Page 217 - CITS - Welder - Trade Theory

WELDER - CITS

Keyhole plasma welding limitations
• more process variables and narrow operating windows

• fit-up is critical
• increased operator skill, particularly on thicker materials Ù high accuracy for positioning
• except for aluminum alloys, keyhole welding is restricted to down hand position
• for consistent operation, plasma torch must be well maintained
Application of the plasma process

Three operating modes possible by varying current bore diameter and gas flow rate
• Micro plasma: 0.05 to 15 amps – used for welding thin sheet down to 0.1mm eg SS bellows and wire
mesh, welding of surgical instruments, repair of gas turbine engine blades, electronic components and micro-
switches etc.
• Medium current: 15 to 200 amps – used as alternative to conventional TIG for improved penetration and
greater tolerance to surface contamination. Generally mechanized due to bulkiness of torch.
• Keyhole plasma: over 100 amps – By increasing cur- rent and plasma gas flow a very powerful beam is
possible which can achieve full penetration in 10 mm stain- less steel. During welding the hole progressively
cuts through the metal with the molten weld pool flowing behind to form the weld bead.
Limitations of plasma arc welding
1 PAW requires relatively expensive and complex equipment as compared to GTAW; proper torch maintenance
is critical
2 Welding procedures tend to be more complex and less tolerant to variations in fit-up, etc.

Plasma cutting principles and advantages

Objectives : At the end of this lesson you shall be able to
• describe the plasma cutting process
• explain the plasma cutting principal
• explain the plasma cutting advantage.

Cutting processes - plasma arc cutting
Plasma arc cutting process, was introduced in the industry in the mid-1950s. The process is used to cut all metals
and non-metals. The common oxy-fuel cutting process (based on a chemical process) is suitable for cutting
carbon steel and low alloy steel cutting only. Materials such as copper, aluminum and stainless steels were earlier
separated by sawing, drilling or sometimes by power flame cutting. These materials are now cut using a plasma
torch, at faster rates and more economically. The Plasma cutting process is basically a thermal cutting process,
free of any chemical reaction, that means, without oxidation. In plasma arc cutting an extremely high temperature
and high velocity constricted arc is utilized.
Principle of operation

Plasma arc cutting is a process resulting from ionizing a column of gas (argon, nitrogen, helium, air, hydrogen or
their mixtures) with extreme heat of an electric arc. The ionized gas along with the arc is forced through a very
small nozzle orifice, resulting into a plasma stream of high velocity (speed up to 600 m/sec) and high temperature
(up to 20000°K). When this high speed is reached, high temperature plasma stream and electric arc strike the
work piece, and ions in the plasma recombine into gas atoms and liberate a great amount of latent heat. This heat
melts the work piece, vaporizes part of the material and the balance is blasted away in the form of molten metal
through the heat (Fig 1).
Plasma cutting system (Fig 2,3,4)
Plasma cutting requires a cutting torch, a control unit, a power supply, one or more cutting gases and a supply of
clean cooling water (in case water-cooled torch is used).

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CITS : C G & M - Welder - Lesson 61-76 CITS : C G & M - Welder - Lesson 61-76