Plasma ARC Welding (PAW) Vivek

Content Introduction How Plasma Welding works? Equipment Welding Modes in PAW Difference between PAW and TIG Control System Advantages Disadvantages Applications

Introduction Arc welding process that produces coalescence of metals by heating them with a constricted arc between an electrode and the work piece (transferred arc) or between the electrode and the water-cooled constricting nozzle (non transferred arc) . Plasma: A gaseous mixture of positive ions, electrons and neutral gas molecules.

A small flow of argon is supplied through the nozzle and its constricting orifice to form the arc plasma. Sometimes diatomic gases like nitrogen or hydrogen are used as plasma gases because of good heat transfer characteristics. Shielding of the arc and weld zone is provided by a gas flowing through an encircling outer nozzle assembly. The shielding gas can be argon, helium, or mixtures. Two welding techniques are possible with PAW: melt-in, and keyhole. The Low current melt-in technique is normally used when welding thin sheet sections. Narrower welds can be made with PAW than with GTAW because of the constricted arc. High current Melt-in can be done on butt or lap joints up to about 1/8 inch thick. PAW can also be used on Titanium and other reactive metals. PAW Welding

How Plasma Welding Works Gas which is heated to an extremely high temperature and ionized so that it becomes electrically conductive. PAW process uses this plasma to transfer an electric arc to the work piece. The metal to be welded is melted by the intense heat of the arc and fuses together. Objective of PAW: To increase the energy level of the arc plasma in a controlled manner. This is achieved by providing a gas nozzle around a tungsten electrode operating on DCEN.

2 Variants of PAW: Transferred arc mode: Non-transferred mode: Arc is struck between the electrode(-) and the work piece(+) Used for high speed welding and Used to weld Ceramics, steels, Aluminum alloys, Copper alloys, Titanium alloys, Nickel alloys. Non-transferred mode: Arc is struck between the electrode(-) and the nozzle(+), thus eliminating the necessity to have the work as a part of the electrical system. Arc process produces plasma of relatively low energy density. Since the work piece in non-transferred plasma arc welding is not a part of electric circuit, the plasma arc torch may move from one work piece to other without extinguishing the arc.

Fig. Arc in Plasma Arc Welding

Equipment Power Supply A DC power source (generator or rectifier) having drooping characteristics and open circuit voltage of 70 volts or above is suitable for PAW. Rectifiers are generally preferred over DC generators. Working with He as an inert gas needs open circuit voltage above 70 volts. This voltage can be obtained by series operation of two power sources; or the arc can be initiated with argon at normal open circuit voltage and then helium can be switched on. High frequency generator and current limiting resistors Used for arc ignition. Plasma Torch Either transferred arc or non transferred arc typed

Shielding gases Shields the molten weld from the atmosphere. Two inert gases or gas mixtures are employed. Argon(commonly used), Helium, Argon+Hydrogen and Argon+Helium, as inert gases or gas mixtures. Helium is preferred where a broad heat input pattern and flatter cover pass is desired. A mixture of argon and hydrogen supplies heat energy higher than when only argon is used and thus permits higher arc alloys and stainless steels. For cutting purposes a mixture of argon and hydrogen (10-30%) or that of nitrogen may be used. Hydrogen, because of its dissociation into atomic form and thereafter recombination generates temperatures above those attained by using argon or helium alone.

Current and gas decay control Welding Parameters: Current 50 to 350 Amps, Voltage 27 to 31 Volts, Gas flow rates 2 to 40 liters/min. (lower range for orifice gas and higher range for outer shielding gas), DCSP is normally employed except for the welding of Al in which cases water cooled copper anode and DCSP are preferred. Temp of Jet 50000°F (28000°C) Current and gas decay control To close the key hole properly while terminating the weld in the structure. Fixture To avoid atmospheric contamination of the molten metal under bead.

Modes in paw Micro-plasma welding Welding Current from 0.1A to 15A. Arc Length is varied up to 20mm Used for welding thin sheets (0.1mm thick), and wire and mesh sections. Medium-plasma welding Welding current from 15A to 100A. Keyhole welding Welding Current above 100A, where the plasma arc penetrates the wall thickness. Widely used for high-quality joints in aircraft/space, chemical industries to weld thicker material (up to 10mm of stainless steel) in a single pass.

Quality and Common Faults Sunken Bead Undercut too much penetration Welding current is too high Travel Speed is too low Bead too small, Irregular Little Penetration Welding Current and Plasma Gas Flow is too low Travel is too fast Undercut and Irregular Edges Plasma Gas Flow is too high Proper Size Bead, Even Ripple and Good Penetration Correct Current, Even torch movement, Proper Arc Voltage and Plasma Gas Flow Quality and Common Faults

Difference between PAW and TIG Plasma Arc Welding Tungsten Inert Gas Welding Two gases are used, One for Plasma Gas and other for Shielding Gas. Only one gas used, which forms plasma as well as shields the arc and molten weld pool. Uses Constricted Arc. Uses Non-Constricted Arc. Temp. of about 11000°C is achieved. Temp. of about 4000°C is achieved. Deep Penetration is achieved. Penetration obtained is not so deeper. No Filler Material is required. More Filler Material is required. Fast Metal Deposition Rate. Metal Deposition Rate is not so faster. Inert Gas Consumption is very high. Inert Gas Consumption is very low. Costly welding equipment. Less costly welding equipment. Cutting of Hard and Brittle Material is possible. Cutting of Hard and Brittle Material is not possible.

Control System

Advantages Permits faster metal deposition rate and high arc travel speed as compared to TIG Uniform penetration with high welding rate is possible Stability of arc and Excellent weld quality Can produce radiographic quality weld at high speed Can weld steel pieces up to about half inch thick, square butt joint Useful for semi automatic and automatic processes. Process is very fast and clean Requires less operator skill due to good tolerance of arc to misalignments; High penetrating capability (keyhole effect);

Disadvantages Special protection is required as Infrared and UV Radiations is produced Consumption of Inert Gas is high Needs high power electrical equipment. Gives out ultraviolet and infrared radiation. Operation produces a high noise of the order of 100dB. Expensive equipment; Can weld only upto 25mm thickness. High distortions and wide welds as a result of high heat input (in transferred arc process). More chances of Electrical hazards.

applications Aerospace Industries Cryogenics Foodstuff and Chemical Industries Machine and Plant Construction Automobiles and Railways Ship Construction Tank Equipment and Pipeline Construction etc.

Base Metal weldable by Plasma Arc Process: Easily Weldable: Al, Cu-base alloys (Cu, Cu-Ni), Magnesium, Ni-base alloys (Inconel, Nickel, Monel), Precious Metals, Steels (Low Carbon Steel, Low Alloy Steel, Stainless Steel, Tool Steels), Titanium (upto 8mm thick) etc... Acceptable but weldable with care: Cast, Malleable, Nodular Iron, Wrought Iron, Lead, Tungsten etc... Possible but not Popular: Bronzes, Brass, Nickel Silver, Lead, Zinc etc...

P Protected electrode, offers long times before electrode maintenance (usually one 8 Hr Shift) L Low amperage welding capability (as low as 0.05 amp) A Arc consistency and gentle arc starting produce consistent welds, time after time S Stable arc in arc starting and low amperage welding M Minimal high frequency noise issues, HF only in pilot arc start, not for each weld Arc energy density reaches 3 times that of GTAW. Higher weld speeds possible W Weld times as short as 5 millisec (.005 sec) E Energy density reduces heat affected zone, improves weld quality Length of arc benefit due to arc shape and even heat distribution D Diameter of arc chosen via nozzle orifice