基于CA-LB多场耦合的钛合金激光快速成形熔池凝固枝晶生长机理研究

Titanium alloy is widely used on the aerospace and some other fields. The laser rapid forming of titanium alloy has the advantages of saving material and time, low cost, high flexibility and excellent combination properties of components and has recently been the chief tendency of titanium alloy fabrication technique. Currently the formation mechanisms of micro-grain morphology and microdefect during the solidification of titanium alloy laser rapid forming are still on the stage of exploratory experiments. Lacks of exact multi-field coupling theoretical model and prediction of microstructure during laser molten pool solidification make it difficult to precisely control the quality and combination mechanical properties of components. Ti-6Al-4V titanium alloy is used as the cladding material and the cellular automaton method and Lattice Boltzmann method are coupled to build the multi-field coulping mathmatical models of flow, temperature, solute concentration and dendritic growth fields during molten pool solidification. The characteristics of solidifation dynamic evolution are studied in laser molten pool from microscale and the dendritic morphology of titanium alloy and formation mechanism of microdefect are explored by the method of combining theoretical model, simulation with experiments. The influence mechanisms of macro-parameters, such as laser power, scanning speed, distance from focused point and powder feed amount, on the microstructure morphology are illuminated so that it can realize to properly predicte and control the forming process, optimize the structure and retard defects and provide the theoretical basis and guidence for optimizing laser process parameters and improving the qulity of manufactured components.

钛合金广泛应用于航空航天等领域，钛合金激光快速成形技术具有省材、节时、低成本、柔性程度高和加工零件综合性能优异等特点，成为近年来钛合金制造技术发展的总趋势。目前钛合金激光快速成形凝固微观晶粒形态和缺陷形成机理仍处在实验摸索阶段，缺少描述凝固过程多场耦合的理论模型和对微观组织的正确预测，致使成形零件质量和综合力学性能难以实现精确控制。本课题以Ti-6Al-4V钛合金为熔覆材料，采用元胞自动机法和格子Boltzmann法耦合，建立熔池凝固流场、温度场、溶质浓度场和枝晶生长等多场耦合数学模型，运用理论、模拟和实验相结合的方法，从微观尺度研究熔池凝固动态演化特性，揭示熔池内钛合金凝固枝晶形貌和缺陷形成机理，阐明激光功率、扫描速度、离焦量和送粉量等宏观工艺参数对微观组织形态的作用机制，从而实现正确预测和控制成形过程、优化组织、抑制缺陷的目的，为优化激光工艺参数、提高成形零件质量提供理论依据。

钛合金激光快速成形技术具有省材、节时、低成本、柔性程度高和加工零件综合性能优异等特点，研究激光熔池凝固微观晶粒形态、晶粒形成模型、形成机理及工艺参数对晶粒形态的作用规律，可以更好地通过控制凝固过程实现对微观晶粒形态的控制，达到提高材料综合力学性能的目的。目前钛合金激光快速成形凝固微观晶粒形态形成机理仍处在实验摸索阶段，对微观晶粒形貌进行正确预测和考察激光工艺参数对微观晶粒生长机理的影响具有重要意义。.以Ti-6Al-4V为熔覆材料，运用模拟和实验相结合的方法，研究激光熔池凝固过程多场耦合作用下微观晶粒生长机理，考察激光工艺参数对微观晶粒生长的影响机制。研究结果表明：激光熔池快速凝固得到晶粒粗大、内部均为马氏体网篮组织；熔池内晶粒均沿温度梯度方向生长，且冷却速率、温度梯度较大时，柱状晶粒有向等轴晶转变趋势（长径比趋于1）；激光单道熔覆中，晶粒受温度梯度方向的影响与扫描速度呈一夹角生长。对熔凝组织的合金元素分布及含量进行测定，在晶界处和晶粒内部无明显的组织偏析。基体至熔覆层显微硬度逐渐增大，随能量密度的增大，熔凝组织显微硬度随之降低。构建了适合于TC4钛合金激光快速成形熔池微观组织模拟的形核模型和晶粒生长动力学模型，将宏观有限元FE法和微观元胞自动机CA法耦合，模拟TC4钛合金激光快速成型熔覆凝固过程中微观组织形貌。模拟结果表明熔池中的晶粒以粗大柱状晶为主，生长方向与激光热源的移动方向保持一定的角度，大多数晶粒沿熔池温度梯度的反方向生长，且相邻晶粒生长具有连续性，没有明显的熔池边界，模拟和实验结果基本符合。高温度的液态金属由熔池中心出发沿熔池边缘区域往熔池底部流动，再借由熔池中心处高温液态金属流失形成的负压而逆行向上流动，形成完整的环流。液态金属在流动的过程加快了热量的传递，导致熔池底部及边缘区域温度升高，改变熔池的形貌。.项目研究已发表SCI论文1篇，录用SCI论文1篇、核心期刊论文2篇，专著1部，申报发明专利2项。项目研究揭示了微观晶粒形态分布规律，可以通过优化工艺参数预测提高钛合金件性能，为钛合金激光快速成形工艺提供工程实用的科学依据。