GMAW FundamentalsGas Metal Arc Welding Metal Inert Gas
Safety Electrocution hazard� Skin burns from flying metal� Skin burns from direct light from arc� Skin burns from indirect light from arc� Cotton clothing and leather gloves� Helmet to protect eyes from light� Safety glasses when chipping slag �� Ventilation to remove dangerous fumes� Do not weld near water� Do not weld near combustible materials� Keep welding cables and jobs free grease� Protect bystanders from arc rays
Introduction GMAW is defined as arc welding using a continuously fed consumable electrode and a shielding gas. GMAW is also known as MIG (Metal Inert Gas). Produces high-quality welds Yields high productivity
Advantages Large gaps filled or bridged easily Welding can be done in all positions No slag removal required High welding speeds High weld quality Less distortion of work piece
Disadvantages Hard to reach locations are less easily welded because of bulky torch and cables Wind or air drafts may compromise gas shielding Reactive metals (i.e. titanium) may need special shielding provisions High heat may be uncomfortable to welders Correct parameter selection learning needs dedicated training Equipment is more complex and expensive than that of alternative processes
Several tips must be consider in selecting mode of transfer Type, intensity and polarity of welding current Electrode size Electrode composition Electrode extension Shielding gas mix composition
Types of Metal TransferThe basic GMAW process includes three distinctive process techniques:1. Short Circuit (Short Arc)2. Globular Transfer3. Spray Arc Transfer
Modes of GMAW Transfer
Short Circuit (Short Arc) Operates at low voltages and welding current Small fast-freezing weld puddle obtained Useful in joining thin materials in any position, as well as thick materials in vertical and overhead positions Metal transfer occurs when an electrical short circuit is established this cycle can repeat itself between 20 and as much as 250 times per second.
Short Circuit A - Electrode is short circuited to base metal. No arc, and current is flowing through electrode wire and base metal. B - Resistance increases in electrode wire causing it to heat, melt and “neck down”. C - Electrode wire separates from weld puddle, creating an arc. Small portion of electrode wire is deposited which forms a weld puddle. D - Arc length and load voltage are at maximum. Heat of arc is flattening the puddle and increasing the diameter tip of electrode. E - Wire feed speed overcomes heat of arc and wire approaches base metal again. F - Arc is off and the short circuit cycle starts again.
Advantages All-position capability, including flat, horizontal, vertical-up, vertical-down 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(s).
Globular Transfer Welding current and wire speed are increased above maximum for short arc Droplets of metal have a greater diameter than the wire being used Spatter present Welding is most effectively done in the flat position when using globular transfer
Globular transfer is often ahigh voltage, highamperage, high wire feedspeed transfer, and is theresult of using CO2shielding gas (or 75% AR-25% CO2) with parametershigher than the short-circuiting range
Advantages Uses inexpensive CO2 shielding gas, but is frequently used with argon/CO2 blends. Is capable of making welds at very high travel speeds. Inexpensive solid or metal-cored electrodes. Welding equipment is inexpensive.
Limitations Higher spatter levels result in costly cleanup. 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%. Less desirable weld appearance than spray arc transfer Welding is limited to flat positions and horizontally fillet welds Welding is limited to metal 1/8 inch (3 mm) or thicker
Spray Arc Transfer Occurs when the current and voltage settings are increased higher than that used for Globular Transfer Used on thick sections of base material, best suited for flat position due to large weld puddle Spatter is minimal to none Uses 5% to 10% co2 mix with argon or oxygen. >Forms very small droplets of metal >Very good stability >Very little spatter
Spray arc transfer “sprays” a streamof tiny molten droplets across thearc, from the electrode wire to thebase metal.Spray arc transfer uses relativelyhigh voltage, wire feed speed andamperage values, compared to shortcircuit transfer.
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 semiautomatic, robotic, and hard automation 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% CO2.
Pulse Spray Transfer GMAW-P was developed for two demanding reasons: control of weld spatter and the elimination of incomplete fusion defects common to globular and short-circuiting transfer. The welding current alternates between a peak current and a lower background current. This faster-freezing weld puddle is what allows the pulsed- spray transfer to be used fort thinner metals, better control on out-of-position work. allows for larger wire sizes to be used on varied metal thicknesses.
In pulse spray transfer (GMAW-P) thewelding power source’s pulse controlpulses the welding output withhigh peak currents (amperage) which areset at levels which will cause the transferto go into a spray. The backgroundcurrent (amperage) is set at a level thatwill maintain the arc,but is too low for any metal transfer tooccur.
Pulsed arc transfer
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 and offers an engineered solution for the control of weld fume generation. Reduced levels of heat induced distortion and tendency for arc blow Ability to weld out-of-position and 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. Is combined for use with Tandem GMAW or other multiple arc scenarios. Capable of arc travel speeds greater than 50 inches per minute (1.2 M/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.
Manual GMAW Equipment Threemajor elements are : 1.) Welding torch and accessories 2.) Welding control & Wire feed motor 3.) Power Source GMAW equipment can be used either manually or automatically
GMAW Schematic Diagram
WIRE CONTROL & WIRE FEED MOTORPOWER SOURCE
Welding Torch & Accessories The welding torch guides the wire and shielding gas to the weld zone. Brings welding power to the wire also Major components/parts of the torch are the contact tip, shielding gas nozzle, gas diffuser, and the wire conduit
Others types of torch
GAS DIFFUSER NOZZLETRIGGER CONTACT TIPINSTALLEDCOMPONENTS
Welding Control & Wire Feed Motor Welding control & Wire feed motor are combined into one unit Main function is to pull the wire from the spool and feed it to the arc Controls wire feed speed and regulates the starting and stopping of wire feed Wire feed speed controls Amperage
Types of Wire Feed Motor
Types of WFM Roller
Types of Wire Feeder
Power Source Almost all GMAW is done with reverse polarity also known as DCEP Positive (+) lead is connected to the torch Negative (-) lead is connected to the work piece Provides a relatively consistent voltage to the arc Arc Voltage is the voltage between the end of the wire and the work piece
Contact Tip To Work DistanceIn constant current, the CTWD (contact tip to work distance) determines the arc length.As the CTWD increases the arc length increases, and as theCTWD decreases the arc length decreases. This presented aproblem for semiautomatic welding because it is difficult tomaintain the same CTWD. To compensate for this problem an arc voltage controlled wire
Constant voltage power source designs provide a specificarc voltage for a given pre-selected wire feed speed Thevolt-amp curve, or slope, is comparatively flat. As theCTWD increases with these types of power sources, thereis a decrease in the welding current. As the CTWDdecreases there is an increase in the welding current. Thearc in this case becomes a series circuit, and the CTWDprovides resistance to current. In either scenario, thevoltage remains the same and the arc length remains thesame
POSITIVE NEGATIVETERMINAL TERMINAL
Shielding Gases Purpose of shielding gas is the protect the weld area from the contaminants in the atmosphere Gas can be Inert, Reactive, or Mixtures of both Gas flow rate is between 25-35 CFH Argon, Helium, and Carbon Dioxide are the main three gases used in GMAW
Properties of Gases Affect the performance of the welding process include: 1) Thermal properties at elevated temperatures. 2) Chemical reaction of the gas with the various elements in the base plate and welding wire. 3) Effect of each gas on the mode of metal transfer.
Types of shielding gases InertGas 1. Argon 2. Helium 3. Ar + He Active Gas 1. Carbon Dioxide 2. Inert gas + any type of active gas
Argon is an inert gas which is used bothsingularly and in combination with othergases to achieve desired arc characteristicsfor the welding of both ferrous and non-ferrous metals.Carbon Dioxide Pure carbon dioxide is not aninert gas, because the heat of the arc breaks downthe CO2 into carbon monoxide and free oxygen.This oxygen will combine with elementstransferring across the arc to form oxides which arereleased from the weld puddle in the form of slagand scale.
Helium is an inert gas which is used onweld applications requiring higher heatinput for improved bead wetting, deeperpenetration and higher travel speed. InGMAW it does not produce as stable an arcas argon. Compared to argon, helium has ahigher thermal conductivity and voltagegradient
FLOW METER CFH PRESSURE ADJUSTMENT KNOB CYLCINDER PRESSURE GAUGE