RU2165831C2 - Plasma fusion method - Google Patents

Abstract

FIELD: repairing and strengthening mounting surface of working chamfer of exhaust valves of internal combustion engines by means of powdered alloys with predetermined properties. SUBSTANCE: method comprises steps of selecting parameters of electric current pulse according to conditions of accelerated heating (at period of that pulse) of melt along predetermined width of dressing and displacing melt towards periphery of formed welding bath and at pause between electric current pulses - holding and accelerated cooling of melt metal in dressing zone. Flowrate of plasma generating gas at period of electric current pulse and between electric current pulses is maintained constant. EFFECT: possibility for increasing width of layer fused at one pass, high quality of fusion. 5 dwg, 1 ex

Description

 The invention relates to welding equipment and can be used for plasma surfacing, mainly the landing surface of the working facet of the exhaust valves of internal combustion engines with powder alloys with desired properties in engineering and repair enterprises of various industries.

 In some cases, an increase in the width of the layer deposited in one pass is required. For this, methods are most often used that are associated with an increase in the diameter of the plasma-forming nozzle or the transverse oscillation of the plasma torch. But these methods are not always advisable to apply for the following reasons: increasing the diameter of the nozzle reduces the concentration coefficient of the heat source and leads to a change in the temperature distribution in the plasma jet cross section, which is sometimes not allowed, and the use of an arc control device by mechanical movement complicates the equipment’s construction, makes its more bulky.

 In plasma surfacing, two types of energy are used - thermal and mechanical. The main component of the mechanical energy of the plasma arc is the gas kinetic pressure, and the ratio between the thermal and mechanical energy of the arc determines the quality and productivity of the surfacing process and requires targeted regulation. The control of each of these components allows you to expand the control range of the parameters of the deposited layer.

 A known method of surfacing valves, providing for the simultaneous impact on the melt of a directed magnetic field and centrifugal force [1].

 This method is convenient for induction surfacing, because the inductor can generate the necessary magnetic field. In plasma surfacing, the formation of a magnetic field requires additional devices that dramatically increase the overall dimensions of the plasma torch, which is unacceptable when surfacing valves of small sizes (almost all cars).

 A known method of plasma surfacing, involving the action of an arc in the presence of filler material on the base material [2].

 This method uses a current source with a rectangular characteristic "welding current - voltage" that maintains a constant voltage in the range of the used welding current values, in order to increase the stability of arc burning and stabilize the thermal effect on the melt, which in some cases is not enough, since this solution does not expand the boundaries of quality control and the shape (width) of the deposited layer.

 A known method of plasma surfacing, taken as a prototype, providing for the impact of a pulsed arc in the presence of filler material on the base material [3].

 In this method, the pulse change is synchronized with the change in the plasma-forming gas, and during the current pulse period, the gas flow rate is reduced, and during the current pause period it is increased. This method partially solves the problem by increasing the thermal effect at the time of the current pulse. However, a simultaneous decrease in gas consumption does not allow to fully use the possibilities of the mechanical action of the arc on the metal melt to increase the width of the layer deposited in one pass.

 The problem to which the invention is directed, is to increase the width of the layer deposited in a single pass, achieved without compromising the quality of surfacing.

 It is solved in a plasma surfacing method, which provides for the action of a pulsed arc in the presence of a filler material on the base material, in which the parameters of the current pulse are selected from the conditions for accelerated heating of the melt at a given cutting width during the current pulse, and during the pause of the current, accelerated cooling of the molten metal in cutting, while the flow rate of the plasma-forming gas during the periods of the pulse and pause current is kept constant.

 The choice of the parameters of the current pulse from the conditions for the accelerated heating of the melt at a given width during the period of the pulse provides a sharp increase in the amount of heat released on the arc at the time of the current increase, while increasing the gas kinetic pressure of the plasma jet and the pulsation of the gas-dynamic pressure of this jet, which in aggregate sharply increases force action of the arc on the surface of the melt, and the displacement of the melt to the periphery of the formed weld pool leads to forced filling of this a melt of a pre-prepared groove, the resulting effect of which is to increase the width of the deposited in one pass layer without the formation of defects in it, that is, the stability of filling the groove for surfacing increases.

 The implementation of the retention and accelerated cooling of the molten metal in the groove during the period of the current pause prevents overheating of the base metal and leakage of molten metal from the groove, and also promotes accelerated crystallization of the weld metal in a small volume. In this regard, shrinkage porosity in the deposited layer is reduced, since the volume of the melt in the weld pool decreases.

 Maintaining a constant flow of plasma-forming gas during periods of impulse and pause current allows you to change the total force impact on the molten metal without further complicating the equipment.

 Thus, to increase the width of the deposited in a single pass layer, the application of the proposed method of plasma surfacing does not require changing the diameter of the nozzle, the use of mechanical devices for the transverse movement of the plasma torch, etc.

 The proposed method is illustrated by the drawings shown in FIG. 1-5.

In FIG. 1 shows a current waveform of a pulsed plasma arc, where 1 is the welding current, A; t is the time, s; t _c - time cycle, s; t _and - the time of the current pulse, s; t _p - time pause current, sec.

 In FIG. 2 is a diagram of the formation of a weld pool during a current pulse period, on which are indicated: water-cooled thrust bearing 1, billet 2, metal melt (weld pool) 3, high-temperature zone of the arc 4, shielding gas 5, plasma-forming nozzle 6 and electrode 7.

 In FIG. 3 is a diagram of the formation of a weld pool during a pause of current with the same notation as in FIG. 2.

 In FIG. 4 - contour of the weld pool during the current pulse, top view.

 In FIG. 5 - contour of the weld pool during a pause of current, top view.

 The method is as follows. In the presence of all conditions of the readiness of the installation for surfacing, an automatic cycle is started by the operator’s command. The gas distribution system delivers plasma-forming, protective and transporting gases. The auxiliary arc power supply unit forms an indirect arc on duty, providing stable excitation of the main arc after loading the part to be surfaced (for example, rotation part), and heating (melting) the powder during welding to reduce overheating of the base metal. The power supply unit of the main arc forms an arc of direct action between the electrode and the product, additionally heating the surface of the workpiece, preparing it for surfacing.

 Then in the process of surfacing produce a periodic change in the force of the arc on the melt due to the use of pulsed current (Fig. 1).

 At the time of increasing current there is an increase in the amount of heat released on the arc, while increasing gas-dynamic pressure, while the force effect of the arc on the surface of the melt sharply increases (Fig. 2).

 This complex effect at the moment of a current pulse causes an increase in the temperature of the weld pool while simultaneously displacing the melt from the central part of the bath to its periphery and forces the previously prepared cutting to be filled with the melt. The width of the deposited in a single pass layer is thus increased compared with other known methods.

 During the current decline (pause), the weld pool is rapidly cooled (Fig. 3), while the base metal does not overheat, and the molten metal does not flow out from the groove, which ensures a high quality of the deposited layer.

 Example. Plasma surfacing of the internal combustion engine exhaust valve was made of steel 55Kh20G9AN4 (EP303) with filler material, which was used as a complex alloyed cobalt-based powder of the Stellite F type, with a particle size distribution of 50-160 microns. Argon was used as the plasma gas.

 The diameter and channel length of the plasma torch nozzle were 3 and 2.5 mm, respectively, and the arc length was 8 mm.

The auxiliary arc current is constant - 60 A, and the main arc current is pulsed, with the following parameters: cycle time - t _c = 0.04 s, pulse time - t _and = 0.02 s, pause time - t _p = 0.02 from.

 The value of the current value of the current: in a pulse - 110 A, in a pause - 80 A.

 The voltage of the main arc (average value) is 25 V, and the auxiliary (average value) is 15 V.

 The consumption of the additive powder (average value) is 9 g / min.

The speed of rotation of the workpiece is 4 rpm, and its angle of inclination is 47 ^o .

 Gas consumption (constant during the surfacing period): plasma forming - 1.5 l / min, transporting - 2.5 l / min, protective - 6 l / min.

At the end of the deposition cycle, a two-stage quality control of the deposited layer was carried out:
visual - showed a good formation of the deposited bead, the uniformity of the spreading of the melt and filling it with a groove;
metallographic - showed a decrease in pore size from 60-80 microns to 30-40 microns, a decrease in grain size in the heat affected zone from 300-350 microns to 280-320 microns.

 The width of the weld deposit in one pass using a nozzle with a diameter of 3 mm increased from 4.5 mm to 7 mm.

BIBLIOGRAPHY
1. USSR author's certificate N 1540984, B 23 K 13/00, B 23 P 6/00, 1990

 2. Application of Japan N 4-36787, cl. B 23 K 10/02, 1986

 3. USSR author's certificate N 1683927, B 23 K 10/00, 1991

Claims (1)

 A method of plasma surfacing, involving the action of a pulsed arc in the presence of filler material on the base material, characterized in that the parameters of the current pulse are selected from the conditions for the accelerated heating of the melt at a given cutting width during the period of this pulse and forcing it to the periphery of the formed weld pool, and in the period current pauses - holding and accelerated cooling of the molten metal in the groove, while the flow of plasma-forming gas during the periods of the pulse and pauses of the current is kept constant.

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