Morphological stability of globular crystal during semi-solid processing

With the intensive development of semisolid metal processing technology, an important near-net-shape processing technology, the interaction between melt flow and solidification microstruc-ture becomes gradually one of the important fundamental research fields in materials science. The most important characteristic of semisolid processing is as follows: the solidification microstructure changes markedly under either mechanical stirring or electromagnetic stirring, from dendritic growth under traditional conditions to non-dendritic or globular growth. However, understanding and modeling of the nucleation and crystal growth during semisolid solidification are more difficult than in conventional casting processes due to complicated effects of strong convection. Hence, to date, the formation mechanism of this kind of globular microstructure has not yet been much studied. In the present work, morphological stability of globular crystal was experimentally studied using a succinonitrile-5%H₂O (molar fraction) transparent alloy under different undercoolings and stirring rates. Succinonitrile-5%H₂O transparent alloy was heated to 55°C (5.1°C above the liquidus temperature) and held for 30 min. The melt was then cooled to a temperature below the liquidus temperature at a cooling rate of 0.1°C/min and a series of stirring rates. In situ observation was performed using a stereomicroscope and JVC video camera. The results show that the incubation time for the formation of globular crystal decreases rapidly with the increase of stirring rate. When the stirring rate is low, the incubation time for the formation of globular crystal decreases obviously with the increase of undercooling. When the stirring rate is high, the effect of undercooling on the incubation time for the formation of globular crystal is weak. With the increase of stirring rate, the solid fraction of globular crystal increases at first, and then decreases. When the stirring rate increases to a certain value, the globular crystal will completely disappear. There is a critical undercooling for the transition of growth behaviour of globular crystals. Under the present experimental condition, when the undercooling is larger than the critical value, the size of globular crystal can increase to above 100μm without globular/dendritic transition. But when the undercooling is less than the critical value, globular crystal will grow to a definite size much smaller than 100μm. According to the growth behaviors of globular crystal, the semi-solid microstructure could be refined well under an optimized stirring rate and undercooling.