【作者】 梁红玉；

【导师】 毛协民； 任安锋；

【作者基本信息】 上海大学 ， 材料学， 2006， 博士

【摘要】 为了大幅度提高汽车、赛车等的使用性能，对其核心部件----发动机功率密度及自重的要求越来越高。我国几代汽车技术的发展，伴随着发动机功率成倍增长的同时，发动机的体积和重量也越来越大，由此产生了严重的结构超重问题，致使发动机的性能受到影响，成为制约汽车工业发展的关键因素之一。采用新的高性能材料制造发动机零部件，减轻发动机运动部件质量和传动阻力等是提高发动机性能的有效途径。铝硅合金作为耐磨材料，在机械工业中得到了广泛应用。特别是含硅量在18％-26wt％的过共晶铝硅合金具有密度小、热膨胀系数低、导热性好、足够的高温强度和耐磨性等特点，是理想的发动机轻质耐磨材料。但是，采用普通铸造工艺生产过共晶铝硅合金时，粗大的硅相严重割裂了基体的连续性，使合金的强度、韧性显著下降。当硅量超过14wt％时，即使变质处理也很难消除硅相的不利影响。随着现代工业的发展，尤其是汽车、航空、航天工业的特殊需要，要求铝硅合金进一步提高耐磨性、耐热性，并大幅度降低线收缩率及密度。在合金成分上表现为高硅含量及合金化。显然，常规的合金材料及铸造工艺远远不能满足要求。近几年研制开发的快速凝固新材料为航空、航天工业用高性能材料开辟了一条新路。本文应用快速凝固粉末冶金法（RS／PM）制备了高耐磨超低膨胀系数高硅铝合金材料，对其进行了系统的分析研究，取得了规律性认识，并应用该材料试制了大功率发动机缸套，得到了具有实用价值的研究成果。论文自行设计制造了雾化制粉实验装置，以此为基础研究各种工艺参数对粉末材料特性的影响规律，雾化正交实验及验证实验结果表明：各种雾化工艺参数对合金雾化效果有很大影响：喷嘴间隙δ是影响雾化效果的显著因子，气体流量Q是影响雾化效果的第二显著因子，金属液过热度为非显著因子；实验得到最佳喷嘴间隙取δ=0.55mm，较佳的气体流量Q为34 m³／h，陶瓷管内径值为6.4 mm，最佳喷嘴角度为25°，金属液过热度100℃。论文以群体动力学模型为基础，在充分考虑合金的热物性参数，过饱和度以及第二相形核率变化的条件下，提出了一个描述雾化过共晶Al-Si合金液滴快速凝固过程中组织演变的数学模型，并将其与液滴的运动方程与传热方程相耦合，对雾化合金液滴的冷却凝固过程进行模拟分析，并通过实验进行了验证。结果表明：随着合金液滴尺寸的减小，平均冷却速度增加，当熔滴尺寸足够小时，熔滴温度的变化趋势及合金液滴的组织将发生突变，过共晶Al-Si合金液滴发生亚稳共晶组织转变的临界尺寸为：d_lim=[6．Nu．K_g（T_x-T_g）／ρ．L．（df／dt）]^1／2；增加雾化气体的初速度，降低熔体过热度，会使初生相的析出受到抑制，有利于得到亚稳组织。论文在快速枝晶及共晶生长理论模型基础上，充分考虑了过冷熔体中等轴凝固的生长特性，借用最高界面生长温度判据，建立了共晶合金等轴凝固界面响应函数模型：IRF（ν）=max(T_pri（ν），T_eut（ν）)；通过该模型分析了Al-Si合金系快速等轴凝固过程中的组织竞争生长，绘制了非平衡组织选择图，研究表明：在快速等轴凝固过程中，Al-Si系合金存在着α相、Si相及（α+Si）共晶组织三个生长区，当Si的含量介于12％至25％之间时，将会出现α相及（α+Si）共晶两种亚稳组织，当Si含量大于25％或者小于12％时，只可能形成亚稳的（α+Si）共晶组织；计算结果与实验结果基本吻合，说明所建立的共晶合金等轴凝固界面响应函数模型可以较好地预测Al-Si系合金快速等轴凝固过程中的非平衡组织选择，对其它共晶系合金同样具有一定的指导意义。论文以自行设计制造的小型冷压及热挤压模具为基础进行小试样挤压过程物理模拟，研究粉末材料致密化和金属流变规律及热挤压工艺参数对材料微观组织的影响。结果表明：粉末冷压坯的挤压可以分为填充致密、稳定挤压、紊流挤压等三个阶段；粉末颗粒尺寸越大，粉末的冷压制性能越好，越容易获得表面质量高且形状完整的冷压坯料，同时大尺寸粉末颗粒的热挤压棒材质量好，表面光洁无裂纹；但是，粉末颗粒尺寸越小，所获得的热挤压棒材密度越高，材料内部微观组织细小且均匀分布，且挤压棒材的力学性能越好。综合考虑粉末的利用率及粉末颗粒内部的组织形态力学性能等因素，使用混合粉末比使用单级粉末挤压具有一定的优势，实验证明，使用颗粒半径小于147μm的混合粉末，选择适当的挤压温度、挤压比、挤压模芯角度等参数可以得到高质量的挤压棒材。温度对热挤压制品微观组织影响较大，合金中Si含量越高，初晶硅相随挤压温度升高而长大的趋势越明显。Al-Si合金粉末材料中细小弥散分布的Si相在挤压过程中发生聚集和长大的规律符合LSW粗化动力学理论；挤压变形系数和制品横截面形状主要通过改变粉末变形程度和粉末体结合状态来影响挤压制品的微观组织及力学性能。挤压比为16时，Al-Si合金粉末之间的孔隙已经基本消除，粉末体结合已接近良好状态，较大的挤压比是获得理想微观组织及性能的必要条件。在现有实验条件下，Al-30％Si合金最佳热挤压工艺参数为：挤压温度520℃，挤压比16，模芯角90°。总之，使用快速凝固制粉＋热挤压工艺制备的Al-Si合金与未经过任何变质处理自由凝固条件下制备的合金相比，力学性能得到了显著提高；随着合金中Si含量的提高，挤压制品的抗拉强度、硬度、耐磨性相应提高，延伸率略有下降；细化初晶硅相使其细小均匀分布，改善初晶硅相的形态使其与基体的结合力进一步提高，将有利于提高材料的力学性能、摩擦磨损性能。应用高硅铝合金材料代替传统38CrMoAl材料生产大功率发动机缸套，并采用缸套环向加筋且筋上有径向约束的方法可以使缸套的各种性能满足实际工作要求。其室温抗拉强度大于400 N／mm²，高温250℃抗拉强度大于300 N／mm²，520℃挤压制品的平均磨损量为2mg，平均摩擦系数为0.3，室温热导率及热膨胀系数分别达到130 W／m／k及1.5×10^-5K^-1，其力学性能优于有报道的国外技术水平，综合性能优于铸铁及钢制缸套材料。更多还原

【Abstract】 In order to enhance the operational properties of the automobiles, karts greatly, the power density should be improved and the weight of the engine should be lightened. As the development of the automobiles, the power increase with the increasing of the weight and volume of the engine more and more, which result in overweight and severe weakening operational properties of the automobiles. The efficient way of improving properties of the engine is to manufacture the parts of the engine with new and high-properties material, which can lighten the mass of the moving parts and reduce the power transmission resistance of the engine.As a wear-resistant material, the A1-Si alloy was used in the mechanical industry extensively. Being the low density and coefficient of thermal expansion, the high coefficient of heat conductivity , high-temperature strength and well wear-resisting property, the hypereutectic A1-Si alloys, especially the alloys with the percent of Si is 18%-26%, were the ideal light and wear-resisting materials of the engine. But with the ordinary casting process, the properties of the alloys were weakened greatly by the coarsened primary Si phases which make the matrix of the hypereutectic Al-Si alloy be rent severely. When the percent of Si exceed 14%, the properties of the alloys could not be improved even through the modification.As the development of modem industry, especially the needs of the motor and aerospace industry ask the A1-Si alloys be improved in the properties of wear resistance, heat resistance, linear expansion coefficient and density, then the composition of the alloys should be high-Si content and high-Alloying. Evidently, the ordinary alloy and casting process could not meet the needs. The rapid solidification materials developed in the recent years offer a new way to meet the needs for the development of the motor and aerospace industry.In this paper, the high-silicon aluminum alloys which have the properties of well wear resistance and low-thermal expansion coefficient were prepared by RS/PM. Through systematic research and investigation the regularity knowledge on the alloys was acquired. And the alloy was successfully used to manufacture the cylinder sleeve of the high power engine.The gas-atomizer arrangement was designed and manufactured, the influence of the parameters on the character and microstructures of the powder particles was studied with orthogonal experiments. It was discovered that the slot width of the gas nozzle was the key factor to influence quality of the powder particles, the gas flow rate was the second and the degree of the superheat was the third factor. The best combination of the atomization parameters is that——the diameter of the ceramic nozzle was 6.4mm, the angle of gas nozzle was 25°, the gas flow rate was about 34m³/h and the slot width of the gas nozzle was 0.55mm, the superheat was 100℃. Based on the population dynamics model considering the continuous varieties of thermo-physical parameters, supersaturation, nucleation rate, a model which was compiled with both the droplets heat transfer controlling equation and the droplets motion controlling equation has been developed to describe the microstructure evolution of hypereutectic A1-Si alloy during rapid solidification. The results of the solution to the model for the A390 alloy show that with decreasing droplet size, the average cooling rate increases rapidly. And when size of a droplet arrives at enough small, its temperature and microstructure varieties break out. The results also show that the primary Si phases of the powder particles in the microstructure have not been extinct until its size is less than a critical size, which is d_lim=[6.Nu.k_g（T_x-T_g）/ρ.L.（df）／（dt）]^1／2. With increasing the initial gas velocity and decreasing the superheat of the melt droplet, the nucleation and growth of primary phases are suppressed and the microstructure becomes into the metastable state.Meanwhile the results of atomization experiments of hypereutectic Al-Si alloys also show a good agreement of the experiments with the theoretical calculations. That is to say, the model can be used to predict satisfactorily microstructures evolution of the hypereutectic A1-Si alloys and the model would be useful for the microstructure predictions to other eutectic alloys.On the basis of the model for rapid growth of dendrite and eutectic crystals and the criterion of the highest temperature at the interface growth, a interface response function IRF（v）=max(T_pn.（v）,T_eut（v）) for the growth of a crystal of a eutectic alloys during equiaxed rapid solidification was established. With the IRF, the competitive growth between the primary and eutectic phases of Al-Si alloys was investigated. Then a microstructure-selection map of Al-Si during non-equilibrium solidification was drawn. The map shows that there are three growth zone （primaryα-Al、primary Si phase and （α+Si） eutectic zone） in the Al-Si alloy. When the percentage of the Si phase is 12%-25%, the metastable microstructures wereα-Al plus （α+Si）, and when the percentage of the Si phase below 12% or exceed 25%, the metastable microstructure was the eutectic of （α+Si） only. The calculation results was shown a good agreement with that of atomization experiments. The IRF model can be used to predict satisfactorily the microstructure selection and the evolution of the microstructures of the Al-Si alloy system during non-equilibrium solidification. Meanwhile the IRF model would be of benefit for the microstructure predictions during the non-equilibrium solidification of other eutectic alloys.With moulds of cold compacting and extrusion the extrusion processes of the specimen were simulated. And the regular principles of the densification and rheidity of the powders during the extrusion process were studied. The results show that the extrusion processes of the ingot include three stages, which are the packing densification stage, the stable extrusion stage and the turbulent extrusion stage. The bigger the powder particles, the better the performance of cold compacting. At the meantime, the quality of the alloy bar extruded from bigger powder particles was good enough without cracking. From the point of density, the smaller the powder particles, the higher the density of the alloy bar. On the other hand, the microstructures and mechanical properties of the small powder particles were better than that of the bigger particles.For practice application, mixed powders of various sizes （＜147μm） were better than the powders of single size. In order to improve the quality of alloy bar, some methods should be used in hot extrusion, such as coating with aluminum film, lubricating, increasing temperature of extrusion, increasing the extrusion area ratio （＞16） and so on.The experiments show that influence of the extrusion temperatures on microstructures of alloy bar were more greater and the growth tendency of primary Si phases increased with the increasing percent of Si phases. The results of measurements proved that the growth law of the dispersed primary Si phases conformed to the LSW dynamics theory. The extrusion area ratio and the shape of transverse section can change the microstructures and mechanical properties of alloy bar by influencing deforming and combination extent of powder particles. When the extrusion area ratio was 16, most of the porosities among the powder particles were meanly disappeared and the combination of the powders was firm. So the greater extrusion area ratio should be necessary for refine microstructures. For the mixed powders of A1-30%Si alloy, the best extrusion temperature , the extrusion area ratio and the angle of mould were 520℃、16 and 90°respectively.To sum up, compared with the alloy prepared by normal casting without any modification the mechanical properties of the alloy prepared by RS/PM increased greatly. The tensile strength, hardness, and the wear resistance of the alloy increased and the extensibility of the alloy decreased with the increasing percentage of Si phases for the alloy prepared by RS/PM. It is by reason of refining and improving morphology of primary Si phases to increase the mechanical properties of the alloy and the lower extensibility of the Si phases.The high-silicon aluminum alloy prepared by RS/PM was used to manufacture cylinder sleeve of high power engine. The test measurements show the various properties of the cylinder sleeve including the circumferential reinforcement and the radial restraint could meet the needs of the actual work of the automobile. The tensile strength was over 400N/mm2, high temperature tensile strength was over 300N/mm², the mass loss of alloys bar extrusion at 520℃was only 2mg. Its coefficient of friction was 0.3, the thermal conductivity and coefficient of thermal expansion at room temperature were 130 W/m/k and 1.5×10^-5K^-1 respectively. As you seen, the mechanical properties of the alloy were better than that reported by foreign countries and the synthesis properties of the high silicon aluminum alloy prepared by RS/PM were better than that of cast iron and steel materials.更多还原