CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Welding Current on Microstructure and Properties of AZ31B Magnesium Alloy MIG Welded Joints
Literature Overview
Published in 2020 and supported by the Fujian Provincial University Industry-Academia Cooperation Project (Grant No. 2019H6021), this study from Fujian University of Technology, the Fujian Key Laboratory of New Material Preparation and Forming Technology, and Fujian Kunfu Co., Ltd. investigates the influence of welding current on the microstructure and mechanical properties of AZ31B magnesium alloy MIG welded joints. Magnesium alloys are increasingly used in lightweight structural applications, and understanding welding process effects is critical for their broader adoption.
Core Technical Content and Key Findings
Welding Parameters and Material Characteristics
AZ31B is a widely used wrought magnesium alloy with a composition of approximately 3% Al, 1% Zn, and 0.2% Mn in a magnesium matrix. The alloy exhibits good formability and moderate corrosion resistance but is sensitive to welding parameters due to the low melting point of magnesium (650°C) and its high thermal conductivity.
| Parameter | Low Current | Medium Current | High Current |
|---|---|---|---|
| Welding current (A) | 80-100 | 120-150 | 180-220 |
| Voltage (V) | 14-16 | 18-20 | 22-24 |
| Travel speed (mm/min) | 300-400 | 400-500 | 500-700 |
| Heat input (kJ/mm) | 0.6-0.8 | 1.0-1.4 | 1.8-2.5 |
| Wire diameter (mm) | 1.0 | 1.0 | 1.0 |
| Shielding gas | 99% Ar + 1% CO2 | 99% Ar + 1% CO2 | 99% Ar + 1% CO2 |
Microstructural Evolution
The welding current significantly affected the microstructure of the weld joint:
- Weld metal: Higher currents produced coarser grain structures due to increased heat input and slower cooling rates. The weld metal consisted of α-Mg matrix with Mg17Al12 and Mg2Zn11 intermetallic phases. At higher currents, these intermetallic phases became coarser and more continuous.
- Heat-affected zone (HAZ): The HAZ width increased with welding current, from approximately 1.5-2.5 mm at low current to 4.0-5.5 mm at high current. The HAZ exhibited distinct sub-regions:
- Recrystallized zone: Complete grain recrystallization with fine grains
- Partially recrystallized zone: Mixed recrystallized and unrecrystallized grains
- Grain growth zone: Coarsened grains with some precipitate dissolution
- Intermetallic phases: The Mg17Al12 phase, which forms at grain boundaries, became more continuous and thicker at higher welding currents. This continuous intermetallic network is detrimental to ductility and fracture resistance.
Mechanical Properties
| Property | Base Metal | Low Current Weld | Medium Current Weld | High Current Weld |
|---|---|---|---|---|
| Tensile strength (MPa) | 260 | 180-200 | 160-180 | 140-160 |
| Elongation (%) | 8-10 | 6-8 | 5-7 | 3-5 |
| Hardness (HV) | 60-65 | 45-50 | 40-45 | 35-40 |
| HAZ width (mm) | - | 1.5-2.5 | 2.5-4.0 | 4.0-5.5 |
| Defect density | None | Low | Moderate | High |
The mechanical properties decreased with increasing welding current due to:
- Coarser grain structures reducing strength
- Continuous intermetallic networks reducing ductility
- Increased porosity and defects at high currents
- Greater HAZ softening reducing joint strength
Process Analysis and Defect Control
Defect Formation Mechanisms
The study identified several defect types that are current-dependent:
- Porosity: Hydrogen porosity increased with current due to greater moisture pickup and gas evolution. Magnesium's high affinity for hydrogen makes it particularly susceptible to porosity.
- Hot cracking: Intergranular hot cracking occurred at higher currents due to the continuous Mg17Al12 phase network restricting grain boundary sliding during solidification.
- Undercut and lack of fusion: These defects were more common at lower currents due to insufficient heat input, but could also occur at very high currents due to excessive arc force.
Optimal Current Window
The study identified an optimal current range of 120-150 A for AZ31B MIG welding, which provided:
- Acceptable mechanical properties (tensile strength > 160 MPa, elongation > 5%)
- Manageable defect levels
- Reasonable welding productivity
- Controlled HAZ width
Connection with Cladding and Bimetal Applications
Although this study focuses on magnesium alloy welding rather than cladding, several principles are relevant to cladding and bimetal fabrication:
- Heat input control: The sensitivity of mechanical properties to heat input is universal. In cladding applications, controlling heat input is critical for managing dilution, residual stresses, and microstructural quality.
- Intermetallic phase control: The formation of brittle intermetallic phases at high temperatures is a common concern in dissimilar metal welds. Understanding intermetallic formation mechanisms helps predict and control the metallurgical compatibility of cladding interfaces.
- Defect prevention: The study's analysis of defect formation mechanisms provides insights that can be applied to cladding operations, where defects such as porosity, lack of fusion, and cracking are critical quality concerns.
Key Questions and Reflections
The study raises several questions for cladding engineers:
- How does welding current affect the dilution level and metallurgical bond quality in cladding applications involving magnesium-based materials?
- Can the optimal current window identified for AZ31B welding be adapted for cladding magnesium alloys on steel substrates?
- What are the implications of intermetallic phase formation for the long-term performance of magnesium-based cladding layers?
The research provides valuable insights into the welding of magnesium alloys. For cladding engineers, the transferable insights include the importance of heat input control, the role of intermetallic phases in determining joint properties, and the need for careful process optimization to avoid defects.
Study Insights and Implications
This study provides systematic data on the effects of welding current on AZ31B magnesium alloy weld quality. The findings have several implications for engineering practice:
- Process optimization: The identification of an optimal current window (120-150 A) provides practical guidance for welding magnesium alloys. Engineers should conduct similar parameter studies for their specific applications to identify optimal process windows.
- Quality control: The correlation between welding parameters and defect formation enables better quality control through process monitoring. Real-time monitoring of current, voltage, and travel speed can help prevent defects.
- Material selection: The study highlights the sensitivity of magnesium alloy properties to welding parameters. Engineers should consider this sensitivity when selecting materials for welded applications, particularly in critical structural or pressure-containing applications.
- Standards compliance: The study's data can inform the development of welding procedure specifications (WPS) and welding procedure qualifications (WPQ) for magnesium alloys, which are currently less well-developed than for steels and aluminum alloys.
The research underscores the importance of systematic process development for lightweight alloys. As magnesium alloys gain wider acceptance in structural applications, the welding community must develop comprehensive process knowledge and standards to ensure reliable and safe welded joints.