NAME 345 Welding Technology Lecture 08 Gas Metal Arc Welding (GMAW) Metal Inert Gas (MIG/MUG) Welding

Transcription

1 NAME 345 Welding Technology Lecture 08 Gas Metal Arc Welding (GMAW) Metal Inert Gas (MIG/MUG) Welding Md. Habibur Rahman Lecturer Department of Naval Architecture & Marine Engineering Bangladesh University of Engineering & Technology Dhaka-1000, Bangladesh

2 Gas Metal Arc Welding (GMAW) Definition Gas Metal arc welding is also known as MIG (Metal Inert Gas) welding or MAG (Manual Metal Arc Welding). It is a semi-automatic or automatic arc welding process in which a continuous and consumable wire electrode and a shielding gas are fed through a welding gun. An electric arc forms between the consumable wire electrode and the workpiece metal(s), which heats the workpiece metal(s), causing them to melt and join. MIG welding is a gas shielded metal arc welding process using the heat of an electric arc between a continuously fed, consumable electrode wire and the material to be welded. 2

3 Gas Metal Arc Welding (Contd.) MIG Welding A wire of copper coated mild steel is fed continuously from a reel through a gun with a melting rate up to 5m/min. Current through the ware ranges from 100A to 400A depending upon the diameter of the wire CO 2 is principally used apart from argon or argon helium mixture as shielding gas. A constant voltage, direct current power source is most commonly used with GMAW, but constant current systems, as well as alternating current, can be used. Manual MIG/MAG welding is often referred as a semi-automatic process, as the wire feed rate and arc length are controlled by the power source, but the travel speed and wire position are under manual control. The process can also be mechanized when all the process parameters are not directly controlled by a welder, but might still require manual adjustment during welding. When no manual intervention is needed during welding, the process can be referred to as automatic. 3

4 Schematic/ Circuit Diagram of MIG Welding GMAW Circuit Diagram: (1) Welding torch, (2) Workpiece, (3) Power source, (4) Wire feed unit, (5) Electrode source, (6) Shielding gas supply Schematic Diagram of Gas Metal Arc Welding (MIG Welding) 4

5 Principles of Gas Metal Arc Welding (GMAW) The gas metal arc welding (GMAW) process configuration consists of several components and consumables. The main components and consumables are (1) welding power source, (2) remote controller, (3) wire feeder, (4) welding torch (5) shielding gas cylinder and regulator, and (6) welding wire; in addition, a water circulator for a water-cooled welding torch. With the GMAW process using a constant-voltage power source, the electrode wire is feed at a constant speed that matches the welding current while the arc length is remained almost constant by the self-correction mechanism of the power source. The electrode wire is fed into the arc through the wire feeder, conduit tube, and welding torch. During welding, the arc and molten pool are shielded with a shielding gas to prevent them form adverse effects of nitrogen and oxygen in the atmosphere. The type of welding wire chooses the proper kind of shielding gas to ensure intended usability and weldability. 5

6 Manual GMAW Equipment There are three major elements in GMAW. These are i. Welding Torch and Accessories ii. Welding Control and Wire Feed Motor iii. Power Source GMAW equipment can be used either manually or automatically. 6

7 i. Manual GMAW Equipment (Welding Torch and Accessories) The welding torch guides the wire and shielding gas to the weld zone. It also brings welding power to the wire. Different types of welding torches have been designed to provide maximum welding utility for different types of applications. They range from heavy duty torches for high current work to lightweight torches for low current and out-of-position welding. In both types, water or air cooling and curved or straight front ends are available. Major components/parts of the torch are a. Contact tube (or tip) b. Shielding gas cup or nozzle c. Wire conduit and liner d. One-piece composite cable 7

8 i. Manual GMAW Equipment (Welding Torch and Accessories) a. Contact tube (or tip): The wire guide tube, also called contact tube, is made of copper and is used to bring welding power to the wire as well as direct the wire toward the work. The torch (and guide tube) is connected to the welding power source by the power cable. Because the wire must feed easily through the guide tube and also make good electrical contact, the bore diameter of the tube is important. The instruction booklet, supplied with every torch, lists the correct size contact tube for each wire size. The tube, which is a replaceable part, must be firmly locked to the torch and centered in the shielding gas cup. b. Shielding gas cup or nozzle: The shielding gas cup directs a protective mantle of gas to the welding zone. Large cups are used for high-current work where the weld puddle is large. Smaller cups are used for lowcurrent welding. 8

9 i. Manual GMAW Equipment (Welding Torch and Accessories) c. Wire conduit and liner: The wire conduit and its liner are connected between the torch and wire drive (feed) rolls. They direct the wire to the torch and into the contact tube. Uniform wire feeding is necessary for arc stability. When not properly supported by the conduit and liner, the wire may jam. The liner may be either an integral part of the conduit or supplied separately. In either case, the inner diameter and material of the liner are important. When using steel wire electrodes, a steel spring liner is recommended. Nylon and other plastic liners should be used for aluminum wire. The literature supplied with each torch lists the recommended conduits and liners for each wire size and material. d. One-piece composite cable: It connects the wire conduit and liner with the wire guide tube and shielding gas nozzle. Tigger Installed Components Nozzle Gas Diffuser Contact Tip 9

10 ii. Manual GMAW Equipment (Welding Control and Wire Feed Motor) The welding control and wire-feed motor are often supplied in one package (wire feeder). Their main function is to pull the welding wire from the spool and feed it to the arc. The control maintains pre-determined wirefeed speed at a rate appropriate to the application. The control not only maintains the set speed independent of load, but also regulates starting and stopping of wire feed on signal from the torch switch. Shielding gas, water, and welding power are usually delivered to the torch through the control box. Through the use of solenoids, gas and water flow are coordinated with flow of weld current. The control determines the sequence of gas flow and energizing of the power supply contactor. It also allows some gas to flow before and after arc operation. 10

11 iii. Manual GMAW Equipment (Power Source) Almost all GMAW is done with reverse polarity also known as DCEP. The positive (+) lead is connected to the torch while the negative ( ) lead is connected to the workpiece. Arc length is set by adjusting the power source voltage. Power source may also have one or two additional adjustments for use with other welding applications. Most power sources require either 230V or 460V AC input power. Positive Terminal Negative Terminal 11

12 Gas Metal Arc DCEP Welding: Wire Positive, Work Negative Generally used for gas metal arc welding Provides maximum heat input into work allowing relatively deep penetration to take place Assists in removal of oxides from plate Low current values produce globular transfer of metal from electrode On carbon steel shielding gas must contain minimum of 80% argon Ferrous metals need addition of 2 to 5% oxygen to gas mixture 12

13 Modes of Metal Transfer MIG Welding Metal transfer refers to how filler metal is deposited to the base metal to form the weld bead. The common modes of metal transfer are a. axial-spray arc transfer, b. globular transfer, c. short-circuit metal transfer, and d. pulsed current transfer. The mode of metal transfer is determined by many mitigating factors: i. Base Metal Type ii. iii. iv. Filler Metal Composition Electrode Diameter Polarity v. Arc Current vi. vii. Arc Voltage/ Arc Length Shielding Gas Composition viii. Welding Position 13

14 a. Axial-Spray Arc Transfer MIG Welding Spray transfer is a high current density process that rapidly deposits weld metal in droplets of a size equal to or less than the electrode diameter. The droplets are directed axially in a straight line from the center of the electrode to the weld puddle. Unlike short-circuit transfer, once the arc is established, it is on at all times. The arc is very smooth and stable. The result is little spatter and a weld bead of relatively smooth surface. The arc (plasma) energy is spread out in a cone-shaped pattern (figure). This results in good wash characteristics at the weld bead extremities but yields relatively shallow penetration (shallow depth of fusion). Penetration is deeper than that obtained in SMAW but less than can be obtained with the high energy globular transfer. 14

15 a. Axial-Spray Arc Transfer (Contd.) The axial-spray transfer mode is established at a minimum current level for any given electrode diameter (current density). This current level is generally termed the transition current. A well-defined transition current exists only with a gas shield containing a minimum of 80% argon. The reason argon is used because it helps the electrode to create smaller amounts of molten metal which gives a little more control of the process. At current levels below the transition current, the droplet size increases (larger than the electrode dia.). The arc characteristics are quite unstable in this operating range. This type of MIG welding metal transfer can be used on thick metal due to the fact that it will penetrate the metal deeply. A good spray transfer will make a hissing sound, rather than a crackle or popping sound. Another characteristics of a good spray transfer is a clean arc from the welding gun to the base metals. 15

16 a. Axial-Spray Arc Transfer (Contd.) Advantages High deposition rates. High electrode efficiency of 98% or more. Employs a wide range of filler metal types in an equally wide range of electrode diameters. Excellent weld bead appearance. High operator appeal and ease of use. Requires little post weld cleanup. Absence of weld spatter. Excellent weld fusion. Lends itself to semi-automatic, robotic, and hard automatic applications. Limitations Restricted to the flat and horizontal welding positions. Welding fume generation is higher. The higher-radiated heat and the generation of a very bright arc require extra welder and bystander protection. The use of axial spray transfer outdoors requires the use of a windscreen(s). The shielding used to support axial spray transfer costs more than 100% CO 2. 16

17 b. Globular Transfer Globular transfer (gas shield with CO 2 ) is basically uncontrolled short-circuit in which metal transfer across the arc is in the form of irregular and gravity-assisted large globules/droplets (larger than the dia. of the electrode) directed across the arc in irregular fashion (figure) resulting in a considerable amount of spatter. Spatter is minimized when using a CO 2 shield by adjusting the welding conditions so that the tip of the electrode is below the surface of the molten weld metal and within a cavity generated by the force of the arc. The CO 2 arc is generally unstable in nature and characterized by a cracking. It represents a weld bead surface that is rough in appearance (ripple effect) in comparison to a bead obtained with axial spray transfer. 17

18 b. Globular Transfer (Contd.) MIG Welding Since most of the arc energy is directed downward and below the surface of the molten weld metal, the weld bead profile exhibits extremely deep penetration with a washing action at the weld bead extremities that is less than that obtained in the axial spray transfer mode. Relative stability of the CO 2 arc can be established at higher current levels using a buried arc. When helium-rich gas mixtures are used, a broader weld bead is produced with a penetration depth similar to that of argon, but with a more desirable profile. Welding is most effectively done in the flat position when using globular transfer. 18

19 b. Globular Transfer (Contd.) Advantages It uses inexpensive CO 2 shielding gas, but is frequently used with argon/co 2 blends. It is capable of making welds at very high travel speeds. It uses inexpensive solid or metal-cored electrodes. Welding is most effectively done in the flat position when using globular transfer. globular transfer runs at high wire feed speeds and amperages for good penetration on thick metals. Welding equipment is inexpensive. Limitations Globular transfer is limited to flat and horizontal fillet welds. Less fusion is often common because the spatter disrupts the weld puddle. Since, globular transfer uses more wire, it is generally considered less efficient. Higher spatter levels result in costly cleanup. It reduces operator appeal. It is prone to cold lap or cold shut incomplete fusion defects, which results in costly repairs. Weld bead shape is convex, and welds exhibit poor wetting at the toes. High spatter level reduces electrode efficiency to a range of 87 93%. 19

20 c. Short-Circuit Metal Transfer MIG Welding Short circuiting metal transfer, known by the acronym GMAW-S, is a mode of metal transfer, whereby a continuously fed solid or metal-cored wire electrode is deposited during repeated electrical short-circuits. Short circuit transfer is a transfer used when a lower voltage is used for MIG welding. Short circuit transfer occurs when the wire arcs and contacts the metal creating short circuits. During this short circuit, the wire contacting the metal heats up and drips into the joint by creating a puddle. Then another arc begins and the process keeps repeating many times a second. 20

21 c. Short-Circuit Metal Transfer (Contd.) The easiest way to tell if the transfer is short circuit is by the sound. The sound greatly resembles, an egg hitting an extremely hot frying pan. It is a very crisp and fast crackling sound. Typically short circuit transfer is used on thin metals or sheet metals. This mode of metal transfer typically supports the use of ( mm) diameter electrodes shielded with either 100% CO 2 or a mixture of 75-80% argon, plus 20-25% CO 2. The low heat input attribute makes it ideal for sheet metal thickness materials. The useable base material thickness range for short-circuiting transfer is typically considered to be ( mm) material. Other names commonly applied to short circuiting transfer include short arc micro-wire welding, fine wire welding, and dip transfer. 21

22 c. Short-Circuit Metal Transfer (Contd.) 22

23 c. Short-Circuit Metal Transfer (Contd.) Advantages All-position capability, including flat, horizontal, vertical-up, verticaldown, and overhead. Handles poor fit-up extremely well, and is capable of root pass work on pipe applications. Lower heat input reduces weldment distortion. Higher operator appeal and ease of use. Higher electrode efficiencies, 93% or more. Limitations Restricted to sheet metal thickness range and open roots of groove joints on heavier sections of base material. Poor welding procedure control can result in incomplete fusion. Cold lap and cold shut are additional terms that serve to describe incomplete fusion defects. Poor procedure control can result in excessive spatter, and will increase weldment cleanup cost. To prevent the loss of shielding gas to the wind, welding outdoors may require the use of a windscreen. 23

24 d. Pulsed Arc Transfer MIG Welding The pulsed-arc process is, by definition, a spray transfer process wherein spray transfer occurs in pulses at regularly spaced intervals rather than at random intervals. In the time between pulses, the welding current is reduces and no metal transfer occurs. Pulsed-arc transfer is obtained by operating a power source between low and high current levels. The high current level or peak forces an electrode drop to the workpiece. The low current level or background maintain the arc between pulses. The pulse provides a stable arc and no spatter, since no short-circuiting takes place. This also makes the process suitable for nearly all metals, and thicker electrode wire can be used as well. The smaller weld pool gives the variation greater versatility, making it possible to weld in all positions. The pulsed arc equipment effectively combines two power sources into one integrated unit. 24

25 d. Pulsed Arc Transfer (Contd.) One side of the power source supplies a background current which keeps the tip of the wire molten. The other side produces pulses of a higher current that detach and accelerate the droplets of metal into the weld pool. The frequency is the number of times the period occurs per second, cycles per second. The frequency of the period increases in proportion to the wire feed speed. Taken together they produce an average current, which leverages its use in a wide material thickness range. The transfer frequency of these droplets is regulated primarily by the relationship between the two currents. Pulsed arc welding occurs between ±50 220A, arc volts, and only with argon and argon based gases. It enables welding to be carried out in all positions. 25

26 d. Pulsed Arc Transfer (Contd.) MIG Welding Advantages Absent or very low levels of spatter. More resistant to lack of fusion defects than other modes of GMAW metal transfer. Excellent weld bead appearance. High operator appeal. Offers an engineered solution for the control of weld fume generation. Reduced levels of heat induced distortion. Ability to weld out-of-position. Lower hydrogen deposit. Reduces the tendency for arc blow. Handles poor fit-up. When compared to FCAW, SMAW, and GMAW-S, pulsed spray transfer provides a low cost high-electrode efficiency of 98%. Lends itself to robotic and hard automation applications. Capable of arc travel speeds greater than 50 inches per minute (1.2m/ min.) Limitations Equipment to support the process is more expensive than traditional systems. Blends of argon based shielding gas are more expensive than carbon dioxide. Higher arc energy requires the use of additional safety protection for welders and bystanders. Adds complexity to welding. Requires the use of windscreens outdoors. 26

27 Stickout The stickout is the distance from the end of the contact tube to the end of the wire.. Insufficient stickout will cause the wire to fuse to the contact tube. Excessive stickout will cause the wire to overheat and it will melt into irregular pieces. 27

28 Shielding Gases The purpose of shielding gas is to protect the weld area from the contaminants in the atmosphere. Gas can be inert, reactive, or mixtures of both. Argon, helium and CO 2 are the main three gases used in GMAW. Shielding gas will also have a pronounced effect upon the following aspects of the welding operations and the resultant weld: i. Arc characteristics ii. Mode of metal transfer iii. Penetration and weld bead profile iv. Speed of welding v. Undercutting tendency 28

29 Selection of Shielding Gas MIG Welding A summery for typical usage for the various shielding gases based upon the metal being welded is shown. 29

30 Selection of Shielding Gas for GMAW with Spray Transfer No. Metal Shielding gases Advantages 01. Aluminum Argon 35% argon + 65% helium 25% argon + 75% helium 0 to 1 in. (0 to 25 mm) thick; best metal transfer and arc stability; least spatter. 1 to 3 in. (25 to 76 mm) thick; higher heat input that straight argon; improved fusion characteristics with 5XXX series Al Mg alloys. Over 3 in. (76 mm) thick; higher heat input; minimizes porosity. 02. Magnesium Argon Excellent cleaning action. Improves arc stability; produces a more fluid and controllable weld puddle; good Argon coalescence and bead contour; minimizes undercutting; permits higher speeds than + 1-5% O 03. Carbon Steel 2 pure argon. Argon Good bead shape; minimizes spatter; reduces chance of cold lapping; can not weld % CO 2 out-of-position. 04. Low-alloy steel Argon + 2% O 2 Minimizes undercutting; provides good toughness. Argon Improves arc stability; produces a more fluid and controllable weld puddle; good 05. Stainless steel + 1% O 2 coalescence and bead contour; minimizes undercutting on heavier stainless steels. Argon Provides better arc stability, coalescence, and welding speed than 1% O 2 mixture for + 2% O 2 thinner stainless steel materials. Provides good wetting; decreases fluidity of weld metal for thickness up to 1/8 in. (3.2 Argon Copper, nickel mm). 06. and their alloys Argon Higher heat inputs of 50 & 75% helium mixtures offset high heat dissipation of + helium heavier gases. 07. titanium Argon Good arc stability; minimum weld contamination; inert gas backing is required to prevent air contamination on back of weld area. 30

31 Selection of Shielding Gas for GMAW with Short-Circuiting Transfer No. Metal Shielding gases Advantages 01. Carbon Steel 02. Stainless Steel 03. Low-alloy Steel 04. Aluminum, copper, magnesium, nickel and their alloys Less than 1/8 in. (3.2 mm) thick; high welding speeds without burn thru; minimum Argon distortion and spatter. 75% argon More than 1/8 in. (3.2 mm) thick; minimum spatter; clean weld appearance; good + 25% CO 2 puddle control in vertical and overhead positions. CO 2 90% helium + 7.5% argon + 2.5% CO % helium % argon +4- Deeper penetration; faster welding speeds. No effect on corrosion resistance; small heat affected zone; no undercutting; minimum distortion. 5% CO 2 characteristics; and bead contour; little spatter. Minimum reactivity; excellent toughness; excellent arc stability; wetting 75% argon Fair toughness; excellent arc stability; wetting characteristics, and bead contour; little + 25% CO 2 spatter. Argon & argon + helium Argon satisfactory on sheet metal; argon-helium preferred on thicker sheet material (over 1/8 in. [3.2 mm]). 31

32 Electrode Grouping MIG Welding Electrodes are also grouped according to there performance characteristics. Fast freeze Mild steel Quick solidification of weld pool Deep penetration Recommended for out of position welds Deep penetrating arc Fast fill Highest deposition rate Stable arc Thick flux Flat position and horizontal laps only Fill freeze General purpose electrodes Characteristics of fast freeze and fast fill Low Hydrogen Welding characteristics of fill freeze Designed for medium carbon and alloy steels 32

33 Selecting Electrode Size The optimum electrode diameter is determined by the thickness of the base metal, the welding position and the capacity of the welding power supply. A smaller diameter is usually recommended for out of position welding. When completing root passes in V-joints, a smaller diameter maybe used and then a larger diameter is used for the filler passes. A diameter of 3/32 in. or 1/8 in. can be used on metals up to 1/4 in. thick without joint preparation. The diameter of the electrode should not exceed the thickness of the metal. 33

34 Electrode Storage MIG Welding Electrodes are damaged by rough treatment, temperature extremes and moisture. They should be kept in their original container until used. They should be stored in a heated cabinet that maintains them at a constant temperature. The storage of low hydrogen electrodes is very critical. Designed to reduce underbead cracking in alloy and medium carbon steels by reducing the amount of hydrogen in the weld pool. The flux is hydroscopic--attracts moisture H 2 O. Moisture in the flux also causes excessive gasses to develop in the weld pool and causes a defect in the weld caused worm holes. 34

35 Electrode Angle MIG Welding The electrode angle influences the placement of the heat. Two angles are important: Travel Work The travel angle is the angle of the electrode parallel to the joint. The correct travel angle must be used for each joint. Beads = 15 from vertical or 75 from the work. Butt joint = 15 from vertical or 75 from the work. Lap joint = 45. T joint = 45. Corner = 15 from vertical or 75 from the work. 35

36 Electrode Angle (Contd.) MIG Welding The work angle is the angle of the electrode perpendicular to the joint. The appropriate angle must be used for each joint. Beads = 90 Butt joint = 90 Lap joint = 45 T joint = 45 Corner = 90 The work angle may need to be modified for some situations. For example, a butt joint with two different thickness of metal. 36

37 Arc Length The arc length is the distance from the metal part of the electrode to the weld puddle. The best arc length is not a fixed distance, but should be approximately equal to the diameter of the electrode. Arc length can be adjusted slightly to change the welding process. Excessive length Excessive spatter Reduced penetration Poor quality weld Insufficient length Electrode sticks Narrow weld Poor quality weld 37

38 Speed of Travel The speed of travel (in. per min.) is an important factor when arc welding. The best speed of travel (welding speed) is determined by several factors. The size of the joint, The type of electrode, The size of the electrode, The amperage setting on the machine Deposition rate of the electrode (cubic inches per minute) The deposition rate of an electrode will change with the welding amperage. 38

39 Speed of Travel (Contd.) The ideal speed can be calculated using the volume of the joint and the deposition rate of the electrode. Step 01: Determine the area of the weld. (Assuming 1/16 in. penetration. Area = in in. bh = = in Step 02: Knowing the deposition rate of the electrode, determine the welding speed (Deposition rate = 2.5 in. 2 /min). in. 2.5 in3 1.0 in. = = 40 min. min in2 min. 39

40 Speed of Travel (Contd.) The correct welding speed in indicated by the shape of the ripples. Too slow = excessive width, excessive penetration. Too fast = narrower width, elongated ripple patter, shallow penetration. Recommended = width 2 3 times diameter of electrode, uniform ripple pattern, full penetration. 40

41 Some common MIG welding defects Spatter: Droplets of electrode material that land outside the weld fusion area and may or may not fuse to the base metal. Porosity: Small volumes of entrapped gas in solidifying weld metal. Spatter Surface Porosity Crater Porosity 41

42 Some common MIG welding defects (Contd.) 42