中气孔缺陷的发展过程。通过对气孔形成过程的原位观察，有望获得有关气孔缺陷行为随影响因素变化的基本知识。到目前为止，在焊接过程中进行现场分析在技术上是不可能的。将所获得的知识转移到其他钎焊工艺和材料组上，有助于提高RAB钎焊部件的密封性和使用寿命。

The aim of the research project is the fundamental investigation of RAB brazing processes, which lead to porous defects in the brazing joints. With the gained knowledge, a reduction of the porosity of RAB brazed components is aimed. For this, the high speed X-ray radiation technique is used for the first time as an examination method for in situ analyses of the material flow in high temperature brazing applications. The existing X-ray radiation system is adapted for this purpose to the requirements of high temperature brazing. This new examination method enables to identify the occurring temperature-dependent processes of the movements in the brazing gap with non-destructive in situ observation methods without the limitations of optical light examination processes. The high speed X-ray radiation further allows to analyze the boundary layers that were wetted at different times during the brazing process. The different areas that were identified are selectively analyzed metallographically and strength examinations are conducted, whereby the relations between different local pore structures, reaction layers, structures, and resulting strengths can be correlated with the material dynamics during the brazing process. In addition to the in situ examination of the material movements in the brazing gap, high-speed X-ray radiation offers the possibility to analyze the wetting attempts in an unprecedented quality. The temporal high-resolution observation of open wetting samples inside view allows an exact determination of the wetting angle at any temperature and dwell time. Problems of the depth of field, as known from optical hot-stage-microscopy, do not occur. A combined examination of the propagation velocity in open wetting attempts, in brazing gaps with different width, and at different temperatures will determine the influence of the wettability, the brazing temperature, and the gap width on the propagation velocity, pore size, and pore density. These cognitions lead to an improved understanding of the development of porous defects in RAB brazed ceramic composites.By in situ observation of the pore formation fundamental knowledge about their behavior in dependence of the influencing factors is expected. So far, it was technically not possible to conduct in situ analyzes during the brazing process. A transfer of the gained knowledge even on other brazing processes and material groups can contribute to an increase in the tightness and service life of RAB brazed components.