Understanding and Mitigating Water Droplet Impingement Erosion

水滴撞击侵蚀（WDIE）是由水滴与固体表面的高速碰撞引起的。这种现象在燃气轮机和蒸汽轮机、直升机和喷气发动机中都会遇到。尽管已经进行了大量的努力来解决这个问题，但WDIE过程仍然没有得到很好的理解，因为这些努力主要是针对水滴或固体材料的单独研究。但是，为了理解这种现象，应该同时考虑水滴和固体表面。因此，本项目将通过实验研究与数值模拟相结合的方式进行尝试。*到目前为止，准确地将WDIE实验室测试结果与使用中的叶片寿命联系起来并不容易。让事情变得更加复杂的是，比较不同小组使用不同钻机获得的WDIE结果很困难。由于设计和试验条件的不同，直接比较是非常困难的。这使得不同实验室产生的大量数据对行业没有用处。例如，在文献中，一些数据是根据暴露时间报道的，而没有准确地计算撞击固体表面的液滴的实际数量。其他论文报告了固体样品的材料损失与旋转次数的关系，这也没有考虑到液滴的实际数量和它们的大小分布。在这项工作中，我们将开发一种更具代表性的方法，可以比较使用不同钻机获得的结果。这也将有助于工业界将实验室测试结果与实际叶片寿命联系起来。*大量研究人员试图将WDIE解释为一个疲劳过程。然而，申请人的小组已经研究了三种已知的可以改善疲劳性能的工艺。它们是激光冲击强化（LSP）、低塑性抛光（LPB）和超声波纳米晶表面改性（UNSM）。选择这些工艺是因为每一种工艺都通过不同的属性来改变表面，从而提高疲劳寿命。虽然这三种工艺都会产生深而高的压残余应力，但与UNSM工艺不同，LPB工艺会导致表面硬化，但没有任何明显的微观结构变化。在这些工艺中，只有UNSM能显著改善Ti64合金的WDIE。这意味着疲劳机制并不是WDIE中唯一主导的损伤机制。这也表明，为了提高抗WDIE性能，需要压缩残余应力、微观组织变化和表面硬化之间的协同作用。我们希望进一步了解不同WDIE阶段的潜在损伤机制。例如，为了了解残余应力和微观组织变化的综合影响，将研究lpb处理的17-4PH不锈钢的WDIE性能，因为它没有表现出明显的应变硬化。

Water droplet impingement erosion (WDIE) is caused by high speed collision between water droplets and solid surfaces. This phenomenon is encountered in gas and steam turbines, helicopters, and jet engines. Although large efforts have been performed to address this problem, the WDIE process is still not well understood because these efforts have been mostly directed towards studying the water droplets or the solid material separately. However, in order to understand this phenomenon, the water droplets and the solid surface should be considered at the same time. Therefore, this will be attempted in this project through combining experimental studies with numerical modeling.* To date, it is not easy to accurately relate the WDIE lab test results to the in-service blade life. What makes things more complicated is the difficulty in comparing WDIE results obtained by different groups using different rigs. Direct comparison is very difficult because of different design and testing conditions. This makes the large data produced by different labs not useful for the industry. For example, in the literature some data is reported in terms of time of exposure without accurate account of the actual amount of droplets impacting the solid surface. Other papers report material loss versus number of rotations of the solid sample which again does not take into account the actual number of droplets and their size distribution. In this work we will develop a more representative method that will enable comparing results obtained using different rigs. This will also help the industry in relating the lab test results to the actual blades life.* Large number of researchers tried to explain WDIE as a fatigue process. Nevertheless, three processes that are known to improve fatigue performance have been studied by the applicant's group. These are laser shock peening (LSP), low plasticity burnishing (LPB) and ultrasonic nanocrystalline surface modification (UNSM). The choice of these processes was because each one modifies the surface through different attributes that result in improving the fatigue life. While all the three result in deep and high compressive residual stresses, LPB causes surface hardening but without any noticeable microstructural changes unlike the UNSM process. Among these processes, only UNSM causes noticeable WDIE improvement in Ti64 alloy. This means that fatigue mechanism is not the only dominating damage mechanism in WDIE. This also suggests that for improving WDIE resistance, a synergy between compressive residual stress, microstructural changes and surface hardening is required. We would like to further understand the underlying damage mechanisms at different WDIE stages. For instance, to understand the combined effect of residual stresses and microstructural changes, WDIE performance of LPB-treated 17-4PH stainless steel will be studied because it does not show significant strain hardening.