1. Resistance welding Principle of resistance welding
(Joule heating + Pressure)
The metals are melted by Joule heat (i.e., ).
Pressure is imposed to confine the molten nugget between the interfaces.
Therefore, the process parameters are the current, time and pressure.
Resistance Spot Welding (RSW)
Is most widely used.
In the figure of RSW,
the R at the interface is larger than
that of the bulk metals.
The resistance welding is
the fusion welding or SSW?
RSW: fusion ?
Butt & Flash W: SSW ?
Fig. 9.2 Schematics of resistance spot welding

2. Resistance welding processes
Various processes have been developed: RSW, Seam welding, Projection welding,
Butt welding, Flash welding, HFRW, HFIW, etc.
Feature of each process is how to concentrate the resistance heat (or current)
Fig. 9.1 Resistance welding processes

3. Determination of process parameters in RSW: Weldability Lobe
Two issues of RSW: Welding condition and Electrode life
(You cannot see the molten nugget)
To determine welding parameters, the weldability lobe is used.
Use a fixed pressure (step 1)
Use a fixed weld time (step 2)
Change current and conduct welding
Measure the nugget diameter
Change weld time and repeat step 2
Make weldability lobe (Weld current vs. time)
Change pressure and repeat step 1.
To measure the nugget size,
a simple peel test is used in Fig. 9.5.
The nugget diameter and behavior of tearing
indicate the bond strength & quality of welds.
Fig. 9.3 Construction of weldability lobe
Fig. 9.5 Peel test coupon

4. Determination of electrode life in RSW: Oscillating weldability lobe
Electrode life is important in weld quality and productivity. (Note: electrode itself is cheap)
Prediction of electrode life is done experimentally using the oscillating weldability lobe.
To determine oscillating weldability lobe in Fig. 9.4,
Use fixed parameters (I, t and P)
Conduct RSW
Conduct the peel test
Measure the nugget size
Proper nugget diameter is
The electrode should be replaced
after the determined life.
Electrode lives of coated steel & Al
are very short…
Al & Cu are difficult to weld.
Why?
Fig. 9.4 Electrode life test showing the relationship
between nugget size and number of welds

5. Monitoring nugget size - Dynamic resistance
It is difficult to monitor the nugget size in real-time because you cannot see the nugget.
The resistance changes during the weld time depending on the contact area, resistivity
and indentation in Fig. 9.6.
Thus, nugget size can be measured indirectly by measuring the resistance in real time.
The idea is very nice…
But for coated metals, the resistance is affected by the coating in Fig. 9.7.
Therefore, its application is limited!
Fig. 9.6 Dynamic resistance curve
of plain carbon steel
Fig. 9.7 Comparison between dynamic resistance curves
of plain carbon, galvanized and stainless steel

6. Monitoring nugget size – Electrode displacement
Another way of monitoring the nugget size is the Electrode displacement.
- When the metals are heated, they are expanded.
- When the nugget becomes larger, indentation occurs due to softened metals.
Therefore, the nugget size can be monitored indirectly
by measuring the displacement of the electrodes.
The principle is very nice, and it is not affected by the coating.
The disadvantage is the proper sensor
and its attachment.
Fig. 9.8 Electrode displacement
with upper and lower tolerance limits

7. Inclined Spin Electrode
Process developed by Nippon Steel Co.
Good and Simple idea to elongate
the electrode life.
Mathematically, electrode life is extended
by 6 times.
In practice, the life is longer by 20 times.
due to self-dressing effects.
Cons
Since rotating the electrode is difficult,
it is not used in practice.
Note: You can propose such nice ideas
if you are interested in and
focus on the subject continuously!!
Fig. 9.9 Inclined spin electrode system
to increase electrode life

8. Resistance seam welding
Electrode of the seam welding is a roller or wheel.
Continuous welds are made by seam welding.
Mash seam welding uses Joule heating and high pressure (forging)
so that the overlapped seam has flat surface -> Solid state welding..
It is applied to TBW (Taylored Blank Welding).
Fig Results of various seam welds
Fig Mash seam weld

9. HFRW & HFIW
Principle: Concentrate the resistance heat by HF current (Skin and Proximity effects)
Forge the molten parts by suqeeze roll
At final stage, welds are made with SSW!
HFRW & HFIW are widely used to fabricate the pipe.
For small diameter: HFIW or HFRW
For medium diameter: HFRW
For large diameter (>1m): Arc welding
Fig High frequency resistance and high frequency induction welding

10. Parallel gap welding
Parallel gap welding is used for micro-joining and
involves accessing the weld area from the same direction with both electrodes.
- In parallel gap welding, the gap between the electrodes is typically less than 1 mm
with both electrodes contacting the same part.
- The current flow through the weld materials is accomplished
through the variation in material and interface resistances as the weld occurs.
- When the weld starts, the majority of the current flows though the material
in contact with the electrodes.
Because current always follows the path of least resistance between the electrodes,
the current flow will initially be through the upper material.
- As this material heats, the material resistance increases.
Fig.9.13 Parallel gap welding

11. Summary of Resistance Welding
Pros
Fast (within 1/6s or 10 cycles) and high productivity.
No consumables are needed.
Cons
Difficult to monitor & control the nugget size.
(The automotive industry is now replacing RSW to laser welding)
High hardness due to quenching