Crater cracks are the most common aluminum weld defect, followed by longitudinal hot cracks along the weld centerline. Both are preventable with the right filler selection, joint design, and welding technique. If you’re pulling cracked aluminum welds off the table, the cause is almost always one of four things: unfilled craters, wrong filler for the base alloy, excessive restraint, or concave bead profiles.
How Aluminum Cracks Differently Than Steel
Steel cracks in the HAZ from hydrogen embrittlement (cold cracking). Aluminum doesn’t suffer from hydrogen-induced cracking because its crystal structure handles dissolved hydrogen differently. Instead, aluminum cracks hot, during solidification, when the weld metal is transitioning from liquid to solid.
During solidification, the last liquid to freeze forms thin films along grain boundaries and at the weld centerline. If the solidifying metal contracts (it always does) and the remaining liquid can’t feed the shrinkage, those films tear apart into cracks. The composition of the weld deposit determines how much liquid film forms and how wide the solidification temperature range is. Wide solidification ranges mean more cracking.
Type 1: Crater Cracks
Crater cracks form at the termination point of every weld bead. They’re so common in aluminum that any uncorrected crater should be considered cracked until proven otherwise.
Mechanism: When the arc stops, the puddle freezes from the outside edges inward. The center solidifies last and has no liquid metal to feed the shrinkage. The result is a concave crater with a star-shaped or linear crack at the center.
Prevention:
- Fill the crater. Reduce travel speed at the end of the bead and add extra filler to build a convex profile. The excess filler acts as a reservoir to feed solidification shrinkage.
- Use downslope. Set your TIG machine’s downslope to 5-10 seconds so amperage ramps down gradually. This slows solidification and gives the puddle time to feed itself.
- Back-step at terminations. Reverse direction 1/2 inch before stopping to re-melt and fill the crater area.
- Run-off tabs. On production joints, extend the weld onto a run-off tab and cut it off after welding. The crater forms in the tab, not in the workpiece.
If you find a crater crack, grind it out completely. Inspect with dye penetrant to confirm full removal before rewelding.
Type 2: Longitudinal (Centerline) Hot Cracks
Centerline cracks run along the middle of the weld bead, sometimes the full length of the joint. They’re a solidification cracking problem caused by weld chemistry, bead shape, or both.
Mechanism: Columnar grains grow from each fusion line toward the center of the weld. Where they meet at the centerline, low-melting-point liquid films get trapped. Shrinkage stress tears these films apart.
Causes and fixes:
| Cause | Why It Cracks | Fix |
|---|---|---|
| Wrong filler metal | Dilution puts weld chemistry in crack-sensitive range (0.5-2% Si or 1-3% Mg) | Switch to ER4043 (high Si) or ER5356 (high Mg) to move past the sensitive range |
| Concave bead profile | Thin center section solidifies last and can't resist shrinkage stress | Add more filler to produce a slightly convex bead (crown height 10-15% of bead width) |
| Deep, narrow weld bead | Excessive depth-to-width ratio concentrates stress at centerline | Target a width-to-depth ratio of 1.5:1 or greater |
| High joint restraint | Surrounding structure prevents contraction, concentrating stress in solidifying weld | Reduce restraint, use balanced weld sequence, preheat to reduce thermal gradient |
| High travel speed | Creates a teardrop-shaped puddle with a long tail where cracking initiates | Slow down to maintain an elliptical puddle shape |
| Contamination | Oil, cutting fluid, or oxide inclusions create nucleation sites for cracks | Clean joint thoroughly; degrease and wire brush immediately before welding |
The Crack-Sensitive Composition Ranges
This is the key concept. Aluminum alloys are most prone to hot cracking at specific alloy content ranges:
| Alloy System | Peak Crack Sensitivity | Low Sensitivity (Filler Target) |
|---|---|---|
| Al-Si | 0.5-2.0% Si | Below 0.3% or above 4.5% Si |
| Al-Mg | 1.0-3.0% Mg | Below 0.8% or above 4.5% Mg |
| Al-Cu | 1.0-3.0% Cu | Below 0.5% or above 4.5% Cu |
| Al-Mg-Si (6xxx) | 0.5-1.0% Mg2Si | Add Si via ER4043 or Mg via ER5356 |
6xxx-series alloys (like 6061) are naturally in the crack-sensitive range because they contain roughly 1% Mg2Si. This is exactly why they need a high-alloy filler: ER4043 adds enough silicon, or ER5356 adds enough magnesium, to move the weld deposit out of the danger zone. Welding 6061 with no filler (autogenous weld) practically guarantees cracking.
Type 3: Transverse Cracks
Transverse cracks run perpendicular to the weld direction. They’re less common than centerline cracks and usually indicate a problem with restraint or base metal chemistry rather than the welding procedure.
Causes:
- Excessive restraint from fixtures or thick surrounding structure
- High-magnesium alloys (5083, 5456) with excessive heat input causing wide HAZ
- Concave weld beads that can’t resist transverse shrinkage stress
Prevention: Reduce restraint, increase bead convexity, and on restrained multi-pass joints, use a balanced welding sequence (alternate sides) to distribute shrinkage stress.
Type 4: Liquation Cracking in the HAZ
Liquation cracks form in the HAZ, just outside the fusion line, in the partially melted zone (PMZ). Low-melting constituents at grain boundaries melt during welding and open up under shrinkage stress.
This type of cracking is most common in:
- 2xxx-series alloys (copper-bearing), especially 2024
- 7xxx-series alloys (zinc-bearing), especially 7075
- 6xxx-series alloys on highly restrained joints
Prevention: Avoid high heat input (keeps the PMZ narrow), reduce restraint, and select alloys with low liquation cracking susceptibility. For 6061, ER4043 filler reduces PMZ cracking better than ER5356 because the silicon provides a wider solidification range with better backfilling.
Type 5: Stress Corrosion Cracking (SCC)
SCC is a service failure, not a welding defect, but welding creates the conditions for it. It requires three simultaneous factors: a susceptible microstructure, tensile stress, and a corrosive environment.
Welded 5xxx alloys with more than 3% magnesium (5083, 5086, 5456) are susceptible to SCC if sensitized. Sensitization occurs when the alloy is held at 150-350F for extended periods, causing magnesium to precipitate as Al3Mg2 at grain boundaries.
Prevention in welded joints:
- Control interpass temperature (below 300F on 5xxx alloys)
- Avoid service temperatures above 150F continuous for sensitized alloys
- Specify H116 or H321 tempers for marine plate (these include IGC testing)
- Stress-relieve welded assemblies if the operating environment is aggressive
Bead Shape Rules
Weld bead shape has a direct influence on cracking. Follow these guidelines:
Convex is better than concave. A slightly convex bead (crown height 10-15% of bead width) resists centerline cracking because the center is the thickest part and the last to solidify. A concave bead is thinnest at the center, making it the weakest point during solidification shrinkage.
Width-to-depth ratio above 1.5:1. Deep, narrow beads concentrate columnar grains at a sharp centerline, trapping liquid films. Wider, shallower beads allow grains to meet at a broader interface with less stress concentration.
Consistent bead profile. Changes in bead width or height create stress concentrations where cracks initiate. Keep travel speed and wire feed steady.
Repair of Cracked Aluminum Welds
When you find a crack, you must remove it entirely before depositing new weld metal. Welding over a crack never works because the crack tip propagates through the new deposit.
Repair procedure:
- Locate the crack extent. Use dye penetrant testing (PT) to find both ends of the crack and any branching. Surface-breaking cracks often extend deeper than they appear.
- Remove the crack. Grind with a carbide burr or rotary file. Go 1/4 inch past each end of the crack and deep enough to eliminate it entirely. On thick sections, use air-arc gouging with carbon electrodes and argon shielding.
- Verify removal. Apply PT again after grinding. If any indications remain, grind deeper.
- Prepare the excavation. Smooth the cavity to a U-shape (not a V-shape, which concentrates stress at the root). Clean with acetone.
- Reweld. Use the correct filler for the base alloy. Fill craters. If the original crack was caused by wrong filler, restraint, or technique, address the root cause.
Quick Troubleshooting Reference
| Symptom | Most Likely Cause | First Fix to Try |
|---|---|---|
| Star cracks at weld stops | Unfilled craters | Add downslope; fill crater with extra filler |
| Crack down center of bead | Wrong filler or concave bead | Switch to ER4043; add more filler for convex profile |
| Cracking on every bead on 6061 | Autogenous (no filler) or diluted deposit | Add proper filler; never autogenous weld 6061 |
| Cracks appear during cooling, not welding | High restraint + concave beads | Reduce fixture clamping; tack weld in sequence |
| Fine cracks next to fusion line | Liquation cracking in PMZ | Reduce heat input; switch to ER4043 filler |
| Cracking months after fabrication | Stress corrosion cracking (SCC) | Check for sensitization; verify alloy/temper |
For the filler metal compatibility chart that determines which filler prevents cracking on your specific base alloy, see the aluminum filler selection chart.
For alloy-specific procedures, check the 6061-T6 welding guide and the 5083 marine aluminum guide.
Back to the main aluminum welding guide.