CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of TIG-MIG Composite Welds in AZ31B Magnesium Alloy Profiles
Literature Overview and Material Significance
This 2020 publication in the journal Hot Working Technology, authored by Liu Baoshuan, Jiang Yinglong, Li Long, Guo Yangyang, and Pan Houhong from Southwest Jiaotong University, investigates the microstructure and mechanical properties of composite welds produced by combining TIG and MIG processes in AZ31B magnesium alloy profiles. AZ31B is a widely used Mg-Al-Zn alloy system known for its excellent specific strength, good corrosion resistance, and relatively good castability. However, magnesium alloy welding remains one of the most challenging operations in the industry due to the material's low melting point (approximately 450°C), high thermal conductivity, susceptibility to oxidation, and the formation of brittle intermetallic compounds.
Process Configuration and Welding Challenges
The TIG-MIG composite welding approach for magnesium alloys addresses several inherent challenges. TIG welding provides a stable, controlled arc that is well-suited for root passes and thin sections, while MIG welding offers higher deposition rates for filling and cap passes. The combination allows engineers to optimize each pass for its specific function within the weld build-up sequence.
| Parameter | TIG Welding | MIG Welding |
|---|---|---|
| Current range | 60–120 A | 100–200 A |
| Arc voltage | 10–16 V | 16–22 V |
| Travel speed | 150–300 mm/min | 300–600 mm/min |
| Shielding gas | 100% Ar or He/Ar mix | 100% Ar or He/Ar mix |
| Filler wire | AZ91 or AZ31 | AZ91 or AZ31 |
| Typical plate thickness | 2–6 mm | 4–12 mm |
The primary welding challenges for AZ31B include:
- Hydrogen porosity: Magnesium has a high solubility for hydrogen at elevated temperatures, and rapid cooling can lead to hydrogen-induced porosity.
- Oxide inclusion: Magnesium oxide (MgO) has a very high melting point (approximately 2852°C) and forms a tenacious layer on the molten weld pool surface.
- Hot cracking: The Mg-Al system has a wide solidification range, making it susceptible to hot cracking during solidification.
- Weld pool instability: The low surface tension of molten magnesium can lead to weld pool collapse and spatter.
Microstructural Analysis and Phase Evolution
The microstructure of the TIG-MIG composite weld in AZ31B typically exhibits distinct zones with different characteristics. The weld metal deposited by TIG shows a finer grain structure due to the lower heat input, while the MIG-deposited weld metal shows a coarser structure with potentially more intermetallic phases.
The primary phases in AZ31B welds include:
- α-Mg matrix: The primary solid solution phase
- β-Mg₁₇Al₁₂: A brittle intermetallic compound that forms at grain boundaries and can significantly reduce ductility
- MgO inclusions: Oxide particles that act as crack initiation sites
- Hydrogen pores: Gas cavities formed by dissolved hydrogen during solidification
The distribution and morphology of the β-phase are critical to mechanical performance. Excessive β-phase at grain boundaries can lead to intergranular fracture and reduced toughness. Heat treatment, such as solution treatment followed by aging, can dissolve some of the β-phase and improve mechanical properties.
Mechanical Property Evaluation
The mechanical properties of the composite weld are evaluated through standardized tests:
| Test Method | Typical Result for AZ31B Weld |
|---|---|
| Tensile strength | 180–250 MPa |
| Yield strength | 100–150 MPa |
| Elongation | 5–15% |
| Vickers hardness | 50–80 HV |
| Impact energy | 5–20 J (Charpy V-notch) |
The TIG-MIG composite weld typically shows a slight reduction in mechanical properties compared to the base metal, which is expected due to the formation of intermetallic phases and the absence of precipitate strengthening that exists in the as-extruded base material. The reduction is generally more pronounced in the HAZ where the thermal cycling can cause over-aging of precipitates.
Engineering Practice and Quality Assurance
For engineering applications, the quality of AZ31B welds must be assessed through a combination of NDT and destructive testing. Recommended NDT methods include:
- Ultrasonic testing (UT): Effective for detecting volumetric defects such as porosity and inclusions
- Radiographic testing (RT): Suitable for thin sections where UT may be difficult
- Dye penetrant testing (PT): Essential for detecting surface cracks and porosity
- X-ray fluorescence (XRF): For verifying filler wire composition
The acceptance criteria for magnesium alloy welds should follow standards such as GB/T 20449, ASTM F2974, or EN 13445, depending on the application. For pressure vessel applications, the weld procedure must be qualified in accordance with the applicable pressure vessel code, such as NB/T 47014 or ASME IX.
Key Reflections and Process Improvement
The study highlights several important aspects of magnesium alloy welding that are relevant to broader cladding and bimetal fabrication practices. First, the composite TIG-MIG approach demonstrates that combining welding processes can overcome the limitations of any single process. This philosophy is applicable to other challenging material combinations, such as titanium-to-steel or copper-to-steel joints.
Second, the microstructural analysis reveals that the welding process parameters have a profound influence on the final microstructure and, consequently, the mechanical properties. Fine-tuning of heat input, travel speed, and interpass temperature can significantly improve weld quality. For example, reducing the interpass temperature below 150°C can help control the β-phase formation and improve ductility.
Third, the study underscores the importance of filler metal selection. The choice between AZ31 and AZ91 filler wires has a significant impact on weld composition and microstructure. AZ91 filler contains more aluminum and produces a weld with a higher β-phase content, which may be beneficial for strength but detrimental to ductility.
Summary
The investigation of TIG-MIG composite welds in AZ31B magnesium alloy profiles provides valuable insights into the microstructural evolution and mechanical performance of magnesium alloy welds. The composite process approach offers a practical solution to the challenges of magnesium alloy welding, combining the precision of TIG with the productivity of MIG. For engineers working on bimetal products and pressure vessels, the lessons from this study extend to any dissimilar material welding scenario where microstructural control is critical. The emphasis on understanding the relationship between process parameters, microstructure, and properties is fundamental to developing reliable welding procedures for demanding applications.