Why Do Bonding Problems Happen — and What Can We Do to Prevent Them?
Recently, we’ve talked a lot about the different materials that can be used for resistance spot welding electrodes — materials that, alone or in combination, need to stand up to high currents and large pressing forces. But of course, that’s not the whole story. Even if you have a 100% infiltrated copper tungsten electrode, in a ratio of say, 30% copper and 70% tungsten, providing the best of both materials — that is, the outstanding stability and strength of tungsten at high temperatures, and the great thermal and electric conductivity of copper . . . even then, things can go wrong.
That’s because beyond choosing the materials for your spot welding electrode, you must also have good electrode design and construction if you’re going to get consistent, high-quality welds — not to mention, get the longest life from your electrode investment. You need not only the correct materials and tip and shank profiles, but also proper bonding in the manufacture of the electrode itself.
Why Bonding of Electrodes Matters
Deficiencies in electrode design and bonding can cause variations in the weld quality of end products, including weak or failed welds. This is because defective joins in the electrodes themselves reduce thermal conductivity and cause the electrical resistance of the electrode to fluctuate. These deficiencies can also decrease the life of the electrode, requiring more frequent repair or replacements.
For example, electrodes made from tungsten or molybdenum alloys with a copper shaft are great for many resistance spot welding applications. After all, tungsten and molybdenum have high melting points and are stable at high temperatures, while a copper electrode shaft dissipates heat quickly. However, problems arise when a manufacturer joins these alloys to their copper shaft by using a brazing process.
When the brazing filler is not distributed evenly at the join between the electrode material and the electrode holder, it results in cavities that have an impact on electrical resistance and reduce thermal conductivity. The end result is inconsistent bonding quality, with the cavities reducing the effective bonding area of the electrode. In addition, voids caused by inadequate capillary action or incorrect alignment of the parts during the brazing process can also lower the bonding strength of electrodes.
Electrode tip heating issues are another problem that can be caused by deficiencies in electrode bonding. That’s because when weld current passes through an electrode, the heat that is produced is widespread — it’s within the body of the electrode body, the electrode tip, the tip-to-part interface, the parts beings welded, and the part-to-part interface. With each subsequent weld, the residual heat in the electrode tip builds before stabilizing at some average value, depending on the welding rate and the weld energy.
While residual tip heat is usually not a big deal when you’re welding manually — and therefore, at a slow rate — it can be a big problem in automated welding environments, where the welding rate can reach one weld per second or faster. Here, residual electrode tip heat can cause welding problems such as:
- Increased part deformation
- Part cracking
- Rapid tip oxidation
- Rapid part material/plating buildup on the tip
- Reduced weld strength
- Severe tip-to-part sticking
- Severe tip geometry wear
- Shorter tip life
Excess weld current density can also result in severe electrode sticking, tip area deformation, and tip length bending — all of which, in turn lessen, the bonding ability of electrodes. In addition, surface contaminants such as oil, grease, rust/oxides, scale, or dirt can form a barrier that prevents proper brazing in electrode manufacture — resulting in the actual dropout of the electrode material from its shaft.
How to Avoid Faulty Electrode Bonding
Fortunately, we can help many of problems noted above by creating a seamless join between electrode and shaft. With a better method of bonding an electrode to its holder, you can get more consistent electrical resistance and thermal conductivity, more reproducible welding performance, and longer electrode service life. One such method of seamless joining is the non-defective bonding (NDB) of electrode assemblies made from tungsten, molybdenum, and their alloys, including copper tungsten, silver tungsten, and silver tungsten carbide.
A seamlessly bonded electrode is one that has no filler between the shaft and the electrode tip, for a join that is nearly 100% — creating a more effective bonding area that, in turn, produces a stronger and more consistent weld between the work pieces. In addition, seamlessly bonded electrodes optimize thermal cycling, to reduce heat load and electrode consumption.
In the case of NDB, the method retains the characteristics of the electrode material components while fusing the materials in a mold to form a bond of only a few micrometers. Unlike brazed electrodes, NDB electrodes provide constant thermal conductivity and electrical resistance due to the seamless join between the electrode and shaft.
Put Electrode Bonding Up There with Material Choice
Without the right welding electrode design and construction, your end product can be plagued by production delays due to process inefficiency, electrode failure, or weak welds. Proper bonding of your electrodes — using a process such as NDB — is as important as your choice of materials in ensuring consistent weld quality, minimum electrode sticking, and maximum electrode life.
Simply stated, the better the bond between the materials comprising the electrodes themselves, the better and more reproducible your welding results will be. If you’d like to learn more about the NDB method and its advantages in the manufacture of resistance spot welding electrodes, you can download our free white paper on the non-defective bonding of resistance spot welding electrodes.