超声激励下薄层金属凝固过程晶粒细化机制及工艺过程调控

Ultrasonic energy was applied into the liquid solder through the base materials during Ultrasonic-assisted brazing process. The oxidation film on base metal was removed by acoustic cavitation, which led to connection between the solder and difficult-wetting materials. However, due to the solder itself, especially the limitation of low-temperature solder performance, the mechanical properties of brazed joints are poor. It can significantly improve the mechanical properties of brazed joints by ultrasonic cavitation and acoustic streaming during solder solidification which can achieve the grain refinement. Acoustic streaming and acoustic cavitation effect is mainly due to grain refinement of ultrasound. However, the mechanism of grain refinement in the process of metal solidification by acoustic cavitation and acoustic streaming is unclear. At the same time, ultrasonic wave propagation in a narrow gap structure has multiple sound reflections, interference of sound waves and other features. All of these characters make it more complex internally generated within the acoustic effect as compared with the large volume of space, so it is unable to make use of existing research results. The project analyzes the formation and evolution of acoustic.effects in narrow-gap structure during the solder solidification process. To reveal the grain refinement mechanism of ultrasonic assisted brazing during the solder solidification process, and establish the mathematical model of the relationship with "temperature - ultrasonic intensity - solder solid-liquid ratio- grain size", propose a kind control technology of brazing seam grain refining within process, which can realize accurate microstructural control during ultrasonic assisted brazing.The research project is to provide the new idea of mechanical properties of brazing joint improvement, and it is more significant to enrich the ultrasound grain refreshment theory.

超声波复合钎焊将超声波能量通过母材引入液态钎料中，利用声空化效应去除母材氧化膜，实现钎料与难润湿材料的连接。但由于钎料本身，尤其中低温钎料性能的限制，存在钎焊接头力学性能较低的问题。将超声波引入钎料凝固过程中利用声空化及声流效应可实现钎缝金属晶粒细化，显著提高钎焊接头力学性能。但目前金属凝固过程中声空化及声流细化晶粒机制仍不清楚，超声波在窄间隙结构内的传播过程存在多次声反射、声波干涉等特征，使得其内部产生声学效应与大体积空间内相比更加复杂。本项目分析窄间隙结构中钎料凝固过程声学效应形成及演变规律，揭示超声复合钎焊钎料凝固过程晶粒细化机制并建立“温度-超声强度-钎料固液比例-晶粒尺寸”四者关系的数学模型，提出一种超声复合钎缝晶粒细化工艺过程调控技术，实现超声复合钎焊接头组织精确控制。本项目的研究为提高钎焊接头力学性能提供新的思路，更是对超声细化晶粒作用的理论丰富。

超声辅助钎焊可实现在大气环境下无钎剂钎焊SiC颗粒增强铝基复合材料（SiCp/Al-MMCs），并利用超声空化及声流效应改善陶瓷颗粒与钎料间的润湿性、提高焊缝结晶过程形核率，一定程度上提高接头力学性能，但其力学性能仍受限于钎料自身性能、焊缝金属在凝固结晶过程长大粗化、SiC颗粒偏聚等强度恶化因素，常难以应用于强度要求高的结构中。为了突破传统超声复合钎焊连接SiCp/Al-MMCs的局限性，本研究基于50vol.% SiCp/6061Al的超声辅助钎焊，提出了在液态钎料凝固过程中附加二次超声实现晶粒细化及组织调控的工艺方法。分析了声场与温度场复合作用下母材表面的振动响应规律，声场与流场、温度场耦合作用下窄间隙内半固态钎料运动行为及声学效应变化规律；研究了超声对钎缝金属凝固过程中晶粒组织变化规律及其对接头力学性能的影响，揭示了超声辅助钎焊钎料凝固过程显微组织细化机制。研究结果表明，在钎料冷却至半固态时施加二次超声可实现α-Al相的进一步细化，调控后钎缝金属晶粒尺寸仅为常规超声辅助钎焊接头的二分之一。钎缝中SiC颗粒数量显著增加且分布均匀，接头的力学性能大幅提升，调控后接头剪切强度最高可达245.17MPa，较未进行超声调控处理时提高了75%。本研究为优化钎焊接头组织结构，提高钎焊接头力学性能，拓宽复合材料钎焊应用领域提供了重要研究基础了发展新思路。