Resistance Welding Process | Pipe Welding Equipment | Resistance Welding Dissimilar Metals

Resistance Welding:

Resistance welding is a process in which two or more parts are welded by the coordinated and regulated use of heat and pressure. The heat in the resistance welding process is generated by the resistance offered by the work pieces to the flow of low voltage high density electric current.

01-resistance welding machine - what is resistive welding

The pressure for welding is applied through the contacting electrodes. It may be obtained by mechanical means, springs, air pressure or hydraulically with the help of a pressure cylinder and piston arrangement. The pressure generally used range from 30 to 55 MPa.

The high density current is produced by using transformer in the welding machine circuit. The transformer reduces the voltage to around 4 to 12 volts and raises the amperage to produce a current density of 45 to 52 VA / mm2 of the area to be welded based on a time of around 10 seconds.

AC current has been found to be most convenient for resistance welding because it is possible to obtain any desired combination of current and voltage by using suitable transformer with different settings.

01-resistance welding process - resistance welding equipment

History:

Sometime in the year 1885, Professor Thompson invented a process called electric resistance welding. He discovered that to weld metals together, one could fire an electric current through the metals while they were tightly clamped together. When the current passed through the metals, it would create such a high heat that the metals would melt and run together and a weld would be made.

The total heat generated in the work pieces and electrodes can be expressed as:

H = I2 R t K

H = I V t K

Where,

H – Heat generated in the workpiece, Joules

I – Current, Amperes (5000 to 20000 A), although voltage is low (below 10 V)

R – Resistance, Ohms

t – Time over which the current is supplied, seconds (0.1 to 0.4 sec)

V – Voltage, volts

K – A correction factor to account for the loss of heat due to radiation, conduction and convection from the electrodes and work pieces.

The current flowing to develop the joint determines the rate of heat generation at the joint. High welding currents are required in resistance welding to develop the necessary heat as the ohmic resistance of any resistance welded joint is low.

Process Variables:

A typical resistance welding cycle consists of:

1. Squeeze

2. Weld

3. Hold periods

01-resistance welding - resistance welding parameters

Operation:

The pressure is applied and built up to the desired value over a period of time. After the proper pressure value has been attained, current of required magnitude is passed for pre-set period of time. Further the interface resistance between upper sheet and lower electrode and lower sheet are kept low by proper contact pressure. Heat is rapidly generated at this interface where it is trapped and slowly dissipated. In a properly controlled weld the welding heat is first generated at pin points on sheet interfaces and then subsequently a weld nugget is formed gradually.

Weld Defects:

01-resistance weld defects - nugget defects - spot weld defects

Advantages:

1. Resistance welding is a production welding process most suitable for light gauge sheets which can be overlapped

2. Operation is quite fast, practically all metals can be resistance welded.

3. Cleaner workspace with less contaminants

Shielded Inert Gas Metal Arc (SIGMA) Welding | Metal Inert Gas (MIG) Welding | Gas Metal Arc Welding (GMAW)

This process also known as Shielded Inert Gas Metal Arc (SIGMA) welding, Metal Inert Gas (MIG) welding or Gas Metal Arc Welding (GMAW) uses a shielded arc struck between a bare metal electrode and the workpiece. The metal electrode is provided in the form of a wire reel.

01-mig welding filler wire - mig welding guide

History:

It was first used in the USA in the mid 1940s. The GMAW process was developed and made commercially available in 1948, although the basic concept was actually introduced in the 1920’s. In its early commercial applications, the process was used to weld aluminium with an inert shielding gas, giving rise to the term “MIG” (metal inert gas) which is still commonly used when referring to the process.

01-mig welding stainless steel - mig welding process

Metal can be transferred in three ways:

1. Spray

2. Globular

3. Short circuiting

Spray transfer:

Small droplets of molten metal from the electrode are transferred to the weld area at rates of several hundred droplets per second. The transfer is spatter free and very stable. High DC current and voltage and large diameter electrodes are used, with argon or argon rich gas mixture used as the shielding gas. The avg current required can be reduced by pulsed arc which are high amplitude pulses superimposed over a low, steady current and the process can be used in all welding positions.

Globular transfer:

Carbon dioxide rich gases are utilized and globules propelled by the forces of the electric arc transfer the metal, resulting in considerable spatter. High welding current are used with greater weld penetration and welding speed. Heavier sections are joined in this process

Short circuiting:

The metal is transferred in individual droplets at rates of more than 50 per second as the electrode tip touches the molten metal and short circuits. Low current and voltages are utilized.

01-mig welding metal transferring methods

MIG Welding Specifications:

  • Power requirements are 2 KW
  • Welding thickness are less than 6 mm.
  • Weld thickness formed are 5 mm to 50 mm.

Equipment:

  • DC output power source
  • Wire feed unit
  • Torch
  • Work return welding lead
  • Shielding gas supply, (normally from cylinder)

01-mig welding process - smaw process - sigma welding

Operation:

The metal electrode is provided in the form of a wire reel. It is fed continuously through the feed mechanism at the rate at which it is being consumed so as to maintain the welding arc between its end and the base metal. The electrode wire (Aluminium, Magnesium, Copper and Steel) is drawn from the wire reel by motor driven feed rolls and forced through the filler wire and gas conductor.

01-mig welding torch parts - mig welding how to

The arc is shielded by an inert gas (Argon, Helium, Carbon dioxide) atmosphere provided by the gas flowing through the nozzle of the holder through which electrode wire also passes.

  • Aluminium – Argon
  • Magnesium – Helium
  • Copper Alloys – Argon – Helium Mix
  • Steel – Ar – Co2 mix preferred

The welding arc is controlled by the current setting, the sed of wire feed, and the characteristics of the welding source. To start welding the trigger switch is depressed. This starts the gas flow and purges the lines. The filler wire which protrudes 10 to 15mm from the gas nozzle is then scratched on the work to initiate the arc.

Advantages:

1. High current density and the corresponding higher rate of metal deposition. The deposition rates in this process are much higher than TIG welding.

2. Highest arc stability

3. High quality welds with good and finished appearances.

4. It produces sound welds at high speeds.

5. Smaller heat affected zones than TIG welding.

Disadvantages:

1. High spatter and Unstable arc

2. MIG welding is not used for steels thinner than 5 mm.