In this article we will discuss about:- 1. Meaning of Gas Metal Arc Welding (GMAW) 2. Components of GMAW 3. Modes of Materials Transfer 4. GMAW Practices 5. GMAW Welding using Fluxed Wire 6. Safety in GMAW 7. Characteristics of GMAW 8. Advantages of GMAW 9. Limitations of GMAW 10. Recent Advances in Welding Technology.
Meaning of Gas Metal Arc Welding (GMAW):
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 these 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 thin metal welds and poor fit joints.
It uses consumable electrode which is fed 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 on special reel or spool) from a spool and feed it into the arc.
Complete gas metal-arc welding outfit is shows in Fig. 9.46. CO2 and organ/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 proofs 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.
Components of GMAW:
The description of the major components used in GMAW is given below:
i. Welding Gun:
It resembles a pistol with a trigger to start 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 workpiece.
ii. 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.
iii. Gas Supply:
The gas is supplied from cylinder through a flow meter, regulator and automatically operated solenoid valve.
iv. Welding Machines:
A owner source with constant arc voltage characteristic is connected to the contact tube in the welding gun and to the workpiece.
v. Water Supply System:
The gun may be water cooled in which case a supply of water with suitable flow control is provided.
vi. 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 flew or gas supply is inadequate, the unit will be switched off automatically.
vii. 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 behaviour.
viii. Shielding Gases:
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, well deposition characteristics, i.e., penetration, width of fusion, shape of reinforcement, speed of welding, and undercutting 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 in 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 [Refer Fig. 9.47 (a)].
(i) Spray arc welding
(ii) Short circuiting method or dip transfer arc welding.
2. Gas metal-arc welding with magnetised flux [Refer Fig 9.47(6)].
3. Metal-arc welding with flux-cored welding wire [Refer Fig. 9.47 (c)].
Almost any metal (mild steel, stainless steel, aluminium, bronze etc. can be welded by one or more or these processes. Hard surfacing can also be performed using this process.
Modes of Materials Transfer:
Metal transfer across the arc may take place by one of the following methods:
i. Short Circuiting Transfer:
In this mode the bottom of electrode converts to small drops which are transferred to weld pool at a steady rate of 20 — 200 times/sec.
This type of transfer occurs for low currents (less than 200 amps.) and small electrode diameters. Freezing weld pool is small and is generally suited for joining thin sections, for out-of-position welding, and for large root gaps. Deposition rate is around 0.9 to 2.7 kg/hr.
ii. Globular Transfer:
Globular transfer is characterised by a drop size of diameter greater than that of the electrode.
Globular transfer takes place with DCRP, when current density is relatively low, for all shielding gases.
When CO2 is used as shielding gas even at high current density, only globular, non-axial transfer takes place. Deposition rate is around 1.8 to 3.2 kg/hr.
iii. Spray Transfer:
In the spray transfer welding, the metal is transferred across the arc as a fine spray of several hundred minute droplets per second. This process is used mainly on normal metal joints and/or good fit up joints, and for faster welding.
It consists simply of using a sufficiently heavy current flow for a certain size electrode wire, so that the electrode wire will divide into very small droplets of welding wire metal, and will travel forcibly across the arc. Usually argon gas with 5% oxygen is used as inert gas which reduces spatter (obtained with CO2) and enables a reduction in current, and produces a neater appearance of the weld. It is best suited for welding thick parts (minimum 3 mm thickness for aluminium). Deposition rate is of the order of 2.7—5.4 kg/hr.
iv. Pulsed Spray Transfer:
It is variation of spray transfer arc welding in which controlled droplet transfer is obtained at current levels commonly associated with short circuiting transfer. A pulsating power supply is used with a low back ground level to maintain an arc and a high level in excess of the transition current for the particular electrode.
The combination of the two current levels produces a stable spray arc with droplet transfer each time the current exceeds the transition point. Pulsed spray transfer is used for the out-of-position welding of thicker steel section when short circuiting transfer is difficult to use.
It used argon-based shielding gases to achieve a true spray transfer. Argon-oxygen and argon-C02 mixtures are also used to obtain a stable are and good droplet transfer characteristics. Deposition rate is of the order of 0.9—2.7 kg/ hr.
v. Rotating Spray Transfer:
When current, voltage and electrode extension are more then used for conventional spray transfer, the lower portion of electrode melts over a considerable length and rotates in a helical pattern under the influence of the magnetic field surrounding the arc.
As the arc rotates, a controlled stream of droplets is transferred from the electrode tip to the weld pool over a relatively wide area. The arc energy is thus spread out, causing the penetration pattern to broaden. Argon atmosphere with a 2- 5% oxygen or less than 10% CO2 addition is used.
The current necessary to induce rotational mode of transfer increases rapidly with electrode diameter and decreases with increase in electrode extension. Generally electrode wires (0.9—11 mm diameter) with 19-32 mm extension are used to keep current at 320-380 A with deposition rates 9-11 kg/hr in flat position.
Dip Transfer. In this method, the metal transfers across the arc in larger drops at the rate of 100 drops/sec or less. The drops are large enough to short circuit the electrical flow across the arc. This process is used for welding thin metal joints and for poor fit-up joints. In this method the current values are much lower.
For this process special are current power supplies are needed to overcome the short circuit moment that occurs across the arc. The speed of travel is greater compared to the spray-arc method.
This type of welding can be successfully done on most of the commercial metals if the proper welding wire and proper gases are chosen and the proper settings are made. It can be done in all the positions, i.e., in fiat position or vertical position or an inclined or overhead position. CO2 is generally used as the inert gas.
To obtain high quality steel welds, weld should be made on ‘killed’ steel (fully deoxidised steel). Most of the electrode wires for mild steel welding have deoxidising elements in the welding wire metal such as Mn, Si, Al, titanium, and zirconium. For the same size wires if it is desirable to use more current, the wire feed speed should also be increased.
When changing to a larger diameter wire, either the current must be increased or the wire feed speed must be reduced, or both. These relationships are very important and the manufacturer’s recommendations should be followed.
GMAW Welding Using Fluxed Wire:
Flux is used in GMAW welding as an aid to improve the quality of weld.
It may be used in 3 ways:
(a) As a suspension find powder mix in the inert gas.
(b) As a flux powder coating on the metal electrode as it leaves the torch nozzle. Magnetism is used to hold the flux to the wire. The flux contains iron particles which are attracted to the magnetised metal electrode,
(c) As a flux inside a tubular metal electrode.
Its use eliminates the need for the gas shield equipment and also the cost of the gas.
Safety in GMAW:
In the inert gas arc welding as the arc is more exposed and therefore more intense, the operator should use a dark lens and must be very careful to protect his skin from the arc rays. The machine must be kept in good condition for best results. Water leaks should be repaired immediately as we equipment or wet floor increases the chance of electric shock.
Good ventilation in welding station is essential as the influence of the ultraviolet rays from the arc tends to change oxygen into ozone and nitrogen into nitrogen oxides. Leather and/ or wool clothing is recommended. All other welding precautions should be strictly adhered to.
The following factors which affect GMAW process need to be controlled properly:
i. Welding current,
ii. Arc voltage,
iii. Electrode size,
iv. Extension and inclination travel speed,
v. Weld joint position.
Characteristics of GMAW:
Some of the characteristics of GMAW are as follows:
i. Slag inclusions are eliminated because of absence of flux.
ii. There is no possibility of cracking in the weld or HAZ due to hydrogen because process provides protection against H2. Cost can be lowered for plain carbon steels with CO2 shielding.
iii. Weld defects may occur if welding procedures are not correct.
Advantages of GMAW:
Two main advantages of GMAW are given below:
a. GMAW does not require the high degree of operator skill.
b. Continuous welding at higher speeds and in all positions with deeper penetration is possible.
Limitations of GMAW:
Followings are some of the limitations of GMAW:
i. The welding equipment is more complex, costly and less portable, and used indoors.
ii. It is difficult to weld in small corners or difficult-to- reach places.
iii. The metallurgical and mechanical properties of the joint may be affected due to high cooling rate.
Recent Advances in Welding Technology:
Welding technology and fabrication techniques are undergoing major changes for achieving higher productivity and improved quality. These objective are being met by technological innovations breaking through the barriers of design, material selection, fabrication processes and also equipment and power sources. The largest strides are being met in the field of automation, controls and use of industrial robots.
GMAW welding equipment have undergone major improvements of exploit the capabilities of the process fully. Conventional pulsed GMAW systems and synergic GMAW systems have been introduced.
Industrial Robots for GMAW welding and microprocessors in GMAW welding equipment for seam tracking and adaptive controls are being used. The flux cored wire process (with and without gas shielding) is fast replacing the solid wire GMAW.
Custom-built submerged arc welding equipment with better controls and multi-wire systems linked with manipulators are finding more applications.
Mechanisation of GTAW welding system and use of pulsed GTAW is also being tried.
The use of welding robots has made spectacular advances particularly in hazardous applications and productivity-cum-quality requirements.
There has been rapid change-over to the automatic and semi-automatic welding processes. Manual metal arc welding (MMAW) process, though flexible and apparently cheap, involves considerable welder skill and fatigue, frequent interruption, stub-end losses and low productivity.
Lot of advances have been possible due to a significant development in the field of welding power sources, the use of solid state control devices like thyristors and development of transistorised power sources. This has resulted in faster response and better characteristics eliminating fluctuation in voltage and current leading to better arc stability.
This also enhances tolerance to input supply fluctuations and gives capabilities for pulsing etc. Other developments are the invertor power sources, and square-wave power sources which have great potential for saving material and energy cost.
Synergic Welding System:
The word ‘synergic’ means ‘working together’. In the context of CO2 welding the synergic system means the wire feed speed works together with pulse parameter. This is to say, there is a combined relationship between the wire feed speed and all relevant pulse parameters (pulse repeat frequency, duration and background current levels). The optimum pulse parameters are microprocessed.