Study On Forming Technology And Mechanism Of Rheodiecasting Of Al-Si Alloy

Rheodiecasting technology of aluminium alloy has broad application prospects in the field of material forming.The preparation,transfer and forming of semi-solid slurry are the three key links of this technology.At present,the slurry prepared by the main stirring pulping method has better grain sphericity due to strong stirring,but poor macro-uniformity,coarse grain due to slow heat dissipation.The slurry prepared by the main flow pulping method has good macroscopic homogeneity and finer grains due to fast heat dissipation,but poor micro-homogeneity,poor grain sphericity due to insufficient stirring.In order to obtain semi-solid slurry with fine grains,good roundness and uniform macro-and micro-structure,a semi-solid slurry preparation device of 'flow & stirring' was developed independently.The slurry is centrifugally driven by rotating turntable,and the slurry in motion is strongly stirred by stirring column fixed on the turntable.The viewpoint is put forward in this study that the crystallization process of Al-Si alloy melt without flow and stirring is a comprehensive process of growth of primary ?-Al phase columnar dendrites and dendrite disintegration caused by the interaction between adjacent columnar dendrites.The formation of spherulites is a result of the interference between the growth directions of adjacent dendrite arms during the crystallization process.This interference occurs directly in,rather than later,the process of dendrite formation.In this study,the solidification structure of A356 alloy solidified under the condition of rapid cooling at 1000?/s showed that when the ratio of adjacent parallel columnar dendrite spacing d to the width t of columnar dendrite trunk was different,the structure between columnar dendrite trunks had different morphology,and the structure at the end or intersection of adjacent non-parallel columnar dendrites also underwent certain morphological transformation,resulting in spherulites with regular distribution.This confirms the above viewpoint.As spherulites can also be produced under solidification conditions without flow and stirring,it can be seen that the formation of spherulites does not depend entirely on flow and stirring conditions.This study also holds that the two mechanisms of spherulite formation,dendritebreakup and continuous nucleation,can be established under different conditions,but not mutually exclusive.Different flow or stirring conditions have different effects on the formation of microstructures in the solidification process of Al-Si alloy.The higher the flow or stirring speed,the weaker the effect of dendrite breakup mechanism and the stronger the effect of continuous nucleation mechanisms.The solidification experiments of A356 alloy and ADC12 alloy semi-solid slurry were carried out on a self-developed 'flow & stirring' semi-solid slurry preparation device.By observing the water-quenching solidification structure of A356 alloy and ADC12 alloy after flow and stirring,it was found that under the conditions of flow and stirring,with the increase of the rotating speed of turntable,the heat dissipation effect of slurry was strengthened,the copious nucleation was promoted and the grain size was refined.With the increase of the rotating speed of turntable,the temperature field and material field inside the slurry underwent a transition from macro-uniformity to micro-uniformity.When the pouring temperature was 620?,the mold temperature was 300?,and the number of mixing columns was 4 groups,and the rotating speed of turntable was lower than 600 rpm,with the increase of it,the influence of dendrite breakage was greater,the disorder degree inside the slurry increased,and the roundness of the grains became worse.When the rotating speed of turntable was higher than 600 rpm,with the increase of it,the uniformity degree inside the slurry increased and the formation of dendrites was inhibited gradually.It tended to copious nucleation,and the grain roundness gradually improved.Under the conditions of flow and stirring,with the increase of the group number of stirrer rods,the sizes of grains decreased and the roundness improved gradually.These experimental phenomena coincide with the above viewpoint.The 'flow & stirring' semi-solid slurry preparation device was applied to the rheodiecasting experiment,and the comparison of liquid die casting process and rheodiecasting process was made on the same die.The semi-solid slurry preparation process parameters of A356 alloy were as follows: pouring temperature is 620 ?,mold temperature is 300 ?,rotating speed of turntable is 1400 rpm,group number of stirrer rods is 4;the semi-solid slurry preparation process parameters of ADC12 alloy were as follows: pouring temperature is 590?,mold temperature is 200?,rotating speed of turntable is 1400 rpm,group number of stirrer rods is 4.The processparameters of die casting were: specific pressure 68 MPa,injection rate 2m/s,die temperature 250?.The results of testing and analyzing showed that,compared with the liquid die casting process,the semi-solid die casting process significantly improved the microstructure uniformity,especially the mean diameter uniformity of grains.The optimization rate of semi-solid die casting process for the mean diameter uniformity of the as cast microstructure of A356 alloy,the T6 heat-treated microstructure of A356 alloy,the as cast microstructure of ADC12 alloy,the T6heat-treated microstructure of A356 alloy weres 54.7%,50.3%,42.2% and 36.7%,respectively.Compared with the liquid die casting process,the semi-solid die casting process significantly improved the tensile strength,elongation after fracture and hardness of castings,especially the elongation after fracture.The optimization rate of elongation after fracture of the as cast A356 alloy die casting sample,the T6heat-treated A356 alloy die casting sample,the as cast ADC12 alloy die casting sample and the T6 heat-treated ADC12 alloy die casting sample by semi-solid die casting process were 56.4%,37.5%,31.3% and 50.0% respectively.