Electric heating – Metal heating – By arc
Patent
1999-09-20
2000-10-17
Paschall, Mark
Electric heating
Metal heating
By arc
21912146, 21912154, 21912157, 219 7616, B23K 1000
Patent
active
061335428
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a method of coating materials by plasma powder build-up welding of additive materials in powder form. In addition the invention concerns a plasma torch with bar-type cathode and a gas duct of a burner nozzle, the gas duct surrounding the cathode and a nozzle opening of narrower cross-section being arranged downstream of the gas duct, and a method of coating the bar-type cathode or a cathode portion.
The operation of coating iron-based materials with materials in powder form on a nickel, cobalt and/or copper basis has long been implemented in industry. When applying materials which oxidise easily such as for example aluminum, titanium or alloys thereof, the coating operation is very severely disturbed and possibly rendered entirely impossible to implement, due to the oxides which are formed at the surface.
Even attempts to conduct those operations under a protective gas failed. The only possible way of implementing such a coating operation is the very expensive and complicated procedure of operating under vacuum, which has never resulted in use on a large technical scale.
In accordance with JP-A-03 248777 a welding current is produced between a non-consumable electrode and a plate comprising an aluminum alloy by an alternating current source. The aluminum alloy contains between 15 and 55% by weight of Cu and serves as a build-up material, for the formation of which an alternating current plasma arc is used.
EP-A-0 452 494 describes a plasma torch with a cylindrical electrode and an insert portion disposed therein. The end edges of the electrode are slightly rounded in an embodiment which forms part of the state of the art.
DE-A-35 44 657 discloses a heavy-current electrode comprising a heat-conducting and current-conducting material, which has a coating to reduce burning-away of the electrode on the main body of the electrode. That coating is applied by a plasma spraying process. The spraying powder mixture for the coating comprises W and at least 4% by weight of ThO.sub.2 or CeO.sub.2 or W and at least 1% by weight of oxides, carbides or borides with a high melting temperature and a low electron work function.
In consideration of that factor the inventor has set himself the object of permitting coating of materials which oxidise easily--in particular light metals or alloys--in a simple and inexpensive fashion.
That object is attained by the teachings of the independent claims; the appendant claims set forth preferred configurations.
In accordance with the invention--after numerous attempts at applying to an aluminum alloy a coating with an additive material in powder form by means of powder build-up welding with a plasma transferred arc (referred to as PTA)--there is proposed the use of an alternating current in a frequency range of between about 10 and 100 Hz or a direct current as an energy carrier with a superimposed alternating current at a frequency of between about 10 and 1000 Hz or in the frequency range of between about 1 and 200 KHz; in this case a base point of the plasma arc is to be produced at a spherical or ball-shaped portion formed on an electrode, prior to use thereof. By virtue of that procedure, the oxide skin which is formed due to the oxygen in the air or the residual oxygen in the protective gas, at the surface of the base material or of the additive material in powder form, is destroyed or broken up, and it becomes possible to carry out coating operations even on or with those easily oxidising materials, and to achieve a good weld quality.
It is also advantageous to endeavour to achieve a high degree of mixing with the base material and also it is advantageous in accordance with the invention to so select the flow speed of the issuing gases that the slags or oxides which are formed by virtue of the residual oxygen are torn by the gas flow and deposited at the edge of the molten bath.
Also in accordance with the invention is a plasma torch with a gas duct of a burner nozzle, the gas duct being arranged upstream of a nozzle opening of narrower cross-section, and
REFERENCES:
patent: 4125754 (1978-11-01), Wasserman et al.
patent: 4983807 (1991-01-01), Yamada et al.
patent: 5686001 (1997-11-01), Wrabel et al.
Dvorak Michael
Robert Patrice
Castolin S.A.
Paschall Mark
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