Method of control and apparatus for hot-wire welding

A method and an apparatus for controlling the hot-wire welding process is disclosed in which the heating power for a hot-wire is automatically regulated to a proper value. The wire-heating power after fusing of a filler wire by being overheated is set to a level lower than the heating power immediately before wire fusing, and the wire-heating power is again increased gradually from the set value until the wire is fused by being overheated. By repeating this process, the molten state of the filler wire in the hot-wire TIG welding is automatically kept in optimum condition without requiring any operation of the welding operator or his intuition. Further, during the energization of the filler wire, the instant of fusing by overheating of the wire is detected, and the detected signal is used to sharply reduce or suspend the wire current immediately, whereby spatters which otherwise might be caused by fusing of the overheated wire is prevented or greatly reduced, and the adhesion of the spatters to the arc electrode is prevented, thereby permitting a hot-wire welding operation continuously over a long time. Furthermore, a pulsed current is used for heating the filler wire, and the wire terminal voltage during de-energization of the wire is detected, thereby detecting whether the wire is in contact with a base metal. If the wire is not in contact with the base metal, a current is not supplied during the next pulse application period, so that the current is prevented from flowing from a tungsten electrode to the wire while the wire is not in contact with the base metal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a hot-wire welding process, or more in 
particular to a method of control and apparatus for a hot-wire welding 
process in which magnetic arc blow and spatters are eliminated and 
maintain the heating power of the hot-wire at a proper value 
automatically. 
2. Description of the Prior Art 
FIG. 1 shows a configuration of a welding apparatus used conventionally for 
hot-wire TIG welding process. 
A tungsten electrode 2 making up an arc electrode in the TIG welding torch 
1 and a base metal 3 are connected to a welding arc power supply 4 
directly, and an arc 5 is formed with the tungsten electrode 2 as a 
negative electrode in an argon shielding gas. A welding filler wire 6 is 
supplied from a wire feeder 7 through a conduit 8 and a contact tip 9 
connected therewith to an arc forming section and is contacted with the 
base metal 3. The contact tip 9 and the base metal 3 are connected to a 
wire heating power supply 10. A direct or alternating current is supplied 
to the filler wire 6 thereby to generate Joule heat, thus improving the 
melting rate of the filler wire 6. 
The optimum molten state of the filler wire 6 in the hot-wire TIG welding 
process is such that the filler wire 6 is completely molten immediately 
before entering a molten pool 11 and the molten metal continues to drop 
without interruption. In such a case, the metal is melted and the molten 
metal transfers to the molten pool 11 as if hot water is poured from a 
kettle, thus forming a superior bead 12. 
For this purpose, it is necessary to control the filler wire 6 to be 
energized and heated in the extension e between the contact tip 9 and the 
base metal 3 in such a manner as to strike a balance between power 
supplied thereto and the wire melting amount. 
In the hot-wire TIG welding process, however, it is conventionally known 
that with the increase in wire energization current, an electromagnetic 
force is generated with the arc current, which causes what is called 
"magnetic arc blow", making the welding difficult. 
As a measure against this inconvenience traditionally known and employed 
from old days as a common technical knowledge, an arc current as high as 
possible is selected to increase the stiffness of the TIG arc and the wire 
heating current is set to less than one half of the arc current to heat 
the wire with AC rather than DC current. In order to increase the wire 
melting rate, however, it is necessary to increase the wire current. As a 
result, if a proper arc current is selected for a given object of welding 
to obtain a wire melting rate of, say, 20 g/min, a magnetic arc blow may 
occur continuously unless the arc is set to a very short length less than 
1.5 mm, thus making the welding work difficult. This conventional method, 
therefore, has been limited in applications. 
In the prior art, the power for heating the wire is controlled in such a 
manner that while the extension e is kept as constant as possible during 
the welding work, the wire melting conditions are observed by the welding 
operator to adjust the wire power in accordance with the wire feed rate, 
in sole dependence upon the intuition and labor of the welding operator: 
In contrast, the U.S. Pat. No. 4,6l4,856 proposes a method of controlling 
the applied power to a value matching the wire feed rate by measuring the 
power applied to the extension e during the welding work. 
Specifically, in view of the fact that the amount of heat necessary for 
melting the wire is proportional to the applied power in principle, the 
applied power is supplied in proportion to the wire feed rate. 
As a result, by manually adjusting the above-mentioned proportionality 
appropriately, a great variation in extension e or a considerable change 
in wire feed rate in the welding can be met allowable in a considerable 
degree. 
Even though proper conditions are determined in this way to keep subsequent 
conditions, the arc current, arc length or the angle of inserting the 
filler wire 6 into the molten pool 11 cause a change in the amount of heat 
transferred to the filler wire 6 from the arc 5 or the molten pool 11, so 
that the melting condition somewhat changes, thereby causing a deviation 
from the proper meling state. 
If the applied power deviates toward an excessive side for the wire feed 
rate, a so-called spattering phenomenon takes place, and the arc 5 is 
disturbed with a current passing through the arc between the tip of the 
filler wire 6 and the base metal 3 or the tungsten electrode 2. These 
phenomena disturb welding operation very much. In the event that the 
applied power becomes too small for the wire feed rate, on the other hand, 
the apparent arc conditions remain substantially unchanged and the welding 
operator continues to proceed with his work without noticing the condition 
change, with the result that an unmolten wire 13 is left in the deposited 
metal 14 often forming a welding defect as shown in FIG. 2. To prevent 
these troubles, the welding operator is required to observe the molten 
part of the wire or the arc as frequently as possible during this welding 
work, checking to see whether the proper welding conditions have been kept 
to be met while adjusting the applied power. This decision as to whether 
the proper conditions are maintained depends on the intuition of the 
welding operator. 
SUMMARY OF THE INVENTION 
Accordingly, it is the main object of the present invention to provide 
specific means for solving the above-mentioned problems, or in particular, 
to provide specific means for maintaining the melting condition of the 
filler wire at an optimum state fully automatically in the hot-wire TIG 
welding process without relying on the intuition or requiring any 
operation by the welding operator. 
A further object of the present invention is prevented from being formed or 
reduced greatly with hot-wire to provide a welding apparatus in which 
spatters are melting under excessive heating, thereby making hot-wire 
welding possible continued over a long time. 
In order to achieve the above-mentioned main object, according to the 
present invention, there is provided a method of controlling a hot-wire 
welding apparatus configured of an arc power supply and a wire-heating 
power supply, in which the wire-heating power after fusing the wire by 
excessive heat is set lower than the heating power immediately preceding 
to the fusing of the wire, and the wire-heating power is again gradually 
increased from the setting until the wire is again fused under excessive 
heat. 
Further, according to the present invention, there is provided a hot-wire 
welding apparatus configured of an arc power supply and a wire-heating 
power supply, which comprises a wire-heating power control circuit whereby 
a wire-heating power after fusing of the wire under excessive heat is set 
lower than the heating power immediately preceding to the fusing, and the 
wire-heating power is then gradually increased from the setting until the 
wire is again used under excessive heat. 
Also, in order to achieve the further object of the invention, there is 
provided a hot-wire TIG welding apparatus comprising a circuit for 
detecting the time point immediately before fusing or the moment of fusing 
of the wire during energization of the filler wire and a circuit for 
sharply reducing or interrupting the energization of the wire current 
immediately upon receipt of a signal from the fusion detector circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be explained in detail below 
with reference to the accompanying drawings. 
FIG. 3 shows an embodiment of a wire-heating power supply and a control 
circuit according to a first embodiment of the present invention. This 
first embodiment uses a triac system which can be configured at a very low 
cost as a wire-heating power supply for producing a pulse current. This 
circuit is operated as a power supply replacing the wire-heating power 
supply 10 among the component parts of the hot-wire TIG welding apparatus 
of the prior art shown in FIG. 1. 
In this embodiment, as shown in FIG. 3, the primary of a main transformer 
15 is connected to a triac 16, and the secondary thereof to a full-wave 
rectifier circuit including diodes 17, 18, making up a wire-heating 
current-forming circuit 19. This wire-heating current-forming circuit 19 
has the triac 16 controlled by a gate pulse-forming circuit 20 to produce 
a wire current with the AC power supply of commercial frequency subjected 
to phase control. In order to prevent continuous magnetic arc blow or 
mitigate a magnetic arc blow to thereby facilitate the welding work which 
otherwise would be hampered by the magnetic arc blow, the application of a 
gate pulse to the triac 16 is always controlled in such a way that a 
de-energization period lasts from zero degree to 90 degree and from 180 
degree to 270 degree in commercial frequency phase. The phase control 
angle for energizing the triac 16 is thus assumed in the remaining period 
from 90 degree to 180 degree and from 270 degree to 360 degree to energize 
the filler wire 6 shown in FIG. 1 in a manner to produce a wire-heating 
power commensurate with the wire feed rate. 
The wire terminal voltage (output terminal voltage) represented by Vw 
during the wire de-energization period assumes almost 0V as shown in the 
period x of FIG. 4 when the filler wire 6 is in contact with the base 
metal 3, and a negative voltage below -1V by detecting the voltage of a 
plasma flame when the wire tip is away from the base metal 3 and in 
contact with the arc plasma as shown in the period y of FIG. 5. Taking 
advantage of this characteristic, a wire contact detector circuit 21 for 
detecting whether the tip of the wire 6 is in contact with the base metal 
3 is configured appropriately. The outpu voltage Va of the wire contact 
detector circuit 21 takes a "high" (H) level when the wire 6 is in contact 
with the base metal 3, and a "low" (L) level when it is away from the base 
metal. This signal is applied to an on-off control circuit 22 in the gate 
pulse-forming circuit 20 and operates not to form the next energization 
pulse if the wire 6 is detached from the base metal 3. 
The energization phase control voltage-forming circuit 23, on the other 
hand, is supplied with a signal from the voltage ramping rate voltage 
reduction margin setting circuit 24, together with an output voltage Va of 
the wire contact detector circuit 21, and produces a voltage Vb for 
determining the energization phase of the triac 16 to the energization 
phase control circuit 25 in the gate pulse forming circuit 20. This output 
voltage Vb gradually increases when the output voltage Va is high, that 
is, when the wire 6 is in contact with the base metal 3, while when the 
voltage Va is reduced to low level (L), that is, when the wire 6 comes 
away from the base metal 3, the voltage is decreased to a predetermined 
value. 
FIG. 6 shows changes in the voltage signal and output current with time 
diagramatically. The energization phase control circuit 25 in the gate 
pulse-forming circuit 20 controls the firing angle in such a manner as to 
increase the energization current in accordance with the magnitude of the 
input voltage Vb. With the increase in the energization current, the 
resistance of the extension e indicated in FIG. 1 and the wire-heating 
power determined by the energization current for the extension e naturally 
increase, and when the applied wire-heating power increases as compared 
with the wire feed rate, the wire 6 is overheated and molten like a fuse. 
The wire 6, once molten, comes away from the base metal 3, and the 
resultant change in the output voltage Vw is detected by the wire contact 
detector circuit 21, thereby reducing the output Va to low level. As a 
result, the wire energization is prohibited, and the energization phase 
control voltage-forming circuit 23 reduces the output voltage Vb by a 
predetermined value. When the tip of the wire 6 comes into contact with 
the base metal 3 and the voltage Va rises to high level to resume the wire 
energization, the power applied to the wire 6 is set to a value slightly 
lower than the optimum heating power required for the wire feed rate. 
In this operation, if the wire feed rate is reduced during the welding 
work, the wire 6 tends to be overheated, and therefore the wire is fused 
so frequently that the voltage Vb gradually drops, resulting in the 
wire-heating power being reduced. If the wire feed rate increase, by 
contrast, the wire-heating power tends to be insufficient, and therefore, 
although the wire is not fused, the energization phase control 
voltage-forming circuit 23 operates to increase the voltage Vb gradually, 
with the result that the wire-heating power continues to increase until 
the wire is fused. 
According to the present invention, the functions remain the same even when 
the wire extension e, changes. Specifically, with the increase in the 
extension e, the resistance value therethrough increases. When the 
wire-heating power supply has a constant-voltage output characteristic, 
therefore, the crest value of the current decreases given a constant 
energization phase, so that the output power decreases, thus making fusing 
difficult. As a result, the output voltage Vb gradually increases to 
increase the energization phase, thereby leading to a heating power 
commensurate with the wire feed rate. When the extension e shortens, by 
contrast, the resistance value in the extension e is decreased, and for 
the same energization phase the current crest value increases, causing an 
excessive applied power to facilitate the fusing Each time of the 
detachment the voltage Vb drops and the energization phase decreases, with 
the result that the initial heating power commensurate with the wire feed 
rate is achieved. 
The degree of Vb drop each time of wire detachment, or the rate at which 
the output voltage Vb increase can be determined by controlling the 
variable resistor provided in the voltage ramping rate/reduction degree 
setting circuit 24 for selection and control at a proper value before or 
during the welding process. 
In this manner, the control operation is performed following the changes in 
wire feed rate and extension e during the welding operation, and 
therefore, it is possible to keep a value very near to the proper wire 
heating power automatically. 
FIG. 7 is a diagram showing a configuration of the wire contact detector 
circuit 21 of FIG. 3. 
In FIG. 7, the AC source voltage Vin of commercial frequency is applied to 
a zero-cross signal forming circuit 211 thereby to form a pulse signal for 
phase zero. On the basis of this signal, a wire de-energization period 
setting circuit 212 produces a "high" signal for the period of .pi./2, 
during which a sample-hold circuit 213 keeps the wire voltage Vw 
conducting and applies it to a comparator circuit 214. Then, when the wire 
energization period is started and the output of the wire de-energization 
period setting circuit 212 becomes "low", the immediately-preceding value 
of Vw is held by a sample-hold circuit 213. The comparator circuit 214 
compares the signal from the sample-hold circuit 213 with the voltage from 
the reference voltage setting circuit 2l5, to determine whether the wire 6 
is contact with the base metal 3, and forms an output signal Va. 
FIG. 8 is a diagram showing a configuration of the energization phase 
control voltage-forming circuit 23 and the voltage ramping rate/voltage 
reduction range setting circuit 24 shown in FIG. 3. 
In FIG. 8, the output Va of the wire contact detector circuit 21 is applied 
to a timer T in the energization phase control voltage forming circuit 23. 
The time T produces a signal Q which becomes "high" when the output Va 
changes from "high" to "low" level, and a signal Q subject to the opposite 
change. When the signal Q is "high", a first analog switch AS1 in the 
energization phase control forming circuit 23 turns on, so that a 
low-voltage source E, a first voltage ramping rate control VR1 and a 
capacitor C ae connected in series thereby to promote the charging of the 
capacitor C. 
In FIG. 1, when the wire 6 coming away from the base metal 3 is detected to 
such an extent that the output Va becomes "low", the signal Q becomes 
"high" during the period t.sub.0, and the second analog switch AS2 turns 
on, so that the capacitor C is grounded through a second voltage reduction 
range setting resistor VR2 and thus proceeds to discharge, with the result 
that the voltage across the capacitor C drops. The voltage of the 
capacitor C is produced as an energization phase control voltage Vb 
through an amplifier Amp 1 inserted in the energization phase control 
voltage forming circuit 23. 
FIG. 9 shows the manner in which the output voltage Va, Vb and the analog 
switches AS1, AS2 change with time. 
In the apparatus shown in FIG. 3, the cycle of fusing by overheating the 
wire 6 depends on mainly the change in the power applied to the wire 6 and 
the feed rate of the wire 6. In order that the fusing of the wire 6 by 
overheating may be accompanied by very small spattering, it is desirable 
to reduce the frequency thereof. For this purpose, in the event that it is 
necessary to meet rapid changes in extension e or the rate of feed of the 
wire 6, the rate of increase and the degree of reduction in the voltage V 
is increased thereby to cause frequent fusing of the wire 6. In the case 
where a change in the extension e or the feed rate of the wire 6 is not 
frequent, on the other hand, the increase rate and the degree of drop of 
the voltage Vb is reduced thereby to reduce the frequency of fusing by 
overheating of the wire 6. The change in the power applied to the wire 6 
is determined normally by manual control of the variable resistor of the 
voltage ramping rate/reduction range setting circuit 24 before or during 
the welding process. This control may also be performed automatically by 
means of a circuit for controlling the wire-heating power in such a manner 
that the intervals of fusing by overheating of the wire 6 approaches a 
designated value. In this case, a time interval of fusing by overheating 
of the wire 6 so predetermined, so that when the actual fusing interval is 
longer than the predetermined value, the wire-heating power is increased 
at a higher rate, while if the actual fusing interval is short, the 
wire-heating power is increased at a lower rate. 
In similar fashion, automatic control of the applied power for heating the 
wire 6 is alternatively possible by a circuit for controlling the 
wire-heating power in such a way that the interval of wire fusing under 
excessive heat is adapted to approach a predetermined value, in which an 
interval of wire fusing by overheating is predetermined and when the 
actual interval of fusing is longer than the predetermined value, the 
power for heating the wire 6 is increased by a predetermined amount, while 
the actual interval is shorter, the wire-heating power is reduced by a 
predetermined amount. 
If a high wire current flows the moment of fusing of the wire overheated, 
spatters are liable to form. If the wire current is comparatively low 
below 100A, or no current flows, on the other hand, no spatter is formed. 
In the case where a wire-heating power supply is used for producing a 
pulsed current, it is possible to reduce the spatter formed remarkably by 
use of a circuit for controlling the fusing by overheating of wire or the 
detachment of the wire from the base metal to occur during the period when 
the wire current is sufficiently low during or immediately before 
suspension of wire energization. This is made possible by designating a 
cycle of wire fusing by overheating and control in such a manner that the 
occurrence of fusing coincides with the start of the suspension of wire 
energization. 
Further, the method according to the present invention, in which the 
phenomenon of the filler wire 6 detaching from the base metal 3 or the 
phenomenon that occurs during detachment of the filler wire 6 from the 
base metal 3 by heating by energization is detected electrically, is of 
course applicable, by changing the proportionality constant, to a control 
apparatus proposed by U.S. Pat. No. 4,614,856 in which the heating power 
is controlled to produce a wire-heating power by measuring the power 
actually applied to the extension e and controlling it to a value 
proportional to the feed rate of wire 6. Although resultant control 
apparatus is considerably complicated, the advantages are obtained that 
the response to a sudden change in the feed rate of wire 6 or the 
extension e is further improved. 
In similar manner, power control is also possible by configuring a control 
circuit by which the square of the effective value of the wire-heating 
current is proportional to the wire feed rate and the proportionality 
constant thereof is changed in accordance with the method of the present 
invention. In this way, the response to an abrupt change in the extension 
e of the filler wire 6 is performed by the means according to the present 
invention, and therefore is inferior to the response in the 
above-mentioned case. In spite of this, the response to the change in wire 
feed rate remains superior as in the above-mentioned embodiment. The 
advantage in this case is that the need of detecting the wire voltage from 
a point very near to the ends of the extension e in order to detect the 
wire-heating power in the U.S. Pat. No. 4,614,854 described above, which 
is very troublesome in the welding work, is eliminated. 
The present invention, of which description is made above with reference to 
a wire-heating power supply with a system using the triac 16, is not 
limited to such a method, but the invention is applicable to a method 
using a power supply for pulse energization heating system provided with a 
wire-heating current suspended for a given period of time. FIG. 10 shows a 
wire terminal voltage of a system using an inverter-type power supply as a 
second embodiment of the present invention. During section d in FIG. 10, 
the steady welding work is proceeding with the wire 6 in contact with the 
base metal 3 and the voltage remains virtually zero during the wire 
current suspension. During the period j, the wire 6 is detached from the 
base metal 3, and during the wire current suspension period (f), the tip 
of the wire detects the potential in the arc plasma. When the detachment 
of the wire tip from the base metal according to this invention is 
detected, the energization at next pulse period (during the section g) is 
interrupted, thereby preventing the disturbance of the arc. 
The foregoing description concerns a method of control to keep condition of 
almost proper heating power by causing complete overheating of the filler 
wire to the point of fusing. A fusing frequented by spattering, however, 
is not desirable. To obviate this, another method of control is proposed, 
in which the moment immediately before fusing is detected from a change in 
wire current, wire voltage or output terminal voltage or the wire 
resistance value determined from it. 
FIG. 11 shows a third embodiment of the wire-heating power supply and the 
control circuit 3 according to the present invention, in which a 
controller 26 for detecting the condition immediately before fusing is 
added to the wire-heating power supply and the control circuit included in 
FIG. 3. 
In FIG. 11, the same reference numerals as in FIG. 3 designate the same 
component parts as in FIG. 3. 
In FIG. 11, the controller 26, in response to an output signal Vc of a 
current differentiation sensor 27 for producing a differentiated waveform 
of the wire current Iw, detects the condition immediately before fusing 
from the change in output signal Vc, and sends a firing signal to the 
gates of thyristors 28, 29 directly coupled to the secondary of the main 
transformer 15. At the same time, a signal Vd is applied to the on-off 
control circuit 22 thereby to suspend energization of the triac 16 until 
the lapse of a predetermined time period. This predetermined time period t 
is set with reference to the signal Va from the wire contact condition 
detector circuit 21 as a time length from re-contact of the wire 6 with 
the base metal 3, if away therefrom, or from the detection of the point 
immediately before fusing, if not away therefrom, until the wire 6 is fed 
by 0.5 mm. In this way, the output of the main transformer 15 is shunted 
into the thyristors 28 and 29 immediately before fusing, resulting in a 
sharp reduction in output current, so that the overheated wire 6 is fused 
off sharply in a manner not to form any spatter. Energization is resumed 
with the wire 6 fully in contact in the molten pool, and the applied power 
involved is slightly reduced, thereby controlling the heating power in a 
manner similar to the above-mentioned case of fusing. 
FIG. 12 shows changes with time of the heating current Iw for the additive 
wire 6, the output terminal voltage Vw of the heating power supply, the 
current It flowing in the thyristor 28 and the output voltage Vc of the 
sensor 27 for producing a differentiation of the wire current Iw for the 
apparatus shown in FIG. 11. At the time point a immediately before fusing 
by excessive heat, the resistance value of the filler wire 6 in the 
extension e sharply increased due to the rise in specific resistance with 
temperature increase and reduction in sectional area with the result that 
the wire current Iw sharply decreases, and at the same time a lower load 
reduces the voltage drop in the power supply, resulting in an increased 
output terminal voltage Vw. 
The current waveform with the wire 6 energized in contact with the base 
metal 3 is shown by k in FIG. 12, and a differentiated waveform thereof 
designated by numeral l. When the wire 6 is almost near the fusing state 
by being overheated during energization of the wire 6, the wire current 
decreases considerably quicker than under normal conditions. As a result, 
the differentiation waveform of the wire current detected by the current 
differentiation sensor 27, unlike the wire energization waveform 1 under 
normal conditions, takes a form of a sharp high level as shown by n. A 
comparator with a threshold level slightly higher than the output under 
normal conditions of the differentiation sensor 27 is disposed in the 
fusing detection control circuit 26 and supplied with the signal output n, 
thereby easily detecting that the wire 6 is likely to be fusing. 
In view of the fact that the wire resistance sharply increases causing a 
change in current and voltage in this way immediately before fusing, the 
condition immediately before fusing can be detected with considerable 
accuracy. 
In FIG. 12 when it is detected that the condition immediately before fusing 
exists near the time point a, the energization of the wire is stopped at 
once. At this time the wire is retained to be contacted with the base 
metal 3, and the wire is detached from the base metal 3 at the time point 
b. As the wire is not energized at the time point b, any spatter is not 
generated when the wire is detached from the base metal. 
Actually, in the prior art, it takes 0.8 milli-seconds for the wire current 
to be reduced to zero from 0.8 ms fusing start. According to third 
embodiment using the shorting of a thyristor by contrast, the wire current 
is reduced to zero within 0.1 millisecond after fusing start. 
As explained above, according to the third embodiment of the present 
invention, the wire current is reduced to zero within a very short time a 
compared with the conventional method after wire fusion, and therefore the 
spatters, which otherwise is caused by fusing of the overheated wire 6, is 
extremely reduced. In an advantage specific to the third embodiment, the 
wire 6 is fused to place the thyristor in conductive state, so that the 
energization continues till the zero cross point with a current waveform 
substantially similar to the normal waveform of the curren flowing in the 
main transformer 15, after which the energization is prohibited at the 
primary triac 16 again before the tip of the wire 6 comes into contact 
with the base metal 3. As a result, the imbalance in current between the 
positive and negative half waves of the current flowing in the main 
transformer 15 is reduced further, thereby leading to the advantage of 
lesser cases of magnetic polarization the main transformer due to the DC 
components. 
FIG. 13 shows a fourth embodiment of the wire-heating power supply and the 
control circuit according to the present invention. A triac 35 is 
connected to the secondary output terminal of a main transformer 15 and 
the secondary output immediately after fusing is shorted at the moment of 
the fusing to reduce the current supplied to the wire. 
The present invention is not limited to the above-mentioned wire-heating 
power supply using a triac in most cases, but may be applied with equal 
effect to a wire-heating power supply of inverter type, etc. using a 
transistor. 
As a fifth embodiment of the present invention, FIG. 14 shows an example of 
a wire current waveform Iw and an output voltage waveform Vw of a circuit 
of inverter type used as a wire-heating power supply. These waveforms 
basically take a current waveform similar to square wave, and the wire 
current is reduced considerably rapidly at the time of fusing of the wire 
energized and overheated. Therefore, it is also possible to detect near 
the moment of the fusing easily from a differentiated current waveform, 
thus making it possible to de-energize the wire 6 promptly by use of the 
primary transistor for high-speed switching such as at 20 KHz. In spite of 
the periodical reduction in current due to a pulse current form, the 
normal pulse current form is not affected at all even if the periodical 
reduction is detected erroneously as an overheated fusing signal. 
Apart from the foregoing description of the detection of fusing by 
overheating of the wire 6 from a differentiate waveform of the wire 
current, similar detection is also possible from the change in wire 
voltage Vw as shown in FIG. 14 in the case where the wire current takes a 
form similar to a square wave like in the inverter power supply. 
Specifically, when the secondary current decreases with the approach of 
fusing by overheating, the voltage drop in the heating power supply and in 
the power cables decreases and the secondary output terminal voltage of 
the power supply rises, so that detection is possible from a change in the 
differentiated or absolute value of the output voltage immediately before 
or at the moment of fusing. 
Also, the invention may be embodied as a filler wire heating power supply 
for producing a continuous AC current or continuous DC current has so far 
been normally used for the hot-wire welding process. As still another 
alternative, a filler wire-heating power supply for producing an AC pulse 
current may be also used. 
Instead of the above-mentioned combination of the hot-wire heating power 
supply with the TIG arc for hot-wire TIG arc welding process, the present 
invention may be applied also to a hot-wire welding process combining a 
consummable electrode arc for the hot-wire welding process in which 
spatter does not damage the electrode, thus substantially eliminating 
spatters from the hot wire. 
The wire terminal voltage Vw, which is generally detected between the base 
metal 3 and a contact tip for energizing the wire 6, may alternatively be 
detected as an output terminal voltage of the wire-heating power supply. 
It will thus be understood from the foregoing description that according to 
the present invention, manual operation is substantially eliminated for 
keeping an optimum wire-heating power in the hot-wire welding process. 
More specifically, a proper wire-heating power can be stably and 
automatically maintained regardless of changes in such conditions a 
welding factors for the hot-wire welding process including arc current, 
arc length, material or shape or feed rate of the filler wire, extension 
or the position or angle of insertion into the molten pool. As a 
consequence, the disadvatage of the hot-wire TIG welding process in which 
an unmolten wire liable to be formed remains in the weld metal is 
completely eliminated. Further, a proper heating power is kept 
automatically by matching these various chnages in welding conditions, 
thereby greatly facilitating the semi-automatic hot-wire welding operation 
which has so far encountered considerable difficulties. 
Further, in the prior art control system for supplying the wire-heating 
power corresponding to the wire feed rate, a power detection device 
comprising a Hall element or the like for detecting the wire-heating power 
in addition to a wire feed rate detector, and also, it is necessary to 
detect the wire voltage by such a detector placed as near to the ends of 
the extension as possible to determine a heating power as accurate as 
possible, thereby complicating the wiring work for the welding process. In 
many embodiments of the present invention, in contrast, detection of the 
wire-heating power is not required, but it is only necessary to detect the 
output terminal voltage of the wire-heating power supply in place of the 
voltage across the extension, thus greatly simplifying the control device 
and reducing it's cost. 
Furthermore, according to the present invention, a proper wire-heating 
power is maintained full automatically, and spatters are virtually 
eliminated, thereby making possible continuous hot-wire welding work over 
a long time.