MIG Welding (GMAW)
Set up a MIG machine, choose wire and shielding gas, run a sound bead, and fix feeding and arc problems.
Table of contents
MIG welding feeds a solid wire through a gun while a shielding gas protects the weld. Its formal name is gas metal arc welding (GMAW). It is the most common process in production shops because you can weld for a long time without stopping, it leaves no slag on solid wire, and it is quick to learn. Good MIG welds still take real skill, because a weld can look fine and not be fused.
What You Will Be Able to Do
- Name the parts of a MIG setup and set drive roll tension.
- Choose a wire and shielding gas for the metal.
- Set voltage and wire feed speed, and tune them by sound.
- Choose a transfer mode, and run a bead with the right angles.
- Fix common wire feeding and arc problems.
How the Equipment Works
| Part | What it does |
|---|---|
| Power source | Holds a constant voltage (CV), usually DCEP |
| Wire feeder | Pushes wire from the spool with drive rolls |
| Liner | Guides the wire through the gun cable |
| Contact tip | Passes welding current into the wire |
| Nozzle | Directs gas around the arc |
| Regulator and flowmeter | Set the gas flow |
DCEP means direct current, electrode positive. Solid MIG wire runs on DCEP. If a weld looks awful on a new machine, check the polarity first.
Setting Up the Wire
- Match the drive rolls, liner and contact tip to the wire diameter.
- Use V-groove rolls for steel wire, and U-groove rolls for soft aluminum wire.
- Thread the wire and close the rolls.
- Set tension just tight enough for smooth feeding without slipping or deforming the wire. To test, feed the wire against a block of wood: the wire should bend and coil without the rolls slipping, but the rolls should slip if the wire is stopped at the tip.
- Trim the wire to about 3/8 to 1/2 inch stickout.
Too much tension crushes the wire and grinds it into the liner. Too little makes it stutter.
Warning: Keep the gun away from your face and hands when you feed wire. The wire is live when the trigger is pressed, and it comes out fast and sharp.
Choosing Wire
| Wire | Used on | Notes |
|---|---|---|
| ER70S-6 | Carbon steel | Most common, handles light scale |
| ER70S-3 | Clean carbon steel | Fewer deoxidizers |
| ER308L | 304 stainless | L means low carbon |
| ER309L | Stainless to carbon steel | Used for mixed metals |
| ER4043 | Most aluminum alloys | General purpose |
| ER5356 | 5000-series aluminum | Higher strength, better anodize match |
In ER70S-6, ER means it can be used as an electrode or a rod, 70 is the minimum tensile strength of 70,000 psi, S means solid, and 6 refers to the chemistry. Deoxidizers such as silicon and manganese help clean the puddle.
| Diameter | Typical use |
|---|---|
| 0.023 inch | Thin sheet |
| 0.030 inch | Light fabrication |
| 0.035 inch | General shop work, the most common size |
| 0.045 inch | Heavy fabrication, 1/4 inch and up |
Choosing a Shielding Gas
Shielding gas keeps air out of the weld pool. Air contains oxygen and nitrogen, which cause porosity and brittle welds.
| Gas | Used for | Result |
|---|---|---|
| 75% argon, 25% CO2 (C25) | Carbon steel, short circuit | The everyday standard, good bead, low spatter |
| 90% argon, 10% CO2 | Carbon steel, spray | Smoother arc, less spatter |
| 100% CO2 | Carbon steel | Deep penetration, low cost, more spatter |
| 100% argon | Aluminum | Required, never use it on steel by itself |
| Argon with 2% CO2 or oxygen | Stainless, spray | Stable arc |
| Helium, argon and CO2 tri-mix | Stainless, short circuit | Good wetting, low carbon pickup |
Set the flow by the flowmeter. Most indoor work runs about 20 to 35 cubic feet an hour (CFH). Too little flow lets air in, and too much creates turbulence that pulls air in. If you are in a draft, block it, because raising the flow will not fix wind.
Warning: Argon and CO2 can displace oxygen in pits, tanks and small rooms. Gas can build up with no smell and no warning. Keep shielding gas from collecting anywhere people work.
Understanding Settings
MIG has two main controls.
- Voltage sets the arc length. Higher voltage means a longer arc and a wider, flatter bead.
- Wire feed speed (WFS) sets the amperage. Faster wire means more amps, more heat and more penetration.
These work as a pair. Start with the chart inside the machine door or the wire maker's chart, then fine-tune.
Worked example: For 0.035 inch wire on 1/4 inch carbon steel with C25, start at 21 to 22 volts and 280 to 320 inches per minute (IPM). Run a test bead on scrap of the same thickness. Then listen and look.
Tuning by Sound
A well-tuned short-circuit arc makes a steady crackle, like bacon frying.
| What you hear or see | Likely problem | Change |
|---|---|---|
| Harsh, crackling arc, lots of spatter | Voltage too low for the wire speed | Raise the voltage |
| Wire stubs and pushes the gun back | Wire speed too high or voltage too low | Slow the wire or raise the voltage |
| Long, soft, wandering arc with big drops | Voltage too high or wire speed too low | Lower the voltage or raise wire speed |
Change one setting at a time.
Transfer Modes
Transfer mode is how the metal crosses the arc.
| Mode | Setting | Use |
|---|---|---|
| Short circuit | Low voltage and amps | Thin metal, all positions, root passes |
| Globular | Mid-range | Rarely wanted, high spatter |
| Spray | High voltage and amps, needs argon-rich gas | Thick metal, flat and horizontal only |
| Pulsed spray | Pulsing machine | All positions, less heat, little spatter |
Short circuit can cause lack of fusion on thick metal, because the puddle runs cool. Some codes limit or exclude it for structural work.
Running a Bead
- Get comfortable. Brace your arms and support your gun hand. Make sure you can reach the whole joint.
- Set the angle. Tilt the gun about 10 to 15 degrees from straight up along the line of travel. On a tee joint, aim at the corner with a 45 degree work angle.
- Choose direction. Pushing the gun away from the finished weld gives a flatter bead and better gas coverage. Dragging it toward the finished weld gives a narrower bead with a little more penetration. Most solid-wire work can go either way, but aluminum should always be pushed.
- Hold stickout. Keep 3/8 to 1/2 inch of wire between the tip and the work. Too much stickout cools the arc.
- Start on the joint. Squeeze the trigger and let the puddle form.
- Watch the puddle. Keep the arc on its leading edge. If the puddle gets ahead of you, the weld won't fuse. Speed up.
- Weave only if you need to. A slight side-to-side motion with a pause at each toe helps fill a joint. Wide weaves cause undercut and lack of fusion.
- Fill the crater. Pause briefly at the end, or use the crater setting if your machine has one. An unfilled crater can crack.
Welding Aluminum With MIG
Aluminum wire is soft, and it conducts heat quickly. It behaves very differently from steel:
- Use 100 percent argon.
- Use a spool gun, a push-pull gun or a plastic-lined gun, with U-groove rolls.
- Always push, never drag.
- Travel faster than on steel, and watch for heat building up.
- Clean with solvent, then a stainless wire brush kept only for aluminum, and weld soon after.
Troubleshooting
| Problem | Likely cause | First check |
|---|---|---|
| Porosity | Lost gas or dirty metal | Flow, leaks, drafts, nozzle, cleaning |
| Birdnesting at the drive rolls | Blocked liner or tip, too much tension | Tip size, liner, tension |
| Wire burns back to the tip | Stickout too short, or wire speed too low | Stickout, burnback setting |
| Stuttering wire feed | Worn tip, dirty liner | Replace the tip, blow out or replace the liner |
| Erratic arc | Worn tip or poor work connection | Tip, work clamp on clean metal |
| Excess spatter | Voltage too low, dirty metal | Voltage, cleaning, gas |
| Cold, ropy bead | Too little heat or too fast | Wire speed, travel speed |
Check the contact tip early. It is cheap, it wears fast, and an oval hole causes many arc and feed problems. Keep spatter out of the nozzle, because a clogged nozzle chokes your gas.
Common Mistakes
- Using too long a stickout. It cools the arc and costs you fusion.
- Blasting gas to fix a draft. It pulls in air. Block the wind instead.
- Running 100 percent argon on steel. You get a poor bead shape and poor fusion.
- Going by bead looks on thick steel. A short-circuit bead can sit on top without fusing.
- Ignoring the drive rolls. Crushed wire jams the liner.
Check Your Understanding
Your MIG weld is spattering heavily, and the arc sounds harsh and crackly. The metal is clean. What do you try first?
Raise the voltage in small steps. That sound usually means the voltage is too low for the wire speed.
You switch from steel to aluminum on the same machine. What must you change?
Change to 100 percent argon, aluminum wire, U-groove drive rolls, and a suitable liner or spool gun. Push instead of drag, and clean the metal with a stainless brush.
Your wire keeps tangling at the drive rolls. What are the first things to check?
Check for a clogged or wrong-size contact tip, a blocked liner, and too much drive roll tension. Wire stops at the far end, then bunches up at the rolls.
You run a bead on 1/2 inch plate and it looks nice, but a bend test shows it did not fuse. What might have gone wrong?
Short-circuit settings may have been too cold for the thickness, or the puddle may have run ahead of the arc. Raise the heat or use spray transfer, and keep the arc on the leading edge.
Key Takeaways
- Solid MIG wire runs DCEP on a constant-voltage machine.
- C25 is the everyday gas for carbon steel. Aluminum needs 100 percent argon.
- Voltage sets arc length, and wire feed speed sets amps.
- Set drive roll tension just tight enough for smooth feeding.
- Keep your stickout short, watch the puddle, and fill the crater.
- Check the contact tip first when the arc or feed acts up.