Al-Si合金半固态等温处理

【作者】 刘环；

【导师】 周荣；

【作者基本信息】 昆明理工大学 ， 材料加工工程， 2003， 硕士

【摘要】 半固态金属加工技术是近年来全世界范围内竟相开展研究与开发的一项金属加工技术，自从20世纪70年代美国麻省理工学院学者Flemings发现半固态金属的触变性能后，半固态技术的研究和应用得到了迅速的发展。半固态金属加工技术，简称SSM，就是金属在凝固过程中，进行剧烈搅拌，或控制固-液态温度区间，得到一种液态金属母液中均匀地悬浮着一定固相组分的固液混合浆料。半固态金属具有良好的流动性，易于通过普通加工方法制成产品，并且具有加工温度低，产品质量高等优点。 半固态金属加工的工艺路线主要有两条：一条是将半固态金属浆料在保持其半固态温度的条件下直接进行半固态加工，这被称作流变铸造。另一条是触变成形，它首先要制作半固态坯锭，然后根据产品尺寸下料，再重新加热到半固态温度，进行成形加工。由于直接获得的半固态金属浆料的保存和输送很不方便，因而流变成形技术发展缓慢。生产中主要采用触变成形工艺。 在触变成形工艺中，一个关键问题就是如何制备优质的半固态合金坯锭。目前，制备半固态合金坯锭的方法很多，然而，这些方法中，有的需要大功率的设备，现有设备常常不能满足要求；有的生产率低；有的设备使用寿命短，易造成熔体污染；有的所需条件难以控制。这些方法有其优点，但共同的一个缺点是：工艺复杂，设备投资大。 寻找一个简便、可靠又经济的制作半固态坯锭的工艺是本实验的目的。 实验采用常规铸造得到的亚共晶和过共晶Al-Si合金铸态锭坯，直接进行半固态等温热处理，经过处理后，亚共晶铸锭获得了理想的半固态组织，而过共晶铸锭不但不能获得理想的半固态组织，还会造成晶粒粗化和严重的比重偏析。在实验中研究了变质与不变质两种熔炼工艺、等温温度、等温时间对半固态组织的影响，得到如下结论：(1)对于亚共晶Al-Si合金，经变质处理过的组织，其球化速度快。等温温度越高，获得球状组织的速度越快，球状晶粒也越粗大。等温时间越长，球状晶粒越大，也越圆整。在半固态等温热处理过程中，组织实现了由枝晶向球状的转化，而在这个转化过程中，组织由不平衡向平衡状态过渡，经历了传热、传质等传输过程，传质主要以扩散的形式进行的。(2)对于过共晶Al-Si合金，不宜采用采用半固态等温热处理的方法制备半固态坯锭。昆明理工大学硕士学位论文 采用半固态等温热处理工艺制作半固态坯锭，虽然工艺流程短、容易操作、过程稳定，是一种比较可行的工艺方法，但并不是任何一种成分的合金都适用。更多还原

【Abstract】 Semi-Solid Metal Process has actively been spread all around the world in recent years. Study and apply on semi-solid technology has been developed rapidly since 1970s the scientist Flemings found the thixotropic property of semi-solid metal in Massachusetts Institute of Technology (MIT) in U.S.A.. Semi-Solid Metal Processing, called SSM, is a kind of technology that produces mixture which comprises the solid component suspending in molten metal evenly by agitating or controling the solid-liquid temperature zone. Semi-Solid Metal shows good flowable behaviour so that it could be manufactured easily by general working methods, and it also has some advantages such as low working temperature, high quality etc.There are two ways of Semi-Solid Metal Processing: one way is Rheocasting. Keeping the semi-solid temperature of Semi-Solid Metal, then process it directly. The another way is Thixoforming. In this way, the semi-solid metal is processed to be blocks first, then in terms of the size of the produce ,the blocks are cut , next they are produced to be final shape. Compared with Thixoforming , Rheocasting developed slower . The reason for this is that it is difficult to preserve and convey the semi-solid mixture . Therefore Thixoforming becomes the main way in practice .A key in Thixoforming is how to get high qualitative semi-solid blocks. By now, there are many ways to make blocks. But, some of them need uprated equipment so that the equipment now used could not meet the requirement; some of them have low productivity; some of them have short life and the metal is easily to be polluted; some of them need lots of condition that difficult to be controled. In a word, even though they have many advantages, they have a common shortcoming : complex technology and high equipment investment.Looking for a good way which is simple, dependable and economical to produce semi-solid blocks is the experimental goal.In the experiment, the hypoeutectic and hypereutectic routine casting blocks is directly used to obtain non-dendretic structure by semi-solid isothermal heattreating . After treating , the ideal non-dendretic structure has been obtained in hypoeutectic blocks , but in hypereutectic blocks , it has not. Contrarily , the crystal grain is coarsening and severe specific gravity segregation brought out. In the meanwhile, some factors which affect the formation of non-dendretic structure are studied. Some results are as follows : (1) For hypoeutectic Al-Si alloy , the modified structure become non-dendretic fast. The higher the isothermal temperature is, the shorter the time to obtain non-dendretic structure is needed and the coarser the non-dendretic is. The longer the time keeping the isothermal temperature is, the larger and rounder the non-dendretic is. In the course of the semi-solid isothermal heat treatment, the micro structure fufil the transition. At the same time, the disequilibrium structure experience some procedure such as heat transmission, mass transfer and so on, to become balance. Diffusion is the main mode of mass transfer . (2) For hypereutectic Al-Si alloy , the semi-solid isothermal heat treatment is not suitable to prepare se-mi solid blocks .To make semi-solid blocks by semi-solid isothermal heat treatment, although it has some advantages such as short technological process ,easy-to-move , and the stable processing course .especially little equipment investment ,it is not suitable for every alloy .更多还原