GMAW DEFINED. GMAW (also referred to as Metallic Inert Gas MIG). A continuous consumable wire fed automatically which may or may not be covered by an inert shielding gas. Deep penetration is provided by the use of DCRP for most solid wires and the concentration of heat in a small weld zone..
1. GMAW or MIG GAS METAL ARC WELDING or METAL INERT GAS
2. GMAW DEFINED GMAW (also referred to as Metallic Inert Gas – MIG). A continuous consumable wire fed automatically which may or may not be covered by an inert shielding gas. Deep penetration is provided by the use of DCRP for most solid wires and the concentration of heat in a small weld zone.
3. Types of Metal Transfer Short circuiting/ Short Arc/ Dip Transfer. The arc is short circuited 20 to 200 times per second. Average of 100 for light welding. The electrode (wire) touches the weld puddle creating a short circuit causing the wire to heat and melt off. This happens with solid wire only.
4. Types of Metal Transfer (cont.) Spray Arc-Usually done on thicker metal using larger diameter wires and higher amperage (over 200 amps). A shielding gas mixture of argon with 1 to 5 % oxygen is desireable. Out of position welding is not recommended. Used with solid wire welding.
5. Types of Metal Transfer (cont.) Globular transfer- similar to the short arc process but higher amperages are required and out of position welding is not possible. Used on heavier metals where large deposits are required.
6. Shielding Gases There are six true inert gasses. These are gasses that will not break down during the welding process nor will they chemically react with the molten metal.
7. Six inert gasses Argon, Helium, Krypton, Neon, Xenon and Radon are the true inert gases. The last four are rarely used due to cost and availability. Carbon dioxide is inert at room temperature but changes during the welding process. It then goes back to it’s original state once it starts to leave the weld zone. It does an adaquate job on mild steel.
8. Puposes of shielding gases To displace atmospheric gases such as oxygen, hydrogen and nitrogen.
Oxygen produces undesirable “oxides” which leads to porosity.
Nitrogen produces “nitrides” which cause low ductility and fractures.
Hydrogen causes internal pressures and stress risers also known as under bead cracking.
9. Puposes of shielding gases To help form the weld allowing for out of position welding.
This is much like the shielding process in arc welding. The gasses will help the weld to form correctly and to be free of slag.
10. Shielding gases and their uses. Argon – Aluminum and magnesium
Argon/Helium – Heavy section of Aluminum
Argon/ CO2 – Mild and stainless steel
Helium/Argon/CO2 Mild and stainless
CO2 – Mild steel
Argon/Oxygen – Mild steel, stainless and alloy steels
11. Advantages of the GMAW process: Shorter training time. Easier to learn than most welding processes.
Saving of welding time. No stopping and starting as in SMAW.
Narrower heat affected zone. Less warpage in light gage metal.
No slag and a minimum of splatter.
12. Advantages of the GMAW process (cont.) High deposition rate. 95% of the wire is used in the process.
Fewer impurities in the weld.
Weld a wide range of metal thickness’s with one diameter wire.
Excellent for bridging gaps and misaligned fit ups.
13. Disadvantages of the GMAW process. Metal must be clean to weld on. Solid wire welding provides very little cleaning. Flux cored wire offers some but not a lot.
Prone to wind drafts. Winds may blow the shielding gas out of the zone.
14. Problems /Concerns with GMAW Porosity – An inadaquate gas coverage will cause holes in the weld.
Cold lapping – No fussion on edges (toe) of weld. Caused by wrong set up.
Birdnesting – When the tension for the feed roller is too tight and wire continues to feed internally when the contact tube is closed off.
Bridging – When splatter bridges between the contact tube and the nozzle.
15. GMAW welding wires: Solid wire – comes 0.23” to5/32” in diameter. ER70S (1-6). The higher the last number the more silica in it.
Flux cored – can used with Dual Shield Welding or without gas. Wite is very expensive and the slag must be cleaned off before welding over it.
The wire you use will depend on the metal thickness, position, joint design and specifications for the job.
16. SAFETY Use fully protective clothing; helmet, jacket and gloves.
Weld in a shielded area to prevent “Flashes” to others.
Always say “cover” before starting to weld.
Provide for proper ventilation. Don not stand over the flume (exhaust).
17. SAFETY (cont.) Use a 10-12 gold lense to protect your eyes.
Do not get your gloves wet as you will not be insulted from shock.
Do not walk on any welding leads, especially the GMAW lead.
Do not weld on any metal that is coated (galvanized, zinc, paints, etc.
18. Setting up the GMAW equipment. Visual check of all equipment. Look for damage to the regulator, leads, contact tube, nozzle, etc.
Open the cylinders all the way SLOWLY as inert cylinders are high pressure just like the Oxygen cylinders.
19. Set up cont. Set the regulator to 18 to 25 cfh. Argon requires less pressure because of it’s wieght. Make sure wire feed roller is disengaged as to not waste welding wire.
Use the chart provided to set the WFS and the voltage. If one is not provided, you will have to set up the welder using the trial and error method.
20. Set up cont. Make sure metal is clean from oils and mill scale and work area is free from combustibles.
You should not exceed 19 volts for the work we do in the learning process. Spray arc uses 21 volts or more.
Should sound smooth and consistent. No popping or snapping. Sound is a key factor in setting up the GMAW equipment.
Keep the contact tube and nozzle clean and free from splatter. Use dip gel while the contact tube are hot.
21. Steps to shut down Turn off the gas source.
Release the wire feed roller.
Pull the trigger and relieve the regulator.
Undial the regulator.
Clean up the area, roll up leads (in big loops) cool all metal
22. Setting the WFS and voltage The wire feed speed also controls the amperage. When determining the WFS and voltage, only change one parameter at a time. Example:
Set the voltage low (14-15) and turn the WFS down. As you weld, you or someone else can adjust the WFS up until is sound smooth and consistent. Look at the weld, if it is too tall and lacks penetration then turn the voltage up one volt and turn the WFS down.
23. Setting the WFS and voltage cont. Start welding and turn up the WFS until it sounds as it should. Repeat this process until you have the desired weld qualities ( slightly convex with excellent tie-in on the toe of the weld. Experience will help to determine setting and how to indentify quality welds.