The Plasma Arc Welding Process

Summary

Plasma Arc Welding (PAW) was invented and patented in 1953, by Robert M. Gage, at the Linde/Union Carbide laboratory in Buffalo NY. About 10 years of development and multiple subsequent patents occurred before the devices were brought to market in 1964.

The plasma process is equally suited to manual and automatic applications. It has been used in a variety of operations ranging from high volume welding of strip metal, to precision welding of surgical instruments, to automatic repair of jet engine blades, to the manual welding of kitchen equipment for the food and dairy industry.

How Plasma Welding Works

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 work piece. 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 quality welds.

Plasma gases are normally argon. The torch also uses a secondary gas, argon, argon/hydrogen or helium which assists in shielding the molten weld puddle thus minimizing oxidation of the weld.

Equipment Required List

• Power Supply
• Plasma Console (sometimes external, sometimes built in)
• Water re-circulator (sometimes external, sometimes built in)
• Plasma Welding Torch
• Torch Accessory Kit (Tips, ceramics, collets, electrodes set-up gages)

List of Plasma Welding Features and Benefits

Features, Benefits, and Applications

Features

Benefits

The full list of reasons for using the plasma welding process is lengthy but can be summarized into three main features where customers desire the advantages of at least one feature.

• Precision: The plasma process is generally more precise than conventional Tig (remember that enhanced power supplies can create an arc that is different to a conventional Tig arc) Plasma offers the following advantages over conventional Tig:
  • Stable, concentrated arc
  • Forgiveness in arc length variations (Tig +/- 5%, Plasma +/- 15%)
• Small Part Welding:
  • Low amperage capability (many plasma power supplies go down to .1 amps)
  • Stable at low amps
  • Gentle arc transfer (arc start) with no high frequency noise.
  • Short weld times possible (for spot welds - guidewires, tubes etc.)
• High Production Welding:
  • Long electrode life offers many more hours of welding than Tig before electrode contamination occurs.

In many applications, many of the unique advantages of plasma combine to benefit the overall welding process.

Applications

Small Part Welding: 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.

Sealed Components: Medical and electronic components are often hermetically 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 arc without high frequency electrical noise which could be damaging to the electrical internals

Applications include Pressure and Electrical Sensors, Bellows, Seals, Cans, Enclosures, Microswitches, Valves, Electronic Components, Motors, Batteries, Miniature Tube to Fitting/Flange, Food and Dairy Equipment,

Tool Die & Mold Repair: A whole repair industry has sprung up to assist companies wishing to re-use components with slight nicks and dents from misuse or wear. The ability of modern micro-arc 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 micro-plasma 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.

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. This is especially advantageous in high volume applications where the material outgases or has surface contaminants.

Tube Mill Welding: Tube mills produce tube and pipe by taking a continuous strip of material and rollforming the edges upwards until the edges of the strip meet together at a weld station. At this point the welding process melts and fuses the edges of the tube together and the material exits the weld station as welded tube.

The output of the tube mill depends on the arc welding speed and total time spent welding. Each time the mill shuts down and starts up again there is a certain amount of scrap produced. Thus the most important issues to the tube mill user are:

1. Maximum tube mill weld speed obtainable.
2. Arc stability for optimum weld quality and consistency.
3. Maximum number of hours of welding electrode tip life.

Some tube mills employ plasma welding in order to get a combination of increased weld speed, improved weld penetration and maximum electrode life.

Comparison of GTAW and Plasma Welding Energy Input

The following is from a test made with the GTAW (Tig) and Plasma welding processes on a specific strip of test material in order to establish a comparison of the energy input of poth processes. The test results should be used as a general guideline comparison only as welding engineers can change any of the parameters noted below to achieve a different result.

Test Parameters: Manual welding, no clamping device, Cr/Ni steel, 0.102" thicknes. All values determined with measuring instruments.

   V x A x 60
-----------------
 Speed in cm/min

  12 x 125 x 60
-----------------  =  3.46 KJ
    26 cm/min

  18 x 75 x 60
----------------  =  2.38 KJ
    34 cm/min

In addition to the fact that a higher weld speed is possible, the lower heat input brings the following advantages:

• More consistency
• Less distortion.
• Less stress in welded component.
• Lower risk of damaging any heat sensitive parts adjacent to the weld joint.