Gas Metal Arc Welding (GMAW)

Definition GMAW: A semi-automatic or automatic arc welding process in which a continuous and consumable wire electrode and a shielding gas are delivered to the weld through a welding gun.

GMAW Hazards Electrical shock Hot metal Harmful rays Hot spatter Fumes Shielding gas Pinch points How is the risk of these hazards managed?

Introduction GMAW has the same three requirements of all arc welding. How does GMAW meet these requirements? Shielding Heat Filler Material GMAW Shielding = gas from cylinder Heat = electric arc Filler material = consumable welding wire

GMAW Processes The GMAW process provides four different metal transfer methods. Short circuiting Spray Globular Pulse spray

Short Circuiting Transfer When the gun trigger is activated, the wire starts to move toward the metal. 1. When the wire touches the metal it shorts out. 5. The wire touches the metal and the process continues 2. The high current flow melts the end of the wire and a droplet is formed. 4. As the wire continues to move, the arc restarts. 3. The current pinches off the droplet and the arc stops. Wire speed and voltage must be set correctly for process to work correctly. Wire speed—voltage can be correct and the weld can be too hot or too cold. Process is recognizable by its sharp buzzing or crackling sound Most common method

Spray Transfer Requires high current density. What is current density? The current density of the spray process causes the wire to melt into small droplets (less than the diameter of the wire). The arc forces propels the droplets to the metal. What are the characteristics of the spray transfer method? Ratio of current to the diameter of the wire. The puddle is larger and more fluid. The high current density results in deeper penetration. High current results in high deposition rate. Process results in a hissing or whispering sound. Deep penetration. Why? Would the process work better for thin metal or thick metal? Is the spray transfer method recommended for all welding positions? Why, Why not?

Globular Transfer Low current density results erratic arc and globules of metal larger than the diameter of the wire. Minimum penetration. Excessive spatter Use is limited to submerged arc welding.

Pulse Spray Transfer In the pulse spray transfer the current alternates from the desired welding current to a background current. The background current just maintains the arc. The peak current, time of peak current, background current and time of background current are all adjustable. A = Peak Current B = Background Current C = Peak Time D = Pulses per Second Better control of welding parameters. Out of position welding thin metal What are the advantages of the metal transfer method?

Stickout An important function of all GMAW processes is the stickout. Stickout is the distance from the end of the contact tube to the weld surface. What is a potential problem of insufficient stickout? 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. What is a potential problem of Excessive stickout?

GMAW Equipment

Equipment Requirements Store and deliver gas to the weld at the correct rate. Store and deliver welding wire to the weld at the correct rate. Provide correct current. Automatically or semi automatically control all of the features.

G M A W C o m p o n e n t s What is the function of each part?

GMAW Equipment Power source Wire feeder and controls Welding gun and components Shielding gas delivery system Weld cleaning and finishing tools

GMAW System Parts Hand wheel Flow meter Wire spool Wire Feed roll tension Voltage meter Wire speed meter Wire speed adjustment Voltage adjustment Power switch Gun cable Gun Ground clamp Ground cable Feed rolls Wire guide Wire overrun Safety nut

GMAW Gun H F D B G C E A Trigger Liner Gas diffuser Gas orifice Contact tube Insulation ring Nozzle Welding wire

Flow meter What is the purpose of the flow meter? Two types of flow meters are available. One piece: the gas flow rate is regulated by using a fixed orifice and changing the pressure to change the flow rate. Two piece: the gas flow rate is adjusted by using a constant pressure regulator and changing the flow rate with an adjustable orifice (needle valve). GMAW systems use a flow meter to provide a constant flow rate (ft3/min) of shielding gas. Two piece. Adjustable orifice provides more precise control than adjustable pressure. Which type will provide better control of the gas flow? Why?

Welding Gun Function is to deliver the shielding gas, wire and welding current to the weld pool. MIG guns are rated on amperage capacity. The reason is heat. >400 amps are water cooled. Welding wire is delivered to the gun by the liner. Liner must sized to the welding wire. Small liner = excessive friction which may cause the wire to seize. Large liner = wire can buckle and spring forward causes erratic wire feed. Liner must be correct length. Gun end cutaway Cable Insulation ring Gas diffuser Contact tube Wire Nozzle

Welding Gun--cont. Contact tube Sliding electrical connection. Must match the diameter of the wire Must be replaced frequently The nozzle directs the shielding gas towards the weld pool. Nozzle Contact tube

Power Supply GMAW and FCAW both use a constant voltage power supply. Many different types are available. Industrial rate are usually 100% duty cycle. Non industrial power supplies have sloping duty cycle. May have a built in heat sensitive overload switch. Install power supply so that the cooling air through the machine is not restricted.

Selecting Power Supply Seven factors to consider when selecting a GMAW or FCAW power supply. Type of metal Metal thickness range Type of metal transfer Wire diameter range Primary power available Capacity of power source Control and adjustments

Wire Feeding systems Function is to deliver the welding wire at a constant rate to the gun. Three systems Push Pull Push--pull Two types Constant speed Voltage sensing Two types of construction Internal External

Wire Drive Systems The drive system is responsible for pulling the wire from the spool and then pushing the wire down the cable. The wire drive systems consisted of one or more pair of feed rolls and wire guides. The guides must be the proper size and adjusted close to the feed rolls and aligned with gap between the rolls.

Feed Rolls For a GMAW system to work correctly, the feed rolls must be the correct type and size. The feed roll tension must also be set correctly. U Groove V Groove Knurled V groove Cogged U groove

Consumables—Shielding Gases Carbon dioxide Requires special valve Must use washer between valve and flow meter. Argon Same valve as helium Mixture Same valve as argon

Consumables—Welding wire Solid (Copper clad) Flux core Stainless steel Hard surfacing Aluminum .030. .035, .045 What three wire diameters are most popular?

FCAW

Introduction FCAW (Flux Core Arc Welding) uses a hollow wire that is filled with flux. Same transfer methods as GMAW. Two types Externally shielded Intershielded

Self Shielded Shielding is provided by flux in the core of the wire. Produces a larger volume of fumes. Resistant to contamination caused by sidedrafts. Will have slag to remove. Yes, fresh air is being delivered to the welders breathing zone. This system can be used to deliver cooled air to the welder. Does this fume removal system meet safety codes?

Self Shielded--gun Does not require the use of a nozzle. What is this? GMAW gun FCAW gun What is this? No nozzle? What are the advantages of not using a nozzle? Heat shield _________________ Gun operates cooler Easier to see weld pool Less maintenance, spatter on nozzle _______________ Operator must maintain stickout. What are the disadvantages of not using a nozzle?

Questions