Solidification Behavior And Microstructure Control Of The A356 Aluminum Alloy During Semisolid Rheo-forming Process

The present research aimed at the microstructure control during the solidification process of the aluminum alloy,and the effect of the self-inoculation method?SIM?on the microstructure refinement of the A356 aluminum alloy was taken as the breakthrough point to study the role and refinement mechanism of SIM parameters in aluminum alloy grain refinement process.Then the A356 aluminum alloy semisolid slurry was rapidly prepared combining SIM with semisolid rheological forming method,and the spheroidization mechanism of the primary grains during the isothermal holding process of the slurry and the solidification behavior of the remaining liquid phase during the water-quenched process were studied.After that,the effects of different slurry characteristics on the microstructure,solidification behavior and mechanical properties of the rheo-diecastings were analyzed by combining slurry preparation of the A356 aluminum alloy with High Pressure Die Casting?HPDC?.At last,the solidification behavior of the A356 aluminum alloy semisolid slurry at the conditions of non-equilibrium solidification and near-equilibrium solidification were investigated by controlling different cooling conditions.The results systematically revealed the secondary solidification process of the A356 alumi num alloy semisolid slurry,and provided theoretical and experimental basis for obtaining superoir microstructure and precision parts by using semisolid forming technology for industrial practicing.The results of present research have guiding significance for optimizing the design of semisolid forming process and improving the mechanical properties of semisolid metal forming parts.The main results are as follows:The solidification microstructures of the A356 aluminum alloy were refin ed effectively when the self-inoculants were added into the melt,or the melt was flew through the fluid director.While the SIM,which combined above two processes,was more effective on the microstructure refinement.It could not only obtain small spherical or near-spherical primary?-Al,but also have some refinement effects on the eutectic structure,contribute to the microstructure transformation of the eutectic structure from lamellar to blocky.In addition,the fine non-dendritic microstructures were benefitical to the flow of the intergranular liquid phase,hence obtained the more uniform microstructure.During the pouring process by SIM,melt treatment temperature,self-inoculants amount and the angle of the fluid director were interrelated,and had significant effects on solidified microstructure of the A356aluminum alloy.In terms of the microstructure refinement of the A356 aluminum alloy,the optimum interactive parameters were:the melt treatment temperature was 680?,self-inoculants amount was 5%and the angle of the fluid director was 45^o.The volume fraction and the shape factor of the primary?-Al grains were gradually increasing with the decrease of the melt treatment temperature when the semisolid slurry of the A356 aluminum alloy was prepared by SIM.During the isothermal holding process,the growth rate of the primary?-Al grains conformed to the dynamic equation of D_t³-D₀³=Kt,and the coarsening rate K was increasing with the increase of solid fraction.When the slurry was isothermally held for a short time,primary?-Al grains became spherical.But when the holding time of slurry was too long?such as 10 min?,the sizes of primary particles increased while the merging phenomenon among primary particles occured,which lead to the emergency of“8”shape or“spindle-like”structures.Therefore,the growth of primary?-Al grains in isothermal holding process of the slurry was the results of merging growth and Ostwald ripening.The secondary solidification microstructures were obviously different when the slurry undergone different holding time.When the isothermal holding time was 3 min,the secondary?-Al grains were the smallest and roundest,and the size and amount of the attached secondary grains were the smallest.It was further explained that the morphology of the secondary?-Al grains was greatly influenced by the morphology of the primary?-Al grains,reflecting that the thermal field and concentration field of the slurry were more uniform when the isothermal holding time was 3min.The amount of secondary?-Al grains was gradually increasing with the increase of isothermal holding temperature,which made the eutectic reaction zones restricted.As a result,the final solidified eutectic structures were gradually compact.Compared with the dendrites in HPDC microstructure of the A356 aluminum alloy,the primary?-Al grains in rheo-diecasting?RDC?were fine and spherical.Meanwhile,the secondary?-Al grains formed after the slurry filling into the die were finer than the chilled grains of HPDC.And the morphology of the eutectic structures was obviously changed from blocky and lamellar by HPDC to coralline and fibrous by RDC.The isothermal holding time of the slurry had no obvious effect on microstructures of the remaining liquid phase in RDC,but the isothermal holding temperature affected the secondary solidification microstructure greatly.The solid fraction of the primary?-Al grains was gradually decreasing with the increase of the isothermal holding temperature,causing the increase number of secondary?-Al grains in the unit area.As a result,the merging phenomenon of the secondary?-Al grains was intensified,which made the size of the secondary?-Al grains increased gradually.Moreover,the eutectic structure was also coarsening with the increase of isothermal holding temperature.In RDC process,both primary?-Al grains and secondary?-Al grains grew from stable growth to unstable growth,some small"cell"or"dentate"bulges were formed around primary?-Al grains,and secondary?-Al grains became irregular from the spherical shape.It was measured that the maximum size of secondary?-Al grains in the stable growth was 6.5?m.The microstructures in different positions of the thin-walled plate were also different,the average grain sizes and shape factors of both primary?-Al grains and secondary?-Al grains were decreasing with the increase of filling distance as well as the average eutectic spacing.Compared with HPDC,the mechanical properties of the A356 aluminum alloy were significantly improved by RDC.The ultimate tensile strength was increased from244.16MPa to 274.11MPa,and the elongation was increased from 5.26%to 7.47%.Moreover,the ultimate tensile strength and elongation of RDC were increasing first and then decreasing with the increase of slurry holding time,and reaching the maximum when the isothermal holding time of the slurry was 3min.The difference properties resulted from solid fraction of the slurry,the microstructure of diecastings and internal defects.In the condition of non-equilibrium solidification,the semisolid rheological forming process was mainly influenced by different cooling rates provided by different moulds.The primary?-Al grains were gradually coarsening with the decrease of the cooling rate,and the shape factor was gradually increasing due to the merging phenomenon of primary?-Al grains.Meanwhile,the size of secondary?-Al grains and eutectic spacing were increasing,while the shape factor and the am ount of secondary?-Al grains in the unit area were decreasing.The relationship between secondary?-Al grain size?D and cooling rate G_v conformed to the equation of?=73.812^-0.35 approximately.In the condition of near equilibrium solidification,nucleation could not occur in the remaining liquid phase due to the low cooling rate,therefore,the solidification of the semisolid slurry mainly depended on the stable growth of primary?-Al grains and eutectic reaction.During the process of the slurry cooling to room temperature in the furnace,Mg element was completely migrated into the primary?-Al grains and combined with the over saturated Si element in the grains to precipitate fine needle like microstructure.Compared with the solidification microstructures of remaining liquid phase with different solidification process?such as water quenching,RDC,permanent mould casting,sand moud casting and cooling in the furnace?,it was observed that the isothermal holding time and temperature had significant effect on the secondary solidification microstructures when the cooling rate was in certain range.Secondary solidification microstructures of the slurry were finer than primary solidification microstructures,hence decreased the average grain size of the castings,thus improve the mechanical properties of the castings,theorically.Additionally,the internal defects of the castings could be effectively reduced by controling the shape of secondary?-Al grains.Therefore,in addition to increasing the cooling rate of the mould,the control of the overall microstructure of the semisolid forming parts could be realized by controlling the isothermal holding parameters of the slurry.Then the solidification microstructure of the alloy could be optimized.Finally,the comprehensive mechanical properties of semisolid forming parts could be improved.