塑性加工过程中缺陷修复与动载力学性能变化规律研究

Up to now, evaluation of cavity-type defect healing degree during plastic processes is usually based on the restoration of static mechanical properties such as tensile properties. A new phenomenon has been discovered: when cavity-type defects are disappeared and static tensile properties of defect healing zones are restored completely, their dynamic impact properties are only partially achieved, even the extreme state of nearly zero. Impact properties are the important indicators to identify the safety of load-bearing structures. As a result, the research will focus on the effective healing of internal cavity-type defects and microstructure, find the principle of the overall mechanical properties including impact toughness. The major research contents include: the evolution mechanism between internal cavity-type defects and microstructure with mechanical properties under static or dynamic loads; the methods of defect healing and morphological characteristics of newly formed grain structure during the processes of recovering the mechanical prosperities, and the corresponding model and criterion are established; the change rule between defects recovery and mechanical properties; the influence rule of properties like impact toughness under dynamic load and the external conditions for them to be fully restored to the initial state; the effect of micro pressure stress in high temperature elastic stage on cavity-type defect healing and grain structure; establishing the method of analysis on internal cavity-type defects, microstructure and mechanical properties under static or dynamic load. The results will provide reference and theoretical basis for healing cavity-type defects in heavy forgings and 3D printing workpieces, and for the improvement of the comprehensive mechanical properties, and are really of great academic value for the assurance of quality stability and using reliable reliability.

目前，塑性加工过程中缺陷修复效果一般以拉伸等静载力学性能评价。实验中发现存在着静载力学性能完全恢复，而冲击韧性等动载性能恢复效果差别较大的现象，甚至存在接近零的极端情况。冲击韧性等动载力学性能是反映承载结构件能否安全使用的重要指标。为此，本项目将重点研究有效修复材料内部孔隙性缺陷及组织的演化机制，掌握全部力学恢复过程中可能存在的修复方式，以及残余微小孔隙性缺陷和新生晶粒组织的形貌和特征，建立相应的分析模型和判据。获得内部孔隙性缺陷和微观组织与静、动载力学性能的关系，冲击韧性等动载性能的影响规律和完全恢复到初始状态的外部条件，以及高温弹性区微压力对孔隙性缺陷和基体晶粒组织修复的基础，建立加工工艺方法—内部孔隙性缺陷和组织—静、动态力学性能之间的分析方法。为修复大型锻件、3D打印工件内部缺陷，提高其静、动载等综合力学性能提供理论依据，对保证其质量稳定性和使用可靠性具有重要的学术价值。

冲击韧性、疲劳强度等动载力学性能是评价承载锻件使用寿命和质量水平的重要技术指标。以大型锻件为背景，针对塑性加工过程中缺陷修复变化过程与动载力学性能的变化规律开展研究，掌握了温度、塑性变形量、修复时间等影响因素与金属组织、动载力学性能之间的变化关系，得到了内部缺陷演变与锻件力学性能变化的规律，提出了强度韧性配比的合理范围，掌握了金属组织状态和拉伸等静载，冲击韧性、疲劳强度等动载性能综合控制的工艺条件。研究成果对提高水电、核电、军工等大型锻件使用寿命，解决“卡脖子”关键技术问题提供了理论依据和技术支撑。.高温塑性变形过程中，孔隙性缺陷和组织变化过程为孔隙性缺陷部分接触→孔隙部分闭合→孔隙消失→结合面形成细小晶粒→晶粒长大→与基体组织均匀。力学性能恢复过程是拉伸等静载→冲击韧性→疲劳强度。其对应关系基本表现为孔隙性缺陷锻合后，拉伸等静载强度恢复到初始状态，此时，晶界呈“一”字形分布，冲击韧性、疲劳强度等难以恢复。当结合面的再结晶晶粒组织长大，“一”字形晶界或粗大的三角形晶界基本消失后，冲击韧性能够得到较大恢复，疲劳强度恢复效果不明显。当原缺陷结合面晶粒组织与基体组织均匀后，并且与初始晶粒度相同时，拉伸等静载、冲击韧性和疲劳强度等动载力学基本得到恢复。温度、塑性变形量和保温时间等是影响力学性能恢复的主要因素。高温塑性变形时，一次镦粗或拔长变形锻件拉伸强度基本恢复。一次镦粗加一次拔长变形锻件冲击韧性能够大部分恢复。两次镦粗加两次拔长工艺基本能够保证静、动载力学性能得到恢复。强度韧性值合理配比范围有利于提高锻件在役使用寿命。.研究成果中静动态力学性能数据已用于世界上最大的白鹤滩水电站1000MW水电机组镜板、主轴锻件设计和生产，并被国家能源局批准成为行业技术标准的重要内容。.提出了拉伸强度、冲击韧性、疲劳强度等性能和组织均匀性效果作为评价锻件质量的依据，为核电大型锻件、锻造工艺和技术标准提供了理论基础，对于提高产品质量和使用寿命具有重要的意义。