F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
F T ra n sf o                                                                                                             ...
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

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Unit7 Shielded Gas Arc Welding

  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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

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