Resistance Welding
Commonly used resistance welding processes:
• Resistance Spot Welding (RSW),
• Resistance Seam Welding (RSEW),&
• Resistance Projection Welding (PW) or
(RPW)
• Resistance welding uses the application of
electric current and meclhanical pressure to
create a weld between two pieces of
metal. Weld electrodes conduct the electric
current to the two pieces of metal as they are
forged together.
• The welding cycle must first develop sufficient
heat to raise a small volume of metal to the
molten state. This metal then cools while under
pressure until it has adequate strength to hold
the parts together. The current density and
pressure must be sufficient to produce a weld
nugget, but not so high as to expel molten metal
from the weld zone.
• High Frequency Resistance Welding (HFRW)
P,ercussion Welding (PEW) and Stud Welding
(SW), too.
H = 12 Rt
K
Electrode
K- energy losses through radiation &
conduction
Weld
Nugget
•resistances of the electrodes
•electrode- w/p contact resistance
•resistance of the individual parts to
be welded
•w/p-w/p contact resistance
(maintained high)
Resistance Welding
Benefits
•
High speed welding
•
Easily automated
•
Suitable for high rate
production
•
Economical
HAZ
Electrode
• Resistance Welding Limitations
• Initial equipment costs
•
•
Lower tensile and fatigue strengths
Lap joints add weight and material
Common Resistance Welding Concerns
•Optimize welding process variables.
•Evaluate current welding parameters and
techniques.
•And thus eliminate common welding problems and~
discontinuities - such as
Resistance Welding Problems and
Discontinuities
•
•
•
•
•
•
•
•
•
Cracks
Electrode deposit on work
Porosity or cavities
Pin holes
Deep electrode indentation
Improper weld penetration
Surface appearance
Weld size
Irregular shaped welds
1
RESISTANCE SPOT WELDING
C
'°'
,l!.L.-...L.:....:_--#¢
Bt ~
i
-
,---,
~
t
I
' ..... __ _.,,.,
.... -
D
I
t,
I
y
A - Throat deeth
B - Hom spaang
C - Centerline of rocker ann
D • Lower arm adjustment
E - Air cylinder
.
F-Air valve
G • Upper horn
•
E
- .I
/
o~l--l1Htt1..--- R
---0
•
:i ~ C
H- Lowerhom
M - Rocker arm
N - Secondary flexible .conductor
R - Current regulator (tap switch)
S - Transformer secondary
T - Electrode holder
W - Electrode
Y - Foot control
AIR OPERATED ROCKER ARM SPOT WELDING MACHINE
RESISTANCE SPOT WELDING
(b}
•
ELECTRODE DESIGNS FOR EASY ACCESS INTO COMPONENTS
~
RESISTANCE SEAM WELDING
(a)
Electrode wbeell
(b)
Electrode 7,b:tl
'
at
(c)
(d)
RESISTANCE PROJECTION WELDING
(a)
Force
Flat electroda
Sbe~t
I
~
I
Weldnuaets
WOl'ltpiece
Projectionl
Force
(c)
(b)
Nut
Bolt
Weld nuget
HIGH FREQUENCY BUTT WELDING OF TUBES
(b)
(a)
Higb-frequeJlcy
coil
~-Current .
·Squeer.e roll
Squeczie
roll
FLASH WELDING
(a)
Azc
t
(c)
POOR
GOOD
•
FOR SOLID RODS & TUBES
DESIGN GUIDELINES
~
RESISTANCE STUD WELDING
(•) T
(b)
St d _....,_ Ceramic
u
ferrule
I
Pull
f
(c)
Push
!
(d)
,-..,c.::::i.Molten we).J
metal
Workpiece
(base metal)
The sequence of operation~ in stud welding, whict"I is used for
welding bars, threaded rods, and various fasteners onto metal plates.
DISTORTION
• Welding involves highly localized heating of the meta.I
being ioined together.
• The temperature distribution in the weldment is
nonuniform.
• Normally, the ,veld metal and the heat affected zone (HAZ)
are at temperatures substantially above that of the
unaffected base metal.
• Upon cooling, the weld pool solidifies and shrinks,
exerting stresses on the surrounding weld metal and HAZ.
• If the stresses produced from thermal expansion and
contraction exceed the yield strength of the parent metal,
localized plastic deformation of the metal occurs.
• Plastic deformation results in lasting change in the
component dim ensions and distorts the structure. This
causes distortion of ,veldments.