蠕墨铸铁最佳高温疲劳强度的预测方法、组织特征与损伤机制研究

The cylinder head of the diesel engine with high power density is always subjected to frequent impact of high pressure gas and vibration at elevated temperatures, which is obviously high-cycle fatigue (HCF) damage at high temperatures; however material properties of cylinder head under service condition are seldom reported, especially for these at high temperatures. Therefore, in this project, the compacted graphite cast iron (CGI) used to make cylinder head of high power density diesel engine is employed. The samples with different contents of ferrite and pearlite are prepared by heat treatment technologies and the tensile, conventional and in-situ HCF tests are carried out at different temperatures (room temperature to 500˚C). Fracture surface morphologies after tensile and conventional fatigue tests will be observed and analyzed by scanning electron microscope (SEM), laser confocal scanning microscope (LCSM) and MeX software. The influences of temperatue and microstructure on tensile strength and fatigue strength and corresponding damage mechanisms will be investigated systemically and the quantitative relationships among fatigue strength, tensile strength, testing temperature and microstructure will be established referring to the experimental data. On the basis of these in-situ fatigue experimental results, digital image correlation (DIC) technology and finite element method (FEM) will be employed to analyze microstructure and stress distribution at typical crack sites, to find out the prediction method, microstructure characteristics and intrinsic damage mechanism of the optimum fatigue strength at elevated temperatures. These investigations will provide valuable experimental data and reasonable theoretical explanation for fatigue strength prediction and optimum designing of CGI with superior mechanical properties, even for the reliability improvement of cylinder head.

高功率密度柴油机缸盖长期承受高温高频爆压冲击和震动，势必经历高温高周疲劳工况，然而服役工况下材料性能缺少系统研究。为此，本项目以缸盖蠕墨铸铁为研究对象，采用热处理方法制备多种铁素体-珠光体含量的样品，每组样品进行不同温度(室温至500˚C)的拉伸实验，从中选择四组样品进行不同温度常规和原位高周疲劳实验，采用激光共聚焦显微镜、扫描电镜和MeX软件观察分析拉伸与常规疲劳试样断口特征，研究实验温度和微观组织对蠕墨铸铁抗拉强度、疲劳强度及其损伤机制的影响规律。结合文献数据，尝试建立疲劳强度、抗拉强度、温度和组织的定量关系；基于原位疲劳实验结果，选择典型开裂位置采用数字图像技术和有限元方法进行应力和组织分析，深入分析蠕墨铸铁的疲劳损伤机制随温度和组织的变化规律，并探寻最佳高温疲劳强度的预测方法及对应的组织特征和内在损伤机制，为高性能铸铁疲劳强度预测、优化设计和缸盖可靠性的提高提供实验证据与理论指导。

二十一世纪以来，发动机不断向大容量、大功率、高效能和轻量化等方向发展，极易导致可靠性降低。特别是高功率密度柴油机热端构件长期承受高强度的循环机械与热载荷，势必产生疲劳失效导致服役寿命严重不足，成为制约高端装备设计、制造和广泛使用的瓶颈。因此厘清高温服役性能及损伤机制的变化规律，建立高温疲劳强度(寿命)简便预测方法具有重要的理论与工程意义。.本项目以典型共晶铸造材料(缸盖蠕墨铸铁与活塞铝硅合金)为主要研究对象，采用热处理或调整成分方法制备多种不同组织含量的样品，进行不同温度的拉伸实验与疲劳实验，分析疲劳性能及损伤机制随温度和组织的变化规律，探寻最优高温疲劳性能的组织特征和内在损伤机制；基于“化繁为简、以易预难”的思路，分析大量实验与文献数据，建立疲劳强度/寿命与实验温度、组织特征、抗拉强度等易测性能的定量关系，已取得重要研究成果如下：.1.对于蠕墨铸铁，当蠕化率和成分相同或相近时，随着微观组织的变化，室温疲劳强度随抗拉强度增加满足抛物线关系，最佳的微观组织为上贝氏体；当铁素体含量较高时，疲劳强度随温度的升高而逐渐降低，当珠光体含量较高时，疲劳强度随温度的升高先略微增加再逐渐降低。基于前期提出疲劳强度与抗拉强度一般关系，提出系列疲劳强度与组织特征、实验温度等的定量关系。.2.研究蠕墨铸铁和铝硅合金不同温度的低周与热机械疲劳性能，发现两种疲劳损伤机制相似：对于蠕墨铸铁，没有明显的氧化和蠕变作用，主要是疲劳损伤为主导，高铁素体疲劳寿命更长；对于铸铝合金，蠕变和疲劳损伤是主要的损伤形式，氧化作用可以忽略，降低初生硅尺寸有利于疲劳寿命增加。基于疲劳损伤机制和预测方法(能量法)相似性，提出了通过开展相对少量的低周疲劳和热机械疲劳实验来预测热机械疲劳寿命的简便方法。 .上述系列方法及搭建的预测系统正在内燃机、航空发动机、燃气轮机等热端部件推广应用，为发动机构件疲劳性能预测与优化提供实验证据与理论指导。