超声驱动金属熔滴脉动铺展及液/固界面润湿调控研究

Wetting behavior of metal droplet/solid substrate is one of the key problems in many material metallurgy processes. "Only by wetting, could spreading occur" is a basic standpoint of the traditional wetting theory. On basis of the preliminary investigation, we found that the application of ultrasonic energy could result in pulse spreading of a meltal droplet in the absence of wetting condition, and thus put forward a hypothesis: “ultrasonic agitation enables to re-balance the interface forces at the triple line and drives the metal droplet to spread over the base metal before wetting occurs”. Comprehensive study on 3D characteristics of vibration field of base metal surface, dynamic behavior and mechanisms of spreading of agitated metal droplet, acoustic cavitation in the liquid metal and its effect on the liquid/solid interface interaction will be carried out.It is aimed to reveal the physical nature and discipline of non-wetting spreading of metal droplet and ultimately control the wetting behavior at liquid/solid interface. The results of this project would be an enrichment of the current wetting theory, and provide alternative methods for surface metallization and coating of hard-to-wet materials such as ceramic, glass and metal matrix composites (first spreading and then wetting), as well as references to how to optimize the processes of ultrasonic soldering and 3D manufacturing based on droplet deposition.

金属熔滴/固相基底之间的润湿行为是许多材料冶金过程重点关注的问题之一。“润湿方能铺展”是传统润湿理论的基本观点。基于前期的研究，申请人发现外加超声能量可以使金属熔滴在不润湿基体的条件下发生铺展，进而提出“超声激励可改变金属熔滴固-液-气三相界面的力学平衡状态，驱动其在固体母材表面先铺展后润湿”的设想，通过深入研究固体母材表面振动场的三维动态特性，受激熔滴动态铺展行为及其机制，金属液薄层中的声空化效应及其对液/固界面相互作用过程的影响，旨在揭示超声激励下金属熔滴“非润湿铺展”的物理本质及其规律，以实现对铺展-液/固界面润湿行为的最终调控。本项目可丰富现有的润湿经典理论，同时为难润湿材料（如陶瓷、玻璃、金属基复合材料等）的表面金属化、涂层技术提供新的解决方案（可先铺展，后润湿），也可为超声波钎焊过程、熔滴沉积3D制造等技术的优化提供借鉴。

实现对固体母材表面金属熔滴铺展及润湿行为的有效控制，对焊接、表面金属化等材料冶金过程具有重要的意义。本项目结合有限元计算和高速摄影技术，首先系统揭示了固体母材表面的三维振动场，发现超声在母材表面传播的非均匀性，尤其存在振幅接近于0的“盲区”，提出了通过改变超声频率、施振位置、母材几何形状等优化母材表面振动的方法。其次，直观、实时表征了声场作用下钎料液滴在固体母材表面的动态铺展行为，发现其具有速度超快、雾化伴随、前沿非润湿以及周期性铺展/回缩交替进行的非连续特征，与静态条件下钎料液滴铺展过程的差异极大。计算结果表明，超声附加压力梯度为具有一定不对称性的类正弦变化函数，是钎料液滴实现铺展的内在驱动力，进而阐明了钎料液滴声致铺展的物理本质，并建立铺展的动力学模型。最后，揭示了钎料液滴铺展后固/液界面空化泡的初生、分布及其演化规律，空蚀行为对氧化膜去除过程的影响，掌握了金属液滴铺展-润湿的调控规律。本项目的研究成果丰富了金属熔滴润湿/铺展的理论，纠正了人们对声致铺展认识的误区，对超声钎焊等工程具有重要的指导意义，很好地完成了预定的研究任务。