The shearing/cooling roll (SCR) process was adopted to prepare semi-solid A2017 alloy. The formation and evolution of non-dendritic microstructures in semi-solid A2017 alloy were studied. It is shown that the microstructures of semi-solid billets transform from coarse dendrites into fine equiaxed grains as the pouring temperature of molten alloy decreases o.r roll-shoe cavity height is reduced. From the inlet to the exit of roll-shoe cavity, microstructure of semi-solid slurry near the shoe surface is in the order of coarse dendrites, degenerated dendrites or equiaxed grains, but fine equiaxed grains are near the roll surface. Microstructural evolution of semi-solid slurry prepared by SCR process is that the molten alloy nucleates and grows into dendrite firstly on the roll and shoe's surface. Under the shearing and stirring given by the rotating roll, the dendrites crush off and disperse into the melt. Under the shearing and stirring on semi-solid slurry with high volume fraction of solid, the dendrite arms fracture and form equiaxed grain microstructures. 相似文献
Semi-solid metal processing (SSM) is a modern metal forming technology offering net-shape metal components of complex geometry in a one-step operation. The process relies on the thixotropic behaviour of metals with non-dendritic microstructures in a wide semi-solid temperature range. The beneficial effects are currently exploited in aluminium and magnesium alloys. This alternative manufacturing route to casting and forging has generated high expectations regarding high melting alloys, although no conclusive results have been achieved so far. Current studies focus on a deep understanding of the fundamental basics of alloys in the semi-solid range, e.g. rheology, microstructural evolution, and the consequences of forming, e.g. relations between process parameters, microstructure, and properties. This critical assessment aims at defining the important needs for further development of SSM, and assessing current knowledge. 相似文献
Non-dendritic microstructures are generally obtained in metals after semi-solid deformation (deformation during solidification); however, dendritic growth is preferred without deformation. The fragmentation of dendrites is recognized as an essential contributing factor to non-dendritic microstructures. However, the underlying mechanism of fragmentation needs to be clarified in depth. It is infamously hard for researchers to carry out a direct observation of this process. Moreover, a comprehensive numerical survey of this process is not trivial. The present research reported a new method to model dendritic growth during semi-solid deformation. The motion and deformation of the solid coupled with liquid flow in the melt were treated as the two-phase flow because plastic materials could be formulated as non-Newtonian fluids. The vector-valued phase-field formulation and the self-constructed Navier–Stokes solver made it possible to simulate the growth, motion, deformation, fragmentation and agglomeration of two dendrites coupled with liquid flow in the melt. Computational results suggest that fragmentation can occur when the grain boundary is wet and penetrated by the melt, giving new supporting evidence to a previously proposed mechanism for the fragmentation of dendrites.
The compressive behaviour of Sn-Pb alloys is studied with materials of conventionally dendritic structure and non-dendritic (rheocast) structure obtained through mechanical stirring during solidification. The alloys are found to deform similarly in the fully solid state but their behaviour becomes very different in the semi-solid range depending on the solidification mode. The holding time in the semi-solid state before compression also affects the mechanical properties: the influence of these two parameters is discussed in terms of the initial structure of the alloy and its evolution. Advantages of using semi-solid materials in metal forming processes are also presented. 相似文献
Integrated effects of undercooling and solute drag on recrystallisation mechanism of rapid solidification microstructure were investigated in highly undercooled Ni-Cu alloys. The equiaxed grained microstructures were prepared by fluxing method and subsequent quenching. Annealing the microstructures of the as-quenched alloys, substantial recrystallisation growth was observed. Applying solute trapping model of undercooled melt and solute drag model of solid-state transformation, it can be inferred that the microstructural evolution was dominated by nucleation and growth of recrystallisation process which is strongly dependent on the initial undercooling of the alloy melt and the solute drag force of the solid-state transition process. 相似文献