Formation and Evolution of Non-dendritic Microstructures of Semisolid Alloys Prepared by Shearing/Cooling Roll Process

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.

Morphological evolution of non-dendritic microstructure during solidification under stirring

Morphological evolution of non-dendritic microstructure during the solidification of succinonitrile-5%water(SCN-5%Wat) transparent alloy under mechanical stirring was experimentally investigated.The incubation time for the formation of non-dendritic microstructures decreased rapidly and the microstructure was gradually refined with the increase of stirring rate and cooling rate.When the stirring rate and cooling rate were low,the incubation time for the formation of non-dendritic microstructures decreased obviously with the increase of the melt undercooling.When the stirring rate was high,the effect of the melt undercooling on the incubation time for the formation of non-dendritic microstructures was weak.The morphology of primary microstructures had an important influence on the subsequent morphological evolution of these microstructures during the isothermal holding of the melt without stirring.It was found that when there were dendrites in the undercooled melt,the dendrites would be dissolved and the final microstructure would be replaced by the re-nucleated globular crystals if the stirring time was long enough.

EFFECT OF PREDEFORMATION MANNER ON SEMI-SOLID STRUCTURE OF AZ61 MAGNESIUM ALLOY IN SIMA PROCESS

The effect of predeformation manner, predeformation ratio and isothermal heat-treat parameter on the non-dendrite structure of AZ61 magnesium alloy in SIMA process was studied. Under coequal heat-treat condition, the impact of the hot upsetting pre- deformation on semi-solid microstructure in SIMA process was compared with that of the cold compressive predeformation. The results indicate that non-dendrite microstructure in AZ61 magnesium alloy billets can be obtained by hot or cold upsetting predeformation in SIMA process, although their mechanisms of evolution are different. Increasing hot or cold upsetting predeformation ratio can enhance the effect and quality of the non-dendrite microstructure formed before storage energy up to saturation, but the proper isothermal temperature and holding time should be selected.

Experimental studies and phase field modeling of microstructure evolution during solidification with electromagnetic stirring

Thixocasting requires manufacturing of billets with non-dendritic microstructure.Aluminum alloy A356 billets were produced by rheocasting in a mould placed inside a linear electromagnetic stirrer.Subsequent heat treatment was used to produce a transition from rosette to globular microstructure.The current and the duration of stirring were explored as control parameters.Simultaneous induction heating of the billet during stirring was quantified using experimentally determined thermal profiles.The effect of processing parameters on the dendrite fragmentation was discussed.Corresponding computational modeling of the process was performed using phase-field modeling of alloy solidification in order to gain insight into the process of morphological changes of a solid during this process.A non-isothermal alloy solidification model was used for simulations.The morphological evolution under such imposed thermal cycles was simulated and compared with experimentally determined one.Suitable scaling using the thermosolutal diffusion distances was used to overcome computational difficulties in quantitative comparison at system scale.The results were interpreted in the light of existing theories of microstructure refinement and globularisation.

Effects of trace Cr on as-cast microstructure and microstructural evolution of semi-solid isothermal heat treatment ZC61 magnesium alloy

The content and kind of trace elements in magnesium alloys have important effects on their ascast and semi-solid microstructures. In this research work, effects of trace Cr on as-cast and semi-solid microstructures of ZC61 magnesium alloy were investigated by metal mold casting and semi-solid isothermal heat treatment. The results show that the addition of Cr can refine the α-Mg phase without generating a new phase, noticeably change the eutectic phase, and decrease the average size of solid particles at the same isothermal heat treatment conditions. Non-dendritic microstructures of all alloys are constituted of α_1-Mg phases, α_2-Mg phases and eutectic phases after water quenching. With isothermal temperature increased or holding time prolonged, the eutectic microstructure(α-Mg+MgZn_2+CuMgZn) at the grain boundaries in as-cast alloy is melted preferentially and then turned into semi-solid non-dendritic microstructure by processes of initial coarsening, microstructure separation, spheroidizing and final coarsening. Especially when the ZC61-0.1 Cr alloy was treated at 585 ℃ for 30 min, the ideal non-dendritic microstructure can be obtained, and the corresponding solid particle size and shape factor were 37.5 μm and 1.33, respectively. The coarsening process of solid α-Mg phase at higher temperature or longer time, which is affected by both combining growth and Ostwald ripening mechanism, is refrained when Cr is added to the ZC61 alloy.

Effect of Ce-rich mish metals and electromagnetic stirring on microstructure of AZ91D magnesium alloy

The influence of Ce-rich mish metal（MM） and electromagnetic stirring（EMS） on microstructure evolution of AZ91D magnesium alloy was investigated by optical microscopy, scanning electron microscopy（SEM）, energy dispersion spectrum（EDS） and X-ray diffraction（XRD）. The microstructure of AZ91D and alloying AZ91D is different. Except dominant primary α（Mg） and β-Mg 17Al 12 in AZ91D, a new phase Al 11MM3 is observed in alloyed AZ91D. Increasing Ce-rich MM, primary α grains can be refined, the content and grain size of β obviously decrease, furthermore, its morphology degrades from discontinuous network to separated particles, and Al 11MM3 amount substantially increases and acicular Al 11MM3 aggregates into clusters at grain boundaries. In EMS process, α morphology evolves from rosette to sphere, and then spherical-like with the decrease of isothermal stirring temperature. Moreover, Al 11MM3 mainly distributes inside primary grains and eutecticum in form of short-rod. EMS can alter not only the primary morphology but Al 11MM3.