AWS D1.2 — Structural Welding Code for Aluminum
AWS D1.2 is the structural welding code for aluminum alloys. It governs procedure qualification, welder performance testing, fabrication requirements, and inspection criteria for structural aluminum components using GMAW and GTAW processes with strict preheat limits to prevent hot cracking and strength loss in heat-treatable alloys.
Key distinction: Unlike AWS D1.1 for steel where hydrogen-induced cracking drives preheat requirements, D1.2 addresses hot cracking (solidification cracking) through controlled heat input and filler metal selection. Preheat is capped at 250°F — exceeding this damages heat-treatable aluminum alloys.
What Is AWS D1.2?
AWS D1.2 is the structural welding code for aluminum, covering 5xxx-series (Al-Mg) and 6xxx-series (Al-Mg-Si) alloy families. Unlike D1.1 for steel, D1.2 limits preheat to 250 degrees F maximum because excessive heat causes hot cracking and strength loss in heat-treatable aluminum alloys.
AWS D1.2/D1.2M — Structural Welding Code — Aluminum — is the American Welding Society standard that governs the welding of structural aluminum components. The current edition is AWS D1.2:2014. It covers procedure qualification, welder qualification, fabrication, and inspection requirements for aluminum structures subjected to design stress. The standard applies to wrought and cast aluminum alloys in structural applications including building frames, trusses, bridges, crane structures, and architectural components.
D1.2 is organized similarly to D1.1 but addresses the fundamentally different metallurgical behavior of aluminum compared to steel. Aluminum has high thermal conductivity (roughly four times that of steel), no visible color change before melting, a narrow solidification temperature range that promotes hot cracking, and sensitivity to overheating in heat-treatable temper conditions. These properties require different welding approaches, different qualification variables, and different inspection criteria than steel codes.
The standard covers several welding processes for structural aluminum. Gas metal arc welding (GMAW) is the primary process for production welding due to its higher deposition rates and suitability for automated applications. Gas tungsten arc welding (GTAW) provides precise heat control for thinner sections, root passes, and critical joints. Plasma arc welding with variable polarity (PAW-VP) and friction stir welding (FSW) are also covered. Stud welding is included for specific fastening applications. Shielded metal arc welding (SMAW) is not included because aluminum SMAW electrodes produce hygroscopic flux residue that causes corrosion and is impractical for structural quality requirements.
Preheat Requirements in D1.2
D1.2 limits maximum preheat to 250 degrees F (120 degrees C), and holding times at temperature shall not exceed 15 minutes. This is the opposite philosophy from D1.1, where preheat prevents hydrogen cracking by slowing cooling. In aluminum, excessive preheat causes hot cracking and overaging of heat-treatable alloys.
Preheat in aluminum welding serves a different purpose than in steel. In steel welding under D1.1, preheat slows the cooling rate to prevent hydrogen-induced cold cracking. In aluminum, the primary concern is removing moisture from the joint area and bringing the base metal to a temperature that reduces thermal shock, not preventing hydrogen cracking. Aluminum has such high hydrogen solubility in the liquid state that hydrogen escapes during solidification rather than becoming trapped in the weld metal as it does in steel.
AWS D1.2 establishes a maximum preheat temperature of 250°F (120°C), and holding times at this temperature shall not exceed 15 minutes. This upper limit and time restriction exist because exceeding them causes overaging in heat-treatable alloys (6xxx series), grain growth in all alloys, and significant mechanical property degradation. A 6061-T6 plate preheated above 250°F can lose 30 to 50 percent of its yield strength permanently, with no recovery possible without full solution heat treatment and artificial aging.
The minimum preheat for most applications is simply to remove moisture and bring the metal above the dew point. In cold weather conditions (below 32°F / 0°C), preheating to a moderate temperature prevents condensation on the joint surfaces. Temperature measurement should use contact thermometers or temperature-indicating crayons rated for aluminum. Infrared thermometers require emissivity correction for reflective aluminum surfaces to provide accurate readings.
Hot Cracking and Filler Metal Selection
Hot cracking is the primary weld defect concern in aluminum, not hydrogen cracking. Filler metal selection is critical: ER4043 (Al-Si) resists hot cracking better on 6xxx alloys, while ER5356 (Al-Mg) provides higher strength and better color match for 5xxx alloys. D1.2 Table 4.1 specifies filler metal requirements.
Hot cracking (solidification cracking) is the dominant cracking mechanism in aluminum welding and the primary reason D1.2 requires specific filler metal qualification. Hot cracks form when the weld metal solidifies and contracts, and the remaining liquid film between solidifying grains cannot sustain the tensile strain. The crack typically appears at the weld centerline or in the crater at the end of a weld pass.
Filler metal selection is the primary engineering control for hot cracking in aluminum. The two most common structural aluminum fillers are ER4043 (aluminum-silicon) and ER5356 (aluminum-magnesium). ER4043 contains approximately 5% silicon, which lowers the solidification temperature range and provides better fluidity, reducing hot cracking susceptibility. ER5356 contains approximately 5% magnesium, which provides higher weld metal strength and better corrosion resistance but has a wider solidification range. The choice between them depends on the base alloy, the service environment, and whether the weldment will be anodized (ER5356 anodizes to match base metal color, while ER4043 turns dark).
D1.2 requires filler metal compatibility with the base metal alloy. Welding 6061 base metal with ER4043 filler produces a weld with lower strength than the base metal but excellent crack resistance. Using ER5356 on 6061 provides higher weld strength but slightly higher crack susceptibility. Welding 5xxx base metals (5083, 5086, 5456) requires 5xxx filler metals — using 4043 on 5xxx alloys can produce a brittle Al-Mg2Si intermetallic compound in the weld that reduces ductility and toughness.
Alloy Families in D1.2
D1.2 covers two primary alloy families. 5xxx series (Al-Mg) alloys like 5083 and 5086 are non-heat-treatable, work-hardened, and used in marine and pressure vessel applications. 6xxx series (Al-Mg-Si) alloys like 6061 and 6063 are heat-treatable and used in structural extrusions and architectural applications.
5xxx Series (Aluminum-Magnesium)
The 5xxx alloys are non-heat-treatable, meaning their strength comes from solid solution strengthening and work hardening rather than precipitation hardening. Alloys such as 5083, 5086, 5454, and 5456 are commonly used in structural applications requiring corrosion resistance, including marine structures, chemical storage tanks, and transportation equipment. These alloys maintain good strength after welding because the heat-affected zone (HAZ) reverts to the annealed (O temper) condition, and the annealed strength of 5xxx alloys is relatively close to the work-hardened strength. Filler metals for 5xxx alloys are typically ER5183, ER5356, or ER5556.
6xxx Series (Aluminum-Magnesium-Silicon)
The 6xxx alloys are heat-treatable and widely used in structural extrusions, architectural applications, and light-gauge structural members. Alloys 6061-T6 and 6063-T6 are the most common structural grades. These alloys experience significant strength loss in the HAZ during welding — typically 40 to 50 percent of the T6 condition yield strength — because the welding heat overages the magnesium-silicon precipitates that provide the T6 temper strength. The as-welded strength of the HAZ governs the design capacity of the joint. Some strength recovery occurs through natural aging over several weeks, but full recovery requires post-weld solution heat treatment and artificial aging, which is rarely practical for fabricated structures.
Procedure Qualification Under D1.2
D1.2 requires all welding procedures to be qualified by testing. Unlike D1.1, there is no prequalified WPS path for aluminum — every WPS must be supported by procedure qualification testing with destructive examination. Essential variables include alloy family, filler metal, welding process, and shielding gas composition.
AWS D1.2 requires all welding procedure specifications to be qualified by testing. Unlike D1.1, which provides a prequalified WPS path under Clause 5 for steel, D1.2 has no prequalified exemption — every procedure must be supported by procedure qualification testing. The qualification test coupon must be welded using the WPS parameters and then tested per the applicable acceptance criteria, typically including tensile tests, bend tests, and macroetch examination.
Essential variables in D1.2 include base metal alloy group, filler metal classification, welding process, shielding gas composition, position, thickness range, preheat temperature, and joint design. A change in any essential variable beyond the qualified range requires re-qualification with a new test coupon. The qualification ranges for thickness, position, and base metal groups are defined in the standard and determine how broadly a single procedure qualification can be applied.
Welder performance qualification requires each welder or welding operator to demonstrate the ability to produce sound aluminum welds using a qualified WPS. The test requires producing a test coupon in the applicable position that passes bend testing or radiographic examination. Aluminum welding requires significantly different technique than steel — the high thermal conductivity causes rapid heat dissipation, requiring higher travel speeds and different torch angles to maintain the weld pool.
How D1.2 Compares to Other AWS Structural Codes
D1.2 governs aluminum structural welding while D1.1 governs carbon steel. The fundamental difference: aluminum welding prevents hot cracking (preheat limited to 250 degrees F maximum) while steel welding prevents hydrogen cracking (preheat required per Table 5.11). D1.2 uses GMAW and GTAW; D1.1 also allows SMAW, SAW, and FCAW.
D1.2 vs D1.1 (Steel)
D1.1 governs structural steel welding where the primary metallurgical concern is hydrogen-induced cracking in the heat-affected zone. D1.1 addresses this through mandatory preheat tables (Table 5.11) that require up to 400°F based on carbon equivalent, process hydrogen level, and material thickness. D1.2 limits preheat to 250°F maximum because overheating damages aluminum. D1.1 prequalifies WPS procedures under Clause 5 for common steel joint configurations — D1.2 requires qualification testing for every procedure. D1.1 permits SMAW, SAW, GMAW, and FCAW — D1.2 covers GMAW, GTAW, PAW-VP, FSW, and stud welding but prohibits SMAW.
D1.2 vs D1.6 (Stainless Steel)
D1.6 covers structural stainless steel welding. Both D1.2 and D1.6 share the characteristic that preheat must be carefully limited rather than aggressively applied. D1.6 limits interpass temperature to 350°F for austenitic stainless steels to prevent sensitization. D1.2 limits preheat to 250°F to prevent strength loss. Both codes require procedure qualification testing without a prequalified path. The atmospheric contamination control required for aluminum (moisture) differs from stainless steel (surface contamination causing loss of corrosion resistance).
D1.2 vs D1.9 (Titanium)
D1.9 covers structural titanium welding. Both aluminum and titanium require careful atmosphere control during welding, but for different reasons. Aluminum requires clean, dry surfaces to prevent porosity from hydrogen and oxide inclusions. Titanium requires inert atmosphere shielding on both sides of the weld and trailing shields to prevent oxygen and nitrogen contamination that causes embrittlement. Both codes prohibit SMAW. D1.9 most commonly uses GTAW but also permits GMAW, PAW, EBW, and LBW, while D1.2 uses GMAW, GTAW, PAW-VP, and FSW.
| Aspect | D1.2 (Aluminum) | D1.1 (Steel) |
|---|---|---|
| Base metals | 5xxx/6xxx aluminum alloys | Carbon and low-alloy steels |
| Preheat max | 250°F (no min table) | Table 5.11 lookup |
| Primary concern | Hot cracking prevention | Hydrogen cracking prevention |
| Filler metal | ER4043, ER5356 (A5.10) | AWS A5.1/A5.18/A5.20 |
| Processes | GMAW, GTAW | SMAW, GMAW, FCAW, SAW, GTAW |
| Prequalified WPS? | No — all require testing | Yes (Clause 5) |
Related Standards Guides
Frequently Asked Questions
AWS D1.2 limits preheat to a maximum of 250 degrees Fahrenheit (120 degrees Celsius), and holding times at this temperature shall not exceed 15 minutes before welding begins. Exceeding this temperature or hold time can cause grain growth and significant strength loss in heat-treatable alloys such as 6061-T6 and 6063-T6. Unlike steel where higher preheat is often beneficial, aluminum preheat must be carefully controlled to avoid metallurgical damage.
Aluminum has extremely high hydrogen solubility in the liquid state but very low solubility in the solid state, so hydrogen escapes during solidification rather than becoming trapped as it does in steel. The primary cracking mechanism in aluminum is hot cracking (solidification cracking), which occurs when the weld metal shrinks during solidification and the remaining liquid film between grains cannot sustain the tensile strain. Filler metal selection is the primary control — 4043 and 5356 fillers are designed to reduce hot cracking susceptibility.
No. AWS D1.2 does not cover shielded metal arc welding (SMAW) for structural aluminum applications. The permitted processes are GMAW (MIG), GTAW (TIG), PAW-VP (plasma arc welding with variable polarity), FSW (friction stir welding), and stud welding. GMAW is the most common process for production aluminum welding due to higher deposition rates, while GTAW is preferred for thinner sections and root passes where precise heat control is needed.
AWS D1.1 covers structural steel welding while D1.2 covers structural aluminum welding. The metallurgical concerns are fundamentally different — D1.1 addresses hydrogen-induced cracking through preheat tables (Table 5.11), while D1.2 addresses hot cracking through filler metal selection and controlled heat input. D1.2 limits preheat to 250 degrees Fahrenheit (120 degrees Celsius) maximum, while D1.1 requires preheat up to 400 degrees Fahrenheit for high-carbon-equivalent steels. D1.2 does not permit SMAW, while D1.1 prequalifies SMAW procedures.
AWS D1.2 covers wrought and cast aluminum alloys used in structural applications, primarily from the 5xxx series (aluminum-magnesium, such as 5083, 5086, 5454, and 5456) and the 6xxx series (aluminum-magnesium-silicon, such as 6061, 6063, and 6082). The 5xxx alloys are non-heat-treatable and maintain strength after welding, while the 6xxx alloys are heat-treatable and experience strength loss in the heat-affected zone unless post-weld heat treatment is applied.