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                                              UNIT 7                   SHIELDED GAS ARC WELDING




                                                                   OBJECTIVES




                                               General Objective: To understand the principles of shielded gas arc
                                                                  welding i.e. TIG and MIG welding.

                                               Specific Objectives : At the end of the unit you will be able to :



                                                               Ø        Identify the principles of shielded gas arc
                                                                        welding i.e. TIG and MIG welding.

                                                               Ø        Elaborate on the TIG and MIG welding
                                                                        principles,  welding procedures, welding
                                                                        machines, gas, etc.

                                                               Ø        State the advantages and disadvantages of TIG
                                                                        and MIG compared to manual arc welding.

                                                               Ø        State the weaknesses of TIG and MIG welding
                                                                        and how to prevent them.
                                                                   .
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                                                                         INPUT




                                          7.0. INTRODUCTION

                                                The objective of welding is to produce a welding joint that contains the
                                          same mechanical properties as the base metal. The objective can be achieved
                                          if the molten metal is free from atmospheric air. If not, nitrogen and oxygen
                                          gases in the atmosphere will be absorbed by the melting pool. The welding
                                          produced will have small pore that will weaken the weld.
                                                To prevent the welding, molten metal and the end of the filler rode and
                                          electrodes from atmospheric air pollution before the molten metal become
                                          solid inert gas is blown out from the welding point. These gases will cover
                                          the welding pools, the filler rod points and electrode tips to avoid oxidation.


                                          7.1. TUNGSTEN INERT GAS (TIG)

                                                The welding of aluminium and magnesium alloys by the oxy-acetylene
                                          and manual metal arc processes is limited by the necessity to use a corrosive
                                          flux. The gas shielded, tungsten arc process enables these metals and a wide
                                          range of ferrous alloys to be welded without the use of a flux. The choice of
                                          the either a.c. or d.c. depends upon the metal to be welded.         For metals
                                          having refractory surface oxides such as aluminium and its alloys,
                                          magnesium alloys and aluminium bronze, a.c. is used whilst d.c. is used for
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                                          carbon and alloy steels, heat-resistant and stainless steels, cooper and its
                                          alloys, nickel and its alloys, titanium, zirconium and silver.
                                                The arc burns between a tungsten electrode and the work piece within
                                          a shield of the inert gas argon, which excludes the atmosphere and prevents
                                          contamination of electrode and molten metal. The hot tungsten arc ionizes
                                          argon atoms within the shield to form a gas plasma consisting of almost
                                          equal numbers of free electrons and positive ions. Unlike the electrode in the
                                          manual metal arc process, the tungsten is not transferred to the work and
                                          evaporates very slowly, being classed as ‘non-consumable’. Small amount of
                                          other elements are added to the tungsten to improve electron emission.



                                                                            Gas flow


                                                         Torch



                                                                              Water outlet
                                                                                                    Welding
                                                                                                    machine

                                                  Work piece


                                                                              Water inlet




                                                               Figure 7.1. TIG welding equipment
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                                                                                                             Electrode
                                                                                                             (tungsten)




                                                                                                       Inert/noble
                                                                                                           gas


                                                              Filler rode
                                                                                                       Shielded gas


                                                                                                     arc
                                               Direction of travel
                                                                                                  80 – 90o
                                                                        20 – 30o
                                                      Melting pool


                                                                                     Work piece

                                                   Figure 7.2. TIG in progress. The tungsten does not melt into the
                                                   puddle for filler. This is a nonconsumable electrode.

                                               7.1.1. Preparation of Metal.


                                                         Gas tungsten-arc processes must start with clean metal which
                                               has the proper joint design i.e., V, U, or J. Mechanical and chemical
                                               cleaning are often necessary to prepare the base metal. The edges of
                                               the joint should be shaped to permit adequate fusion and penetration.
                                               It is common practice to reduce or bevel the adjoining edges to 1.6 mm
                                               thickness.
                                                        A strip (backup bar) to support the back side of the base metal
                                               should be used when needed. This is especially helpful on aluminium
                                               since it aids in shielding. The backup bar may be removed after
                                               welding.
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                                               7.1.2. Joint Fit.


                                                     Good joints make it easier to obtain a good weld. In production
                                               work, carefully fitted joints can help save money and can help the
                                               welding operator develop standardized welding techniques.                  Root
                                               opening (distance apart) and angle of bevel are two major factors
                                               requiring close tolerance when fitting joints.


                                               7.1.3. Welding Machine.


                                                     Gas tungsten-arc welding requires a conventional welding
                                               machine, with the following accessories:
                                                            1. Torch, lead cable, and hoses.
                                                            2. Inert gas supply and flow meter for measuring amount
                                                                   of shielding gas.
                                                            3. Water cooling system for water-cooled torches.
                                                                   Air-cooled torches are limited to 150 ampere capacity.
                                                            4. High-frequency spark unit attached to the output leads
                                                                   of the power supply (to start and stabilize arc).


                                                     The finished weld will be greatly affected by type of current and
                                               polarity. For example, aluminium is welded with alternating current
                                               plus superimposed high-frequency current (ACHF). Stainless steel is
                                               welded with direct current straight polarity (DCSP). Improper
                                               electrical connections will cause (a) the electrode to overheat, (b) poor
                                               penetration, or (c) insufficient cleaning effect upon the base metal.
                                                     Current selection must be made with care. When an electrode is
                                               connected to the negative terminal (DCSP), electrons pass through the
                                               arc to bombard the base plate (Fig. 7.3).
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                                                            Welding              Electrode
                                                            machine




                                                                                                              Direction of electron
                                                                                                              travel
                                                                             Positive surface
                                                                             particles travel
                                                                                                                           Work piece




                                                                                                        Deep penetration

                                                  Figure 7.3 Power supply with direct current straight polarity



                                                      This causes nearly 70% of the arc heat to accumulate in the
                                               base metal to assist fusion and penetration. When the electrode is
                                               made positive (DCRP), a cleaning effect is created on the surface of the
                                               base plate (Fig. 7.4).


                                                       Welding
                                                                               Electrode
                                                       machine




                                                                                                             Direction of electron
                                                                         Positive surface                    travel
                                                                         particles travel


                                                                                                                       Work piece




                                                                                            Shallow penetration


                                                  Figure 7.4 Power supply with direct current reverse polarity


                                                      In welding aluminium this method is used to remove surface
                                               oxidation. While an electrode positive connection furnishes a cleaning
                                               effect, it also heats the tungsten electrode. The electrode may get hot
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                                               enough to melt, transfer to the weld pool, and contaminate the base
                                               metal. When this happens, the electrode must be removed, its end
                                               broken off, and it must be ground to shape.
                                                     Alternating current offers the advantages of both direct current
                                               straight polarity (DCSP) and direct current reverse polarity (DCRP).
                                               Gas tungsten-arc welding of aluminium and magnesium requires an
                                               AC power supply (Fig. 7.5).
                                                     Gas tungsten-arc welding is not recommended for metal more
                                               than 20 mm thick. Welds have been completed on 25 mm thick plate
                                               but require a great deal of time and, consequently, are expensive.
                                               Most applications are less than 12 mm thick, and require less than 500
                                               amperes of current.




                                                           Welding               Electrode
                                                           machine




                                                                              Surface
                                                                              particles lifted                        Electron flow


                                                                                                                              Work piece




                                                                                                 Medium penetration



                                                           Figure 7.5 Alternating current power supply
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                                                7.1.4. Welding Torch.


                                                       The welding torch has a round collet which compresses to hold
                                                the electrode and a nozzle to control the gas (Fig. 7.2). Water-cooled
                                                torches    are     used   when      current    values   exceed     150    amperes.
                                                Maintenance of either torch is more time consuming than with the
                                                metal-arc process. Careful selection of nozzle size, proper shaping of
                                                the working end of the electrode and correct extension of electrode
                                                beyond nozzle are important. Nozzle size influences the flow of gas.
                                                End shape of electrode and extension of electrode beyond nozzle control
                                                the stability of the arc. Further, it is important that electrode diameter
                                                match current value (Table 7.1). If the current is too high for the
                                                diameter of an electrode, the life of the electrode will be reduced. When
                                                the current is too low for a given electrode diameter, the arc will not be
                                                stable.

                                            Table 7.1. Selection of nozzle size and electrode size for gas tungsten-arc
                                            welding


                                           Electrode      Nozzle or            WELDING CURRENT IN AMPERES
                                             Size         Cup Sizes                 ACHF                 DCSP             DCRP
                                          (Diameter,                       Pure         Thoriated        Pure or          Pure or
                                            Inches)                       Tungsten      Tungsten        Thoriated        Thoriated
                                             0.020           4,5            5-15              5-20        5-20               *
                                             0.040           4,5           10-60            15-80        15-80               *
                                             1/16            4-6           50-100          70-150        70-150           10-20
                                             3/32            5-7          100-160        140-235        150-250           15-30
                                              1/8            6-8          150-210        225-325        250-400           25-40
                                          *Not applicable.
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                                                     The end of the electrode should remain bright, as if it was
                                               polished. On some metals, such as aluminium and magnesium, the end
                                               is contaminated when starting or by touching the base plate.
                                               Contamination can be burned off by welding on a scrap plate of metal,
                                               or it can be removed by grinding (Fig. 7.6). The electrode should be
                                               adjusted to extend beyond the nozzle a distance equal to the electrode
                                               diameter (Fig. 7.7)




                                                             15o                       30o
                                                                                       45o


                                                                   Grind here



                                                         DCSP                   DCRP         AC


                                           Figure 7.6 Electrode shapes for gas shielded tungsten-arc welding




                                                       3/8” max



                                                                      Electrode diameter

                                           Figure 7.7. Adjustment of electrode from nozzle
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                                               7.1.5. Shielding Gas.


                                                      Gas used with this process produces an atmosphere free from
                                               contamination and also provides a path for arc transfer. The path
                                               creates an environment that helps stabilize the arc. The gas and arc
                                               activity also perform a cleansing action on the base metal. Both argon
                                               and helium are generally used for this process but argon is preferred
                                               because it is cheaper and provides a smoother arc. Helium, however,
                                               helps produce deeper penetration (Table 7-2).


                                               7.1.6. Filler Metal.


                                                     Filler metals are selected to meet or exceed the tensile strength,
                                               ductility, and corrosion resistance of the base metal. The usual practice
                                               is to select a filler metal having a composition similar to that of the
                                               base metal. For most efficient application, select clean filler metals of
                                               proper diameter; the larger the diameter of the filler metal, the more
                                               heat is lost from the weld pool.
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                                          Table 7.2 Selection of gases for manual application of tungsten-arc welding.

                                          Metal             Shielding Gas        Remarks
                                          Aluminium         Argon                Easy starting
                                                                                 Good cleaning action.
                                                            Helium               Faster and more penetration.
                                                            Argon-10% helium     Increase in penetration over pure argon.
                                          Stainless steel   Argon                Better control of penetration (16 gauge
                                                                                 and thinner).
                                                            Argon-helium         Higher welding speeds.
                                                            mixtures
                                          Copper       and Argon                 Easy to control penetration and weld
                                          nickel                                 contour on sheet metal.
                                                            Argon-helium         Increases heat into base metal.
                                                            Helium               Highest welding speed.


                                          7.2. TIG WELDING TECHNIQUES


                                                   After the base metal has been properly cleaned and clamped or tacked
                                          together, welding can be started. On aluminium, the arc is usually started by
                                          bringing the electrode near the base metal at a distance of about one
                                          electrode diameter so that a high-frequency spark jumps across the gap and
                                          starts the flow of welding current. Steel, copper alloys, nickel alloys, and
                                          stainless steel may be touched with the electrode without contamination to
                                          start the arc. Once started, the arc is held stationary until a liquid pool
                                          appears.     Filler rod can be added to the weld pool as required (Fig. 7.8).
                                          Highest current values and minimum gas flow should be used to produce
                                          clean, sound welds of desired penetration (Table 7-3).
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                                                               Table 7.3     Operating data for TIG

                                          Material               Aluminium      Stainless Steel   Magnesium      Deoxidized
                                                                                                                   Copper
                                          Type of Current          ACHF             DCSP              ACHF         DCSP
                                          1.6mm electrode
                                          Current:                  60-80           80-100             60         110-140
                                          Argon:                   15 cfh           11 cfh            13 cfh       15 cfh
                                          Passes:                     1               1                 1             1
                                          3.2mm electrode
                                          Current:                 125-145         120-140             115        175-225
                                          Argon:                   17 cfh           11 cfh            19 cfh       15 cfh
                                          Passes:                     1               1                 1             1
                                          4.7mm electrode
                                          Current:                 190-220         200-250         120-175        250-300
                                          Argon:                   21 cfh           13 cfh            19 cfh       15 cfh
                                          Passes:                     1               1                1,2       1 at 257.4*
                                          *Preheat to temperature indicated.


                                                   The shielded gas is pure argon and pre-heating is required for drying
                                          only to produce welds of the highest quality. All surfaces and welding wire
                                          should be degreased and the area near the joint and the welding wire should
                                          be stainless steel wire brushed or scrape to remove oxide and each run
                                          brushed before the next is laid.
                                                   The angles of torch and filler rod are shown in Fig. 7.8.        After
                                          switching on the gas, water, welding current and HF unit, the arc is struck
                                          by bringing the tungsten electrode near the work (without touching down).
                                          The HF sparks jump the gap and the welding current flows.            Arc length
                                          should be about 3 mm. Practice starting by laying the holder on its side and
                                          bringing it to the vertical position, but using the ceramic shield as a fulcrum
                                          can lead to damage to the holder and ceramic shield. The arc is held in one
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                                          position on the plate until a molten pool is obtained and welding is
                                          commenced, proceeding from right to left, the rod being fed into the forward
                                          edge of the molten pool and always kept within the gas shield. It must not be
                                          allowed to touch the electrode or contamination occurs. A black appearance
                                          on the weld metal indicates insufficient argon supply.



                                                                      15o




                                                                                            Direction of
                                                                                     30o    travel




                                                               Figure 7.8. Example of TIG

                                                 The flow rate should be checked and the line inspected for leaks. A
                                          brown film on the weld metal indicates presence of oxygen in the argon while
                                          a chalky white appearance of the weld metal accompanied by difficulty in
                                          controlling the weld indicates excessive current and overheating. The weld
                                          continues with the edge of the portion sinking through, clearly visible, and
                                          the amount of the sinking which determines the size of the penetration bead
                                          is controlled by the welding rate.


                                          7.3.   METAL INERT GAS (MIG)


                                                 It is convenient to consider, under this heading, those applications
                                          which involve shielding the arc with argon, carbon dioxide (CO2) and
                                          mixtures of argon with oxygen and/or CO2, since the power source and
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                                          equipment is essentially similar except for gas supply. With the tungsten
                                          inert gas shielded arc welding process, inclusions of tungsten become
                                          troublesome with currents above 300 A. The MIG process does not suffer
                                          from these advantages and larger welding current giving greater deposition
                                          rates can be achieved.           The process is suitable for welding aluminium,
                                          magnesium alloys, plain and low-alloy steels, stainless and heat-resistant
                                          steel, copper and bronze, the variation being filler wire type of gas shielding
                                          the arc.
                                                 The consumable electrode of bare wire is carried on the spool and is fed
                                          to a maually operated or fully automatic gun through an outer flexible cable
                                          by motor-driven rollers of adjustable speed, and rate of burn-off of the
                                          electrode wire must be balance by rate of wire feed.                                      Wire feed rate
                                          determines the current used.
                                                 In addition, a shielding gas or gas mixture is fed to the gun together
                                          with welding current supply, cooling water flow and return (if the gun is
                                          water cooled) and a control cable from gun switch to control contractors.
                                          A d.c. power supply is required with the wire electrode connected to the
                                          positive pole ( Fig. 7.9).
                                                                Arc welding                           Gas flow
                                                                power supply                          meter




                                                                                                   Spool of         Inert gas
                                                                                      Welding
                                                                                                   electrode        cylinder
                                                                                      power
                                                                                      cable        wire

                                                                                                                    Electrode
                                                                                                                    feed        Contactor lead,welding
                                                                                                                    rools       current,electrode, and
                                                                                                                                inert gasto welding
                                                                                                                                gun

                                                                       Contacto                 Control head
                                                                       r cable                  forelectrode feed
                                                                                  Ground        and gas supply
                                                                                  cable




                                                                 Figure 7.9 . MIG welding equipment
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                                                During this process an electric arc is used to heat the weld zone. The
                                          electrode is fed into the weld pool at a controlled rate and the arc is shielded
                                          by a protective gas such as argon, helium, or carbon dioxide (Fig. 7.9). Gas
                                          metal-arc welding can be either the short-circuiting process or the spray-arc
                                          process (Fig. 7.10).
                                                                                                 Inert/noble gas




                                                                                                      Shielded gas
                                                                            Arc



                                                           Melting pool
                                                                                    Work piece

                                                                          Figure 7.10. MIG in progress


                                                The short-circuiting arc process (short arc) operates at low currents
                                          and voltages. For example, 18-gauge sheet metal can be welded at 45 amps
                                          and 12 volts.




                                                                              Work piece



                                               Figure 7.11. Mechanics of the short circuiting transfer process as
                                               shown between the electrode and work piece. Electrode dips into pool
                                               an average of 90 times a second
                                                In contrast, the spray-arc process uses high currents and voltages, e.g.,
                                          Arc action is illustrated in Fig. 7.12. This results in high heat input to the
                                          weld area, making possible deposition rates of more than 0.4 lb per minute.
                                          (The deposition rate is the weight of filler metal melted into the weld zone
F T ra n sf o                                                                                                                  F T ra n sf o
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                                          per unit of time.) Most applications of the spray-arc process are in thick
                                          metal fabrications, e.g., in heavy road-building machinery, ship construction,
                                          and beams for bridges.


                                                           Electrode maintains steady arc length




                                                                   Work piece




                                               Figure 7.12. Mechanics of the spray-arc transfer
                                               process as shown between the electrode and work



                                                All metal inert-gas (MIG) welding is classified as semi-automatic, since
                                          the electrode feeds into the weld according to a preset adjustment. After
                                          making an initial adjustment, the welding operator merely moves the gun
                                          along the joint. For effective applications, the welding operator needs
                                          information concerning power requirements, welding gun, selection of
                                          shielding gas, type of filler metal, and job procedures.


                                                7.3.1. Power Requirements.


                                                       Conventional power supplies used for shielded metal-arc
                                                welding are not satisfactory. A welding machine designed for the MIG
                                                process is called a constant potential power source; it produces a
                                                constant voltage and also permits the operator to adjust electrode feed
                                                rates. The adjustments on the power supply are voltage, slope (limits
                                                current), and wire feed rate. Welding current is established by
F T ra n sf o                                                                                                                 F T ra n sf o
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                                                selecting a wire feed rate. Slope adjustment to limit current is not a
                                                problem with spray-arc type transfer. However, in short-circuiting arc
                                                processes, limitations on short-circuit current are essential to prevent
                                                excessive spatter.
                                                       The electrode feed mechanism, an important part of the welding
                                                machine, consists of a storage reel for electrode wire and a power drive
                                                which feeds the electrode into the weld at a controlled rate.

                                                             Table 7.4 Shielding mixtures for MIG

                                                   Metal                Shielding Gas                 Remarks
                                          Aluminium and copper       Argon + helium        High heat input
                                                                     20-80% mixture        Minimum of porosity
                                          Copper                     Argon + nitrogen      Good heat input on copper
                                                                     25-30% mixture
                                          Carbon steels              Argon + oxygen        Stabilizes arc
                                          Low alloy steels           3-5% mixture          Reduces spatter
                                                                                           Causes weld metal to flow
                                                                                           Eliminates undercut
                                                                                           May require electrode to
                                                                                           contain deoxidizers
                                          Low alloy steels           Mixture of argon, Increases toughness of weld
                                                                     helium and carbon deposit
                                                                     dioxide
Unit7 Shielded Gas Arc Welding
Unit7 Shielded Gas Arc Welding
Unit7 Shielded Gas Arc Welding
Unit7 Shielded Gas Arc Welding
Unit7 Shielded Gas Arc Welding
Unit7 Shielded Gas Arc Welding
Unit7 Shielded Gas Arc Welding
Unit7 Shielded Gas Arc Welding
Unit7 Shielded Gas Arc Welding
Unit7 Shielded Gas Arc Welding
Unit7 Shielded Gas Arc Welding
Unit7 Shielded Gas Arc Welding

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Unit7 Shielded Gas Arc Welding

  • 1. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/1 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c UNIT 7 SHIELDED GAS ARC WELDING OBJECTIVES General Objective: To understand the principles of shielded gas arc welding i.e. TIG and MIG welding. Specific Objectives : At the end of the unit you will be able to : Ø Identify the principles of shielded gas arc welding i.e. TIG and MIG welding. Ø Elaborate on the TIG and MIG welding principles, welding procedures, welding machines, gas, etc. Ø State the advantages and disadvantages of TIG and MIG compared to manual arc welding. Ø State the weaknesses of TIG and MIG welding and how to prevent them. .
  • 2. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/2 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c INPUT 7.0. INTRODUCTION The objective of welding is to produce a welding joint that contains the same mechanical properties as the base metal. The objective can be achieved if the molten metal is free from atmospheric air. If not, nitrogen and oxygen gases in the atmosphere will be absorbed by the melting pool. The welding produced will have small pore that will weaken the weld. To prevent the welding, molten metal and the end of the filler rode and electrodes from atmospheric air pollution before the molten metal become solid inert gas is blown out from the welding point. These gases will cover the welding pools, the filler rod points and electrode tips to avoid oxidation. 7.1. TUNGSTEN INERT GAS (TIG) The welding of aluminium and magnesium alloys by the oxy-acetylene and manual metal arc processes is limited by the necessity to use a corrosive flux. The gas shielded, tungsten arc process enables these metals and a wide range of ferrous alloys to be welded without the use of a flux. The choice of the either a.c. or d.c. depends upon the metal to be welded. For metals having refractory surface oxides such as aluminium and its alloys, magnesium alloys and aluminium bronze, a.c. is used whilst d.c. is used for
  • 3. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/3 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c carbon and alloy steels, heat-resistant and stainless steels, cooper and its alloys, nickel and its alloys, titanium, zirconium and silver. The arc burns between a tungsten electrode and the work piece within a shield of the inert gas argon, which excludes the atmosphere and prevents contamination of electrode and molten metal. The hot tungsten arc ionizes argon atoms within the shield to form a gas plasma consisting of almost equal numbers of free electrons and positive ions. Unlike the electrode in the manual metal arc process, the tungsten is not transferred to the work and evaporates very slowly, being classed as ‘non-consumable’. Small amount of other elements are added to the tungsten to improve electron emission. Gas flow Torch Water outlet Welding machine Work piece Water inlet Figure 7.1. TIG welding equipment
  • 4. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/4 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Electrode (tungsten) Inert/noble gas Filler rode Shielded gas arc Direction of travel 80 – 90o 20 – 30o Melting pool Work piece Figure 7.2. TIG in progress. The tungsten does not melt into the puddle for filler. This is a nonconsumable electrode. 7.1.1. Preparation of Metal. Gas tungsten-arc processes must start with clean metal which has the proper joint design i.e., V, U, or J. Mechanical and chemical cleaning are often necessary to prepare the base metal. The edges of the joint should be shaped to permit adequate fusion and penetration. It is common practice to reduce or bevel the adjoining edges to 1.6 mm thickness. A strip (backup bar) to support the back side of the base metal should be used when needed. This is especially helpful on aluminium since it aids in shielding. The backup bar may be removed after welding.
  • 5. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/5 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 7.1.2. Joint Fit. Good joints make it easier to obtain a good weld. In production work, carefully fitted joints can help save money and can help the welding operator develop standardized welding techniques. Root opening (distance apart) and angle of bevel are two major factors requiring close tolerance when fitting joints. 7.1.3. Welding Machine. Gas tungsten-arc welding requires a conventional welding machine, with the following accessories: 1. Torch, lead cable, and hoses. 2. Inert gas supply and flow meter for measuring amount of shielding gas. 3. Water cooling system for water-cooled torches. Air-cooled torches are limited to 150 ampere capacity. 4. High-frequency spark unit attached to the output leads of the power supply (to start and stabilize arc). The finished weld will be greatly affected by type of current and polarity. For example, aluminium is welded with alternating current plus superimposed high-frequency current (ACHF). Stainless steel is welded with direct current straight polarity (DCSP). Improper electrical connections will cause (a) the electrode to overheat, (b) poor penetration, or (c) insufficient cleaning effect upon the base metal. Current selection must be made with care. When an electrode is connected to the negative terminal (DCSP), electrons pass through the arc to bombard the base plate (Fig. 7.3).
  • 6. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/6 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Welding Electrode machine Direction of electron travel Positive surface particles travel Work piece Deep penetration Figure 7.3 Power supply with direct current straight polarity This causes nearly 70% of the arc heat to accumulate in the base metal to assist fusion and penetration. When the electrode is made positive (DCRP), a cleaning effect is created on the surface of the base plate (Fig. 7.4). Welding Electrode machine Direction of electron Positive surface travel particles travel Work piece Shallow penetration Figure 7.4 Power supply with direct current reverse polarity In welding aluminium this method is used to remove surface oxidation. While an electrode positive connection furnishes a cleaning effect, it also heats the tungsten electrode. The electrode may get hot
  • 7. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/7 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c enough to melt, transfer to the weld pool, and contaminate the base metal. When this happens, the electrode must be removed, its end broken off, and it must be ground to shape. Alternating current offers the advantages of both direct current straight polarity (DCSP) and direct current reverse polarity (DCRP). Gas tungsten-arc welding of aluminium and magnesium requires an AC power supply (Fig. 7.5). Gas tungsten-arc welding is not recommended for metal more than 20 mm thick. Welds have been completed on 25 mm thick plate but require a great deal of time and, consequently, are expensive. Most applications are less than 12 mm thick, and require less than 500 amperes of current. Welding Electrode machine Surface particles lifted Electron flow Work piece Medium penetration Figure 7.5 Alternating current power supply
  • 8. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/8 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 7.1.4. Welding Torch. The welding torch has a round collet which compresses to hold the electrode and a nozzle to control the gas (Fig. 7.2). Water-cooled torches are used when current values exceed 150 amperes. Maintenance of either torch is more time consuming than with the metal-arc process. Careful selection of nozzle size, proper shaping of the working end of the electrode and correct extension of electrode beyond nozzle are important. Nozzle size influences the flow of gas. End shape of electrode and extension of electrode beyond nozzle control the stability of the arc. Further, it is important that electrode diameter match current value (Table 7.1). If the current is too high for the diameter of an electrode, the life of the electrode will be reduced. When the current is too low for a given electrode diameter, the arc will not be stable. Table 7.1. Selection of nozzle size and electrode size for gas tungsten-arc welding Electrode Nozzle or WELDING CURRENT IN AMPERES Size Cup Sizes ACHF DCSP DCRP (Diameter, Pure Thoriated Pure or Pure or Inches) Tungsten Tungsten Thoriated Thoriated 0.020 4,5 5-15 5-20 5-20 * 0.040 4,5 10-60 15-80 15-80 * 1/16 4-6 50-100 70-150 70-150 10-20 3/32 5-7 100-160 140-235 150-250 15-30 1/8 6-8 150-210 225-325 250-400 25-40 *Not applicable.
  • 9. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/9 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c The end of the electrode should remain bright, as if it was polished. On some metals, such as aluminium and magnesium, the end is contaminated when starting or by touching the base plate. Contamination can be burned off by welding on a scrap plate of metal, or it can be removed by grinding (Fig. 7.6). The electrode should be adjusted to extend beyond the nozzle a distance equal to the electrode diameter (Fig. 7.7) 15o 30o 45o Grind here DCSP DCRP AC Figure 7.6 Electrode shapes for gas shielded tungsten-arc welding 3/8” max Electrode diameter Figure 7.7. Adjustment of electrode from nozzle
  • 10. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/10 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 7.1.5. Shielding Gas. Gas used with this process produces an atmosphere free from contamination and also provides a path for arc transfer. The path creates an environment that helps stabilize the arc. The gas and arc activity also perform a cleansing action on the base metal. Both argon and helium are generally used for this process but argon is preferred because it is cheaper and provides a smoother arc. Helium, however, helps produce deeper penetration (Table 7-2). 7.1.6. Filler Metal. Filler metals are selected to meet or exceed the tensile strength, ductility, and corrosion resistance of the base metal. The usual practice is to select a filler metal having a composition similar to that of the base metal. For most efficient application, select clean filler metals of proper diameter; the larger the diameter of the filler metal, the more heat is lost from the weld pool.
  • 11. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/11 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Table 7.2 Selection of gases for manual application of tungsten-arc welding. Metal Shielding Gas Remarks Aluminium Argon Easy starting Good cleaning action. Helium Faster and more penetration. Argon-10% helium Increase in penetration over pure argon. Stainless steel Argon Better control of penetration (16 gauge and thinner). Argon-helium Higher welding speeds. mixtures Copper and Argon Easy to control penetration and weld nickel contour on sheet metal. Argon-helium Increases heat into base metal. Helium Highest welding speed. 7.2. TIG WELDING TECHNIQUES After the base metal has been properly cleaned and clamped or tacked together, welding can be started. On aluminium, the arc is usually started by bringing the electrode near the base metal at a distance of about one electrode diameter so that a high-frequency spark jumps across the gap and starts the flow of welding current. Steel, copper alloys, nickel alloys, and stainless steel may be touched with the electrode without contamination to start the arc. Once started, the arc is held stationary until a liquid pool appears. Filler rod can be added to the weld pool as required (Fig. 7.8). Highest current values and minimum gas flow should be used to produce clean, sound welds of desired penetration (Table 7-3).
  • 12. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/12 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Table 7.3 Operating data for TIG Material Aluminium Stainless Steel Magnesium Deoxidized Copper Type of Current ACHF DCSP ACHF DCSP 1.6mm electrode Current: 60-80 80-100 60 110-140 Argon: 15 cfh 11 cfh 13 cfh 15 cfh Passes: 1 1 1 1 3.2mm electrode Current: 125-145 120-140 115 175-225 Argon: 17 cfh 11 cfh 19 cfh 15 cfh Passes: 1 1 1 1 4.7mm electrode Current: 190-220 200-250 120-175 250-300 Argon: 21 cfh 13 cfh 19 cfh 15 cfh Passes: 1 1 1,2 1 at 257.4* *Preheat to temperature indicated. The shielded gas is pure argon and pre-heating is required for drying only to produce welds of the highest quality. All surfaces and welding wire should be degreased and the area near the joint and the welding wire should be stainless steel wire brushed or scrape to remove oxide and each run brushed before the next is laid. The angles of torch and filler rod are shown in Fig. 7.8. After switching on the gas, water, welding current and HF unit, the arc is struck by bringing the tungsten electrode near the work (without touching down). The HF sparks jump the gap and the welding current flows. Arc length should be about 3 mm. Practice starting by laying the holder on its side and bringing it to the vertical position, but using the ceramic shield as a fulcrum can lead to damage to the holder and ceramic shield. The arc is held in one
  • 13. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/13 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c position on the plate until a molten pool is obtained and welding is commenced, proceeding from right to left, the rod being fed into the forward edge of the molten pool and always kept within the gas shield. It must not be allowed to touch the electrode or contamination occurs. A black appearance on the weld metal indicates insufficient argon supply. 15o Direction of 30o travel Figure 7.8. Example of TIG The flow rate should be checked and the line inspected for leaks. A brown film on the weld metal indicates presence of oxygen in the argon while a chalky white appearance of the weld metal accompanied by difficulty in controlling the weld indicates excessive current and overheating. The weld continues with the edge of the portion sinking through, clearly visible, and the amount of the sinking which determines the size of the penetration bead is controlled by the welding rate. 7.3. METAL INERT GAS (MIG) It is convenient to consider, under this heading, those applications which involve shielding the arc with argon, carbon dioxide (CO2) and mixtures of argon with oxygen and/or CO2, since the power source and
  • 14. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/14 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c equipment is essentially similar except for gas supply. With the tungsten inert gas shielded arc welding process, inclusions of tungsten become troublesome with currents above 300 A. The MIG process does not suffer from these advantages and larger welding current giving greater deposition rates can be achieved. The process is suitable for welding aluminium, magnesium alloys, plain and low-alloy steels, stainless and heat-resistant steel, copper and bronze, the variation being filler wire type of gas shielding the arc. The consumable electrode of bare wire is carried on the spool and is fed to a maually operated or fully automatic gun through an outer flexible cable by motor-driven rollers of adjustable speed, and rate of burn-off of the electrode wire must be balance by rate of wire feed. Wire feed rate determines the current used. In addition, a shielding gas or gas mixture is fed to the gun together with welding current supply, cooling water flow and return (if the gun is water cooled) and a control cable from gun switch to control contractors. A d.c. power supply is required with the wire electrode connected to the positive pole ( Fig. 7.9). Arc welding Gas flow power supply meter Spool of Inert gas Welding electrode cylinder power cable wire Electrode feed Contactor lead,welding rools current,electrode, and inert gasto welding gun Contacto Control head r cable forelectrode feed Ground and gas supply cable Figure 7.9 . MIG welding equipment
  • 15. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/15 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c During this process an electric arc is used to heat the weld zone. The electrode is fed into the weld pool at a controlled rate and the arc is shielded by a protective gas such as argon, helium, or carbon dioxide (Fig. 7.9). Gas metal-arc welding can be either the short-circuiting process or the spray-arc process (Fig. 7.10). Inert/noble gas Shielded gas Arc Melting pool Work piece Figure 7.10. MIG in progress The short-circuiting arc process (short arc) operates at low currents and voltages. For example, 18-gauge sheet metal can be welded at 45 amps and 12 volts. Work piece Figure 7.11. Mechanics of the short circuiting transfer process as shown between the electrode and work piece. Electrode dips into pool an average of 90 times a second In contrast, the spray-arc process uses high currents and voltages, e.g., Arc action is illustrated in Fig. 7.12. This results in high heat input to the weld area, making possible deposition rates of more than 0.4 lb per minute. (The deposition rate is the weight of filler metal melted into the weld zone
  • 16. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/16 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c per unit of time.) Most applications of the spray-arc process are in thick metal fabrications, e.g., in heavy road-building machinery, ship construction, and beams for bridges. Electrode maintains steady arc length Work piece Figure 7.12. Mechanics of the spray-arc transfer process as shown between the electrode and work All metal inert-gas (MIG) welding is classified as semi-automatic, since the electrode feeds into the weld according to a preset adjustment. After making an initial adjustment, the welding operator merely moves the gun along the joint. For effective applications, the welding operator needs information concerning power requirements, welding gun, selection of shielding gas, type of filler metal, and job procedures. 7.3.1. Power Requirements. Conventional power supplies used for shielded metal-arc welding are not satisfactory. A welding machine designed for the MIG process is called a constant potential power source; it produces a constant voltage and also permits the operator to adjust electrode feed rates. The adjustments on the power supply are voltage, slope (limits current), and wire feed rate. Welding current is established by
  • 17. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/7/17 he he k k lic lic SHIELDED GAS ARC WELDING C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c selecting a wire feed rate. Slope adjustment to limit current is not a problem with spray-arc type transfer. However, in short-circuiting arc processes, limitations on short-circuit current are essential to prevent excessive spatter. The electrode feed mechanism, an important part of the welding machine, consists of a storage reel for electrode wire and a power drive which feeds the electrode into the weld at a controlled rate. Table 7.4 Shielding mixtures for MIG Metal Shielding Gas Remarks Aluminium and copper Argon + helium High heat input 20-80% mixture Minimum of porosity Copper Argon + nitrogen Good heat input on copper 25-30% mixture Carbon steels Argon + oxygen Stabilizes arc Low alloy steels 3-5% mixture Reduces spatter Causes weld metal to flow Eliminates undercut May require electrode to contain deoxidizers Low alloy steels Mixture of argon, Increases toughness of weld helium and carbon deposit dioxide