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Gas Metal Arc Welding Equipment, Setup, and Operation

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Presentation on theme: "Gas Metal Arc Welding Equipment, Setup, and Operation"— Presentation transcript:

1 Gas Metal Arc Welding Equipment, Setup, and Operation
Chapter 10 Gas Metal Arc Welding Equipment, Setup, and Operation

2 Objectives List the various terms used to describe gas metal arc welding Discuss the various methods of metal transfer including the axial spray metal transfer process, globular transfer, pulsed-arc metal transfer, buried-arc transfer, and short-circuiting transfer GMAW-S List shielding gases used for short-circuiting, spray, and pulsed-spray transfer

3 Objectives (cont'd.) Describe the more commonly used GMA welding filler metals Define deposition efficiency, and tell how a welder can control the deposition rate Define voltage, electrical potential, amperage, and electrical current as related to GMA welding Tell how wire-feed speed is determined and what it affects

4 Objectives (cont'd.) Discuss how the GMAW molten weld pool can be controlled by varying the shielding gas, power settings, weave pattern, travel speed, electrode extension, and gun angle Describe the backhand and forehand welding techniques List and describe the basic GMAW equipment Explain how the arc spot weld produced by GMAW differs from electric resistance spot welding and the advantages of GMA spot welding

5 Introduction 1920s: metal arc welding process using an unshielded wire was being used 1948: first inert gas metal arc (GMA) welding process developed Used to weld aluminum using argon gas for shielding Later, carbon dioxide and dioxide were used as shielding gases May be semiautomatic, machine, or automatic

6 Weld Metal Transfer Methods
Several modes of transferring filler metal Short-circuiting transfer (GMAW-S) Axial spray transfer Globular transfer Pulsed-arc transfer (GMAW-P) Selecting the mode depends on: Welding power source and type of shielding gas Wire electrode size and material type and thickness Best welding position

7 Short-Circuiting Transfer GMAW-S
Low currents allow liquid metal at electrode tip to be transferred Direct contact with molten weld pool Most common process used with GMA welding: On thin or properly prepared thick sections of material On a combination of thick to thin materials With a wide range of electrode diameters With a wide range of shielding gases

8 Globular Transfer Generally used on thin materials and at a very low current rang Transfers molten ball metal across arc Little control over where metal lands Rarely used alone Used in combination with pulsed-spray transfer

9 Axial Spray Metal Transfer
Wire tip projects very small drops Projected axially across arc gap to molten weld pool Hundreds of drops per second Drops are propelled by arc forces at high velocity Spray transfer process conditions Argon shielding DCEP polarity Transition current

10 FIGURE 10-5 Axial spray metal transfer
FIGURE 10-5 Axial spray metal transfer. Note the pinch effect of filler wire and the symmetrical metal transfer column. Larry Jeffus

11 Pulsed-Arc Metal Transfer
Dual pulsed current Pulse of high current: axial spray transfer mode Lower pulse of current: should not transfer any weld metal Advantage Ease of controlling the weld Synergic systems Interlock power supply and wire feeder

12 Pulsed-Arc Metal Transfer Current Cycle
FIGURE 10-7 Mechanism of pulsed-arc spray transfer at a low average current. © Cengage Learning 2012

13 Pulsed-Arc Metal Transfer Current Cycle (cont’d.)
Components Ramp up Overshoot High pulse current High pulse time Ramp down Step-off current Background current Pulse width Advantages Lower average currents All position Less distortion Reduced spatter High-quality welds Several others Disadvantages Cost and complexity

14 Shielded Gases for Spray or Pulsed-Spray Transfer
Axial spray transfer Required: shielded gas containing argon Helium/argon mixtures may contain as much as 80% helium Adding small amounts of oxygen Provides a stable site for the arc Amount of oxygen needed to stabilize arcs in steel varies with the alloy

15 Buried-Arc Transfer Carbon dioxide is very forceful
Wire tip can be driven below surface of molten weld pool Spatter produced by the arc: trapped in the cavity Useful for high/speed mechanized welding of thin sections Compressor domes for hermetic air-conditioning and refrigeration equipment or automotive components

16 GMAW Filler Metal Specifications
Key points GMA welding filler metals: available for a variety of metals Some steel wire electrodes have a thin copper coating Protects electrode from rusting Improves electrical contact Burns off or is diluted into weld pool

17 Wire Melting and Deposition Rates
Wire melting rates, deposition rates, and wire feed speeds Affected by the same variables Wire melting rate: measured in inches per minute or pounds per hour Deposition rate: nearly always less than melting rate Deposition efficiency: ratio of amount of weld metal deposited to wire used

18 Welding Power Supplies
Important terms Voltage Electrical potential Amperage Electrical current GMAW power supplies Constant-voltage, constant potential-type machines SMAW power supplies Constant-current-type machines

19 Speed of the Wire Electrode
Selected in inches per minute (ipm) Wire speed control dial: used to control ipm To accurately measure wire-feed ipm: Snip off wire at contact tube Squeeze trigger for six seconds Release and snip off the wire electrode Measure number of inches of wire that was fed Multiply its total length by ten

20 Power Supplies for Short-Circuiting Transfer
There is a slight decrease in voltage as amperage increases Rate of decrease is called slope Voltage decrease per 100-ampere increase Slope is called volt-ampere curve Machine slope is affected by circuit resistance Slope increases: short-circuit current and pinch effect are reduced

21 Molten Weld Pool Control
GMAW molten weld pool can be controlled by varying several factors Shielding gas Power settings Weave pattern Travel speed Electrode extension Gun angle

22 Shielding Gas Shielding gas selected affects the weld
Method of metal transfer, speed, weld contour, etc. Also consider metal to be welded Commonly used shielding gases Argon Argon gas blends Helium Carbon dioxide Nitrogen

23 FIGURE 10-15 Effect of shielding gas on weld bead shape.
© Cengage Learning 2012

24 Power Settings Weld bead is affected by several factors
Voltage Amperage Welds require a balance of voltage and amperage Wire-feed speed affects amperage Increasing voltage changes arc length

25 Weave Pattern GMA welding process
Greatly affected by electrode tip and weld pool location Keep arc and electrode tip directed into molten weld pool Most of the SMAW weave pattern can be used for GMA welds

26 Travel Speed Location of arc inside molten weld pool is important
Speed cannot exceed ability of arc to melt base metal Too high: results in overrunning of weld pool Too low: can restrict fusion to base plate

27 Electrode Extension Distance from contact tube to arc
Measured along the wire Adjustments in this distance Cause a change in resulting wire bead GMA welding currents are relatively high Length of wire increases: voltage increases

28 FIGURE 10-16 Electrode-to-work distances. © Cengage Learning 2012

29 Gun Angle Relation of the gun to the work surface
Can be used to control the weld pool. Forehand/perpendicular/backhand welding Forehand technique: pushing the weld bead Backhand welding: dragging the weld bead Perpendicular: gun angle is at approximately 90° to work surface

30 Equipment Basic GMAW equipment Gun Electrode feed unit
Electrode supply Power source Shielded gas supply Control circuit Related hoses, liners, and cables

31 Power Source May be transformer-rectifier or generator type
Transformers Stationary Commonly require a three-phase power source. Engine generators Ideal for portable use or where sufficient power is not available

32 Electrode (Wire) Feed Unit
Push-type feed system Wire rollers clamped securely against the wire to push it through the conduit Rollers have smooth or knurled U-shaped or V-shaped grooves Soft wires are easily damaged by knurled rollers Difference between push-type and pull-type: size and location of drive rollers Electrode must have enough strength to be pushed through the conduit without kinking

33 Electrode (Wire) Feed Unit (cont’d.)
Pull-type feed system Smaller higher-speed motor located in the gun Wire moves through conduit Possible to move soft wire over great distances Gun is heavier and more difficult to use Rethreading wire takes more time Operating life of motor is shorter Not commonly used

34 Electrode (Wire) Feed Unit (cont’d.)
Push-pull-type feed system Synchronized system Motors located at both ends of electrode conduit Can be used to move any type of wire Ability to move wire over long distances Faster rethreading Increased motor life System is more expensive

35 Electrode (Wire) Feed Unit (cont’d.)
Linear electrode feed system Does not have gears or conventional-type rollers Uses a small motor with a hollow armature shaft Changing roller pinch changes speed at which the wire is moved Bulky system of gears is eliminated Motor operates at a constant high speed Wire may become twisted as it is moved through the feeder

36 Electrode (Wire) Feed Unit (cont’d.)
Spool gun Compact, self-contained system Allows welder to move freely Major control system is mounted on welder Feed rollers and motor are found in the gun Very soft wires can be used Small spools of wire required: often expensive Guns are small but feel heavy

37 Electrode (Wire) Feed Unit (cont’d.)
Electrode conduit Guides welding wire from feed rollers to the gun Power cable and gun switch circuit wires are contained in a conduit Steel conduit may have a nylon or Teflon liner Failure to attach conduit can cause misalignment

38 Electrode (Wire) Feed Unit (cont’d.)
Welding gun Attaches to end of power cable, electrode conduit, and shielded gas hose Trigger switch starts and stops weld cycle Contact tube transfers welding current to electrode Gas nozzle directs shielded gas onto weld

39 GMA Spot Welding GMAW spot weld Allows welds to be made:
Starts on one surface of one member and burns through the other Fusion occurs and small nugget is left on metal surface Allows welds to be made: In thin-to-thick materials When only one side of the materials to be welded is accessible When there is paint on the interfacing surfaces

40 Summary Keys to producing quality GMA welds
Equipment, setup, and adjustments Advantage of GMA welding process Ability to produce long, uninterrupted welds Selecting proper method of metal transfer Normally done by shop foreman or supervisor Welder must be proficient with each method of metal transfer Practice and develop skills

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