Electric arc spraying

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PATENTS AND REGISTRATION OFFICE

PUBLISHING WRITING No. 337 975 ^lntcl B 23 k 27/00 ^kl· 49 h 27/00

Patent Application. No. 6541/59 Received 1θΥίϊ'1959 Validity Day 10 VII 1959

Ans. generally available on 1 day t 96g

Ans. published and the pamphlet published 23 VIII 1971

Priority requested from 11 VII 1958 (United States, 747,938)

UNION CARBIDE CORPORATION, NEW YORK, NY USA

Inventor: DM Yenni, WC MoGill and JW Lyle

Agent: PU Brain

The invention relates to a method and apparatus for coating solid bodies of electrically conductive or conductive material by means of plasma jet spraying.

A method and apparatus for coating by plasma spraying are known, wherein a fusible wire electrode is fed into an outer portion of a plasma jet starting from a nozzle to a workpiece. The plasma beam is achieved by conducting the arc together with. a gas flow through a constricted channel whose cross-sectional area is no larger than the cross-sectional area of ​​an equivalent non-enclosed arc. The elbow used is formed between a non-fusible rod electrode disposed with its arc-forming end inside a gas nozzle and opposite polarity electrode means provided with either the fusible wire alone or both the fusible wire and an electrically conductive workpiece. The high temperature and the collimated plasma beam rapidly melts the wire or wire electrode, in case the wire is also connected to the welding current as a live conductor as it is introduced into the plasma jet outside the nozzle and transmits the molten metal into a fine jet of small droplets which the underlying workpiece so that a seated coating is obtained or a weld joint is filled. In cases where the workpiece is connected to the electrical circuit, the surface receiving the beam becomes a target for the arc, whereby considerably higher temperatures are achieved on the workpiece surface than would otherwise be the case.

Such a method and apparatus have been improved according to the invention by feeding the fusible wire (or wire electrode) into the contracted plasma jet inside the gas nozzle. This method has advantages primarily in the form of improved control of focusing, setting and the quality of the small droplets of the molten metal beam. The wire is suitably arranged at or near the point of the nozzle's smallest cross-section, whereby maximum or nearly maximum torque transfer is obtained from the hot, high velocity arc gas to the molten coating material.

The invention thus relates to a method of coating solid bodies of electrically conductive or conductive material by means of plasma spraying. The invention also relates to a device for carrying out the method according to the invention.

The characteristics of the method and apparatus according to the invention are to be understood by the following claims.

The invention is also illustrated by means of the accompanying drawing.

In the accompanying drawing, Fig. 1 shows a partial view, generally in cross-section, of a device according to the invention and in Fig. 2, and 4 similar views of modifications of the device.

In Fig. 4, the device A comprises a rod electrode 10, which is coaxial to a nozzle electrode 11. Such electrodes are connected to an electric current source 12 over lines 13 and 14, respectively. A gas flows down through the annular space between the rod electrode 10 and the bore 15 of the nozzle electrode 11. Such a gas may be any suitable gas such as argon, helium, nitrogen or hydrogen. It is preferred that a certain amount of hydrogen is present in the gas mixture, since hydrogen from a metallurgical point of view increases the melting of the wire due to the increased amount of heat produced by the formation of atomic hydrogen in the arc and subsequent regeneration on or near the extruded material.

The fusible wire 16 is fed by rollers 17 through a side channel P into the wall of the nozzle electrode 11 and into the nozzle channel 18. According to the drawing, the wire is inserted at an acute angle, but this is not necessary. The vertical axis position at which the thread is inserted is selected in view of the size and special design of the device. Thus, the wire is arranged as close as possible to the point of gnaximal arc contraction or resp. maximum moraent concentration 19 · The wire 16 is in electrical contact with the nozzle electrode 11 and thus becomes an electrode when it projects into the nozzle channel 18. The arc '19, which is initially passed between the electrodes 10 and 11, then shows a tendency to be partially transferred to the wire 16 The molten metal from the wire 16 is then ejected as a high velocity plasma jet 20 formed by the plasma jet and the molten metal. The nozzle 11 is cooled below its melting point by passing coolant, such as water, from an inlet 21 through a duct 22 to an outlet 23. A suitable solid body S is provided under device A during plasma jet spraying 20.

The feed rate of the wire is adjusted in accordance with the electrical energy consumption so that the molten end of the wire is kept substantially at the center of the nozzle channel. In this case, the metal spraying will be substantially in line with the longitudinal axis of the nozzle channel 18. An excessively slow feed rate results in the spraying of large particles at an angle to such an axis, the angle being adjacent to the direction from which the wire is being fed. An increase in feed rate in addition to optimal conditions results in the same spraying as at too slow feed rate, but it will be located on the other side of such a shaft. Both of these conditions are obviously undesirable.

Ideal wire feed rates of 635 cm / min for steel, 355 cm / min for an alloy with 80% Ni and 20% Cu, and 762 cm / min for aluminum have been successfully used with a 1.6 mm diameter wire and a total power of 10 kV direct current and with the rod electrode as negative pole. The device, under the above conditions, had a nozzle diameter of 3.2 mm and a divergent bore of 30 ° between horizontal angles.

In the embodiment of Fig. 1, the nozzle electrode and the fusible wire also have the same electrical potential. Such a circuit enables efficient self-regulation in connection with the wire feed, whereby the molten wire end is substantially retained in the center of the nozzle channel. As the thread extends into the longitudinal portion of the nozzle channel, it begins to conduct a larger portion of the current. If it extends beyond the center of the channel, the melting rate is increased by the increased current plus the resistance heat along the extended portion of the wire plus a greater exposure to the high energy flow and the wire melts faster back to the central portion. If the thread speed and the extended portion of the thread decrease, the thread will absorb less current and. The nozzle electrode thus comes to absorb more current. The overall effect of this will be a decrease in the melting rate.

The longitudinal nozzle channel extending on the other side of the thread focuses and. effectively controls the position of the flow of the molten droplets. The diverged outlet duct effectively reduces inappropriate clogging caused by deposits of molten metal particles inside the nozzle. A divergent channel in the nozzle electrode also causes the electrode surface to spread out as well. current density is reduced. This reduces erosion at high current levels. Also, a divergent channel in the nozzle electrode allows an outlet gas velocity overburdened, under certain conditions, which further accelerates the molten material in the spray to achieve greater impacts to the workpiece and results in denser coatings or welds. For the reasons stated above, it is therefore appropriate that the nozzle outlet has a larger cross-sectional area relative to the nozzle area at the point of entry of the wire.

Additional gas protection for reducing the atmospheric air pollution of the outgoing flow is obtained by introducing protective gas at the nozzle outlet through a hollow inlet device 28 at the nozzle outlet end.

Figure 2 shows an improved embodiment of the present invention. In this case, the fusible wire electrode 16 is electrically isolated from the nozzle electrode 11 by a tubular electrical insulator 24, which is arranged in a side channel P. Through conduit 13, the main current is supplied from the current source 12 to the rod electrode 10 and through conduit 25 to the fusible electrode 16. The nozzle electrode 11 communicates with the current source over a resistor 26 which holds the nozzle at a lower potential than that of the fusible wire.

The device of Fig. 2 can operate at higher wire feed rates than that of Fig. 1, since higher energy levels can be supplied to the wire without damaging the nozzle. This becomes important if wire feed speeds as high as 45.4 kg / h or higher are desired.

An ignition arc is maintained between the rod electrode and the nozzle electrode so that the work can be started and so that an arc is retained if the wire feed is interrupted for any reason. The electrical contact from line 25 to wire 16 may be located outside the burner so that the resistance heat along the wire is increased.

The embodiment of Fig. 3 shows a nozzle extension 27, ₅ The same purpose as the expanded nozzles of Figures 1 and 2 is to focus and direct the flow of the gas and the molten particles to a designated point or surface and to reduce air pollution of the molten particles.

A further modification of the invention is shown in Fig. A. The arc current from the electrode 10 is divided into the nozzle anode 11, the molten wire electrode 16 and the unit piece 29 by suitably adjusted ballast resistors 26 and 30.

The following examples describe the use of the device according to the invention for the application of metal coatings to metal sheets.

Example 1

Spraying of a wire consisting of an alloy of 80% Ni and 20% Cu, median arc torch.

The device of Fig. 1 is used except that a nozzle with a straight bore is used, i.e. a non-divergent outlet. A gas mixture of 5.66 nk argon / hour and 0.382 m 2 hydrogen / hour was passed downward around a 3.2 mm diameter thorium-coated tungsten electrode and through a (non-divergent) channel of the 3.2 mm diameter nozzle electrode. An arc of 75 volts DC and 150 amps was formed between such electrodes, the rod electrode having a negative pole. A single-alloy thread with 80%

Ni and 20% Cu and with a diameter of 1.6 mm were fed through a channel in the side of the nozzle at a rate of 101 cm / min. Additional hydrogen shielding gas at the rate of 1.42 m 2 / h was introduced into the nozzle outlet. The hot effluent gas and the extruded molten metal from the wire electrode were then struck against a cylindrical cold rolled steel rod of diameter 1.27 cm which rotated and located 2.54 cm from the outlet of the burner nozzle. The resulting applied alloy was dense, strongly adhered to and had less than 1% pores and less than 1% oxide contamination.

Example 2

Spraying of steel wire with arc burner.

The device of Figure 2 was used. A gas mixture of 5.66 m 2 argon / h and 0.396 m 2 hydrogen / h was passed downward around a 3.2 meter diameter thorium coated tungsten electrode and out through the channel in a 3.2 mm diameter nozzle electrode with a dividing outlet at an angle of 30 mm. °. An arc of. 80 volts and 110 amperes DC were formed between the rod electrode (negative pole) and the molten wire electrode plus the nozzle electrode. The fusible wire electrode occupied 100 amperes while an arc current of amperes was supplied from. nozzle electrode. The fusible wire electrode was a 1.6 mm diameter carbon steel welding rod and fed at a speed of 444.5 cm / min. Additional hydrogen shielding gas was introduced at a rate of 1.42 m 2 / hour at the nozzle outlet. The hot outgoing gas stream and the molten metal particles from the fusible wire electrode were then struck against a rotating carbon steel rod R of diameter 1.27 cm and located 2.54 cm from the burner nozzle. The resulting steel-to-steel coating was dense, adhered well, had less than 5% porosity and less than 1% oxide contamination.

The nozzle channels described above have circular cross sections of other shapes such as rectangular, square or oval can be used without departing from the spirit of the invention.