工业纯钛等通道球形转角膨胀挤压（ECAEE-SC）成形机理及晶粒细化机制研究

In order to slove the problems of low forming efficiency, poor grain refining ability and inhomogeneous deformation of traditional ECAP method, a novel process named equal channel angular expansion extrusion with spherical cavity (ECAEE-SC) is proposed for commercially pure titanium (CP-Ti). By setting a spherical cavity and an expansion channel in the ECAP channel, the ECAEE-SC process integrates shear strain and positive strain to achieve combined, continuous and high efficient severe plastic deformation (SPD). Based on the comprehensive using of theoretical analysis, numerical simulation and experimental investigation, the analytical expressions of extrusion load and stress under variable thermal field during ECAEE-SC process is established, and the equivalent strain accumulation formula is obtained. A thermal-mechanical coupled rigid-viscoplastic finite element model of CP-Ti under ECAEE-SC process is established to reveal its plastic flow characteristics and process forming mechanism. The microstructure evolution including grain morphology, grain boundary structure and oriented texture of CP-Ti during ECAEE-SC process is investigated. The influence of dislocation-twin interaction induced by simple shear of ECAEE-SC on grain refinement is clarified, and the grain refinement mechanism of CP-Ti under ECAEE-SC process is revealed. The results can provide a scientific basis for the microstructure controlling of high performance titanium materials. Moreover, it is of great significance for enriching and developing new SPD technology as well as promoting its engineering application.

针对传统ECAP法成形效率不高、细晶能力不强、变形不均匀等难题，以工业纯钛为研究对象，提出一种等通道球形转角膨胀挤压（ECAEE-SC）新工艺。该工艺耦合剪切应变与正应变于一体，通过在ECAP通道内设置球形空腔和膨胀通道，实现材料复合、连续、高效剧烈塑性变形。综合运用理论分析、数值模拟和实验研究等技术手段，建立变温度场下ECAEE-SC挤压力和变形应力解析表达式，获得变形过程等效应变累积理论计算公式；建立热力耦合作用下工业纯钛ECAEE-SC变形刚粘塑性有限元模型，研究其塑性流变特征与工艺成形机理；探究ECAEE-SC变形过程中工业纯钛的晶粒特征、晶界结构和织构演变规律，阐明简单剪切应变诱导下位错-孪晶界交互作用对材料晶粒细化的影响规律，揭示超细晶工业纯钛ECAEE-SC变形晶粒细化机制。研究结果为高性能钛材组织调控提供科学依据，对于丰富和发展SPD新技术，促进其工程应用具有重要意义。

本项目突破传统ECAP挤压成形全过程通道截面尺寸相等的思路，将“球形分流”和“膨胀挤压”结构引入ECAP模具，提出一种叠加耦合剪切应变与正应变的复合SPD工艺——等通道球形转角膨胀挤压（ECAEE-SC）。建立了考虑摩擦条件下ECAEE-SC挤压力解析表达式，推导获得了变形过程中累积等效应变的理论计算模型，基于热扭转模拟试验构建了工业纯钛高温热变形本构方程和热加工图，采用有限元模拟研究了热力耦合条件下工业纯钛ECAEE-SC塑性变形行为，并对模具结构和工艺参数进行了优化设计。在此基础上，开展了工业纯钛等温ECAEE-SC工艺实验验证，对所制备的超细晶工业纯钛进行了微观结构表征和力学性能测试，探讨了变形过程中工业纯钛微观组织和力学性能的演变规律，阐明了多效应耦合作用下ECAEE-SC挤压工业纯钛成形机理和晶粒细化机制。.研究结果表明，ECAEE-SC工艺集镦粗、剪切和挤压等多种变形于一体，坯料单道次变形后累积应变量约为3.01。球形转角和膨胀通道的结构设计提高了金属流动性和变形均匀性，增大了材料内部的静水压力和变形效率，实现了“一次挤压，多种工艺，连续变形”的复合成形效果。变形初期，材料内部板条组织与变形孪晶相互交割，位错滑移和孪生变形两种机制同时存在且交替进行，晶粒细化是两者共同作用的结果。变形后期，在连续剧烈塑性剪切和动态回复的双重作用下，位错滑移逐渐取代孪生变形成为主要的塑性变形机制。.项目研究为高性能钛材的制备加工和组织调控开辟了一种新思路，对于发展关键基础材料先进制造工艺，促进剧烈塑性变形技术工程应用具有一定的参考价值。