Effect Of Low-Voltage Alternating Current Pulse On The Solidification Behavior Of Al And Al-Si Alloys

The electric current pulse（ECP）technique as new technology of material solidification processing,which provides a new way to achieve solidification under extreme conditions,has attracted considerable attention.To date,the internal law of solidification structure evolution affected by ECP parameters is not clear.The mechanism of microstructure refinement with ECP still remains controversial.Furthermore,the study on regulating metal solidification with ECP mainly focuses on refining solidification structure and its mechanism under capacitor energy storage electric pulse at present.The study on the solidification technology of other forms ECP different from capacitor energy storage electric pulse,on the one hand,will improve the effect law of ECP parameters on the solidification structure,on the other hand,will be helpful to understand the mechanism of grain refinement with ECP.In the present work,a low-voltage alternating electric current pulse（LAECP）is applied during the solidification process of pure Al and Al-Si alloys which are widely used light structural materials.The experimental investigation of morphology,size and distribution of primary phases in macro and micro structures of metals under the condition of different LAECP parameters and solidification parameters was carried out.Furthermore,the role of a series of electric effects:melt flow driven by Lorentz force under LAECP,free energy change（ΔGe）based on different current distribution,Joule heating,etc.on grain refinement and morphological transition of primaryα-Al phase and Si phase and Segregation of primary Si phase was discussed in detail.The main conclusions are as follows:（1）The capability of forming fine equiaxed grain（FEG）has been inspected,when LAECP parameters or the electrode placement are different.And the source of fine crystals has been analyzed.The results show that the refinement of pure Al solidification macrostructure is affected by both the pulse current density and intensity,but the formation of FEG depends more on the intensity of LAECP.In addition,Grain refinement in pure Al under LAECP is related to the configuration and position of two electrodes.The upper and lower configurations of the electrode have the best capacity of FEG formation.The source of FEG in pure Al under LAECP is the result of competition and synergy between the detachment of crystal nuclei due toΔGem which is motion terms inΔG_e and the detachment of crystal nuclei and dendrite fragmentation due to the forced melt flow.Moreover,fall-off of the crystal nuclei on the mold wall takes precedence over dendrite fragmentation by forced melt convection in pure Al.The results of microstructure embedded with mesh tube also show that current effects such as Joule heat and solution structure fluctuations can be ignored in the formation of pure Al FEG.（2）The evolution and mechanism of solidification structure of hypoeutectic Al-7 wt.%Si（Al-7Si）alloy under LAECP were discussed through systematic experiments.The results indicate that the mean grain size of equiaxed grain in solidification macrostructure of Al-7Si alloy under LAECP decreases with the increase of pulse current density.And the minimum average particle diameter is about 1.0 mm.The grain refinement mainly occurs in nucleation stage and early growth stage of primary phase.Further,the source of FEG in Al-7Si alloy under LAECP is the result of competition and synergy between the decrease of nucleation barrier caused by the decrease of solid-liquid interface energy leads to the increase of nucleation rate of alloy,the detachment of crystal nuclei due toΔGem and the detachment of crystal nuclei and dendrite fragmentation due to the forced melt flow.Furthermore,the morphology of theα-Al dendrite in solidification microstructure of hypoeutectic Al-7Si alloy under LAECP turns into rosiness from dendritic with the increase of pulse current density.The morphology evolution ofα-Al dendrite is mainly attributed to melt flow driven by Lorentz force,and has nothing to do with current effects such as Joule heat which only depend on the pulse current density in the range of pulse current density selected for the experiment.Theα-Al morphology can be predicted through calculating the Reynolds number of alloy melt,after Al-7Si alloy was treated with different current density LAECP.（3）The columnar to equiaxed transition（CET）in the unsteady directional solidification structure of Al-7Si alloy with LAECP was explored.The results show that the LAECP can promote the CET during the directional solidification of Al-7Si alloy.When the origin of fine equiaxed grains is attributed to the detachment of crystal nuclei due toΔGem and the forced melt flow,a realistic criterion for CET should be based on a critical cooling rate encompassing tip growth rates and the critical transition cooling rate is about 0.14 K·s^-1-0.16 K·s^-1 in the present study.When the forced melt flow can induce dendrite fragmentation,the directional solidification macrostructure includes the FEG region and the region which both columnar and equiaxed grains coexisted, and the formation of a complete equiaxed crystal region can be determined by Gu<12.5V_L⁰.⁷⁵.The mixed microstructure of FEG and columnar crystals is attributed to the fluctuation of temperature field caused by forced convection at macro and micro scales.（4）The LAECP has a significant effect on the distribution,morphology,and size of the primary Si phase in the solidification structure of hypereutectic Al-Si alloy.As the effective values of LAECP increasing,morphologies of primary Si change from star-shaped and plated-like to plated-like and polygonal in hypereutectic Al-30 wt.%Si（Al-30Si）alloy.For hypereutectic Al-20 wt.% Si（Al-20Si）alloy,morphologies of primary Si transform firstly from coarse star-shaped and plated-like to fine plated-like and polygonal and finally become finer polygonal with increasing the effective values of LAECP.It is speculated that the reduction of solid-liquid interface energy caused by LAECP inhibits the condensation of tetrahedral or octahedral clusters in the alloy melt into plated-like crystal nucleus or star-shaped crystal nucleus.And the star- shaped crystal nucleus was suppressed more significantly.As a result,the morphology of primary Si is not completely consistent in alloys with different Si content.The suitable LAECP leads to remarkably segregation of primary Si in Al-30Si and Al-20Si alloys ingots.There was no significant change in the distribution of primary Si phase in the Al-14 wt.%Si（Al-14Si）alloys ingots.It shows that the segregation of the primary Si phase under LAECP is related to the content of Si element.The segregation of primary Si is attributed to forced convection induced by Lorentz force in alloy melt and mushy zone and electric current-associated free energy change related with the position of crystal nuclei(ΔG_em).