Gas metal arc welding

Gas Metal Arc Welding (GMAW)

1 Introduction

Welding is the fabrication process of joining two metal pieces permanently by applying heat or pressure or both. Joining takes place by melting and fusing: melting the base metals and applying the filler metal. This is achieved by melting workpieces and adding a filler material to form a pool of molten material (weld pool) that cools to become a strong joint. (Wikipedia)

Some advantages of welding are that it produces a strong and tight joining between two pieces, is cost effective, simple and can be mechanized and automated. However welding results in internal stresses, distortions and changes in microstructure in the weld region.

GMAW is currently one of the most popular welding methods, especially in industrial environments because it has lead to simplification of the welding process. GMAW is said to be one of the easiest welding processes to learn and perform. This is because in the process, the power source virtually does all the work by adjusting welding parameters to handle differing conditions. GMAW is extensively used in sheet metal industry and automobile industry. It has replaced riveting and resistance spot welding. It has also found applications in robot welding, where robots handle the workpieces and the welding gun to increase the consistency and the manufacturing process rate.

The change in welding trends from SMAW to GMAW in the small and medium fabrications, mainly in the automotive industry. The reason is the attempt of manufacturers to maintain quality and decreasing cost.Lateron it was noted that GMAW was not preferred previously because of the limitation of incomplete fusion, which was not preferred for bridge and structural fabrications. However, with increase in the technology in GMAW, such as advancement in pulsed spray mode of transfer of metal, applications of the welding process has increased. Then there were some advancement in pulse metal arc welding by producing spray transfer at low mean currents. In a GMAW, process good thermal and electrical conductivities act as a drawback because such properties lead to excessive heating of base metals. Hence, limiting the use of the gas metal arc welding process. Pulsed gas metal arc welding (GMAW-P) addresses such problems. Another advancement in GMAW is the use of double electrode in the welding process to increase the manufacturing speed. The DE-GMAW allows increasing the melting current while controlling the base metal current at a desired level. They also developed a model to correlate the change in resistance in the base metal required to achieve the desired base metal current. Then came upon the process of laser hybrid welding, which is popular in automotive industry today. They describe the interaction of laser with GMAW during welding and discuss various variables involved during the process. Later on engineers reviewed the droplet transfer models of solid wire. They described the simulation models: SFBT (static force balance theory), PIT (pinch instability theory) and VOF theory. They concluded that results produced by VOF were in confirmation with the experimental results. With advancement in technology of GMAW, its applications have touched new horizon. Therefore, it is important to study about this process in detail.

The purpose of this report is to present the process variables, effect of process variables and equipment on the welding process, and sensing and control systems. Advantages, disadvantages and applications of GMAW process are briefly discussed.

This report is expected to help future researchers in their research endeavors by acting as a literature review and a guide to the gas metal arc welding process.

2 Methodology

This section will briefly describe the types of welding processes and then concentrate on the GMAW process. Different methods of GMAW process, the modes of metal transfer are introduced in this section. Emphasis is laid on the process variables in the GMAW process, equipment used, and the sensing and control systems. In designing a welding process, the effect of different process variables must be considered. Each application has its unique requirements and limitations. These will be in relation to the parameters that can be controlled or not. Also, the equipment used is of great concern in terms of its simple operation and control and uses. It is important to know the process variables that will have affect on the welding process and their relationship with other parameters.

The emerging necessity of the welding processes is its automation. With the use of robots in welding, it has become necessary to automate the whole process and monitor and control the operation and quality of weld. Different sensing and control systems under research and recently introduced in the industry are discussed.

2.1 Welding processes

Various welding processes have been developed and used in the welding industry depending upon their application, energy source, such as mechanical, electrical, chemical or optical, metals to be welded, location of metals, cost, etc.

The three broad classes are:

• Solid state welding
• Fusion welding
• Soldering and Brazing

2.1.1 Solid state welding

Solid state weldingis a welding process, in which two work pieces are joined under a pressure providing an intimate contact between them and at a temperature essentially below the melting point of the parent material (Kopeliovich). Two materials bind by diffusion of the interface atoms. The processes that come under this class are:

• Forge Welding (FOW)
• Cold Welding (CW)
• Friction Welding (FRW)
• Explosive Welding (EXW)
• Diffusion Welding (DFW)
• Ultrasonic Welding (USW)

Although these processes have advantages, they require thorough surface preparation like degreasing, oxide removal and brushing or sanding. In addition, these processes are expensive.

2.1.2 Soldering and brazin.

Soldering and brazing involve melting the filler metal, which then flows into the space between the closely fitted base metals and solidifies. In soldering, the melting point of the filler metal is below 800°F while in brazing it is above this temperature. In both these processes, the melting point of the filler metal is below that of base metals. The filler metal is distributed between the properly fitted parts by capillary attraction.

Some disadvantages are removal of flux residuals to prevent corrosion, no gas shielding may cause porosity of the joint, large sections cannot be joined, filler materials may contain toxic components and expensive filler materials.

2.1.3 Fusion welding

Fusion welding involves the partial melting of two members welded by a heat source and amalgamated into one piece. The thermal energy required for fusion is usually supplied by chemical or electrical means. It may use a filler material like a consumable electrode or a wire. Fusion welding uses a protective layer like gas shielding or flux, which melts and forms a viscous slag on the weld metal that solidifies and removed later.

2.2. Gas metal arc welding (GMAW)

Gas metal arc welding (GMAW) or metal inert gas (MIG) welding or metal active gas (MAG) welding is a semi-automatic or automatic arc welding process, which joins metals by heating them to their melting point with an electric arc. A continuous, consumable electrode wire and a shielding gas are fed through a welding gun. MIG involves use of an inert gas while MAG uses active gas like oxygen or carbon dioxide.

2.2.1 GMAW process

Gas metal arc welding process usually comprise of a constant voltage, direct current (constant current or alternating current systems can also be used) arc burning between a thin bare metal wire electrode and the work piece. The arc and weld area are encased in a protective gas shield, fed through the welding gun. A continuous, consumable wire electrode is fed from a spool, through the welding torch/gun, which is connected to the positive terminal into the weld zone.

2.2.1.1 Parameters. The parameters of GMAW process are:

• Shielding gas
• Electrode size
• Electric parameter: voltage and current (continuous current is used)
• Feed rate (of electrode)
• Travel speed

The shielding gas like carbon dioxide or a mixture of carbon dioxide and argon helps protect the molten metal from reacting with the atmosphere. Molten metal when exposed reacts with oxygen, nitrogen and hydrogen in the environment. Shielding gas flows through the gun and cable assembly and out of the gun nozzle with the welding wire to shield and protect the molten weld pool. The risk of reacting of metal with atmosphere limits the use of GMAW indoors because outdoors wind can blow the shielding gas away from the work piece and result in reaction.

The consumable wire commonly is copper colored mild steel, which has been electroplated with a thin layer of copper to protect it from rusting, improve electrical conductivity, increase contact tip life, and improve arc performance.

2.2.2 GMAW methods. GMAW can be performed in three different ways:

• Semiautomatic Welding – wire feeding is controlled by the equipment and the movement of welding gun is by hand. Also called hand-held welding.
• Machine Welding – a gun is connected to a manipulator (not hand-held). Manipulator controls are adjusted constantly by an operator.
• Automatic Welding – welds without the constant adjusting of controls by a welder or operator.

2.2.3 Mode of metal transfer in GMAW. GMAW use four different modes to transfer metal from the electrode to the work piece. These are:

Globular mode of transfer

• Short-circuit transfer
• Globular transfer
• Spray transfer
• Pulse-spray transfer

2.2.3.1 Short-circuit transfer

Short circuit transfer refers to the welding achieved by short-circuiting (touching) welding wire with the base metal between 90 – 200 times per second. The wire feed speeds, voltages, and deposition rates are usually lower than with other types of metal transfer such as spray transfer. This facilitates welding thin or thick metals in any position.

A typical Short Circuit Cycle can be summarized in following steps:

Electrode is short-circuited to base metal. No arc and current is flowing through electrode wire and base metal.

Resistance in electrode wire increases causing it to heat, melt and neck down.

Electrode wire separates from weld puddle, creating an arc.

Small portion of electrode wire is deposited, which forms a weld puddle.

Arc length and load voltage are at maximum.

Heat of arc flattens the puddle and increases the diameter tip of electrode.

Wire feed speed overcomes heat of arc and wire approaches base metal again.

Short circuit cycle starts again.

2.2.3.2 Globular Transfer

Globular transfer refers to the state of transfer between short-circuiting and spray arc transfer. Large globs of wire are expelled from the end of the electrode wire and allowed to enter the weld puddle. This type of mode of transfer results when welding parameters such as voltage, amperage and wire feed speed is somewhat higher than the settings for short circuit transfer.

2.2.3.3 Spray Arc Transfer

Spray arc transfer refers to spraying a stream of tiny molten droplets across the arc, from the electrode wire to the base metal. Spray arc transfer uses relatively high voltage, wire feed speed and amperage values, compared to short circuit transfer. Inert argon rich shielding gas is used for best results.

2.2.3.4 Pulse-spray Transfer

In the pulse-spray transfer mode, the power supply is made to cycle between a high spray transfer current and a low background current. It is different from the spray transfer in that it allows the super cooling of the weld pool during background cycle. In each cycle one droplet transfers from the electrode to the weld pool. The low background current allows pulse-spray mode of transfer to weld out of position on thick sections with higher energy than the short-circuit transfer, thus producing a higher average current and improved sidewall fusion. It can be used to lower heat input and reduce distortion when high travel speeds are not needed or cannot be achieved because of equipment or throughput limitations.

2.2.4 Process Variables

The process variables of the GMAW affect the welding efficiency and weld quality. These variables either act alone by affecting the final product or they interact with each other and affect weld penetration, bead geometry. It is important to study these variables and have their set limits for a desired welding process and good overall weld quality. The enough penetration, high heating rate and rightwelding profile make the quality of welding joint. These are affected by welding current, arc voltage,welding speed and protective gas parameters. Table 1 shows the effect of different process variables on penetration depth, deposition rate, bead size and bead width.

Table 1: Effect of process variables on penetration, deposition rate, bead size and bead width.

a change will occur if current is maintained by wire feed speed. http://products.asminternational.org/hbk/index.jsp

The process variables are listed and discussed below:

• Welding current (electrode feed speed)
• Polarity
• Arc voltage (arc length)
• Travel speed
• Electrode extension
• Electrode orientation (gun angle)
• Electrode diameter

2.2.4.1 Welding current

Welding current is the electrical amperage in the power system as the weld is being made. In GMAW constant voltage power sources (voltage) are used, therefore, amperage is thought to be controlled by wire feed speed. Welding current is read from the power source meter or a separate ammeter is often used. The total welding amperage or current supplied to the arc is determined by the wire feed rate, open circuit voltage setting and the slope setting on the welding power source (Figure 1 and Figure 2). The faster the wire feed speed, higher is the welding amperage. However, the wire feed speed only determines the balance between the welding current and the load voltage at the arc. When all other variables are held constant, an increase in welding current results in an increase in the depth and width of penetration, deposition rate, and weld bead size.

This makes GMAW arc was made self-regulating. i.e. if the welder pulls the torch away from the workpiece—raising the arc length and arc voltage—the power supply drops the arc current to burn off wire at a slower rate until the preset arc voltage was re-established. If the welder pushed the torch toward the work—shortening the arc and reducing the arc voltage—the power supply quickly raised the welding current to burn off more wire until the preset arc voltage was re-established.

2.2.4.2 Polarity

Polarity describes the electrical connection of the electrode (welding gun) with the terminal of a power source. When the gun power lead is connected to the positive terminal, the polarity is designated as direct current electrode positive (DCEP).

When the electrode or the gun power is connected to the negative terminal, the polarity is designated as direct current electrode negative (DCEN). When alternating current (AC) is used, the polarity changes every half cycle of 50 or 60 Hz. In GMAW usually DCEP is used because it yields a stable arc, smooth metal transfer, relatively low spatter, good weld bead characteristics and deep penetration for a wide range of welding currents.

On the other hand, DCEN results in the molten droplet size tends to increase and the droplet transfer becomes irregular (Figure 3). This increases large grain spatter. Some wires with unique chemical composition have been developed for DCEN, which give excellent results on galvanized sheets.

Variable polarity gas metal arc welding (VP-GMAW) is the current trend in the welding industry. Inverter pulse power supplies allow to combine DCEP and DCEN polarities in varying amounts (Figure 4). In their research, Harwig et al. (2000) showed that VP-GMAW could be used for welding thin gage aluminium sheets. They noted that during DCEN polarity, droplet formation takes place and it is transferred across the arc by DCEP polarity. They said that DCEN could be added up to 60 % to the current, beyond that the arc becomes unstable.

2.2.4.3. Arc Voltage

Arc voltage is the amount of voltage present between the electrode and workpiece. Arc voltage and arc length are used interchangeably. Arc voltage is an approximate means of defining the physical arc length in electrical terms. However, one physical arc length could yield different arc voltage readings, depending on factors such as shielding gas, current, and electrode extension. If all these variables are kept constant, arc voltage and arc length can be correlated i.e. with increase in voltage setting, arc length increases. The welders are interested in arc length, but arc voltage is easy to monitor and must be specified in welding procedures. Therefore, it’s the arc voltage that is most commonly used term than the arc length.

Arc voltage controls the height and width of the weld. Any increase in arc voltage from specific value, flattens the bead and increase the width of the fusion zone. Very high voltage results in porosity, spatter (unstable arc), and undercut. However, a voltage less than required will result in narrower weld bead with higher crown i.e. wire stubs on the work. Therefore, voltage must be set midway between high/low voltages.

2.2.4.4. Travel speed

is the speed at which a welder moves the electrode along the joint to make a weld. Technically it’s the linear rate at which the arc is moved along the weld joint. Weld penetration is always maximum at intermediate travel speed, when all other conditions are constant. If low travel speed is used, the arc will impinge on the molten weld pool than working on the base metal and hence affect penetration efficiency. Large increase in travel speed will result in less thermal energy on the base metal. At high speed, the rate of melting of base metal is increased first and then decreased. If travel speed is increased any further, undercutting along the edges of weld bead may occur because of insufficient deposition of filler metal in the path melted by the arc.

High speed GMAW as signifies uses high travel speeds. Rapid Arc Company uses pulsed GMAW for faster travel speeds, low spatter, out of position operation and lower heat input. They achieve high travel speeds by using lower arc voltage i.e. shorter arc length, this reduces spatter and washed out bead profile, allowing high torch travel speed. They divided rapid waveform into four parts (Figure 5):

• Pulse: A sudden increase in current increases arc energy, and forms and squeezes a molten droplet extending from the end of the electrode.
• Puddle Rise: The ramp down of current relaxes the plasma force, depressing the puddle, allowing it to rise up towards the droplet.
• Short : The arc collapses, and the droplet contacts the weld puddle.
• Puddle Repulsion: immediately following a short breaking into an arc, a gentle plasma boost pushes the puddle away and conditions the electrode tip. This ensures reliable separation of the wire tip and the puddle resulting in a stable rhythm of the cycle.

2.2.4.5 Electrode orientation

Electrode orientation is the angle of the electrode axis with respect to the travel direction. This is called the travel angle. On the other hand, it could be the angle of the electrode axis with the work surface. This is called the work angle. When the electrode points in a direction opposite to the travel direction, it results in a trail angle and is called the backhand welding technique. When the electrode points in the direction of travel, it results in a lead angle and is called the forehand welding technique.

The maximum penetration is achieved for trailing travel angle between 5 to 15° (from perpendicular). This also provides a narrow, convex surface configuration and shielding of the molten weld pool. However, the leading travel angle provides the welder better visibility and a flatter weld surface. This is more commonly used technique. For materials such as aluminum, a leading angle is preferred, because it provides a cleaning action ahead of the molten weld metal, which promotes wetting and reduces base-material oxidation. This is because the leading angle of the electrode pushes the molten metal and slag ahead of the weld.

When producing fillet welds in the horizontal position, the work angle should be about 45° to the vertical member.

2.2.4.6. The electrode extension

The distance between the last point of electrical contact (usually the gun contact tip or tube) and the end of the electrode. An increase in the amount of this extension causes an increase in electrical resistance. This, in turn, generates additional heat in the electrode, which contributes to greater electrode melting rates. Without an increase in arc voltage, the additional metal will be deposited as a narrow, high-crowned weld bead. The optimum electrode extension generally ranges from 6.4 to 13 mm for short-circuiting transfer and from 13 to 25 mm for spray and globular transfers.

2.2.4.7. The electrode diameter

Influences the weld bead configuration. A larger electrode requires a higher minimum current than a smaller electrode does to achieve the same metal transfer characteristics.Higher currents, in turn, produce additional electrode melting and larger, more-fluid weld deposits. Higher currents also result in higher deposition rates and greater penetration, but may prevent the use of some electrodes in the vertical and overhead positions.

2.2.5 Equipment

The basic assembly of GMAW containing the components of equipment can be seen in Figure 6. It is important to study each part of the equipment to reach the required quality of the weld. Each application of GMAW will have a specific requirement for each part. Therefore, these can be controlled or modified to change the welding process to achieve good welding efficiency and quality.

The fundamental equipment for a typical GMAW installation includes:

• Welding gun
• Electrode feed unit
• Welding control mechanism
• Power source
• Electrode source
• Regulated Shielding gas

For automatic welding equipment the wire feed unit and the current contact and gas barrel are combined in a single welding head (Figure 7). For Semi automatic welding flexibility is generally achieved by separating the wire feed unit from the torch and passing wire, gas, current and cooling water through the flexible conduit. Wire-feeding complexities must be considered using these systems. High powered motors are required to push wire for several yards. Ferrous metal wires can be fed through smooth, flexible and rigid spiral steel wire-feed tubes. But, aluminium and non-ferrous metals are difficult to feed through tubes until they are nylon lined. The wire feed difficulties increase with decreasing wire diameter.

Welding current in GMAW equipment is introduced to the wire by passing it to a copper tube. A variation in point of current pick up can alter the resistance between contact and arc and cause variations in burn-off rate because of its effect on overall circuit resistance. With high currents or high resistance metals the current contact tube is shortened or fitted with small tip diameter tip to reduce variation.

Water cooking is required for equipments that have automatic welding heads and those that work at about 250 A. Water cooling and chromium-plated surface make removal of fume and spatter from the nozzle easier.

• POWER CABLE (NEGATIVE)
• POWER CABLE (POSITIVE)
• WELDING VOLTAGE
• & CURRENT DETECTION
• 115 VAC IN
• TO PRIMARY POWER 230/460/575 V
• COOLING WATER IN
• SHIELDING GAS IN
• TO CARRIAGE DRIVE MOTOR
• 115 VAC IN TRAVEL START/STOP
• WIRE FEED MOTOR
• SHIELDING GAS IN
• COOLING WATER IN
• COOLING WATER OUT

The parts of GMAW are discussed below in detail.

2.2.5.1Welding gun

Responsible for delivering the electrical current to the electrode, and directs it to the work piece and allows the flow of shielding gas to the weld area. The choice of welding gun is critical, but often ignored over power source, wire feeder, and shielding gas, which are most costly. Proper choice can give good welds and productivity. Different types of guns are used for different applications: heavy duty guns for high current and high volume production, and light guns for low current and out of position welding. Figure 8 shows the most commonly used gun, which is air cooled. Water cooled gun is used for high current requirement. Welding guns are rated on their current-carrying capacity. If inert gas is used, the gun rating is reduced to a much lower extent. A welding gun can be equipped with its own electrode feed unit.

Parts of welding gun

• Back end is the power pin that connects the gun and power cable to the wire feeder. This connection must be tight. A loose connection between the gun and the feeder can cause electrical resistance throughout the entire system. This will result in overheating, which may damage the gun or the wire feeder. This may also cause gas leakage and poor conductivity that can lead to an erratic arc and poor weld quality. Usually a supportive strain relief is provided at the connection between the power cable and wire feeder. This helps in good wire feeding, which results in a stable arc and quality welds. There is another option of selecting a gun with multiple feeders for various GMAW applications reducing overall cost.
• Contact tube: is used to transmit welding current to the electrode and to direct the electrode towards the work. It is usually made of copper or a copper alloy and connected electrically by power cable to the power source. The tube hole for wire input is of 0.13 to 0.25 mm larger than the wire being use, larger for aluminium and non ferrous metals. Nylon lining is used for non ferrous metals and aluminium electrodes. This inner surface of the tube has to be changed in case of excessive wear, which may result in poor electrical contact.
• Consumables (nozzles and tips): The nozzle in the welding gun directs the shielding gas into the welding area. An even flow must be maintained to protect molten weld from the environmental gases. Larger nozzles are used for high current work with large weld pool and small nozzles used for low current work.

Consumables are selected based on longevity instead of price. This reduces costs of replacement parts and changeover time. Non-threaded, large-base contact tips that fit securely to the diffuser provide good electrical conductivity and heat transfer. It is important to use heavy-duty tips and nozzles that provide good gas coverage to help ensure good arc starts, less spatter, and less rework and cleanup.

• Electrode conduit and liner support: protect and direct the electrode from the feed rolls to the gun and contact tube. They are connected to a bracket adjacent to the feed rolls on the electrode feed motor. It is necessary to maintain uninterrupted electrode feeding for good arc stability.

The liner is the most critical component of the GMAW gun because of the problems that can arise from it. A steel liner is used for steel and copper electrodes, whereas nylon liners are used for soft electrode materials, such as aluminum and magnesium. Replacing the liner consumes time and maintenance costs. The liner can be a source of gas leakage leading to contamination of the weld pool. Usually gas seal or solid O-ring connection is used at the back of the liner. Liner must be selected specifically for the wire diameter.

• Power cable: smallest and shortest cable possible is used without limiting the welding needs. These will reduce operator fatigue, minimize clutter, and help prevent excessive coiling that can lead to poor wire feeding. It is made sure that the power cable fits tightly into the wire feed system to maintain proper conductivity.
• Trigger: is the only moving part on a GMAW gun. It can fail because of mechanical motion. Some guns have trigger options such standard, locking, dual-pull, and dual-schedule switches. These options may allow for better work suitability and help increase productivity by making welding more comfortable.
• Neck and Handle: a welding gun may have a fixed, rotatable, and flexible neck. The neck can be of different lengths and angles to provide flexibility when welding in various positions or tight quarters. Rotatable necks, allow you to weld out of position more comfortably without changing the gun handle or sacrificing quality. Flexible necks can be adjusted for different positions, which save changeover time and eliminate the need to inventory specialty guns for each application. Necks with hard plastic or metal covers are used to protect them from damage, which maybe lead to shorts and failures. A small ventilated handle makes it easier to weld with comfortable operation.

Cost can be reduced by choosing small gun with low amperage and reducing operator fatigue and downtime. A smaller gun will result in low weight, better maneuverability and comfort. This may also reduce carpal tunnel syndrome, linked with heavy guns. Costs can be cut by using shorter power cables by minimizing downtime and wire feeding problems.

The National Electrical Manufacturers Association (NEMA) in the US and the European Conformity (CE) in Europe has established the GMAW gun criteria. A GMAW gun rated to 300 A exceeds up its rated capacity if used at 400 A and 100% duty cycle and can be welded at 400 A and 50% duty cycle. The advantages of using a more easy to use and lighter gun results in improved productivity as it does not cause fatigue to the welder. A small and lighter gun has the ability to handle the same heat and current loads while offering improved operator comfort.

2.2.5.2The electrode feed unit

The wire feeder or the electrode feed unit consists of an electric motor, output shaft, drive rolls, and accessories for maintaining electrode alignment and pressure (Figure 9). The main function is to pull the welding wire from the spool and feed it to the arc. The electrode feed motor is usually direct-current and provides mechanical energy for pushing the electrode through the gun and to the work. The components are connected directly or remotely to the speed control unit. The feed unit can be a part of the welding gun (Figure 10) or separate from the gun such as the dual-feed unit. The feed unit and the welding gun can be electrically coupled to provide a push-pull system. This is usually used for aluminium when it is difficult to pass the wire through the wire conduit.

Various types of feed rolls are available such as knurled, “U” groove, “V” groove, and flat. For harder wires such as steel, the knurled design is used. It allows maximum frictional force to be transmitted to the wire with a minimum of drive roll pressure. Knurled rolls are not used for soft metals like aluminium because they will cause the wire to flake and hence, clogging the gun or liner. For softer wires, the “U” groove or “V” groove type is used for the application of uniform pressure around the wire without deforming it. The flat rolls are used for smaller-diameter wires and in combination with a “U” or “V” groove.

2.2.5.3The welding control mechanism

The welding control and the electrode feed motor for semiautomatic operation are usually integrated. The control mechanism is usually independently powered by 115 V ac. The control is responsible for maintaining pre-determined wire-feed speed at an appropriate rate required by the specific application. It also regulates the start and stopping of the wire feed when signaled by the gun trigger.

The control unit is also responsible for delivering shielding gas, water, and welding power to the gun. Gas and water flow are regulated to coincide with the weld start and stop by using solenoid valves. The control mechanism can also sequence gas flow starts and stops and energize the power source contractor. The control mechanism may allow some gas to flow before welding starts (preflow) and after welding stops (post flow) to protect the molten weld puddle.

2.2.5.4 The welding power source

The power source provides electrical power (20 to 80 V) to the electrode and workpiece in order to produce an arc. Almost all GMAW applications use DCEP. In this positive lead is connected to gun while the negative lead is connected to the workpiece. The wire feed speed and, hence, current, is regulated bythe welding control (Figure 11). Therefore, the arc length is adjusted through the power source by setting the power source voltage.

The power source can be static type or rotating type. In static type incoming power (120 to 480 V) is reduced to welding voltage by a transformer or solid-state inverter. The rotating type power source is provided by a rotating generator driven by a motor or internal combustion engine. Static type can respond to the variable arc conditions rapidly. Static type is used in shops and rotating type in field operations. These can be designed to produce constant potential or constant current output.

The constant potential output can be pulsed at constant or variable frequency. Solid-state electronic power sources give further control over pulsing variables like frequency, pulse width, etc.

In conjunction with a constant-speed wire feeder, the constant-voltage power source balances the variations in the contact-tip-work-distance that occur during normal welding operations. This is achieved by instantaneously increasing or decreasing welding current to increase or decrease the electrode burn off rate. The initial arc length is established by adjusting the voltage at the power source and no other changes are required during welding. The wire feed speed, which is also the current control, is then set by the operator and adjusted as necessary. We can control the slope and the inductance in the systems that are designed with short-circuiting type of metal transfer.

2.2.5.5 Electrode source

Welding electrode is an alloy of two or more elements that is fed to the weld. If its steel, the alloy is carbon and iron with impurities like sulphur and phosphorous. The elements added to the electrode act as either alloying agents or deoxidizers, and scavengers. The GMAW process uses a continuously fed electrode that is consumed at relatively high speeds. Therefore, the electrode source must provide a large volume of material that can readily be fed to the gun to ensure maximum process efficiency. This source is usually in the form of a spool or coil that can hold from 7 to 27 kg (15 to 60 lb) of wire that has been wound to allow free feeding without kinks or tangles. Larger spools are also available, and material can be provided in drums of 340 to 455 kg (750 to 1000 lb). Small spools of 0.45 to 0.9 kg (1 to 2 lb) are used for spool-on-gun equipment.

The type of electrode used has an effect on the transfer efficiencies of the alloying and deoxidizing agents. For example:

• Solid electrode wire in argon-oxygen gas shield is the most efficient.
• Solid electrode wire in CO2 shielding gas is efficient.
• Flux-cored electrode wire in CO2 is least efficient.

Another aspect to consider is the deposition rate of the electrode, which will depend on several variables such as type and size of the electrode, type of power source, shielding gas, method of metal transfer and position of the weld joint.

There are five major factors that influence the choice of the electrode:

• Base plate chemical composition
• Base plate mechanical properties
• Shielding gas employed
• Type of service or applicable specification requirements
• Type of weld joint design.

2.2.5.6 Regulated shielding gas supply

The shielding gas prevents the contamination of the weld pool by nitrogen, oxygen and water vapors present in the atmosphere. Some defects caused by these are:

• Excess nitrogen in solidified steel reduces the ductility, impact weld strength, cause cracking and welds porosity.
• Excess oxygen in steel combines with carbon to form carbon monoxide (CO) causing porosity. Also, combine with other elements in steel forming compounds that produce inclusions in the weld metal.
• Hydrogen in water vapor and oil, combine with iron or aluminum, leading to porosity and “under bead” weld metal cracking.

A system is used to provide constant shielding gas, argon, helium and carbon dioxide, at certain pressure and flow rate during welding. The system consists of a regulator that reduces the source gas pressure to a constant working pressure, regardless of the variations at the source. The regulator can be a single-stage or dual-stage type and has a built in flowmeter. The gas mixtures like small amounts of oxygen, nitrogen and hydrogen with inert gases come in single cylinders. The shielding gas source can be a high-pressure cylinder or a liquid-filled or a bulk-liquid tank. Mixing devices can be used to get the correct required proportion.

Argon, helium and carbon dioxide can be used alone, in combinations or mixed with others to provide defect free welds in a variety of applications. Agron is most generally used for any metal. For thick materials, with high diffusivity, helium is preferred because it gives high arc voltage and deeper penetration. An oxygen-free nitrogen source is used for copper.

2.2.6 Sensing and Control system

Nowadays GMAW has become more of an automated process where welding process is almost performed by a robot. Therefore, it has become a grown interest of the industry to develop a reliable sensing a control system, which can detect any defects and correct them automatically. Industry has outlined three levels of on-line quality control (Fotona et al., 2004).

• First level: automatically on-line detection of bad welds in production.
• Second level: locate type of defect and its causes such as disturbances in shielding gas supply, wire feed rate and/or welding geometry, etc.
• Third level: change the parameters as soon as possible to reduce loss and produce quality weld. “A sensor is a detector, if it is capable of monitoring and controlling welding operation based on its own capacity to detect external and internal situations affecting welding results and transmit a detected value as a detection signal. The whole control device is defined as a sensor system” (Nomura, 1994).

The properties that a sensor should have are:

• Capability to maintain accuracy to specified welding process.
• Freedom from influence of welding process-induced disturbances such as light, heat, fume, spatter and electromagnetic.
• Low cost and durability.
• Easy maintenance.
• Compact size and light weight.
• Wide range of applications.

A control system is designed to control the welding process based on the information obtained from the sensor (Nomura, 1994). The control system is classified according to the application it is fabricated for:

• Seam tracking control of a welding line.
• Adaptive control for welding conditions.
• Seam tracking control and adaptive control.
• Welding monitoring.

Literature review of the current research going on in this field and prevalent sensors in the industry was undertaken. These sensors are discussed below.

2.2.6.1 Optical sensing system for seam tracking

Bae et al. (2002) developed a visual sensing system for automatic GMAW for root pass of steel pipe. The root pass procedure in welding steel pipes is difficult to automate. To attain automatic welding of steel pipe, root pass has to be automated first. This requires adjusting the position of torch and welding conditions of different root states. Authors tested seam tracking and weld pool control system for root pass welding of steel pipe. The system consisted of five-axis pipe welding manipulator with its controller, hardware logic for detecting the short-circuit and visual sensing system. The manipulator allowed welding torch and vision sensor to have automatic access to welding position with predetermined configurations when pipe was selected. The visual sensing system composed of CCD camera, lenses and filters, a frame grabber and image processing algorithms.

The researchers used short-circuit metal transfer mode to acquire image of the weld pool and its position by using vision sensor. The sensor was placed tangent to the surface of the welding position. The image from the sensor was processed to detect shape and size of the weld pool, from which the relative distance between the torch position and weld seam was determined and the gap size of groove measured. The controller used this information to determine a new welding position, permitting the torch to advance in that position and apply new welding conditions in real time. Pipe welding system constituted a positioned for turning the pipe, a five axis welding manipulator, a welding power source, a control panel and a control computer

2.2.6.2 On-line optical monitoring system

Saforza and Blasiis (2002) developed a non-intrusive sensor for monitoring arc welding process. It comprised of four electro-optic modules, which were able to detect infra-red, visible, and ultraviolet radiation from the interaction zone. During the welding process, three different signals, infrared, ultraviolet and electronic temperature are simultaneously stored.

This system can on-line detect process defects in the production phase, which can be repaired immediately. This is advantageous over non-destructive controls, such as radiographic and ultrasonic tests. The system is versatile and flexible in that it can be applied to all kinds of steels by substitution of two electro-optic detectors.

The sensor is made of two different modules: the electro-optical module and the signals amplification electronic components. A diode laser emitting in the visible zone allows the system to be pointed on the working zone. The distance between the sensor and the working zone has to be about 1 m to ensure a good collection of radiation. Plasma radiation is collected and selected by optical system constituted of lenses, neutral filters, optical fibers, interferential filters and band pass filters. A protecting window and an attenuator filter are used to avoid the detector saturation. The radiation is focused on fibers bundle, then each fiber delivers the radiation to four different systems formed by filters and photodiode detectors. The system is controlled by software that has been realized by assembler.

2.2.6.3 Acoustic signals for on-line monitoring in short circuit gas metal arc welding

Fotona et al. (2004) presented a non-contact automated data acquisition system for monitoring a gas metal arc welding (GMAW) process based on arc acoustics. This method is not widely accepted in the industry because the acoustic waves produced during welding have not been correlated with welding conditions and the final weld quality. For each welding process the waves produced may be different.

The acoustic waves produced by GMAW provide information about the behavior of the arc column, the molten pool and droplet transfer. The researchers measured acoustic waves in the surrounding air and in the parts being welded by employing a microphone and PZT sensor. Influences on sound generation were tested by using two unalloyed carbon steels: DIN RSt13 with 0.1 % carbon and DIN Ck45 with 0.45 % carbon. In addition two types of shielding gases were experimented, CO2 and Crystal gas mixture (90 % Ar, 10% CO2). The distance between welding nozzle and welding part was varied by welding on the slope.

The signals obtained were processed to get time and frequency domain descriptors and relationship between the descriptors and weld process characteristics evaluated

They measured acoustic waves from the welding part by piezoelectric sensor with resonance frequency of 2.8 MHz. The sensor was fixed to the welding specimen using a wave guide. Signals were transferred to oscilloscope and then to PC.

The results suggested that acoustic waves generated during the welding process are by short circuiting and arc reignition. The type of shielding gas affects the acoustic parameters. Wire extension length had influence at lengths larger than 12 mm. Carbon on the test pieces did not significantly affect the acoustic waves. Acoustic waves produced during droplet transfer are greater than those produced during microstructure changes. Acoustic signals efficiently monitor arc nonregularity or in its behavior such as extinguishing (Figure 15) and burn-through events, which effect weld quality. The authors concluded that acoustic method is useful to assess welding process stability and detect defects in arc behavior.

2.2.6.4 Infrared sensing system

Infrared sensing system has been used as an in-direct penetration depth indicator. Other techniques used are ultrasonic sensing, weld pool oscillation, optical sensing and radiographic sensing. Infrared sensing is preferred for monitoring of thermal-welding processes. Combined with image analysis seam tracking and bead width control systems are used. It has been widely implemented on GMAW processes.

Theory: The theory of infrared sensing is better explained by Fan et al. (2003). At a particular point adjacent to the weld, the temperature reaches a maximum value and decreases with time. If constant heat input is supplied, maximum temperature reached increases with distance covered from the start of the weld and becomes constant at steady state. For moving heat sources it is called a quasi-stationary thermal state. The isotherms remain unchanged and move with heat source. The limits of isotherms are lines parallel to the bead. The isotherms are affected by changes in base metal properties, plate thickness, joint fit-up and cetera. These changes in the limits of isotherms of thermal distribution are measured and used for monitoring and control.

2.2.6.5 Process sensing of hybrid laser welding and GMAW processes

Travis et al. (2004) indicated that commercially no single sensor can reliably detect full spectrum of weld states. Therefore, the concept of fusing is used, i.e., a variety of sensor inputs are integrated to make use of advantages of each sensor. They investigated the use of multiple sensors to predict the state of weld. This was done in three steps: determining the type of signals generated and emitted during the welding process and why they fluctuate; developing equipment for detecting and monitoring changes in signals; and methods of analyzing sensor output and interpret it. Method was developed for sensing laser-arc hybrid welding using four different sensors: arc current and voltage of GMAW, infrared sensor, and an ultraviolet sensor to monitor electromagnetic emissions from hybrid

2.2.6.6 An on-line welding quality and process control system

Lin and Fischer developed a neural network system to monitor the quality of the weld head in manual arc welding. The quality indicators they used were bead width, bead height, and penetration depth, along with the absence of spatter. They combined two Modified Cerebellar Model Articulation Controller (MCMAC) neural networks and Linear Discriminant Function (LDF) to establish: predict quality measurements; classification measurement of the arc welding process; and corrective estimation of controllable system. They compared it with conventional GMAC and hack-propagation neural networks. They used parallel multiple input state variables and Linear Neighborhood Sequential Training (LNST) algorithm making MCMAC faster. They tested the system with welding arc sound signals and gave satisfactory accuracy to be used in real-world.

2.2.6.7 Magnetic image detection

Hayashi et al. (2008) detected stainless steel welding part inside a multi layered tube structure by non-destructive evaluation technique, magnetic image detection. They used a cylindrical stainless-steel sample, fabricated into a tube by rolling and welding using arc welding and argon as shielding gas. They obtained the magnetic images by a system consisting of an exposure coil, magnetoresistive (MR) sensor, lock-in amplifier, x-y stage, revolving stage with a horizontal level stage and personal computer. The MR sensor was able to measure magnetic field generated from sample at low frequencies between 50 Hz and 1kHz. They measured normal components, tangential components of the sample surface by scanning the MR sensor and magnetic characteristics recorded. They were able to detect the difference in the permeability between weld area and the base metal.

2.2.6.8 H-infinity robust control system

Jou (2002) proposed a visual sensing system for controlling the width and length of the weld pool measured by a CCD camera attached to the welding torch as a front face description of the weld pool geometry. The control system used length and width as the output variables and current and voltage as control variables and welding current and reciprocal of feed speed as input variables. The researcher found the development of an H-infinity control system for welding processes to be challenging because it requires an effective description of the uncertainties. The system is difficult to operate because of complex algorithms. Therefore, advanced digital processors have to be used for operator convenience.

2.2.6.9 High power laser and GMAW system

Kavacevic (2001) at the Research Center for Advanced Manufacturing (RCEAM), Southern Methodist University is developing a rapid prototyping welding processes for depth of penetration and heat buildup in layer. The researchers at RCAM concluded that there are two disadvantages in GMAW, uncertain detachment time and inconsistent droplet size. They developed a new sensing and control system for metal transfer in GMAW using high-frame-rate-digital camera assisted by He-Ne laser and a real-time image-processing algorithm to monitor droplet formation. A PCI frame grabber and a Pentium III PC give good real time droplet formation monitoring. This system is claimed to accurately control the height and width ratio of bead layer generated by GMAW.

2.2.6.10 Non contact laser sensing system

In a press release of April 9, 2009 Servo-Robot has reported a Robo-Find laser-based sensing system that locates, detects and measures weld joints without making contact with part (Figure 18). It senses part position and variations and corrects it in less than 1sec. It includes Sense-i/D or SF/D laser camera and a video camera for remote programming. Setup is done through a laptop.

2.2.6.11 Intelligent pipe welding

Adaptive intelligent systems by Zhang has developed a control technology that can adjust welding parameters in butt welding of pipe for in field applications. For mechanized operation, welder has control torch movement and control fluctuations in travel speed and standoff distance. Their control system adjust welding parameters by observation of the weld pool surface by a torch sensor which measures a weld pool depth from arc electrical signals using a regular torch. The quasi-keyhole technology responds to real-time fluctuations in welder operation. Hence, increasing the speed of operation.

2.2.7 Advantages, disadvantages and applications of GMAW

Gas metal arc welding is the most widely used welding process in the world today. It is used on all thicknesses of steels, aluminium, nickel, stainless steel, etc. MAG can be used for both steel and unalloyed, low-alloy and high-alloy based materials. MIG can be used for welding aluminium and copper materials. GMAW is a versatile method, which offers many advantages. The technique is easy to use, gives high productivity and requires no slag cleaning. In most of its applications, gas metal arc welding is a simple welding process. However, weld quality can vary between operators because the process depends on a number of external factors.

Few advantages, disadvantages and applications of GMAW are briefly listed below:

2.2.7.1 Advantages

• Welding process is fast due to continuously fed electrode as compared to GTAW or stick electrode welding.
• High level of productivity.
• Easily automated.
• It can produce joints with deep penetration.
• Thick and thin, both types of work pieces can be welded effectively.
• Large metal deposition rates are achieved by MIG welding process.
• The process is easy to learn and can be easily mechanized.
• No flux is used. Welding produces smooth, neat, clean and spatter free welded surfaces, which require no further cleaning. This helps reduce total welding cost.

2.2.7.2 Disadvantages

• The process is slightly more complex than GTAW or stick electrode welding because a number of variables: electrode stick out, torch angle, welding parameters, type and size of electrode, welding torch manipulation, etc, are required to be controlled effectively to achieve good results.
• Welding equipment is more complex, costly and less potable.
• Air drifts may disperse the shielding gas, it cannot work well in outdoor welding applications.
• Weld metal cooling rates are higher than with the processes that deposit slag over the weld metal.

2.2.7.3 Applications

• The process can be used for the welding wide variety of materials like carbon, silicon and low alloy steels, stainless steels, aluminium, magnesium, copper, nickel, and their alloys, titanium.
• for welding steel tools and dies.
• For manufacture of refrigerator parts.
• In industries like aircraft, automobile, pressure vessel, and ship building.

3. Conclusion

Gas metal arc welding is the most widely used welding process in the industry because it is a fast process as compared to GTAW and produce joints with greater penetration. GMAW has high productivity and can be easily automated. In addition, large deposition rates can be achieved in the welding process producing clean spatter free welded surfaces. Most importantly, GMAW process is easy to learn and mechanize. The type of GMAW method and the mode of metal transfer govern the application it is being designed for. For each application, the efficiency and quality of weld can be controlled by controlling the process variables: welding current, polarity, arc voltage, travel speed, electrode extension, electrode orientation and electrode diameter.

An increase in welding current results in an increase in the depth and width of penetration, deposition rate, and weld bead size.

Direct current electrode positive (DCEP) polarity is usually used because it yields a stable arc, smooth metal transfer, relatively low spatter, good weld bead characteristics and deep penetration for a wide range of welding currents. Variable polarity gas metal arc welding (VP-GMAW) is emerging for welding thin gage aluminium sheets. In DCEN polarity the droplet formation takes place and it is transferred across the arc by DCEP polarity.

Arc voltage controls the height and width of the weld. It is set between the high/low voltages. Higher voltage will result in flat bead and increase in fusion zone. Very high voltage results in porosity, spatter, and undercut. However, a voltage less than required results in narrower weld bead.

Weld penetration is maximum at intermediate travel speed, when all other conditions are constant. If low travel speed is used, the arc will impinge on the molten weld pool than working on the base metal and affect penetration efficiency.

High speed GMAW use high travel speeds. Pulsed GMAW is used for faster travel speeds, low spatter, out of position operation and lower heat input. High travel speeds are achieved by using lower arc voltage i.e. shorter arc length, this reduces spatter and washed out bead profile, allowing high torch travel speed.

Travel angle between 5 to 15° (from perpendicular) results in maximum penetration. Travel angel can be leading or trailing. For aluminum, a leading angle is preferred. When producing fillet welds in the horizontal position, the work angle should be about 45° to the vertical member. Much work is being done to develop sensors for leading angle welding so, that the operator has better understanding of the weld conditions.

The optimum electrode extension ranges from 6.4 to 13 mm for short-circuiting transfer and from 13 to 25 mm for spray and globular transfers.

Electrode diameter influences the weld bead configuration. A larger electrode requires a higher minimum current than a smaller electrode does to achieve the same metal transfer characteristics.

In a welding process, it is important to consider the specific application the process is being designed for, operation conditions for the welder and the cost. Equipment plays and important part in improving the efficiency of the welding process and quality of the weld. Equipment parts such as welding gun, electrode feed unit, welding control mechanism, power source, electrode source and shielding gas regulator, can be selected and modified according to the specific application.

The choice of welding gun is critical, but often ignored over power source, wire feeder, and shielding gas, which are most costly. Proper choice can give good welds and productivity.

The electrode feed motor is usually direct-current and provides mechanical energy for pushing the electrode. The feed unit can be a part of the welding gun or separate from the gun such as the dual-feed unit. The feed unit and the welding gun can be electrically coupled to provide a push-pull system when it is difficult to pass the wire through the wire conduit.

The control unit is maintains pre-determined wire-feed speed at an appropriate rate required by the specific application. It also regulates the start and stopping of the wire feed when signaled by the gun trigger. It delivers shielding gas, water, and welding power to the gun.

The electrode source is usually in the form of a spool or coil that can hold from 7 to 27 kg of wire that has been wound to allow free feeding without kinks or tangles. Larger spools and material in drums of 340 to 455 kg are present in the market. Small spools of 0.45 to 0.9 kg are used for spool-on-gun equipment.

The shielding gas regulator reduces the source gas pressure to a constant working pressure, regardless of the variations at the source. It can be a single-stage or dual-stage type with a built in flow meter.

There is an increasing demand in the industry for fully-automatic GMAW process, extensive research is being undertaken to develop a sensing and control system that will accomplish three levels of quality control i.e. detecting the defect as early as possible, knowing the cause of defect and controlling the parameters. Some of the techniques have been discussed and their applications elaborated. Automated systems have been developed and used in the industry for specific applications, but