Spot Welding

Questions and Answers

This question comes up frequently. Many look for a formula to set the squeeze and hold time for resistance welding.

Several articles have been written on this subject in this blog:

HOW DO YOU SET THE PROPERSQUEEZE TIME IN A RESISTANCE WEDING SCHEDULE?

IS THERE A FORMULA TO DETERMINE THE SQUEEZE TIME IN RESISTANCE WELDING?

IS SQUEEZE TIME IMPORTANT?

HOW WILL HOLD TIME CHANGE THE RESISTANCE WELD STRENGTH?

Spot Weld Schematic

The answer is there is no formula for squeeze or hold time. Squeeze time is meant to close the electrodes on the part. Every machine does this at a different speed due to part configuration the equipment and distance to travel. Hold times are dictated by similar variations beyond control. The only fact is that a good weld requires bothsqueeze and hold time to function properly and produce a good strong weld nugget.

Proper squeeze ensures the part is held tightly with the proper force to contain the weld and prevent expulsion during the weld cycle. Hold cools the weld under force ensuring the nugget solidifies properly under pressure to minimize voids and maximizes strength. Any compromise on the time of these two functions risks the proper nugget generation and formation.

The proper time for each has to be developed for each application and machine. There are pressure monitors that can detect when force has been reached or not and tell the control to proceed to weld in place of a fixed squeeze. Some machines are so slow that 10 cycles of hold results in 20 cycles before it opens. Each application dictates its own values.

The front office wants faster weld schedules. The front office wants no rework. No squeeze and no hold is faster but will generate more rework. There is a happy medium

A formula does not apply.

Reference: RWMA – Resistance Welding Manual 4th Edition

Resistance brazing is not spot welding so the electrode face geometry does not have to concentrate the heat into a small area. Frequently the electrode face is machined to conform to the surface that it is in contact with. Frequently the electrode will contact the entire work piece surface. In some cases the electrode faces is milled out. The work piece is nested into the face of the electrode in order to insure proper part placement and alignment. A small vacuum is sometimes pulled through the center of the electrode to hold the part in place before the electrode closes. This is very common for small part brazing.

The electrode material is frequently RWMA Class 2. It could be ETP copper and in many cases carbon electrodes are used.

Refractory faced electrodes2

When resistance brazing contact materials, refractory faced electrodes RWMA Class 11, 13 and 14 as well as Class 2 are commonly used.

The first choice of course should be the least expensive, stocked product. After the coppers that would be Class 2. It is easily machined to fit shapes and is stocked as electrodes and bar stock.

I am not sure of carbon electrode pricing and availability. It should fit in here somewhere for large amounts of heat between Class 2 and the refractories.

If the resistance braze generates large amounts of heat for a long time. (Some resistance brazes are 10 - 20 seconds or longer - not cycles. I have seen some over one minute long.)  The electrodes can get very hot. If the Class 2 is struggling (excessive wear and sticking) then turn to the refractory faced electrodes Group B, Class 10 – 14. The heat and temperature will not phase them. The only caution is that they are brazed assemblies. Their face can glow red but don’t let that red reach the braze joint holding them on the base material. To keep the braze joint from failing pulse the heat once in a while momentarily, if needed.

Of the refractories Class 10 is not stocked and not readily available.

The first choice should be Class 11. This is a copper tungsten material available as faced electrodes and bar stock. It is machinable and heat resistant with great strength and wear at high temperatures.  This is the best choice for most that need a refractory material, when Class 2 can't handle the heat or sticks too much.  Class 12 does not add that much heat resistance for the cost.  Class 14 ups the price substantially and is worth it if you have to have it.  But most can get by with Class 11 for the cost difference.  Don't forget hard Carbon.  It is pretty forgiving and works well and glows red for a long time.

Class 12 is the next level of copper tungsten with more tungsten and less copper. It also is stocked as faced electrodes and bar stock. It has a little more strength and heat resistance than Class 11 and is still machinable.

Class 14 would be the next level of heat and strength resistance. This is pure Molybdenum. Very strong and heat resistant. It is machinable with care. Costs are increasing and stock is not as available with this material.

Class 13 very heat resistant and strong. This is pure tungsten. Heat does not bother this material. Very costly, Not machinable, Must be ground, Attachment by mechanical means is best. This is for specialty applications only.

In summary use copper, or copper alloy – Class 2 when possible

Long resistance brazes mean heat buildup this pushes the capabilities of standard material and then one turns to:

Hard carbon is a candidate for long hot welds

RWMA Refractory Group B Class 10 – 14 are candidates

Class 11 (Copper/Tungsten) is most frequently used.

Class 14 (Molybdenum) in some applications

Class 13 (Tungsten) as a last resort if it is the only electrode that will work

Reference: CMW Inc. Product Resistance Welding Products Catalog
                   Tuffaloy Products Catalog
                   RWMA – Resistance Welding Manual 4th Edition

Pressure, current and time (PCT) are the basic functions performed by the resistance welder. These functions are controlled or initiated by the weld controller. It initiates each step when told to start the weld process by input from the foot switch or automation PLC. The controller allows time for each step to operate and controls the current amplitude. In simplified form this is Squeeze, Weld and Hold. The squeeze sequence allows the pressure system to build up the force to contain the weld. The weld function is the actual current flow and is totally controlled by the weld controller. It regulates the amplitude and time of current flow. Hold is the period which allows the weld nugget to cool down and solidify under force. The controller regulates this time.

MFDC systems over the last twenty years have been the dominant new welding system. It has found usage in most new installations. That is not to say that AC systems do not have their place in the industry. They do and will continue to be used. Below is a comparison of the two systems pro & con:

Comparison of MFDC and AC 2

COMPARISON OF MFDC VS AC

A more complete comparison can be found in another article in this blog:

COMPARE MFDC vs AC IN RESISTANCE WELDING

If we look at projection welding. Projection welding has worked well with AC for years.

MFDC works best when:
• There is a power shortage
• Need short weld time
• There is a need to restrict the heat effected zone
• There is a requirement to reduce the weight or size of the weld equipment

AC works best when:
• The MFDC need parameters listed above are not necessary
• AC equipment is available to repurpose to a new application.
• Personnel are familiar with the AC equipment. No training necessary.
• Equipment has long life
Repurposing or bringing in a new piece of equipment similar to the previous and its long life is where AC performs best.

 Ring Projection 30 Ka 2                                   projection nut weld

MFDC RING PROJECTION                                             AC PROJECTION WELD

WELD @ 30 Ka for 100 ms

MFDC Inroads:

MFDC has made improvement with applications that push the envelope for AC.
Examples are Larger spud welding with high power needs and nut welding where short weld time has resulted in better weld quality. In the large spud welders, MFDC delivers large amounts of power from a balanced three-phase input with very short weld times. In nut welding some of high strength steels weld best with short 10 – 30 millisecond weld times. This requires MFDC. These short weld times reduce the embrittlement of the weld.

To summarize, the marketplace that uses robots use MFDC systems to reduce weight. Some use MFDC for technical reasons (short heat times).

AC Facilities:

Those facilities not using robots have a choice and often they repurpose used AC systems or stick with familiar AC systems that cost less, unless there is a technical/power reason to upgrade to MFDC.

 

Reference: RWMA – Resistance Welding Manual 4th Edition
AWS – Welding Journal, Q & A July 2019

 

An inquiry came in suggesting that two forces are responsible for creating a resistance weld. Let’s examine this question. To do so we will look at two previously published articles in this blog:

“WHAT IS AC RESISTANCE WELDING”

AND

“WHAT IS THE DEFINITION OF RESISTANCE WELDING”

Per the definition of the Resistance Welding Manufacturers Alliance:

RESISTANCE WELDING IS THE JOINING OF METALS BY APPLYING

                                        PRESSURE
                                     AND PASSING
                                        CURRENT
                                FOR A LENGTH OF
                                           TIME
    THROUGH THE METAL AREA WHICH IS TO BE JOINED

 

To expand upon this PRESSURE is provided by mechanical means of pneumatic or hydraulic cylinders, servos, cams and sometimes manually. Normally it is measured in pounds, kilos or equivalent. It is applied in sufficient amount to contain and control the weld to a specific desired area, before the current is applied. CURRENT is supplied by the control and transformer. The transformer does just that transforms relatively small current input from the buss to many thousand amperes needed for resistance welding at people safe low voltages. The control determines the amount of CURRENT and the TIME.
Heat is generated according to JOULES LAW

Joules Law

The heat is generated by the current squared, times the resistance, multiplied by the time. It is the current flowing through the area to be joined for a length of time and the resistance at that joint which generates heat to make a joint. The force holds all of this together and forges the joint as it forms and cools it down.

In answer to the question posed the resistance weld is formed per Joules law and the application of pressure and current for a length of time. This is commonly referred to as “PCT”. Three forces or factors control the resistance welding process. The welding machine provides this through this through the:

Pressure System – (P)Pneumatic or hydraulic cylinders or servos

Power System - (C) of the transformer and conductors

Controls – (C & T) Meters the time and amount of current and machine operations

AC resistance welding uses AC current and transformers for the power in resistance welding. Most rocker arm and press welders have been manufactured with AC power supplies.

The equipment can also be DC direct current.

In the last thirty years MFDC mid frequency direct current has become the go to equipment for automated robotic systems and many other applications.

There are many articles published in this forum about the advantages and disadvantages of the AC, DC and MFDC systems. One can find these by performing a search on the home page of this forum.

Reference: RWMA – Resistance Welding Manual 4th Edition

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