02 Gas Metal Arc Welding Notes.

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About gas metal arc welding

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  • Gas Metal Arc Welding & Flux Cored Arc Welding

    GAS METAL ARC WELDING

    Introduction:

    Gas metal arc welding (GMAW), was initially referred to as metal inert gas (MIG metal

    inert gas) welding is an arc welding process that uses an arc between a continuous filler

    metal electrode and the weld pool. The process incorporates shielding from an externally

    supplied gas and is used without the application of pressure.

    MIG was primarily developed for welding aluminium and other non-ferrous materials in

    the 1940s. However, further process developments, such as operating at low current

    densities, pulsed current and the use of reactive gases (particularly carbon dioxide) or gas

    mixtures led to its use for a broader range of materials. It was the use of both inert and

    reactive gases or gas mixtures that made the term Gas Metal Arc Welding (GMAW) as

    the formally accepted name of the process.

    Further developments during the 1950s and 1960s gave the process more versatility and

    as a result, it became a highly used industrial process. Presently, GMAW is commonly

    used the automotive and shipbuilding industries, where it is preferred for its versatility

    and speed.

    The GMAW process can be implemented in Semi-automatic or Automated operations.

    All commercially available important metals, listed below, can be welded in all positions

    with this process by selecting the appropriate combination of shielding gas, electrodes

    and welding process variables.

    Materials welded by GMAW:

    Carbon steels

    High strength low alloy steels (HSLA)

    Stainless steels

    Aluminium and its alloys

    Copper and its alloys

    Nickel and Nickel based alloys

    Cast irons

  • Principles or Operation of GMAW

    Figure 1: GMAW Process

    The GMAW Process utilizes the automated feeding of a continuous, consumable

    electrode that is shielded by externally supplied gas. The process is illustrated

    schematically in Figure 1.

    The basic equipment components required are a welding gun and cable assembly, an

    electrode (wire) feeder unit, a welding power source and the gas supply with the

    associated apparatus. A typical semi-automatic GMAW set up is illustrated in the

    schematic in Figure 2.

    The gun guides the electrode, conducts electrical current and directs shielding gas to the

    work-piece. Most commonly, a constant voltage power source, providing essentially a

    flat volt-ampere (V-A) curve, is used in conjunction with a constant speed electrode feed

    unit to regulate the arc length. Alternatively, a constant current power source (providing a

    drooping V-A curve can also be used with an electrode feed unit that is arc-voltage

    controlled.

  • Figure 2: GMAW Set up

    The constant voltage power source & constant speed wire feed combination is self

    regulatory. Any changes in the gun position caused by the welders hand movement, can

    initiate a self regulating action to maintain the arc length.

    For example, when the gun-to-workpiece distance is suddenly increased, the arc length

    momentarily becomes longer and this causes a reduction in current, thereby momentarily

    reducing the electrode melting rate. Since the feed remains at the same speed, the arc

    length decreases and the current increases until the melting rate again equals the feed

    rate.

    This self regulation of the GMAW arc is depicted in figure 3.

    Figure 3: Self regulation of the GMAW arc with Constant voltage power source and

    constant speed wire feed

  • Metal Transfer Modes

    The characteristics of GMAW are best described in terms of the basic means by which

    metal is transferred from the electrode to the workpiece, or the mode of metal transfer as

    this is referred to.

    There are four primary methods of metal transfer in GMAW, called globular, short-

    circuiting, spray, and pulsed-spray, each of which has distinct properties and

    corresponding advantages and limitations.

  • The mode of transfer is determined by a number of factors:

    1. Magnitude, type and polarity of welding current

    2. Electrode diameter

    3. Electrode composition

    4. Electrode extension and

    5. Shielding gas composition

    Short-circuiting Transfer

    This mode of transfer, known as short-circuiting or short-arc GMAW, occurs when

    carbon dioxide is the shielding gas, the electrode diameter is smaller, and the current

    density is low. The metal is transferred from the electrode only during the period in

    which the electrode is in contact with the weld pool. No metal is transferred across the

    arc. The electrode contacts the weld pool in the range of 20- to 200 times per second.

    Molten droplets forms on the tip of the electrode, but instead of dropping to the weld

    pool, they bridge the gap between the electrode and the weld pool as a result of the

    greater wire feed rate. This causes a short circuit and extinguishes the arc, but it is

    quickly reignited after the surface tension of the weld pool pulls the molten metal bead

    off the electrode tip. This process is repeated 20 to 200 times per second, making the arc

    appear constant to the human eye.

    As a result of the lower current, the heat input for the short-arc variation is reduced,

    making it possible to weld thinner materials while decreasing the amount of distortion

    and residual stress in the weld area. This transfer is generally suited for joining of thin

    sections, for out-of-position welding and bridging large root openings

    This type of metal transfer is slow and it is difficult to maintain a stable arc, because it

    depends on achieving a consistent and high short-circuiting frequency, which can only be

    accomplished with a good power source, suitable welding conditions, and significant

    welder skill.

    The composition of the shielding gas has a dramatic effect on the surface tension of the

    molten metal, arc characteristics and penetration. CO2 produces high spatter levels

    compared to Argon and Helium but it also promotes deeper penetration. To achieve a

    good compromise between spatter and penetration when welding carbon and low alloy

    steels, mixtures of CO2 and argon are often used. Addition of Helium or Argon may

    increase penetration in nonferrous metals.

    Globular Transfer

    GMAW with globular metal transfer is often considered the most undesirable of the four

    major GMAW variations, because of its tendency to produce high heat, a poor weld

    surface, and spatter. The method was originally developed as a cost efficient way to weld

    steel using GMAW, because this variation uses carbon dioxide, a less expensive shielding

    gas than Argon.

  • As the weld is made, a ball of molten metal from the electrode tends to build up on the

    end of the electrode, often in irregular shapes with a larger diameter than the electrode

    itself. When the droplet finally detaches either by gravity or short circuiting, it falls to the

    workpiece, leaving an uneven surface and often causing spatter.

    As a result of the large molten droplet, this mode of transfer is generally limited to flat

    and horizontal welding positions.

    Spray Transfer

    Spray transfer GMAW occurs when the molten metal from the electrode is propelled

    axially across the arc in the form of minute droplets. With Argon-rich gas shielding it is

    possible to produce a very stable, spatter-free axial spray transfer mode. The mode

    requires Direct current with a positive electrode (DCEP) and a current level above a

    critical value termed the spray transition current. Below this level, the transfer is globular.

    In this variation, molten metal droplets (with diameters smaller than the electrode

    diameter) are rapidly passed along the stable electric arc from the electrode to the

    workpiece, essentially eliminating spatter and resulting in a high-quality weld finish.

    However, high amounts of voltage and current are necessary, which means that the

    process involves high heat input and a large weld area and heat-affected zone.

    As a result, it is generally used only on work-pieces of thicknesses above about 6 mm

    (0.25 in). Because of the large weld pool, it is often limited to flat and horizontal welding

    positions, but when a smaller electrode is used in conjunction with lower heat input, its

    versatility increases. The maximum deposition rate for spray arc GMAW is relatively

    high; about 60 mm /sec (150 in/min).

    Pulsed-spray Transfer

    A more recently developed method, the pulse-spray metal transfer mode is based on the

    principles of spray transfer but uses a pulsing current to melt the filler wire. This allows

    one small molten droplet to fall with each pulse. The pulses allow the average current to

    be lower, decreasing the overall heat input and thereby decreasing the size of the weld

    pool and heat-affected zone while making it possible to weld thin work-pieces.

    The pulse provides a stable arc and no spatter, since no short-circuiting takes place. This

    also makes the process suitable for nearly all metals, and thicker electrode wire can be

    used as well.

    The process also requires that the shielding gas be primarily argon with a low carbon

    dioxide concentration.

    Additionally, it requires a special power source capable of providing current pulses with a

    frequency of between 30 and 400 pulses per second.

  • However, the method has gained popularity, since it requires lower heat input and can be

    used to weld thin workpieces, as well as nonferrous materials.

    Process Variables:

    Power source characteristics: As stated before, a constant voltage power source is used for GMAW process. The V-A

    characteristics of the power source are depicted in figure 5 below.

    Figure 5: Constant voltage power supply V-A curve

    As is seen in the figure, a small change in arc voltage will cause a large change in current

    thereby increasing or decreasing the melt-off rate of the electrode. This compensates for

    the variations in contact tip-to-workpiece distance by instantaneously increasing or

    decreasing the welding current.

    Shielding Gases

    As we know, the primary function of the Shielding gas is to exclude the atmosphere from

    contact with the molten metal. This is necessary since most metals, when molten from

    oxides and to a lesser extent, nitrides. Oxygen also reacts with carbon in the molten metal

    to form CO and CO2 gases. These reaction gases may produce defects or discontinuities

    such as slag inclusions, porosity and weld metal embrittlement.

    In addition to provide a protective environment, shielding gas and flow rate have a

    pronounced effect on:

    Arc characteristics

    Modes of metal transfer

  • Penetration and weld bead profile

    Speed of welding

    Undercutting tendency

    Cleaning action and weld metal mechanical properties

    The choice of a shielding gas therefore depends on several factors, most importantly:

    the type of material being welded and

    the process variation being used

    Figure 7: Bead contours and penetration patterns with various shielding gases

    Bead contours and Penetration patterns for various shielding gases are shown in figure 7.

    Carbon dioxide provides maximum penetration while Helium produces uniformly

    distributed arc energy.

    Pure inert gases such as argon and helium produce excellent results for nonferrous

    welding and therefore are used with most nonferrous metals, However, with steel these

    pure gases cause an erratic arc, encourage spatter (Helium) or do not provide adequate

    weld penetration (Argon).

    Pure carbon dioxide, on the other hand, allows for deep penetration welds but encourages

    oxide formation, which adversely affect the mechanical properties of the weld. Its low

    cost makes it an attractive choice, but the arc is harsh, spatter is unavoidable, making

    welding thin materials difficult.

  • Figure 8: Relative effect of Oxygen versus Carbon dioxide additions to the Argon shield

    Mixtures therefore are used to combine the properties to give optimum results.

    Oxygen (1 5%) and Carbon dioxide (3 25%) additions to Argon produce noticeable

    improvement in arc stability and reduces the tendency for undercut. Carbon dioxide

    additions to Argon may also enhance the weld bead configuration by producing a well

    defined pear-shaped profile as is seen in fillet welds shown in figure 8.

    As a result, argon and carbon dioxide are frequently mixed in a 75%/25% or 80%/20%

    mixture, which reduces spatter and makes it possible to weld thin steel workpieces.

    Argon is also commonly mixed with other gases, such as oxygen, helium, hydrogen, and

    nitrogen. The addition of up to 5% oxygen encourages spray transfer, which is critical for

    spray-arc and pulsed spray-arc GMAW.

    An argon-helium mixture is completely inert, and is used on nonferrous materials.

    Gas Flow Rate

    The desirable rate of gas flow depends primarily on weld geometry, speed, current, the

    type of gas, and the metal transfer mode being utilized.

    Welding flat surfaces requires higher flow than welding grooved materials, since the gas

    is dispersed more quickly.

    Faster welding speeds mean that more gas must be supplied to provide adequate

    coverage.

    Higher current requires greater flow, and generally, more helium is required to provide

    adequate coverage than argon.

    Gas flow rates used for different metal transfer modes are listed in the table below for a

    quick reference.

  • Metal Transfer Mode Gas Flow required

    Short circuiting & Pulsed spray

    modes (small weld pools)

    10 L/min (20 ft/h)

    Globular transfer 15 L/min (30 ft/h) is preferred

    Spray transfer (higher heat input,

    larger weld pool)

    20-25 L/min (4050 ft/h)

    Electrode

    The electrodes (filler metals) for GMAW are specified by various filler metal

    specifications (AWS for instance)

    They define requirements for sizes and tolerances, packaging, chemical composition, and

    in some cases, mechanical properties.

    The process variables that affect weld penetration, bead geometry and overall weld

    quality are:

    Welding amperage (electrode feed speed)

    Polarity

    Arc Voltage

    Travel speed

    Electrode extension beyond the contact tip (stick out)

    Electrode orientation (work, travel angles)

    Weld joint position

    Electrode diameter and

    Shielding gas composition and flow rate

    Process Variations

    The gas metal arc process has been adapted to provide specific characteristics for a wide

    range of applications. Some of these variations are:

    Gas Metal Arc Sport Welding

    Narrow Groove Gas metal arc welding

    Gas Metal Arc Brazing

    Tandem gas metal arc welding

    Only Gas metal arc brazing is discussed here within the scope of the lecture.

  • Gas Metal Arc Brazing:

    In gas metal arc braze welding, a copper based electrode (e.g. aluminium bronze or

    silicon bronze) is used instead of a steel electrode to join steel.

    As the copper alloy has a lower melting temperature than steel, less heating of the base

    metal is required in order to deposit a weld bead, and little or no melting of the base

    metal occurs.

    Because of the low heat input, braze welding is some times used to join heat sensitive

    materials such as cast iron, for welding of thin sheet steel to help prevent melt through.

    Gas metal arc brazing is also used to join galvanized steels.

    The lower heat input reduces the amount of coating that is melted away and the copper

    based weld bead furnishes better corrosion resistance than that provided by a carbon steel

    weld bead.

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