| It is the base and key of semi-solid forming that breaking dendritic crystals in casting structure to obtain refined near- spherical or globular non-dendritic microstructures. With the deep development of semi-solid processing forming, lots of new semi-solid billet fabrication technologies, such as Near-Liquidus Casting, new MIT, Rotating Tube and Strain Induced Melt Activating (SIMA), emerge in an endless stream and form a fine complexion that all flowers are in bloom. Among them, SIMA, the organic combination of plastic forming and solidification processing, is one of important approach of fabricating semi-solid billet under solid state condition, with high theoretical research value and application foreground.Al-Si alloy, with low melting point, extensive application fields, quietly fits for semi-solid metals processing forming, meanwhile, the fabrication and research on their billets are the most extensive and successful. However, it is such a high requirement for controlling temperature because of narrow interval between solid and liquid phases near eutectic point that it is very hard to fabricate semi-solid billets. At present, comparing with paying too much attention to hypoeutectic or hypereutectic alloys, little attention to such near eutectic point alloy as AlSi9Mg was paid.Based on SIMA, the excellent Al-Si alloy semi-solid billets were successfully fabricated by means of strain inducement and isothermal treatment for AlSi9Mg poured in the range of near-liquidus. Through using orthogonal test for three times and combining alloy phase structure analysis based on qualitative study on evolution regular of dendritic crystal structure in holding temperature at near-liquidus, the effects of combination action of near-liquidus casting, strain inducement and isothermal treatment on the formation and morphology of primaryα-Al phase non-dendritic structure were investigated quantitatively, and the optimal match relation between the processing parameters of each course and the microstructure parameters of semi-solid was obtained, also the test was given.The results show that : the compound fabrication technology, benefits to improve the formation and morphology of non-dendritic structure, decreases the requirement for strain-induced deformation volume and fully develops their own merits of near-liquidus casting and SIMA, so as to prepare more homogeneous and globular primaryα-Al phase non-dendritic structures in AlSi9Mg alloy, compared by the single near-liquidus casting or SIMA , which has Average equal-area-circle diameter of 35-40μm and shape factor of greater than 0.75; the optimal match of processing parameters and microstructure parameters obtained by orthogonal test is in good agreement with the result by tests., that is to say that it is the optimal match of processing parameters of compound fabrication technology when the pouring temperature is615°C, cold deformation volume is 30%, isothermal treatment temperature is 586°C and isothermal treatment time is25min, which can obtain the microstructure with Average equal-area-circle diameter of 38.93um and shape factor of greater than 0.794.Through proper controlling of the processing parameters of strain inducement and isothermal treatment during the course of drawing near-liquidus casting into SIMA, finally a new compound fabrication technology is obtained. Meanwhile, the definition and superiority of compound fabrication technology based on SIMA are disclosed, so as to realize the billet fabrication technology of semi-solid from "single" to "compound", which has important science meaning and popularization value for obtaining high quality and environmental protection billets continuously and for promoting their utility.
|
PDF Full Text Request