Electric heating – Metal heating – Nonatmospheric environment at hot spot
Reexamination Certificate
1998-03-20
2002-10-08
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
Nonatmospheric environment at hot spot
C219S074000, C219S086800, C219S086250, C219S086330
Reexamination Certificate
active
06462298
ABSTRACT:
DESCRIPTION OF BACKGROUND OF THE INVENTION AND RELATED TECHNOLOGY
1. Technological Field
The present invention relates to a long-life welding electrode and a fixing structure for the same, a welding head, and to a welding method.
2. Background Technology
Conventionally, types of electrodes for welding are divided to those each having a sharp tip and those each having a flat tip. And in electrodes for welding having a sharp tip, the form remarkably changes and the durability is low, while, in those each having a flat chip, the form little changes, but the arc discharge characteristic is remarkably degraded, and for the reasons as described above, in both types of electrode for welding, a frequency of exchange is high and the work efficiency is low.
In a conventional type of electrode
901
having a flat tip as shown in FIG.
9
(
b
), there are a number of points at which a distance between the welding electrode
901
and a welded object
910
becomes shortest (arc grounding point:), and a point, from which arc discharge is generated (arc discharge point: ∘), is generated at various points, so that an arc used for welding can not be kept stable.
Also, a welding electrode is worn out along a form of an equipotential surface, and it has been found out that a welding electrode having a sharp tip (with an angle from 30 to 60 degrees), is easily worn out and it is impossible to executed welding for a long time under stable conditions.
On the other hand, it has been tried to develop a technology of adding around 2 weight % of ThO
2
(thoria) in a mother material for a welding electrode (W) for the purpose to improve durability of the electrode.
When thoria is added, sometimes the durability may be improved, but the durability is not always improved. That is to say, the effect provided by adding thoria is not constant.
Conventionally, a welding electrode is fixed with a screw. That is to say, as shown in
FIG. 2
, a welding electrode
201
is fixed to a fixing base
202
by inserting the welding electrode
201
into an inserting section
204
of a fixing base
202
having the inserting section
204
and also passing a setting screw
203
through a screw hole provided in a side of the fixing base
202
.
With the conventional type of fixing structure as described above, however, a welding electrode is degraded.
Generally argon is used as a welding gas, and argon is poor in its thermal conductivity, and high current value is required for welding, so that a temperature of the welding electrode increases and durability of the electrode is worsened.
Further, a welding gas is supplied via a welding power supply unit with a resin-based material emitting a large quantity of gas used in piping in the power supply unit, and also a resin-based material is used for a tube for connection among a gas supply system, a welding power supply unit, and a welding head, so that atmosphere is worsened during welding and the welding electrode may be oxidized and degraded (Refer to FIG.
6
).
Engineers are required for replacement of a degraded electrode, and many engineers are required for the conventional type of electrodes each frequently requiring replacement, and also a long time is required for this work for replacement, so that the work efficiency is low, which does now allow welding with high reliability.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide an electrode for welding and a fixing structure for the same, a welding head, and a welding method which make it possible to improve durability of a welding electrode, because of forms and materials thereof as well as of atmosphere for welding, to largely reduce a frequency of exchange of welding electrodes and also reduce a number of engineers and a time required for exchanging welding electrodes, to improve the work efficiency, and to enable wending with high reliability for a long time.
(1) A welding electrode according to the present invention is characterized in that a tip section thereof has a curved surface. More specifically, this curved surface should preferably be like a form of equipotential surface which is vertical against an electric line of force generated between a welding electrode and a material to be welded.
With the configuration as described above, as shown in FIG.
9
(
a
), an arc discharge position is kept constant, and wearing of the electrode can be suppressed by uniformly generating a current generated from a welding electrode, which makes it possible to improve durability of the welding electrode.
It is preferable from the view point of further improving durability of a welding electrode that the curved surface is an arch-formed one with a diameter from 0.05 mm or more to 0.3 mm or less.
(2) The welding electrode according to the present invention is characterized in that at least one type of oxide selected from a group comprising lanthana, yttrium, and ceria is added to a mother material for the electrode.
The present inventors made strenuous efforts to understand how a life of a welding electrode is decided. As a result, it was found out that a life of wiring (&tgr;) used in the semiconductor industry is applicable to a life of a welding electrode, and the equation for a life of wiring (&tgr;) is as follows:
&tgr;=(
E
0
/(&rgr;
J
2
))exp(
Ea/kT
)
wherein J indicates a current density, &rgr; indicates are sistivity of wiring, E
0
indicates a constant specified to wiring, K indicates a Boltzmann constant, T indicates a temperature of wiring, and Ea indicates energy for activation.
Herein, assuming that a material for the welding electrode is constant, a resistivity &rgr; of the welding electrode, E
0
, Ea, and k are kept unchanged, and also assuming that the temperature T on the welding electrode when a type of welding gas and a welding current are changed with a distance between the welding electrode and an object to be welded and a melting area of the object to be welded kept unchanged is constant, a life (&tgr;) of the welding electrode is expressed by the following equation:
&tgr;=(1
/J
2
)
A
wherein A=(E
0
/&tgr;) exp (Ea/kT), and from this expression it can be understood that the life of a welding electrode is inverse proportion to a square of the current density. Herein, if a form of the welding electrode's tip is as shown in FIG.
1
and is kept unchanged, it is possible to substitute the current density for a current value, and also as shown in
FIG. 3
describing below, if a distance between the welding electrode and an object to be welded is fixed and a melting area of an object to be welded and a form of the welding electrode are kept unchanged, a life of the welding is decided only by a current value.
Also, a current value during welding is lowered because, when a gas with a high thermal conductivity (such as hydrogen or helium) is added, an arc polar is made thinner because of the thermal pinch effect and a density of electrons irradiated to the object to be welded becomes higher.
As described above, a life of a welding electrode is decided by a current density, and as a current density is smaller, a life of a welding electrode becomes longer.
A current density is expressed by the expression of J=AT
2
exp (−&PHgr;/kT). Herein J indicates a current density, A indicates a thermoelectron discharge constant, T indicates a temperature of an electrode, K indicates a Boltzmann constant, and &PHgr; indicates a work function. For this reason, to improve the thermoelectron performance during arc discharge, a work with a small work function should be used. For that purpose, an oxide with a small work function may be added to a mother material for a welding electrode. However, if a melting point of the added oxide is low, the oxide is evaporated during welding, and when the welding electrode is used frequently, the electrode is degraded. So by using an oxide with a small work function and also with a high melting point and a high boiling point to improve the thermoelectron discharge performance as well as to prevent evaporation of a weldi
Nakamura Osamu
Nitta Takahisa
Ohmi Tadahiro
Shirai Yasuyuki
Fujikin Inc.
Knuth Randall J.
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