TIG can be used to weld copper, titanium, even two dissimilar metals, and is handy for making tricky welds (e.g. s-curves, or welds on round things)..
TIG generates heat via an arc of electricity jumping from a (tungsten metal) electrode to the metal surfaces you intend to weld - usually aluminum or steel.
TIG stands for Tungsten Inert Gas, after the tungsten electrode, and the sheath of inert gas (argon or an argon mixture) surrounding it.
Big thanks to Mose O'Griffin, who narrated, taught, and demonstrated.
Also, If you're interested in MIG welding, see this instructable:
Step 1: Choose the Electrode
For aluminum, the best choice is a pure tungsten rod.
You can alternately choose from any number of tungsten alloys (including thoriated tungsten - which is radioactive!) which are uniquely suited to welding particular alloys of metal.
For reference, this example uses the specific alloy 6061 Aluminum (the "steak and potatoes" or "normal" type of aluminum)
Step 2: Grind the Electrode
Do this, especially if the rod is a brand-new cylinder and doesn't have a pointed or rounded tip yet.
The tip will become rounded due to heat as you weld.
Lincoln recommends a balled tip for AC welding, and a pointed tip for DC welding.
The pointed tip will give a smaller, more directed arc. The arc will tend to dance around, when from a rounded tip.
Step 3: Insert the Electrode Into Its Collet
The tip of the electrode should be about 1/4" away from the protective sheath, but not much more.
Skip this if you already have your electrode ready.
Shown below are two different types of electrode holders.
Step 4: Choose the Settings
AC is for aluminum - choose "AC".
Set the "Cleaning/Penetrating" setting to the more "Penetrating" side - around 7 on a scale from 1 to 10.
Set the "Air on" to about 5 seconds, if you have the option to do so.
This is the amount of time the gas stays on after the arc stops, to keep your weld from oxidizing/rusting.
Set the "Max Amps" pretty high - perhaps around 250.
For specific settings, check out Miller's settings calculator.
For the curious:
Cleaning/Penetrating is a modification to the positive/negative ratio of the AC that changes the depth of your weld (cleaning is more shallow).
DCEP means "DC, Electrode Postive". This setting is used for balling the tungsten tip, or stick welding.
DCEN means "DC, Electrode Negative", and is for welding steel.
The difference between the two DC settings is the direction the electricity flows - to the metal from the electrode, or to the electrode from the metal. This makes a big difference in the amount of heat the metal absorbs, and the width and depth of the weld.
Step 5: Turn on the Gas
For steel, you would use an argon/carbon dioxide mixture.
The gas is important to keep the weld from becoming corroded, as metal will rust (or in the case of aluminum, oxidize) ridiculously quickly at the high temperatures the metal reaches.
Step 6: Prepare Metal and Welding Table
Beauty Tip: Use a wire brush to scrub the surfaces of the metal. It's good practice to keep a dedicated aluminum brush (separate from what you use to clean steel). If you want really nice-looking welds, you can also wipe down the welding rods with acetone.
If you don't care about the way the welds look when finished, don't bother and you'll be fine. However, your welds will be a little weaker and not as pretty.
Clamp your metals so they'll stay where you want them when you weld.
And, if you have the stuff, spray down your welding table with anti-spatter (so if any metal leaks off, it doesn't stick where it lands, which is important if you're trying to keep your surface flat).
Step 7: Get Dressed Up!
Use thick leather welding gloves, and a welding helmet, and closed-toe shoes.
To avoid sunburn, wear a long-sleeved shirt or jacket, or a welding coat.
Clean your helmet - the better you can see what you're doing, the better you can weld.
Some people like auto-darkening welding helmets. Others don't like relying on the device's reaction time - you have to invest a lot, to get a good-quality, fast one.
We're using always-dark helmets.
You might use a bright flashlight, if you want to see what you are doing without lifting your helmet.
Step 8: One Last Check
Step 9: Weld!
Jam down on the foot-pedal to quickly dump a bunch of current and heat into the metal.
The idea is to very quickly heat the metal and start the weld pool. You'll know it when you see it - the metal becomes fluid.
Start on an edge.
When the pool has formed, touch the rod in.
If you heat the metal for too long, it will warp. The longer the metal is heated, the more it will warp.
This is a "tack" weld, to hold the metal piece in place, so you can take off the clamps and do big long bead welds.
Once you have a weld pool started, you can ease off the pedal a little, to control the amount of heat and current being applied to the metal.
Welds shrink as they cool, so you can alternate sides to keep the welds even, and to keep one side of metal from getting too hot and warping. Hot metal can warp a great deal, and if you don't change sides, you might find yourself even 1/4" off of where the metal ought to be.
Step 10: Draw a Bead
Start a weld pool by jamming down the pedal fast to form a weld pool.
Then lighten up on the pedal, to regulate the current.
If the metal starts to burn/melt away, too much current is being dumped in, back off the foot pedal.
If the metal gets a flaky, but not liquid look to it, put more power in (step on the pedal harder).
To weld aluminum, move the electrode towards the rod, while feeding rod into the pool. The rod should be to the side of the electrode that the weld will form or grow towards. This is known as "leading" the electrode.
To weld steel, the electrode travels first down the weld line, and the rod chases after, "following" the heat.
Step 11: Types of Welds
The easiest type of weld is the "fillet", two metals jointed at right angles. (The one shown in this instructable).
The next is the "lap" weld, which is two metals resting flat against each other.
Trickier is the "butt" weld, where two metals touch along the edges - it is difficult to keep the electrode traveling in a straight line along the joint, and tough to keep the weld pool going well on both.
Corners require some skill, as the heat is not dissipated evenly.
You can create "cosmetic" or "strength" welds.
Cosmetic welds tend to look more even over long lengths. Create them by making a weld pool, dipping in the rod, and then moving to the next point. Use them on highly visible joints, like on bicycles.
Strength welds are a lot stronger - use them for things that aren't designed to be seen, or are designed for strength rather than beauty. Anything that needs to bear a load (e.g., a gas cannister or propane tank) will have a strength weld. These are the welds where you simply draw the weld pool along continuously, while constantly feeding rod in.