热锻过程中金属空洞缺陷的修复行为及其对高温开裂的影响机理研究

Most of the studies on the healing behavior of the void defects in hot forging process are qualitative, as the relationship between the healing effect and the factors (including the temperature, maintaining time, stress-strain states and so on) is not obtained quantitatively. Moreover, the existing ductile fracture criterions have not fully considered the suppressive effect of the void healing behavior on the accumulative damage in plastic deformation, which indicates that those criterions are not quite suitable for describing the fracture behavior of the materials in the hot forging process with alternately tensile-compressive deformation. In order to solve the above problems, the hot forging experiments with defects healing and the theoretical analysis of the void evolution are applied to study the contents as following: (1) the influence mechanism of the grain growth and recrystallization on the void healing behavior, and the quantitative relationship between the void healing extent and the factors including temperature, maintaining time, stress state and deformation history; (2) the quantitative description of the competition mechanism between the void growth and the void healing; (3) the ductile fracture criterion with considering the effect of void healing behavior, which can be applied to predict the fracture occurrence in the multi-stage hot forging process with complex deformation history.

针对热锻过程中金属空洞缺陷修复行为的研究大多处于定性分析阶段，缺陷修复程度与温度、保温时间和应力应变状态等因素之间的定量关系还不明确。而且，现有的韧性开裂模型尚未充分考虑空洞修复对损伤累积的抑制作用，因此对复杂变形路径下的热开裂描述会有所偏差。为解决上述问题，本项目拟通过预置空洞材料的热变形实验和空洞演化理论分析方法研究以下内容：(1)研究高温下晶粒长大与再结晶对空洞愈合修复的作用机理，分析变形温度、保温时间、受力状态、变形历史等因素对空洞缺陷愈合修复的影响规律，提出空洞修复程度与各因素之间的定量函数关系；(2)分析复杂变形路径下空洞缺陷的长大汇聚与愈合修复两种演化机制之间的竞争作用，揭示空洞缺陷的愈合修复对材料损伤累积的量化影响；(3)将空洞愈合修复程度的描述方法引入到空洞形状和体积演化表达式中，建立适用于复杂变形路径下的高温韧性开裂模型，进而指导热锻工艺制定，以消除缺陷并减少开裂发生。

金属材料在高温塑性变形中，内部空隙缺陷的演化行为包含两方面，一是空隙的形核、长大和汇聚导致裂纹出现；另一方面，空隙缺陷在高温条件和合适的应力应变状态下会发生闭合并愈合修复，能使材料的连续性和力学性能得到一定程度的恢复。在高温变形过程中，损伤累积与空隙缺陷愈合修复会相互竞争，有效利用空隙缺陷的愈合修复行为，有利于延缓甚至避免金属热开裂行为的发生。本项目针对大型锻件常用的SUS316LN和SUS630不锈钢材料进行分析，通过预置空隙试样的热变形实验研究了高温下空隙缺陷的愈合修复机理，分析了变形温度、保温时间等因素对空隙缺陷愈合修复的影响规律，提出空隙修复程度与各因素之间的定量关系；分析了高温变形中空隙缺陷的演化行为，并建立了高温流变模型；通过引入空洞愈合修复程度的描述方法，建立了考虑空隙愈合修复行为的高温韧性损伤判据，在热锻工艺规划中具有一定应用前景。