Stock material or miscellaneous articles – All metal or with adjacent metals – Having composition – density – or hardness gradient
Reexamination Certificate
2002-07-11
2003-06-24
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Having composition, density, or hardness gradient
C219S1370WM, C219S146220, C428S687000, C428S385000, C428S386000, C428S469000, C428S472100, C428S702000, C428S923000, C428S925000
Reexamination Certificate
active
06582833
ABSTRACT:
RELATED APPLICATIONS
This application claims the priority of Japanese Patent Applications No. 2001-212326 filed on Jul. 12, 2001 No. 2002-62438 filed on Mar. 7, 2002 and No. 2002-188327 filed on Jun. 27, 2002 which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Ti-base wire rod used for forming molten metal in welding and thermal spraying.
2. Description of the Related Art
Shield arc welding is generally employed for welding of Ti-base metal members made of Ti metal or Ti alloy, in which Ti-base wire rod composed of an industrial pure Ti is used, and a portion to be weld is covered with an inert gas in order to prevent Ti from being oxidized. In a typical case of MIG (metal inert gas) arc welding shown in
FIG. 5
, arc AR is generated between a Ti-base wire rod
201
for use in welding and a work piece WP made of pure titanium or titanium alloy in an atmosphere of inert gas IG such as argon, helium or so. The welding is proceeded while feeding the wire rod
201
with the aid of a feed roller
202
so that the end of such rod is melted in the arc AR. A reference numeral
205
herein represents a gas nozzle (torch) for injecting the inert gas IG from the end thereof, which nozzle has at the base end thereof a flexible conduit tube
204
. A reference numeral
206
herein represents an electrode chip (contact chip) fixed to the torch
205
, which is responsible for holding of the wire rod
201
and for supplying electric current to such wire rod
201
, WM represents a welding bead, and MP represents a molten pool. The MIG arc welding is advantageous in raising the efficiency of the welding, suppressing welding failure thanks to a deeper weld zone resulted from an improved welding energy, and facilitating welding at narrow places with a down-sized end portion of the torch
205
.
On the other hand, it has been a general practice to form a coated layer with Ti-base metal by thermal spraying for the purpose of corrosion prevention for large-sized members. In the field of thermal spraying, a thermal sprayed layer is formed using a similar Ti-base wire rod as described in the above by thermal spraying process, typically by arc thermal spraying process. In the arc thermal spraying process, two Ti-base wire rods are fed to holders for current supply in parallel to thereby form an arc discharge gap between both ends of such rods, and a molten metal formed there is then sprayed using an inert gas such as nitrogen or argon, or using air as a medium so as to allow a thermal sprayed layer to deposit on a target work piece. The Ti-base wire rods are respectively fed through conduit tubes to a thermal spray gun similarly to the case of welding.
In recent years, there are accelerated trends in increasing feeding speed of the wire rod
201
aiming at higher efficiency in the Ti welding process and shorter period before completion of the welding process. In such situation, a large friction between the surface of the wire rod
201
and the conduit tube
204
undesirably interferes smooth feeding of the wire rod
201
, which may at worst result in clogging or buckling of the wire rod
201
within the conduit tube
204
.
In particular, surface of the conventional Ti-base welding wire rods are generally finished by mechanical or chemical polishing so as to produce a metallic gloss merely for the purpose of improving the appearance. Thus finished wire rods are however poor in feeding smoothness due to their coarse surfaces. Such wire rods having the metallic glossy appearance are also disadvantageous in that being causative of less stable arc during the welding as expected from their appearance, and desirable bead shape cannot be obtained in particular by MIG arc welding using an automatic welder since the arc trembles searching for a stable point. This may be ascribable also to that distance between the end of the wire rod and a work piece to be welded can finely fluctuate due to irregularity in the rod feeding speed.
In MIG arc welding of Fe-base members, surface of a Fe-base wire rod for welding is often plated with Cu or coated with a lubricating oil. In contrast, in the welding of Ti which is a labile metal, provision of such Cu plating or coating of such lubricating oil on the surface of the wire rod will not be practical since it may degrade strength of the weld joint due to possible formation of a brittle Cu—Ti-base intermetallic compound or carbide. While there is another long-established measures for stabilizing the arc during welding by introducing carbon dioxide gas or oxygen in the shield gas, such measures is still unsatisfactory since a large amount of oxygen uptake from the shield gas into the welding bead will occur, which may degrade elongation of the weld joint in particular for the case of Ti welding.
Also Ti thermal spraying essentially suffers from the same problem as in the welding. In arc thermal spraying for example, the arc is formed between two Ti-base wire rods, so that irregularity in the feeding speed of either one of such wire rods will vary gap distance for the arc discharge to thereby destabilize the arc. It can thus be concluded that the problem in the arc stability can occur more frequently than in welding.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a Ti-base wire rod for forming molten metal, which is excellent both in feeding smoothness and arc stabilizing property in welding or thermal spraying, and is capable of ensuring desirable mechanical properties of the resultant weld portion, and quality of obtained thermal sprayed layer.
To solve the foregoing problems, a Ti-base wire rod for forming molten metal of the present invention is such that being serially melted from an end user heating to thereby produce molten metal comprising Ti or Ti alloy, wherein at least a portion including the surface of the wire rod comprises Ti metal or Ti alloy mainly composed of Ti, and the surficial portion of the wire rod including the surface thereof has formed therein an oxygen enriched layer having an oxygen concentration higher than that in the inner portion, and having a thickness larger than that of native oxide film possibly formed on the Ti metal in the air under ordinary temperature. The ordinary temperature is, for example, 20° C.
Since Ti is a labile metal, the surface thereof is readily passivated in the air at ordinary temperatures to thereby produce native oxide film. The native oxide film can suppress internal corrosion of the metal, which ensures Ti metal or Ti alloy mainly comprising Ti an excellent corrosion resistance. The native oxide film mainly comprises TiO
2
having an average oxygen concentration of approx. 40 wt %, which concentration is of course higher than that of the inner portion comprising Ti metal. The native oxide film thus can be rated as a kind of oxygen enriched layer. The thickness of the native oxide film is however considerably small, which is as thin as approx. 40 to 100 nm.
What is formed on the surface of the wire rod in the present invention is an oxygen enriched layer having a thickness larger than that of the native oxide film. By intentionally forming on the surface of the wire rod the oxygen enriched layer with the thickness larger than that of the native oxide film, feeding smoothness of the wire rod through a conduit tube or so will considerably be improved, which results in improved stability of arc typically during arc welding or arc thermal spraying. More specifically, this will ensure the following effects (the same will apply to a second aspect of the present invention described later):
(1) improved feeding smoothness of the wire rod through the conduit tube will remarkably reduce apprehensions for clogging or buckling of the wire rod, which successfully reduces frequency of process interruption and raises process efficiency in welding or thermal spraying. More specifically, while the coefficient of dynamic friction of conventional Ti wire rod having a polished surface is 0.5 to 0.6 or around, the present i
Chujoya Makoto
Hanajima Shigeo
Naito Bunji
Noda Toshiharu
Ohashi Kohachiro
Daido Tokushuko Kabushiki Kaisha
Koehler Robert R.
Townsend & Banta
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