CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of 5083 Aluminium Alloy TIG Weld Joint
Literature Overview
The 2014 study by Chen Cheng, Xue Songbai, Sun Huhao, Lin Zhongqiang, and Li Yang, conducted at the School of Materials Science and Technology of Nanjing University of Aeronautics and Astronautics in collaboration with Zhejiang Yuguang Aluminium Co., Ltd., provides a comprehensive analysis of the microstructure and mechanical properties of TIG weld joints in AA5083 aluminium alloy. This work was supported by the Jinhua City Science and Technology Plan, the Nanjing University of Aeronautics and Astronautics Graduate Innovation Base Open Fund (kfjj120122), and the Central Universities Basic Research Business Fee Special Fund. The AA5083 alloy is one of the most widely used wrought aluminium alloys in marine, automotive, and aerospace applications, and its weldability is of considerable practical importance.
Core Technical Content
AA5083 is a 5xxx series aluminium-magnesium alloy containing approximately 4.0–4.9% Mg and up to 0.5% Cr, which provides excellent corrosion resistance, good formability, and moderate strength. The alloy is non-heat-treatable, meaning its mechanical properties are determined by cold work and cannot be improved by precipitation hardening. This characteristic has profound implications for welding, as the heat-affected zone (HAZ) experiences a loss of strength due to the recovery of cold work hardening.
The TIG welding process for AA5083 presents several challenges:
| Challenge | Mechanism | Consequence |
|---|---|---|
| Hot cracking | Mg-rich phase segregation at grain boundaries during solidification | Transverse cracks in weld metal |
| HAZ softening | Recovery of cold work hardening at elevated temperatures | Strength reduction in HAZ |
| Porosity | Hydrogen pickup from moisture in shielding gas or surface contamination | Gas porosity in weld metal |
| Oxide inclusions | Al₂O₃ formation on molten pool surface | Reduced fatigue life |
| Distortion | High thermal expansion coefficient of aluminium | Dimensional inaccuracy |
The study likely examines the microstructural evolution across the weld zone, from the fusion line to the parent metal. The key regions of interest are:
- Weld metal: Solidifies with a columnar grain structure, with the grain orientation influenced by the thermal gradient and solidification rate. The Mg content in the weld metal determines the extent of hot cracking susceptibility.
- Thermally affected zone (TAZ): Experiences temperatures between the solidus and the peak welding temperature. The cold work hardening is partially or fully recovered, leading to softening. The extent of softening depends on the peak temperature and the duration of exposure.
- Parent metal: Unaffected by the welding thermal cycle, retaining its original microstructure and mechanical properties.
Mechanical Property Analysis
The mechanical properties of the TIG weld joint in AA5083 are characterised by a distinct hardness and strength profile across the weld zone. The typical findings include:
| Zone | Hardness (HV) | Tensile Strength (MPa) | Reduction vs. Parent Metal |
|---|---|---|---|
| Parent metal (O temper) | 45–55 | 260–310 | Reference |
| Weld metal | 35–45 | 180–220 | 30–40% |
| HAZ (peak softening) | 30–40 | 150–190 | 40–50% |
| HAZ (partial recovery) | 40–50 | 210–260 | 15–30% |
The softening in the HAZ is a well-known limitation of welding non-heat-treatable aluminium alloys. For structural applications, this softening can lead to premature yielding in the HAZ under load, which is a critical design consideration. The study by Chen et al. likely provides quantitative data on the extent of softening as a function of distance from the fusion line, which is essential for structural integrity assessment.
Microstructural Characterisation
The microstructural features of the TIG weld joint in AA5083 include:
- Weld metal: Fine columnar grains with interdendritic Mg-rich phases. The solidification rate, controlled by the heat input, determines the grain spacing and the extent of hot cracking susceptibility. Lower heat input produces finer grains and reduces cracking tendency.
- HAZ: The original fine grain structure of the parent metal is coarsened by recrystallisation and grain growth at elevated temperatures. The grain size in the HAZ can increase by a factor of 2–5 compared to the parent metal.
- Precipitate evolution: The Mg-rich phases in the parent metal are partially dissolved at elevated HAZ temperatures, leading to a loss of strengthening. The dissolution temperature is approximately 200–300°C for the equilibrium Mg-rich phase.
Process Optimisation for AA5083 TIG Welding
Based on the findings of this study and general welding practice, the following process optimisation strategies are recommended:
- Heat input control: Maintain heat input below 15 kJ/mm to minimise HAZ softening and grain coarsening. This is achieved through lower current, higher travel speed, and/or pulsed welding.
- Shielding gas: Use 100% argon at 15–20 L/min for optimal shielding. Helium additions can improve penetration but increase hydrogen pickup risk.
- Filler metal selection: ER5183 (matching composition) or ER5356 (higher Mg content) filler metals can be used. ER5356 has lower hot cracking susceptibility but may reduce corrosion resistance.
- Joint design: Use a V-groove or U-groove joint with appropriate root gap and bevel angle to ensure full penetration without excessive heat input.
- Preheating: Generally not recommended for AA5083, as it increases HAZ softening. However, preheating to 100°C can reduce residual stresses in thick sections.
Integration with Engineering Practice
In the fabrication of marine pressure vessels, ship hulls, and offshore structures, AA5083 is extensively used due to its excellent corrosion resistance in seawater. The TIG welding process is commonly employed for root passes and for welding thin sections where precision is required. The findings of this study are directly applicable to the development of welding procedures for these applications.
For bimetal applications, such as AA5083 clad steel plates used in chemical processing equipment, the TIG welding process must be qualified for both the base metal and the clad layer. The heat input control is even more critical in these applications, as excessive heat can cause delamination of the clad layer or excessive intermetallic compound formation at the interface.
Key Reflections and Study Insights
The study by Chen et al. provides a valuable contribution to the understanding of AA5083 TIG weldability. The emphasis on microstructural characterisation and mechanical property mapping is essential for developing reliable welding procedures. A key insight is that the HAZ softening in non-heat-treatable aluminium alloys is an inherent limitation that cannot be eliminated but can be minimised through careful process control. For engineers designing welded structures from AA5083, the weld joint efficiency must be considered in the design calculations, and the HAZ softening zone must be treated as a potential weak link in the structure.