The shunt effect in resistance spot welding occurs when part of the welding current flows through previously made, closely spaced welds. This diverted current reduces the nugget size of the new weld, weakens joint strength, and can lead to weld defects. The phenomenon is especially important in multi‑spot welding applications such as automotive body assembly.
This study investigates how the shunt effect behaves under different welding conditions. The following factors were analyzed:
- Weld pitch (spacing)
- Material strength
- Sheet thickness
- Number of welds in sequence
- Material–thickness combinations in three‑sheet stacks
1. Weld pitch
Weld pitch is known to be the most influential on the shunting effect. To isolate the influence of weld pitch on shunting behavior, two consecutive spot welds were simulated at fixed weld pitches of 20 mm, 30 mm, and 40 mm. All cases used identical welding parameters targeting a 5 mm nugget diameter in the first weld, ensuring that any variation in the second weld resulted solely from changes in pitch distance.
Effect of Pitch on Shunting Current
- Increasing the weld pitch effectively reduces the shunting current through the previously made weld.
- Shunting current is high (due to higher initial contact resistance in the new weld) at the start of the welding process and then drops as welding continues.
Nugget Size Reduction
| Weld pitch | 20 mm | 30 mm | 40 mm |
|---|---|---|---|
| First weld | 5.01 | 4.97 | 5.00 |
| Shunted weld | 4.32 | 4.42 | 4.47 |
| Size reduced | 14% ⇓ | 11% ⇓ | 10.6% ⇓ |
- Weld nugget size decreases more in the shunted weld with a shorter weld pitch due to higher shunting current through the previously made weld.
2. Material Strength
To compare how different steels respond to shunting, the same two‑weld configuration was simulated using DC06, DP600, and USIBOR1500 sheets. The weld pitch was held constant at 20 mm, and each material was welded using its own optimized current level to achieve a 5 mm nugget in the first weld. Welding time and force remained constant across all materials.
Shunting behavior
- The shunting current loss is higher in lower-strength steel due to its lower resistance in the sheet material.
- Shunting current is high at the start for all materials and reduces as welding continues due to the dynamic resistance in the new weld.
Nugget Size Reduction
| Material | DC06 | DP600 | USIBOR1500 |
|---|---|---|---|
| First weld | 5.01 | 5.15 | 5.14 |
| Shunted weld | 4.32 | 4.85 | 4.85 |
| Size reduced | 14% ⇓ | 5.8% ⇓ | 5.6% ⇓ |
- The shunt effect is more significant with the mild sheet (due to lower resistance in the sheet material) than that with high-strength steel when welded at the same weld pitch (20 mm).
3. Sheet thickness
The effect of sheet thickness was evaluated by simulating two‑weld sequences using 1.0 mm and 1.5 mm sheets for both DC06 and DP600. The weld pitch was fixed at 20 mm, and identical welding parameters were applied within each material grade. This allowed direct comparison of how bulk resistance and thickness influence shunting current and nugget reduction.
Shunting current
- The thicker sheet combination has a higher shunting current loss than the thinner sheet combination.
- Shunting current is higher at the start for all thickness cases, then drops as welding continues.
Nugget size reduction
| Sheet thickness | 1.0 mm | 1.5 mm |
|---|---|---|
| First weld | 5.01 | 5.47 |
| Shunted weld | 4.32 | 4.37 |
| Size reduced | 14% ⇓ | 20% ⇓ |
| Sheet thickness | 1.0 mm | 1.5 mm |
|---|---|---|
| First weld | 5.15 | 5.38 |
| Shunted weld | 4.85 | 4.90 |
| Size reduced | 6% ⇓ | 9% ⇓ |
- The shunted weld nugget size in thicker sheets decreased more than in thinner sheets due to higher shunting current loss.
4. Number of Welds
For a better understanding of how the shunt effect evolves in real production scenarios, a series of six spot welds was placed from left to right with the same weld pitch of 25 mm. As each new weld forms, all existing welds would result in accumulated shunting current loss.
Nugget size reduction
- The nugget sizes are continuously decreasing through each weld.
- The nugget shapes lean more towards the existing welds.
| 1st | 2nd | 3rd | 4th | 5th | 6th |
|---|---|---|---|---|---|
 |
 |
 |
 |
 |
 |
| 4.91 mm | 4.38 mm | 4.17 mm | 3.97 mm | 3.97 mm | 3.59 mm |
| - | 10.8% ⇓ | 15.1% ⇓ | 19.1% ⇓ | 19.1% ⇓ | 26.9% ⇓ |
| Order | Cross section | Nugget size | Size reduced |
|---|---|---|---|
| 1st | |
4.91 mm | - |
| 2nd | |
4.38 mm | 10.8% ⇓ |
| 3rd | |
4.17 mm | 15.1% ⇓ |
| 4th | |
3.97 mm | 19.1% ⇓ |
| 5th | |
3.97 mm | 19.1% ⇓ |
| 6th | |
3.59 mm | 26.9% ⇓ |
5. Three-sheet Combination
More complex welding cases in three-sheet combination were studied to evaluate how shunt current influences nugget formation. The study was conducted with different material-thickness combinations (DC-HSLA-DP) at three different weld pitches.
Shunt Sensitivity
| DC-HSLA | HSLA-DP | |
|---|---|---|
| First weld | 4.78 | 6.15 |
| Shunted weld | 3.42 | 5.69 |
| Size reduced | 28.5% ⇓ | 7.5% ⇓ |
- Nugget size at mild steel sheet interface (DC-HSLA), due to lower resistance in mild steel, reduces more than the nugget between higher strength steel sheets (HSLA-DP).
Effect of pitch
- Increasing the weld pitch reduced shunting effectively.
- Nugget size at mild steel sheet varies more significantly compared to the weld nugget between higher-strength sheets.
| Weld pitch | 20 mm | 30 mm | 40 mm |
|---|---|---|---|
| First weld | 4.78 | 4.86 | 4.86 |
| Shunted weld | 3.42 | 3.74 | 4.31 |
| Size reduced | 28.5% ⇓ | 23.0% ⇓ | 11.3% ⇓ |
| Weld pitch | 20 mm | 30 mm | 40 mm |
|---|---|---|---|
| First weld | 6.15 | 6.07 | 5.97 |
| Shunted weld | 5.69 | 5.70 | 5.85 |
| Size reduced | 7.5% ⇓ | 6.1% ⇓ | 2.0% ⇓ |
Summary
The shunt effect diverts current through nearby existing welds, reducing current in the new weld, thereby causing smaller nugget. The case studies show how it depends on different factors:
- Weld pitch
Shorter weld pitch increases the shunt effect and reduces nugget size. - Material strength
Mild steel shows higher shunt loss than high-strength steel. - Sheet thickness
Thicker sheets increase shunt current loss. - Number of existing welds
Shunt effect accumulates with increasing number of welds. - Multi-sheet stacks of different steels
Weld nugget at mild steel reduces more than that between high strength steels.
