Al-si Alloy Semi-solid Isothermal Treatment

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 .