Plasma is formed by passing an electric current through plasma gas. This heats the plasma gas to an extremely high temperature so that it ionizes and becomes electrically conductive. In some cases, the plasma can reach temperatures in the region of 25,000° C (45,032° F). PAW then uses this plasma to transfer an electric arc to a workpiece. The intense heat of the plasma jet melts the metal to be welded or produces a keyhole. This concentrated energy makes it suitable for both plasma welding and plasma cutting applications.
One of the key features of the plasma welding process is the design of the plasma welding torch. The electrode - typically a thoriated tungsten electrode - is housed within the torch nozzle and the nozzle opening is constricted. A pilot arc is initiated between the torch electrode and nozzle tip. This arc plasma is then transferred to the metal to be welded. Because the plasma gas and arc are forced through the constricted orifice of the nozzle, plasma welding delivers intense heat to a smaller area (known as the heat-affected zone). This precision has the potential to produce exceptionally high-quality welds. The design of the welding torch also results in better arc stability. Because plasma arc welding operates at very high temperatures, the torches are often water-cooled to prevent damage to the nozzle, although some are air-cooled.
The following gases or mixtures are required for plasma arc welding:
It is extremely important that the welder chooses a plasma shielding gas that does not adversely affect the welding process or the weld metal produced. We offer a number of dedicated shielding gases and mixtures blended specifically to enhance plasma welding outcomes. These are marketed under different family names in different regions, but include our PURESHIELD® argon family, suited to plasma welding of all metals. Our ALUSHIELD® series is better suited to thicker sections, providing consistent plasma jet performance on all materials. The Linde HYDROSTAR® and HELISTAR® lines include compositions tailored to high-alloyed stainless steels. STAINSHIELD® and SPECSHIELD® mixes are primarily targeted at plasma welding of austenitic stainless steels, whereas STARGOLD® and HELISTAR blends are ideal for plasma welding of aluminum.
Contact your local Linde representative to check availability of our plasma jet gases, shielding gases, root backing gases, trailing gases and welding supplies in your region.
The plasma arc can be established in two different ways:
Plasma welders can also adjust the current, the plasma gas flow rate and the orifice diameter to support different operating modes offering variations in precision and power:
The unique plasma arc torch design creates a more focused beam than TIG torches, typically producing higher-quality welds. There are a number of other similarities and differences, however.
PAW stands for plasma arc welding. Inspired by the TIG welding process, Robert M. Gage invented it in 1953 at the Linde/Union Carbide laboratory in Buffalo, New York. The process was patented and brought to market in 1964.
Yes, the PAW welding process requires a shielding gas. It also requires a plasma gas. The plasma gas exits directly from the torch and is separate from the shielding gas flow. In addition, some applications require a back-purge or trailing gas.
The basic equipment requirements for PAW welding are a power supply (either AC or DC), a plasma torch, a control console between the power source and the torch, a gas supply system for the plasma gas, the shielding gas and possibly the root backing and trailing gas, a wire feeder system (if filler wire is used) as well as leads and connectors.
Services are provided by the torch conduit that carries welding current, plasma gas and shielding gas plus water hoses for water-cooled torches. It also carries the control cable for switching on the process.
Similar to TIG welding, the electrode in plasma welding is generally made out of thoriated tungsten. Unlike other GMAW methods, the electrode is contained within the body of the torch.
Like tungsten inert gas (TIG) welding, plasma arc welding (PAW) is generally associated with lower fume emission rates (FER) than metal inert gas (MIG) or metal active gas (MAG) welding processes. Good general ventilation will often keep FER well within acceptable limits. However, it can produce significant amounts of gaseous fume such as ozone.
Use of helium or helium/argon or hydrogen-containing shielding gases can help mitigate exposure to gaseous fume, as these mixtures tend to generate less ozone than pure argon or argon-rich non-hydrogen-containing shielding gases.
Contributing to Occupational Health and Safety in Plasma Welding
Our COMPETENCE, PERFORMANCE and PREVENTION Lines of shielding gases can help increase productivity and lower occupational health and safety risks through less-emissive welding gases with the ability to reduce FER directly at the source.
We also offer a broad range of personal protective equipment (PPE) including respiratory equipment and welding helmets. Radiation from a plasma arc can be more intense than that from other open arc processes. This makes welding helmets with the correct filter shade particularly important to protect welders against 'arc eye'.