Welding Processes

1. Introduction
The manufacture of virtually all sophisticated modern products involves joining together many individual components. Where a permanent join is required, welding is often a good option. Other possible processes such as brazing, soldering, and use of adhesives will are considered in the Design module.

Welding processes can be split into two broad categories:

Fusion processes The surfaces of two components to be joined are cleaned, placed close together and heated while being protected from oxidation. A pool of molten metal forms and connects the components, a filler rod may be used to add metal to the joint.
This category covers a very wide range of processes, some of which are considered in more detail later
Solid phase processes The metals to be joined do not melt, they are heated, usually by friction heating generated by sliding the parts together under a normal load, this softens the metals and removes surface contamination. The sliding is then stopped, the normal load is increased and the two surfaces join together.
Friction welding is the main process in this class and is widely used to join axisymmetric components in two different types of steels. Examples include engine valves where a heat resistant alloy head is required, but a steel that will slide well in the guide is needed for the stem.

2. Fusion Welding Processes
2.1 Potential Problems
Although the majority of welding is carried out on steels, other metals, particularly aluminium, are increasingly being joined by welding. To ensure that the designers intent is met a number of precautions need to be considered when specifying the metals and process.

Distortion - As the parts are heated to their melting point, they will undergo significant thermal expansion and on cooling, contraction. To minimise the adverse effects of this it will often be necessary to use jigs, tacking and a suitable welding sequence or procedure, ie 'back stepping'.

Oxidation - At the elevated temperatures involved in fusion welding, metals oxidise rapidly. Oxidation would either prevent joining or result in a sub-standard joint. Oxidation is prevented in a number of ways depending upon the process. In oxy-acetylene welding, the gases produced by combustion in the flame prevent oxidation. In the metal inert gas (MIG) and tungsten inert gas (TIG) processes, the arc is shrouded by a supply of inert gas such as helium, argon, carbon dioxide or a mixture of these. In the electric arc consumable electrode process the rod is coated with a flux which forms a protective layer and gas in the weld area as the rod is consumed.

Loss of Mechanical Properties - Where a metal has been subjected to a thermal process, welding will frequently result in loss of property - strength or hardness - in the area adjacent to the weld, called the heat affected zone (HAZ).

Embrittlement - One source of embrittlement when welding steel is where consumables are damp, which may result in the weld taking up hydrogen. This problem can be avoided by ensuring that all metal to be welded and consumables are stored in dry conditions.
An alternative cause of embrittlement is when alloy steels are welded or when a 'plain' steel with a carbon content above 0.2% is welded. In these instances the self cooling of the parts may result in martensitic zones adjacent to the weld which are unacceptably brittle.
For general fabrication work involving plain steel, it is normal to use specify a carbon content of less than 0.15%.
When welding alloy steels appropriate pre-heating and slow post cooling must be carried out to ensure that the cooling rate is too slow to result in any martensitic transformation.

Joint Orientation
As most welding processes rely on a weld pool being formed between the parts to be joined, the process is most straightforward when the weld pool is stable, that is when it is horizontal. This will not be the case when vertical and overhead joints are required. These require much more skill and special electrodes/coatings may be used.

2.2 Oxy Acetylene Welding
In this process heat is provided by the combustion of Acetylene in Oxygen which gives a flame temperature of about 2700oC. This can be used to weld steel with a thickness up to about 5 mm. A filler rod is normally applied to assist in making the join. The process is widely used in jobbing shops with manual operation and requires a fair bit of skill, particularly when welding aluminium and thinner gauge steel.

2.3 Arc Welding
Several welding processes are based on heating with an electric arc, only a few are considered here, starting with the oldest, simple arc welding, also known as shielded metal arc welding (SMAW) or stick welding.
In this process an electrical machine (which may be DC or AC, but nowadays is usually AC) supplies current to an electrode holder which carries an electrode which is normally coated with a mixture of chemicals or flux. A earth cable connects the workpiece to the welding machine to provide a return path for the current. The weld is initiated by tapping ('striking') the tip of the electrode against the workpiece which initiates an electric arc. The high temperature generated (about 6000oC) almost instantly produces a molten pool and the end of the electrode continuously melts into this pool and forms the joint.

The operator needs to control the gap between the electrode tip and the workpiece while moving the electrode along the joint.

In the shielded metal arc welding process (SMAW) the 'stick' electrode is covered with an extruded coating of flux. The heat of the arc melts the flux which generates a gaseous shield to keep air away from the molten pool and also flux ingredients react with unwanted impurities such as surface oxides, creating a slag which floats to the surface of the weld pool. This forms a crust which protects the weld while it is cooling. When the weld is cold the slag is chipped off.

The SMAW process can not be used on steel thinner than about 3mm and being a discontinuous process it is only suitable for manual operation. It is very widely used in jobbing shops and for on site steel construction work. A wide range of electrode materials and coatings are available enabling the process to be applied to most steels, heat resisting alloys and many types of cast iron.

2.4 Metal Inert Gas (MIG) or Gas Metal Arc Welding (GMAW)
In this process a filler metal is stored on a spool and driven by rollers (current is fed into the wire) through a tube into a 'torch'. The large amount of filler wire on the spool means that the process can be considered to be continuous and long, uninterrupted welds can easily be made. An inert gas is also fed along the tube and into the torch and exits around the wire. An arc is struck between the wire and the workpiece and because of the high temperature of the arc a weld pool forms almost instantly. In this process they key issues are selecting the correct gas mixture and flow rate and the welding wire speed and current. Once these have been set, the skill level required is lower than with the oxy acetylene process, and it can readily be automated and MIG welding is now commonly carried out by robots. The MIG process is widely used on steels and on aluminium. Although the inert gas shield keeps the weld clean, depending upon the process settings, there may be spatter of metal globules adjacent to the weld which detracts from its appearance unless they are removed.

2.5 Flux Cored Arc Welding (FCAW)
This is similar to MIG welding except that the electrode is in the form of a hollow tube filled with flux that may include metal powder(s). No inert gas is needed.

2.6 Tungsten Inert Gas (TIG) or Gas Tungsten Arc Welding (GTAW)
This process uses a non consumable tungsten electrode is used and an arc struck between this and the workpiece surface.

An inert gas is used to shield the weld area and filler rod may be used. The process is well suited to joining non - ferrous metals, including aluminium, refractory and special metals and is effective for joining thin section metals. A high degree of skill is needed, but high quality welds can be produced.

2.7 Other Processes
A number of other specialised processes are in use including: electron beam welding (EBW) which needs to be carried and in a vacuum and laser beam welding.

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David J Grieve, 23rd February 2009.