Arc welding machine and method

An arc welding machine includes a transformer a diode for rectification connected to the secondary winding of the transformer, two capacitors connected in series with each other and in parallel with the diode, and the first and second switching elements connected in series with each other and in parallel with the diode. This machine also includes an electrode having one end connected at a location intermediately of the two switching elements and the other end opposed to an object to be welded, first and second drive circuits connected to the first and second switching elements, respectively, an oscillating circuit for determining the width of pulses of the first and second drive circuits, and a timer circuit for inputting to the oscillating circuit signals required to make the oscillating circuit periodically repeat oscillating and stop operations. The object is connected at a location intermediately of the two capacitors. The second drive circuit outputs inverted signals of the first drive circuit. By this construction, an alternating current and a direct current alternately flow between the electrode and the object during welding.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to an arc welding method suited for 
factory automation, and more particularly to an arc welding method suited 
for use in welding a material such as, for example, aluminum or magnesium, 
which requires the removal of an oxide layer during welding. The present 
invention also relates to an arc welding machine for effecting this 
method. 
2. Description of the Prior Art 
FIG. 1 schematically depicts a conventional arc welding machine for 
effecting arc welding processes in an inert gas (not shown) with an 
alternating current being supplied from a power supply system 4 so as to 
flow between an electrode 1 and an object 3 to be welded. In a period 
represented by T.sub.ED, the electrode 1 is negative whereas the object 3 
is positive. This period is characterized in that the penetration is deep, 
the rate of consumption of the electrode 1 is small, and arc sounds are 
small, as shown in Table 1. 
On the other hand, in a period represented by T.sub.EF, the electrode 1 is 
positive whereas the object 3 is negative. When the material of a base is 
aluminum, magnesium or the like, an oxide obstructs welding. The reason 
for this is that the melting point of the oxide is higher than that of the 
base. The period T.sub.EP is characterized by an oxide removal action. 
TABLE 1 
______________________________________ 
Oxide Layer Consumption of 
Arc 
Removal Action Penetration 
Electrode Sound 
______________________________________ 
T.sub.EN 
No Deep Small Small 
##STR1## 
##STR2## 
T.sub.EP 
Yes Shallow Large Large 
##STR3## 
##STR4## 
______________________________________ 
As described above, in applications where the melting point of an oxide is 
higher than that of a base, welding is performed by the use of an 
alternating current with T.sub.EN and T.sub.EP being alternated. In 
general, the alternating current required for the welding has a frequency 
of about 100 Hz, and the magnitude thereof is slightly greater during 
T.sub.EN than during T.sub.EP. 
However, this kind of conventional arc welding method employing an 
alternating current during welding is not suited for use in fillet welding 
or Uranami welding (penetration welding) because the directivity of arcs 
is not stable. Furthermore, higher harmonics caused by the inversion 
enlarges arc sounds, and the electrode heating action during T.sub.EN 
results in rapid consumption of the electrode. 
SUMMARY OF THE INVENTION 
The present invention has been developed to overcome the above-described 
disadvantages. 
It is accordingly an object of the present invention to provide an improved 
arc welding machine and method which is superior in the directivity of 
arcs and can obtain deep penetration. 
Another object of the present invention is to provide an arc welding 
machine and method of the abovedescribed type which can reduce the rate of 
consumption of an electrode and arc sounds. 
In accomplishing these and other objects, an arc welding method according 
to the present invention comprising the steps of: producing an alternating 
current; periodically converting the alternating current to a direct 
current; and alternately applying the alternating current and the direct 
current to an object to be welded. The frequency of alternation of the 
alternating current and the direct current is rendered to be 0.5-10 Hz 
whereas the ratio of periods during which the alternating current flows 
through the object is rendered to be 30-80%. 
In order to effect the above-described method, an arc welding machine 
according to the present invention comprises a transformer having a 
primary winding and a secondary winding, a rectification means connected 
to the secondary winding of the transformer, two capacitors connected in 
series with each other and in parallel with the rectification means, and 
first and second switching elements connected in series with each other 
and in parallel with the rectification means. This machine further 
comprises an electrode having one end connected at a location 
intermediately of the two switching elements and the other end opposed to 
an object to be welded, a first drive means connected to the first 
switching element, a second drive means connected to the second switching 
element, an oscillating means for determining the width of pulses of the 
first and second drive means, and a timer means for inputting to the 
oscillating means signals required to make the oscillating means 
periodically repeat oscillating and stop operations. The second drive 
means outputs inverted signals of the first drive means. The object to be 
welded is connected at a location intermediately of the two capacitors. 
By the above-described construction, an alternating current and a direct 
current alternately flow between the electrode and the object during 
welding. 
Preferably, the arc welding machine also comprises a pulse-width regulating 
means for regulating the width of pulses of the oscillating means. 
According to the present invention, because the effect of the direct 
current is added to that of the alternating current, the pole at a 
hot-cathode is rendered to be stable, thereby enhancing the directivity of 
arcs and producing the cooling effect of the electrode. Furthermore, 
because the number of inversion of the electric current is small, arc 
sounds are relatively small. Also, the alternation of the alternating 
current and the direct current produces nicelooking wavelike beads because 
the features of the former are wide beads and shallow penetration whereas 
those of the latter are narrow beads and deep penetration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 schematically depicts an arc welding machine according to the 
present invention. This machine is provided with a power supply system 14, 
which produces a waveform different from that of the conventional power 
supply system 4 shown in FIG. 1. The power supply system 14 alternately 
supplies an alternating current and a direct current during a period 
represented by T.sub.AC and during a period represented by T.sub.DC, 
respectively, as shown in FIG. 3. 
The ratio of the period of the alternating current and the frequency of 
alternation of the alternating current and the direct current are 
expressed by: 
EQU T.sub.AC /(T.sub.AC +T.sub.DC).times.100, and 
EQU 1/((T.sub.AC +T.sub.DC), respectively. 
The relationship among the width of beads the width of cleaning, the depth 
of beads, and the rate of consumption of an electrode 1 was investigated 
with the ratio of the period of the alternating current and the frequency 
of alternation of the alternating current and the direct current being 
changed. The width of cleaning is the width of that portion of an object 3 
to be welded from which an oxide layer has been removed. 
FIG. 4 is a graph obtained as a result of experiments in which the speed of 
welding and the frequency of the alternating current were set to 30 cm/min 
and 100 Hz, respectively, with an electric current of 200 amperes being 
applied to an aluminum plate of 6 mm thick. 
With the increase of the ratio of the period of the alternating current, 
both the width of cleaning and the width of beads become wide whereas the 
depth of beads becomes shallow. On the other hand, the rate of consumption 
of the electrode 1 increases. In view of this fact, the lower limit and 
the upper limit in the ratio of the period of the alternating current were 
respectively set to 30% above which the width of cleaning is wider than 
the width of beads and to 80% below which the increase in the rate of 
consumption of the electrode 1 is small and the depth of beads is not so 
shallow. 
FIG. 5 is a graph indicative of the relationship between the depth of beads 
and the frequency of alternation of the alternating current and the direct 
current. Similar to the experimental conditions set to obtain the result 
shown in FIG. 4, welding was performed at a speed of 30 cm/min with an 
electric current of 200 amperes being applied to an aluminum plate of 6 mm 
thick. The ratio of the period of the alternating current was set to 50%. 
When the frequency of alternation is below 10 Hz, the beads present a 
wavelike external appearance, as shown in FIG. 6. When the distance 
between two adjoining waves and the speed of welding are rendered to be l 
and V, respectively, a relationship expressed by the following formula 
establishes. 
EQU l=V.times.(T.sub.AC +T.sub.DC) 
Accordingly, if the frequency of alternation is increased, the pitch of 
waves becomes narrow. In general, V=30 cm/min. Accordingly, if the 
frequency of alternation is greater than 10 Hz i.e., if 1/(T.sub.AC 
+T.sub.DC)&gt;10 Hz, l=0.5 mm, and therefore, it is considerably difficult to 
obtain wavelike beads. 
FIG. 7 is a circuit diagram embodying an arc welding machine according to 
the present invention. This circuit includes a transformer 7 having a 
primary winding and a secondary winding, a diode 5 for rectification 
connected to the secondary winding of the transformer 7, two capacitors 6a 
and 6b connected in series with each other and in parallel with the diode 
5, and two switching elements 8a and 8b connected in series with each 
other and in parallel with the diode 5. The switching elements 8a and 8b 
are connected with respective drive circuits 9a and 9b. The drive circuit 
9a is directly connected with an oscillating circuit 10 so that output 
pulses from the oscillating circuit 10 may be inputted thereinto. On the 
other hand, the drive circuit 9b is connected with the oscillating circuit 
10 via an invertor 12 so that the output pulses from the oscillating 
circuit 10 may be inverted in polarity by the invertor 12 prior to the 
input thereof into the drive circuit 9b. The oscillating circuit 10 
determines the period of pulses of the drive circuits 9a and 9b. To the 
oscillating circuit 10 are connected a timer circuit 11 periodically 
repeating ON and OFF and a pulse-width regulating circuit 13 for 
regulating the width of pulses generated by the oscillating circuit 10. A 
load for welding i.e., an object 3 to be welded is connected at a location 
intermediately of the two capacitors 6a and 6b whereas one end of an 
electrode 1 is connected at a location intermediately of the two switching 
elements 8a and 8b. The other end of the electrode 1 is opposed to the 
object 3 so that arcs 2 may be generated during welding. 
The oscillating circuit 10 is operable in association with the operation of 
the timer circuit 11. When the timer circuit 11 is ON, the oscillating 
circuit 10 oscillates. In contrast, when the timer circuit 11 is OFF, the 
oscillating circuit 10 stops oscillations. The timer circuit 11 and the 
oscillating circuit 10 repeat such operations. 
When the timer circuit 11 is ON, the drive circuits 9a and 9b output 
signals to the switching elements 8a and 8b, respectively, based on 
signals outputted from the oscillating circuit 10, thereby alternately 
turning on and off the switching elements 8a and 8b. In this way, the 
alternating current flows during the period T.sub.AC, as shown in FIG. 3. 
In contrast, when the timer circuit 11 is OFF, the oscillating circuit 10 
stops oscillations. Thereafter, the switching element 8a is kept on 
whereas the switching element 8b is kept off. Accordingly, the electric 
current flows in the direction of capacitor 6a - object 3 - arc load 
2.fwdarw.electrode 1.fwdarw.switching element 8b. This means that the 
direct current flows during the period T.sub.DC with the electrode 1 being 
negative, as shown in FIG. 3. 
As described above, according to the present invention, the alternating 
current and the direct current alternately flow in compliance with ON and 
OFF of the timer circuit 11. Accordingly, the ratio of the period of the 
direct current (=T.sub.OFF /(T.sub.ON +T.sub.OFF)=T.sub.DC /(T.sub.AC 
+T.sub.DC)) can be freely selected in accordance with the setting of the 
time period of the timer circuit 11. 
Even if the ratio of the period of the direct current is fixed, the width 
of pulses can be narrowed during the oscillating operations of the 
oscillating circuit 10 by the operation of the pulse-width regulating 
circuit 13. By doing so, the width of pulses of the drive circuit 9a can 
be narrowed whereas that of the drive circuit 9b can be widened. 
Accordingly, because the ON-period of the switching element 8b is 
extended, the ratio of the period during which the electrode 1 is negative 
is enlarged, as similar to the case where the ratio of the period of the 
direct current is enlarged. As a result, the ratio of the period of 
hot-cathode discharge becomes large, and the directivity and the stability 
of arcs are enhanced. Also, the electrode heating effect becomes small, 
thereby restraining the rate of consumption of the electrode 1. 
As is clear from the above, the arc welding method according to the present 
invention utilizes the welding current periodically alternating the period 
of the alternating current and that of the direct current. In this method, 
the frequency of alternation and the ratio of the period of the 
alternating current are rendered to be 0.5-10 Hz and 30-80%, respectively. 
Because the directivity of arcs are enhanced, this method is suited for 
use in fillet welding or Uranami welding. Furthermore, the rate of 
consumption of the electrode and arc sounds are relatively small. In 
addition, beads present a nicelooking wavelike external appearance. 
Because of these reasons, the method according to the present invention is 
suitable for factory automation. 
Although the present invention has been fully described by way of examples 
with reference to the accompanying drawings, it is to be noted here that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless such changes and modifications otherwise depart 
from the spirit and scope of the present invention, they should be 
construed as being included therein.