CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of AZ91D Magnesium Alloy TIG Welded Joints
Literature Overview and Research Context
This 2009 publication from Inner Mongolia University of Technology, authored by Liu Jun, Dong Junhui, Wu Yongjun, and Meng Xianchao, investigates the microstructure and mechanical properties of AZ91D magnesium alloy TIG welded joints. The research was supported by the Inner Mongolia Autonomous Region Higher Education Research Project (Grant No. NJZY070601) and published in Light Alloy Fabrication Technology. AZ91D, a widely used wrought magnesium alloy containing approximately 9 wt.% aluminum and 1 wt.% zinc, is extensively employed in automotive, aerospace, and consumer electronics applications due to its excellent specific strength and weight reduction potential. However, magnesium alloys are notoriously difficult to weld due to their high reactivity with oxygen and nitrogen, low melting point, and susceptibility to hot cracking.
The TIG welding of AZ91D presents unique challenges related to the formation of oxide films, the limited solubility of aluminum and zinc in the solid solution, and the tendency for solidification cracking in the weld zone. This research provides valuable insights into the metallurgical behavior of AZ91D during TIG welding and offers guidance for the development of reliable welding procedures for magnesium alloy components.
Welding Process Parameters and Shielding Considerations
The TIG welding parameters for AZ91D are carefully selected to minimize the adverse effects of the high reactivity of magnesium with the atmosphere. The shielding gas coverage is critical, with pure argon providing adequate protection at flow rates of 15–20 L/min. The welding current is typically in the range of 80–150 A for sheet thicknesses of 2–5 mm, with a travel speed of 300–600 mm/min. The heat input, calculated as the product of voltage, current, and time divided by travel speed, is maintained in the range of 0.4–0.8 kJ/mm to limit the thermal cycle severity and reduce the risk of hot cracking.
| Parameter | Typical Value for AZ91D TIG Welding |
|---|---|
| Welding Current | 80–150 A |
| Arc Voltage | 12–16 V |
| Travel Speed | 300–600 mm/min |
| Heat Input | 0.4–0.8 kJ/mm |
| Shielding Gas | 100% Argon |
| Gas Flow Rate | 15–20 L/min |
| Tungsten Electrode | Pure tungsten, 2.4 mm |
| Filler Wire | AZ91D or AZ91 |
| Preheat Temperature | 100–200 °C |
The use of a pure tungsten electrode is preferred over thoriated or lanthanated tungsten electrodes to minimize the risk of tungsten inclusion and to avoid the potential for thorium contamination, which is particularly critical for magnesium alloys that are used in biomedical and food-contact applications. The preheat temperature of 100–200 °C helps to reduce the thermal gradient and minimize the risk of solidification cracking, while also helping to remove surface moisture that could lead to porosity formation.
Microstructural Evolution in the Weld Zone
The microstructure of the AZ91D TIG welded joints is characterized by three distinct zones: the weld nugget, the heat-affected zone, and the unaffected base metal. The weld nugget microstructure consists of α-Mg solid solution dendrites with interdendritic Mg17Al12 (β-phase) eutectic. The grain structure is typically columnar near the fusion boundary and transitions to equiaxed in the center of the weld, driven by the thermal gradient and the solidification rate. The β-phase morphology in the weld nugget is finer and more uniformly distributed than in the base metal, due to the higher cooling rates and the modified composition resulting from the dilution with the filler metal.
In the heat-affected zone, the microstructure is characterized by a gradient of thermal effects. Near the fusion boundary, the β-phase particles undergo partial dissolution and coarsening, leading to a reduction in the volume fraction of the β-phase and a slight coarsening of the remaining particles. Further from the fusion line, the thermal cycle is insufficient to cause significant β-phase dissolution, but it is sufficient to promote grain growth and the coarsening of secondary phases. The grain structure in the HAZ is typically coarser than in the base metal, which can lead to a reduction in the mechanical properties, particularly in terms of toughness and fatigue resistance.
| Zone | Dominant Phases | Grain Structure | Tensile Strength (MPa) | Elongation (%) |
|---|---|---|---|---|
| Base Metal | α-Mg, Mg17Al12 | Equiaxed | 260–280 | 8–10 |
| Weld Nugget | α-Mg, fine β-phase | Fine columnar/equiaxed | 220–250 | 6–8 |
| HAZ | α-Mg, coarse β-phase | Coarse equiaxed | 200–230 | 5–7 |
Mechanical Properties and Failure Analysis
The mechanical properties of the AZ91D TIG welded joints show a reduction in tensile strength and elongation compared to the base metal, which is typical for magnesium alloy welds. The reduction in tensile strength is attributed to the coarsening of the β-phase particles in the HAZ and the formation of a finer, more dispersed β-phase in the weld nugget that provides less strengthening than the base metal microstructure. The elongation is reduced due to the presence of columnar grains in the weld nugget and the coarsening of the grain structure in the HAZ, which reduces the ductility and the ability of the joint to accommodate plastic deformation.
The failure analysis of the welded joints typically reveals a mixed mode of fracture, with ductile fracture in the weld nugget and brittle fracture in the HAZ. The brittle fracture in the HAZ is attributed to the coarsening of the β-phase particles and the reduction in the volume fraction of the β-phase, which reduces the ability of the HAZ to accommodate plastic deformation. The use of post-weld heat treatment, such as solution treatment followed by aging, can improve the mechanical properties of the welded joints by dissolving the coarse β-phase particles and redistributing the aluminum and zinc elements in the matrix.
Engineering Implications and Study Insights
The TIG welding of AZ91D magnesium alloy presents a significant challenge for engineers working on lightweight structural components, particularly in the automotive and aerospace industries. The key to achieving reliable welds lies in the careful control of the welding parameters, the use of appropriate shielding gas, and the application of post-weld heat treatment to optimize the microstructure and mechanical properties. The findings from this research highlight the importance of the β-phase morphology and distribution in determining the mechanical properties of AZ91D welded joints. For engineers designing magnesium alloy components, the selection of welding parameters to minimize the thermal cycle severity and the application of post-weld heat treatment to refine the β-phase distribution are critical steps in ensuring the reliability and performance of the welded joints. This research provides a valuable foundation for the development of welding procedures for AZ91D and other magnesium alloys, contributing to the advancement of lightweight structural engineering.