Aluminum alloy weldability varies dramatically across the different alloy series, making proper classification understanding and filler metal selection essential for producing sound, code-compliant welds. As aluminum finds increasing use in transportation, construction, aerospace, and marine applications, welding engineers must understand how each alloy family responds to thermal cycling and what filler metals produce optimal results.
Aluminum Alloy Classification System
The Aluminum Association designates wrought alloys using a four-digit system where the first digit indicates the principal alloying element. This classification directly determines weldability:
1xxx Series: Pure Aluminum
Alloys with 99.00% or higher aluminum content. These are the most weldable alloys, with minimal hot cracking susceptibility. They are primarily used in chemical equipment and reflective applications. Typical filler: 1180 or 1100 matching filler.
2xxx Series: Copper-Alloyed
Copper is the primary alloying element (2-10%). These alloys, including the famous 2024 and 2014, are generally considered unweldable by conventional fusion welding due to high hot cracking susceptibility. They are primarily joined by mechanical fastening or friction stir welding in aerospace applications.
3xxx Series: Manganese-Alloyed
Manganese (0.5-1.5%) provides moderate strength increases over pure aluminum. Alloys like 3003 are non-heat-treatable and exhibit good weldability. Common in sheet metal work, chemical equipment, and heat exchangers. Filler: 4043 or 5356.
4xxx Series: Silicon-Alloyed
Silicon lowers the melting point and improves fluidity. 4043 is both a base alloy and the most commonly used filler metal for general-purpose aluminum welding. 4047 (12% Si) is used for brazing applications. These alloys have good weldability when properly matched with appropriate fillers.
5xxx Series: Magnesium-Alloyed
Magnesium (up to 5.5%) provides solid solution strengthening. The 5xxx series, especially 5083, 5086, and 5454, are the workhorse alloys for welded structures including marine vessels, transportation tanks, and architectural applications. 5xxx alloys offer excellent weldability and are the preferred choice for structural welding.
6xxx Series: Magnesium and Silicon
These heat-treatable alloys (notably 6061 and 6063) contain both magnesium and silicon, forming Mg₂Si precipitates. 6061 is the most commonly welded structural alloy, used extensively in frames, bridges, and architectural structures. Weldability is good with proper filler selection, though strength reduction in the heat-affected zone (HAZ) must be considered.
7xxx Series: Zinc-Alloyed
Zinc is the primary strengthening element. Most 7xxx alloys (7075, 7050) are highly susceptible to hot cracking and are generally not recommended for fusion welding. However, 7003 and 7005 can be welded with appropriate filler metals and procedures. These alloys find primary use in aerospace where mechanical fastening is preferred.
Hot Cracking Mechanisms
The primary welding challenge with aluminum alloys is hot cracking (solidification cracking). This occurs when:
- The alloy has a wide freezing range (difference between liquidus and solidus temperatures)
- Insufficient silicon or other low-melting constituents are present to fill grain boundaries
- Weld restraint is high, creating tensile stresses during solidification
- The weld pool contains contaminants that form low-melting eutectics
The 4043 filler metal works well for many alloys precisely because its 5% silicon content provides a low-melting eutectic that flows into grain boundaries during solidification, preventing crack propagation.
Filler Metal Selection Criteria
The American Welding Society (AWS) System
Aluminum filler metals are designated by the AWS A5.10 specification using a four-digit system similar to base alloys. Common filler metals include:
- ER4043: 5% Si – general purpose, excellent fluidity, good for 3xxx, 5xxx, 6xxx
- ER5356: 5% Mg – higher strength, better color match after anodizing, for 5xxx and 6xxx
- ER4047: 12% Si – brazing filler, low melting point
- ER5183: 4.8% Mg + 0.8% Mn – high strength for marine applications
- ER5556: 5% Mg + 0.15% Cr – high strength for 5083 and similar
Selection Decision Matrix
The primary factors in filler metal selection include:
- Base alloy compatibility: The filler must produce a crack-free weld with the specific base alloy
- Service environment: Marine environments require Mg-bearing fillers (5xxx series) for corrosion resistance
- Strength requirements: 5xxx fillers generally produce higher strength welds than 4xxx fillers
- Post-weld treatment: If anodizing is required, 5xxx fillers match the base metal color better
- Operating temperature: For service above 65°C (150°F), avoid high-Mg fillers (>3.5% Mg) due to stress corrosion concerns
- Welding process: GTAW and GMAW may require different filler sizes and temper conditions
Filler Metal Selection by Base Alloy
Welding 5xxx Series (Marine and Structural)
For 5083, 5086, and 5454:
- Primary choice: ER5356 or ER5183
- Alternative: ER5556 for highest strength requirements
- Avoid: ER4043 for marine applications (Mg₂Si formation reduces corrosion resistance)
Welding 6xxx Series (Structural and Architectural)
For 6061 and 6063:
- General purpose: ER4043 (best crack resistance, good appearance)
- Higher strength: ER5356 (when strength is priority over crack resistance)
- Marine exposure: ER5356 or ER5183
- Anodized finish: ER5356 (matches 6063 color after anodizing)
Welding Dissimilar Alloys
When joining different aluminum alloys, filler selection must consider the interaction with both base metals:
- 5xxx to 6xxx: Use ER5356
- 3xxx to 5xxx: Use ER5356 or ER4043
- Any to cast alloy: Use ER4043 (best flow characteristics)
Welding Process Considerations
GTAW (TIG) Welding
Gas Tungsten Arc Welding produces the highest quality aluminum welds. Key parameters:
- AC current with high-frequency start for cleaning action
- 100% argon shielding gas
- Ceriated or lanthanated tungsten electrodes (2% thoriated where permitted)
- Filler wire diameter matched to joint thickness
GMAW (MIG) Welding
Gas Metal Arc Welding offers higher deposition rates for production work:
- DC electrode positive (DCEP) polarity
- Argon or Ar/He mix shielding gas
- Push gun technique (10-15 degree push angle)
- Spool gun or push-pull system for soft aluminum wire
Pre-Weld Preparation
Proper preparation is critical for aluminum welding quality:
- Cleaning: Remove all oil, grease, and oxide layer using stainless steel wire brushing or chemical cleaning within 24 hours of welding
- Preheating: For alloys thicker than 6mm or 5xxx series with Mg content above 3.5%, preheat to 93-149°C (200-300°F) to prevent cracking
- Joint design: Aluminum requires wider joint angles (60-70° included) compared to steel due to higher thermal conductivity
- Fixturing: Use adequate clamping to control distortion, especially for thin sheets
Post-Weld Considerations
Aluminum weldments require specific post-weld treatment:
- Heat treatment: 6xxx alloys lose strength in the HAZ and may require re-aging (T6 temper restoration)
- Stress relief: Complex fabrications may require thermal stress relief at 315-343°C (600-650°F)
- Inspection: RT (radiographic) and PT (penetrant) are the most common NDT methods for aluminum welds
CoreMetal Supply for Aluminum Welding Applications
CoreMetal Steel supplies aluminum sheet, plate, coil, and extrusion profiles in all major alloy series (1xxx through 6xxx) for welded fabrication projects. Our inventory includes 5083-H116/H321 marine plate, 6061-T6 structural plate and sheet, 5052-H32 general-purpose sheet, and 3003-H14 chemical equipment sheet.
For your aluminum welding projects, contact CoreMetal Steel with your material requirements and we will provide competitive pricing with full mill test certification and timely delivery to your fabrication facility.