1. TWI Training & Examination Services EWF/IIW Diploma Course
RESISTANCE WELDING
TWI Training & Examination Services
EWF/IIW Diploma Course

2. Resistance spot welding principle

3. Resistance seam welding principle

4. Projection welding principle

5. Spot welding - heat generation
Electrical resistance in the welding circuit
a & g - electrode material resistance
b & f - contact resistance electrode to base metal
c & e - base metal resistance
d - base metal interface resistance

6. Spot welding equipment
Pedestal welding machine
Portable welding machine

7. Spot welding equipment
Resistance electrodes functions
Base metal thickness
conduct the welding current to the work  made from Cu alloys
transmit force to the workpiece  Cr, Mo or W additions
dissipate heat from the weld zone
Metal to be welded
fix the current density in the weld zone  size depends on base metal thickness

8. Welding cycles
squeeze - assure proper contact before welding current is applied
hold - weld nugget solidifies and cools
upslope - avoid overheating and expulsion at the beginning of weld
downslope - control weld nugget solidification

9. Welding cycles
gradually build up the heat at the interface while welding thick steel sheet
forging force - to consolidate weld nugget, reduce porosity and hot cracking

10. Resistance welding parameters
Welding current
Q=I2Rt  the most important parameter
can be AC (most of the applications) or DC
DC normally produced from stored electrical energy
affected by impedance variations caused by magnetic metals introduced into the secondary loop of the machine
excessive current result in deep indentation

11. Resistance welding parameters
Weld time
depends upon the rate of heat generation/dissipation
excessive time lead to exceeding the melting temperature
Welding pressure
affects the contact resistance
as the pressure increase, the heat generated decrease
if pressure is increased, current and/or time must increase too

12. Spot weld testing
Peel test
Tensile shear test

13. Spot weld testing
Tension test

14. Projection welding
Projection welding machine
Projection welds

15. Resistance welding advantages/disadvantages
1) High speed
1) Disassembly is difficult
2) Lap joints add weight; also eccentric load!
2) Less skill required
3) Adaptable to automation
3) Equipment cost higher than arc welding equipment
5) Leak tight welds (seam welding!)
4) Unfavorable line power demands
5) Abrupt changes in joint contour cannot be leak tight (seam welding)
6) Built-in notch gives poor fatigue resistance