RESISTANCE WELDING: Spot Welding, Seam Welding, Projection Welding, Resistance Butt Welding, Flash Butt Welding, Percussion Welding, High Frequency Resistance Welding, High Frequency Induction Welding

RESISTANCE WELDING

In resistance welding the metal parts to be
joined are heated by their resistance to the flow
  of an electrical current. Usually this is the
only source of heat, but a few of the welding operations combine resistance
heating with arc heating, and possibly with combustion of metal in the arc. The
process applies to practically all metals and most combinations of pure metals
and those alloys, which have only a limited plastic range, are welded by
heating
the parts to fusion
(melting). Some
alloys, however, may
welded without fusion; instead, the parts are heated to a plastic state
at which the applied pressure causes their crystalline structures to grow
together. The welding of dissimilar metals
may be accomplished by melting both
metals frequently only the metal with the lower melting point is melted, and an
alloy bond
   is formed at the surface of the unmelted
metal.


In resistance welding processes
no fluxes are employed, the filler metal is rarely used and the joints are
usually of the lap type. The amount of heat generated in the workpiece depend
on the following factors:
(1) 
Magnitude of the current,
(2)  Resistance of the current conducting path, and Mathematically,
H = IVt
= I(IR)t= I2Rt

Where, H  =  heat
generated in joules, 
I = current
in Amp.,
R = resistance in ohms, t = time of current flow in seconds.

Types of Resistance welding


The major types of resistance welding are given as under:
(1) Spot Welding
(2) Seam Welding
(3)  Projection Welding
(4)  Resistance Butt Welding
(5)  Flash  Butt Welding
(6)  Percussion Welding
(7)  High Frequency Resistance Welding
(8)  High Frequency Induction Welding

Some of the above important welding processes are
discussed as under,

Spot Welding

In this process overlapping
sheets are joined by local fusion at one or more spots, by the concentration
of current flowing between two
electrodes. This is the most widely used resistance welding process. A typical resistance spot welding machine is shown in Fig.
It essentially consists of two
electrodes, out of which one is fixed. The other electrode is fixed to a rocker arm
(to provide mechanical advantage) for transmitting the mechanical force from a pneumatic
cylinder. This is the simplest type
of arrangement. The other possibility is that of a pneumatic or hydraulic
cylinder being directly connected to the electrode without any rocker arm. For welding large assemblies such as car
bodies, portable spot welding machines are used. Here the electrode holders and
the pneumatic pressurizing system are present in the form of a portable
assembly which is taken to the place, where the spot is to be made. The
electric current, compressed air and the cooling water needed for the
electrodes is supplied through cables and hoses from the main welding machine
to the portable unit. In spot welding, a satisfactory weld is obtained when a
proper current density is maintained. The current density depends on the contact area between the electrode and the work-piece. With the continuous
use, if the tip becomes
upset
and- the contact area
increases,  the
current  density 
will be lowered and
consequently the weld is obtained over a large area. This would not be able
to melt the metal and  hence  there 
would 
be no proper 
fusion.
A resistance  welding 
schedule is the sequence
of events that normally
take place in each of the welds.
The events are:


1. The squeeze time is the time required
for the electrodes to align and clamp the two
work-pieces together under them and provide the necessary electrical contact.

2. The weld time is the time of the
current flow through the work-pieces till they are heated to the melting temperature.

3. The hold time is the time when the pressure is to be maintained on the molten metal without the electric current.
During this time, the pieces are expected to be forged welded.

4. The off time is time during which, the
pressure on the electrode is taken off so that the plates can be positioned for the next spot.


Resistance spot welding machine setup
Resistance spot welding machine setup


Before spot welding one must make sure that
(i)  The
job is clean, i.e., free from grease,
dirt, paint, scale,
oxide etc.


(ii)   Electrode tip surface is clean, since
it has to conduct the current into the work with as little loss as possible. Very fine emery cloth may be used for routine
cleaning.

(iii)   Water is running through
the electrodes in order to

(a)  
Avoid them
from getting overheated and thus damaged,

(b)  
Cool the weld.

(iv)  Proper welding current has been set on the current selector
switch.

(v)  Proper time has been set on the weld-timer.

Spot welding electrodes


Spot welding electrodes are made of materials which have
(1) 
Higher electrical and thermal
resistivities, and

(2)  Sufficient strength
to withstand high pressure at elevated temperatures.

Copper base alloys such as
copper beryllium and copper tungsten are commonly used materials for spot
welding electrodes. For achieving
the desired current density, It
is important to have proper electrode shape for which three main types of spot
welding electrodes are used which are pointed, domed and flat electrodes.

Applications of Spot Welding

(i) It
has applications in automobile and aircraft industries


(ii)  The attachment of braces, brackets,
pads or clips to formed sheet-metal parts such as cases, covers or trays is another application of spot welding.

(iii) Spot welding of two 12.5 mm thick
steel plates has been done satisfactorily as a replacement for riveting.

(iv) Many
assemblies of two or more sheet metal stampings that do not require gas tight
or liquid tight joints can be more economically joined by spot welding than by
mechanical methods.
(v)  Containers and boxes frequently are spot welded.

Resistance Seam Welding

It is a continuous type of spot welding wherein
spot welds overlap
each other to the desired extent. In this process
coalescence at the faying surfaces
is produced by the heat obtained from the resistance to electric current
(flow) through the work pieces held together under pressure by circular
electrodes. The resulting weld is a series of overlapping resistance-spots welds made
progressively along a joint by rotating the circular electrodes. The principle of seam welding
 and resistance seam welding process
set up is shown in Fig. The seam welding is
similar to spot welding, except that circular rolling electrodes are used to
produce a continuous air-tight seam of overlapping welds. Overlapping
continuous spot welds seams are produced
by the rotating electrodes and a regularly
interrupted current.


Principle of seam welding process & Resistance seam welding process setup
Principle of seam welding process & Resistance seam
welding process setup


Application Resistance Seam Welding

1. It
is used for making leak proof joints
in fuel tanks of automobiles.
2. Except for copper and high copper
alloys, most other metals can be seam welded.
3. It
is also used for making
flange welds for use in watertight tanks.


Resistance Projection Welding

Fig.shows the projection welding. This
process is a resistance welding process in 
which two or more than two spot welds are made
simultaneously
by making raised
portions
or projections on predetermined locations on one of the
workpiece. These projections act to localize the heat of the welding circuit.
The pieces to be welded are held in position under pressure being maintained by
electrodes. The projected contact
spot for welding should be 
approximately equal to the weld metal thickness. The welding of a nut on the auto- motive chasis is an example of projection
welding.
Resistance projection welding
Resistance projection
welding


Resistance Upset Butt and Flash Butt Welding

This welding is also used for
joining metal pieces end to end but it has largely replaced the butt-welding
method for weld articles small cross-sections. It can be used for thick sections also. Initially the current
is switched on and then one end the moveable part to be welded is brought
gently closer to the fixed end of the other part to localize heat at the ends
and thus raises the temperature of the ends quickly to the welding
heat. On acquiring contact of fixed end and moveable end with each other,
the moveable end is then pressed against one another by applying mechanical pressure. Thus the molten
metal and slag to be squeezed out in
the form of sparks enabling the pure metal to form the joint and disallowing
the heat .to spread back. The principle
of upset butt welding and flash butt welding are depicted in Fig
. In this resistance welding
single phase A.C. machines are commonly employed. 


Resistance  upset butt welding
Resistance 
upset butt welding



Resistance flash butt welding
Resistance flash butt welding


The merits and demerit of
flash welding over simple butt-welding are follows:

Merits

1. It is comparatively much faster than butt welding.

2. This method utilizes less current in
comparison to butt welding as the small portion of the metal is only being heated for getting a good weld

3. Created joint by this welding is much
stronger than the butt welding joint. Also the
strength of the weld produced is high even more than that of the base
metal. The end of the metal pieces to
be welded in this welding need not be squared as it is   the basic requirement in butt-welding.

4. A high
degree of accuracy can
be easily achieved in terms of length
alignment
of weld.

Demerits

1. The periodic maintenance of machine
and replacement of insulation is needed as flashing particles of molten metal
are thrown out during welding which may enter into the slide ways and insulation of the set up.


2. Welder has
to take enough care against possible fire hazard due to flashing during welding.

3. Additional stock has to be provided
for compensating loss of metal during f1ashing and upsetting. This increases to the cost of weld.

4. Cost
of removal of flash weld metal by trimming, chipping, grinding, etc. will increase
to the welded product.

1. Surface of the jobs where they come in
contact with the gripping surfaces, should be clean otherwise they will restrict
the flow of electric current.

2. The available power, opening between the jaws
of the gripping clamps and upsetting pressure of the welding set limit
the size and cross sectional area of the jobs to be welded.

Applications of Resistance Welding

All conducting forged metals
can be easily be flash welded. A number of dissimilar metals
can also be welded by controlling the welding conditions carefully. Metals generally welded metal by the process involves lead,
tin, antimony, zinc, bismuth and
their alloys, low carbon steels, stainless steel, alloy
steels, tool steels, copper alloys, aluminium alloys, magnesium alloys nickel
alloys, molybdenum alloys,
and titanium alloys.
This process is used in automobile
industry, welding of solid and
tubular structural assemblies, etc. in
air-craft industry, welding of band saw blades, welding of tool steel
drills, reamers and taps etc. to mild steel or alloy steel shanks,
welding of pipes
and tubes.

 Common
Advantages of Resistance Welding

Some common advantages of resistance welding include:
(a)  It is well suited
for mass production.
(b)  It is economical in operation, since nothing is consumed except
electrical power.
(c)  Skilled welders
are not required.
(d) 
Welds are
quickly made.
(e)  It is possible to weld dissimilar metals. Some disadvantages of
resistance welding include:
(a)  High initial
cost of the resistance welding
equipment
(b)  Certain resistance welding processes are limited to lap joints.
A lap joint has an inherent service between the two metal pieces, which causes stress concentrations in
applications where fatigue is present. This service may also cause trouble
when corrosion is present

0 thoughts on “RESISTANCE WELDING: Spot Welding, Seam Welding, Projection Welding, Resistance Butt Welding, Flash Butt Welding, Percussion Welding, High Frequency Resistance Welding, High Frequency Induction Welding”

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