Manufacturing firms can substantially increase their productivity, while improving the quality and appearance of welded joints in different types of specialized metal fabrication through the use of automated plasma welding. This innovative welding technology recently introduced in Canada by Air Liquide.

Automated plasma welding was recently introduced to North America , and is considered an enhancement of the TIG (Tungsten Inert Gas) process. It involves a free electric arc in an inert or neutral gas, and is an Advanced Fabrication Technology introduced by Air Liquide Canada.

This welding process meets the special needs of manufacturers who produce pipe and piping systems, vessels and storage tanks for the petrochemical, textile, transportation and agri-food industries.

Automated plasma welding is not a recent invention. It has been used for several decades in Europe for welding stainless steel, carbon steel and titanium.

In response to the growing demand from advanced manufacturing industries for more efficient fabrication processes, Air Liquide Canada decided to emphasize automated plasma welding systems. The process provides significant productivity gains on the factory floor because of a savings in time and materials.

A very precise process
The term plasma refers to gaseous environments in temperatures above 3,000°C.

Plasma is the fourth state of matter after solids, liquids and gases. It is made up of excited molecules, electrons, ions and atoms, and often is found in the natural environment, generated for example by lightning.

In welding, however, the meaning of plasma has shifted in recent decades to refer to the state of an electric arc that is constricted by a nozzle.

In plasma welding, the energy for the weld is provided by an electric arc that passes through an atmosphere of inert plasma gas between a non-consumable, or infusible, electrode and the workpiece.

The wire feed is carried out automatically outside the plasma torch. The arc is constricted mechanically and kinetically as it passes through a nozzle, a narrow cylindrical opening that directs the arc and increases the voltage of the arc column to reach temperatures of 10,000–16,000°C.

Plasma welding processes use gas mixtures of argon, hydrogen, CO2 or helium for welding different materials. With this technique, carbon and stainless steel, other corrosion-resistant metals, nickel alloys, titanium, zirconium, tantalum and even noble metals can be welded easily with unparalleled accuracy.

Quick, high-quality welds

Plasma torches are moving from cutting processes to welding.

Plasma welding can be used in two different fusion modes, but for thicker materials, the most efficient fusion mode provides a full penetrating arc.

In this mode, called “Keyhole”, the arc blows a hole through the entire thickness of the workpiece, creating a shape that resembles a keyhole while it travels through the joint.

Due to surface tension, the molten metal in front of the arc is pushed towards the back, to create tiny waves. The density of energy generated by the plasma arc is much higher by far than in other processes and allows it to be applied to smaller and thicker areas of metal.

Plasma welding is used primarily in metallurgy, the chemical industry and engineering, due to the high temperatures generated by its powerful concentrated beam and its 100 percent penetration capacity.

The plasma process can be used to create complete circular and longitudinal welds in one pass and from a single side, at varying welding speeds – 1 cm/min 5 to 50 cm/min or 6 cm/min to 20 in./min – depending on the thickness of the base material.

One of the important features of the plasma welding process is that it allows users to create single square groove welds on base materials ranging in thicknesses from 3 mm to 10 mm (1/8 in. to 3/8 in.) without requiring multiple filling passes.

The automation of the process in a vertical or horizontal position and the low-profile bead shape also reduce post-weld operations, producing weld beads that are flawless in appearance.

Where and why is plasma used?
The plasma welding process meets the high quality and productivity standards of many industries that use stainless steel and noble metals as base materials for welding very large cylindrical storage tanks, from a flat position or standing on a bracket.

Plasma welding is also well suited for the fabrication of spools of pipes upstream of the installation site.

It allows subassemblies to be prepared and welded in the factory from tubes, elbows, flanges and other parts, which are then fitted together. These subassemblies are used particularly by shipyards and offshore drilling platform manufacturers, as well as refineries, thermal power plants, chemical and agrifood plants, and gas distribution facilities.

Plasma welding has significant advantages, including increased welding speed and low distortion, which reduces if not eliminates post-weld straightening operations.

Firms also appreciate the reduced use of filler metal, the high radiographic quality of welds and the reproducibility of quality.

The high quality of the visual aspect and the chemical composition of the weld, which are key in the agri-food industry, guarantee against future corrosion problems.

More than any other process, plasma welding respects the chemical composition of the base materials and eliminates the preparation of joints and thus drastically reduces welding time, since a square butt joint preparation can be used for thicknesses to 10 mm (3/8 in.)

The narrowness of the heat zone and assurance of full and regular penetration due to the plasma jet’s ability to make a single pass over the joint end to end, results in flawless-looking welds on both sides.

The primary reason for using plasma welding, however, is that it reduces welding time compared with manual welding by a factor of four to five, depending on the technique and materials used.

Equipment design for increased productivity

The plasma welding process requires a complete and customized automated system, which generally consists of a column and boom arm synchronized with two welding power sources and a positionner.

Complete turnkey systems, which include all the above-mentioned components plus torches, accessories, integration of the firm’s special requirements and on-site installation, can be provided to users.

To ensure the equipment is working optimally, a control box allows the operator to control welding parameters such as current value, voltage, wire speed, and gas flowrate.

Since the system is automated, the process requires only one operator who does not have to be a skilled welder. In fact, the best results have been obtained with operators having very little welding experience but a general background in mechanics and electricity.