Method to control an arc welding system to reduce spatter

An electric arc welder and a method for performing a pulse welding process producing reduced spatter. The welder produces a current between an advancing electrode and a workpiece. The welder includes a short-detecting capability for detecting a short condition upon occurrence of a short circuit between the advancing electrode and the workpiece. The welder may also include a switching module in the welding circuit path of the welder having an electrical switch and a resistive path. Times of occurrence of short intervals can be tracked and a blanking signal can be generated based on the tracked short intervals to anticipate a next short interval in a next pulse period of the pulsed welding process. The blanking signal can be used to reduce a welding current in the welding circuit path by introducing additional resistance into the welding circuit path via the switching module, for example.

TECHNICAL FIELD

Certain embodiments relate to pulsed electric arc welding equipment and processes. More particularly, certain embodiments relate to anticipating or reacting to short circuits formed between a welding electrode and a workpiece during a pulsed electric arc welding process by reducing the output current during the time of the short to reduce spatter.

BACKGROUND

In electric arc welding, a popular welding process is pulse welding which primarily uses a solid wire electrode with an outer shielding gas. MIG welding uses spaced pulses which first melt the end of an advancing wire electrode and then propels the molten metal from the end of the wire through the arc to the workpiece. A globular mass of molten metal is transferred during each pulse period of the pulse welding process. During certain pulse periods, especially in applications where the welding electrode operates very close to the workpiece, molten metal contacts the workpiece before being entirely released from the advancing wire electrode. This creates a short circuit (a.k.a., a short) between the advancing wire electrode and the workpiece. It is desirable to eliminate or clear the short rapidly to obtain the consistency associated with proper pulse welding. However, clearing a short can result in undesirable spatter being generated. Such spatter causes inefficiencies in the welding process and can result in molten metal being spattered over the workpiece which may have to be removed later using a grinding tool, for example.

Further limitations and disadvantages of conventional, traditional, and proposed approaches will become apparent to one of skill in the art, through comparison of such approaches with embodiments of the present invention as set forth in the remainder of the present application with reference to the drawings.

BRIEF SUMMARY

Embodiments of the present invention comprise an electric arc welding system and methods for reducing spatter during a pulsed electric arc welding process. Spatter is reduced during a welding operation by reducing the welding output current during a time when a short occurs between the welding electrode and the workpiece. In one embodiment, a switching module, including an electrical switch and a resistive path, is incorporated into the return welding current path of a power source of the electric arc welding system. During non-shorting conditions of the pulse welding operation, the electrical switch is closed or on, allowing welding current to freely return to the power source by experiencing minimal resistance through the switch. However, when a short is anticipated or occurs during the welding process, the electrical switch is opened or turned off, forcing the welding current to have to go through the resistive path of the switching module, causing the level of the welding current to be lower than it otherwise would be. The lower current generated during the short interval results in less spatter being created when the short is cleared. The time of occurrence of a short during the pulse periods may be tracked and a blanking interval, overlapping the interval of time corresponding to an anticipated short, may be applied such that the switch is open during the blanking interval.

These and other features of the claimed invention, as well as details of illustrated embodiments thereof, will be more fully understood from the following description and drawings.

DETAILED DESCRIPTION

During an arc-welding process, when the distance between the tip of the electrode and the workpiece is relatively small, molten metal may be transferred via a contact transfer process (e.g., a surface-tension-transfer or STT process) or a free-flight transfer process (e.g., a pulsed welding process) with a tethered connection. In a contact transfer process, a molten metal ball on the tip of the welding electrode makes contact with the workpiece (i.e., shorts) and starts to “wet into” the molten puddle on the workpiece before the molten metal ball begins to substantially separate from the tip of the electrode.

In a free-flight transfer process, the molten metal ball breaks free of the tip of the electrode and “flies” across the arc toward the workpiece. However, when the distance between the tip of the electrode and the workpiece is relatively short, the molten metal ball flying across the arc can make contact with the workpiece (i.e., short) while a thin tether of molten metal still connects the molten metal ball to the tip of the electrode. In such a tethered free-flight transfer scenario, the thin tether of molten metal tends to explode, causing spatter, when the molten metal ball makes contact with the workpiece as illustrated inFIG. 6herein, due to a rapid increase in current through the tether.

FIG. 1illustrates a block diagram of an example embodiment of an electric arc welding system100incorporating a switching module110in a welding output return path and providing welding outputs121and122. The system100includes a power converter120capable of converting an input power to a welding output power. The power converter120may be an inverter-type power converter or a chopper-type power converter, for example. The system100further includes a wire feeder130capable of feeding a welding electrode wire E through, for example, a welding gun (not shown) that connects the welding electrode wire E to the welding output121.

The system100also includes a current shunt140operatively connected between the power converter120and the welding output121for feeding welding output current to a current feedback sensor150of the system100to sense the welding output current produced by the power converter120. The system100further includes a voltage feedback sensor160operatively connected between the welding output121and the welding output122for sensing the welding output voltage produced by the power converter120. As an alternative, the switching module110could be incorporated in the outgoing welding current path, for example, between the power converter120and the current shunt140, or between the current shunt140and the welding output121.

The system100also includes a high-speed controller170operatively connected to the current feedback sensor150and the voltage feedback sensor160to receive sensed current and voltage in the form of signals161and162being representative of the welding output. The system100further includes a waveform generator180operatively connected to the high speed controller170to receive command signals171from the high speed controller170that tell the waveform generator how to adapt the welding waveform signal181in real time. The waveform generator180produces an output welding waveform signal181and the power converter120is operatively connected to the waveform generator180to receive the output welding waveform signal181. The power converter120generates a modulated welding output (e.g., voltage and current) by converting an input power to a welding output power based on the output welding waveform signal181.

The switching module110is operatively connected between the power converter120and the welding output122which is connected to the welding workpiece W during operation. The high speed controller170is also operatively connected to the switching module110to provide a switching command signal (or a blanking signal)172to the switching module110. The high speed controller170may include logic circuitry, a programmable microprocessor, and computer memory, in accordance with an embodiment of the present invention.

In accordance with an embodiment of the present invention, the high-speed controller170may use the sensed voltage signal161, the sensed current signal162, or a combination of the two to determine when a short occurs between the advancing electrode E and the workpiece W, when a short is about to clear, and when the short has actually cleared, during each pulse period. Such schemes of determining when a short occurs and when the short clears are well known in the art, and are described, for example, in U.S. Pat. No. 7,304,269, portions of which are incorporated herein by reference. The high-speed controller170may command the waveform generator180to modify the waveform signal181when the short occurs and/or when the short is cleared. For example, when a short is determined to have been cleared, the high-speed controller170may command the waveform generator180to incorporate a plasma boost pulse (see pulse750ofFIG. 7) in the waveform signal181to prevent another short from occurring immediately after the clearing of the previous short.

FIG. 2illustrates a diagram of an example embodiment of a portion of the system100ofFIG. 1, including the switching module110in the welding current return path. The power converter120may include an inverter power source123and a freewheeling diode124. The welding output path will have an inherent welding circuit inductance210due to the various electrical components within the welding output path. The switching module110is shown as having an electrical switch111(e.g., a power transistor circuit) in parallel with a resistive path112(e.g., a network of high power rated resistors).

During a pulse period of the welding waveform, when no short is present, the electrical switch111is commanded to be closed by the switching command signal172from the high-speed controller170. When the electrical switch111is closed, the electrical switch111provides a very low resistance path in the output welding return path, allowing welding current to freely return to the power converter120through the switch111. The resistive path112is still present in the welding output return path, but most of the current will flow through the low resistance path provided by the closed switch111. However, when a short is detected, the electrical switch111is commanded to be opened by the switching command signal172from the high-speed controller170. When the electrical switch111is opened, current is cut off from flowing through the switch111and is forced to flow through the resistive path112, resulting in the level of the current being reduced due to the resistance provided by the resistive path112.

FIG. 3illustrates a schematic diagram of an example embodiment of the switching module110ofFIG. 1andFIG. 2. The switching module110includes the transistor circuit111and the resistor network112as shown. The switching module110may include a circuit board for mounting the various electrical components of the module110including the transistor circuit111, the resistor network112, LEDs, and status logic circuitry, for example.

FIG. 4illustrates a flowchart of a first example embodiment of a method400for preventing spatter in a pulsed electric arc welding process using the system100ofFIG. 1. Step410represents operation where the switch111of the switching module110is normally closed (no short condition). In step420, if a short is not detected, then the switch111remains closed (no short condition). However, if a short is detected then, in step430, the switch111is commanded to go through an opening and closing sequence during the short interval (i.e., the time period over which the electrode is shorted to the workpiece).

The opening/closing sequence in step430starts by opening the switch111when the short is first detected. The switch111remains open for a first period of time (e.g., a first 10% of the short interval). This decreases the output current quickly so the short does not break right away causing a large amount of spatter. After the first period of time, the switch is again closed and the output current is ramped during a second period of time to cause the molten short to begin to form a narrow neck in an attempt to break free from the electrode and clear the short. During this second period of time, as the current is ramping, a dv/dt detection scheme is performed to anticipate when the short will clear (i.e., when the neck will break). Such a dv/dt scheme is well known in the art. The switch111is then opened again just before the short is about to clear (e.g., during the last 10% of the short interval) in order to quickly lower the output current once again to prevent excessive spattering when the neck actually breaks (i.e., when the short actually clears).

In step440, if the short (short between the electrode and the workpiece) is still present, then the switch111remains open. However, if the short has been cleared then, in step450, the switch111is again closed. In this manner, during a short condition, the switch111goes through an opening/closing sequence and the current flowing through the welding output path is reduced when the switch is open, resulting in reduced spatter. The method400is implemented in the high-speed controller170, in accordance with an embodiment of the present invention. Furthermore, in accordance with an embodiment of the present invention, the system100is able to react at a rate of 120 kHz (i.e., the switching module110can be switched on and off at this high rate), providing sufficient reaction to detection of a short and detection of the clearing of the short to implement the method400in an effective manner.

In accordance with a somewhat simpler alternative embodiment, instead of going through the opening/closing sequence described above with respect toFIG. 4, the current of the welding circuit path is decreased, in response to detection of a short between the advancing wire electrode and the workpiece, by opening the switch111for at least a determined period of time, thus increasing the resistance in the welding circuit path. For most pulse periods, the determined period of time is of a duration allowing for the short to clear without having to first increase the current of the welding circuit path. During a given pulse period, if the short clears before the determined period of time has expired as desired, then the process proceeds to the next part of the pulse period. However, if the short does not clear within the predetermined period of time then, immediately after the determined period of time, the switch111is closed again, causing the current of the welding circuit path to once again increase and clear the short. In such an alternative embodiment, the switch111is simply opened for at least part of the determined period of time in response to the detection of the short. In most pulse periods, the current does not have to be increased to clear the short.

Furthermore as an option, when the short between the advancing wire electrode and the workpiece is detected, a speed of the advancing wire electrode can be slowed. Slowing the speed of the advancing wire electrode helps to clear the short more readily by not adding as much material to the short as otherwise would be added. To slow the speed of the advancing wire electrode, a motor of a wire feeder advancing the wire electrode may be switched off and a brake may be applied to the motor. The brake may be a mechanical brake or an electrical brake, in accordance with various embodiments.

FIG. 5illustrates an example of a conventional pulsed output current waveform500resulting from a conventional pulsed electric arc welder that does not use the switching module110ofFIGS. 1-3in accordance with the method400ofFIG. 4, or the simpler alternative method described above. As can be seen from the waveform500ofFIG. 5, after a peak pulse510is fired, a short may occur starting at time520, for example, that lasts until time530, for example, when the short is cleared The times520and530define a short interval540. As can be seen inFIG. 5, peak pulses510are fired at regular intervals during the multiple pulse periods or cycles of the welding process. During any given cycle or pulse period, a short condition may or may not occur. In a conventional system, when a short occurs, there is very little resistance in the welding output path compared to the inductance. Current continues to flow even if the power source is turned off.

Referring again toFIG. 5, during the short interval540, the output current tends to increase due to the lack of an arc between the electrode E and the workpiece W (the resistance becomes very low), and due to the fact that the welding circuit inductance210acts to keep current flowing in the welding output path, even when the power converter120is phased back to a minimum level. The current tends to increase until the short is cleared (i.e., until the molten metal short breaks free of the electrode E). However, at such increased current levels, when the short breaks or clears, the increased current levels tend to cause the molten metal to explode causing spatter.

FIG. 6illustrates the exploding spatter process that was discovered using high speed video technology in a free-flight transfer process having a tethered connection. A high peak pulse (e.g.,510) causes a ball of molten metal610to push out towards the workpiece W creating a narrow tether620between the ball610and the electrode E. As the ball610flies toward the workpiece W across the arc, the tether620narrows and, eventually, a short occurs between the electrode E and the workpiece W through the tether620. This condition tends to occur for almost every pulse period in an operation where the welding electrode operates very close to the workpiece. In particular, it was discovered that for a free-flight transfer pulse welding process, the tether620creates an incipient short and a large amount of current can begin to flow through the narrow tether620. The increasing current level finally causes the relatively thin molten tether620to explode creating spatter630as shown inFIG. 6. However, by incorporating the switching module110and the method400(or the simpler alternative) as described above herein, the spatter630that is created can be greatly reduced.

FIG. 7illustrates an example of a pulsed output current waveform700resulting from the pulsed electric arc welder100ofFIG. 1that uses the switching module110ofFIGS. 1-3in accordance with the method400ofFIG. 4. As can be seen from the waveform700ofFIG. 7, after a peak pulse710is fired, a short may occur starting at time720, for example, that lasts until time730, for example, when the short is cleared. The times720and730define a short interval740. As can be seen inFIG. 7, peak pulses710are fired at regular intervals during the multiple pulse periods or cycles of the welding process. During any given cycle, a short condition may or may not occur. However, when the distance between the tip of the electrode and the workpiece is relatively small, a short can occur on almost every cycle.

Referring again toFIG. 7, during the short interval740, the switch111of the switching module110is opened when the short first occurs and again when the short is about to clear, causing the output current to flow through the resistive path112and, therefore, causing the current level to reduce. As an example, the switching signal172may be a logic signal that goes from high to low when a short is detected, causing the switch to open. Similarly, when the short is cleared, the switching signal172may go from low to high to close the switch111again. When the switch111is opened, the resistive path112puts a load on the welding output path allowing the freewheeling current to drop quickly to desired levels. The current tends to reduce until the short is cleared and, at such reduced current levels, when the short breaks or clears, the molten metal tends to pinch off in an unexplosive manner, eliminating or at least reducing the amount of spatter created. Also, in the waveform700ofFIG. 7, the plasma boost pulse750, which is used to help prevent another short from occurring immediately after the short that was just cleared, is more prominent and potentially more effective.

FIG. 8illustrates a flowchart of another example embodiment of a method800for preventing spatter in a pulsed electric arc welding process using the system100ofFIG. 1. In accordance with an embodiment, the method800is performed by the controller170. The high-speed controller170tracks the times of occurrence of the shorts and/or the clearing of the shorts and provides an estimate of when the short interval940(the time between the occurrence of a short and when the short is cleared) (seeFIG. 9) will occur during at least the next pulse period. From this estimate, a blanking interval960(seeFIG. 9) can be determined which is used to generate the blanking signal172.

In step810of the method800, the system100detects the occurrence of shorts and/or the clearing of those shorts during the repeating pulse periods of the pulsed welding waveform, according to known techniques. In step820, the time of occurrence of the detected shorts and/or clearings within the pulse periods are tracked (e.g., by the high-speed controller170). In step830, the location and duration of the short interval940(seeFIG. 9) for a next pulse period is estimated based on the tracking results. In step840, an overlapping blanking interval960for at least the next pulse period is determined based on the estimated location of the short interval for the next pulse period. In step850, a blanking signal (a type of switching signal)172is generated (e.g., by the controller170) to be applied to the switching module110during the next pulse period.

FIG. 9illustrates an example of a pulsed output current waveform900resulting from the pulsed electric arc welder100ofFIG. 1that uses the switching module110ofFIGS. 1-3in accordance with the method800ofFIG. 8. As can be seen from the waveform900ofFIG. 9, after a peak pulse910is fired, a short may occur starting at time920, for example, that lasts until time930, for example, when the short is cleared. The times920and930define a short interval940. As can be seen inFIG. 9, peak pulses910are fired at regular intervals during the welding process. During any given cycle, a short condition may or may not occur. However, during a welding process where the arc length is relatively short (i.e., where the wire electrode is operated relatively close to the workpiece), shorts can occur in almost every pulse period.

In accordance with the method800, the times of occurrence of the short and/or clearing of the short within the pulse period are determined and tracked from pulse period to pulse period. In this manner, the controller170may estimate the location of the short interval that will likely occur in the next or upcoming pulse periods. However, at the beginning of a pulsed welding process, before any substantial tracking information is available, the location of the short interval may be a stored default location based on, for example, experimental data or stored data from a previous welding process. The blanking signal172can be adapted or modified to form a blanking interval960within the blanking signal172which temporally overlaps the estimated short interval940for the next pulse period(s). Ideally, the blanking interval960starts shortly before the short interval940of the next pulse period (e.g., before the time920) and ends shortly after the short interval940of the next pulse period (e.g., after the time930), thus the temporal overlap. In one embodiment, only the times of occurrence of a short are tracked, not the clearing of the shorts. In such an embodiment, the duration of the blanking interval is set to last long enough for the short to clear, based on experimental knowledge.

In this manner, the actual occurrence of a short during the next pulse period does not have to be detected before the switch111of the switching module110can be opened. As the pulsed welding process progresses, the location of the short interval may drift or change as the distance between the wire electrode and the workpiece drifts or changes, for example. However, in this embodiment, since the location of the short interval is being tracked over time, the location of the blanking signal can be adapted to effectively follow and anticipate the short interval. By opening the switch111during the blanking interval960, the current drops and it is expected that the tether will occur and break during the blanking interval960.

Experimental results have shown that, using the switching module110as described herein in a particular pulsed welding scenario, the welding output current level at the point of clearing the short can be reduced from about 280 amps to about 40 amps, making a tremendous difference in the amount of spatter produced. In general, reducing the current below 50 amps seems to significantly reduce spatter. In addition, travel speeds (e.g., 60-80 inches/minute) and deposition rates are able to be maintained.

Other means and methods of reducing the welding output current level during the time period when a short is present between a welding electrode and a workpiece are possible as well. For example, in an alternative embodiment, the control topology of a welding power source may be configured to control the output current to a highly regulated level during the time of the short. The power source can control the shorting current to a lower level (e.g., below 50 amps) during a shorting interval to reduce the spatter. For example, referring toFIG. 1, the switching module110can be disabled or eliminated, allowing current to freely flow in the welding output circuit path. The controller170is configured to command the waveform generator180to modify a portion of the output welding waveform signal181of the welding process during the blanking interval to reduce the welding output current through the welding output circuit path. Therefore, in this alternative embodiment, the controller170reduces the current during the blanking interval through the waveform generator180and the power converter120, instead of via the switching module110. Such an alternative embodiment can work quite well if the inductance210of the welding circuit is sufficiently low.

In summary, an electric arc welder and a method for performing a pulse welding process producing reduced spatter are disclosed. The welder produces a current between an advancing electrode and a workpiece. The welder includes a short-detecting capability for detecting a short condition upon occurrence of a short circuit between the advancing electrode and the workpiece. The welder is controlled to reduce the current between the advancing electrode and the workpiece during the time of the short to reduce spatter of molten metal when the short clears.

An embodiment of the present invention comprises a method for reducing spatter in a pulsed arc-welding process. The method includes tracking times of occurrence of short intervals during pulse periods of a pulsed arc-welding process using a controller of a welding system. The tracking may be based on at least one of detecting occurrences of shorts during pulse periods of the pulsed welding process and detecting clearing of shorts during pulse periods of the pulsed welding process. The method further includes estimating a temporal location of a short interval for at least a next pulse period of the pulse welding process based on the tracking. The method also includes determining a blanking interval for at least a next pulse period based on the estimating. The method may further include generating a blanking signal for at least a next pulse period based on the blanking interval. The method may further include increasing a resistance of a welding circuit path of the welding system during the blanking interval in response to the blanking signal to reduce a welding current through the welding circuit path during the blanking interval. Increasing the resistance may include opening an electrical switch of a switching module disposed in the welding circuit path. In accordance with an embodiment, the electrical switch is in parallel with a resistive path within the switching module. The method may include reducing a welding current through a welding circuit path of the welding system during the blanking interval for at least a next pulse period by modifying a portion of a waveform of the welding process during the blanking interval, wherein the waveform is generated by a waveform generator of the welding system. In accordance with an embodiment, the blanking interval is temporally wider than and temporally overlaps an expected short interval of at least a next pulsed period.

An embodiment of the present invention comprises a system for reducing spatter in a pulsed arc-welding process. The system includes a controller configured for tracking times of occurrence of short intervals during pulse periods of a pulsed arc-welding process of a welding system. The controller is further configured for estimating a temporal location of a short interval for at least a next pulse period of the pulsed welding process based on the tracking. The controller is also configured for determining a blanking interval for at least a next pulse period based on the estimating. The controller may also be configured for generating a blanking signal for at least a next pulse period based on the blanking interval. In accordance with an embodiment, the blanking interval is temporally wider than and temporally overlaps an expected short interval of at least a next pulse period. The system may further include a switching module disposed in a welding circuit path of the welding system and operatively connected to the controller. The switching module is configured to increase a resistance of the welding circuit path of the welding system during the blanking interval in response to the blanking signal to reduce a welding current through the welding circuit path during the blanking interval. The switching module includes an electrical switch and a resistive path in parallel. The controller may be configured for commanding a waveform generator of the welding system to reduce a welding current through a welding circuit path of the welding system during the blanking interval for at least a next pulse period by modifying a portion of a waveform of the welding process during the blanking interval. The controller may further be configured to detect occurrences of shorts during pulse periods of the pulsed welding process, and to detect occurrences of clearing of shorts during pulse periods of the pulsed welding process.

An embodiment of the present invention comprises a method for reducing spatter in a pulsed arc-welding process. The method includes detecting a short during a pulse period of a pulsed arc-welding process using a controller of a welding system. The method further includes increasing a resistance of a welding circuit path of the welding system for a first period of time to reduce a welding current through the welding circuit path in response to detecting the short. The method also includes decreasing the resistance of the welding circuit path of the welding system for a second period of time immediately after the first period of time to increase the welding current through the welding circuit path. The method further includes increasing the resistance of the welding circuit path of the welding system for a third period of time immediately after the second period of time to reduce the welding current through the welding circuit path in anticipation of clearing the short. Increasing the resistance may include opening an electrical switch of a switching module disposed in the welding circuit path. Decreasing the resistance may include closing an electrical switch of a switching module disposed in the welding circuit path. The method may further include detecting that a short has cleared, and decreasing the resistance of the welding circuit path of the welding system in response to detecting that the short has cleared.

An embodiment of the present invention comprises a method for reducing spatter in a pulsed arc-welding process. The method includes detecting a short between a workpiece and an advancing wire electrode during a pulse period of a pulsed arc-welding process using a controller of a welding system. The method further includes decreasing a current of a welding circuit path of the welding system for at least a portion of a determined period of time in response to detecting the short wherein, during most pulse periods of the pulsed arc-welding process, the determined period of time is of a duration allowing for the short to clear without having to first increase the current of the welding circuit path. Decreasing the current may include increasing a resistance of the welding circuit path. Increasing the resistance may include opening an electrical switch of a switching module disposed in the welding circuit path, wherein the switching module includes the electrical switch in parallel with a resistance path. The method may further include increasing the current of the welding circuit path of the welding system immediately after the determined period of time if the short has not cleared. Increasing the current may include decreasing a resistance of the welding circuit path. Decreasing the resistance may include closing an electrical switch of a switching module disposed in the welding circuit path, wherein the switching module includes the electrical switch in parallel with a resistance path. The method may further include slowing down a speed of the advancing wire electrode in response to detecting the short between the electrode and the workpiece. Slowing down the speed of the advancing wire electrode may include switching off a motor of a wire feeder advancing the wire electrode and applying a brake to the motor. The brake may be a mechanical brake or an electrical brake, in accordance with various embodiments.

While the claimed subject matter of the present application has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the claimed subject matter. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the claimed subject matter without departing from its scope. Therefore, it is intended that the claimed subject matter not be limited to the particular embodiment disclosed, but that the claimed subject matter will include all embodiments falling within the scope of the appended claims.