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A crater is the small depression left when a weld pool freezes without enough filler metal to replace the metal that contracts as it cools. On a MIG weld, it may look like a dimple at the end of the bead, but it can also contain a crack that spreads into the weld or base metal. The fix is not simply “weld longer.” You need to control the start, maintain the arc long enough for the joint to fill, and finish with a deliberate taper.
Why MIG Weld Craters Form
When you release the gun trigger, the arc stops immediately but the molten pool is still shrinking. If the pool is large and you remove the arc at once, the center sinks. High heat, a slow travel speed, excessive wire feed, and stopping abruptly all make the depression worse.
Starts can have the opposite problem. A cold start may leave a lump of unmelted wire, poor fusion, or a shallow section at the beginning of the weld. Dirty steel, an excessively long stickout, and poor grounding make starts less consistent. On thin sheet, lingering to fill a crater can burn through the work, so crater control must be matched to material thickness.
Prepare the Joint Before Striking the Arc
Clean the weld area to bright metal. Remove paint, mill scale, rust, oil, and zinc at least 1 inch on either side of the joint. Use a dedicated stainless brush for stainless steel; a brush previously used on carbon steel can contaminate it. Clamp the parts tightly and place the work clamp on clean metal close to the joint. A weak or distant ground can cause arc instability that looks like a technique problem.
For a butt joint, leave a consistent gap rather than trying to bridge an accidental wide opening. On light-gauge steel, a small gap can improve penetration but may require short stitch welds. On thicker material, beveling and multiple passes may be necessary. Do not expect a single crater-control trick to compensate for an underprepared joint.
How to Start a MIG Weld Cleanly
Set the machine for the wire diameter, material, shielding gas, and thickness. Use the manufacturer’s voltage and wire-speed chart as a starting point, then test on scrap of the same thickness. A stable arc should produce a consistent frying or sizzling sound, not violent popping and long pauses.
- Position the gun so the contact tip is about 3/8 inch from the work for common solid-wire short-circuit MIG. Keep the stickout consistent.
- Use a work angle near 90 degrees across the joint and a travel angle of roughly 5 to 15 degrees in the direction of travel.
- Begin moving slightly before or as you press the trigger. This prevents a large blob from forming at the starting point.
- Pause briefly—often about 0.25 to 0.5 second—at the start if the joint needs tie-in, then travel at a steady speed.
For a fillet weld, aim the arc at the root rather than at one leg. Starting on a small run-on tab is even better when appearance and consistency matter. The weld begins on the tab, reaches full size before entering the joint, and ends on another tab. Cut the tabs off afterward. This is common fabrication practice, but unnecessary for many brackets and repairs.
How to Stop Without Leaving a Crater
The simplest method is to keep the gun moving while reducing the amount of heat and filler deposited at the end. As you approach the stopping point, shorten the final movement or make a small backstep into the previous bead, then release the trigger while the pool is nearly filled. Do not stop travel first and then hold the arc in place; that usually creates an oversized molten pool and a deeper depression.
For a short weld, use one of these methods:
- Backstep: Weld to the end, move back about 1/4 inch over the hot bead, and release the trigger while still moving. This fills the termination and blends it into the bead.
- Small weave or loop: Make a tight, controlled loop at the end to add filler without dwelling in one spot. This is useful on medium-thickness steel, but risky on thin sheet.
- Trigger feathering: If the machine has crater-fill or burnback controls, reduce current or wire feed for a fraction of a second before the arc stops. Use a short test bead first; excessive reduction can leave a cold, unfused end.
- Overlap: On a long weld, restart about 1/2 inch behind the previous end, weld through it, and continue forward. This is more reliable than trying to make one uninterrupted bead.
Never pull the gun away sharply while the wire is still burning into the pool. Keep the nozzle positioned over the termination until the arc has stopped, then hold the gun still for a moment to let shielding gas protect the cooling weld. A post-flow period is more important with aluminum or stainless steel than with ordinary mild steel, but it does not fill a crater by itself.
Choose the Ending Technique for the Material
| Material or situation | Useful stopping method | Main risk |
|---|---|---|
| Thin sheet, about 18 to 22 gauge | Short stitch welds, slight backstep, and quick trigger release | Burn-through from dwelling to fill the crater |
| 1/8-inch mild steel | Steady travel with a short backstep or small loop | Cold overlap if the restart is too far behind |
| Thicker plate or multi-pass work | Fill the end, grind if needed, then overlap the next pass | Crater crack or lack of fusion at a restart |
| Aluminum MIG | Use machine crater-fill settings or a controlled taper | Hot, wide pool and burn-through; contamination also causes porosity |
Machine Settings and Equipment That Help
A welder with adjustable inductance, burnback, and crater-fill controls gives you more ways to fine-tune starts and stops. Crater-fill controls are helpful for production work, but they are not essential for occasional repairs. A basic MIG machine set correctly on clean steel can produce sound terminations with a backstep and overlap.
If you are buying equipment, compare MIG welders with crater-fill control when you regularly weld aluminum, thin sheet, or repeated production parts. For general home fabrication, spend first on a stable machine with a useful wire-speed range and a good duty cycle rather than paying extra for features you will not use.
Use the correct shielding gas and flow rate. For mild steel with solid wire, a common starting point is 75 percent argon and 25 percent carbon dioxide at roughly 20 to 25 cubic feet per hour indoors. Excessive flow can create turbulence and draw air into the weld. Outdoors, wind can destroy shielding; use a screen or change processes rather than simply turning the flow higher.
Keep a proper auto-darkening welding helmet, gloves, flame-resistant clothing, and ventilation in place. Crater control often requires watching the puddle closely, but never lower your helmet or remove protection to inspect a weld while it is hot.
Inspect and Repair the Finished Weld
After the weld cools, clean off spatter and inspect the termination under good light. A sound end should be blended into the bead without a sharp notch, visible crack, pinhole, or deep dish. A shallow cosmetic depression may be acceptable on a noncritical joint, but a structural part deserves a repair.
Grind out a cracked crater to sound metal, feather the edges, and reweld with a short overlap. Do not merely place a small blob over the defect; that can trap the crack underneath. If the joint is safety-critical, follow the applicable welding procedure or have it inspected by a qualified professional.