【作者】 刘清梅；

【导师】 翟启杰；

【作者基本信息】 上海大学 ， 钢铁冶金， 2007， 博士

【摘要】 传统导入方式的超声波处理虽然能够显著改善金属凝固组织和强化金属材料性能，但是由于存在诸多缺点，比如容易卷入夹杂物、对于高温合金的操作困难等，使得该技术在工业应用上有很大的难度。本文采用自主研发的“自吸式变幅杆”超声侧部导入技术，解决了超声波导入难题，并研究了该超声处理技术对铝基和铁基合金凝固特性、组织和性能的影响，探讨了其中的基础理论问题。为了探索超声波侧部导入处理工艺的可行性，本文首先采用此技术处理了低熔点亚共晶A1-7.3％Si合金，系统研究了此工艺对合金凝固特性和组织的影响。采用微分热分析方法对合金的凝固初始温度、共晶反应温度和时间进行了分析。研究表明，200W超声波处理对熔体的凝固初始温度影响小，随着输入的超声波功率进一步提高到600W后，合金的凝固初始温度发生大幅度降低，对应于宏观组织细化和初生α相形貌的显著改善。超声波功率从0W变化到700W时，平均晶粒尺寸与输入的超声波功率呈负指数递减规律。本文考察了超声波处理对材料力学性能的影响，结果表明超声波处理提高了该合金的抗拉强度和延伸率。为了探讨超声波处理对高熔点合金的影响，本文选用成分简单的二元铁碳合金(T10钢)为研究材料，结合超声波侧部导入工艺分析超声波对高熔点合金凝固特性和组织的影响。微分热分析结果表明，功率小于300W时，超声波处理对凝固初始温度基本没有影响。随着输入功率的提高，凝固初始温度逐渐降低。组织结果显示，随着超声波功率的提高T10钢晶粒细化程度越发明显，分布更加均匀。力学性能测试结果显示，超声波处理后试样抗拉强度增大，延伸率提高。断口扫描结果表明，超声波处理使断口的断裂特征由典型的脆性断裂逐渐转变为准解理断裂。在上述研究基础上，结合企业需求，深入研究了不同超声波功率处理对1Cr18Ni9Ti奥氏体不锈钢凝固过程、组织及性能的影响。微分热分析显示，随着超声波处理功率的增大，熔体的凝固初始温度逐渐降低，凝固时间缩短，包晶反应温度也随着超声波功率的提高而降低，而且包晶反应前期近平衡凝固时间随着功率的提高大大缩短。通过数学分析定量的得到超声波处理过程凝固初始温度与功率的关系。随后采用EBSD电子背散射衍射技术对晶界和晶粒内部缺陷进行分析。结果表明，超声波处理增加了组织中的晶界、亚晶界面积和位错密度，并且随着输入功率的提高，凝固组织发生由柱状晶向等轴晶的转变。功率从0W到700W，输入功率与奥氏体相含量呈线性递增关系，700W超声波处理时组织近似为全奥氏体。分析了超声波处理对奥氏体不锈钢微观组织的影响。结果显示，随着超声波功率的提高发生了柱状枝晶向等轴晶或球形晶的转变，相应的二次枝晶臂间距变小。通过EDS能谱技术测试了不锈钢中主要元素在不同位置的分布，结果表明施加超声波加速了固／液界面前沿的溶质扩散，减小溶质富集边界层厚度，从而降低了合金元素偏析。力学性能测试结果表明，超声波处理对不锈钢试样的延伸率影响不大，但能显著提高合金抗拉强度。对合金抗拉强度沿着超声波传播距离上的衰减进行分析，表明合金抗拉强度与传播距离呈负指数递减关系。抗腐蚀性测试显示超声波处理提高了奥氏体不锈钢的抗腐蚀性能。结合凝固理论和声学理论，对超声波处理细化机理进行了系统分析。分析表明空化气泡在膨胀过程导致微区过冷和气泡闭合形成的高压提高了合金形核速率，使组织细化。而气泡闭合形成的强冲击力造成的型壁晶粒剥落、超声波处理后熔体内热起伏造成枝晶臂颈部熔断也有效地促进了凝固组织的进一步细化。分析了超声波作用对金属理论结晶温度的影响，推导出了超声波处理下合金理论结晶温度与超声波功率的数学关系式，并根据实验所测的金属凝固初始温度，得到超声波处理后合金过冷度增大。结合牛顿冷却模型和流体学理论，推导了超声波处理条件下金属冷却速率与超声波功率和处理时间的关系。建立了凝固熔体中的超声波传播过程的“衰减模型”，探讨了超声波衰减与传播距离和熔体实时温度的数学关系。采用自主研发的超声波衰减测试设备采集了亚共晶A1-7.3％Si合金凝固过程中不同温度和传播距离下的衰减值，得到衰减分布函数。通过实验数据对“衰减”模型进行验证，表明理论模型和实验结果相符。更多还原

【Abstract】 It is well known that ultrasonic treatment can refine solidification structure effectively and strengthen mechanical properties of metals. However, due to some disadvantages of the traditional ultrasonic treatment process, such as inclusion immixture, difficult manipulation to high-melting metals and so on, the application of ultrasonic treatment in the industry field has great difficulties.A novel ultrasonic treatment process, i.e. ultrasonic side-introduced technology with "self-attraction amplitude rod" is developed to solve the problems of traditional ultrasonic treatment and extend its applications. The aim of this work is to systemically investigate its application to metal solidification.In order to investigate the feasibility of side-introduced ultrasonic treatment, we examined the effects of ultrasonic treatment on solidification characteristics and structure of low-melting A1-7.3%Si alloy. Differential heat analysis method is selected for measuring solidification initial temperature, eutectic reaction temperature and time of A1-7.3%Si alloy during solidification. The experimental results show that solidification initial temperature of the melt has few changes until the inputting ultrasonic power reaches 600W. With the further increase of ultrasonic power, solidification initial temperature is decreased, causing grain refinement and morphology development of primaryαphase. From 0W to 700W, average grain size can be expressed as the negative exponential of ultrasonic power. Mechanical tests show that both tensile strength and elongation percentage are enhanced.Next, we evaluate the contribution of ultrasonic treatment on solidification characteristics and structure of high-melting T10 steel. Results of differential heat analysis show that once the ultrasonic power is beyond 300W, solidification initial temperature decreases with the increment of ultrasonic power. Microstructure study of T10 steel reveals that ultrasonic treatment results in drastic refinement of grain with more uniform distribution. The significant structure variation obviously has an effect on mechanical properties. Mechanical test demonstrates that ultrasonic treatment can improve both tensile strength and ductility remarkably. Furthermore, fracture characteristics of samples are transformed from brittle fracture to quasi-cleavage fracture.Based on the above investigation, we investigate the effect of ultrasonic treatment on solidification characteristics, structure and properties of 1Crl 8Ni9Ti austenite stainless steel so as to meet the demand of the industry field. The differential heat analysis suggests that the solidification initial temperature decreases and solidification time shortens with the application of ultrasonic during solidification. The application of ultrasonic treatment on 1Crl 8Ni9Ti also makes the peritectic reaction temperature decrease. The relation between solidification initial temperature and ultrasonic power is quantitatively determinate. Analsis results of EBSD show that with the increase of ultrasonic power, important structure changes occur with the equiaxed grain being substituted for column one. Moreover, not only the area of grain and subgrain boundary, but also the dislocation density increases. From 0W to 700W, austenite content is ratio to linear increase of ultrasonic power. When the power is 700W, the structure is almost austenite phase. Meanwhile, the investigation of the effect of ultrasonic treatment on rnicrostructure of 1Crl 8Ni9Ti is carried out. Ultrasonic treatment plays the role in the columnequiaxed dendrite transformation. According to EDS results, we exhibit the distribution of main elements in different positions. It is further revealed that ultrasonic substantially suppressed the zonal segregation of Cr and Ni due to the acceleration of solute diffusion in the front of solid/liquid surface, which leads to the decrease of solute boundary layer thickness. The experimental results of mechanical properties indicate that the ultrasonic treatment improves tensile strength remarkably and slightly increases elongation percentage of 1CrlSNi9Ti austenite stainless steel. Tensile strength can be expressed as the negative exponential of propagation distance of ultrasonic. In addition, the stainless steel has better corrosion resistance after ultrasonic treatments.On the basis of the solidification and acoustic theory, the refinement mechanism of ultrasonic treatment during solidification is discussed. It is indicated that micro-area supercooling during cavitation bubble expansion and high pressure induced by bubble close give rise to the increase of nucleation rate of melts, causing the refinement of structure. In addition, shock force generated by cavitation bubble and heat pulse caused by ultrasonic also make great contributions to improve dendrite morphology and refinement structure. The effect of ultrasonic treatment on theory crystallization temperature of the metal is studied. Calculation shows that theory crystallization temperature rises after the application of ultrasonic treatment. According to the experimental results of solidification initial temperature, it can be concluded that ultrasonic treatment causes the increase of degree of supercooling. By means of Newton cooling model and fluidics, we analyze the relation between cooling rate and ultrasonic power.Attenuation model of ultrasonic propagated into the liquid metal is built to reveal the mathematic relation among ultrasonic attenuation, propagation distance and melt temperature. Meanwhile, in virtue of self-developed attenuation-testing device, we collect ultrasonic attenuation of Al-7.3%Si alloy with different propagation distances during solidification and fit the function of attenuation distribution of the alloy. Verification between the experiment results and the theory model shows that they are in good agrerment with each other.更多还原