Welding 6061-T6 aluminum destroys the T6 heat treatment in the heat-affected zone, dropping tensile strength from 42 ksi to roughly 18-24 ksi in the softened area. That’s a 40-55% strength loss that no filler metal choice or welding technique can prevent. The metallurgy is straightforward: the T6 temper comes from precipitation hardening (solution heat treatment + artificial aging), and the welding heat dissolves those hardening precipitates in the HAZ.
If you’re designing a welded 6061 structure, you have two choices: design around the reduced HAZ strength, or plan for post-weld heat treatment (PWHT) to restore full T6 properties. Most shop fabrication takes the first approach. Aerospace and structural applications sometimes justify the second.
Why 6061-T6 Loses Strength When Welded
The “T6” designation means the aluminum was solution heat treated at 990F, quenched, then artificially aged at 320-350F for several hours. This process creates a fine dispersion of Mg2Si precipitates that pin dislocations and give 6061-T6 its 42 ksi tensile strength and 35 ksi yield strength.
Welding heats the HAZ above 400F. Between 400-600F, the precipitates coarsen and lose their strengthening effect. Above 900F, they dissolve completely back into solid solution. The result is a softened band running parallel to the weld, typically 1/2 to 1 inch wide on each side, where the material reverts to something close to T0 or T4 condition.
| Condition | Tensile Strength (ksi) | Yield Strength (ksi) | Elongation (%) |
|---|---|---|---|
| 6061-T6 (base metal) | 42-45 | 35-40 | 10-12 |
| 6061-T4 (naturally aged) | 30-35 | 16-21 | 20-22 |
| 6061-T0 (annealed) | 18 | 8 | 25-30 |
| HAZ after welding | 18-24 | 8-16 | Variable |
| As-welded (ER4043 filler) | 24 | 11 | -- |
| As-welded (ER5356 filler) | 27-30 | 14-17 | -- |
The weld metal itself is also weaker than T6 base metal. With ER4043 filler, expect about 24 ksi tensile in the weld. ER5356 runs slightly higher at 27-30 ksi. Neither filler restores base metal strength without PWHT.
Filler Metal Selection: ER4043 vs ER5356
Both fillers are AWS-approved for 6061, but they serve different purposes.
ER4043 (Al-5Si)
ER4043 contains 5% silicon, which improves fluidity and reduces hot cracking sensitivity. It’s the default filler for 6061 in most fabrication shops. Benefits include:
- Excellent flow and wetting, producing smooth bead profiles
- Lower crack sensitivity than ER5356 on restrained joints
- Good color match on non-anodized parts
- Slightly lower as-welded strength (24 ksi tensile)
Use ER4043 when the weldment will be post-weld heat treated, because the silicon content doesn’t interfere with the aging response.
ER5356 (Al-5Mg)
ER5356 contains 5% magnesium and delivers higher as-welded strength. Choose it when:
- The part operates in a saltwater or chemical environment (better corrosion resistance)
- The finished part will be anodized (ER4043 turns dark gray; ER5356 color-matches 6061)
- You need maximum as-welded strength without PWHT
- AWS D1.2 structural code requires it for the specific joint
ER5356 has a stiffer puddle that doesn’t flow as freely, and it’s more prone to hot cracking on highly restrained joints. Avoid it on casting alloys or anything with more than 0.5% silicon in the base.
| Property | ER4043 | ER5356 |
|---|---|---|
| Main alloy element | 5% Silicon | 5% Magnesium |
| As-welded tensile (ksi) | 24 | 27-30 |
| Crack sensitivity | Lower | Higher |
| Puddle fluidity | Excellent | Moderate |
| Anodize color match | Poor (dark gray) | Good (matches 6061) |
| Saltwater corrosion | Moderate | Excellent |
| Best for PWHT? | Yes | No benefit |
TIG Welding 6061-T6: Settings and Technique
TIG on AC is the standard process for 6061-T6 under 3/8 inch thick. The AC cycle provides cathodic cleaning to break through the aluminum oxide layer while DCEN delivers penetration.
| Material Thickness | Tungsten Dia. | Filler Dia. | Amperage Range | AC Balance (% EN) | Cup Size |
|---|---|---|---|---|---|
| 1/16 in (0.063) | 3/32 in | 1/16 in | 60-80A | 65-70% | #6-#7 |
| 1/8 in (0.125) | 3/32 in | 3/32 in | 100-130A | 65-70% | #7-#8 |
| 3/16 in (0.188) | 1/8 in | 1/8 in | 150-190A | 68-75% | #8-#10 |
| 1/4 in (0.250) | 1/8 in | 1/8 in | 180-240A | 68-75% | #10-#12 |
| 3/8 in (0.375) | 5/32 in | 5/32 in | 240-300A | 70-75% | #10-#12 |
Argon flow: 20-25 CFH through a gas lens. Larger cups with gas lenses provide better coverage and let you see the puddle clearly. Pure argon is standard; helium additions (up to 25%) increase heat input for thicker material.
Key technique points for 6061-T6:
- Push angle, not drag. Pushing the torch keeps shielding gas ahead of the puddle.
- Move fast. A fast travel speed concentrates heat in a narrow zone, minimizing the width of the softened HAZ. Lingering spreads the damage.
- Fill craters. Ramp down amperage at the end of each pass using your foot pedal or programmed downslope. Crater cracks are the most common defect in aluminum welding.
- Keep it clean. Stainless steel brush dedicated to aluminum only. Acetone degrease. No shop rags that touched cutting oil.
MIG Welding 6061-T6
MIG (GMAW) makes sense on 6061-T6 over 3/16 inch thick where production speed matters. Use a spool gun or push-pull gun with 100% argon at 25-30 CFH.
| Material Thickness | Wire Dia. | Voltage | Wire Feed Speed (ipm) | Transfer Mode |
|---|---|---|---|---|
| 3/16 in | 0.035 in | 21-23V | 350-400 | Spray |
| 1/4 in | 0.035 in | 23-25V | 400-500 | Spray |
| 3/8 in | 0.047 in | 25-27V | 300-400 | Spray |
Spray transfer is required for aluminum MIG. Short-circuit transfer produces cold lap defects and incomplete fusion that hides inside the joint. Pulse MIG is the best option for thinner sections (1/8 to 3/16 inch) where spray transfer runs too hot.
Preheat Guidelines
Preheat on 6061-T6 is a balancing act. Too little preheat on thick sections causes lack of fusion. Too much preheat expands the softened HAZ and further reduces joint strength. Stay under 400F maximum.
| Thickness | Preheat Temp | Notes |
|---|---|---|
| Under 1/4 in | None required | Ambient temp is fine |
| 1/4 - 1/2 in | 200-300F | Improves fusion, reduces required amps |
| Over 1/2 in | 300F max | Exceeding 400F widens HAZ excessively |
Monitor interpass temperature with a contact pyrometer. Keep it under 350F between passes. If the material gets hotter than that, stop and let it cool. Aluminum doesn’t change color as it heats, so you can’t tell by looking at it.
Joint Design for Reduced HAZ Strength
Since the HAZ will always be weaker than the base metal on as-welded 6061-T6, the joint design needs to compensate. Standard approaches include:
- Thicker sections at the joint. If the part needs 3/16 inch wall for service loads, use 1/4 inch at the weld joint to account for HAZ softening.
- Double-V grooves on thicker material balance distortion and allow full-penetration welds without excessive heat input from one side.
- Back-step welding on long joints controls distortion and distributes heat more evenly.
- Move the joint. If possible, relocate the weld away from the highest-stress area of the structure.
AWS D1.2 (Structural Welding Code - Aluminum) requires that allowable stresses be based on the as-welded strength of the HAZ, not the T6 base metal strength. For 6061-T6 welded with ER4043 or ER5356, the design tensile stress is based on roughly 24 ksi, not 42 ksi.
Post-Weld Heat Treatment (PWHT)
Full PWHT can restore 6061 to T6 condition after welding. The process involves:
- Solution heat treatment: Heat the entire weldment to 985-995F and hold for 1-2 hours (depending on section thickness).
- Quench: Water quench immediately. The part must reach the quench tank within 10-15 seconds to prevent premature precipitation.
- Artificial aging: Reheat to 320-350F and hold for 8-18 hours.
This restores the Mg2Si precipitate structure throughout the HAZ. The catch: you need an oven large enough for the entire assembly, and the water quench can cause distortion on complex shapes. Partial heat treatment doesn’t work; the entire weldment must go through the cycle.
For parts welded with ER4043, the weld metal responds well to PWHT and gains significant strength. For ER5356, the magnesium-rich deposit doesn’t benefit as much from aging, so the weld remains the weakest link even after heat treatment.
Common Defects and How to Avoid Them
Crater cracks form at the end of a weld bead when the puddle solidifies too fast. Always ramp down amperage and add extra filler to fill the crater. If you pop the arc off a full crater, it cracks almost every time.
Porosity comes from hydrogen contamination. Sources include moisture on the base metal, dirty filler rod, damp shielding gas, and hydrocarbons (cutting fluid, marker ink). Dew point on the base metal is the most common cause in humid shops. Wipe the joint with acetone and weld within minutes of cleaning.
Lack of fusion happens when the oxide layer isn’t broken and the filler metal sits on top without actually fusing into the base. Increase amperage, use AC balance with enough EP to clean the oxide, and confirm your joint prep removed all heavy oxide before striking the arc.
Hot cracking in 6061 usually means the wrong filler for a restrained joint. Switch from ER5356 to ER4043 if cracking persists, or redesign the joint to reduce restraint.
When to Pick a Different Alloy
If the welded joint strength of 6061-T6 doesn’t meet your design requirements and PWHT isn’t practical, consider switching to a 5xxx-series alloy. 5083 marine-grade aluminum is strain-hardened rather than precipitation-hardened, so the HAZ strength loss is much less severe. The 5xxx series retains 85-90% of its base metal strength after welding, compared to 50-60% for 6061-T6.
For the full picture on matching filler to your specific alloy combination, see the aluminum filler selection chart.
Back to the main aluminum welding guide for process fundamentals and oxide removal techniques.