INTRODUCTION TO WELDING PROCESSES

INTRODUCTION TO WELDING PROCESSES

Modern welding technology started just before the end of the 19th century with the development of methods for generating high temperature in localized zones. Welding generally requires a heat source to produce a high temperature zone to melt the material, though it is possible to weld two metal pieces without much increase in temperature. There are different methods and standards adopted and there is still a continuous search for new and improved methods of welding. As the demand for welding new materials and larger thickness components increases, mere gas flame welding which was first known to the welding engineer is no longer satisfactory and improved methods such as Metal Inert Gas welding, Tungsten Inert Gas welding, electron and laser beam welding have been developed. In most welding procedures metal is melted to bridge the parts to be joined so that on solidification of the weld metal the parts become united. The common processes of this type are grouped as fusion welding. Heat must be supplied to cause the melting of the filler metal and the way in which this is achieved is the major point of distinction between the different processes. The method of protecting the hot metal from the attack by the atmosphere and the cleaning or fluxing away of contaminating surface films and oxides provide the second important distinguishing feature. For example, welding can be carried out under a shield comprising of a mixture of metal oxides and silicates which produce a glass-like flux, or the whole weld area may be swept clear of air by a stream of gas such as argon, helium or carbon dioxide which is harmless to the hot metals.

There are certain solid phase joining methods in which there is no melting of the electrodes, though heat is produced in the process. The melted and solidified cast metal is normally weaker than the wrought metal of the same composition. In the solid phase joining such melting does not occur and hence the method can produce joints of high quality. Metals which are dissimilar in nature can also be readily welded by this process. In the normal process joining of dissimilar metals will present problems because of the brittle intermetallic compounds formed during melting. Since the work pieces are closely pressed together, air is excluded during the joining process.

The welding processes those we shall discuss are gas welding, arc welding which includes manual metal arc welding (MMA), tungsten inert gas shielded arc welding (TIG), gas metal arc welding (MIG, MIG/CO₂), submerged arc welding (SAW), etc. High energy density processes like electron beam welding, laser beam welding, plasma welding are also dealt with. Pressure welding and some special welding techniques like electro-slag welding etc. are also be discussed in detail.

Gas welding

• oxygen-acetylene welding

Fusion arc welding

• Shielded metal arc welding (SMAW)
• Submerged arc welding (SAW)
• Flux cored arc welding (FCAW)
• Gas shielded arc welding (MIG, TIG)
1. MIG welding (gas metal arc welding)
2. Pulsed MIG welding
3. Hot wire MIG
4. Plasma MIG
5. TIG welding
6. Pulsed TIG welding
7. Hot wire TIG
8. Spot TIG

Electrical method

• Electric resistance welding

(a)   spot welding

(b)   seam welding,

(c)    projection welding,

(d)   upset butt welding and

(e)    flash butt welding

• Electro-slag welding (ESW)
• Induction pressure welding

Energy method

• Electron beam welding (EBW)
• Laser beam welding
• Plasma welding

Special methods

• Explosive welding (EW)
• Friction welding
• Diffusion bonding

Though the different processes have their own advantages and limitations and are required for special and specific applications, manual metal arc welding continues to enjoy the dominant position in terms of total weld metal deposited. The TIG process produces the finest quality weld on all weldable metals and alloys. The arc temperature may be upto 20,000 K. Although TIG welding produces the highest quality welds, it is a slow and expensive process. Metal inert gas welding process (MIG) is economical with consumable electrode fed at a predetermined rate.

Plasma arc welding (PAW) has made substantial progress in utilising the high heat energy of an ionised gas stream. The jet temperature can be as high as 50,000 K. Foils down to a thickness of 0.01 mm can also be welded in this process and hence this process is more useful in electronic and instrumentation applications.

All the processes like TIG, MIG and PAW can be successfully used for either  Semi-automatic or automatic applications. But they are all open arc processes where radiation and comparatively poor metal recovery put a limit on using high currents. High productivity and good quality welds can be achieved by submerged arc welding process with weld flux and wire continuously fed. The slag provides the shielding of the weld pool with provision for addition of alloying elements whenever necessary.

Electron beam welding and laser welding are classified under high energy density processes.

For efficient welding the power source should provide controlled arc characteristic necessary for a particular job. In one case a forceful deeply penetrating arc may be required, while in another case, a soft less penetrating arc may be necessary to avoid ``burn through''. The welding process will also require a particular type of power source.

 Table 1.1 gives the power source required for widely used welding process.

BASIC WELDING TERMS

What is Arc Welding?
Arc welding is a method of joining two pieces of metal into one solid piece. To do this, the heat of an electric arc is concentrated on the edges of two pieces of metal to be joined. The metal melts, while the edges are still molten, additional melted metal is added. This molten mass then cools and solidifies into one solid piece.

Welding Consumables

Stick Electrode A short stick of welding filler metal consisting of a core of bare electrode covered by chemical or metallic materials that provide shielding of the welding arc against the surrounding air. It also completes the electrical circuit, thereby creating the arc. (Also known as SMAW, or Stick Metal Arc Welding.)
MIG Wire  Like a stick electrode, MIG wire completes the electrical circuit creating the arc, but it is continually fed through a welding gun from a spool or drum. MIG wire is a solid, non-coated wire and receives shielding from a mixture of gases. (Process is also known as GMAW, or Gas Metal Arc Welding.)
Cored Wire (Flux-Cored Wire)  Cored wire is similar to MIG wire in that it is spooled filler metal for continuous welding. However, Cored wire is not solid, but contains flux internally (chemical & metallic materials) that provides shielding. Gas is often not required for shielding. (Process is also known as FCAW, or Flux-Cored Arc Welding.)
Submerged Arc  A bare metal wire is used in conjunction with a separate flux. Flux is a granular composition of chemical and metallic materials that shields the arc. The actual point of metal fusion, and the arc, is submerged within the flux. (Process is also known as SAW, or Submerged Arc Welding.)
Stainless Steel Stainless steel electrodes and wire are used for welding applications where corrosion resistance is required. Stainless steel consumables are designed to match the composition of stainless steel base metals.
Hardfacing A stick of electrode or cored wire that is designed not to fuse two pieces of metal together, but to add a layer of surface metal to a work-piece in order to reduce wear. An example of this is the shovel on an excavator.

Welding Equipment

Stick Welders Heating the coated stick electrode and the base metal with an arc creates fusion of metals. An AC and/or DC electrical current is produced by this machine to create the heat needed. An electrode holder handles stick electrodes and a ground clamp completes the circuit.
TIG Welders  A less intense current produces a finer, more aesthetically pleasing weld appearance. A tungsten electrode (non-consumable) is used to carry the arc to the workpiece. Filler metals are sometimes supplied with a separate electrode. Gas is used for shielding. (Process is also known as GTAW, or Gas Tungsten Arc Welding.)
MIG Welders and Multi-Process Welders Constant Voltage and Constant Current welders are used for MIG welding and are a semi-automated process when used in conjunction with a wire feeder. Wire is fed through a gun to the weld-joint as long as the trigger is depressed. This process is easier to operate than stick welding and provides higher productivity levels. CC/CV welders operate similarily to CC (MIG) welders except that they possess multi-process capabilities - meaning that they are capable of performing flux-cored, stick and even TIG processes as well as MIG.
Engine Driven Welders Large stick or multi-process welders are able to operate independent of input power and are powered by a gasoline, diesel, or LPG engine instead. Ideal for construction sites and places where power is unavailable.
Wire Feeder / Welders For MIG welding or Flux-Cored wire welding, wire feeder welders are usually complete and portable welding kits. A small built in wire feeder guides wire through the gun to the piece.
Semiautomatic Wire Feeders For MIG welding or Flux-Cored welding, semiautomatic wire feeders are connected to a welding power source and are used to feed a spool of wire through the welding gun. Wire is only fed when the trigger is depressed. These units are portable.
Automatic Wire Feeders For MIG, Flux-Cored, or submerged arc welding, automatic wire feeders feed a spool of wire at a constant rate to the weld joint. They are usually mounted onto a fixture in a factory/industrial setting and are used in conjunction with a separate power source.
Magnum Guns / Torches MIG welding guns and TIG welding torches are hand-held welding application tools connected to both the wire feeder and power source. They direct the welding wire to the weld joint and control the wire feed with the use of a trigger mechanism.

Cutting

Plasma Cutters A constricted cutting arc is created by this machine, which easily slices through metals. A high velocity jet of ionized gas removes molten material from the application.
Oxyfuel Gas Cutting Oxyfuel gas cutting process involves preheating the base metal to a bright cherry red, then introducing a stream of cutting oxygen which will ignite and burn the metal.

Welding Automation / Robotic Welding

Robotic Welding Systems The combination of a robotic arm, a welding power source and a wire feeder produces welds automatically using various programs, welding fixtures and accessories.
Environmental Systems Also known as fume extraction, these systems are often incorporated into a robotic fixture to remove welding fumes natural to the process from the welding environment. Usually a vacuum unit, they can be portable or mounted onto a wall.