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At the end of a TIG weld, simply releasing the torch trigger can drop the current too quickly. The molten puddle contracts, pulls away from the weld edges, and leaves a crater. On aluminum and other crack-sensitive alloys, that crater can become a starting point for a visible crack or leak.
The cure is to set a controlled down-slope: a timed reduction in welding current that gives you a few seconds to fill the puddle before the arc stops. Down-slope is useful, but it cannot fix poor torch movement, insufficient filler, or a welding machine that is set up incorrectly.
What down-slope does
Down-slope reduces amperage gradually after you release the torch switch or remote control. Instead of going from full current to zero instantly, the machine tapers the current over a selected period, commonly between 1 and 10 seconds.
During that taper, keep the torch over the end of the weld and add filler metal as the puddle shrinks. The filler replaces the metal that would otherwise pull inward and form a depression. You should finish with a slightly crowned surface, not a deep dimple.
Down-slope is different from post-flow. Post-flow keeps shielding gas over the hot tungsten and weld after the arc goes out; it does not fill the crater. A practical TIG setup needs both functions.
Set a practical starting time
For most manual TIG work, start with a down-slope of 2 to 4 seconds. That is long enough to fill a normal termination without forcing you to hold the torch in place for an awkward amount of time.
| Material and joint | Typical starting down-slope | What to watch for |
|---|---|---|
| Thin stainless, 0.040–0.080 inch | 1–2 seconds | Excess heat and a widened end crater |
| Mild steel, 1/8 inch | 2–3 seconds | Whether the final filler addition wets both edges |
| Aluminum, 1/8–1/4 inch | 3–5 seconds | Cracking, oxide contamination, and a collapsing puddle |
| Heavy or highly restrained joints | 4–8 seconds | Overheating and a long, soft termination |
These are starting points, not universal settings. A small lap joint may need less time than a thick butt joint, even at the same amperage. If your welder has separate downslope settings for a torch switch and a foot pedal, set the one used by your control method.
Use the correct hand movement
As you approach the planned stopping point, shorten the arc slightly and slow your travel speed. Keep the tungsten centered over the puddle. Start adding small amounts of filler during the current taper, rather than waiting until the puddle has almost disappeared.
For a typical 1/8-inch steel weld running at roughly 100 to 130 amps, one or two small filler additions during a 2- to 3-second downslope are often enough. Do not dump a large blob into the puddle. Excess filler can create a high lump, trap shielding gas, or freeze before it bonds to both sides of the joint.
When the crater is filled, hold the torch still until the arc extinguishes. Continue shielding gas for at least 6 to 10 seconds on stainless steel and aluminum, and longer if the tungsten remains visibly hot. Pulling the torch away immediately can discolor the weld and contaminate the electrode.
Torch switch versus foot pedal
A two- or four-step torch control can make crater filling easier. In two-step mode, pressing the switch starts the arc and releasing it begins downslope. In four-step mode, you can press to start, release to weld, and press again to begin the programmed taper. The exact sequence varies by welder, so verify it in the manual.
A foot pedal or fingertip amperage control gives more direct control. You can manually reduce current while adding filler, then release the control after the crater is full. This is often better for changing joint thickness or awkward positions, but it takes practice and costs more.
If you are buying a machine, compare the available control features rather than focusing only on maximum amperage. A basic AC/DC TIG welder with downslope control is enough for many repair and fabrication jobs. A programmable four-step sequence or remote amperage input is worth paying for if you weld frequently.
Common causes of crater defects
Down-slope set too short: The current falls faster than you can add filler. Increase the time by one second and practice keeping the torch steady.
Down-slope set too long: The weld spreads wider and overheats the joint end. Reduce the time or travel farther while the current is tapering. A long taper is not automatically better.
No filler at termination: Current reduction alone does not replace lost metal. Add filler until the puddle is level with, or slightly above, the surrounding bead.
Stopping over the edge: Ending directly at a corner or at the edge of a plate encourages the puddle to run away. If possible, weld slightly past the joint, fill the crater there, and remove the run-off tab afterward.
Contaminated tungsten or shielding: A dirty electrode, excessive arc length, or drafts can make the puddle unstable during downslope. Use a clean gas lens or cup, keep the arc around 1/16 inch on thin work, and protect the weld from airflow.
Wrong AC settings: On aluminum, excessive cleaning action can make the puddle wide and fluid, while insufficient cleaning can leave oxide in the crater. Adjust AC balance and frequency according to the machine and material rather than trying to solve every defect with downslope.
Equipment choices and a simple test
A foot pedal is a useful upgrade when you regularly weld aluminum, thin sheet, or joints with changing heat requirements. A TIG welding foot pedal is not automatically universal, so check the connector and compatibility with your welder before buying.
For repeatable work, use a machine with adjustable pre-flow, downslope, and post-flow. An inexpensive welder is fine if it has stable arc control and the settings you need; paying for high amperage will not improve crater filling by itself. A TIG welding practice coupon kit can be useful, but clean scrap of the same alloy works just as well.
Practice on identical scrap at your intended amperage. Mark a stopping point, weld to it, start downslope, add filler in small increments, and inspect the termination after cleaning. If there is a depression, increase filler or slow the taper. If the end is wide and overheated, shorten downslope or move slightly faster. A sound termination has smooth edges, no pinhole, and no visible radial crack.