Study On Manufacturing Technology And Application Of Semi-solid Slurry Of Al-Si Alloys

In recent years,issues of energy consuming and environment pollution are becoming more and more concerned.There is enormous space for application of aluminum alloys to reduce weight in automobile industry.Semi-sloid die casting with merits of stable laminar die filling,little solidification shrinkage,integral components and heat-treatable,is regarded as a potential pathway to boost application of aluminum alloys in lighting automobiles.However,this technique dose not earn extensively popularization and application up to now.The primary cause is that present slurry preparation technology can't fulfill the requirement of high quality and high efficiency of production.This article takes common aluminum alloys A357 and 319s as objects of study,and adopts the thermal equilibrium method to preparae slurry.This is achieved based on technology of controlling solidification processes.Systemic study of the manufacture of aluminum semi-solid slurry,formation and evolution mechanism of spherical microstructure,filling behavior of slurry,microstructure and mechanical properties of semi-solid diecasting components have been studied.The results obtained in this work can be considered to be a guide line in controlling solidification among aspects of semi-solid slurry manufacture and engineering application of semi-solid rheo-diecasting.The main research contents and achievements are as follows:Cooling behavior of liquid metal during the manufacturing slurry was analysed by thermal equilibrium method.A theoretical model of heat transfer process balance method,has been deduced.Tthe results show that the slurry temperature field is function of the pouring temperature and the initial temperature of crucible for the given alloy and crucible material.The influence patterns of process parameters on microstructure of the A357 alloy were studied based on the thermal equilibrium method.The results are that:(1)Pouring temperature has a maximum impact on slurry microstructure.As decrease of pouring temperature,molten metal obtains increased degree of supercooling adjacent liquidus.Free crystals nucleis formed with inner flow of molten metal and are reserved,which makes the ultimate spherical microstructure finer and more uniformly distributed.The linear intercept method was applied to measure the grain size.When the pouring temperature changes from 690? to 630? the mean grain sizes decreases from 820?m to 248?m at edge,from 706?m to 223?m at medium between edge and center,and from 659?m to 186?m at center.(2)When pouring temperature is higher than 660?,the slurry in crucible has low apparent viscosity.Now,as the increase of rotating speed,convection of molten metal is strengthened,and microstructure is obviously refined.When pouring temperature is lower than 645?,the slurry has high apparent viscosity.Now,it's difficult for rotating movement to generate obvious convection effect,so increase of rotating speed has little influence on microstructure.(3)As the increase of rotating time,grains with small sizes dissolved and disappeared,while grains with big sizes grows in Ostwald ripen way.The growth rate of 22.47pm3/s was obtained by calculating using the LSW model.(4)Under the same condition,mass of slurry mainly affects the degree of supercooling of molten melt.The less is slurry mass,the more obvious the chilling effect during pouring period,and more grain numbers,and finer grain.Theoretical model depicting the nucleation and growth of grains during thermal equilibrium slurry forming process is established.Concepts of initial and secondary nucleation are proposed to describe the nucleation mechanism.Based on the concepts of transient nucleation and free movement of the nuclei,pouring process has the chilling effect that makes nucleation in the molten metal occur under supercooling,and the nuclei can be reserved.These numerous nuclei are the main reason for having obtained a fine and spherical microstructure.Following the above discussed initial nucleation process,the movement of the nucleus formed closed to the crucible are affected by conditions of the undercold molten metal.Generally speaking,homogeneus temperature and composition distribution in the molten is desirable to obtain a fine and homogeneously distributed grain structure,which can be achieved by slow cooling rate and rotating the crucible.The mechanism is that slowing cooling rate and rotating the crucible improve the stability of the interface between the nuclei and liquid metal surrounded and facilitate the uniform growth of the nuclei along all directions.Relationship between parameters in the thermal equilibrium process and slurryflow stability is established.At a given pouring temperature,slurry stability is mainly determined by solid fraction.With decrease of slurry temperature,i.e.increase of solid fraction,the slurry flow stability initially increases and then decreases.At a given temperature of slurry,flow stability is mainly determined by morphology of the primary particles.With decreasing of pouring temperature,where the morphology of grains changes from dendrite to globular and then particles clusters,stability initial increases and then decreases.The optimal parameters for 319s slurry to ensure stable flow of slurry during filling process are 630 ?-640? of pouring temperature and 35%-45%of solid fraction.By using these optimal parameters,a comples shape turbocharger impeller has been successfully manufactured.