PREPARED BY: VIJAY PATEL 09CAD12
Principle of plasma welding
The basic principle of plasma welding is shown in Fig..If a high voltage is applied between the electrode and the workpiece while the plasma gas is flowing, the gas is ionized and becomes conductive,then a plasma arc is generated. Since the plasma arc is restricted by the nozzle, it had higher energy density, compared to MAG and TIG and can be used as a heat source of ultra high temperature (above20,000°C) having high heat concentrating performance. If the plasma current, gas composition, gas flow rate, etc. are controlled selectively, this arc can be used for boring (cutting) and refilling (welding) easily and the cost is less than the laser. These features are basis of the welding system we have developed. For the concrete welding process of the one-side plasmaspot welding, see Fig.
Pictured above is a plasma arc on the left and a TIG arc on the right. Below, the diagram shows the schematic difference between the plasma arc on the left and the TIG arc on the right. Note the cylindricalshape of the plasma arc when compared to the conical shape of Tig arc.
How it work?
A plasma is a gas which is heated to an extremely high temperature
and ionized so that it becomes electrically conductive. Similar to GTAW (TIG), the plasma arc welding process uses this plasma to transfer an electric arc to a workpiece. The metal to be welded is melted by the intense heat of the arc and fuses together. In the plasma welding torch a Tungsten electrode is located within a copper nozzle having a small opening at the tip. A pilot arc is initiated between the torch electrode and nozzle tip. This arc is then transferred to the metal to be welded. By forcing the plasma gas and arc through a constricted orifice, the torch delivers a high concentration of heat to a small area. With high performance welding equipment, the plasma process produces exceptionally high qualitywelds. Plasma gases are normally
argon. The torch also uses a secondary gas which can be argon, argon/hydrogen, or helium, which assists in shielding the molten weld puddle thus minimizing oxidation of the weld.
A plasma welding torch has an intricate internal design to carry plasma pilot gas, shield gas, water cooling hoses, and power cables for plasma pilot arc current and weld current.
Table 1
Comparison of one-side welding methods and their per
Formances
No. Evaluation item Plasma Arc (MAG)
YAG laser
1 One-side weldability Overlapped parts Limited filletwelding Overlapped parts
2 Applicable clearance
between sheets 0 – 2.0 mm 0 – 1.0 mm Max. 0.2 mm
3 Applicable total thicknessof sheets 1.4 – 4.6 mm 1.6 – 1.4 – 3.0 mm
4 Strain Strain is little Thermal strain is left Strain is very little
5 Welding or core wire Core wire is not welded
since it does not contact. Core wire can be welded since it conta Core wire is not welded
since it does not contact
6 Welding strength: JIS
Grade A
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7 Productivity (Welding time) 1.5 – 3.5 sec/spot 1.5 – 3.5 sec/spot 1.0 – 2.5 sec/spot
8 Equipment cost (Index) 3.2 1 13.6 – 22.7
9 Running cost (Index) 1.9 1 3.7
Table 2
Basic specifications of plasma welding system
Unit Item Specification
Power supply unit Rated input voltage 3-phase 200 V
Rated input power Approx. 32 kVA
Rated output current DC120 A
Torch unit Electrode Embedded tungsten
Nozzle Inside diameter: 2.2
Cooling method Cooling with water
Table 3
Capacity of plasma welding system
Item Capacity
Applicable thickness Max. 4.6 mm
Applicable clearance between sheets 0 – 2.0 mm
Welding speed 2.0 sec (When thickness is 2.0 mm)
Allowable welding angle 0 (Horizontal) – 100°
Gas and plasma properties
Argon, helium, hydrogen, oxygen, nitrogen and carbon dioxide as well as mixtures of
these gases are used for welding, cutting and coating. The meaningfulness of the simulation
results depend strongly on taking appropriate values for the most important parameters of
transport and thermodynamic properties of the gases, which are:
• molar mass,
• density,
• specific heat and enthalpy,
• viscosity,
• radiatiative properties,
• ordinary diffusion coefficient,
• electric conductivity and
• thermal conductivity.
These parameters are in LTE
This penetration-self-adapive can be define here in six part,
Nw we can see step by step
Features and benefits
There are a range of features and benefits to plasma arc
welding. Plasma arc welding features a protected electrode which allows for less electrode contamination. This is especially advantageous in welding materials that out gas when welded and contaminate the unprotected GTAW electrode. Plasma arc welding has forgiveness in arc length changes due to arc shape and even heat distribution. This results in the arc stand off distances not being as critical as in GTAW. Plasma arc welding gives a good weld consistency and no AVC is needed in 99% of applications, sometimes even with wirefeed. The arc transfer is gentle and consistent so it provides for welding of thin sheet, fine wire, and miniature components where the harsh GTAW arc start would damage the part to be welded. Offering a stable arc in welding reduces arc wander so
the arc welds where it is aimed allowing weld tooling in close proximity to the weld joint for optimum heat sinking. There is only minimal high frequency noise used in plasma arc welding to start the pilot arc, thus plasma can be more easily used with NC controls with less fear of the arc starting noise causing glitches in any electrical equipme nt. Another benefit lies in welding applications involving hermetic sealing of electronic components where the GTAW arc start would cause electrical disturbances possible damaging the electronic internals of the component to be welded. Arc energy in plasma welding can reach three times that of TIG welding causing less weld distortion and smaller welds with higher welding speeds. Welding time can be as short as 0.005 sec, ideal for spot welding of fine wires, while the accurate weld times, combined with precision motion devices, provide for repeatable weld start/stop positionsin seam welding. Low amperage arc welding (as low as0.05A) allows welding of miniature components and good control in downsloping to a weld
edge. The arc diameter chosen via the nozzle orifice used assists in predicting the weld bead size.
Applications
The plasma process can gently yet consistently start an arc to the
tip of wires or other small components and make repeatable
welds with very short weld time periods. This is advantageous
when welding components such as needles, wires, light bulb filaments, thermocouples, probes, and some surgical instruments, When dealing with hermetically sealed medical and electronic components, sealed via welding, the plasma process provides the ability to
1) reduce the heat input to the part;
2) weld near delicate insulating seals;
3) start the arcwithout high frequency
electrical noise which could be damaging to theelectrical internals.
A whole repair industry has sprung up to assist companies wishing
to reuse components with slight nicks and dents from misuse or wear. The ability of modern microarc power supplies to gently start a low amperage arc and make repairs has provided users with a unique
alternative to conventional repair and heat treatment. Both the micro-TIG and microplasma welding processes are used for tool, die, and mold repair. For outside edges the plasma process offers great arc stability and requires less skill to control the weld puddle. To reach inside corners and crevices the TIG process allows the tungsten
welding electrode to be extended in order to improve access. In strip metal welding, the plasma process provides the ability to consistently transfer the arc to the workpiece and weld up to the edges of the weld
joint. In automatic applications no arc distance control is necessary
for long welds and the process requires less maintenance
to the torch components.
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