Welding
Definition:-
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics,
by causing fusion, which is distinct from lower
temperature metal-joining techniques such as brazing and soldering,
which do not melt the base metal.
In addition to
melting the base metal, a filler material is typically added to the joint to
form a pool of molten material (the weld pool) that
cools to form a joint that is usually stronger than the base material. Pressure may
also be used in conjunction with heat or by itself, to produce a weld.
Welding also requires a form of shield to protect the filler metals or melted
metals from being contaminated or oxidized.
Welding methods:-
- Oxy-fuel welding - also known as oxyacetylene welding or oxy welding, uses fuel gases and oxygen to weld and cut metals.
- Shielded metal arc welding - also known as "stick welding or electric welding", uses an electrode that has flux around it to protect the weld puddle. The electrode holder holds the electrode as it slowly melts away. Slag protects the weld puddle from atmospheric contamination.
- Gas tungsten arc welding - also known as TIG (tungsten, inert gas), uses a non-consumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by an inert shielding gas such as argon or helium.
- Gas metal arc welding - commonly termed MIG (metal, inert gas), uses a wire feeding gun that feeds wire at an adjustable speed and flows an argon-based shielding gas or a mix of argon and carbon dioxide (CO2) over the weld puddle to protect it from atmospheric contamination.
- Flux-cored arc welding (FCAW) - almost identical to MIG welding except it uses a special tubular wire filled with flux; it can be used with or without shielding gas, depending on the filler.
- Submerged arc welding - uses an automatically fed consumable electrode and a blanket of granular fusible flux. The molten weld and the arc zone are protected from atmospheric contamination by being "submerged" under the flux blanket.
- Electroslag welding (ESW) - a highly productive, single pass welding process for thicker materials between 1 inch (25 mm) and 12 inches (300 mm) in a vertical or close to vertical position.
- Electric resistance welding (ERW) - a welding process that produces coalescence of laying surfaces where heat to form the weld is generated by the electrical resistance of the material. In general, an efficient method, but limited to relatively thin material.
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Shielded metal arc weldingShielded metal arc welding (SMAW), also known as manual metal arc welding (MMA or MMAW), flux shielded arc welding or informally as stick welding, is a manual arc welding process that uses a consumable electrode covered with a flux to lay the weld.An electric current, in the form of either alternating current or direct current from a welding power suppply, is used to form an electric arc between the electrode and the metalsto be joined. The workpiece and the electrode melts forming a pool of molten metal (weld pool) that cools to form a joint. As the weld is laid, the flux coating of the electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination.Because of the versatility of the process and the simplicity of its equipment and operation, shielded metal arc welding is one of the world's first and most popular welding processes. It dominates other welding processes in the maintenance and repair industry, and though flux-cored arc welding is growing in popularity, SMAW continues to be used extensively in the construction of heavy steel structures and in industrial fabrication. The process is used primarily to weld iron and steels (including stainless steel but aluminium, nickel and copper alloys can also be welded with this method.
Operation:-
To strike the
electric arc, the electrode is brought into contact with the workpiece by a
very light touch with the electrode to the base metal then is pulled back
slightly. This initiates the arc and thus the melting of the workpiece and the
consumable electrode, and causes droplets of the electrode to be passed from
the electrode to the weld pool. Striking an arc, which varies widely based upon
electrode and workpiece composition, can be the hardest skill for beginners.
The orientation of the electrode to workpiece is where most stumble, if the
electrode is held at a perpendicular angle to the workpiece the tip will likely
stick to the metal which will fuse the electrode to the workpiece which will
cause it to heat up very rapidly. The tip of the electrode needs to be at a
lower angle to the workpiece, which allows the weld pool to flow out of the
arc. As the electrode melts, the flux covering disintegrates, giving off
shielding gases that protect the weld area from oxygen and otheratmospheric gases. In addition, the flux provides
molten slag which covers the filler metal as it travels from the electrode to
the weld pool. Once part of the weld pool, the slag floats to the surface and
protects the weld from contamination as it solidifies.
Once hardened, it
must be chipped away to reveal the finished weld. As welding progresses and the
electrode melts, the welder must periodically stop welding to remove the
remaining electrode stub and insert a new electrode into the electrode holder.
This activity, combined with chipping away the slag, reduces the amount of time
that the welder can spend laying the weld, making SMAW one of the least
efficient welding processes. In general, the operator factor, or the percentage
of operator's time spent laying weld, is approximately 25%.
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1 Comments
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