缸体和缸盖是柴油发动机系统最重要的组成部分。汽车运行过程中交变的燃烧热载荷及频繁的加减速操作使它们在工作过程中承受着很强的循环热应力作用。高周和低周热疲劳是缸体和缸盖主要的失效形式。由于良好的导热性能、铸造性能和低的生产成本，灰铸铁一直是生产柴油发动机缸体/盖的首选材料。然而随着商用汽车向高性能及轻量化发展的趋势，发动机的爆压及功率密度越来越高，壁厚要求越来越薄，对缸体和缸盖的性能要求已逼近传统灰铸铁材料的极限。在保证抗拉强度大于280 MPa的同时，进一步提升灰铸铁的导热系数及耐热疲劳性能是解决该问题的方向。为此，本文围绕高导热高强度灰铸铁材料的制备及其耐热疲劳性能展开了研究。 基于热力学计算和等效介质理论，建立了灰铸铁导热系数的预测模型。对不同化学成分及石墨形态，模型预测结果与实测数据吻合较好。进一步定量地分析了合金元素对灰铸铁导热系数的影响程度，并得到制备高导热高强度灰铸铁的条件，即选用Mo为固溶强化元素，降低其它合金元素尤其是Si的含量，保证一定的碳当量，保证三级以上石墨长度。 研究了孕育过程对灰铸铁组织及性能的影响，并提出了高导热高强度灰铸铁的孕育方式。SrFeSi孕育剂可产生数量和尺寸适中的MnS-SrO颗粒。这些颗粒可同时促进初生枝晶和共晶石墨的形核，使抗拉强度和导热系数同时提高。通过合理的化学成分设计和孕育工艺，得到了抗拉强度为285~347 MPa，导热系数为55.5~61.8 W·m-1·K-1的灰铸铁。 基于自适应模糊神经网络法，建立了灰铸铁的组织特征与抗拉强度和导热系数之间的关系。计算了灵敏度水平并对组织特征的影响程度分别进行了排序。明确了降低石墨含量的同时提高石墨长度是制备导热高强度灰铸铁材料的最佳途径。 采用Uddeholm自约束法测试了高导热高强度灰铸铁材料的耐热疲劳性能。发现热疲劳裂纹在灰铸铁内主要沿石墨周围被氧化的基体扩展，石墨的含量和形态对热应力和裂纹扩展的影响具有双重性。高的石墨含量和长的石墨尺寸一方面会提高导热系数而降低热应力，另一方面却使基体的氧化更严重从而促进了热疲劳裂纹的扩展。实验结果表明，抗拉强度为285 MPa，导热系数为61.8 W·m-1·K-1时灰铸铁的耐热疲劳性能最佳。
Cylinder blocks and heads are vital components for the combustion system of diesel engine. With alternating firing load and frequent start-stop events, cylinder blocks and heads are subjected to periodic thermal stress. High-cycle and low-cycle thermal fatigue are thus the primary failure modes. With excellent thermal conductivity, castability and the best compromise between feasibility and cost, gray cast iron remains the privileged materials for the production of cylinder blocks and heads. However, with the aim of increased capability and lightweight construction, a demand combined higher peak pressure, higher power density and thinner thickness has been evolved both for cylinder blocks and heads. Such requirements have already reached the limits of conventional gray cast iron. Further improving the thermal conductivity and thermal fatigue resistance as well as maintaining the tensile strength higher than 280 MPa are considered as a solution. Thus, this thesis focused on the preparation and the thermal fatigue resistance of gray cast iron with improved thermal conductivity and tensile strength. On the basis of computational thermodynamics and effective medium theory, a numerical model was built to predict the thermal conductivity of gray cast iron. For various chemical compositions and morphology of graphite flakes, a good agreement was found between the predictions and the experimental measurements. The effects of alloying elements were quantitatively studied. The preparation condition for gray cast iron with improved thermal conductivity and tensile strength was obtained, namely strengthening the matrix with Mo, reducing the addition of other alloying elements especially Si, keeping appropriate carbon equivalent and having grade 3 or longer graphite flakes. Effects of inoculation on the microstructure and properties of gray cast iron were investigated. Based on that, a method was proposed to inoculate the gray cast iron with high thermal conductivity and tensile strength. By providing appropriate size and number of MnS-SrO particles for the nucleation of primary austenite and eutectic graphite, SrFeSi can inoculate gray cast iron with high tensile strength and thermal conductivity. Depending on reasonable chemical compositions and proposed inoculation, high performance gray cast iron with 285~347 MPa and 55.5~61.8 W·m-1·K-1 was obtained.Based on the adaptive neuro-fuzzy network system, a numerical model was developed to link the microstructural characteristics and properties of gray cast iron. Sensitivity levels of tensile strength and thermal conductivity were then calculated to rank the relative importance of microstructural parameters. It was proved that the most effective method to develop gray cast iron with high tensile strength and thermal conductivity is to reduce the graphite content and increase the graphite length. The thermal fatigue tests were performed using Uddeholm self-restraint method. It was observed that the oxidized matrix around graphite flakes plays an important role in crack propagation. Contradictory effects on thermal stress and crack propagation from graphite were also found. Increasing the size and content of graphite flakes can reduce the thermal stress due to higher thermal conductivity but increase the crack propagation because of more oxidized matrix. As shown by experimental results, the gray cast iron with 285 MPa tensile strength and 61.8 W·m-1·K-1 thermal conductivity possesses the best resistance of thermal fatigue damage.
