Recommendations for Selecting Spot Welding Electrodes

Selecting the proper electrode for your spot-welding job can seem daunting. The vast array of shapes, sizes, and materials can make it difficult. However, selecting the correct electrode is critical to consistently making quality welds — Fig. 1.  

The electrodes themselves serve four critical tasks that are crucial to the weld quality of your job: 

• They conduct current to the workpiece. 

• They apply electrode force, which determines the resistance at the various contact points between the electrodes and workpieces and the workpieces themselves. 

• They contain the molten weld nugget. 

• They dissipate heat from the weld zone during and after the weld is complete. 

Answering a few basic questions can help narrow your electrode selection down quickly.

1. Are you making a spot weld or projection weld? 

Is this a spot weld, or is it a projection weld (typically a weld nut or a weld stud)? Spot welds require electrodes with a distinct weld face to control the size of the spot weld itself, while projection welds require larger flat electrodes to support the workpiece(s) and/or the nut or stud. Special electrodes are available for projection welding nuts and studs — Fig. 2. 

2. What is the material that is being welded? 

Resistance welding can be used to weld a wide variety of materials, from common steel and aluminum to coated steels and almost any other type of metal, including copper. Knowing the type of material or materials you are welding is necessary to select the correct electrode alloy for your application. Group A alloys are copper-based alloys used for electrodes and tooling. The most common Group A alloys are as follows: 

• Class 1: These are typically used to spot weld low-resistance materials like aluminum, brass, or bronze. 

• Class 2: These are normally used to spot weld steel. This is the most common electrode material. 

• Class 3: These are usually used for tooling and higher-pressure spot welds. 

Group B alloys are refractory metals typically used in projection welding applications or when spot welding or brazing highly conductive materials such as copper. These materials are usually a facing that is brazed to a Group A material base. See Table 1 for the common uses of all the RWMA Class Alloys. 

3. What nose should I use? 

There are six standard electrode noses for spot welding electrodes for various spot-welding applications. The different noses are available in a variety of weld face sizes — Fig. 3. Noses can be paired for general spot-welding applications or mixed to achieve various requirements. Here is a list of common examples when different nose profiles can be useful: 

• Prevent surface damage on Class A welds. Use a C nose or flat face. 

• Break tough oxide layers on aluminum welds. Use a F nose or radius face. 

• Control the weld nugget location in spot welding different thicknesses of material. Use different diameter weld faces. 

• Weld in a tight space, such as a corner or flange. Use a D nose or an offset face. 

4. What are the specifications of the desired weld? 

 What is the size of the desired weld, and what are the thicknesses of the workpieces? Your engineering drawings for your workpiece should define your minimum nugget size, and your electrode weld face should be larger than your nugget size. Knowing those pieces of information will help you select the proper electrode with the correct weld face size. Reference the RWMA Resistance Welding Manual (https://pubs.aws.org/p/323/rwma-resistance-welding-manual-revised-4th-ed) or AWS C1.1M, Recommended Practices for Resistance Welding (https://pubs.aws.org/p/2152/c11mc112019-amd1-recommended-practices-for-resistance-welding). Both publications are a great starting point for electrode selection and weld schedules. 

5. How will the electrodes be mounted? 

The majority of resistance welding electrodes are usually held in their electrode holders with a friction taper fit. The tapered fit electrodes can be easily removed and have internal holes for cooling. Electrodes are also available with thread mounting. Specialty electrodes can be held with clamping mechanisms or retainer nuts where machine clearances don’t allow traditional electrodes to be installed or where welding forces would otherwise make tapered or threaded mountings impracticable. You want to select electrodes compatible with the current tooling and weld forces. Table 2 shows weld force ranges for the RWMA tapers. 

6. How will the electrodes be cooled? 

Proper cooling is the best way to maximize electrode life. Electrodes should be cooled with clean water at a rate of 0.5 to 1.5 gal/min — Fig. 4. Select electrodes and holders with water tubes that will sit approximately 0.125 in. below the bottom of the electrode.  

For certain applications, such as projection welding, it may not be practical to cool the electrodes internally with water tubes. In these cases, investigate a water-cooling jacket that mounts to the exterior of the electrode — Fig. 5. 

7. Do you have dissimilar materials or metal thicknesses? 

When welding dissimilar materials or metal thicknesses, there will be differences in the resistance of the workpieces to be welded — Fig. 6. The smaller weld face generates more heat; thus, shifting the weld nugget into the thinner sheet. When using the same electrodes to weld two different material thicknesses, the weld nugget will be biased toward the thicker sheet, because the thicker sheet has higher resistance. To shift the weld nugget toward the thinner sheet, it is recommended to use a larger weld face on the thicker sheet. This will reduce the current density and increase the heat dissipation on the thicker sheet. The same result could also be accomplished using an electrode with higher resistance on the lower sheet. The goal with heat balancing is to utilize the electrode material or weld face diameter to compensate for differences in the resistance of the workpiece. You want the weld nugget to penetrate the two workpieces evenly. 

In Conclusion

Once you know basic information about the resistance weld that needs to be performed, you can start the selection process of the electrodes themselves.

This article was written by Mitchell Radford (senior product engineer at Tuffaloy Products Inc., Greer, S.C.) for the American Welding Society. Figs. 1–5 were developed by Tuffaloy, and the two tables contain content from the company’s website.