a.k.das - resistance welding

7/30/2019 A.K.das - Resistance Welding

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*Resistance welding andmodern welding processes*

By : Dr . A K Das

* *

I ndian I nstitute of WeldingANB

Refresher Course - Module 06


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Resistance Welding

This is a group of welding processes in which coalescence isproduced by the heat obtained from the resistance of the work to

electric current in a circuit of which the work is a part, and by theapplication of pressure. The most important resistance weldingprocesses are spot welding, seam welding, projection welding,upset butt welding, and flash butt welding.

The circuit used for resistance spot welding is shown in Fig.1. It

includes a transformer whose primary turns are divided intosections, and taps are taken from the sections to a built-in tap-changing switch. In this way, the welding voltage and weldingcurrent can be adjusted at will on the basis of the followingrelations :

V1/V2 = W1/W2; I2 = V1W2/W1 = const/W1

Where V1 is the primary voltage, V2 is the secondary voltage, W1 isthe primary turns, and W2 is the secondary turns.


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Resistance Welding

The primary voltage is 415-440 V, and the secondary voltage rangesfrom 1 to 12 V, depending on the purpose served by the machine.

The secondary contains few turns. Power is turned on and off by an

automatic interrupter placed in the primary circuit of the machine.

Spot welds can be made on two sides of the work (which is in then

clamped between the electrodes) or, though more seldom, no one side.

A spot weld is made in three steps [Fig.1]. During the first step, the

parts to be joined are clamped between the electrodes. During the

second step, the clamped members are raised to the welding

temperature.


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Resistance Welding


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Resistance Welding

During the third step, the pressure applied to the joint is raised and thework is allowed to cool. It is important to maintain and optimal duration

for each step. If the force is removed too early, this might weaken the

still hot spot weld, and the result would be a low-quality joint.

Present-day spot welding machines can handle steel articles with anoverall thickness of up to 60 mm. The cross-sectional area, number, and

location of spot welds on the work are found by calculation.

Upset butt welding is a resistance welding process in which the parts

previously clamped together are heated by the electric current passingthrough the work to the welding temperature (which is 1200-13000C for

low-carbon steel). Then power is turned off, and the clamping (upset)

force is increased to complete the weld.


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Resistance Welding

In flash butt welding, the abutting surfaces are flashed by an

intense, low-voltage arc and then a clamping pressure is appliedto upset the joint, so that coalescence occurs at the interface.Flash butt welding does not call for careful preparation of the jointfaces which is required in upset butt welding.

Resistance welding is widely used in the production of cars,tractors, farm machines, structural members and completestructures. It can be performed manually and automatically. Oflate, robots have been developed to carry out resistance weldingwithout any human intervention. In upset butt welding it is difficultto distribute heat uniformly throughout the cross-sectional area of

the work. For this reason upset butt welding is limited to partswith a cross-sectional area of not over 200-250 mm2. Bars with a

cross-sectional area of 250-100 000 mm2 are joined by flash buttwelding.


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Fully Automatic Drum Welder


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Flash Butt Welder

Two Wheeler Rims


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Projection Welding Machine


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Resistance Welding

Resistance welded joints have shown performance which fully meetspresent-day requirements.

___________________________________________________________________

TABLE : MECHANICAL STANDARDS FOR SEAM WELDING MACHINES

RATING OF NOMINAL* NOMINAL NOMINAL NOMINAL ELECTRODE SIZE MINIMUM LOWER ARM WORK

TRANSFORMER THROAT ELECTRODE RAM _____________________ CLEARANCE [DIA]

kVA AT THE DEPTH FORCE STROKE THICKNESS DIA ________________________________

RATE OF 50 PERCENT Knurl and Gear Idling

DUTY CYCLE Driven Electrode Electrode

(1) (2) (3) (4) (5) (6) (7) (8)

mm Newton mm mm mm mm mm

50 460

610 4 400 100 9.5 178 280 230

75 760

100 460

150 760 9 800 127 12.5 203 380 280

200 1 070

250 610

920 13 200 150 18.7 254 457 355

400 1220

The normal throat depth shall be measured from the centre-line of point of welding to the nearest point of the interference for flat work or sheet. In the case ofmachines with universal upper-heads, the above measurements shall be taken with the machine arranged for circumferential welding.

Nominal ram strokes shown include vertical adjustments of ram but no vertical adjustments of lower arm.

The maximum length of work at the above minimum diameters shall be 150 mm, less than the nominal throat depth. These dimensions do not apply to theuniversal

head type of seam-welding machine.

________________________________________________________________________________________________________________

R i t W ldi


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Resistance Welding

______________________________________________________________

TABLE : THROAT DEPTH AND MINIMUM SHORT-CIRCUIT CURRENTTRANSFORMER RATING AT THROAT SHORT-CIRCUIT CURRENT

50 PERCENT DUTY CYCLE DEPTH mm MINIMUM VALUE AMPERES

460 16 200

50 610 15 300

760 14 500

460 19 600

75 610 17 900

760 17 900

460 23 500

100 760 22 100

1 070 18 700

460 28 900

150 760 25 500

1 070 23 800

460 32 300

200 760 28 000

1 070 25 500

610 32 300

250 920 28 000

1 220 25 500

610 38 300

400 920 34 000


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Resistance Welding

3.7.5.2 The transformer shall be capable of operatingat not less than 20 percent duty-cycle at the

indicated short circuit current without exceeding

the temperature-rise specified in IS : 4804 (Part I)

1968*.

NOTE For the purpose of compliance with this

clause, it shall suffice to compute the duty-cycle

in accordance with the formula given in 4.3.1 of

IS : 4804 (Part I)-1968*, px being taken as the

product of minimum value of short-circuit

current in kA and the maximum open circuit

secondary voltage.


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Electron-beam welding

This is a fusion welding process. The electron beam is produced by an electron

gun (Fig.2), a vacuum devise, inside which electrons are emitted by a hot

cathode and channeled towards the work acting as anode. The emitted electronsare focused into a dense stream by the magnetic field set up by focusing coils.

The electrons bombard the workpiece and heat it to very high temperatures. The

impinging beam is scanned along the joint by a deflection system.

So that no arc discharge could occur inside the gun, it is exhausted to a very high

vacuum (of the order of 1.33 x 10-2 Pa) maintained by the pumping system of

the electron-beam welding machine.

The electron beam may strike the work continuously or in pulses. The

temperature to which the beam can heat the work can be adjusted by varying the

power density of the beam.

(Figure 2 next slide)


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Electron Beam Welding Electron Beam welding is your

solution for joining exotic materialswith little or no loss in strength.

Joins aluminum, copper, carbonsteels, stainless steels, nickel & cobaltalloys, tantalum, titanium, andrefractory metals

Penetration from 0.001 to 2.000

Lowest total heat input resulting inlow thermal distortion

High depth-to-width ratios, typically10:1

Hard vacuum environment of 1x10^-4 torr

Energy Area: 1x10^7 Watts/cm^2

Electron Beam welding


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The beam length is varied with the aid of the focusing system. Thebeam can be manipulated at a distance of up to 900 mm to the work,which is an obvious advantage when making welded joints in hard-to-reach spots.

The specific heat power of the electron beam is hundreds or eventhousands of times the figure achieved with the usual electric arc.For example, an electric arc with a voltage of 20V, a current of 200A,and a diameter of 10 mm produces a power density of about 4 x

107W m-2. An electron beam operating at a voltage of 100 kV, acurrent of 8 mA, and a diameter of 0.05 mm produces a powerdensity of 4 x 1011 Wm-2, which is 10 000 times greater. Owing to thehigher power density, electron-beam welding can proceed at higherwelding [or travel] speeds, make narrow and deep welds, keep to aminimum the heating of the near-weld zone and, as a consequence,to minimize welding stresses and distortion, and to improve thestrength of the welded joints.

Of late, work has been under way to develop plasma-electron beamand electron-beam welding units in which the beam is coupled outof vacuum into the atmosphere. With this arrangement, electron-beam welding can be used to handle workpieces of an unlimitedsize.


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Electron-beam welding holds out special promise in the

fabrication of especially critical structural members

(girders, columns etc.), high-pressure reservoirs,

assemblies for steam generators and turbines,components for internal-combustion engines, and so on.

Electron-beam welding can advantageously be used in

outer space for the repair of orbital stations and otherjobs.


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Plasma-arc welding and cutting

In this welding process, local heating is accomplished by a plasma stream. The

plasma refers to a high-temperature ionizing gas. The lowest temperature at which

spontaneous ionization can take place is above 55000C. The plasma used for

welding purposes has a temperature of 5500-30 0000C. The manner in which a

plasma stream is generated is illustrated in Fig.3. Power is supplied by a d.c.

source, E. The minus terminal of the source is connected to an electrode, 4, and

the plus terminal to a nozzle, 2. The arc, 5, striking between the electrode and

nozzle is blown outwards by a gas mixture with the formation of a plasma jet, 1.

In torches for plasma-arc welding (Fig.3) one of the electrodes is the workpiece

itself.

As compared with arc welding, cutting and surfacing, the plasma-arc process

offers the following advantages :

(1) High productivity (four and more times greater).

(2) Low degree of deformability of the work (due to high rates of cutting and/or

welding).

(Figure next slide)


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Fig.3 Plasma arc welding; [a] plasma-jet (non-transferred-arc) type; [b] plasma-arc (transferred-arc) type

1

plasma stream; 2

plasma constricting nozzle; 3

plasma gas; 4

electrode; 5

arc column;6workpiece; Epower source


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Plasma cutting process

For plasma cuttingtransferred arc with

high gas flow is used

Inert gases eg Argon,Nitrogen or Argon-H2used with tungstenelectrode.

Used mainly forstainless steel,aluminium, nickelalloys etc.


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Air plasma

Oxidising gases eg air oroxygen can be used with a

hafnium electrode mounted

on a copper holder cutting

MS

Cut quality is slightly poor

due to excessive top edge

melt

Variants

dual gas

- water injection/shroud


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Plasma-arc welding and cutting

(3) Reduced amount of discard in cutting, because the kerf thus

produced is much narrower than in oxygen cutting.

A major disadvantage of the plasma-arc process is noise due to theoperation of the plasma source. Because of this, hand-held plasma

sources are used on a very limited scale. For the most part,

remotely operated plasma sources are employed.


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Laser-beam welding and cutting

In this process the necessary heat is obtained from the concentrated

coherent light beam produced by a laser.

The active body in a laser can be a solid, a gas, a gas mixture, a liquid.Fig.4 shows diagrammatically the arrangement of a ruby laser. Thekey component of this laser is a pump cavity, 1, which encloses a ruby

rod, 2. Installed parallel with the ruby rod is a high voltage flash tube,5. The radiation emitted by the ruby rod is shaped into a narrow beamand directed onto the workpiece by an optical system made up of aprism, 3, a lense, and an interchangeable objective, 4. There isanother optical system consisting of a light source 8, a prism 7, and alense 6, to tune up the laser.

The beam emitted from the ruby rod is focused to a spot from 0.01 to0.1 mm in diameter.

Lasers hold out promise to welding practice as they can join heavy-

gauge steel plates. They are also promising in the metal-cutting field.


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Laser cuttingCO2 lasers use pure Helium &CO2 as lasing gases.

Assist gases are used in laser

cutting to eject the molten

metal from the cut.

Argon/nitrogen used forstainless steel, aluminium,

titanium etc.

Oxygen can be used for MS

and low alloy steels which aids

cutting

Lasers give very high cut

quality and speeds.



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** Thank you **

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a.k.das - resistance welding

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