基于相场法的钛/铝异种合金搅拌摩擦点焊接头脆性相演变机理研究

To explore the fast evolution mechanism of brittle phase in the refill friction stir spot weld joint of titanium and aluminum dissimilar alloys, we present the phase field simulation method to study the super interface reaction and atom diffusion during large hot deformation. Firstly, the dynamic recrystallization (DRX) microstructure on the weld joint is characterized and its evolution dynamics are calculated. The microstructural defects during DRX, including grain boundaries, dislocations and vacancies are quantitatively characterized by high resolution techniques. The thermal and mechanical coupled atom diffusion mechanism is explored and its mathematical relation with hot deformation is established. Secondly, the kinetics affecting brittle phase evolution including the inhomogeneous nucleation and super atomic diffusion are quantitatively evaluated. The deformation and DRX textures which are crystallographic factors affecting brittle phase evolution are also investigated. The interface reaction, atom diffusion and transformation stress coupled phase field model is established. By using the model, the in situ and real-time metallurgical behaviors involved in brittle phase evolution are predicted. The fast evolution mechanism of brittle phase and affecting factors are clarified. Finally, the mathematical relation between key parameters of brittle phase and DRX is obtained. On that basis, the multi-scale modeling method of macroscopic hot deformation, mesoscopic DRX, microscopic atom diffusion and brittle phase evolution is presented. The expected results will make contribution to diffusion and interface reaction theories for solid state welding of dissimilar materials.

针对钛/铝合金搅拌摩擦点焊接头快速演变的脆性相，本项目提出采用相场法研究热塑性变形驱动的原子扩散和界面反应。首先，基于接头动态再结晶组织表征和动力学计算，采用高分辨表征技术定量分析动态再结晶晶界、位错、空位等微观缺陷，探究原子热力耦合扩散机制，建立原子扩散系数与热变形的定量关系；其次，从动态再结晶晶体缺陷引发非均匀形核并提供原子超扩散通道的动力学角度，变形和再结晶织构引起脆性相变体选择的晶体学角度，构建界面反应、原子扩散、相变应力多场耦合的相场计算模型，通过该模型可原位、实时预测脆性相演变的组织特征，定量揭示脆性相快速演变动力学规律及其影响机制；最后，建立脆性相与动态再结晶关键参数的定量关系，即形成热变形-动态再结晶-原子扩散和脆性相演变跨尺度研究方法。该研究将大大丰富异种金属固相连接扩散动力学和界面反应动力学的科学内涵。

焊接具有高温低应变、低温高应变或低温低应变长持续时间的特点，反应组配异种合金连接界面极易产生过量金属间化合物（IMC）脆性相，严重削弱接头承载性能。IMC的精准定量预测和调控是实现异种金属高强度连接必须解决的关键核心难题之一。焊接界面IMC组织场演变具有时间上瞬时、空间上局部、能量上非平衡的综合特征，其准确求解涉及复杂的边界条件以及演变热力学、动力学、晶体学机理，取决于反应型异种合金本征特性及焊接工艺综合影响因素。项目围绕异种合金搅拌摩擦点焊接头脆性相演变动力学规律，采用试验表征、解析建模及相场建模综合方法，定性、定量研究强塑性流动驱动的原子扩散和界面反应。项目研究形成了热变形-DRX-原子扩散和脆性相演变跨尺度研究方法，丰富了异种金属固相连接扩散动力学和界面反应动力学的科学内涵。