V

V.S. Savenko¹, A.G. Silivonec², S.A. Ermokovec²

¹ Doctor of technical sciences, Professor

MSPU named after I.P. Shamyakin, Mozyr, Republic of Belarus

² Undergraduate of physics and engineering faculty

MSPU named after I.P. Shamyakin, Mozyr, Republic of Belarus

THE CONTRIBUTION OF THE PONDEROMOTIVE FACTORS IN THE PHENOMENON OF ELECTROPLASTICITY OF DEFORMATIONAL MAGNESIUM

The paper presents the results of research of influence of pulse current on the implementation of electroplastic deformation. We have chosen magnesium as a material. It is widely used in modern technology due to its low density, which can significantly reduce the weight of components and structures. Magnesium is widely used in the aviation and automotive industries.

The article presents calculations of the ponderomotive factors of its own magnetic field and electric field inside a conductor when implementing a multipass electoplastic rolling of magnesium.

Introduction. There are a number of factors of secondary force pulse current that affect the plastic deformation of the metal under mechanical stress above the yield strength such as the pinch effect occurring under the influence of its magnetic field current and the polarization of electron subsystems of the metal with the creation of the transverse electric field of Hall, preventing further compression of electron plasma.

Skin effect implements the reduction of the amplitude of electromagnetic waves as their penetration into a conducting medium at the flow of high frequency alternating current (AC) through the conductor creating the uneven distribution over the cross section of the current density with a decrease in axial areas and increase near the conductor surface. And it is associated with the he rate of diffusion of the magnetic field in the metal and the presence of characteristic time τ_p of magnetic field penetration in the conductor with current. [1]

It should be noted that during the electroplastic deformation in addition to the secondary pinch and skin effects in transmission pulse of electric current, there is a thermal softening of the lattice, the calculation of its parameter for the considered duration of the current pulses (10^-4 - 10^-5 s) is not significant.

Pinch and skin effects. Electroplastic deformation implemented by multipass rolling of magnesium (35 transitions) when passing through the metal the pulse current density of 10⁵ A/mm²,the duration and pulse repetition rate of 10⁴ s and 600 - 800 Hz. Final geometrical parameters after 35 transitions: thickness is 1mm, width is 4mm.

On the surface of the sample self-magnetic field takes the maximum value (fig. 1a):

, (1)

where is amplitude value of the current in the conductor, then:

, (2)

where is the cross-sectional area of the conductor.

Mechanical pressure on the surface of the sample for the electron plasma compression is equal to (fig. 1b):

, (3)

where is mechanical pressure (Pa), is magnetic permeability for magnesium 1,257·10^-6 .

Maximum axial force from the pinch-effect is equal to:

, (4)

where is maximum axial force, is Poisson's ratio equal to 0,27,
is mechanical pressure, is cross-sectional area of the sample.

A special role in the phenomenon of electroelasticity played by the material of sample and its conductivity, the rate of diffusion of the magnetic field in the sample, which respectively determines the magnitude of the pinch-effect. The value of pinch-effect depends on the circular frequency which is 600 - 800 Hz.

Q factor should be small, such as when is the value of . [3, 4] Pulse duration should be minimal in this case, it was τ = 10^-4s.

The thickness of the skin layer for the sample of magnesium was determined as (5):

, (5)

where is the thickness of the skin layer, is magnetic permeability of magnesium,
is conductivity of magnesium, is pulse repetition rate.

Then, the thickness of the skin layer at 600 Hz: .

The thickness of the skin layer at 800 Hz: .

It is obvious that at sufficiently high frequency ω, the thickness of skin layer can be very small (fig. 2).

а) b)

Figure 1 - a) Сhange of the self-magnetic field of current h on the surface of magnesium at different values of current density Jm; b) Modification of mechanical pressure p on the surface of magnesium at different values of its own magnetic field current n

Figure 2 - Сhange of magnitude of the skin layer δ from the circular frequency ω

Conclusion. To achieve high pinch effect, it is necessary to select such a frequency of current pulse, so that the magnetic field does not have time to substantially penetrate the surface of the sample. When the deformation of the sample occurs due to the pinch effect, magnetic field diffuses the crystal, the speed of diffusion depends on the conductivity of the metal and the frequency of the current. At the same geometry of the samples, the pinch effect is stronger on the material that has high electrical conductivity. Thus, the pulse current in addition to electronic plastic action and exerts a ponderomotive effect on the deformable material (compression of the sample with its own magnetic field and excitation of elastic waves with a repetition rate of pulsed current). [1, 2].

Bibliography

1. Savenko, V. S. Mechanical twinning and electroplasticity of metals in terms of external energy impacts : monograph / V. S. Savenko. – 2nd ed. – Minsk : BGAFK, 2003. – 203 p.

2. Roshchupkin, A. M. About the influence of electric current and magnetic field on interaction of dislocations with point defects in metals / A. M. Roshchupkin, I. L. Bataronov // Solid state physics. – 1988. – V. 30. – № 11. – 3311 p.

3. Baranov, U. V. Physical basis of electropulse and electroplastic treatments and new materials / Y. V. Baranov, O. A. Troitsky, J. S. Abraham,
A. D. Shlyapin – M. : MOSCOW STATE INDUSTRIAL UNIVERSITY, 2001. – 844 p.

4. Troitsky, O. A. Fundamental and applied research of electroplastc deformation of metals / O. A. Troitsky, V. S. Savenko. – Minsk : IVC, 2013. – 375 p.