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1. Welding

Welding is a metal-joining process in which coalescence is obtained by heat and pressure. It may also be defined as a metallurgical bond accomplished by the attracting forces between atoms. Before these atoms can be bonded together, absorbed vapors and oxides on contacting surfaces must be overcome. The number one enemy to welding is oxidation, and, consequently, many welding processes are performed in a controlled environment or shielded by an inert atmosphere. If force is applied between two smooth metal surfaces to be joined, some crystals will crush through the surfaces and be in contact. As more and more pressure is applied, these areas spread out and other contacts are made. The brittle oxide layer is broken and fragmented as the metal is deformed plastically. Coalescence is obtained when the boundaries between the two surfaces are mainly crystalline planes. This process, known as cold welding, will be discussed further in this chapter. The breaking through or elimination of surface oxide layers happens when a weld is made If temperature is added to pressure the welding of two surfaces will be facilitated, and coalescence is obtained in the same manner as cold-pressure welding. As temperature is increased the ductility of the base metal is increased and atomic diffusion progresses more rapidly. Nonmetallic materials on interfacial surfaces are softened, permitting them to be removed or broken up by plastic flow of the base materials. Hot-pressure welds are more efficient but not necessarily stronger if the atom-to-atom bond is the same(1).
Many welding processes have been developed. They differ widely in the manner that 
heat is applied and in the equipment used. The principal processes are listed here(1).

WELDİNG PROCESSES

1. Braze welding

A. Torch

B. Furnace

C. Induction

D. Resistance

E. Dip

F. Infrared

II. Forge welding

A. Manual

B. Machine

1. Rolling

2. Hammer

3. Die

III. Gas welding

A. Oxyacetylen

B. Oxyhydrogen

C. Air acetylene

D. Pressure

IV. Resistance welding

A. Spot

B. Projection

C. Seam

D. Butt

E. Flash


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F. Percussion

G. High frequency

VI. Arc welding

A. Carbon electrode

1. Shielded

2. Unshielded

B. Metal electrode

1. Shielded

a. Shielded arc

b. Atomic hydrogen

c. Inert gas

d. Arc spot

e. Submerged arc

f. Stud

g. Electroslag

2. Unshieldde

a. Bare metal

b. Stud

VII. Special welding processes

A. Electron beam

B. Laser welding

C. Friction welding

D. Thermit welding

a. Pressure

b. Nonpressure

E. Flow welding

F. Cold welding

a. Pressure

b. Ultrasonic

G. explosive welding

H. Diffusion welding





2. Welding processes

2.1. Arc welding

These processes use a welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. They can use either direct (DC) or alternating (AC) current, and consumable or nonconsumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and filler material is sometimes used as well.To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common classification is constant current power supplies and constant voltage power supplies. In arc welding, the voltage is directly related to the length of the arc, and the current is related to the amount of heat input. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the

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electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding. In these processes, arc length is kept constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current. For example, if the wire and the base material get too close, the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance.[2] The type of current used in arc welding also plays an important role in welding. Consumable electrode processes such as shielded metal arc welding and gas metal
arc welding generally use direct current, but the electrode can be charged either positively or negatively. In welding, the positively charged anode will have a greater heat concentration, and as a result, changing the polarity of the electrode has an impact on weld properties. If the electrode is positively charged, it will melt more quickly, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds.[7]


Nonconsumable electrode

processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. However, with direct current, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds. Alternating current rapidly moves between these two, resulting in mediumpenetration welds. One disadvantage of AC, the fact that the arc must be re-ignited after every zero crossing, has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave, making rapid zero crossings possible and minimizing the effects of the problem.






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