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.

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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 nylo