Spot Welding

Questions and Answers

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

The answer is no the base plates should not be hot. If they are the reason must be determined.

Tooling and fixtures are used in virtually all resistance welding operations to hold parts in place for welding. They must be kept clean of flash and debris and be safe for operators. Heat buildup should be minimized. If a fixture is heating up several possible reasons come to the forefront: Inductance, Insulation, weld flash and location.

Metal in Throat

                       BAD                                            GOOD

This exhibits the part in the throat area but magnetic tooling in the throat causes the same problem. It absorbs energy and heats up during the welding process and forces the welder to work harder to make the desired spot weld.

Inductance is an electrical field induced in any magnetic material in or near the throat of an AC resistance welder. Insulated or not any magnetic material in or close to the throat of the AC machine can be affected. It will absorb energy and heat up. This includes the fixture itself or its components (bolts & nuts) or base plate. To prevent this, fixtures, base plates and other components should be fabricated from nonmagnetic materials (aluminum, copper, nonmagnetic stainless or insulators). Inductance is a common problem in the field. Many tool rooms are not aware of the problem (inductance). To check your tool, test your tool or components with a magnet to see if it is fabricated with magnetic material.

Insulation of the fixture is also necessary to prevent any current from flowing through the fixture and heating it up by resistance heating. This means insulating bolts, nuts, and washer with washers and sleeves as well as the mating surfaces underneath or between.

The other housekeeping issue is weld flash. A buildup of weld flash can lead to current flow on an otherwise insulated fixture. A stray current path of this sort can lead to heating of the fixture and loss of power at the welding operation.

The proximity (location) of the weld base to the welding should also be considered. Normally the electrodes handle the greatest heat and cooling functions. If the weld fixture base is too close it may come in contact with large amounts of heat and it may also need some form of heat dissipation. This could be a chill block or direct water cooling.

 

Weld Fixture

Weld Fixture Exhibiting Non Magnetic Stainless Tooling and Insulated Pins and Tools with Water Cooling

Reference: RWMA Manual Chapter 19

 

Improper weld current selection can cause problems during resistance welding. The problems normally show up in the form of expulsion, indentation, electrode sticking, mushrooming or low weld strength. There is an ideal current range for every resistance weld set up. This will produce good weld nuggets with little or no expulsion, modest indentation and minimal electrode wear or sticking. Whenever you push the extremes of the current in the weld window either high or low you risk problems.

Electrode face mushrooming is the normal wear mechanism of a spot welding electrode. To slow this process or prevent excessive mushrooming one must control several factors in the welding process.

 Mushroomed electrode

EXTREME MUSHROOMING

Back up electrodes are used for projection welding electrodes. They are normally used on the lower side and are flat faced. There is no need for a defined weld face since the projection concentrates the weld current and heat into a desired spot for nugget creation.

Have a Question?

Do you have a question that is not covered in our knowledgebase? Do you have questions regarding the above article? Click here to ask the professor.