Welding Company Had a Spot Weld Failure and Unable to Determine Why

The following is an actual trip report documenting an actual consultation with one of WeldComputer Corporation’s customers. Names and addresses have been changed or removed for privacy purposes.

Company visited:

A Welding Company
Somewhere, USA.

Purpose of trip: A Welding Company (AWC) had a spot weld failure and was unable to determine why it happened.  AWC requested an on-site visit from WeldComputer Corporation to determine the cause of the inconsistent welding performance and to provide recommendations on how to correct the problem and prevent bad welds from passing through production without being detected.

Summary: Within the scope of a one day consultation, WeldComputer Corporation 1) provided AWC with a review of recommended machine setup procedures, 2) used a WeldView® Monitor to identify two separate problems with the machine that were responsible for causing weld failures, 3) provided specific recommendations that AWC can implement on its own to correct those problems, 4) demonstrated how to use the monitor to rapidly establish robust weld schedule settings to produce more consistent higher quality welds and 5) demonstrated how to set upper and lower monitor limits to non-destructively evaluate the integrity of every production weld.


Report:

Upon arriving at AWC the WeldComputer representative was shown the welding station where the failure occurred.  The AWC representative reported that two of the welding processes on this machine were identical and that truncated electrodes with a 0.25” face diameter were being used.  The AWC representative wanted to know why the same welding current that produces an undersized weld on one of the units (“Process 1”) results in a good weld on the other apparently identical unit (“Process 2”).

This investigation began by visually examining the electrode contact area on the part surface for Process 1 in comparison to Process 2. These results revealed that Process 1 had approximately a 15% smaller contact area than Process 2. It was also observed that the electrode force on Process 1 was not evenly distributed. This was because the electrode face was not parallel to the part surface. These differences alone could easily explain why different heat settings were needed for these two processes. Replacement electrodes were installed in the Process 1 station and a repeat comparison revealed an improvement in the electrode force distribution on the part surface and a closer surface area match to that of Process 2.

Proper control of electrode contact area is a typical problem experienced by spot welding operations.  In most operations a 15% reduction in current density will result in an undersized weld or no weld. While electrical conductance monitoring can often detect a sizable shift in electrode contact area, thermal expansion monitoring provides the most reliable method of detecting this type of variation. The temperature reached at the site of the weld is directly affected by current density, and current density is inversely proportional to electrode contact area.  Material thermal expansion is an instantaneous response to the average temperature through the axis of the work piece as the weld is being formed. Current meters cannot reliably detect this type of variation because the change in welding current attributed to electrode contact area variation is usually less than the amount of current variation that normally occurs from weld to weld.

A portable WeldView® Monitor was connected to one of the welding stations. The variables monitored included: combination work piece thickness, work piece thermal expansion, cylinder pressure, transformer secondary voltage, conductance, power and current.

During the course of the day, 22 welds were produced. All of these welds used 11 cycles of weld heat. The cylinder pressure reported by the monitor during the 1st weld is displayed below.

The monitor clearly documented that the cylinder pressure was in a transient state throughout the duration of the weld; the cylinder pressure was 51.13 PSI at the initiation of weld heat and drifted up to 57.23 PSI by the time the welding heat was terminated (182.7 ms after the start of the weld), and continued to drift up to 59.76 PSI by the time 336 ms of time lapsed since the beginning of the weld. Unless such a pressure versus time function is orchestrated by a control system for a specific purpose, this is generally an undesirable characteristic to exist in the electrode force system of a resistance welding operation because it is often not repeatable from weld to weld.

The pressure trace from the 1st weld was used as a template to synthesize acceptability limits for the purpose of detecting if the pressure pattern during subsequent welds deviated by more than +/-1 PSI. These upper and lower limit settings are also shown in the above pressure trace.

The thermal expansion response of the 1st weld is displayed below.

The monitor shows that 1) the combination work piece thickness of the parts welded was 2.63 mm, 2) at the time the welding current was terminated (approximately 182.7 ms after the start of the weld) the work pieces had thermally expanded by 250 um, and 3) within 153.3 ms after the termination of welding current, the part had contracted to 125 um.

Based on this expansion response pattern it can be concluded that a weld nugget was formed, but that a larger nugget could be achieved either by increasing the heat or decreasing the electrode pressure. This weld was not produced with the most robust weld schedule settings because the thermal expansion response still had a rapid rate of increase at the time that the weld heat was terminated.

In contrast, weld 21, which was produced with the same heat setting but at a lower electrode pressure, thermally expanded by 282 um, and within 153.3 ms after the termination of welding current, had contracted to 145 um. The actual expansion response of weld 21 is shown below.

This expansion response trace clearly documented that weld 21 achieved larger nugget growth than weld 1. It can also be concluded that this is a more robust welding process because the thermal expansion was no longer increasing when the weld heat was terminated (i.e. maximum thermal expansion had already been achieved prior to termination of weld heat).

The 2nd weld produced had a response that was comparable to the 1st weld; the cylinder pressure was 55.58 PSI at the initiation of weld heat and had drifted up to 57.52 PSI by the time the welding heat was terminated, and continued to drift up to 60.0 PSI by the time 336 ms of time lapsed since the beginning of the weld. At the time the welding current was terminated the work pieces had thermally expanded by 252 um, and within 153.3 ms after the termination of welding current the part had contracted to 124 um.

The 3rd weld produced exhibited a considerable process shift; at the time the welding current was terminated the work pieces had thermally expanded by only 167 um, providing clear evidence that an undersized weld was produced. The actual expansion response trace from the monitor is shown below.

Upon examining the cylinder pressure response it was easy to conclude that the undersized weld occurred because the electrode force had increased by more than 15 percent. As shown in the pressure trace below, the pressure was reasonably consistent over the duration of this weld, but had shifted to more than 66 PSI.

Inconsistent cylinder pressure observed after making only three welds provided clear evidence of a pneumatic control problem with this weld machine. An examination of the apparatus revealed that the root cause of this problem is due to a deficiency in the machine pneumatic design. Specifically, the existing system operates the cylinder that controls the electrode force by turning on an electrically controlled valve that switches pressure to the input of a pressure regulator, and the output of the pressure regulator is connected directly to the cylinder.

A mechanical pressure switch, also connected to the regulator output, senses when pressure is present and provides a signal to the control to proceed with the welding sequence. This pneumatic arrangement causes the pressure regulator to undergo a startup transient at the initiation of every weld.  Upon supplying air to the input of the pressure regulator, it takes several seconds to achieve a stable regulated output pressure.

The pattern of the startup transient can be expected to vary as a function of input pressure, temperature, humidity and other factors.  The control, by waiting for the pressure switch to close before proceeding to weld, further increases the pressure variability during the weld.  This is because the actual PSI value that actuates the mechanical pressure switch can be widely variable, particularly when it is subjected to a slowly increasing pressure.

The problem with this pressure system can easily be remedied by connecting line pressure directly to the input of a high flow capacity precision regulator and installing the valve that electronically switches pressure to the cylinder on the output side of the pressure regulator. These changes should be implemented before placing this machine back into production.

On the 4th weld produced, the cylinder pressure had shifted back to the same pressure response pattern that occurred during the first two welds. This was evidenced by the pressure trace shown below.

Surprisingly, instead of producing an expansion response comparable to the first two welds, this weld resulted in an expansion of only 82 um, providing clear evidence that virtually no nugget growth occurred.  A destructive test of this weld confirmed that no nugget was produced.

Since the pressure pattern for weld 4 was virtually identical to weld 1 and weld 2 (which resulted in acceptable nugget growth), a conclusion had to be reached that the failure of weld 4 could not be attributed to inconsistent pressure.

The 5th weld produced had a similar pressure pattern to welds 1, 2 and 4; the expansion response reverted to the expected pattern, which was comparable to the expansion responses produced by weld 1 and weld 2. This ruled out the possibility that the failure of weld 4 could be related to a change in electrode geometry.

The monitor reported that the current for weld 4 was no less than the current produced for the other welds, so this ruled out the possibility of a problem with the welding control or the welding transformer.  In fact, a comparison of the total RMS current recorded by the WeldView® Monitor from all 22 welds produced throughout the day revealed that the total RMS current for weld 4 was noticeably greater than the other 21 welds.  This comparison is shown below.

Upon observing the above information the AWC representative went to a computer workstation to examine the current for all of these welds that was recorded by an existing monitoring system that was supplied by the control manufacturer.  That system showed no indication of a current increase on weld 4.

It should be noted that even though the WeldView® Monitor used for these tests has more than 10 times the dynamic range of other monitors, in most instances only severe welding problems can be detected by measuring current.  Accurate conductance monitoring is a superior method of detecting a problem with the weld machine secondary circuit.  On the machine being examined voltage probes were connected to the transformer secondary cables at the points where the cables attached to the copper fixtures on the machine welding station. This provided the ability to measure the conductance of the weld machine fixture and work piece combination during each weld.

A comparison of the conductance recorded by the WeldView® Monitor from all 22 welds revealed that the conductance of the apparatus was substantially higher during weld 4 than it was during the other 21 welds.  The most dramatic increase occurred approximately 4.2 ms after the start of the weld. This comparison is shown below.

After examining the work piece thermal expansion, cylinder pressure, current and conductance information from all of the welds, it can be concluded that weld 4 failed because the majority of the secondary current from the welding transformer traveled through a pathother than the site of the weld. The machine fixtures have several actuating and positioning devices that are either part of or in close proximity to the conducting path of the weld machine secondary circuit.  All of these fixtures, devices and insulators should be carefully inspected to determine where an abnormal physical contact occurred that allowed thousands of amps of current to flow in parallel with the weld.

Recommendations:

1) Modify the pneumatic cylinder force control to prevent the regulator from undergoing a startup transient upon initiation of every weld.

2) Identify and correct the mechanical problem with the machine that created a parallel current path with the weld being produced.

3) Monitor each welding process. Remove the guesswork from resistance welding.

4) Use the monitor to verify production welding parameters. Immediately detect an incorrect pressure setting or heat setting that would result in faulty welds.

5) Use the monitor to verify proper machine performance. Save time and money by avoiding unnecessary maintenance procedures and by knowing immediately when maintenance is needed.

6) Use the monitor to verify the quality of every production weld. Improve quality assurance by preventing sporadic bad welds from being shipped to customers that cannot be prevented with periodic destructive tests.

7) Eliminate destructive testing. Increase productivity and save money.