Microstructure and Mechanical Properties of AZ31 Magnesium Alloy TIG Welds Under Varying Heat Input

Literature Overview

The study by Liao Huimin, He Hongyan, Yang Yang, and Zeng Ming, published in Materials in Mechanical Engineering (Volume 41, Issue 3, 2017, pp. 24-28), investigates the effects of welding heat input on the microstructure and mechanical properties of TIG welded AZ31 magnesium alloy joints. The research was conducted on 2 mm thick hot-rolled AZ31 magnesium alloy plates welded using AC TIG (GTAW) at varying current levels. This work is particularly relevant to the growing application of magnesium alloys in lightweight structural components for automotive and aerospace industries.

Experimental Methodology and Key Findings

The researchers employed AC TIG welding on 2 mm hot-rolled AZ31 magnesium alloy plates with varying welding currents. The heat input was systematically varied to establish the relationship between process parameters and weld quality. Metallographic examination, microhardness profiling, and tensile testing were performed on the welded joints.

Microstructural Evolution

The base metal microstructure consists of a single-phase α-Mg solid solution, which is consistent with the hot-rolled condition of the AZ31 alloy. In the weld zone and heat-affected zone (HAZ), the microstructure comprises α-Mg solid solution with dispersed β-Mg₁₇Al₁₂ intermetallic phases. The β-Mg₁₇Al₁₂ phase is a hard, brittle intermetallic compound that forms during solidification and subsequent cooling.

As welding heat input increases, two significant microstructural changes occur: first, the grain size in the weld zone increases due to enhanced grain growth during the extended high-temperature exposure; second, the volume fraction of β-Mg₁₇Al₁₂ phase increases because higher heat input promotes more complete melting and slower cooling rates that favor intermetallic precipitation.

Mechanical Property Response

The tensile strength of the welded joint exhibits a characteristic peak behavior with respect to heat input. At low heat input, the strength is limited by incomplete fusion and potential lack of penetration. As heat input increases toward the optimal value, better fusion and more uniform microstructure contribute to strength improvement. Beyond the optimal heat input, grain coarsening and excessive β-Mg₁₇Al₁₂ precipitation lead to strength degradation.

The optimal welding current of 90 A yields a tensile strength of 223 MPa, which represents 89.2% of the base metal strength. This weld strength ratio is considered acceptable for many engineering applications of AZ31 magnesium alloy, although it falls short of the ideal target of 95% or higher.

Fracture Analysis

The fracture location predominantly occurs in the HAZ rather than in the weld metal or base metal. The fracture mode is characterized as quasi-cleavage, indicating a brittle-to-ductile transition behavior. The HAZ is the weakest region because it experiences peak temperatures below the melting point, resulting in grain growth without the homogenizing effect of full melting. The β-Mg₁₇Al₁₂ phases along grain boundaries in the HAZ act as crack initiation sites.

Process Optimization Insights

The microhardness distribution reveals an interesting pattern: the weld zone exhibits the highest hardness, followed by the base metal, with the HAZ showing the lowest hardness. This counterintuitive result can be explained by the fact that the HAZ experiences peak temperatures that are insufficient for complete recrystallization but high enough to promote grain growth and stress relief. The weld zone achieves higher hardness due to rapid solidification and the presence of β-Mg₁₇Al₁₂ precipitates.

The quasi-cleavage fracture in the HAZ suggests that further improvement in ductility could be achieved through post-weld heat treatment. Solution treatment followed by aging could dissolve the β-Mg₁₇Al₁₂ phases and allow controlled precipitation, thereby improving the toughness of the HAZ. However, such treatment may be impractical for large structural components.

Engineering Practice Considerations

For magnesium alloy welding in production environments, the following practical guidelines emerge from this study:

1. Heat input control is critical: The optimal window is narrow, and process monitoring must ensure consistent current levels throughout the weld.
2. Shielding gas purity must be maintained: Magnesium is highly reactive with oxygen and nitrogen, and any contamination leads to oxide inclusions and porosity.
3. Joint design should minimize HAZ weakness: Backing plates or backing gas shielding can reduce HAZ oxidation and improve joint integrity.
4. Post-weld inspection should focus on the HAZ: Since fracture initiates in the HAZ, non-destructive testing should be optimized to detect defects in this region.

Study Insights and Implications

This research provides valuable quantitative data on the heat input sensitivity of AZ31 magnesium alloy TIG welds. The identification of 90 A as the optimal current for 2 mm plates offers a practical starting point for process development. However, I note that the study does not address the effects of welding speed independently of current, which would provide a more complete understanding of the heat input parameter space.

The finding that fracture occurs in the HAZ with quasi-cleavage morphology has significant implications for fatigue design. Magnesium alloy welded joints in cyclic loading applications would require careful fatigue assessment, and the HAZ should be treated as the critical region for crack initiation. The 89.2% weld strength ratio, while acceptable for static loading, may be insufficient for fatigue-critical applications where the stress concentration at the HAZ/weld interface could accelerate crack growth.

Future research should explore the effects of pulsed TIG welding on AZ31 microstructure, as pulsed techniques can decouple heat input from current amplitude and potentially achieve finer microstructures with reduced β-Mg₁₇Al₁₂ precipitation.