SU1632670A1 - Method of plasma treatment - Google Patents

Abstract

The invention relates to arc processing and can be used for plasma arc cutting of metals. The purpose of the invention is to increase processing performance. A direct polar current with an amplitude of 0.7 to 4.0 kA with a frequency equal to the frequency of natural oscillations of the arc current is applied to the arc. The imposition of pulsations on the arc current improves the performance of arc processing by increasing the cutting speed at a high cathode resource. 3 dw., 1 tabl

Description

The invention relates to arc processing, in particular to plasma treatment methods, and can be used for plasma arc cutting of metals.

The aim of the invention is to increase the productivity of arc processing by increasing the cutting speed.

FIG. 1 shows a diagram of an apparatus for implementing the method; in fig. 2 - dependence of the cutting speed of the cut-off on the oscillation frequency v modulating the arc current; in fig-. 3 - the dependence of the maximum cutting speed аЈЈЙ, e on the amplitude of the modulated arc current D7.

A device for implementing the plasma treatment method contains a rectifier 1, the positive output of which is connected to product 4 via modulator 2 and rheostat 3. The negative output of rectifier 1 is connected to plasma generator 5. The modulator 2 is controlled by control unit 6 (Fig. 1 ).

The method is carried out as follows.

Without modulating the arc current, VfeiSf is determined for a given thickness of the metal being cut, and the natural frequency of the arc current vc is measured. Then impulses are imposed on the working arc current

modulating current in direct polarity, formed by current modulator 2.

The magnitude of the modulating current D7 is greater than the average arc current of the plasma torch. Since the capacitance discharge coincides in direction with the arc current, the modulation scheme can be called the direct polarity circuit. The indicated modulation of the arc current creates additional local heating of the cut product 4 at the arc attachment point (anode spot). The additional Joule heat released (from the modulating current pulse) accelerates the process of melting the metal and destroying its crystal lattice, and the gas flow blows this melt.

It is known that the pulsed mode of arc burning intensifies the erosion of electrodes. They try to get rid of this by eliminating oscillatory processes on the arc, for example, by inserting an inductance device in the supply circuit. In the proposed plasma torch feeding method, this phenomenon is used to increase the cutting speed, and the cathode erosion in this case does not exceed the usual cathode erosion when implementing methods that exclude pulsed mode arc burning

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about j

The eigenfrequency of Hz oscillations of the arc along the cutting thickness is limited by the following process: in the anodic arc spot, at the top of the product being cut, due to Joule heat release, electron bombardment, convective, conductive and radiant heat exchanges, a steam cushion is created from the material of the cut product, blown off by plasma-forming gas . This process leads to the movement of the anode spot deep into the metal being cut to its lower boundary. As the plasma torch moves, the arc returns to the upper boundary of the metal being cut, and the process repeats again.

Arc locking mode at the moment of imposing a pulse with amplitude A7

current density -Ј --- 108- 109 A / m2. SraTH

Since the discharge in the near-electrode area is narrowed, it is / is higher here than in the rest of the arc. Therefore, a gradient of magnetic pressure P - - arises simultaneously with each current pulse.

(c is the speed of sound) directed from the electrode to the arc and ejecting at a speed v the electrode material and plasma from the electrode in the direction of the arc channel. The speed is determined from the ratio

pv2 / 2 f-,

where p is the flux density.

At the same time, the process of ejection of material and plasma from the cathode prevails, since in the anodic anchor zone the density of the material of the steam cushion is 2-3 times higher than in the cathode one.

In addition to the evaporating electrode material, the gradient sucks cold gas from the near-electrode region. Electrode jets, cooling, compress the arc, preventing the arc from expanding with increasing current, which leads to the appearance of an increasing portion of the current – voltage characteristic of the current – voltage characteristic. This means that the arc power increases not only due to an increase in the average current, which increases with an increase in the modulation frequency vn, but also due to an increasing portion of the IVC.

If there is no phase shift between the frequency of the effect of the gradient Pn and vc, the pressure Pm coming from the cathode throws out the anode anchor point in the direction of increasing the arc length, which leads to an increase in voltage and power on the arc, which means that the cutting parameters improve.

In all other cases, the arc is shortened, which leads to deterioration of the cutting parameters.

At the same time, it was experimentally established that when kA an intensive destruction of the thermochemical cathode occurs, which leads to the termination of the cutting process, and thus the productivity of the plasma cutting process decreases.

At 7 kA, cathode destruction does not occur, but the increase in cutting speed when current impulses of direct polarity are applied to the arc at a modulation frequency equal to the natural frequency of oscillation of the arc current is negligible.

The experimental data are given in the table at a modulation frequency of 102 Hz, equal to the natural frequency vc of arc current oscillations for a given power source — a single half-period rectifier with an inductive choke. With other power sources and regime cutting parameters, the whisker will be different.

Example. The steel sheet of st. 3 thickness 1.1 m thick was cut. Plasma-forming gas — air consumption 2.5 x m / s, arc current 70 A, arc voltage 215 V. Current pulse amplitude A. Self-oscillation frequency of the arc 102 Hz. Without modulation, the maximum cutting speed is 0.0087 m / s. When the modulator is switched on at a frequency of 102 Hz, the increase in the maximum cutting speed is 15% (Fig. 2). With a current pulse amplitude of 1100 A, the increase in the maximum cutting speed is 26% (Fig. 3).

Compared with the basic prototype object, the proposed method allows to significantly increase the cutting speed.

Claims (1)

Invention Formula A plasma treatment method in which the natural frequency of the arc current is preliminarily determined and a current is additionally supplied to it with a frequency equal to the frequency of arc current oscillation, characterized in that, in order to increase the processing performance, the arc is supplied with direct polarity amplitude 0.7-4.0 KA. V M CUTTING .Q .0.008 80 120 / m, Hz MLKS m 0.012 0.01 0,008 800 1000 AJ, A Riga 5