Transcript Chapter 10
Chapter 10 Gas Metal Arc Welding Equipment, Setup, and Operation © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning FIGURE 10-5 Axial spray metal transfer. Note the pinch effect of filler wire and the symmetrical metal transfer column. Larry Jeffus © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning Pulsed-Arc Metal Transfer Current Cycle FIGURE 10-7 Mechanism of pulsed-arc spray transfer at a low average current. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning FIGURE 10-15 Effect of shielding gas on weld bead shape. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning Power Settings • Weld bead is affected by several factors – Power settings – Voltage – Amperage • Welds require a balance of voltage and amperage – Wire-feed speed affects amperage – Increasing voltage changes arc length © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning FIGURE 10-16 Electrode-to-work distances. © Cengage Learning 2012 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning Equipment • Basic GMAW equipment – – – – – – – Gun Electrode feed unit Electrode supply Power source Shielded gas supply Control circuit Related hoses, liners, and cables © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 Vshaped 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning GMA Spot Welding • GMAW spot weld – 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 © 2012 Delmar, Cengage Learning 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 © 2012 Delmar, Cengage Learning