Metal – Processing

Welding

Oxy-acetyleneThis uses a mixture of oxygen and acetylene to create a flame

that will at a temperature of around 2500°C at the hottest point. This will clearly be sufficient to melt mild steel at the joint, allowing the melting of a filler rod to fuse the joint edges together.

Advantages of Oxy-Acetylene Welding :·        It's easy to learn.·        The equipment is cheaper than most other types of

welding rigs (e.g. TIG welding)·        The equipment is more portable than most other types of

welding rigs (e.g. TIG welding)·        OA equipment can also be used to "flame-cut" large

pieces of material. Disadvantages of Oxy-Acetylene Welding :·     OA weld lines are much rougher in appearance than other

kinds of welds, and require more finishing if neatness is required.

·     OA welds have large heat affected zones (areas around the weld line that have had their mechanical properties adversely affected by the welding process)

 Materials Suitable for OA Welding:Most steelsBrass

Electric arcAn Electric arc generates sufficient heat to melt mild

steel at the joint edges by creating an electric current through a gap (arc) between the materials being joined and the filler rod (electrode). The electrode is coated in a flux which, when melted, prevents the joint area becoming oxidised.

Today, is commonly used in industries such as the automobile industry for its quality, versatility and speed.

Advantages • Versatility - suitable for a variety of positions

and applications • Capable of relatively high deposition rates • Enables "one process" operation for individual

projects - simplifies training, supervision and logistics

Disadvantages • If the weld is not applied in the right place the

weld could be weak. • Potential for lack-of-fusion type defects if

welding parameters are incorrect or misalignment occurs

• Fume extraction may be required

MIG / TIGMore refined forms of electric arc welding can be used to join thin sheet material. MIG

(Metal Inert Gas) and TIG (Tungsten Inert Gas) welding use a gas jet around the filler wire to prevent oxidation of materials. Different gases are used with different materials, for examples argon is used with aluminium.

TIG Welding Advantages•  Superior quality welds •  Welds can be made with or without filler metal •  Precise control of welding variables (heat) •  Free of spatter •  Low distortion

TIG Welding Problems • Erratic arc • Excessive electrode consumption • Oxidized weld deposit • Arc wandering • Difficult arc starting

MIG Welding Benefits•  All position capability •  Higher deposition rates •  Less operator skill required •  Long welds can be made without starts and stops •  Minimal post weld cleaning is required

MIG Welding Problems

You need for both of these welding methods a wire feed, different thicknesses of wire needed for different welds which also changes the power and speed needed to correctly weld.

Spot & SeamBoth spot welding and seam welding use an

electrical current as the heat source. Spot welding, as the name suggests,

provides a spot of heat to fuse the metal together. It is usual to fins a series of spot welds in a structure like a car body shell.

Advantages ‘spot’• The process involves less maintenance

cost. • Skill of the operator does not influence

the quality of spot welds obtained by this process.

• The process is normally free from smoke and spatter.

Disadvantages ‘spot’• To achieve low cost of construction, the

weld production rate is not very high.• The process proves un-economical as

compared to other welding methods.

Industrial Spot-welder

SeamSeam welding; a typical application for this type of

joining method is thin coated mild steel for food and drinks cans.

Advantages of Seam Welding • It can produce gas tight or liquid-tight joints. • Overlap can be less than for spot or projection

welds. • A single seam weld or several parallel seams may

be produced simultaneously.

Disadvantages of Seam Welding • Welding can be done only along a straight or

uniformly curved line. • It is difficult to weld a thicknesses greater than 3 mm.• A change in the design of electrode wheels is

required to avoid obstructions along the path of the wheels during welding.

Diagram of Seam welder.