Flow Of Liquid Phaseand Crystal Growth Of Alloy Melt Under Electric Fields

Amongst the physical field assisting solidification techniques, including magnetic field, ultrasonic field and micro-gravity field as well as electric field solidification, the last has drawn much attention from many researchers and technicians due to its merits of easy facility, feasible operation and visible effect. In comprison to the normal solidification process modified by chemical additives, however, electric field can also result in refinement of the cast grains and improvement of the microstructures without changing alloy compisition, making surplus casting defects and damaging health and enviroment. In case of the advancement of casting and solidification assisted by the electric field, numerous results reveal that electric field is with the better abillity to refine grains, alter precipitate phase morphologies and distributions for a variaty of metals and alloys. Nevertheless, there are issues which should be resolved in terms of unstable chosen ways of electric current patterns, misunderstanding mechanisms of fluid flow and freezing and application of new generation materials when the electric field was intersected to solidification process.Aiming at the points mentioned above, a complex method which combined numerical and physical simulation and solidification experimental with and without electric field, was utilized subsequently to resolve the influence of electric current treatment on liquid phase flow and its crystal growth of alloys. In numerical simulation, FEM (Finite Element Method) and APDL(ANSYS Parametric Design Language) were chosen to calculate the intensity and distribution of electromagnetic field and flow field of alloy melt, taking Ti-(46-50at%)Al as an instance, inside mould cavities of width-to-thickness ratio in cross-section ranging from 1:1, 2:1, 3:1, 4:1, 5:1 to 9:1. The three excitation modes of electric field, e.g. alternate electric field, pulse electric field and steady electric field, were applied during the calculation respectively. The flow field in the front of the S/L interface was also simulated thereby. In physical simulation, a multi-functional and time-dependent dynamic observation appratus was set up according to the physical similarity laws. Metalloids or alloy-like binary solutions were selected to observe and examine the fluid flow and crystal growth under electric field. Mostly, NH4Cl-89.43at%H2O and SCN-8.38at%ETH were simulated to single phase and NPG-90.45at%SCN to hypoeutectic as well as AMPD-19at%SCN to hyperperitectic in their counterparts of alloys respectively. In experimental, the pouring and solidifying of binary alloys, Al-9at%Si and Ti-50at%Al has been investigated through exerting a steady electric field in which the alloys were cast into slabs with 5:1 in width-to-thickness ratio in cross-section. Finally, samples cutting from the 5:1 slab were analyzed through OM, XRD, SEM and EDS for observing structures and tensile test and microhardness test for measuring mechanical properties.Based on the above simulation and experiment, the results can be drawn into conclusion as followed. A width-to-thickness ratio of 5:1 mould cavity is appropriate for accommodating fluid because of the intensity and distribution of electromagnetic field are favorable in this case, where it leads to a more steady fluid flow in comparison with other mould cavity ratios under any electric field of the three modes. Moreover, the flow droven by steady electric field is much more stable and enduring than the others. Also, the fluid flow in the front of S/L interface presents turbulent votexes around dendrite tips at their same side which consolidates upstream flow in the fluid. Because NH4Cl-89.43at%H2O has large difference in electric conductivity of liquid phase and solid phase, Joule heating effect will play an important role to remelt the drifting crystals and surpress the growth of the second and third arms of equiaxed and columnar dendrites. For SCN-8.38at%ETH metalloid alloy, the Pinch effect and TEMHD (Thermoelectric Magneto- Hydrodynamic) effect simutaneously accelerate the planar to cellular transformation and speed up the cellular growth rate. Because TEMHD effect itself can arise increasing of the undercooling in the front of cellular tips, it leads to branching of cellular crystals. With their growth and further forcing the columnar grains grow inclinedly against the upstream flow in conjunction with Pinch effect. For NPG-90.45at%SCN metalloid alloy, Pinch effect causes the primary NPG phase to remelt and be broken down, and also makes the presented eutectic NPG phase in NPG/SCN eutectoid to branch. For AMPD-19at%SCN metalloid alloy, the steady electric field changes theβperitectic phase growth patern from columnar to granular due to the chemical potential varied by the electric field in both peritectic phase and primary phase, which means the peritectic phase transformation is being supressed, thereafter, the amount ofβproducts are reduced and growth of column grains are unable to develop in the next precipitate procedure. A method of inputing electric current into the mould cavity using electrodes with similar compisition to those of the alloys was created. In performance of the method, a criteria was also deduced for judging homogeneity of a curent conducting inside the melt, i.e. (D/W)/(1/7/) , where it needs to satisfy R≥0.01m, the radius of the binding post of electrode, and L≤10m the length of electric field acting zone. So far as the situation meeting the formula is satisfied, an optimal flow will be achieved consequently. The grain size of Al-9at%Si alloy and Ti-50at%Al alloy is decreased to some extent, which approves and supports the exsiting results on grain refininment of the cast alloys. In case of the eutectic colonies of Al-9at%Si alloy, the branching of eutectic silicon was also found. In the solidification microstrutures of Ti-50at%Al alloy a reduction of peritectic phase was observed, i.e. the peritectic phase was suppressd by elctric current under a electric field.In the end, this paper presents thermodynamic and kinetic interpretion on reseasons why the electric field affects solidification process of the casting alloys. It will make fundations for castings on the basis of electric field solidification technique. It is also expected to pave more ways on application of Al-Si alloy and Ti-Al alloy in aviation, aerospace and automobile industries.