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Workshop Technology, Chapter 4

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Workshop Technology 2, Unit 4 shielded gas arc welding

Workshop Technology 2, Unit 4 shielded gas arc welding

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  • 1. w. A B B Y Y.c Y PD F T ra n sf o UNIT 7 bu to re he k w 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. . lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/1 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y w. A B B Y Y.c om
  • 2. w. A B B Y Y.c Y PD F T ra n sf o bu to re he k lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/2 om w w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y 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 w. A B B Y Y.c om
  • 3. w. A B B Y Y.c Y PD F T ra n sf o bu to re he k lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/3 om w w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y 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 Work piece Water inlet Figure 7.1. TIG welding equipment Welding machine w. A B B Y Y.c om
  • 4. w. A B B Y Y.c Y PD F T ra n sf o bu to re he k w 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. lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/4 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y w. A B B Y Y.c om
  • 5. w. A B B Y Y.c Y F T ra n sf o Good joints make it easier to obtain a good weld. In production work, carefully fitted joints can help save money and can help the 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). bu to re he k w 7.1.2. Joint Fit. welding operator develop standardized welding techniques. lic SHIELDED GAS ARC WELDING rm y ABB PD C to re C lic k he J3103/7/5 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y w. A B B Y Y.c om
  • 6. w. A B B Y Y.c Y PD F T ra n sf o Welding machine bu to re he k w Electrode 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 machine lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/6 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y Electrode Positive surface particles travel Direction of electron 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 w. A B B Y Y.c om
  • 7. w. A B B Y Y.c Y PD F T ra n sf o bu to re he k w 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 machine lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/7 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y Electrode Surface particles lifted Electron flow Work piece Medium penetration Figure 7.5 Alternating current power supply w. A B B Y Y.c om
  • 8. w. A B B Y Y.c Y PD F T ra n sf o 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 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 Size WELDING CURRENT IN AMPERES 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. bu to re he k w 7.1.4. Welding Torch. torches lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/8 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y w. A B B Y Y.c om
  • 9. w. A B B Y Y.c Y PD F T ra n sf o bu to re he k w 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 lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/9 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y 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 w. A B B Y Y.c om
  • 10. w. A B B Y Y.c Y F T ra n sf o 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. bu to re he k lic SHIELDED GAS ARC WELDING rm y ABB PD C to re C lic k he J3103/7/10 om w w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y w. A B B Y Y.c om
  • 11. w. A B B Y Y.c Y PD F T ra n sf o 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. Argon Better control of penetration (16 gauge and thinner). Argon-helium Higher welding speeds. mixtures Copper and Argon nickel Easy to control penetration and weld 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. bu to re he k w Table 7.2 Selection of gases for manual application of tungsten-arc welding. Stainless steel lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/11 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y 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). w. A B B Y Y.c om
  • 12. w. A B B Y Y.c Y PD F T ra n sf o Table 7.3 Material Operating data for TIG Aluminium Stainless Steel Magnesium Deoxidized ACHF DCSP ACHF DCSP Current: 60-80 80-100 60 110-140 Argon: 15 cfh 11 cfh 13 cfh 15 cfh Passes: 1 1 1 1 125-145 120-140 115 175-225 Argon: 17 cfh 11 cfh 19 cfh 15 cfh Passes: 1 1 1 1 190-220 200-250 120-175 250-300 Argon: 21 cfh 13 cfh 19 cfh 15 cfh Passes: 1 1 1,2 1 at 257.4* 1.6mm electrode 3.2mm electrode Current: 4.7mm electrode Current: bu to re he k w Copper Type of Current lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/12 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y *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 w. A B B Y Y.c om
  • 13. w. A B B Y Y.c Y PD F T ra n sf o bu to re he k w 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 o 30 Direction of 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. lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/13 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y 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 w. A B B Y Y.c om
  • 14. w. A B B Y Y.c Y PD F T ra n sf o bu to re he k w 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. lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/14 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y 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). Gas flow meter Arc welding power supply Welding power cable Spool of electrode wire Inert gas cylinder Electrode feed rools Contacto r cable Ground cable Control head forelectrode feed and gas supply Figure 7.9 . MIG welding equipment Contactor lead,welding current,electrode, and inert gasto welding gun w. A B B Y Y.c om
  • 15. w. A B B Y Y.c Y PD F T ra n sf o bu to re he k lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/15 om w w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y 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 w. A B B Y Y.c om
  • 16. w. A B B Y Y.c Y F T ra n sf o 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 bu to re he k lic SHIELDED GAS ARC WELDING rm y ABB PD C to re C lic k he J3103/7/16 om w w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y w. A B B Y Y.c om
  • 17. w. A B B Y Y.c Y PD F T ra n sf o 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 Aluminium and copper Shielding Gas Remarks Argon + helium High heat input 20-80% mixture Copper Minimum of porosity 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 bu to re he k w selecting a wire feed rate. Slope adjustment to limit current is not a Metal lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/17 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y Mixture of argon, Increases toughness of weld helium and carbon deposit dioxide w. A B B Y Y.c om
  • 18. w. A B B Y Y.c Y F T ra n sf o 7.3.2. Selection of Gas. The primary purpose of the inert gas is to shield the weld crater from contamination. Shielding gas may also affect (1) the transfer of metal across the arc, (2) fusion and penetration, (3) the shape of weld deposit, (4) the speed of completing the weld, (5) the ability of filler metal to flow over the surface without undercutting, and (6) the cost of the finished weld. No single inert gas is satisfactory for all welding conditions. Some specific jobs are more efficiently welded with a mixture of gases. For example, low alloy steels are welded with a mixture of argon, helium, and carbon dioxide (Table 7.4). 7.3.3. Filler Metal. Electrodes used for filler metal with the MIG process are much smaller in diameter than those used with the metal-arc process. Sizes may range from 0.4 mm to 5.5 mm in diameter. Small diameter electrodes require high feed rates, from 100 to 1,400 inches per minute. The composition of the electrode usually matches that of the base metal, but for welding high-strength alloys, the composition of the electrode may vary widely from that of the base metal. For example, an aluminium-zinc-magnesium alloy (7039) is welded with an aluminium-magnesium alloy (5356). bu to re he k lic SHIELDED GAS ARC WELDING rm y ABB PD C to re C lic k he J3103/7/18 om w w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y w. A B B Y Y.c om
  • 19. Y F T ra n sf o 7.4. JOB PROCEDURES High-quality welds are obtained by controlling process variables which include current, voltage, travel speed, electrode extension, cleanliness, and type of joint. 7.4.1. Current. Welding current varies with the melting rate of the electrode. Extreme values of current tend to promote defects, but a high current (1.1 mm. electrode at 220 amp) reduces the drop size of the transfer, improves arc stability, and improves penetration. 7.4.2. Voltage. With the MIG welding process, the voltage control determines the arc length. The higher the voltage setting, the longer the arc. A desirable voltage range to establish a short arc is 19-22 volts; defects are more likely to occur outside this range (Fig. 7.14). Curve representing undercutting re to bu y rm he k lic C w Severity of defect (Increase) A B B Y Y.c SHIELDED GAS ARC WELDING Severity of defect (Increase) w. PD ABB to re C lic k he J3103/7/19 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y Voltage Fig. 7.13. Defects related to voltage settings. Curve representing porosity Voltage w. A B B Y Y.c om
  • 20. w. A B B Y Y.c Y PD F T ra n sf o higher voltage is more desirable for flat-position welding than for vertical or overhead welding. Table 7-5 indicates typical voltage values. Table 7-5 Typical arc voltage for MIG using drop transfer and 1/16 inch diameter electrode. Argon Helium Ar-O2 Mixture CO2 1-5%O2 Aluminium 25 30 * * Carbon Steel * * 28 30 Low-alloy Steel * * 28 30 Stainless Steel 24 * 26 * Nickel 26 30 * * Copper 30 36 * * *Not recommended. 7.4.3. Travel Speed. After selecting a current and voltage setting, select the rate of travel. A typical example is 0.6m – 0.76m per minute (in./min). If the rate is changed more than a few mm per minute, weld quality will be greatly affected (Fig. 7.15). bu to re he k w Position of welding will determine voltage needed. For example, a Metal lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/20 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y w. A B B Y Y.c om
  • 21. w. A B B Y Y.c Y PD F T ra n sf o No undercut. Travel speed 26 in/min bu to re he k w Undercutting. Travel speed 32 in/min Fig. 7.15. Undercutting of horizontal fillet on 6.3mm thick aluminium as affected by travel speed. Gas metal arc process was used. Position of welding will affect the travel speed. For example, if the weld direction is dropped 15 degrees from flat so that the position is slightly downhill, travel speed can be increased. 7.4.4. Electrode Extension. Electrode extension is important. The further the electrode extends from the gun to the arc, the greater the electrical resistance between the output terminals. Higher resistance increases the temperature of the electrode, and the resistance-heated electrode uses less current in the weld puddle. In the spray-arc process, the electrode extension should be about 12 mm to 25 mm, for short-circuiting transfer; it should be approximately half this distance. 7.5. lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/21 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y MIG WELDING TECHNIQUES There are three methods of initiating the arc. w. A B B Y Y.c om
  • 22. w. A B B Y Y.c Y F T ra n sf o i. The gun switch operates the gas and water solenoids and when released the wire drive is switched on together with the welding current. ii. The gun switch operates the gas and water solenoids and strikes the wire end on the plate operates the wire drives and welding current (known as ‘scratch start’). iii. The gun switch operates the gas and water solenoids and wire feed with welding current known as ‘scratch start’. As a general rule dip transfer is used for thinner sections up to 6.4 mm and for positional welding, whilst spray transfer is used for thicker sections. The gun is held at an angle of 80o or slight less to the line of the weld to obtain a good view of the weld pool, and welding proceeds from right to left with nozzle held 6 – 12 mm from the work. The further the nozzle is held from the work less the efficiency of the gas shield, leading to porosity. If the nozzle is held too close to the work spatter may build up, necessitating frequent cleaning of the nozzle, while acting between nozzle and work can be caused by a bent wire guide tube allowing the wire to touch the nozzle, or by spatter build-up short-circuiting wire and nozzle. If the wire burns back to the guide tube it may be caused by a late start of the wire feed, fouling of the wire in the feed conduit or the feed rolls being too tight. Intermittent wire feed is generally due to insufficient feed rolls pressure or looseness wire due to wear in the rolls. Excessively sharp bends in the flexible guide tubes can also lead to this trouble. Root run is performed with no weave and filler runs with as little weave as possible consistent with good fusion since excessive weaving tends to promote porosity. The amount of wire projecting beyond the contact tube is important because the greater the projection, the greater the I2R effect and the greater the voltage drop which may reduce the welding current and affect bu to re he k lic SHIELDED GAS ARC WELDING rm y ABB PD C to re C lic k he J3103/7/22 om w w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y w. A B B Y Y.c om
  • 23. w. A B B Y Y.c Y PD F T ra n sf o bu to re he k lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/23 om w w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y penetration. The least projection commensurate with accessibility to the joint being welded should be aimed at. Backing the strips which are welded permanently on to the reverse side of the plate by the root run are often used to ensure sound root fusion. Backing bars of copper or ceramics with grooves of the required penetration bead profile can be used and are removed after welding. It is not necessary to back-chip the root run of the light alloys but with stainless steel this is often done and a sealing run put down. The importance of fit-up in securing continuity and evenness of the penetration bead cannot be over-emphasized. Flat welds may be slightly tilted to allow the molten metal to flow against the deposited metal and thus give a better profile. If the first run has a very convex profile poor manipulation of the gun may cause cold laps in the subsequent run. 7.6. DIRECT CURRENT STRAIGHT POLARITY The welding circuit shown in figure 7.16, is known as a straight polarity circuit. It is understood that the electrons are flowing from the negative terminal (cathode) of the machine to the electrode. The electrons continue to travel across the arc into the base metal and to the positive terminal (anode) of the machine. Approximately two-thirds of the total heat produced with DCSP is released at the base metal while one-third is released at the electrode. The choice of direct current straight polarity depends on many variables such as material of the base metal, position of the weld, as well as the electrode material and covering. Reactor Arc gap Electrode Work piece Cathode Field Holder Anode Figure 7.16. Wiring diagram of a direct current, straight polarity (DCSP) arc circuit w. A B B Y Y.c om
  • 24. w. A B B Y Y.c Y PD F T ra n sf o 7.7. bu to re he k lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/24 om w w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y DIRECT CURRENT REVERSE POLARITY ARC WELDING It is possible, and sometimes desirable, to reverse the direction of electron flow in the arc welding circuit. When electron flow from the negative terminal (cathode) of the arc welder to the base metal, this circuit is known as direct current reverse polarity (DCRP). In this case, the electron returns to the positive terminal (anode) of the machine from the electrode side of the arc, as shown in Figure 7.17. Reactor Arc gap Electrode Work piece Anode Field Holder Cathode Figure 7.17. Wiring diagram of a direct current, reverse polarity (DCRP) arc circuit When using DCRP, one-third of the heat generated in the arc is released at the base-metal and two-thirds is liberated at the electrode. With two-thirds of the heat released at the electrode in DCRP, the electrode metal and the shielding gas are super-heated. This superheating causes the molten metal in the electrode to travel across the arc at a very high rate of speed. Deep penetration results due to the force of the high velocity arc. There is theory that, with a covered electrode, a jet action and/or expansion of gases in the metal at the electrode tip causes the molten metal to be propelled with great impact across the arc. The choice of direct current reverse polarity depends on many variables such as material of the base metal, position of the weld, as well as the electrode material and covering. w. A B B Y Y.c om
  • 25. w. A B B Y Y.c Y PD F T ra n sf o bu to re he k w ACTIVITY 7 7.1. Explain the term nonconsumable electrode. 7.2. What does the term inert signify? 7.3. List the gases used for shielding a welding arc. 7.4. Explain how TIG welding electrodes are shaped. 7.5. How far should the electrode extend beyond the nozzle of the TIG torch? 7.6. lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/25 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y Explain why MIG welding is classified as a semiautomatic process. w. A B B Y Y.c om
  • 26. w. A B B Y Y.c Y F T ra n sf o FEEDBACK ON ACTIVITY 7 7.1. The electrode does not melt into the weld. 7.2. The gas does not combine with the base metal or filler. 7.3. Argon, helium and carbon dioxide. 7.4. The electrode diameter should match the current value. If the current is too high for the diameter of the electrode the life of the electrode will be short. When the current is too low for a given electrode diameter, the arc will not be stable. The end of the electrode should remain bright, as if it was polished. 7.5. The electrode should extend beyond the nozzle a distance equal to the electrode diameter. 7.5. MIG welding is classified as semi-automatic because 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. bu to re he k lic SHIELDED GAS ARC WELDING rm y ABB PD C to re C lic k he J3103/7/26 om w w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y w. A B B Y Y.c om
  • 27. w. A B B Y Y.c Y F T ra n sf o SELF-ASSESSMENT 7 1. From the standpoint of operation, how are TIG and MIG processes different? How are they similar? 2. What polarity does anode signify? 3. In what direction do the electrons travel when using straight polarity? 4. How much of the heat used for arc welding is liberated at the electrode when using straight polarity? 5. Why is it recommended that a tungsten electrode arc be started on a scrap tungsten surface? 6. What would happen if the tungsten electrode were bent off centre? 7. Name two defects that could occur with gas shielded-arc welding processes and explain how each could be avoided. bu to re he k lic SHIELDED GAS ARC WELDING rm y ABB PD C to re C lic k he J3103/7/27 om w w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y w. A B B Y Y.c om
  • 28. w. A B B Y Y.c Y PD F T ra n sf o bu to re he k lic C SHIELDED GAS ARC WELDING w FEEDBACK OF SELF-ASSESSMENT 7 1. rm y ABB to re C lic k he J3103/7/28 om w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y TIG uses a tungsten electrode that does not melt into the weld; because the electrode is shielded and cooled by inert gas flow. A separate filler rod is used as needed MIG uses a continuous electrode which feeds into the weld automatically as an arc is maintained. . They both use inert gas. Electrode (tungsten) Inert/noble gas Filler rode Shielded gas arc Direction of travel 80 – 90o 20 – 30o Melting pool Work piece TIG in progress. The tungsten does not melt into the puddle for filler. This is a nonconsumable electrode. w. A B B Y Y.c om
  • 29. w. A B B Y Y.c Y PD F T ra n sf o bu to re he k lic C SHIELDED GAS ARC WELDING rm y ABB to re C lic k he J3103/7/29 om w w w w Y 2.0 2.0 bu y rm er Y F T ra n sf o ABB PD er Y Inert/noble gas Shielded gas Arc Melting pool Work piece MIG in progress 2. Positive (+) 3. Across the arc into the base metal and to the positive terminal. 4. One-third (1/3) 5. To keep the tungsten electrode clean. 6. Uses more current and electrode will be jagged or contaminated. 7. (a) Eyes and skin – arc is more intense. Wear leather and specially treated cloth. (b) Breathing – provide adequate ventilation. w. A B B Y Y.c om