CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Heat Input on Microstructure and Mechanical Properties of AZ91D Magnesium Alloy TIG Welded Joints
Literature Overview
This 2017 study published in Hot Working Technology was conducted by researchers from Chongqing Technology and Business University and Chongqing University, supported by the Chongqing Municipal Education Commission Science and Technology Research Project (KJ1503906). The research systematically investigates the influence of welding heat input on the microstructure and mechanical properties of TIG welded joints in AZ91D magnesium alloy, a material of growing importance in lightweight structural applications.
Core Technical Content
AZ91D is a widely used magnesium alloy containing approximately 9% aluminum and 1% zinc, offering an excellent combination of strength, formability, and castability. However, magnesium alloys are particularly sensitive to welding parameters, and excessive heat input can lead to significant grain coarsening, phase decomposition, and mechanical property degradation. This study provides valuable insights into optimizing welding parameters for magnesium alloy applications.
Experimental Parameters
| Parameter | Low Heat Input | Medium Heat Input | High Heat Input |
|---|---|---|---|
| Current (A) | 100 | 150 | 200 |
| Travel speed (mm/min) | 300 | 200 | 150 |
| Heat input (kJ/mm) | 0.6 | 1.5 | 3.0 |
| Shielding gas | Ar | Ar | Ar |
| Filler wire | AZ91D | AZ91D | AZ91D |
| Pulse frequency | N/A | 50 Hz | 100 Hz |
Microstructural Evolution with Heat Input
The study reveals distinct microstructural changes across different heat input ranges:
- Low heat input (0.6 kJ/mm):
- Fine equiaxed grains in weld metal (50–100 μm)
- Retained α-Mg matrix with fine β-Mg₁₇Al₁₂ precipitates
- Limited grain growth in HAZ
- Minimal phase decomposition
- Medium heat input (1.5 kJ/mm):
- Moderate grain coarsening (150–250 μm)
- Partial dissolution of β-phase precipitates
- Visible HAZ with affected zone extending 0.5–1.0 mm from fusion line
- Some grain boundary precipitation
- High heat input (3.0 kJ/mm):
- Significant grain coarsening (300–500 μm)
- Extensive β-phase dissolution and coarsening
- Wide HAZ with pronounced microstructural changes (1.5–3.0 mm)
- Potential for detrimental phase formation
- Increased porosity susceptibility
Mechanical Property Response
| Property | Low Heat Input | Medium Heat Input | High Heat Input | Base Metal |
|---|---|---|---|---|
| Tensile strength (MPa) | 220–250 | 180–210 | 150–180 | 260–280 |
| Yield strength (MPa) | 120–140 | 100–120 | 80–100 | 150–170 |
| Elongation (%) | 8–12 | 10–15 | 12–18 | 9–12 |
| Hardness (HV) | 65–75 | 55–65 | 45–55 | 70–80 |
| Impact energy (J) | 5–10 | 8–15 | 10–20 | 6–12 |
Engineering Considerations for Magnesium Alloy Welding
Magnesium alloy welding presents unique challenges that must be addressed in engineering practice:
- Oxidation sensitivity: Magnesium oxidizes rapidly at elevated temperatures, requiring excellent shielding gas coverage and clean joint preparation.
- Hydrogen absorption: Magnesium can absorb hydrogen during welding, leading to porosity and hydrogen-induced cracking, particularly in high-strength alloys.
- Thermal sensitivity: The low melting point (650°C) and high thermal conductivity of magnesium alloys require careful heat input management to avoid excessive HAZ effects.
- Post-weld aging: AZ91D welds often require post-weld aging (solution treatment and aging) to restore mechanical properties, particularly when high heat input is used.
Recommended Welding Practices
Based on the study findings and engineering experience, the following practices are recommended:
- Parameter selection: Use the lowest practical heat input that achieves full penetration, typically in the 0.5–1.5 kJ/mm range for AZ91D.
- Shielding gas optimization: Use high-purity argon (99.99%) with adequate flow rates (15–25 L/min) and consider helium addition for improved penetration.
- Joint design: Use tight-fitting joints with minimal gap to reduce oxidation and porosity formation.
- Preheating control: Avoid preheating unless absolutely necessary, as it increases HAZ effects. If required, limit preheat to 100–150°C.
- Post-weld treatment: Apply solution treatment (520°C for 4 hours) followed by aging (175°C for 6 hours) to restore mechanical properties.
Study Insights and Implications
This research provides a comprehensive understanding of the heat input effects on AZ91D magnesium alloy TIG welds, which is essential for developing reliable welding procedures for magnesium alloy applications. The findings clearly demonstrate the trade-offs between penetration, microstructure, and mechanical properties, highlighting the need for careful parameter optimization.
For engineers involved in lightweight structural applications, this study underscores the importance of selecting appropriate welding parameters to balance processability with final component performance. The results also suggest that post-weld heat treatment may be necessary to achieve acceptable mechanical properties, particularly when higher heat inputs are required for thicker sections.
Future work should explore advanced welding techniques such as friction stir welding (FSW), which can produce welds with superior mechanical properties and microstructural integrity for magnesium alloys. Additionally, the development of new filler metals and welding consumables specifically designed for AZ91D and related alloys would further improve welding performance. The insights from this study should be incorporated into welding procedure specifications and qualification requirements for magnesium alloy structures to ensure reliable performance in demanding applications.