Study On Semisolid Feedstock Of A356 By Controlled Pouring Methods

Effects of pouring temperatures, grain refinement and process parameters such as mould preheat temperatures on grain size and morphology of A356 alloy were investigated in this study. Effects of wall mechanisms and constitutional undercooling on nucleation were studied by inserting a stainless steel gauze in the mould. The fraction solid was determined based on the cooling curve/heating curve measured during solidification/remelting. Microstructure evolution during partial remelting and isothermal holding was investigated. Results indicate that pouring temperature has a significant effect both on grain size and morphology of A356. The microstructure of samples poured at low temperatures consist of part globular grain and fine equiaxed grain structure. For the samples poured at high temperatures, equiaxed grains become big and columnar grains are formed on or near mould wall. The columnar range is increased with the increased temperatures. Pouring temperature has a significant effect on grain size, especial for the samples poured at temperature over 650℃. Grain size increases from 200μm to 980μm when pouring temperature varies from 650℃to 725℃. With the increasing of pouring temperature the grain structure outside the stainless steel gauze varies from fine equiaxed structure to coarse one and to complete columnar structure until the pouring temperature is 725 ℃. While grain structure inside the stainless steel gauze are all equiaxed grain structure. Only the sample poured at 625℃has fine structure, then the grain size increases dramatically with the increased pouring temperature. There is a critical mould preheat temperature existing for the effect of preheat temperature on grain size. A sudden increase in grain size upon reaching a certain mould preheat temperature was found in both samples, occurring at 600℃in the 625℃poured samples and at 550℃in the 650℃poured samples. Typical heating curve and its derivative curve for A356 alloy were measured and the relationship between fraction solid and temperature was calculated. A semisolid window at 578℃～583℃exists for partial remelting of A356 alloy where the fraction solid remains constant at about 42%. The original microstructure has a significant effect on the microstructure evolution during partial remelting and isothermal holding. Very dendritic structures evolve to a more rounded but very irregular morphology even after a long holding time. Equiaxed dendritic materials with a small grain size are more likely to evolve to a globular structure.