【作者】 张永；

【导师】 丁宏升；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2007， 硕士

【摘要】 活塞作为发动机上的重要构件,担负着动力传输的重要任务。Al-Si合金以其较高的比强度及较好的尺寸稳定性,在活塞材料中已逐步占据了统治地位。其性能的每一次提高都必将伴随着机动车的一次“速度革命”。脉冲电场细晶凝固技术始于国外,近些年在国内得到了快速发展。其所能作用的材料及其范围不断扩大,理论研究也不断深入,有望在不久的将来进行大规模工业应用。然而为了提高Al-Si合金的高温及耐磨性能,通常大量增加合金中Si的含量,因此使得合金凝固组织中的共晶Si、初生Si的形态和分布不合理,而且当加入变质剂时又易带来污染、过变质及合金成分不准确等一系列问题。本文针对上述问题,制备了三种不同成分的Al-Si活塞合金试样,且对每种成分合金试样在不同过热度下进行了电脉冲处理(PECT)实验。并采用金相观察、SEM及EDS分析手段,对PECT前后及不同温度PECT下试样的凝固组织进行了分析,主要包括晶粒组织及第二相的形态、分布等。最后对合金试样的相关力学性能进行了测试。结果表明:PECT能显著细化Al-Si活塞合金组织,使得含Si量小于12.6wt%合金晶粒尺寸减小;含Si量大于12.6wt%合金的一次枝晶臂间距(PDAS)的大小与PECT温度相关;合金凝固组织中初生Si、共晶Si及第二相的形态、分布特征在PECT前、后变化明显;结合对合金试样进行的强度、塑性及显微硬度的力学性能测试结果,得出了PECT对Al-Si活塞合金的最佳处理工艺。运用热力学上关于正规溶液模型的假设,将Al-Si活塞合金熔体近似为Al-Si二元体系。根据Al-Si二元系固液两相的平衡条件,基于电脉冲孕育观点,分析了脉冲电场引起的熔体中组元相互作用能的变化对其发生固液相变时吉布斯自由能变化的影响。并结合脉冲电流提高形核率观点,解释了PECT后Al-Si活塞合金组织中初生Si、共晶Si在形貌、分布上变化的原因。更多还原

【Abstract】 As an important part of the engines, pistons play a significant role in bearing and transporting the load. Among piston materials Al-Si alloys are becoming dominant gradually due to their better specific strength and shape stability. It is said that every time the properties of pistons are improved, the“speed up revolution”in engines will take place.The pulse electric field grain refining technique that originates abroad takes much more progress domesticly. It is applied to many kinds of materials, and is investigated more deeply. It is expected to be used extensively in industrial fields. However, in order to improve pistons’wear resistance and high temperature resistance, we should increase the Si content in the Al-Si alloys. But this leads to uneven distributions in the morphology of the primary and eutectic Si phases. When we add chemical substance to modify them, it will also bring us problems of pollution, over-modification, and errors in composition controlling.In light of the issue that implies above, three different compositions of Al-Si piston alloys were prepared, and were subjected to pulse electric current treatment (PECT) at different temperatures. In virtue of optical microscopy (OM), SEM and EDS, we observed and analyzed the solidification microstructures including the morphology of the matrix and the distribution and configuration of the secondary phases. Accordingly, the mechanical properties of the specimen prepared previously were tested.The results indicate that PECT can refine the grains of Al-Si alloys, which refer to decreasing the grain size of the alloys with the Si content less than 12.6wt%. We also found the relations between the PECT temperature and the PDAS (primary dendrite arm spacing) of the alloys with Si content more than 12.6wt%. The morphology and distribution of the primary Si, eutectic Si, and the secondary phases varied obviously. Finally, we derived the best process parameter for PECT on Al-Si piston alloys from the testing of the tensile strength, elongation and microhardness of alloys prepared.In order to elucidate the effect and mechanism of PECT, we thought the melt of Al-Si piston alloy as melt of Al-Si binary alloy approximately, in terms of the thermodynamic regular solution model. On the basis of equilibrium transformation of the solid-liquid phase of Al-Si binary alloy, we demonstrated that the PECT’s affect on the melt could arise from the variations of the interaction energy, which was based on an“electro-pulse modification”idea, and these variations resulted in the changes of Gibbs free energy in solid-liquid phase transformation. On the other hand, according to that PECT can accelerate the nucleation of Al, the growth of the primary Si and eutectic Si are surpressed, then changing their morphology and distributions.更多还原