III. Pre-Weld Preparation: 70% of Success

Aluminum welding abhors “dirt” and “delay.”

1. Thorough Cleaning (Three-Step Method) :
   • Degreasing: Wipe off oil, grease, and fingerprints using acetone or isopropyl alcohol.
   • Film Removal: Scrub unidirectionally with a dedicated stainless steel wire brush. Strictly avoid mixing brushes used for steel (iron contamination causes cracks). Never use grinding wheels—embedded abrasive particles can cause hot cracking.
   • Timing: Weld immediately after cleaning. Aluminum rapidly reforms its oxide layer in air. If stored >24 hours, re-cleaning is necessary.
2. Groove Design & Preheating:
   • Thin Sheets (<3mm) : No bevel. A slight 0.5-1mm chamfer on the backside aids in degassing.
   • Thick Plates (>8mm) : Use a wider bevel angle than steel (e.g., 55° per side) for better torch access. Preheat to 80-120°C; maintain interpass temperature between 60-100°C. Overheating softens the heat-affected zone (HAZ).

IV. Deep Dive into TIG Welding (Most Precise Process)

For manual TIG welding of aluminum, these parameters and techniques are key:

1. Power Source Mode: AC is Mandatory

• Electrode Negative (EN) half-cycle: Provides penetration.
• Electrode Positive (EP) half-cycle: Positive ions bombard the workpiece, physically blasting away the oxide layer (“cathodic cleaning”).

2. Tungsten Electrode & Current

• Electrode: For modern inverter machines, use lanthanated or zirconiated tungsten. Grind to a blunt point with a small flat land, not a perfect ball.
• Current Rule of Thumb: ~1A per 0.025mm (1 mil) of thickness. E.g., for 3mm (~120 mils) plate, set starting current to 120-140A.
• Advanced AC Settings:
  • AC Frequency: Increase (100-200Hz) for a constricted arc, ideal for thin sheets and tight gaps.
  • AC Balance: Typically set EN ratio to 65%-75%. If the weld pool looks dirty, grey, or the oxide appears to be “boiling,” increase EP% (i.e., decrease EN%).

3. Operational Technique

• Look for the “Bright Spot” : After striking the arc, hold briefly. When the molten pool surface changes from dull grey to a bright, mirror-like finish, the oxide film is broken. Now, start adding filler metal and moving.
• Dip/Dab Technique: Keep the filler wire at a low angle (10-15°). Quickly touch the leading edge of the molten pool and quickly withdraw. Aluminum wire melts fast; your feeding frequency must be higher than for steel.
• Crater Filling: Never break the arc abruptly! Use the current downslope function and add an extra dab of filler wire to fill the crater. Otherwise, crater cracks are guaranteed.

V. Filler Metals & Shielding Gas: Matching Pairs

1. Filler Metal Selection (Critical)

• ER4043 (Al-Si 5%) : Excellent fluidity, high crack resistance. Ideal for welding 6xxx series alloys (6061, 6063). Drawback: lower ductility, anodized parts will appear grey/black.
• ER5356 (Al-Mg 5%) : Higher strength, good color match after anodizing, good corrosion resistance. Ideal for welding 5xxx series alloys (5083, 5052).
• Sizing Strategy: Choose the largest diameter practical. Larger diameter wire has less surface area per unit weight, meaning less potential surface contamination (oxides/hydrates). Guideline: <8mm thickness → 1.2mm wire; ≥8mm thickness → 1.6mm wire.

2. Shielding Gas

• 100% Argon (Ar) : Stable arc, easy initiation. Suitable for thicknesses ≤ 8mm.
• Argon-Helium Mix (e.g., Ar 70% + He 30%) : Recommended for thick plates (>8mm) or when porosity is a major concern. Helium has ~9x the thermal conductivity of Argon, significantly increasing penetration, travel speed, and reducing porosity.
• Flow Rate: More is not better. Typical TIG flow: 8-12 L/min (15-25 CFH). Excessively high flow creates turbulence, sucking in atmospheric contamination.

VI. Troubleshooting Common Defects (Field Guide)

1. Heavy Black Soot/Smoke on Weld: Incorrect AC balance (insufficient cleaning action) or gas turbulence. Increase EP balance or check for gas nozzle damage/dirty gas lens.
2. Porosity: Hydrogen is the culprit. Check: Was acetone cleaning thorough? Is the gas hose made of PTFE/Teflon (standard rubber hoses absorb moisture)? Is ambient humidity >70%?
3. Lack of Fusion: Insufficient heat or oxide not removed. For thick plates: either current is too low, or torch angle is too acute (arc not directed at base metal).
4. Cracking: Wrong filler metal or abrupt crater termination. Use 4047/4043 for 6xxx; use 5356 for 5xxx. Always fill the crater.
5. Erratic Wire Feeding (MIG) : Aluminum wire is soft and easily damaged. Check contact tip for wear and correct bore size. Ensure liner is not too long/restrictive. Use a Teflon/PTFE liner to reduce friction.