Influence Of Electric Current Pulses Research Articles

Effect of electric current pulse on size of primary carbides in hypereutectic high chromium cast irons in different solidification stages

To investigate the influence of electric current pulse (ECP) on the size of primary carbides in the hypereutectic high chromium cast irons, an ECP (peak density: 19.66 × 105 A m−2) was applied respectively to samples in different solidification stages (the pre-nucleation stage, the nucleation and initial growth stage, the late growth stage of primary carbides). Besides the latter two stages, the ECP can also refine primary carbides in the pre-nucleation stage. There are a few similar reports in the literature about other alloys. The samples were treated with ECP of different current density in the same stages to better study the refinement mechanism. The results show that the size of primary carbides decreased first and then increased as the ECP current density increased in the first two stages. The size continued to decrease in the last stage. The analysis shows that the main roles of ECP in different stages are different. The main roles of ECP are the free energy effect and the pinch effect both in the first two stages. That is the pinch effect in the last stage. The change of size of primary carbides is the result of competition between two effects of ECP.

Solidification of Pb–Al Alloys Under the Influence of Electric Current Pulses

Continuous solidification experiments are carried out with Pb–Al alloys under the influence of the electric current pulses (ECPs). The results demonstrate that the ECPs mainly affect the microstructure formation through changing the energy barrier for the nucleation of the minority phase droplets (MPDs) and minority phase particles (MPPs) during cooling Pb–Al alloys in the liquid–liquid and liquid–solid phase transformation temperature ranges in advance of the solidification of the matrix liquid. For Pb–Al alloys with Al-rich droplets/particles as the minority phase, the ECPs lower the energy barriers for the nucleation of the MPDs/MPPs and cause a significant increase in the nucleation rate of the MPDs/MPPs and, thus, promote the formation of Pb–Al alloys with a well-dispersed or even nanoparticles dispersed microstructure. The ECPs parameters show an important influence on the microstructure formation of Pb–Al alloys. The refinement extent of the MPDs/MPPs increases with the increase in the peak current density. For a given peak current density, the refinement extent of the MPDs/MPPs increases with the increases in the pulse frequency and pulse width first, and then level off and become asymptotic.

Influence of electric current pulses on the solidification of Cu-Bi-Sn immiscible alloys.

Continuous solidification experiments were carried out with Cu-Bi-Sn alloys under the effects of Electric Current Pulses (ECPs). A model describing the microstructure evolution was developed. The formation of the microstructure in the continuously solidified alloys was calculated. The calculations demonstrated that ECPs mainly affect the solidification process through changing the energy barrier for the nucleation of the minority phase droplets (MPDs). When the matrix liquid has a lower electric conductivity compared to the MPD, the ECPs lead to a decrease in the energy barrier for the nucleation of the MPDs which then promote the formation of a finely dispersed microstructure. When the matrix liquid has a higher electric conductivity compared to the MPD, the ECPs cause an increase in the energy barrier for the nucleation and lead to the formation of a phase segregated microstructure.

Influence of Electric Current Pulses on the Microstructure and Mechanical Properties of X70 Pipeline Steel

The effect of high electric current pulse on the microstructure and macroscopic mechanical behavior of X70 pipeline steel was studied. With the increasing of current density, the grains become refined and the tensile strength has an evident improvement without a decrease in elongation rate. The theoretical analysis shows that two kinds of refinement mechanisms, recrystallization and phase transformation, compete when the exerted current densities differ. Therefore, the ECP treatment should provide a promising method to refine materials and to improve their physical properties by using different current densities.

Effect of electric current pulse on solidification of silicon steel pertinent to twin roll casting

An experimental study with respect to the influence of electric current pulse (ECP) on the solidification structure of molten silicon steel was investigated with a copper mould designed to simulate the twin roll casting process. The experimental results showed that the application of ECP can increase the proportion of equiaxed grains up to 67·5%. The mechanism of exerting ECP is also discussed. It can be confirmed that the application of ECP on twin roll casting could effectively improve the solidification structure of silicon steel strip.

Research on solidification structure refinement of pure aluminum by electric current pulse with parallel electrodes

The influence of electric current pulse (ECP) with parallel electrodes on the solidification structure of pure aluminum has been investigated. The experimental results indicate that the solidification structure cannot be refined when the ECP is applied before the molten metal starts nucleating. However, significant refinement of the solidification structure can be achieved by applying ECP during the nucleation stage. In addition, under the same cooling conditions, ECP with parallel electrodes has better refinement effect than ECP with up–down electrodes. The refinement mechanism of ECP with parallel electrodes has also been proposed.

The Influence of Electric Current Pulses on the Microstructure of Magnesium Alloy AZ91D

The microstructure evolution of magnesium alloy AZ91D solidified with different electric current pulses and cooling rates was investigated and a new method, Low-voltage Electric Current Pulses (LVECP), to produce semi-slurry magnesium alloy was developed in this paper. The experimental results showed that the electric current pulses during solidification changed morphology of dendrites and the equiaxed, non-dendritic grains formed. The size of the primary a-Mg particles in semi-solid AZ91D alloy and the sphericity of the particles decreased with increase of discharging the voltage and treating time of LVECP. The increase of the cooling rate during the solidification of AZ91D alloy with LVECP promoted the formation of finer a-Mg particles, but the value of the sphericity of the particles rised. The formation of equiaxed, nondendritic structure by LVECP might be attributed to the electric current pulses increase the nucleation rate, restrained growth of the dendrites, and made dendrite arms remelted during the solidification of AZ91D alloy.

The influence of electric current pulses on the microstructure of the MnBi/Bi eutectic

The MnBi/Bi eutectic was directionally solidified with freezing rate oscillations caused by electric current pulses. The influence of current pulsing on microstructure depended on the ampoule translation rate V and pulsing conditions. Continuous electric current had a small effect on the microstructure. Current pulsing with the solid positive had a substantial influence, greater than with the solid negative. For V⩾2 cm/h, current pulsing transformed sections of the ingot from a quasi-regular microstructure to irregular MnBi, broken lamellae or completely void of MnBi. The reverse was true at 1.1 cm/h. For the quasi-regular portions, the average MnBi rod spacing λ decreased almost linearly with the average current. This result contradicts the hypothesis that freezing rate fluctuations were responsible for the effect of convection on MnBi–Bi microstructure. The decrease of λ with current pulsing was less at high V, where the rods were more rounded. Current pulsing also increased the roundness of the MnBi rods. The percent MnBi in the grains with a quasi-regular structure decreased with increasing current pulsing amplitude.

Influence of electric-current pulses on the operation of sources of twinning dislocations in bismuth single crystals

The influence of electric-current pulses on the operation of different sources of twinning dislocations is discussed. It is established that an electric current stimulates it and decreases the critical stresses needed for the onset of generation of dislocation loops.

Influence on Gravity on the Microstructure of the MnBiBi Eutectic

Larson and Pirich showed that the MnBi fiber spacing in the Mn–Bi system is finer when the alloy is solidified in space or in the presence of a magnetic field. In completion of a program aimed at explaining these results, the influence of electric current pulses on the MnBi fiber spacing is determined. Small, frequent electric current pulses cause fiber spacing to increase. The fiber spacing is roughly proportional to the length of the current pulse and its amplitude. An additional piece of information is the recent results by Smith, showing no influence of low gravity on the MnBi fiber spacing. Consequently now the Larson-Pirich results are believed to be caused by a fluctuating freezing rate on earth, resulting from time-dependent buoyancy-driven convection. The fiber spacing adjusts more readily to a sudden decrease in freezing rate than it does to a sudden increase in freezing rate. That is, the MnBi fibers are terminated more readily than they branch. If the reverse is true, then a fluctuating freezing rate would cause a decrease in fiber spacing rather than an increase.

Influence of electric current pulses on the mobility and multiplication of dislocations in Zn-monocrystals

The method of selective etching was applied to study the influence of electric current pulses with duration 2 × 10−4 s and current density more than 20 MA/m2 on the pyramidal dislocation mobility in Zn monocrystals at 77 and 293 K. It was established that the increase of dislocation mobility under the influence of current pulses is accompanied by multiplication of dislocations at current densitiesj > 102 MA/m2. The obtained results are discussed with the account of electron-dislocation interaction, heating and Frank-Read source operation under the electron wind influence.

Effect of prior cold work on the influence of electric current pulses on the recrystallization of copper

Les resultats experimentaux obtenus montrent que l'application concurrente d'impulsions de courant electrique pendant le recuit isochrone du cuivre deforme a froid augmente les vitesses de restauration et de recristallisation et conduit a un affinement de la grosseur du grain et a une reduction de la frequence de maclage