Presented by: Putra Perdana Bandi ARC WELDING Presented by: Putra Perdana Bandi

Electrode Polarity When welding with direct current, it is customary for the negative pole to be connected to the electrode and the positive pole to the workpiece

For thin sheet welding and when using heavily coated electrodes, the reverse polarity is adopted.

Arc temperature is about 400-600 ˚C lower at the negative pole than at the positive pole.

Welding tools Various types of electrode holder (welding tongs) are available.

The hand-held shield has an interchangeable glass window.

The special green glass has clear glass in front of it to protect it against spatter and enable the molten flux to be observed correctly.

A protective hood is used instead of the hand-held shield if the welder needs both hands for his work.

Leather or asbestos gloves and aprons protect skin and clothing against radiated heat and spatter.

Slag is removed from the seam with a pick or sledge hammer, and a wire brush used for final cleaning.

Electrodes The task of the electrode is to produce the arc and supply the material needed to fill the weld seam

The electrode should as far as possible have the same properties as the workpiece (for example strength and elongation)

The electrodes are clamped into the welding tongs and connected to the current source by a cable.

The following types of electrode are used: bare (bright) metal electrode for normal welding work; cored electrodes with a non-metallic core sheathed or coated electrodes, which can be lightly coated (by dipping) or heavily coated (by repeated dipping and pressing).

For light metals, other non-ferrous metals, cast iron and alloyed materials etc, special electrodes are needed.

For flanged seams, carbon electrodes are used

The coating material melts with the electrode and produces slag.

This is lighter than steel, and therefore floats on the molten pool and together with the resulting gases creates a protective cover round seam so that oxygen and nitrogen cannot penetrate.

Methods of welding We must distinguish between building up a surface by welding and welding joint to connect two workpieces together.

When building up by welding, individual areas on the workpiece are welded to deposit additional material from the welding rod, for example worn slide ways, bearing pins or shafts.

Another application is the production of wear-resistant edges or faces on cheaper base materials by building up a surface of higher-quality material.

The same method is used for cutting tools, on which a better-quality tool steel is attached to a cheaper base material.

Welded joints, on the other hand, always incorporate a welded seam, which is basically triangular in shape.

Welding cast iron Cast iron has to be welded only during maintenance or repair work.

In contrast to steel, cast iron has almost no elongation and can therefore absorb only low tensile stresses.

The welder must have an accurate picture of the stress patterns in order to prevent defective welds.

To make the weld area softer and machinable, care should be taken to cool it down slowly after welding, so that the carbon can precipitate out as graphite flakes

Shielded-arc inert gas welding Shielded gas welding is a form of arc welding in which the arc and the molten weld metal are enclosed within a protective atmosphere.

The protective gas is supplied to the welding point by the welding torch.

It ensures that no oxygen or nitrogen from the atmosphere can penetrate the molten pool, no alloying components are burnt, no fluxes are required and no slag has to be removed.

Furthermore, protective gas welding can take place at a high speed, so that only a small heated zone is produced and the risk of workpiece distortion is minimised.

The welding torch is guided either by hand or mechanically, if fully mechanised equipment is used.

The electrode is mostly water-cooled, but for welding torches used on thin sheet air cooling can be sufficient

Tungsten inert gas (TIG) welding The arc is struck between a tungsten electrode, which scarcely burns away at all, and the workpiece.

The welding rod is guided laterally by hand in the pool.

Depending on the workpiece, either direct or alternating current is used.

The protective gas is argon, a rare gas which is chemically inert.

TIG welding is mainly suitable for sheet, section and pipes up to a thickness of approx. 5 mm, made from heat-resistant, acid-resistant or stainless steels, copper or its alloys and alumunium and its alloys.

Metal inert gas (MIG) welding The arc is stuck here between a melting wire electrode and the workpiece.

The wire electrode is fed to the welding torch through a flexible tube from a coil of wire, using a feed motor

Direct current is used for welding, supplied to the wire electrode just ahead of the welding point.

The protective gas is argon

High current density at the relatively thin wire electrode ensures excellent melting performance, a high welding speed and deep penetration.

MIG welding is mainly used for thick sheets of high-alloy steel, copper and its alloys and alumunium and its alloys

Metal active gas (MAG) welding This is a development of the MIG welding process.

Instead of argon, a cheaper protective gas is used: carbon dioxide or a made-up mixture of argon, carbon dioxide and oxygen.

The MAG process is particularly suitable for welding unalloyed and low-alloy steel sheet, either on an industrial scale or as a manual operation.

It is favoured in automobile repair and construction shops because even thin sheet, for example body panels, can be welded quickly and with little distortion.