ELECTRIC WELDING PART – 08 – ARC WELDING PRINCIPLE AND PROCESS

ARC WELDING

Arc welding is one of several fusion processes for joining metals. 

By applying intense heat, metal at the joint between two parts is melted and caused to intermix - directly, or more commonly, with an intermediate molten filler metal.

PRINCIPLE

Electric arc is luminous electrical discharge between two electrodes through ionized gas. The arc is formed between the actual work and an electrode (electrode may be stick or wire) that is manually or mechanically guided along the joint.
MAJOR COMPONENTS OF ARC WELDING

1. Power supply (AC or DC)

2. Welding electrode

3. Work piece

4. Welding leads (electric cables) connecting the electrode and

work piece to the power supply.

ARC WELDING MACHINES

a) D.C. welding machines

(i) Motor Generator set            
(ii) Transformer with rectifiers

b) A.C. welding machines.

PROCESS

1. When the electrode being near to the work piece, an arc is developed which closes the electrical circuit.

2. The arc temperature may reach 5500°C which is enough for fusion of the work piece edges and joining them

3. Filling rod is used as an outside metal, which melts and fills the weld pool.

4. The outside metal may be a consumable electrode which fills the weld pool.

5. Filler metal and work piece are of similar chemical compositions.

6. Neutral shielding or inert gases such as argon, helium are used to prevent contamination by oxide and nitride when the molten reacts with the surrounding atmosphere.

7. Shielding is provided in the weld zone as a flux coating on the electrode.

8. When a long join is required the arc is moved along the joint line.

9. The front edge of the weld pool melts the welded surfaces when the rear edge of the weld pool solidifies forming the joint.

10. Solidification is the process of becoming hard or solid by cooling or drying or crystallization.

WELDING ELECTRODES

In arc welding an electrode is used to conduct current through a work piece to fuse two pieces together.

Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding or shielded metal arc welding, or non-consumable, such as in gas tungsten arc welding.

1. Electrode can be a rod or a wire which is used to carry current between the tip and the work piece.

2. It may be specially prepared rod or wire that carry current as well as melts and supplies filler metal to the joint.

Most welding in the manufacture of steel products uses the second type of electrode.

TYPES OF ELECTRODES

1. Bare electrodes

Bare welding electrodes are made of wire compositions required for specific applications.

2. Light coated electrodes

Light coated welding electrodes have a definite composition.

A light coating has been applied on the surface by washing, dipping, brushing, spraying, tumbling, or wiping.

The coatings improve the characteristics of the arc stream.

3. Shielded arc or heavy coated electrodes

Shielded arc or heavy coated welding electrodes have a definite composition on which a coating has been applied by dipping or extrusion. The electrodes are manufactured in three general types: those with cellulose coatings; those with mineral coatings; and those whose coatings are combinations of mineral and cellulose.

4. Tungsten electrodes

Non consumable welding electrodes for gas tungsten-arc (TIG) welding are of three types: pure tungsten, tungsten containing 1 or 2 percent thorium, and tungsten containing 0.3 to 0.5 percent zirconium.

5. D.C arc welding electrodes

Direct current shielded arc electrodes are designed either for reverse polarity (electrode positive) or for straight polarity (electrode negative), or both.

Many, but not all, of the direct current electrodes can be used with alternating current. Direct current is preferred for many types of covered, nonferrous, bare and alloy steel electrodes.

6. A.C arc welding electrodes

Alternating current is used in atomic hydrogen welding and in those carbon arc processes that require the use of two carbon electrodes. It permits a uniform rate of welding and electrode consumption.

In carbon-arc processes where one carbon electrode is used, direct current straight polarity is recommended, because the electrode will be consumed at a lower rate.

7. Non-consumable Electrodes

There are two types of non-consumable welding electrodes.

The carbon electrode is a non-filler metal electrode used in arc welding or cutting, consisting of a carbon graphite rod which may or may not be coated with copper or other coatings.

The tungsten electrode is defined as a non-filler metal electrode used in arc welding or cutting, made principally of tungsten.

The following materials are commonly used for coating

1. Titanium oxide                    2. Ferro-manganese

3. Silica, flour                          4. Asbestos clay

5. Calcium carbonate               6. Cellulose with sodium silicate

ADVANTAGES OF COATED ELECTRODES

1. Due to increase in melting rate, the operation of welding becomes faster.

2. The coating contains compounds of potassium and sodium, which supports arc stabilization.

3. It produce slag (slag means the scum formed by oxidation at the surface of molten metals) over the weld which reducing the changes in brittleness, smooth surface and protects from atmospheric contamination.

4. During welding, sputtering of metal is prevented.

FACTORS TO BE CONSIDERED FOR SELECTION OF ELECTRODES

1. The type of the metal to be welded.

2. The welding position.

3. The type of electric supply (A.C or D.C.)

4. The polarity of the welding machine.