CLADDING TECHNOLOGY SHANXI CO., LTDPost Weld Heat Treatment Effects on Variable Polarity TIG Welded AA2219 Aluminum Alloy Joints
Literature Overview
This paper, published in the Transactions of the Nonferrous Metals Society of China in 2014, examines the influence of post weld heat treatment (PWHT) on the properties of variable polarity gas tungsten arc welded (VP-TIG) joints of AA2219 aluminum alloy. The research was conducted by scholars from the School of Materials Science and Engineering at Tianjin University and the Tianjin Key Laboratory of Modern Connection Technology, supported by the National Natural Science Foundation of China (Project No. 51275343). AA2219 is a widely used aerospace aluminum alloy known for its excellent strength-to-weight ratio and good weldability, making it a critical material for aircraft fuselage, wing structures, and other high-performance applications.
Core Technical Content
AA2219 Alloy Characteristics
AA2219 is an Al-Cu-Mg-Li alloy that belongs to the 2xxx series. It is characterized by:
- High strength in the T87 temper condition (yield strength approximately 517 MPa, ultimate tensile strength approximately 586 MPa)
- Good fatigue resistance
- Susceptibility to stress corrosion cracking in the T8 temper
- Excellent weldability compared to other high-strength aluminum alloys
- Low tendency for hot cracking due to the presence of lithium
Variable Polarity TIG Welding
Variable polarity TIG (VP-TIG) welding alternates between direct current electrode negative (DCEN) and direct current electrode positive (DCEP) polarity during the welding cycle. This technique offers several advantages over conventional DCEN TIG welding:
- Cathodic cleaning effect: During the DCEP phase, the oxide layer on the workpiece surface is removed, improving weld quality and reducing the need for mechanical cleaning between passes.
- Reduced tungsten erosion: The DCEN phase dominates the cycle, minimizing tungsten consumption while still achieving the cleaning benefit.
- Improved weld penetration: The variable polarity cycle can produce deeper and more uniform penetration compared to pure DCEN welding.
- Reduced hydrogen porosity: The cleaning effect helps remove oxide films that can trap hydrogen, reducing porosity formation.
Post Weld Heat Treatment Conditions
| PWHT Condition | Temperature (°C) | Duration (h) | Cooling Method | Purpose |
|---|---|---|---|---|
| Solution treatment | 495-510 | 1-4 | Water quench | Dissolve precipitates |
| Aging (T4) | 160-175 | 4-8 | Air cool | Precipitate strengthening |
| Overaging (T7) | 200-230 | 2-6 | Air cool | Improve SCC resistance |
| Stress relief | 250-300 | 1-2 | Air cool | Reduce residual stress |
Microstructural Evolution and Mechanical Properties
Microstructural Zones in VP-TIG Welded AA2219
| Zone | Microstructure | Heat Treatment Response |
|---|---|---|
| Weld metal | Equiaxed alpha-Al grains, CuAl2 particles | Precipitates dissolve during solution treatment |
| Fusion boundary | Fine grains, possible Cu-rich phases | Homogenization during PWHT |
| HAZ | Recrystallized or partially recrystallized grains | Strengthening through aging |
| Base metal (T87) | Fine precipitates (T1, S1, theta prime) | Dissolution during solution treatment |
The weld metal in AA2219 typically exhibits a lower strength than the base metal due to the loss of precipitate strengthening during the welding process. The heat-affected zone (HAZ) may also experience softening, particularly in the region where the temperature exceeds the solution treatment temperature but is insufficient to allow full recrystallization.
Effect of PWHT on Mechanical Properties
Post weld heat treatment can significantly improve the mechanical properties of VP-TIG welded AA2219 joints:
| Property | As-Welded | After T4 Treatment | After T7 Treatment | Base Metal (T87) |
|---|---|---|---|---|
| UTS (MPa) | 250-350 | 400-480 | 380-450 | 586 |
| Yield Strength (MPa) | 150-220 | 300-380 | 280-350 | 517 |
| Elongation (%) | 10-18 | 8-14 | 10-16 | 12 |
| Hardness (HV) | 45-65 | 85-110 | 80-100 | 130-145 |
The improvement in strength after PWHT is attributed to the re-establishment of precipitate strengthening. During solution treatment, the coarse precipitates formed during welding are dissolved, and subsequent aging allows the formation of fine, coherent precipitates that effectively impede dislocation motion. The T7 overaging treatment, while producing slightly lower strength than T4, offers improved resistance to stress corrosion cracking, which is particularly important for aerospace applications where AA2219 is susceptible to SCC in the T8 temper.
Engineering Practice and Quality Control
Weld Quality Assessment
For aerospace applications, the quality of AA2219 welded joints is assessed according to stringent standards such as AWS D10.9, ASTM B209, and various aerospace specifications. The following quality criteria are typically applied:
| Inspection Method | Standard | Acceptance Criteria |
|---|---|---|
| Visual examination | AWS D10.9 | No cracks, no excessive undercut |
| Penetrant testing (PT) | ASTM E709 | No indications |
| Ultrasonic testing (UT) | ASTM E2358 | No volumetric defects |
| Radiographic testing (RT) | ASTM E94 | No porosity clusters > 0.5 mm |
| Tensile test | ASTM B557 | Weld metal UTS ≥ 250 MPa |
| Bend test | AWS D10.9 | No cracks on outer surface |
Process Optimization Considerations
The VP-TIG welding process for AA2219 requires careful control of the polarity ratio (the ratio of DCEN to DCEP time). A typical polarity ratio ranges from 70:30 to 85:15 (DCEN:DCEP), depending on the thickness and joint configuration. Too high a DCEP ratio can cause excessive tungsten erosion and contamination, while too low a ratio may not provide adequate cleaning effect.
The shielding gas composition is also critical. Pure argon is the standard shielding gas for aluminum welding, but helium additions (up to 30-50 percent) can improve penetration depth for thicker sections. The gas flow rate should be maintained at 8-12 L/min to ensure adequate protection of the weld pool and the hot solidifying metal.
Key Technical Insights
The most significant finding from this research is the demonstration that PWHT can substantially recover the mechanical properties of VP-TIG welded AA2219 joints, bringing them closer to the base metal properties. This is particularly important for aerospace applications where the welded joint strength directly affects the structural integrity and service life of the component.
The variable polarity TIG welding technique offers a practical solution to the challenges of welding AA2219, particularly for multi-pass welds where interpass cleaning is required. The cathodic cleaning effect reduces the need for mechanical or chemical cleaning between passes, improving productivity and weld consistency.
Study Insights and Implications
This research provides valuable guidance for the fabrication of AA2219 components in aerospace and other high-performance applications. The combination of VP-TIG welding and post weld heat treatment offers a viable approach to achieving acceptable joint properties without resorting to more expensive welding methods such as electron beam welding or laser welding. For engineers involved in pressure vessel and structural component fabrication, the key lessons are: (1) variable polarity TIG welding is a practical and effective method for welding aluminum alloys; (2) post weld heat treatment is essential for restoring mechanical properties; and (3) the selection of PWHT condition (T4 vs. T7) should be based on the specific requirements for strength versus stress corrosion cracking resistance. Future work should focus on optimizing the VP-TIG parameters for thicker sections, investigating the effect of welding sequence on residual stress distribution, and developing automated welding procedures for improved consistency and productivity.