【作者】 李东成；

【导师】 于思荣；

【作者基本信息】 吉林大学 ， 材料加工工程， 2009， 博士

【摘要】 金属基复合材料具有高的比强度、比模量、耐高温、耐磨等特殊性能,早已成为当今世界范围内材料科学的一个重要主攻方向。成型方式是限制复合材料发展的一个关键难点。半固态成型中的应变诱发熔化激活(Strain Induced Melt Activation)法,简称SIMA法,是具有发展前景的方法。目前未见关于SIMA法半固态加工颗粒增强金属基复合材料部件的报导。本文针对铝合金及其复合材料的SIMA法半固态浆料制备方法展开研究,探讨SIMA半固态组织演变机理。为充分掌握基体材料的特性,基体材料采用了商用Al2024棒材和Al-3%Si两种材料,复合材料材料采用了SiC_p/Al-3%Si。本文提出了SIMA半固态浆料的球状晶粒的形成机理、半固态保温的晶粒合并物理模型、半固态保温晶内液相的形成机理、复合材料半固态保温过程增强颗粒对组织形成的影响。同时综合考虑了多向应力状态对组织形成的影响。采用有限元方法模拟了材料内部的热残余应力,模拟时综合考虑了颗粒的形状、大小、体积百分率及气孔的大小、分布对热残余应力的影响。更多还原

【Abstract】 With the development of science and technology, especially the rapid development of the aviation and spaceflight and the appearance of new industries using atomic energy and developing ocean cosmically, more requirements for the structure materials are put forward. Traditional metals and alloys can not meet these demands. Compared with traditional metals and alloys, metal matrix composites (MMCs) are attached importance by material researchers in the world because of their high specific strength, high specific modulus, high temperature endurance, and wear resistance. This type of materials have been ranked as the focus of the new material development in 21st century by The United States, Japan and other developed countries. MMCs have become an important researching direction of the materials science in today’s world.The difficulty restricting MMCs development is their forming method. Semi-solid forming (SSF) is a new method of the material forming. During SSF, the billets are heated to a temperature between solidus and liquidus, i.e., semisolid state, and then are formed by molds. During SSF, the fluidity of materials is good as casting, and the microstructure of products is compact as forging. The method of the strain induced melt activation (SIMA) is a promising method in SSF. The billets made from casting and rolling can be molded after being plasticly deformed and heated to the semisolid temperature. During forming with SIMA, the stirring process is not needed, and a forming process with little or without waste can be achived. Now, SIMA is widely used for alloys with low and high melting point. But, the report aoubt SIMA application on MMCs is not found.The properties of the product made by SSF are affected direct by the preparation of the semi-solid slurry. In this thesis, the research on the preparation of the semi-solid slurry of alluminium alloy and its particles reinforced composites by SIMA was carried out. The evolution mechanism of the semi-solid structure was studied in order to provide the theoretical basis for the SIMA semi-solid forming of particles reinforced aluminium alloy composites. This is significant to extend the application of MMCs reinforced by paticles.The researches in this dissertation were divided into the following aspects:Firstly, the researches for the SIMA semisolid forming of aluminium alloys were carried out. In order to fully grasp the characteristics of the matrix material, commercial A12024 bar and Al-3%Si alloy were used as the matrix materials. The results are as follows:1. The SIMA semisolid microstructure after heating deformed plastically A12024 alloy is globular. When the plastic strain is from 0.1 to 0.8, the grain size decreases from 76μm to 52μm, i.e., the grain size decreases with inceasing level of the plastic deformation.2. For as cast Al-3%Si alloy holding at 630°C, the microstructure changes from big dendritic crystal to small grain, and finally to sphere.3. For single direction compressed Al-3%Si alloy, with increasing amount of the deformation, the non-directional as cast dendritic structures are elongated, the degree of fracture of dendritic grains increases, the orientation of the microstructure increases gradually, the defects between crystals increase, and the deformation energy storage increases.4. When Al-3%Si alloy with different compressed deformation is held at 630°C, with the increase in holding time, the diameter of grains inceases. The change of the grain size is minimum when the holding time is between 30 min and 60 min. The roundness of grains inceases with holding time. After the temperature is held for 60 min, the roundness does not change on the whole. When the ratio of deformation is 50%, the maximum grain size increases with the holding time. And when the ratio of deformation is 20%, the maximum grain size changes very little with the holding time. When the level of deformation is low, the degree of the grain fracture is small, and there are big grains not breaked up.5. It is similar to A12024, when Al-3%Si alloy with different compressed deformation is held at 630°C, the grain size decreases with increasing level of deformation. When the ratio of the deformation is less than 20%, the grain size decreases sharply with increasing level of deformation. When the ratio of the deformation is 50% or 70%, the grain sizes decrese gently. With the increase of the level of the deformation, the roundness increases. The more the amount of the deformation is, the better the roundness is, and the easier the globular structure is obtained. The evolution process of the microstructure is: the polygonal grains with different shape and size→the even grains→the globular grains→the coarse grains.Secondly, the evolution mechanism of the SIMA semisolid microstruture was studied from the point of view of thermodynamics and dynamics on the basis of the study on the matrix SIMA semisolid microstructure. The results are as follows:1. The evolution mechanism of the SIMA semi-solid globular grains was presented for the first time. The main idea is: the slippage inside the dendritic grain→the grain slide and rotation→the melting of the grain boundary→the melting of the sharp angle of grains. On the basis of the study on the SIMA semisolid microstructure by single- and multi-directional compression, the law of the effect of the hydrostatic stress on the SIMA semisolid grain size was obtained. The hydrostatic stress restrains the slippage inside a grain. The bigger the hydrostatic stress is, the more the restraining effect is, and the bigger the grain size by SIMA is.2. A physical model of the coalition of two grains is presented for the first time. The crystal boundary between the contacted grains with small orientation difference dispears during being held at a semi-solid temperature. The coalition of grains is a spontaneous process. The mechanisms of the grain coalition during being held at a semi-solid temperature include two aspects: one is that two grains or one of two grains turn under force; the other is the movement of atoms.3. The static shapes of liquid in a grain are presented for the first time. During being held at a semi-solid temperature, the static shape of liquid in a grain is ellipsoid or sphere.4. The mechanism of the spheroidization and growth of solid in semi-solid state is presented for the first time. For a grain with different positive curvature, the position with big radius will grow, and the radius will decrease; the position with small radius will melt, and the radius will increase; the evolution process of the grain is to dig the high position and to fill the low position. For a grain with positive and negative curvature at the same time, the place with positive curvature will melt, and the place with negative curvature will grow; the grain evolution is to dig the high place and to fill the vacancy. When a globular grain is near a grain with small curvature radius, the grain sizes will be polarized.Thirdly, the researches on SIMA semisolid forming of SiCp/Al-3%Si composites were carried out. On the basis of the investigation on matrix SIMA semisolid forming, the effets of the reinforcement in the composites were taken into account. The results are as follows:1. Through the study on the SIMA semisolid microstrcture of SiCp/Al-3%Si composites, the SIMA semisolid microstructure feature of the composites was discovered. For SiCp/Al-3%Si composites held at semisolid states, the reinforcement SiC particles will be in the liquid. The solid growth is restricted by reinforcement or other solids.2. The results of the composites with different level of compression are similar to the matrix alloy. With the increase of the holding time at semi-solid temperature, the grain size of the matrix increases, the roundness increases, and the fractal dimension of the grain decreases.3. For the same holding time, with increasing level of deformation, the grain size increases, the roundness increases, and the fractal dimension decreases.4. For the same volume fraction, the bigger the reinforcement size is, the more irregular the solid grains are, the bigger the fractal dimensions are, the smaller the roundness is, and the bigger the grain size is.5. For the same reinforcement size, the bigger the volume fraction of reinforcement is, the more uniform the solid grains are, the bigger the grain size is, the bigger roundness is, and the smaller the fractal dimentions are.Fouthly, the thermal stress of the composites was analysed by three dimensional elastic-plastic finite element method (FEM). The effects of the pore and reinforcement were taken into account to analyse the resisual stress for the first time. The resuls are as follows:1. For different size of the globlar reinforcements, the matrix of the composites yields in different degree. The place of the maximum stress of matrix is at the surface contacting with reinforcement. With the increase of the distance to reinforcement, the stress of matrix decreases at different degree. The smaller the grain size is, the more rapidly the stress decreases. For the same size of the reinforcement, the bigger the volume fraction of the reinforcement is, the bigger the residual stress is, and the bigger the variation scope of the stress between two particles is. For the same reinforcement size, the bigger the angle between two sides of the polygon is, the more severe the stress concentration is, and the level of the stress increases in different degree.2. For the models of different pore diameter and different distance between pore and reinforcement, the matrix yields at the surface contacting with reinforcement and pore. The stress state is compressive on the surface contacting with reinforcement, and is tensile on the surface contacting with pore. The elasitic area appears between reinforcement and pore. The matrix yields at the point of pore surface at which the distance to reinforcement is minimum. With the increase of the distance to reinforcement, the stress decreases to elasitic state. With the increase of the pore diameter, the minimum stress of Mises decreases.更多还原