- last post: 01.01.0001 12:00 AM PDT
Posted by: john masterchief
Unidirectionally Solidified Al-Cu Eutectic Alloy under High Intensity Pulsed Magnetic Field Keywords: pulse magnetic field, Al-Cu eutectic, directional solidification Abstract. The pulsed electric current produced by discharging of capacitors pass through the double solenoids with ferrosilicon core, the instant strong pulsed magnetic field with the oscillating and declining characters was made between two ferrosilicon cores. The effect of high-pulsed magnetic field on the unidirectionally solidified Al-Cu eutectic microstructure with 10ᄉm/s withdrawn velocity was investigated. Under the high intensive pulsed magnetic field, the Al-Cu eutectic solidified morphology experience three evolution stages that are regular columnar structure to breaking off fine grain, coarsening dendrite to newly regularization columnar structure with increasing of 015.5J charge energy. It is found that rich copper phase evidently come out inter eutectic cells and the eutectic spacing decrease in newly regularization specimen. The induced electric field caused by high pulsed magnetic field in metallic melt brought into oscillating solute electro-migration in front of solidification interface, which has the effect of promoting solute diffusion and reducing the constitutionally supercooling region. Introduction Pulsed magnetic field and electric current field have been widely used to control and modify the solidification process to obtain fine as-cast grain size, such as pulse electric discharging (PED) and electromagnetic stirring "Rheocasting process"[1]. The modification of solidified morphology is normally related with Lorentz-force bringing forced convection in melt and breaking off the arms of dendrites or Joule heating causing re-melting [2, 3]. But little efforts have been done about effects of high-pulsed magnetic field on dynamic and thermal-dynamic behavior at the liquid/solid interface during solidification. In the present work, the high-pulsed magnetic field is directly imposed on Al-Cu eutectic liquid/solid interface to determine the relationship between the magnetic field strength and unidirectionally solidified microstructure. Experimental procedure A Bridgman directional solidification apparatus with pulse electric source was used. The temperature gradient is 381K/cm when furnace temperature is set to 900. The rod-like specimen of Al-33.2wtCu eutectic alloy with a diameter of 7.0 mm was placed in alumina crucible with 120mm length. According to measured cooling curves, the growth rate of 10ᄉm/s was selected to ensure the high-pulsed magnetic field act on the liquid/solid interface. Two capacitor banks having 30ᄉF and 50ᄉF capacitances were used in the AC electric source. The charging time and discharging time of the capacitor were respectively set as 3 s and 0.25s. A digital oscilloscope which measures the voltage drop, shows that the oscillating frequency is 1000Hz and oscillating cycle is 118ms. Pulse electric current passes through a pair of solenoid coils with a ferrosilicon core of 300mm in length. The space between two ferrosilicon cores is 15mm. The relative location of ferrous core and solidified specimen is shown in Fig.1 A Hall probe with gauss meter was used to measure magnetic field strength between two ferrosilicon cores without the furnace assembly. Table 1 gives the main experimental parameters. Results and Discussion The magnetic field distribution Applying Biot-Savart law to calculate the magnetic field strength and distribution of magnetic force line between ferrous cores, the reference coordinate system and calculation results are show in Fig.1. In the calculation, the solenoid coil with ferrosilicon cores were simplified as thin coils and parallel to each other. The radius of coil is r and the distance is 2d. The same direction and magnitude of pulsed electric current are assumed to pass through the two coils, i= Uo e L - t Liquid Ferrosilicon core X Z Ferrosilicon core Solid u Fig.1 Relative location between ferrous cores and unidirectionally solidified specimen, and distribution of magnetic force lines between ferrous cores Table 1 Main experimental Peak Unit pulse Specimen charging voltage, t is charging charging discharging Number time, is oscillating voltage, V energy, J coefficient, is angle 1 -- -- frequency, L is circuit 2 255 2.6 inductive reactance. The 3 399 6.3 function of magnetic field 2 1 4 (t, z) = ir 622 15.5 1 strength along horizontal B + 2 3 22 2 r + (z + d ) r 2 + (z - d )2 ᄉ direction with time can be derived as: sin t , where U0 is parameters Peak electric current, A -- 11.7 18.2 28.6 3 2 Peak strength of magnetic field, T -- 0.9 1.4 2.1 [ ][ ] (1) Where ᄉ is magnetic permeability, which is near 1H/m in aluminum liquid and air. r=19ᅲ103m 2d=15ᅲ103m. Field source discretization methods are used to obtain discrete solution of magnetic field component in the calculation of magnetic field strength, assuming that the outline of coil is the discrete perimeter and electric density in discrete field source is uniform. The calculation results shown in Fig.1 indicate that magnetic field strength is a certain centrosymmetric distribution. The magnetic strength boosts up with the increase of charging voltage. The measured results of maximum magnetic field strength are listed in Table 1. Unidirectionally solidified structures. The test specimen was sectioned along longitudinal and transverse solidification directions for metallographic study. The microstructures observed by optical microscope are shown in Fig.2 which shows the growth features of unidirectionally solidified morphology of Al-Cu eutectic under different capacitor bank voltage. Where, u is the solidification direction of s
, that is Fick's first law, flux formula of solute flow induced by EPG satisfies (4) J c = - D L C L - Where CL and are solute gradient and induced EMF gradient respectively, is the Hamilton operator. is a constant related to effective action forces of EPG on solute atoms. If the temperature gradient is assumed to be constant within the solute diffusion boundary layer, according to the solute mass equilibrium relation, the following formula should exist: Gu (5) D C + = L L m c L Here G is the temperature gradient, mL is the slope of liquidus. The maximum width of constitutionally supercooled region in front of the solidification interface can be obtained as: TL (max) = G c + c m L (6) D L The IEP caused by the pulsed magnetic field would influence the solute movement and solute distribution in front of the solidification interface in terms of formula (6). When c m L < 0 , the D L width of constitutionally supercooled region is decreased, which has the effect of stabilizing solidification interface. The effect of EPG will be much more prominent with increasing discharging capacitor tank voltage. The stronger solute flow caused by EPG with increasing magnetic field strength will decrease effective diffusion distance of solute, which contributes to coarsen dendrite. The rich copper phase enriched on grain boundary under instantly high-pulsed magnetic field, which may be related to the difference of electrdynamic potential between inner-grain and inter-grain. Its physical and chemical mechanism needs to be investigated furthermore. Conclusions The directionally solidified microstructure of Al-33.2wt %Cu eutectic experience several evolving stages with increasing magnetic field strength under instantly high-pulsed magnetic field. The morphologies change from original regular columnar structure, fine grain of breaking off dendrite, coarsening dendrite structure, and finally to renewedly regularization of columnar structure. But it is obvious that the rich copper phase enrich inter eutectic cells in renewly regularization structure. The induced electrodynamic potential gradient in metallic melt caused by pulsed magnetic field brought into solute flow in front of the solidification interface, which has the effect of reducing the width of constitutionally supercooled region and stabilizing solidification interface.
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