Zhuojin Pipe Fitting Co., LtdAutomatic TIG Welding of AZ31 Magnesium Alloy: Microstructure, Texture, and Mechanical Properties
Literature Overview
This paper by Liu Hong-tao and colleagues, published in the International Journal of Minerals, Metallurgy and Materials (Vol. 24, No. 1, 2017), presents a comprehensive study of the automatic TIG welding (ATIGW) of AZ31 magnesium alloy using a six-axis robot. The authors employed optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD) to investigate the microstructure evolution and texture changes across the weld joint. The study also reports microhardness distribution, tensile properties, and fracture characteristics. The research was supported by the National Key Research and Development Plan of China and multiple provincial funding agencies.
Background: Magnesium Alloy Weldability and AZ31 Properties
AZ31 is one of the most widely used wrought magnesium alloys, with approximately 3 wt% Al and 1 wt% Zn. It offers an excellent combination of low density (1.74 g/cm³), good formability, and adequate strength, making it ideal for lightweight structural applications in automotive and aerospace industries. However, magnesium alloys present unique welding challenges:
- Low melting point (650 °C) and low boiling point (1090 °C) result in severe vaporization of magnesium during welding, leading to dilution and porosity.
- High thermal conductivity (72 W/m·K) requires high heat input for adequate penetration, but excessive heat input leads to grain coarsening and loss of mechanical properties.
- Thermodynamically unstable oxide film (MgO, Al₂O₃) is easily removed by arc cleaning but can re-form rapidly in the weld pool, causing porosity and lack of fusion.
- Hydrogen absorption from moisture in the atmosphere leads to hydrogen porosity, which is particularly detrimental to the fatigue life of magnesium alloy welds.
The use of automatic TIG welding with a six-axis robot addresses several of these challenges by providing consistent, repeatable welding parameters and minimizing human variability. The robotic system can maintain precise torch position, travel speed, and torch angle throughout the weld length, which is critical for achieving uniform weld quality.
Microstructural Evolution Across the Weld Joint
The EBSD analysis revealed distinct microstructural regions across the AZ31 weld joint, each with characteristic features:
Base Metal (BM)
The base metal exhibited a typical hot-rolled texture with the {0002} basal plane largely parallel to the sheet rolling plane. The maximum pole density was 9.45, indicating a moderate texture strength. The grain structure consisted of elongated grains in the rolling direction, with an average grain size of 30–50 μm.
Heat-Affected Zone (HAZ)
The HAZ showed coarse recrystallized grains with an average grain size of 80–120 μm, significantly larger than the base metal. This grain coarsening is attributed to the peak temperature in the HAZ exceeding the recrystallization temperature of AZ31 (approximately 200–250 °C), combined with the slow cooling rate associated with the high thermal conductivity of magnesium. The coarse grains in the HAZ are a concern for mechanical properties, as they reduce the yield strength and fatigue resistance of the joint.
Fusion Zone (FZ)
The fusion zone exhibited epitaxial growth of columnar grains, growing from the HAZ into the weld center. The columnar grains had an average width of 20–40 μm and a length of 100–200 μm. The epitaxial growth indicates that the solidification was controlled by the thermal gradient, with the columnar grains growing preferentially along the direction of maximum heat extraction.
| Region | Grain Structure | Average Grain Size | Texture Characteristic |
|---|---|---|---|
| Base Metal (BM) | Elongated, rolled | 30–50 μm | {0002} basal plane parallel to rolling plane; pole density 9.45 |
| HAZ | Coarse recrystallized | 80–120 μm | Partially retained rolling texture; weakened texture |
| Fusion Zone (FZ) | Columnar, epitaxial | 20–40 μm width | c-axis inclined ~25° to welding direction; pole density 12.9 |
Texture Analysis and Implications
The EBSD texture analysis revealed substantial changes in crystallographic texture between the base metal and the fusion zone. In the base metal, the {0002} basal plane was largely parallel to the sheet rolling plane, which is characteristic of hot-rolled magnesium alloys. In the fusion zone, the c-axis of the crystal lattice was inclined approximately 25° with respect to the welding direction, and the maximum pole density increased from 9.45 in the BM to 12.9 in the FZ.
This texture change has significant implications for the mechanical properties of the weld joint. Magnesium alloys are highly anisotropic, and the mechanical properties are strongly dependent on the crystallographic orientation relative to the loading direction. The inclined c-axis in the FZ means that the basal slip systems are not optimally aligned with the tensile loading direction, which can reduce the tensile strength and ductility of the weld metal. However, the increased pole density indicates a stronger texture, which may improve the consistency of mechanical properties in the through-thickness direction.
Mechanical Properties and Fracture Characteristics
The microhardness distribution across the weld joint showed a typical pattern:
| Region | Microhardness (HV) | Relative to BM |
|---|---|---|
| Base Metal (BM) | 55–62 | 100% |
| Fusion Zone (FZ) | 48–55 | 85–95% |
| HAZ (coarse grain) | 42–50 | 75–85% |
| HAZ (fine grain) | 50–58 | 90–100% |
The tensile properties of the AZ31 ATIG weld joint showed a tensile strength of 165–180 MPa and an elongation of 8–12%, compared to the base metal values of 210–220 MPa and 20–25%. The joint efficiency, defined as the ratio of weld tensile strength to base metal tensile strength, was approximately 80–85%, which is acceptable for non-critical structural applications but insufficient for safety-critical components.
The fracture analysis revealed that the fracture originated in the HAZ, specifically in the coarse-grained region, and propagated through the HAZ and FZ. The fracture surface exhibited a mixed mode of cleavage and ductile dimples, with the cleavage facets corresponding to the coarse grains in the HAZ. This confirms that the HAZ is the weakest region of the weld joint and the primary site of fracture initiation.
Engineering Practice and Quality Control
For the production welding of AZ31 magnesium alloy using ATIG, the following quality control measures are essential:
- Shielding gas: A high-purity argon gas (≥ 99.99%) should be used as the primary shielding gas, with a flow rate of 15–20 L/min. A back-purge of argon or helium should be used to protect the weld root from atmospheric contamination.
- Welding parameters: The welding current should be optimized to achieve adequate penetration without excessive heat input. Typical parameters for 2–4 mm thick AZ31 plate include: current 100–150 A, travel speed 5–8 mm/s, torch angle 15–25° from vertical, and tungsten electrode diameter 1.6–2.4 mm.
- Preheat: A modest preheat of 50–100 °C can help reduce the thermal gradient and minimize the risk of cracking, but excessive preheat should be avoided to prevent grain coarsening in the HAZ.
- Post-weld treatment: A stress relief annealing at 200–250 °C for 1–2 hours can reduce residual stresses and improve the fatigue resistance of the weld joint. However, this treatment may further coarsen the HAZ grains, so the temperature and duration must be carefully controlled.
- Surface preparation: The base metal surfaces should be cleaned by mechanical grinding or chemical etching to remove the oxide film and any surface contamination. The cleaned surface should be within 1 hour of welding to prevent re-oxidation.
Study Insights and Limitations
This study provides a comprehensive microstructural and mechanical characterization of AZ31 ATIG weld joints, with particular emphasis on the texture evolution across the joint. The use of EBSD for texture analysis is a significant advancement over traditional XRD texture analysis, as it provides spatially resolved texture information at the grain level.
The key finding that the HAZ is the weakest region of the weld joint, with coarse recrystallized grains and reduced microhardness, has important implications for welding procedure optimization. Future research should focus on developing welding parameters or post-weld treatments that can refine the HAZ grain structure without compromising the weld metal properties. Options include pulsed TIG welding, which can reduce the heat input and minimize HAZ grain coarsening, or the use of a consumable insert or flux to modify the HAZ microstructure.
The joint efficiency of 80–85% is acceptable for many automotive applications, but it is insufficient for aerospace or structural applications where higher joint efficiency is required. Engineers considering the use of ATIG welding for AZ31 magnesium alloy should carefully evaluate the joint efficiency requirements for their specific application and consider alternative joining methods, such as friction stir welding (FSW) or adhesive bonding, if higher joint efficiency is needed.