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Resistance Welding: Solutions
Resistance Welding: Solutions
Roger B. Hirsch, is ‘Elihu Thompson Resistance Welding Award’ winner, President of Unitrol Electronics Inc. of Northbrook, Illinois, and Former Chair of the Resistance Welding Manufacturing Alliance (RWMA), a standing subcommittee of the AWS.
We are welding 14 gage steel making about 25 welds a minute. We can get very good welds for the first 50 or so pieces. Then the welds start to get weaker. If we stop welding for about ½ hour, the welds start off good again and then again become weaker after about 50 pieces. Does this sound like a problem with the welding control?
PHOTO 1: Badly arched lower welder arm where it attached to the welder secondary pad. Loose bolts caused this damage.
While it is always possible that a welding control can have a problem, I doubt that this is the case for your problem. There are two things to look for.
- WATER PROBLEMS: If cooling water flow is low, or water temperature is too high, heat will build up in the electrodes, holders, and other water-cooled components in the welder secondary. This raises the resistance of these components and lowers the current in the weld. The most common problem I see is that you are missing a water tube in the electrode holder, or the cooling tube does not go all the way to the bottom of the hollow of the electrode. In this case the electrode will get hot enough to turn a purple color, and the resistance goes quite high. Letting the welder “rest” allows this hot copper to cool down and lowers resistance.
- Welder secondarycomponent problems. Any connection in the welder secondary (exposed copper components at the front of the welder) that has high resistance will heat up during welding and increase this resistance. Another person contacted me recently with this exact problem on a 250KVA aircraft welder. I suggested he check the secondary connections and he found the lower arm connection to the weld transformer secondary pad was loose and had arched as shown in Photo 1. Because the connection was so badly pitted, the surfaces had to be machined and silvered. Once this was done and the bolts tightened, the welder produced the same current over the entire shift.
I weld stainless steel cross wires. Just by accident it was found that we can increase the strength of a weld by lowering the welding pressure regulator. Will this cause a problem?
PHOTO 2: Stainless steel wires being cross welded.
First let’s see what is causing the increased strength. If you lower the force between the rods, the resistance at the contact point will increase. The energy produced at this joint is covered by the formula: Joules of energy = I2 X r X t
I = current
r = resistance
t = time
If you reduce the force at the weld joint area you increase the resistance and therefore increase the energy expended on that joint.
That’s the good news. But you have to be careful when lowering electrode force. One of the purposes of electrode force is to forget the parts together. Lower force will reduce the amount of forging to reduce the reliability of the weld.
Another purpose of the force is to keep metal within the nugget area during the weld process. Low force will start to produce metal expulsion. This removes some of the metal at the joint, and the flying sparks of hot metal can be a danger to the operator and people in that area. Extremely low force will cause voids inside the weld nugget to lower the total joint area and reduce weld strength. And metal spit out between the wires at the nugget area might need to be ground down as an expensive and unnecessary secondary process.
Additionally, if you lower the electrode force, you also lower the force where the electrodes contact the outside of the parts being welded. This heats up the area under the electrodes more than needed and will degrade the electrode rapidly. The result will be greatly lowered electrode life.
Consult a good cross-wire welding chart to get starting values to use. Take the weld force and weld time on the chart, and only adjust the weld current until you reach good stable results. You can find a good chart in the RWMA Resistance Welding Manual, 4th edition, available from the American Welding Society: pubs.aws.org/p/323/rwma-resistance-welding-manual-revised-4th-ed. The chart is in TABLE 6.1. This is a chart for welding steel rods. For stainless steel rods increase the electrode force about 10%. Using this chart you can even adjust the percent setdown (final height of completed joint).
My company is a 3-shift operation. We weld large cold rolled steel door panels on deep throat welders. This includes welding of stiffeners as well as hinge plates and latch brackets. There are two problems that we have observed.
First, the welds produced during the third shift seem to be stronger than the other two shifts even though the welding programs are locked into all of our welders.
The second is that if we set up a weld when the door panel is pushed into the throat of the welder (between the weld’s arms) and then weld a front reinforcement channel when the door panel is pulled almost out of the welder throat, the welds are different in appearance and strength. We try to make our setups in the middle position of the door, but the results are not reliable. This also makes production of strong welds that do not mar the outer “show” surface very difficult.
Modern welding controls can easily solve these problems.
- LINE VOLTAGE VARIATION: Looking at your first question, the voltage going into your welder from the power company is not constant. It will rise and fall during the day and evening in response to the amount of power being used by others on the line. Typically, voltage at night is the highest when most industrial use is low and, in the summer, air conditioning use is minimized.
The voltage available between the electrodes on the welder secondary is directly proportional to the line voltage. Therefore lower voltage going into the welder will result in proportionally lower secondary voltage and lower weld current. This leaves two choices;
First, you can adjust your welding program settings for each shift. That is not a great fix but can help.
The second is to use welding controls that have automatic voltage compensation systems. These systems, called AVC or LVC, will electronically increase or decrease the voltage on the secondary of the welding transformer to produce a reasonably constant voltage for each of your heat settings. All quality modern resistance welding controls should have this feature available for your use. They should be able to hold the RMS (effective) voltage going into the welding transformer to no worse than +/- 1% variation with a line voltage variation of +/-10%. On a 460 volt line, welder output voltage should be within this 1% window for line voltages as low as 414 volts and as high as 506 volts.
- 2. METAL INTO WELDER THROAT: As you push the metal panel back onto the welder, the steel increases the impedance (AC resistance) of the welder secondary. This means that for the same voltage at the welder secondary a lower current is produced. This lower current reduces the weld strength considerably.
PHOTO 3: Large panel pushed all the way into the throat of the welder.
PHOTO 4: Large panel welded all the way out of the throat of the welder.
I worked on a welder that had a 40” throat. The parts being produced were, at times, pushed pretty much the full depth of the arms and at other times only 4” into the area between the arms. With no change to the welding program, the current produced at when the part was pulled almost out of the throat was 9,800 amps. When the part was pushed fully into the throat, the amperage dropped to 7,300 amps. The welds produced at these two locations were very different.
One solution is to rotate the part so that it does not go as deep into the throat. This means handling large heavy sheets and requires a really good support system and a strong operator. This also reduces production output by adding a lot of wasted material handling.
The best solution is to use a welding control that provides some form of constant current function. This function can maintain the welding current at both extremes of part position by electronically changing the welding firing and using feedback to target a desired welding current. Some controls are more successful than others at reaching rock-solid current values between the electrodes. The best will hold the welding current variation to a range of about +/1% with parts fully into and out of the throat.
This becomes even more challenging when you are seam weldingwith rolling electrodes. One process I have worked on over the years is the joining of steel coil ends during the processing of sheet steel into coils. When one billet has been rolled out, the end of the steel strip has to be joined to start of the next rolled sheet. This is often done by having a seam weld wheel assembly move over this splice while the metal stays still.
The sheets can be 8 feet or wider and this changes the secondary impedance dramatically as the wheels travel deeper into the steel sheet. Without some form of constant current ability from the welding control, the welds produced at the end of a seam would be considerably weaker than the starting welds. This can cause these joints to fail as the steel sheet if processed through rollers under high stress on the way to the coiler.
PHOTO 5: Setting a weld program in CONSTANT CURRENT mode using direct amps.
Ironically, many companies own modern resistance welder that have the ability to operate in the constant current mode but do not know it exists. They paid for a quality control and are not taking full advantage of the control’s ability. Check your control to see if it has this ability. If not, it is probably time to upgrade your welder if you want consistent weld quality on large parts.