Aluminum Welding: TIG, MIG, and Friction Stir Techniques

## The Challenges of Welding Aluminum Aluminum presents several welding difficulties that do not exist with steel: **Oxide layer**: Aluminum oxide (Al₂O₃) melts at 2072 °C, while the base aluminum melts at around 660 °C. If the oxide is not removed or disrupted, molten aluminum becomes trapped beneath an unmelted oxide skin, causing lack-of-fusion defects. AC polarity in GTAW and the cleaning action of reverse-polarity cycles break up this oxide during welding. **High thermal conductivity**: At 205 W/m·K (roughly 14 times that of stainless steel), aluminum conducts heat away from the weld zone rapidly. This demands higher heat input to achieve fusion on thicker sections and makes maintaining consistent penetration difficult as the part heats up during multi-pass welding. **Porosity**: Liquid aluminum dissolves hydrogen readily; solid aluminum does not. As the weld pool solidifies, dissolved hydrogen forms gas pores. Sources of hydrogen include moisture, hydrocarbons, and oxide hydration on the wire or base metal surface. Cleanliness is paramount. **Solidification cracking**: Many aluminum alloys (particularly 2xxx and 7xxx heat-treatable series) are susceptible to hot cracking during solidification due to a wide freezing range and high thermal contraction. ## GTAW (TIG) Welding of Aluminum GTAW with alternating current (AC) is the reference process for high-quality aluminum welding. The electrode-positive half-cycle breaks up the oxide layer (cleaning action); the electrode-negative half-cycle provides penetration. Key parameters: - **Tungsten**: pure tungsten (green band) or zirconia-alloyed (white band) for AC. Thoriated tungsten (red band) is reserved for DC applications. - **Shielding gas**: pure argon at 12–20 L/min. Helium additions (25–75% He) increase heat input for thick sections but are more expensive. - **Filler rod**: ER4043 (Al-5Si) for general purpose welding of 6xxx alloys. ER5356 (Al-5Mg) for 5xxx alloys and where higher weld strength or anodizing color match is needed. | Base Alloy | Recommended Filler | Notes | |------------|-------------------|-------| | 1100 | ER1100 or ER4043 | Matching purity or Si-bearing | | 3003 | ER4043 | Good fluidity | | 5052 | ER5356 | Mg-matching | | 6061 | ER4043 or ER5356 | ER4043 resists cracking better | | 7075 | Not recommended | Extremely crack-prone; rivet instead | ## GMAW (MIG) Welding of Aluminum GMAW with spray transfer is the production process for aluminum fabrication. Push angle (10–15° forehand) is essential—dragging the gun traps oxide and gas in the weld pool. - **Wire**: 1.2 mm ER4043 or ER5356. Wire must be stored in sealed packaging and used within days of opening to prevent oxide and moisture buildup. - **Shielding gas**: pure argon or Ar-He blends. Never use CO₂ or Ar-CO₂ mixtures—CO₂ reacts with aluminum. - **Feed system**: use a Teflon liner in the conduit (steel liners shave soft aluminum wire), a U-groove drive roll (not V-groove, which deforms the wire), and keep the conduit length under 3 meters. Pulsed GMAW has become the preferred mode for aluminum, offering spray-transfer quality with reduced heat input and better control on thinner sections (1.5–4 mm) where conventional spray would burn through. ## Friction Stir Welding (FSW) FSW is a solid-state process invented in 1991 at TWI that joins aluminum without melting it. A rotating tool with a shoulder and pin plunges into the joint line and traverses along it, plasticizing the material by friction and mechanically stirring the two sides together. Advantages over fusion welding: - No melting means no porosity, no solidification cracking, and no filler metal needed - Retains more base metal strength in heat-treatable alloys (6061-T6 FSW joints retain 80–90% of base metal UTS vs. 60–70% for GMAW) - Can join 2xxx and 7xxx alloys that are considered unweldable by fusion processes - Produces a fine-grained microstructure in the stir zone with excellent fatigue properties FSW is used for aerospace panels (Eclipse 500 jet fuselage), marine structures, rail car bodies (Hitachi Shinkansen), and Apple laptop enclosures. Limitations include the need for rigid fixturing, backing anvils, and the inability to weld complex 3D geometries that a fusion torch can access. ## Heat-Treatable vs. Non-Heat-Treatable Alloys This distinction is critical for welding: **Non-heat-treatable** (1xxx, 3xxx, 5xxx): strengthened by cold work. The HAZ anneals during welding, creating a soft zone at roughly the annealed strength regardless of the original temper. 5052-H32 welded joint efficiency is about 75–80%. **Heat-treatable** (2xxx, 6xxx, 7xxx): strengthened by precipitation hardening (T4, T6 tempers). Welding dissolves or overages the precipitates in the HAZ. 6061-T6 drops to roughly 165–185 MPa tensile in the HAZ (from 310 MPa base), giving a joint efficiency of about 55–65% for GMAW. Post-weld solution heat treatment and aging can restore near-full properties but requires furnace capacity and quench facilities. ## Pre-Weld Preparation Cleanliness determines weld quality in aluminum more than in any other metal: 1. Degrease with acetone or MEK (no chlorinated solvents—phosgene risk when heated) 2. Remove oxide with a stainless steel wire brush dedicated to aluminum (never use a carbon steel brush) 3. Weld within 4 hours of cleaning; oxide reforms and thickens over time 4. Store filler wire sealed; contaminated wire is the leading cause of porosity in production shops