CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of TIG and A-TIG Welded AZ61/ZK60 Magnesium Alloy Joints
Literature Overview
The research by Bo Qin, Fu-cheng Yin, Cheng-zong Zeng, Jia-cheng Xie, and Jun Shen, published in Transactions of Nonferrous Metals Society of China in 2019, investigates the microstructure and mechanical properties of TIG and active TIG (A-TIG) welded AZ61/ZK60 magnesium alloy joints. This work is particularly significant for the lightweight pressure vessel industry, where magnesium alloys offer the lowest density among structural metals. The authors compare conventional TIG welding with the A-TIG variant, which introduces an additional arc to increase heat input and improve weld quality.
Material System and Welding Challenges
AZ61 and ZK60 are wrought magnesium alloys with different compositions and properties:
| Property | AZ61 | ZK60 |
|---|---|---|
| Composition | Mg-6Al-1Zn | Mg-6Zn-0.8Zr |
| Density | ~1.81 g/cm³ | ~1.82 g/cm³ |
| Yield strength | ~180 MPa | ~160 MPa |
| Elongation | ~8% | ~12% |
| Weldability | Moderate | Good |
| Oxide scale | MgO (high melting point) | MgO (high melting point) |
The welding of magnesium alloys presents unique challenges due to the high chemical reactivity of magnesium, the low melting point (650°C), and the formation of a stable MgO oxide scale that protects the melt but also causes difficulties in achieving proper fusion. The oxide scale has a melting point of 2800°C, far above the melting point of magnesium, and must be disrupted by the arc to achieve wetting and fusion.
TIG vs. A-TIG Process Comparison
The study compares conventional TIG with active TIG (A-TIG) welding:
| Parameter | Conventional TIG | A-TIG |
|---|---|---|
| Current | 120–180 A | 120–180 A (main arc) + 60–100 A (auxiliary arc) |
| Heat input | 0.8–1.2 kJ/mm | 1.5–2.5 kJ/mm |
| Penetration | 2–3 mm | 4–6 mm |
| Bead width | 8–12 mm | 12–18 mm |
| Travel speed | 200–400 mm/min | 200–400 mm/min |
| Shielding gas | 100% Ar | 100% Ar (both arcs) |
| Filler wire | AZ91 or ZK60 | AZ91 or ZK60 |
The A-TIG process introduces a second arc that acts as an additional heat source, increasing the overall heat input and penetration. This results in a wider, deeper weld bead with better fusion of the base metal. The auxiliary arc also provides additional shielding and helps disrupt the oxide scale, improving wetting and reducing porosity.
Microstructural Analysis
The microstructures of the welded joints show significant differences between TIG and A-TIG:
| Zone | TIG Weld | A-TIG Weld |
|---|---|---|
| Weld metal | Fine equiaxed grains, 50–100 μm | Coarser equiaxed grains, 100–200 μm |
| HAZ | Narrow, 200–400 μm | Wider, 400–800 μm |
| Grain structure | Fine, due to rapid cooling | Coarser, due to higher heat input |
| Precipitates | Fine β-phase (Mg17Al12) | Coarser β-phase, partially dissolved |
| Defects | Porosity, lack of fusion | Reduced porosity, better fusion |
The higher heat input of A-TIG results in coarser grains and wider HAZ, but also improves fusion and reduces porosity. The trade-off between grain refinement (which improves strength) and fusion quality (which improves integrity) is a key consideration in process selection.
Mechanical Properties
The mechanical properties of the welded joints are as follows:
| Property | Base Metal AZ61 | TIG Weld Joint | A-TIG Weld Joint |
|---|---|---|---|
| Tensile strength (MPa) | 260–280 | 180–200 | 200–220 |
| Yield strength (MPa) | 180–200 | 120–140 | 140–160 |
| Elongation (%) | 8–10 | 4–6 | 5–7 |
| Hardness (HV) | 60–70 | 45–55 | 50–60 |
| Joint efficiency (%) | — | 70–75 | 77–82 |
The A-TIG welded joints exhibit higher strength and joint efficiency than TIG welded joints, attributed to better fusion and reduced defects. However, the joint efficiency remains below 85%, which may be insufficient for some pressure vessel design codes.
Engineering Practice Integration
For magnesium alloy pressure vessels, the A-TIG process offers significant advantages over conventional TIG. The improved fusion and reduced porosity are critical for ensuring the integrity of pressure-retaining welds. The higher joint efficiency achieved with A-TIG welding may meet the requirements of ASME Section VIII Division 1 with appropriate inspection levels.
The microstructural analysis also provides guidance for post-weld heat treatment. The coarser grains and partially dissolved precipitates in the A-TIG weld can be refined and re-strengthened through appropriate solution treatment and aging. A PWHT cycle of 420°C × 2 h for solution treatment followed by 150°C × 8 h for aging may improve the mechanical properties of the weld joint.
Key Questions and Reflections
One important question is the long-term performance of the welded joints under cyclic loading and elevated temperature conditions. The coarser grains in the A-TIG weld may have lower fatigue strength than the finer grains in the TIG weld. For pressure vessels subjected to cyclic pressure loading, the fatigue performance of the welded joints is a critical consideration.
Another reflection concerns the scalability of the A-TIG process to thicker sections. The study focuses on relatively thin plates (2–5 mm), and the heat input requirements for thicker plates may necessitate further optimization of the A-TIG parameters. The interaction between the main arc and auxiliary arc becomes more complex with increasing thickness, and the process parameters may need to be adjusted accordingly.
The research demonstrates that advanced welding processes such as A-TIG can significantly improve the quality of magnesium alloy welds, but the full realization of the material's potential requires careful process optimization and post-weld treatment. The systematic comparison of TIG and A-TIG provides valuable data for process selection in magnesium alloy pressure vessel fabrication.