Spot Welding

Questions and Answers

Voids and cracks are not desirable in any resistance welding nuggets. In some material systems there is a tendency for cracks to form – such as Aluminum. In other materials oxides and surface impurities can end up in the nugget and voids or cracks form. Some cracks or voids form due to the volume expansion and contraction from liquid to solid state.

Depending upon the material and product and the industry there are specifications. Aircraft has specifications. Automotive has specifications. Individual companies have specifications.

Per AWS D8.1 Specification for Automotive Weld Quality – Resistance Spot Welding of Steel

Automotive specifies that cracks or voids lengths combined in the specified weld nugget area cannot exceed 25% of the nugget width. Therefor 1/3rd the nugget diameter is not acceptable in the automotive industry.

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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

Spot welding is one of the four resistance welding processes as defined by the Resistance Welding Manufacturing Alliance. The four processes are:

            RWMA CD containing 2 day program 390

THE FOUR RESISTANCE WELDING PROCESSES

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There ideal answer is never. Given reality this is not likely. In normal resistance welding on coated material, a spot welding or seam welding wheel is going to react with the coating and wear at some accelerated rate. This will be faster than on bare stock. Dressing the welding surface will be necessary at some point. That point will be determined by the speed of the wear and the equipment being used.

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Yes, changing the transformer turns ratio will change the current. A transformer with a tap switch works on this principal. The tap switch is wired to various points in the primary winding in order to vary the turns ratio. By changing the tap switch position the turns ratio is changed. One can choose the lowest voltage and resultant current in Tap 1 or the highest voltage and current output in the highest Tap. The number of taps and their range depend upon the transformer design.

Tap switch

Press Welder Transformer Tap Switch

Many weld transformers are manufactured with a tap switch. This tap switch gives one the ability to change the turns ratio as needed.  The net result is the output voltage changes up or down as the tap is changed.

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A common use for RWMA Class 20 material is spot welding of galvanized steel. Its properties of anti-sticking and heat resistance have made it one of the choices for this sometimes difficult welding operation. Other materials Class 2 and Class 1 can and are used for galvanized also. What makes this premium material a choice to consider for galvanized steel welding?

It is well known that the zinc/galvanized coating readily alloys with the copper of the electrode and weakens its surface structure and forms a layer of brass on the welding surface. Any electrode alloy which maintains its strength and retards this surface condition is a candidate for galvanized welding.

Resistance to Annealing RWMA Copper Alloys

In this chart Class 20 is shown to resist heat and maintain it strength at elevated temperatures. In resistance welding to take advantage of these properties Class 20 should be used in higher heat applications. In many cases it is used exactly this way. The weld schedules are being run hot. The electrodes are subjected to a lot of heat, potential annealing, mushrooming, brassing and sticking. The weld schedule by choice is being run at the high end of the weld lobe.

Weld Lobe

In the weld lobe shown a good weld can be made at 14 cycles with 9.5-11.5 amperes. Class 20 applications are used when the schedule is set to run at 11.5 as marked. This means the electrode will see expulsion frequently if not all of the time. These companies want to insure they are hot enough to never be cold and fall to the left of the blue line. They choose to use a premium electrode material and run hot to insure nugget integrity. With wear the nugget will still fail but with history they know when and change electrodes or redress before this happens.
The other choice of course is to turn the current down below the expulsion level and operate between the blue and red line with a safety factor.
Other materials with similar properties are:

RWMA CLASS 1 2 20

The other choices are either RWMA Class 2 or Class 1 alloy electrode material depending upon how hot you are running. At the reduced heat operating levels between the red and blue lines these alloys will perform well. Most of industry is welding galvanized and coated materials with RWMA Class 1 and 2.

If you are considering changing electrode material and potentially altering the weld schedule, keep in mind the following:
Pressure, current and time (PCT) are the variables that could be adjusted. This may lead to requalification of the weld set-up depending upon your in house procedures and the amount of the schedule change.

Reference: RWMA Resistance Welding Manual 4th Edition
                   AWS J1.3 Specifications for Materials Used in Resistance Welding electrodes and Tooling

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