冷喷涂热冲击与喷丸效应协同诱导高强铝合金搅拌摩擦焊接头强韧化机制及可调控性研究

Corrosion resistance is one of important indicators of properties for friction stir welded (FSW) aluminum alloy joints, which should be carefully considered during practical applications. Cold spraying (CS), as an advanced solid-state coating technique, is promising to deposit protective coatings for FSW joints because of their excellent corrosion-resistance, and even more important the modification of joint mechanical properties. However, up to now the underlying mechanism for this exciting fact and its influencing factors are not clear. .Therefore, the project will focus on the thermal shock effect (TSE) caused by the high speed and high temperature gas flow and the peening effect (PE) resulting from the high velocity impact of spray particles during CS. Through the design of experiments (DOE) and numerical simulations, the influences of these two effects (TSE and PE) on the residual stresses and microstructure evolution of FSW joints will be systematically investigated. And then the principal inner mechanisms (i.e. microstructure evolution and residual stresses) and the main external inducing factors (i.e. TSE and PE) for strengthening of joints after CS will be clarified. Furthermore, the methods to control the joint properties will be proposed based on the established relationships among the inner mechanisms, external inducing factors and CS process parameters (e.g. driving gas temperature, particle velocity and spray distance). Finally, the controllability of the methods to improve the joint properties will be evaluated through DOE and data analysis. The achievements of this project will help realize the integrated control of corrosion resistance and strengthening of FSW joints, to promote the applications of FSW in many important industries. This project will also be beneficial to exploring the new strengthening methods for mechanical properties of Al alloys.

抗腐蚀性是搅拌摩擦焊（FSW）接头的重要性能指标。冷喷涂（CS）防护涂层不仅能提高FSW焊缝抗腐蚀性，还能改善接头力学性能，但该力学性能改善的内在机制及其外在诱因尚不清楚。因此，本项目抓住上述关键科学问题，将CS过程中高温高速气流的“热冲击效应”与粒子高速撞击的“喷丸效应”作为突破口，采用实验设计与数值计算方法，系统地研究两种效应（外在诱因）对FSW接头残余应力与显微组织（内在机制）的影响规律，进而揭示CS改善FSW接头强韧性的内在主导机制与外在主导诱因；通过建立内在机制、外在诱因与冷喷涂工艺（如气体温度、粒子速度和喷涂距离）三者之间的相关性，获得接头力学性能改善的协同调控方法；最后，基于实验设计与数据分析，评价该力学性能调控方法的可控性。该研究将促进FSW接头抗腐蚀性能与力学性能一体化调控的实现，有利于FSW在各工业领域的进一步推广应用，同时为探索新的力学性能调控方法指明方向。

本研究以3.2mm厚2024-T3高强铝合金FSW接头为研究对象，采用冷喷涂技术在接头表面制备纯铝、铝基复合涂层以及Cu、Ni涂层，分析了冷喷涂对接头显微组织、力学性能和腐蚀防护性能的影响，通过试验设计揭示了冷喷涂对力学性能改善的内在机制。通过分析旋转速度对接头显微组织和力学性能的影响规律，得到最佳焊接参数为：600rpm和200mm/min，此参数下接头具有最高的抗拉强度(约420MPa)和延伸率(约8.7%)；将该参数写的FSW接头作为冷喷涂基体，选用适当的冷喷涂工艺参数在FSW接头上表面制备成形良好的Al-20vol.%Al2O3铝基复合涂层。经冷喷涂处理的接头力学性能得到显著提高且近表面显微组织得到改善，尤其是冷喷涂后接头残余应力降低；为了揭示接头性能改善的机理，采用试验设计的方法将冷喷涂过程中高温高速气流的“热效应”和粒子高速撞击的“喷丸效应”剥离。两种效应单独作用时均对接头近表面显微组织和整个接头的力学性能产生了与冷喷涂过程类似的改善作用，“热效应”对接头的主要贡献在于延伸率的提高，“喷丸效应”的主要贡献在于强度的提高。对比冷喷涂后接头组织性能变化，其两种效应单独作用时对力学性能的变化并不是简单的相加，而是协同诱导实现对接头的强韧化作用；为了揭示不同喷涂粉末对FSW接头力学性能的影响规律，采用相同的喷涂工艺参数，在FSW接头表面制备了成形良好的纯Cu和纯Ni涂层，发现Cu涂层和Ni涂层对接头的力学性能的提高程度明显高于铝基复合涂层；采用电化学测试方法和摩擦磨损实验对比了冷喷涂纯铝涂层和Al-20vol.%Al2O3复合涂层对FSW接头的腐蚀防护情况以及两种涂层的耐磨性，两种涂层均可以对FSW接头提供腐蚀防护，铝基复合涂层的耐蚀性较纯铝涂层差，Al2O3陶瓷相硬度较大，增加了铝涂层的耐磨性。该研究将促进FSW接头抗腐蚀性能与力学性能一体化调控的实现，有利于FSW技术和冷喷涂技术的推广应用。