Electric heating – Metal heating – By arc
Reexamination Certificate
2002-04-25
2003-09-16
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130510
Reexamination Certificate
active
06621049
ABSTRACT:
DETAILED DESCRIPTION OF THE INVENTION
The present invention is related to an apparatus for assessing start-of-weld and steady state welding stability in pulsed arc consumable electrode gas-shielded welding.
BACKGROUND ART
In consumable electrode gas-shielded welding using the pulsed arc method, as shown in
FIG. 8
, a pulsed current having a set periodic cycle is caused to flow in a welding wire, and the heat input of the arc formed between the welding wire and the workpiece melts the tip of the wire. A magnetic pinch force created by the pulse current squeezes out droplets of the molten metal and separates them from the tip of the wire, to be droplet-transferred to the weld site. In a stable welding state under optimum welding conditions, the welding is performed in a one-drop-per-pulse droplet-transfer mode.
This one-drop-per-pulse droplet-transfer pulsed arc welding has been made possible by recent on-going progress in welding inverter/power supply technology in the areas of precision feedback control of output voltage, and high-speed control of output current, the goal being to achieve reduced spatter, improved standards-based welding state repeatability, and higher welding speed.
[In spite of these improvements,] however, a stable arc could not be obtained at the start of a weld, because the welding wire suddenly brought into contact with the workpiece had not yet been heated, and was cold. Because of this, the welding operation at this stage was vulnerable to a variety of weld fault phenomena that cause major problems for automated production lines using welding robots, etc.
One example of such a start-of-weld fault phenomenon is the ‘wire stick’ phenomenon. This problem occurs when there is no momentary insulation breakdown as the welding wire (to which a high voltage is applied) is short-circuited to the workpiece, and thus no arc discharge occurs. This can be caused by poor grounding of the workpiece (which is part of the arc discharge circuit), a faulty welding wire power source, or the formation of an oxide ball on the tip of the welding wire. When a wire stick incident occurs, although the welding wire continues to be fed out and short circuit current continues to flow, no bead is formed, because there is no arc. This creates a discontinuity in the bead for the duration of the wire stick incident, thus causing a weld defect at the start of the weld.
Another start-of-weld fault phenomenon is arc interruption. In the arc interruption phenomenon, when a long duration short (such as a wire stick) occurs at the start of a weld, a very large short circuit current is drawn to release the short. This can cause a large amount of spattering when the arc is reestablished, and/or can cause the welding wire to be blown away. In either case, the arc is momentarily interrupted. The duration of a momentary arc interruption is referred to as ‘arc interruption time.’ Arc interruption prevents a proper arc from being formed, which in turn prevents the weld bead from being formed. This results in a discontinuity in the bead for the duration of the arc interruption time, causing a weld defect near the start of the weld.
Actions that can be taken to suppress such fault phenomena include (1) periodically cleaning the welding wire feed path, (2) periodically replacing the welding wire conduit cable, (3) periodically replacing the contact tip, or (4) taking the above actions after welding defects occur. The problem with this approach, however, is that because weld fault phenomena tend to be abated within a brief period after the start of the weld, once the weld is completed it can be very difficult, using visual inspection, to make pass/fail decisions for minor abnormalities. For this reason, in the past there tended to be differences in decisions made by individual inspectors, which made it hard to develop uniform in-line defect assessment standards.
In Japanese Laid-open Patent Bulletin No. H11-123546, the applicant in the present case proposed a method for assessing start-of-weld welding stability in consumable electrode ‘short circuit’ arc welding, a technique that differs from the pulsed arc consumable electrode gas-shielded welding described above. Because the droplet-transfer mode of this short circuit arc welding method is completely different from that in pulsed arc welding, however, the technology disclosed in the above bulletin cannot not be applied as-is for assessment of start-of-weld welding stability in the pulsed arc welding method. That is, in short circuit arc welding, as shown in
FIG. 7
, the tip of the consumable electrode (hereinafter ‘welding wire’) is melted by the heat input of the arc. A high density current produces a magnetic pinch force that separates molten metal from the tip of the welding wire in droplet form, and short-circuit-transfers it to the workpiece in the droplet ‘contact transfer’ state of the short circuit welding phenomenon. The weld current waveform at the start of the weld is basically astable and aperiodic. Therefore the process of extracting indexes indicative of the above fault phenomena from this waveform would be matter of isolating and extracting fault phenomena-related unstable waveform portions of a waveform that is itself unstable. It was clear that this would be extremely difficult to do. Therefore, for the short circuit arc welding method, rather than extracting fault phenomena indexes as had been done in the past, more importance was placed on extracting, from the above astable, aperiodic welding current waveform, indexes that were related to arc phenomena stability.
Therefore, even though the phenomena that appear when weld defects occur are the same in both welding methods, the drop transfer states, as described above, are completely different, and the processes that lead to defects are therefore also different. Accordingly, it was determined that no common index existed that could be used in both of these welding techniques to assess weld stability at the start of a weld.
Moreover, if one were to attempt to use the start-of-weld welding stability assessment method for short circuit arc welding described in Japanese Laid-open Patent Bulletin No. H11-123546 to perform the same assessment in pulsed arc welding, quantitative detection of instantaneous arc stability phenomena from the periodic pulse waveform would be too difficult to enable an accurate assessment to be performed.
As described above, then, the start-of-weld welding stability assessment method used for short circuit arc welding could not be used to perform a similar assessment for pulsed arc welding. Therefore, for fully automatic welding lines using arc welding robots, etc., as well as for semi-automatic welding lines, the prevention of welding quality defects caused by instability of start-of-weld welding phenomena was still a major problem.
The present invention was devised with the above problem in mind, and it is therefore a first object thereof to provide an assessment apparatus for making quick and accurate pass/fail assessments of welding stability for pulsed arc welding by quantitatively and accurately evaluating start-of-weld welding instability phenomena that occur in consumable electrode pulsed arc welding.
To achieve the above object, a welding stability assessment apparatus for pulsed arc welding according to claim 1 of the present invention is characterized in that, in consumable electrode gas-shielded pulsed-arc welding, wherein a welding voltage is applied between a welding electrode and a workpiece to be welded, for repeatedly supplying a pulse current and a base current, in alternation, and wherein welding is effected by causing a droplet from the welding electrode to be dropped onto the workpiece for each pulse, it comprises:
a detector means for detecting at least one of
welding voltage between the welding electrode and the workpiece,
welding current flowing between the welding electrode and the workpiece, and
power-on time;
a computer means for computing degree of irregularity in values detected by the detector means; and
an assessment m
Central Motor Wheel Co., Ltd.
J.C. Patents
Shaw Clifford C.
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