DESCRIPTION
ARC WELDING METHOD AND ARC WELDING DEVICE
TECHNICAL FIELD
The present invention relates to a method and apparatus for arc
welding in which welding is performed by alternating a short-circuit
state and an arc state.
BACKGROUND ART
A welding operation includes a sputter removal process, which
is considered a time inefficient process. To reduce the numbers of
times of the sputter removal process, it is necessary to reduce
sputtering. A well-known technique to reduce sputtering is a
consumable electrode arc welding in which welding is performed by
feeding a welding wire in alternating forward and backward directions
and by alternating a short-circuit state and an arc state (see, for
example, Patent Literature 1). The following is a description, with
reference to Fig. 9, of an example of a method for controlling arc
welding in which welding is performed by alternating a short-circuit
state and an arc state while feeding a welding wire as a consumable
electrode.
Fig. 9 shows time waveforms of a wire feed speed, a welding
output, and other elements according to the conventional arc welding.
As shown in Fig. 9, in a short circuit period from a time t1 when a short
circuit occurs until a time t2 when an arc occurs, the wire feed speed is
changed from a basic feed speed to a backward feed speed. In an arc
period from the time t2 when the arc occurs until a time t6 when
another short circuit occurs, the wire feed speed is accelerated to
return to the basic feed speed. In the arc period, from the time t2
when the arc occurs until a time t3, which is a first predetermined time,
the welding current is increased to reach a predetermined peak current
IP. After reaching the predetermined peak current IP at the time t3,
the welding current may be kept at the peak current IP for a
predetermined period. From the times t3 until a time t4, the welding
voltage is controlled to be constant, and the welding current based on
the constant voltage is outputted. From the time t4, the welding
current is decreased to reach a predetermined base current IB, which
is lower than the predetermined peak current IP. From a time t5
until the time t6 when the arc period ends, the welding current is kept
at the predetermined base current IB.
As described above, the transfer of molten metal of the wire to
the base material during a short circuit is mechanically secured by
feeding the wire in the backward direction. This regulates the
irregular short-circuit cycle, which is the principal cause of sputtering,
thereby reducing sputtering and stabilizing a short-circuit transfer
welding process.
The above-described conventional arc welding mechanically
secures the transfer of the molten metal of the wire to the base
material during a short circuit. This regulates the irregular
short-circuit cycle, which is the principal cause of sputtering, thereby
reducing sputtering.
In the welding start period, however, no weld pool has yet been
formed in the base material (also referred to as the object to be welded).
This makes it harder to open the short circuit than in the steady-state
welding period (also referred to as the main welding period) in which
the base material has a weld pool. Thus, in the welding start period,
it takes a longer time and a larger welding current to open the short
circuit. This may cause the molten metal of the wire to grow too much,
thereby generating small sputters when the short-circuit opens.
Citation List
Patent Literature
Patent Literature 1: Japanese Patent Unexamined
Publication No. 2007-216268
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and
apparatus for arc welding which can reduce sputtering in the welding
start period. The reduction of sputtering is achieved by making the
increase slopes of a short-circuit current and the current value at the
inflection point, which is the change point of the different increase
slopes of the short-circuit current between the welding start period and
the steady-state welding period.
The method for arc welding of the present invention performs
welding by alternating a short-circuit state and an arc state using a
welding wire as a consumable electrode. In this method, the
short-circuit current is controlled such that the increase slope of the
short-circuit current is different between a steady-state welding period,
which indicates a steady welding state and a welding start period,
which precedes the steady-state welding period.
This method can reduce an increase in the short-circuit current
even when no weld pool has yet been formed in the base material
immediately after welding is started. As a result, the molten metal of
the wire can properly grow, thereby reducing sputtering when the
short circuit opens.
The arc welding apparatus of the present invention performs
welding by alternating an arc state and a short-circuit state between
an object-to-be-welded and a welding wire used as a consumable
electrode. The arc welding apparatus includes the following units: a
switching unit to control a welding output; a welding voltage detection
unit to detect a welding voltage; a short-circuit/arc detection unit to
detect whether the welding is in the short-circuit state or in the arc
state based on the output of the welding voltage detection unit; a
short-circuit control unit to control a short-circuit current in a short
circuit period upon receiving a signal representing a short circuit from
the short-circuit/arc detection unit; an arc control unit to control an arc
voltage in an arc period upon receiving a signal representing an arc
from the short-circuit/arc detection unit; a programmed current setting
unit to allow the operator to set a programmed current; and a
start-period setting unit to set a welding start period, which precedes a
steady-state welding period based on the programmed current set in
the programmed current setting unit. The short-circuit control unit
includes the following units: a basic setting unit for the increase slope
of the short-circuit current to determine the increase slope of the
short-circuit current in the steady-state welding period based on the
programmed current set by the programmed current setting unit; and
a control unit for the increase slope of the short-circuit current to
determine the increase slope of the short-circuit current in the welding
start period by adding or subtracting a predetermined value to or from,
or multiplying a predetermined rate by the increase slope of the
short-circuit current determined by the basic setting unit for the
increase slope of the short-circuit current based on the programmed
current set by the programmed current setting unit. The short-circuit
current is controlled such that the increase slope of the short-circuit
current is different between the steady-state welding period, which
indicates a steady welding state and the welding start period, which
precedes the steady-state welding period.
This structure can reduce an increase in the short-circuit
current even when no weld pool has yet been formed in the base
material immediately after welding is started. As a result, the molten
metal of the wire can properly grow, thereby reducing sputtering when
the short circuit opens.
The arc welding apparatus of the present invention performs
welding by alternating an arc state and a short-circuit state between
an object-to-be-welded and a welding wire used as a consumable
electrode. The arc welding apparatus includes the following units: a
switching unit to control a welding output; a welding voltage detection
unit to detect a welding voltage; a short-circuit/arc detection unit to
detect whether the welding is in the short-circuit state or in the arc
state based on the output of the welding voltage detection unit; a
short-circuit control unit to control a short-circuit current in a short
circuit period upon receiving a signal representing a short circuit from
the short-circuit/arc detection unit; an arc control unit to control an arc
voltage in an arc period upon receiving a signal representing an arc
from the short-circuit/arc detection unit; a programmed current setting
unit to allow an operator to set a programmed current; and a
start-period setting unit to set a welding start period, which precedes a
steady-state welding period based on the programmed current set in
the programmed current setting unit. The short-circuit control unit
includes the following units: a basic setting unit for the increase slope
of the short-circuit current to determine a first increase slope of the
short-circuit current and a second increase slope of the short-circuit
current subsequent to the first increase slope in the steady-state
welding period based on the programmed current set by the
programmed current setting unit, a basic setting unit for an inflection
point of the increase slopes of the short-circuit current for determining
a current value at the inflection point at which the increase slope of
the short-circuit current changes from the first increase slope to the
second increase slope in the steady-state welding period based on the
programmed current set by the programmed current setting unit; a
control unit for the increase slope of the short-circuit current to
determine the first and second increase slopes of the short-circuit
current in the welding start period by adding or subtracting a
predetermined value to or from, or multiplying a predetermined rate
by the first and second increase slopes, respectively, of the short-circuit
current in the steady-state welding period determined by the basic
setting unit for the increase slope of the short-circuit current based on
the programmed current set by the programmed current setting unit;
and a control unit for the inflection point of the increase slopes of the
short-circuit current to determine a current value at an inflection point
in the welding start period by adding or subtracting a predetermined
value to or from, or multiplying a predetermined value by the current
value at the inflection point of the increase slopes of the short-circuit
current in the steady-state welding period determined by the basic
setting unit for the inflection point of the increase slopes of the
short-circuit current based on the programmed current set by the
programmed current setting unit. The short-circuit current is
controlled such that the first and second increase slopes of the
short-circuit current and the current value at the inflection point are
different between the steady-state welding period, which indicates a
steady welding state and the welding start period, which precedes the
steady-state welding period.
This structure can reduce an increase in the short-circuit
current even when no weld pool has yet been formed in the base
material immediately after welding is started. As a result, the molten
metal of the wire can properly grow, thereby reducing sputtering when
the short circuit opens.
As described above, the present invention, the increase slopes of
a short-circuit current and the current value at the inflection point,
which is the change point of the increase slopes of the short-circuit
current are made different between the welding start period and the
steady-state welding period. This method can reduce an increase in
the short-circuit current even when no weld pool has yet been formed
in the base material immediately after welding is started. As a result,
the molten metal of the wire can properly grow, thereby reducing
sputtering when the short circuit opens.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 shows time waveforms of a wire feed speed, a welding
voltage, and a welding current according to a first exemplary
embodiment of the present invention.
Fig. 2 shows time waveforms of a wire feed speed, a welding
voltage, and a welding current in each of a welding start period, a
second start period, and a steady-state welding period according to the
first exemplary embodiment.
Fig. 3 shows an example of the relation between a programmed
current and an increase slope of a short-circuit current according to the
first exemplary embodiment.
Fig. 4 shows another example of the relation between the
programmed current and the increase slope of the short-circuit current
according to the first exemplary embodiment.
Fig. 5 shows an example of the relation between the
programmed current and the current value at the inflection point,
which is the change point of the increase slopes of the short-circuit
current according to the first exemplary embodiment.
Fig. 6 shows another example of the relation between the
programmed current and the current value at the inflection point,
which is the change point of the increase slopes of the short-circuit
current according to the first exemplary embodiment.
Fig. 7 is a schematic configuration view of an arc welding
apparatus according to the first exemplary embodiment.
Fig. 8 shows time waveforms of a wire feed speed, a welding
voltage, and a welding current according to the first exemplary
embodiment.
Fig. 9 shows time waveforms of a wire feed speed, a welding
output, and other elements according to conventional arc welding.
DESCRIPTION OF EMBODIMENT
An exemplary embodiment of the present invention will be
described as follows with reference to drawings. In these drawings,
the same reference numerals are used for the same components, and
hence the description thereof may be omitted.
FIRST EXEMPLARY EMBODIMENT
The present first exemplary embodiment describes a method for
arc welding first, and then describes an arc welding apparatus using
this method.
Figs. 1 and 2 show time waveforms of a wire feed speed, a
welding current, and a welding voltage in consumable electrode arc
welding according to the first exemplary embodiment in which a
short-circuit state and an arc state are alternated. Fig. 2 shows time
waveforms of a wire feed speed, a welding current, and a welding
voltage in each of a welding start period, a second start period, and a
steady-state welding period. The time waveforms of the wire feed
speed, the welding current, and the welding voltage shown in Fig. 1 are
about the steady-state welding period.
As shown in Fig. 1, a time point P1 is when a short circuit
occurs. At the time point P1, a short-circuit initial period is started.
A time point P2 is when the short-circuit initial period is ended. At
the time point P2, it is started to output a first increase slope di/dt of a
short-circuit current (hereinafter, IS1a), which is the increment of the
short-circuit current per unit of time. A time point P3 is an inflection
point ISCa at which the increase slope changes from IS1a to a second
increase slope di/dt of the short-circuit current (hereinafter, IS2a). A
time point P4 is when it is ended to output IS2a. At the time point P4,
a "narrow portion treatment" is performed in which the narrow portion
of the droplet formed between the weld pool and the tip of the welding
wire is detected, and then the welding current is immediately brought
to a low level. A time point P5 is when the droplet moves away from
the wire tip, thereby ending the short-circuit state and generating an
arc. At the time point P5, it is started to output the peak current IP of
the welding current immediately after the arc occurs. A time point P6
is when a transition is started from the peak current IP to the base
current IB. From the time point P6 to a time point P7, either the
welding current or the welding voltage may be controlled. From the
time point P7 to a time point P8, the base current IB is outputted. At
the time point P8, the next short circuit occurs.
In the wire feed control shown in Fig. 1, the wire is fed in
sinusoidally alternating forward and backward directions at a
predetermined frequency and velocity amplitude. A sinusoid is a
basic waveform. When the wire is fed in the forward direction, a short
circuit occurs at or around the time point P1, which is the crest of the
wave. When the wire is fed in the backward direction, on the other
hand, an arc occurs at or around the time point P5, which is the trough
of the wave. When the wire is fed forward, a short circuit is likely to
occur, and when the wire is fed backward, the short circuit is likely to
open. Thus, a short-circuit state and an arc state occurs basically
according to the wire feed control to feed the wire in alternating
forward and backward directions.
As shown in Fig. 2 showing the time waveforms from the start of
welding, welding is performed based on IS1a, IS2a, and the current
value at the inflection point ISCa in a steady-state welding period,
which indicates a steady welding state. IS2a follows IS1a and is
based on a programmed current. The current value at the inflection
point ISCa at which the increase slope changes from IS1a to IS2a is
based on the programmed current.
In the welding start period shown in Fig. 2, the first increase
slope di/dt (hereinafter, IS1b) of the short-circuit current and the
second increase slope di/dt (hereinafter, IS2b) of the short-circuit
current, and the current value at an inflection point ISCb of the
short-circuit current are controlled to be smaller than in the
steady-state welding period as will be described in detail later . The
welding start period, which is, for example, experimentally determined,
can be a period from the start of welding until the wire feed speed
reaches the wire feed speed of the steady welding state. When the
welding start period is ended, a weld pool is properly formed in the
base material.
The second start period shown in Fig. 2 between the welding
start period and the steady-state welding period is a period in which
the increase slopes and the current value at the inflection point in the
welding start period are changed to their equivalents in the
steady-state welding period. The second start period can have a
duration of several hundred milliseconds.
In Fig. 1, the solid lines represent the time waveforms (basic
waveforms) of the welding current and the welding voltage based on
the programmed current in the steady-state welding period. The
dotted line represents an example of the time waveform when the
increase slopes and the inflection point are changed in the welding
start period. In Fig. 1, both the solid and dotted lines are shown in
the same short circuit period for easy comparison.
The following is a description of the control of the steady-state
welding period, which is the basic control for one cycle consisting of a
short circuit period and an arc period from the time points P1 through
P8 (the time waveforms shown by the solid lines) in Fig. 1.
At or around the time point P1, which is the crest of the
sinusoidal wave, the wire is fed in the forward direction under the wire
feed control, and the welding wire comes into contact with the
object-to-be-welded, thereby generating a short circuit. In the
short-circuit initial period between the time points P1 and P2, a
short-circuit initial current, which is lower than the current outputted
when the short circuit occurs, is outputted.
The following is the reason why the current in the short-circuit
initial period from the time points P1 through P2 is made lower than
the current outputted when the short circuit occurs. If the
short-circuit current is increased to a high level immediately after a
short circuit occurs, the short circuit may open at once, and be followed
by another short circuit. This may break the short circuit cycle in
which a short circuit occurs, then opens when a certain time has
passed, and another short circuit occurs. To avoid this, the period to
output a lower current than the short-circuit current is provided
immediately after a short circuit occurs. In this period, a stable short
circuit condition is established first, and then the short-circuit current
is increased to a high level, thereby securing the above-described short
circuit cycle.
The short-circuit initial period and the short-circuit initial
current value are determined through experimental verification or
other methods. Their basic set values are also determined through
experimental verification or other methods to achieve stable welding.
As a result, the ratio of the short circuit period to the arc period can be
50:50 when welding is performed at a certain speed (1 m/min in the
present first exemplary embodiment). The short-circuit initial period
and the short-circuit initial current value are stored in the form of a
table or a mathematical formula in association with the programmed
current in an unillustrated storage unit in the arc welding apparatus.
At the time point P2, the welding wire is in short circuit with
the object-to-be-welded (also referred to as the base material). From
this time point, the actual short-circuit current starts to increase along
IS1a determined based on the programmed current. At the time point
P3, the short-circuit current reaches the current value at the inflection
point ISCa. From this time point, the actual short-circuit current
starts to increase along IS2a determined based on the programmed
current. Note that the basic set values of IS1a from the time points
P2 through P3, IS2a from the time points P3 through P4, and the
current value at the inflection point ISCa of the short-circuit current
at the time point P3 are determined through experimental verification
or other methods to achieve stable welding. As a result, the ratio of
the short circuit period to the arc period can be 50:50 when welding is
performed at a certain period (1 m/min in the present first exemplary
embodiment). The increase slopes IS1a and IS2a, and the current
value at the inflection point ISCa are stored in the form of a table or a
mathematical formula in association with the programmed current in
the unillustrated storage unit in the arc welding apparatus.
From the time points P4 through P5, the narrow portion of the
molten tip of the welding wire is detected and the short-circuit current
is immediately brought to a low level as conventionally known.
At or around the time point P5, which is the trough of the
sinusoidal wave, the wire is fed in the backward direction under the
wire feed control, and the welding wire moves away from the
object-to-be-welded, opening the short circuit. From the time point P5
when an arc occurs in the arc period, the current is increased with a
predetermined slope until it reaches the peak current IP at the time
point P6. Note that it is possible to output the peak current IP for a
predetermined time, if necessary.
From the time points P6 through P7, it is possible either to
output the welding current based on the welding voltage under
controlled voltage conditions, or to output a predetermined current
under controlled current conditions. In either case, it is necessary to
grow the droplet and to maintain an appropriate arc length.
From the time point P7, the welding current is kept at the base
current IB until the next short circuit occurs at the time point P8. At
or around the time point P8, which is the crest of the sinusoidal wave,
the wire is fed in the forward direction under the wire feed control, and
the welding wire comes into contact with the object-to-be-welded,
generating another short circuit. Since the welding current is kept at
the comparatively low base current IB, a short circuit becomes likely to
occur, and a minor short circuit with a low welding current never cause
large sputters.
The peak current IP and the peak current time from the time
points P5 through P6, and the base current IB from the time points P7
through P8 are determined through experimental verification or other
methods. The peak current IP, the peak current time, and the base
current IB are stored in the form of a table or a mathematical formula
in association with the programmed current in the unillustrated
storage unit in the arc welding apparatus.
As described above, the control performed from the time points
P1 through P8 is defined as one cycle of arc welding control, and
welding is performed by repeating this cycle.
The following is a description of the control in the welding start
period shown in Fig. 2.
The following is a description, with reference to Figs. 1 and 2, of
the control of IS1b, IS2b, and the inflection point (current value) ISCb
at which the increase slope changes from IS1b to IS2b in the welding
start period.
The wire is fed in sinusoidally alternating forward and
backward directions at a predetermined frequency and amplitude
based on the programmed current. This condition is defined as the
basic waveform.
In the welding start period, the wire is fed periodically in the
same manner as in the steady-state welding period. In the welding
start period, however, sputtering would be caused if ISlb from the time
points P2 through P3, IS2b from the time points P3 through P4, and
the inflection point ISCb at the time point P3 have the same values as
in the steady-state welding period. The reason for this is as follows.
No weld pool has yet been formed in the base material immediately
after welding is started. Therefore, it is harder to open the short
circuit than in the steady-state welding period in which the base
material has a weld pool.
When the base material has a weld pool, the surface tension and
heat of the molten metal of the weld pool accelerate the opening of the
short circuit. When the base material does not have a weld pool, on
the other hand, it takes time until the short circuit opens because of
the absence of the surface tension and heat of the weld pool. A long
short-circuit time means that the short-circuit current is applied for a
long time. This accelerates the melting of the wire, causing the
molten metal droplet at the tip of the wire to grow too much. The too
grown molten metal droplet generates small particles of molten metal
between the molten metal that is transferred to the base material and
the wire tip when the short circuit opens. The small particles of
molten metal may sputter instead of dropping in the weld pool.
To avoid this, in the welding start period, it is necessary to
control each of IS1b based on the programmed current, IS2b
subsequent to IS1b, and the inflection point ISCb, which is the change
point of the increase slopes of the short-circuit current to be smaller
than in the steady-state welding period. The wire is fed in the
backward direction in the short circuit period. Therefore, the short
circuit can open within an acceptable time even if the increase slopes
and the inflection point are reduced.
The following is a description, with reference to Figs. 3 to 6, of
how to adjust ISlb, IS2b subsequent to IS1b, and the inflection point
ISCb at which the increase slope changes from IS1b to IS2b in the
welding start period to be smaller than in the steady-state welding
period. Figs. 3 and 4 show the relation between the programmed
current and IS1a and IS1b in the present first exemplary embodiment.
Figs. 5 and 6 show the relation between the programmed current and
the inflection points ISCa and ISCb of the short-circuit current in the
present first exemplary embodiment. Although Figs. 3 and 4 show
ISla and ISlb, the same holds true for IS2a and IS2b.
As shown in Fig. 3 showing the increase slope di/dt (IS1) of the
short-circuit current, in the steady-state welding period shown by the
solid line, when the programmed current is 200A, IS1a based on the
programmed current is 150A/msec. In the welding start period shown
by the dotted line, the increase slope is changed from the steady-state
welding period by the absolute value, which is -50A/msec. As a result,
IS2a in the welding start period is 100A/msec. Thus, the increase
slope in the welding start period can be determined by adding or
subtracting (in this case, subtracting) a predetermined value to or from
the increase slope in the steady-state welding period.
Alternatively, as shown in Fig. 4, the increase slope in the
welding start period can be changed from the steady-state welding
period by the rate of change, which is -20%. As a result, ISlb in the
welding start period is 120A/msec. Thus, the increase slope in the
welding start period can be determined by multiplying a
predetermined rate (in this case, 0.8) by the increase slope in the
steady-state welding period.
As shown in Fig. 5 showing the inflection point ISC of the
short-circuit current, in the steady-state welding period shown by the
solid line, when the programmed current is 200A, the current value at
the inflection point ISCa of the short-circuit current based on the
programmed current is 300A. In the welding start period shown by
the dotted line, the inflection point ISCb is changed from the
steady-state welding period by the absolute value, which is -100A. As
a result, the inflection point ISCb of the short-circuit current is 200A.
Thus, the inflection point in the welding start period can be
determined by adding or subtracting (in this case, subtracting) a
predetermined value to or from the inflection point in the steady-state
welding period.
Alternatively, as shown in Fig. 6, the inflection point ISCb in
the welding start period can be changed from the steady-state welding
period by the rate of change, which is -40%. As a result, ISCb in the
welding start period is 180A. Thus, the inflection point in the welding
start period can be determined by multiplying a predetermined rate (in
this case, 0.6) by the inflection point in the steady-state welding
period.
As described above, the increase slopes of the short-circuit
current and the inflection point of the short-circuit current in the
welding start period are controlled to be smaller than IS1a, IS2a, and
ISCa based on the programmed current in the steady-state welding
period. As a result, the short circuit can open within an acceptable
time even with a lower current, thereby reducing sputtering.
In the above-described example, the wire is fed in alternating
forward and backward directions. When the wire is fed only in the
forward direction as conventional cases, no weld pool has yet been
formed in the base material immediately after welding is started. In
such cases, the opening of the short circuit can be accelerated to
shorten the short circuit period by controlling IS1b, IS1b, and ISCb to
be larger than their equivalent values based on the programmed
current in the steady-state welding period.
An example of adjusting the increase slopes of the short-circuit
current and the inflection point in the welding start period is to store
IS1b, IS2b, and ISCb in association with the programmed current in
the unillustrated storage unit. Then, IS1b, IS2b, and ISCb are read
based on the programmed current from the storage unit.
In Figs. 3 through 6, the relation between the programmed
current and IS1a or IS2a based on the programmed current, or the
relation between the programmed current and ISCa based on the
programmed current are shown as straight lines. Alternatively, these
relations can be shown as quadratic curves or other curves.
As shown in Figs. 3 through 6, the variation width of ISla, ISlb,
IS2a, and IS2b based on the programmed current, and the variation
width of the inflection points ISCa and ISCb based on the programmed
current may have an upper limit and a lower limit. This can prevent
too much adjustment of the increase slopes and the current values at
the inflection points of the short-circuit current. Without the upper or
lower limit, IS1a, IS1b, IS2a, IS2b, and the inflection points ISCa,
ISCb based on the programmed current might be changed too small or
too large. This may greatly increase sputtering or cause an unstable
arc.
The increase slopes IS1a, IS1b, IS2a, and IS2b and the values at
the inflection points ISCa, ISCb of the short-circuit current are set
based on the programmed current value of the welding current and
also on at least one of the following items: the diameter, the type, and
the extended length of the wire, which is a consumable electrode to be
fed, and the type of a shielding gas to be fed.
The following is a description of the control of the second start
period provided between the welding start period and the steady-state
welding period as shown in Fig. 2.
As shown in Fig. 2, the second start period is a period to change
IS1b, IS2b, and the inflection point ISCb of the short-circuit current in
the welding start period to their equivalent values based on the
programmed current in the steady-state welding period. More
specifically, the second start period is a period to gradually change
IS1b, IS2b, and the current value at the inflection point ISCb of the
short-circuit current in the welding start period to IS1a, IS2a, and the
inflection point ISCa of the short-circuit current based on the
programmed current in the steady-state welding period. In this case,
the change is performed by a predetermined increment per
predetermined number of short circuits or per predetermined time.
Thus, the second start period prevents IS1b, IS2b, and the inflection
point ISCb of the short-circuit current when the welding start period is
ended from being changed too suddenly to IS1a, IS1b, and the
inflection point ISCa, respectively, of the short-circuit current in the
steady-state welding period. The second start period is determined,
for example, based on the programmed current and either a time to
generate a predetermined number of short circuits or a predetermined
time.
As described hereinbefore, according to the present exemplary
embodiment, the increase slopes and the inflection point of the
short-circuit current in the welding start period are controlled to be
smaller than IS1b, IS2b, and the value at the inflection point ISCb of
the short-circuit current based on the programmed current. Then, in
the second start period, the increase slopes and the inflection point of
the short-circuit current in the welding start period are changed to the
increase slopes and the inflection point of the short-circuit current
based on the programmed current in the steady-state welding period.
This can reduce sputtering throughout the entire welding process from
the welding start period through the steady-state welding period.
The wire is fed in sinusoidally alternating forward and
backward directions, and the increase slopes and the inflection point of
the short-circuit current in the welding start period are controlled to
be smaller than in the steady-state welding period. This control
particularly contributes to the reduction of sputtering in the welding
start period.
Thus, the method for arc welding of the present invention
performs welding by alternating a short-circuit state and an arc state
using a welding wire as a consumable electrode. This method for arc
welding controls a short-circuit current such that the increase slope of
the short-circuit current is different between a steady-state welding
period, which indicates a steady welding state, and a welding start
period, which precedes the steady-state welding period.
This method can reduce an increase in the short-circuit current
even when no weld pool has yet been formed in the base material
immediately after welding is started. As a result, the molten metal of
the wire can properly grow, thereby reducing sputtering when the
short circuit opens.
The short-circuit current may be controlled such that the
increase slope of the short-circuit current in the welding start period is
smaller than in the steady-state welding period.
This method can reduce an increase in the short-circuit current
even when no weld pool has yet been formed in the base material
immediately after welding is started. As a result, the molten metal of
the wire can properly grow, thereby reducing sputtering when the
short circuit opens.
The short-circuit current may be controlled such that the
increase slope of the short-circuit current in the steady-state welding
period and the welding start period includes the first increase slope
IS1a and the second increase slope IS2a subsequent to the first
increase slope.
This method can reduce an increase in the short-circuit current
even when no weld pool has yet been formed in the base material
immediately after welding is started. As a result, the molten metal of
the wire can properly grow, thereby reducing sputtering when the
short circuit opens.
The short-circuit current may be controlled such that the
current value at the inflection point ISCa at which the increase slope
of the short-circuit current changes from the first increase slope ISla
to the second increase slope IS2a is different between the steady-state
welding period and the welding start period.
This method can reduce an increase in the short-circuit current
even when no weld pool has yet been formed in the base material
immediately after welding is started. As a result, the molten metal of
the wire can properly grow, thereby reducing sputtering when the
short circuit opens.
In the steady-state welding period, the first and second increase
slopes IS1a and IS2a of the short-circuit current, and the current value
at the inflection point ISCa may be determined based on the
programmed current. In the welding start period, the first and
second increase slopes IS1b and slope IS2b of the short-circuit current,
and the current value at the inflection point ISCb may be determined
by adding or subtracting a predetermined value to or from, or
multiplying a predetermined value by the first and second increase
slopes IS1a and IS2a, respectively, of the short-circuit current and the
current value at the inflection point ISCa, respectively, in the
steady-state welding period.
As a result, the short circuit can open within an acceptable time
even with a low current, thereby reducing sputtering.
The short-circuit current may be controlled such that the
current value at the inflection point ISCb of the short-circuit current in
the welding start period is smaller than the current value at the
inflection point ISCa of the short-circuit current in the steady-state
welding period.
As a result, the short circuit can open within an acceptable time
even with a low current, thereby reducing sputtering.
The first and second increase slopes IS1a and IS2a of the
short-circuit current and the current value at the inflection point ISCa
in the steady-state welding period and the welding start period may
have an upper limit and a lower limit.
This can prevent too much adjustment of the increase slopes
and the current values at the inflection points ISC of the short-circuit
current.
A second start period may be provided between the welding
start period and the steady-state welding period. In the second start
period, the first and second increase slopes of the short-circuit current
and the current value at the inflection point obtained when the
welding start period is ended may be gradually changed to their
equivalents obtained when the steady-state welding period is started.
This can reduce sputtering throughout the entire welding
process from the welding start period through the steady-state welding
period.
The second start period may be a time to generate a
predetermined number of short circuits or a predetermined time.
This can reduce sputtering throughout the entire welding
process from the welding start period through the steady-state welding
period.
When the welding is performed by alternating the short-circuit
state and the arc state, the welding wire feed speed corresponding to
the programmed current may be defined as an average feed speed, and
the welding wire may be fed in alternating forward and backward
directions at a predetermined frequency and a predetermined
amplitude.
This method can reduce an increase in the short-circuit current
even when no weld pool has yet been formed in the base material
immediately after welding is started. As a result, the molten metal of
the wire can properly grow, thereby reducing sputtering when the
short circuit opens.
The arc welding apparatus performing the above-mentioned arc
welding control according to the present first exemplary embodiment
will be described as follows. Fig. 7 is a schematic configuration view
of the arc welding apparatus, which has the following structure.
As shown in Fig. 7, the electric power from input power 1 is
rectified by primary rectifying unit 2, converted to AD power by
switching unit 3, and is stepped down by transformer 4. Then, the
power is rectified by secondary rectifying unit 5 and DCL6, which is an
inductor, and is applied between welding wire 25 and
object-to-be-welded 28 (also referred to as the base material). As a
result, welding arc 27 occurs. Welding wire 25 is applied with the
power via tip 26.
As shown in Fig. 7, the arc welding apparatus includes
switching unit 3, drive unit 7 for controlling switching unit 3, welding
voltage detection unit 8, welding current detection unit 9,
short-circuit/arc detection unit 10, short-circuit control unit 11, and
arc control unit 16. Welding voltage detection unit 8 is connected
between welding power output terminals. Welding current detection
unit 9 detects a welding output current. Short-circuit/arc detection
unit 10 determines the presence or absence of a short circuit or an arc
based on a signal from welding voltage detection unit 8. Short-circuit
control unit 11 controls the short-circuit current in a short circuit
period upon receiving a signal representing a short-circuit from
short-circuit/arc detection unit 10. Arc control unit 16 controls the
arc voltage in an arc period upon receiving a signal representing an arc
from short-circuit/arc detection unit 10.
As shown in Fig. 7, the arc welding apparatus further includes
programmed current setting unit 19, start-period setting unit 22,
second start-period setting unit 23, setting unit 29 for the average wire
feed speed, basic setting unit 20 for the wire feed frequency, and basic
setting unit 21 for the wire feed amplitude. Programmed current
setting unit 19 allows the operator to set a programmed current.
Start-period setting unit 22 sets a welding start period based on the
programmed current. Second start-period setting unit 23 sets a
second start period based on the programmed current. Setting unit
29 for the average wire feed speed determines the average feed speed of
the welding wire based on the programmed current. Basic setting
unit 20 for the wire feed frequency sets a frequency for the wire feed
control based on the average feed speed. Basic setting unit 21 for the
wire feed amplitude sets an amplitude for the wire feed control based
on the average feed speed. Arc control unit 16 includes basic setting
unit 17 for the peak current and the base current, and control unit 18
for the peak current time. Basic setting unit 17 determines a peak
current and a base current in an arc period based on the signal from
short-circuit/arc detection unit 10. Control unit 18 determines a peak
current time in an arc period.
First, the wire feed control is described as follows.
Basic setting unit 20 for the wire feed frequency and basic
setting unit 21 for the wire feed amplitude each output to wire feeding
motor 24 a wire feed speed at which the wire is fed in sinusoidally
alternating forward and backward directions at a frequency and
amplitude. The wire feed speed is outputted based on the
programmed current value of programmed current setting unit 19, and
the average feed speed, which is the wire feed speed determined by
setting unit 29 for the average wire feed speed.
The relation between the average feed speed, frequency, and
amplitude for the wire feed control and the programmed current is
stored in the form of a table or a mathematical formula in the
unillustrated storage unit, and is determined based on the
programmed current.
Next, the welding control is described as follows.
Welding voltage detection unit 8, which is connected between
the welding power output terminals, outputs a signal corresponding to
the detected voltage to short-circuit/arc detection unit 10. From this
determination result, short-circuit/arc detection unit 10 determines
whether the welding output voltage is below a certain value or not
based on the signal from welding voltage detection unit 8. Unit 10
then determines whether welding wire 25 is in contact with and short
circuited to object-to-be-welded 28 or is in non-contact with it and
welding arc 27 exists. Unit 10 then outputs a determination signal.
Short-circuit control unit 11 includes the following units: basic
setting unit 12 for the increase slope di/dt of the short-circuit current;
control unit 13 for the increase slope di/dt of the short-circuit current;
basic setting unit 14 for the inflection point of the short-circuit
current; and control unit 15 for the inflection point of the short-circuit
current. Basic setting unit 12 for the increase slope di/dt sets the
first and second increase slopes di/dt based on the set programmed
current. Control unit 13 for the increase slope di/dt changes the
increase slopes di/dt set in basic setting unit 12 based on start-period
setting unit 22 and second start-period setting unit 23. Basic setting
unit 14 for the inflection point sets an inflection point at which the
increase slope changes from the first increase slope di/dt to the second
increase slope di/dt based on the set programmed current. Control
unit 15 for the inflection point changes the current value at the
inflection point based on start-period setting unit 22 or the
programmed current.
As described above, the arc welding apparatus having the
above-described structure controls the increase slopes and the
inflection point of the short-circuit current in the welding start period
to be smaller than in the steady-state welding period, thereby reducing
sputtering in the welding start period.
The arc welding apparatus of the present invention performs
welding by alternating an arc state and a short-circuit state between
object-to-be-welded 28 and welding wire 25 used as a consumable
electrode. This arc welding apparatus includes switching unit 3,
welding voltage detection unit 8, short-circuit/arc detection unit 10,
short-circuit control unit 11, arc control unit 16, programmed current
setting unit 19, and start-period setting unit 22. Switching unit 3
controls a welding output. Welding voltage detection unit 8 detects a
welding voltage. Short-circuit/arc detection unit 10 detects whether
the welding is in the short-circuit state or in the arc state based on the
output of the welding voltage detection unit. Short-circuit control
unit 11 controls a short-circuit current in a short circuit period upon
receiving a signal representing a short circuit from short-circuit/arc
detection unit 10. Arc control unit 16 controls an arc voltage in an arc
period upon receiving a signal representing an arc from
short-circuit/arc detection unit 10. Programmed current setting unit
19 allows the operator to set a programmed current. Start-period
setting unit 22 sets a welding start period, which precedes a
steady-state welding period based on the programmed current set in
programmed current setting unit 19. Short-circuit control unit 11
includes basic setting unit 12 for the increase slope of the short-circuit
current and control unit 13 for the increase slope of the short-circuit
current. During welding, the short-circuit current is controlled such
that the increase slope of the short-circuit current is different between
the steady-state welding period, which indicates a steady welding state
and the welding start period, which precedes the steady-state welding
period. Basic setting unit 12 for the increase slope determines the
increase slope of the short-circuit current in the steady-state welding
period based on the programmed current set by programmed current
setting unit 19. Control unit 13 for the increase slope of the
short-circuit current determines the increase slope of the short-circuit
current in the welding start period by adding or subtracting a
predetermined value to or from, or multiplying a predetermined rate
by the increase slope of the short-circuit current determined by basic
setting unit 12 for the increase slope based on the programmed current
set by programmed current setting unit 19.
This structure can reduce an increase in the short-circuit
current even when no weld pool has yet been formed in the base
material immediately after welding is started. As a result, the molten
metal of the wire can properly grow, thereby reducing sputtering when
the short circuit opens.
The short-circuit current may be controlled such that the
increase slope of the short-circuit current in the welding start period is
smaller than in the steady-state welding period. This structure can
further reduce sputtering when the short circuit opens.
The arc welding apparatus of the present invention performs
welding by alternating an arc state and a short-circuit state between
object-to-be-welded 28 and welding wire 25 used as a consumable
electrode. The arc welding apparatus includes switching unit 3,
welding voltage detection unit 8, short-circuit/arc detection unit 10,
short-circuit control unit 11, arc control unit 16, programmed current
setting unit 19, and start-period setting unit 22. Switching unit 3
controls a welding output. Welding voltage detection unit 8 detects a
welding voltage. Short-circuit/arc detection unit 10 detects whether
the welding is in the short-circuit state or in the arc state based on the
output of the welding voltage detection unit. Short-circuit control
unit 11 controls a short-circuit current in a short circuit period upon
receiving a signal representing a short circuit from short-circuit/arc
detection unit 10. Arc control unit 16 controls an arc voltage in an arc
period upon receiving a signal representing an arc from
short-circuit/arc detection unit 10. Programmed current setting unit
19 allows the operator to set a programmed current. Start-period
setting unit 22 sets a welding start period, which precedes a
steady-state welding period based on the programmed current set in
programmed current setting unit 19. Short-circuit control unit 11
includes the following units: basic setting unit 12 for the increase slope
of the short-circuit current; basic setting unit 14 for the inflection
point of the increase slopes of the short-circuit current; control unit 13
for the increase slope of the short-circuit current; and control unit 15
for the inflection point of the increase slopes of the short-circuit
current. Basic setting unit 12 for the increase slope determines a first
increase slope of the short-circuit current and a second increase slope
of the short-circuit current subsequent to the first increase slope in the
steady-state welding period based on the programmed current set by
programmed current setting unit 19. Basic setting unit 14 for the
inflection point of the increase slopes determines a current value at the
inflection point at which the increase slope of the short-circuit
current changes from the first increase slope to the second increase
slope in the steady-state welding period based on the programmed
current set by programmed current setting unit 19. Control unit 13
for the increase slope of the short-circuit current determines the first
and second increase slopes of the short-circuit current in the welding
start period by adding or subtracting a predetermined value to or from,
or multiplying a predetermined rate by the first and second increase
slopes, respectively, of the short-circuit current in the steady-state
welding period determined by basic setting unit 12 for the increase
slope of the short-circuit current based on the programmed current set
by programmed current setting unit 19. Control unit 15 for the
inflection point of the increase slopes of the short-circuit current
determines a current value at an inflection point in the welding start
period by adding or subtracting a predetermined value to or from, or
multiplying a predetermined value by the current value at the
inflection point of the increase slopes of the short-circuit current in the
steady-state welding period determined by basic setting unit 14 for the
inflection point of the increase slopes of the short-circuit current based
on the programmed current set by programmed current setting unit 19.
In the arc welding apparatus, the short-circuit current is controlled
such that the first and second increase slopes of the short-circuit
current and the current value at the inflection point are different
between the steady-state welding period, which indicates a steady
welding state and the steady-state welding period, which precedes the
welding start period.
This structure can reduce an increase in the short-circuit
current even when no weld pool has yet been formed in the base
material immediately after welding is started. As a result, the molten
metal of the wire can properly grow, thereby reducing sputtering when
the short circuit opens.
In the arc welding apparatus of the present invention, the
short-circuit current may be controlled such that the first and second
increase slopes of the short-circuit current and the current value at the
inflection point in the welding start period are smaller than in the
steady-state welding period. This structure can further reduce
sputtering when the short circuit opens.
The arc welding apparatus of the present invention further
includes a second start-period setting unit. In the second start period,
the first and second increase slopes of the short-circuit current and the
current value at the inflection point obtained when the welding start
period is ended may be gradually changed to their equivalents
obtained when the steady-state welding period is started. The second
start-period setting unit determines the second start period provided
between the welding start period and the steady-state welding period
based on the programmed current set by programmed current setting
unit 19. This structure can further reduce sputtering when the short
circuit opens.
The arc welding apparatus of the present invention may further
include setting unit 29 for the average wire feed speed, basic setting
unit 20 for the frequency, and basic setting unit 21 for the amplitude.
In this case, welding is performed by feeding welding wire 25 in
alternating forward and backward directions at a predetermined
frequency and a predetermined amplitude and by alternating a
short-circuit state and an arc state. Setting unit 29 for the average
wire feed speed determines the average feed speed of the welding wire
based on the programmed current set by programmed current setting
unit 19, Basic setting unit 20 for the frequency sets a frequency for
controlling the wire feed by feeding the wire in sinusoidally or
trapezoidally alternating forward and backward directions based on
the average feed speed of the welding wire set by setting unit 29 for the
average wire feed speed. Basic setting unit 21 for the amplitude sets
an amplitude for controlling the wire feed by feeding the wire in
sinusoidally or trapezoidally alternating forward and backward
directions based on the average feed speed of the welding wire set by
setting unit 29 for the average wire feed speed. This structure can
further reduce sputtering when the short circuit opens.
Each component of the arc welding apparatus shown in Fig. 7
may be either formed alone or combined with some other components.
In the present exemplary embodiment, the increase slopes di/dt
and the inflection point of the short-circuit current are stored in the
storage unit by associating them with the programmed current, but
may alternatively be stored by associating them with the wire feed
speed or the wire feed amount. This is because the programmed
current is in proportion to the wire feed speed or the wire feed amount.
In the present exemplary embodiment, the wire is fed by
sinusoidal currents, but may alternatively be fed by trapezoidal
currents as shown in Fig. 8. Thus, feeding the wire in alternating
forward and backward directions at a predetermined frequency and
amplitude has the same effect whether the currents are trapezoidal or
sinusoidal.
INDUSTRIAL APPLICABILITY
According to the present invention, the increase slopes of the
short-circuit current and the current value at the inflection point,
which is the change point of the increase slopes of the short-circuit
current are made different between the welding start period and the
steady-state welding period. This can reduce an increase in the
short-circuit current even when no weld pool has yet been formed in
the base material immediately after welding is started. As a result,
the molten metal of the wire can properly grow, thereby reducing
sputtering when the short circuit opens. Thus, the method and
apparatus for arc welding of the present invention are industrially
useful.
REFERENCE MARKS IN THE DRAWINGS
1 input power
2 primary rectifying unit
3 switching unit
4 transformer
5 secondary rectifying unit
6 DCL
7 drive unit
8 welding voltage detection unit
9 welding current detection unit
10 short-circuit/arc detection unit
11 short-circuit control unit
12 basic setting unit for the increase slope of the short-circuit
current
13 control unit for the increase slope of the short-circuit current
14 basic setting unit for the inflection point of the short-circuit
current
15 control unit for the inflection point of the short-circuit current
16 arc control unit
17 basic setting unit for the peak current and the base current
18 control unit for the peak current time
19 programmed current setting unit
20 basic setting unit for the wire feed frequency
21 basic setting unit for the wire feed amplitude
22 start-period setting unit
23 second start-period setting unit
24 wire feeding motor
25 welding wire
26 tip
27 welding arc
28 object-to-be-welded
29 setting unit for the average wire feed speed

WECLAIM:
1. A method for arc welding in which welding is performed by
alternating a short-circuit state and an arc state using a welding wire
as a consumable electrode, wherein
a short-circuit current is controlled such that an increase slope
of the short-circuit current is different between a steady-state welding
period, which indicates a steady welding state and a welding start
period, which precedes the steady-state welding period.
2. The method of claim 1, wherein
the short-circuit current is controlled such that the increase
slope of the short-circuit current in the welding start period is smaller
than in the steady-state welding period.
3. The method of claim 1, wherein
the short-circuit current is controlled such that the increase
slope of the short-circuit current in the steady-state welding period
and the welding start period includes a first increase slope and a
second increase slope subsequent to the first increase slope.
4. The method of claim 3, wherein
the short-circuit current is controlled such that a current value
at an inflection point at which the increase slope of the short-circuit
current changes from the first increase slope to the second increase
slope is different between the steady-state welding period and the
welding start period.
5. The method of claim 4, wherein
in the steady-state welding period, the first and second increase
slopes of the short-circuit current and the current value at the
inflection point are determined based on a programmed current, and
in the welding start period, the first and second increase slopes
of the short-circuit current, and the current value at the inflection
point are determined by adding or subtracting a predetermined value
to or from, or multiplying a predetermined value by the first and
second increase slopes, respectively, of the short-circuit current and
the current value at the inflection point, respectively, in the
steady-state welding period.
6. The method of claim 4, wherein
the short-circuit current is controlled such that the current
value at the inflection point of the short-circuit current in the welding
start period is smaller than in the steady-state welding period.
7. The method of claim 4, wherein
the first and second increase slopes of the short-circuit current
and the current value at the inflection point in the steady-state
welding period and the welding start period have an upper limit and a
lower limit.
8. The method of claim 4, wherein
a second start period is provided between the welding start
period and the steady-state welding period, and
in the second start period, the first and second increase slopes
of the short-circuit current and the current value at the inflection point
obtained when the welding start period is ended are gradually changed
to the first and second increase slopes of the short-circuit current and
the current value at the inflection point, respectively, obtained when
the steady-state welding period is started.
9. The method of claim 8, wherein
the second start period is a time to generate a predetermined
number of short circuits or a predetermined time.
10. The method of claim 1, wherein
when the welding is performed by alternating the short-circuit
state and the arc state, the welding wire feed speed corresponding to
the programmed current is defined as an average feed speed, and the
welding wire is fed in alternating forward and backward directions at a
predetermined frequency and a predetermined amplitude.
11. An arc welding apparatus for performing welding by alternating
an arc state and a short-circuit state between an object-to-be-welded
and a welding wire used as a consumable electrode, the arc welding
apparatus comprising:
a switching unit to control a welding output;
a welding voltage detection unit to detect a welding voltage;
a short-circuit/arc detection unit to detect whether the welding
is in the short-circuit state or in the arc state based on an output of the
welding voltage detection unit;
a short-circuit control unit to control a short-circuit current in a
short circuit period upon receiving a signal representing a short circuit
from the short-circuit/arc detection unit;
an arc control unit to control an arc voltage in an arc period
upon receiving a signal representing an arc from the short-circuit/arc
detection unit;
a programmed current setting unit to allow an operator to set a
programmed current; and
a start-period setting unit to set a welding start period, which
precedes a steady-state welding period based on the programmed
current set in the programmed current setting unit, wherein
the short-circuit control unit includes:
a basic setting unit determining an increase slope of the
short-circuit current in the steady-state welding period based on the
programmed current set by the programmed current setting unit; and
a control unit determining an increase slope of the
short-circuit current in the welding start period by adding or
subtracting a predetermined value to or from, or multiplying a
predetermined rate by the increase slope of the short-circuit current
determined by the basic setting unit for the increase slope of the
short-circuit current based on the programmed current set by the
programmed current setting unit, wherein
the short-circuit current is controlled such that the increase
slope of the short-circuit current is different between the steady-state
welding period, which indicates a steady welding state and the welding
start period, which precedes the steady-state welding period.
12. The arc welding apparatus of claim 11, wherein
the short-circuit current is controlled such that the increase
slope of the short-circuit current in the welding start period is smaller
than in the steady-state welding period.
13. An arc welding apparatus to perform welding by alternating
an arc state and a short-circuit state between an object-to-be-welded
and a welding wire used as a consumable electrode, the arc welding
apparatus comprising:
a switching unit to control a welding output;
a welding voltage detection unit to detect a welding voltage;
a short-circuit/arc detection unit to detect whether the welding
is in the short-circuit state or in the arc state based on an output of the
welding voltage detection unit;
a short-circuit control unit to control a short-circuit current in a
short circuit period upon receiving a signal representing a short circuit
from the short-circuit/arc detection unit;
an arc control unit to control an arc voltage in an arc period
upon receiving a signal representing an arc from the short-circuit/arc
detection unit;
a programmed current setting unit to allow an operator to set a
programmed current; and
a start-period setting unit to set a welding start period, which
precedes a steady-state welding period based on the programmed
current set in the programmed current setting unit, wherein
the short-circuit control unit includes:
a basic setting unit for the increase slope of the
short-circuit current to determine a first increase slope of the
short-circuit current and a second increase slope of the short-circuit
current subsequent to the first increase slope in the steady-state
welding period based on the programmed current set by the
programmed current setting unit;
a basic setting unit for an inflection point of the increase
slopes of the short-circuit current to determine a current value at the
inflection point at which the increase slope of the short-circuit current
changes from the first increase slope to the second increase slope in the
steady-state welding period based on the programmed current set by
the programmed current setting unit;
a control unit for the increase slope of the short-circuit
current to determine the first and second increase slopes of the
short-circuit current in the welding start period by adding or
subtracting a predetermined value to or from, or multiplying a
predetermined rate by the first and second increase slopes,
respectively, of the short-circuit current in the steady-state welding
period determined by the basic setting unit for the increase slope of the
short-circuit current based on the programmed current set by the
programmed current setting unit; and
a control unit for the inflection point of the increase
slopes of the short-circuit current to determine a current value at an
inflection point in the welding start period by adding or subtracting a
predetermined value to or from, or multiplying a predetermined value
by the current value at the inflection point of the increase slopes of the
short-circuit current in the steady-state welding period determined by
the basic setting unit for the inflection point of the increase slopes of
the short-circuit current based on the programmed current set by the
programmed current setting unit, wherein
the short-circuit current is controlled such that the first and
second increase slopes of the short-circuit current and the current
value at the inflection point are different between the steady-state
welding period, which indicates a steady welding state and the welding
start period, which precedes the steady-state welding period.
14. The arc welding apparatus of claim 13, wherein
the short-circuit current is controlled such that the first and
second increase slopes of the short-circuit current and the current
value at the inflection point in the welding start period are smaller
than in the steady-state welding period.
15. The arc welding apparatus of claim 13, further comprising:
a second start-period setting unit for determining a second start
period provided between the welding start period and the steady-state
welding period based on the programmed current set by the
programmed current setting unit, wherein
in the second start period, the first and second increase slopes
of the short-circuit current and the current value at the inflection point
obtained when the welding start period is ended are gradually changed
to the first and second increase slopes of the short-circuit current and
the current value at the inflection point, respectively, obtained when
the steady-state welding period is started.
16. The arc welding apparatus of claim 11 or 13, further comprising:
a setting unit determining an average feed speed of the welding
wire based on the programmed current set by the programmed current
setting unit;
a basic setting unit for the frequency to set a frequency to
control the wire feed by feeding the wire in sinusoidally or
trapezoidally alternating forward and backward directions based on
the average feed speed of the welding wire set by the setting unit for
the average wire feed speed; and
a basic setting unit setting an amplitude to control the wire feed
by feeding the wire in sinusoidally or trapezoidally alternating forward
and backward directions based on the average feed speed of the
welding wire set by the setting unit for the average wire feed speed,
wherein
welding is performed by feeding a welding wire in alternating
forward and backward directions and by alternating the short-circuit
state and the arc state at a predetermined frequency and a
predetermined amplitude.

ABSTRACT

This arc welding method implements control such that the rate of increase of
the short-circuit current and the bend point where said rate of increase
changes are smaller in a weld start period than in a steady-state welding
period. By thus using short-circuit-current increase rates and bend points of
different sizes in the weld start period and the steady-state welding period,
spatter in the weld start period is reduced.