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