基于熔池热边界的电弧增材形性一体化控制理论研究

Wire and arc additive manufacturing (WAAM) is suitable for near-net forming large-sized complex components with high efficiency and low cost, but is has not been got industrialized applications for the low technology maturity. In view of the engineering requirement for control mechanisms and process parameters design methodology, this project focus on thermal boundary conditions of molten pool and considers the system state of molten pool as a result of the interactions between heat input and thermal boundary conditions, so as to grasp the contradiction between the desired consistent molten pool and the time-varied thermal boundary conditions, and thus address it in a scientific way. Thermodynamic state of molten pool is characterized within the theoretical framework of non-linear non-equilibrium thermodynamics, and the evolution constrained by non-linear time-varied thermal boundary is analyzed using extremum principle and maximum entropy production principle. Design methodology of thermal boundary conditions is clarified after restructing operation of mathematical model of negative entropy flow, during which the characteristic parameters of thermal boundary are introduced by Lagrangian multiplier method. The negative entropy flow is put forward to maintain the steady state molten pool. Finally, the relations of operation parameters → thermal boundary conditions → thermodynamic state of molten pool → shaping and mechanical properties are established through the correlation analysis of thermodynamic state and shaping, mechanical properties. And the process design method is proposed by establishing the processing map, which will provide technical and theoretical support for technology promotion and engineering application.

电弧增材制造技术（WAAM）适用于大尺寸复杂构件高效、低成本近净成形，但该技术成熟度不足，尚处于工业化前夜。针对WAAM对形、性一体化控制机理及工艺设计方法的迫切工程需求，本项目抓住WAAM控形控性对熔池一致性的要求与时变的热边界这一矛盾，将熔池体系视为热输入与熔池热边界相互作用的响应，聚焦熔池热边界特征，通过在非线性非平衡态热力学理论框架下，表征熔池体系热力学状态，并引入最大熵产生原理，从熵产生极值分析的角度描述非线性时变热边界约束下熔池体系的演变路径，继而采用拉格朗日乘子法考虑热边界（成形环境）约束，重构维系熔池体系定态特征的“负熵流”数学模型，阐明热边界设计原理与方法。最后，结合熔池体系状态与WAAM成形、成性的相关性分析，建立“工艺规范→热边界条件→定态熔池体系→形/性特征”的整套工艺设计方法，绘制WAAM形、性一体化控制加工图，为WAAM技术推广及工程化应用提供技术及理论支撑。

金属3D增材成形，伴随着载能束的持续热输入，始终存在非线性时变的熔池热状态与一致的形性控制需求这一矛盾。建立与维持熔池热物理状态一致性成为控形、控性的根本出发点。基于这一思想本项目跳出熔池内部复杂热、质分析这一传统视角，聚焦熔池外部热边界，基于最大熵产生原理热力学描述熔池熔凝环境的演变路径，并构建负熵流以平衡热输入-输出导致的热边界变化，获得建立、维持熔池热边界一致性的工艺控制策略。采用实验研究结合数值模拟的方法，探讨了预热温度、层间停留、随焊冷却、随焊辅助热源等多种形式的热边界控制方案，结果表明上述工艺方案均可改变熔池熔合区两侧温度梯度，调整经熔合区向成形环境的热流输出，规划熔池内部“蓄热”。负熵流的构建与维系，本质上是热积累在熔池内外的再分配问题，及其导致的熔池向成形环境热流输出及随之的热流密度改变。熔池热物理状态及控形、控性的一致性要求即时地调控与熔池热边界相关的工艺变量，动态匹配抵消热边界非线性时变引起的热失配。.熵函数作为熔池体系状态的热力学参量，选取高导热（5356铝合金）、低导热（TC4钛合金），低相变温度点（TC4双相钛合金）、高温相稳定（GH4169镍基高温合金）等典型材料，在熵函数一阶导数、二阶导数为零的点设计实验，分析其凝固组织形态、晶粒粒度及取向特征，并映射到熵函数曲线，以熵函数联系工艺规范、热边界适配规律及组织特征。.本项目基本思想是视熔池为一个整体，建立体系状态与所处热环境二者作用规律，进而构造负熵流分配引起二者变化的热积累，抵消热分配失配引起的状态改变。常用的预热、层间控温、热输入调节等手段，本质上都是优化热积累再分配问题，都可涵盖在项目中提出的负熵流内涵中。因此，本项目中理论模型及工艺优化方法适用多数增材制造技术，如激光选区熔化、激光/电子束/电弧熔丝，并可扩展至其他有持续性热源的热加工技术领域，如激光熔敷、热喷涂、激光热处理等。