【作者】 张蓉；

【导师】 刘林；

【作者基本信息】 西北工业大学 ， 材料加工工程， 2000， 博士

【摘要】 Al-Si过共晶合金是一种重要的铸造合金，广泛应用于航空、航天及汽车制造等领域。但铸造组织中常出现的粗大初生硅相严重地损害了材料的机械性能，所以多年来，初生硅的细化问题得到了人们的普遍关注。本文采用等温液淬、快速凝固等试验手段，使用光学金相、扫描电镜和定量金相的分析方法，对该合金中的初生硅在熔体过热处理中的溶解及冷凝过程中的生长行为进行了系统地研究，探讨了熔体不同处理状态，即不同过热温度及保温时间，对凝固组织的影响。在现有凝固理论的基础上，建立了初生硅在熔体中溶解及生长的动力学模型。考察了在不同熔体处理工艺下，冷却速度与凝固组织之间的相关性，并从显微组织的角度探讨了熔体过热处理对冷凝后合金耐磨性的影响。在实验和理论分析的基础上，取得了以下主要成果： 1．系统研究了在不同冷却速度及熔体过热处理（过热度为700℃-1100℃）条件下，Al-Si过共晶合金中初生硅和共晶组织的变化规律。结果表明：通过调节熔体过热参数和冷却条件可以有效地控制凝固组织。提出了改善初生硅形态和尺寸的具体途径，即： a）增大冷却速度； b）提高熔体处理温度； c）延长熔体保温时间； d）采用循环过热。 2．全面研究初生硅溶解及生长过程。结果表明：初生硅在熔体中有较高的热稳定性，在熔体中的溶解过程是由扩散和界面反应共同控制的。熔体中存在的未溶初生硅颗粒可作为凝固时初生硅相的生长核心。本论文建立的初生硅溶解动力学模型和生长模型可以较好地描述液淬组织中初生硅尺寸的变化规律。 3．通过对合金耐磨性的研究，建立了熔体处理对初生硅形态、尺寸的影响在耐磨性能上的表征。发现熔体过热温度愈高，组织中初生硅粒子越细小，分 张蓉 西北工业大学博士学位论文——布越弥散，基体的硬度越高，有利于改善抗犁削磨损的能力，同时可以改善与初生硅相之间的变形协调性，减缓裂纹的形成和扩展，提高材料的耐磨性。更多还原

【Abstract】 Al-Si hypereutectic alloys are widely used in industries and Hi-Tech areas, butcoarse primary Si phase in casting is detrimental to the mechanical properties of thealloy. Therefore, refinement of primary silicon have been received great interestfrom metallurgists. In this thesis, the dissolution and growth behavior of primarysilicon phase in hypereutectic Al- 17% Si alloy melt is studied systematically bymeans of melt overheating treatment. The quench interrupting and rapidsolidification technique has been used in order to control primary silicon. Thequenched microstructure was obtained from the melt of different holding times withthe overheating temperature ranging from 7000C to 1 1000C. The dynamical modelsfor dissolution and growth of primary silicon were established based on currentsolidification models. The interrelations between cooling rate and solidificationstructures were examined under different melt treatment. The effects of melttreatment on the wear resistance were finally investigated. Based on the experimentsand theoretical analysis, the following main results were obtained.1.The variation of primary silicon and eutectic structure with overheating processfrom 700 to 1 1000C overheating and with different cooling rate (1O5-1O0~C/sec)is systematically studied. The results show that the solidification microstructurecan be controlled by adjusting cooling rate and parameter of melt treatment. Themeasurements of improving morphology and size of primary silicon areproposed:a) Higher cooling rate during solidification;b) Higher overheating temperature of the melt;c) Heating and cooling between liquidus cyclically.?III ?2.The study on the processes and dynamics of dissolution and growth of primarysilicon in the melt reveals that primary silicon is of high heat stability in themelt and its dissolution is controlled both by interface reaction between siliconand the melt and by diffusion in the bulk liquid. By combination of dissolutionand growth model, a more precise model to describe the size and morphology ofprimary silicon in the quenched samples is proposed. The calculated resultsfrom this model are basically agreed with experiment.3.The relationship between melt overheating treatment parameters and wearresistance of the alloy is established. It is concluded that the finer the primarysilicon phase is, the more uniformly distribute in the matrix, the higher the wearresistance. The hardness of matrix increases with increasing of melt overheatingtemperature, which improves the deformation consistency. Therefore, theinitiation and propagation of cracks will be prevented or slowed down, resultingin higher wear resistance.更多还原