Gas metal arc welding (GMAW) is performed by using direct current reverse polarity as it gives both good cleaning action and fast filler metal deposition rates. A rather high welding current is desirable as this extra current breaks up the globules of molten metal into a fine spray. This increases the rate of transfer and gives better control of arc, so that it can be directed accurately into the weld joint. This process can also be used with low current flows for thin metal welds and poor fit joints.
It uses consumable electrode which is feed through the electrode holder into the arc and at the same speed the electrode is melted and deposited in the weld. A small adjustable speed motor is used to remove wire (usually wound own special reel os spool) from a spool and feed it into the arc. Complete gas metal-arc welding outfit is shown in figure. CO2 and argon/CO2 mixtures are often used as shielding gases for welding various types of carbon sheets compared with GTAW. Very little operator skill is required to obtain satisfactory welds.
Initially the process was developed mainly for welding reactive metals such as aluminium and titanium. It is today a versatile process because of high deposition rate, ease of welding in all positions, requirement of less operator skill and adaptability to weld almost all metals to produce welds of high quality without the problems of flux, moisture and slag entrapment.
The description of major components used in GMAW is given below :
Welding gun : It resembles a pistol with a trigger to start to the process. It carries a ceramic nozzle through which wire electrode and shielding gas exit. The nozzle is air cooled for low power guns (upto about 200 amps.) and water cooled for higher power ratings.
A contact tube inside the gun is used for connecting the power source to the wire and the other power lead is connected to the work piece.
Wire drive : The welding wire, drawn from a spool, is fed through drive rolls, the wire feed rate being controlled to maintain a constant arc voltage.
Gas supply : The gas is supplied from cylinder through a flow metal, regulator and automatically operated solenoid valve.
Welding mechanics : A power source with constant arc voltage characteristic is connected to the connect tube in the welding gun and to the workpiece.
Water supply system : The gun may be water cooled in which case a supply of water with suitable flow control is provided.
Control unit : It coordinates the functioning of power supply, wire drive and movement of the gun and regulates the gas supply and flow of water. If the water flow or gas supply is inadequate, the unit will be switched off automatically.
Power sources : Generally a DCRP is used because it results in surface cleaning (removes oxide of metal) of metals such as aluminium. DCSP is used for aluminium or magnesium with a 'buried-arc' or short circuiting metal transfer. AC supply is not convenient due to erratic arc behavior.
Shielding Gases are used to protect the molten metal from contamination from the atmosphere. The choice of the shielding gas is determined by arc and metal transfer characteristics, weld deposition characteristics, i.e., penetration, width of fusion, shape of reinforcement, speed of welding and under cutting tendency.
Helium requires higher arc voltage than argon and thus heat input is higher. Thus helium is used for highly conducting metals like copper. Argon is preferred for thin metals.
Also the argon, being heavier than helium, forms a shielding blanket easily. Therefore a small amount of flow of argon is sufficient compared to helium. Often a mixture of argon and helium is used.
The addition of carbon dioxide to argon for welding steel stabilizes the arc, promotes favourable metal transfer and minimizes spatter. Undercut is reduced or eliminated and porosity is also reduced and high impact strength obtained. CO2 alone is used for welding mild steel.
It results is high welding speed, better joint penetration and sound deposit. However spatter is observed to be high.
This type of equipment is used for various kinds of welding.
1. Gas metal-arc welding [Fig.(a)].
(i) Spray arc welding.
(ii) Short circuiting method or dip transfer arc welding.
2. Gas metal-arc welding with magnetized flux.[Fig.(b)].
3. Metal-arc welding with flux-cored welding wire[Fig.(c)].
(a) With inert gas
(b) Without inert gas
Almost any metal (mild steel, stainless steel, aluminium, bronze etc.) can be welded by one or more of these processes. Hard surfacing can also be peformed using this process.
GMAW has also been used for hyperbaric conditions. On surface, dip transfer welding or short arc welding technique is used to transfer metal across the arc so that it is not influenced by gravity. In dip transfer technique, the wire is allowed to enter the molten weld pool short circuiting the arc. A rapid increase in current heats and eventually ruptures the feed wire, the pool stabilizes itself by surface tension force and the wire is fed forward to repeat the process. However this technique, being low heat input process, is not suited under hyperbaric conditions due to fusion problem for welding of thick section.
Spray transfer technique which uses electromagnetic forces to project molten droplets across the arc, has also not been found suitable for hyperbaric conditions as it much depended on the welder skill.
Presently flux cored wire arc with controlled transfer pulse (CTP) GMA also called 'synergic' welding is used. With advances in electronic technique, it is possible to vary the static and dynamic characteristics of the power supply to suit welding parameters. For a defined consumable with a specific shield gas, metal transfer is principally controlled by the magnitude and direction of pulse of current applied to the arc. As the electric field strength of the arc is dependent on the pressure, increased arc voltages and adjustment to the power supply characteristics are demanded for hyperbaric welding to ensure stable operation. The complex relationship between power supply characteristic, wire feed speed control strategy and variation in welding torch/workpiece distance need to be analyzed to make this system success.