Microstructure and Mechanical Properties of AZ71 Magnesium Alloy TIG Welded Joints

Literature Overview

The research paper by Wu Jingtong, You Guoqiang, Guo Qiang, and Long Siyuan, published in Thermal Processing Technology (2011, Vol. 40, No. 1), investigates the welding of 2.2 mm thick AZ71 magnesium alloy sheets using the TIG welding process. The authors, from Chongqing University and the National Iron and Alloy Materials Engineering Technology Research Center, conducted tensile tests and hardness measurements on the welded joints. The work was supported by the Chongqing Science and Technology Project (Cstc2009AB4010). The study provides valuable data on the weldability of AZ71 magnesium alloy, a material of growing importance in lightweight structural applications.

Material Background and Welding Challenges

AZ71 is a wrought magnesium alloy containing approximately 7 wt% aluminum and 1 wt% zinc, with the balance being magnesium. This alloy is widely used in automotive and aerospace applications due to its excellent combination of strength, ductility, and formability. However, magnesium alloys present unique challenges for welding due to their high reactivity with oxygen and nitrogen, low melting point (650°C), and high thermal conductivity. The AZ71 alloy is particularly susceptible to hot cracking during welding because of the wide solidification range of the Mg-Al system and the formation of low-melting-point eutectic phases at grain boundaries.

The TIG welding process is commonly employed for magnesium alloy welding because it provides a stable arc and good protection of the molten pool with inert gas. However, achieving adequate penetration without excessive heat input is critical, as excessive heat can lead to significant grain coarsening in the heat-affected zone (HAZ) and increased susceptibility to cracking.

Experimental Results and Key Findings

The authors tested different welding currents and found that a current of 90 A produced the best tensile performance, with a joint tensile strength of 281.23 MPa, representing 89.58% of the base metal strength. The following table summarizes the key mechanical properties reported in the study.

The fracture analysis revealed that failure occurred in the weld zone, with a quasi-cleavage fracture morphology approaching cleavage fracture. In contrast, the base metal exhibited a ductile fracture mode with dimple fracture characteristics. This indicates that the weld zone is the weakest region of the joint, which is a common finding in magnesium alloy welds.

The hardness distribution across the joint showed an interesting pattern: the weld zone had the highest hardness, followed by the base metal, and the HAZ had the lowest hardness. This distribution can be explained by the different microstructural evolution in each region. The weld zone, upon solidification, forms a fine dendritic microstructure with finely dispersed Mg17Al12 intermetallic particles, which contribute to higher hardness. The HAZ, on the other hand, experiences significant grain coarsening due to the thermal cycle, which reduces the number of grain boundaries and weakens the Hall-Petch strengthening effect, resulting in lower hardness.

Microstructural Analysis and Failure Mechanism

The quasi-cleavage fracture morphology observed in the weld zone suggests a mixed-mode failure mechanism. The presence of Mg17Al12 intermetallic phases along the grain boundaries in the weld zone can act as crack initiation sites and promote intergranular cracking. The wide solidification range of the weld metal leads to the formation of a mushy zone with a high fraction of liquid at the final stages of solidification, which promotes hot cracking. The quasi-cleavage morphology indicates that the failure involved both cleavage and intergranular mechanisms, with the Mg17Al12 phases playing a role in facilitating crack propagation.

The lower hardness in the HAZ compared to the base metal is attributed to the coarsening of the precipitates and the reduction of dislocation density due to recrystallization during the thermal cycle. In AZ71 alloy, the primary strengthening mechanisms are solid solution strengthening from aluminum and zinc in the magnesium matrix, and precipitation strengthening from Mg17Al12 and MgZn2 phases. In the HAZ, the thermal cycle can dissolve fine precipitates and cause grain growth, leading to a reduction in strength and hardness.

Engineering Practice Implications

For engineering applications involving AZ71 magnesium alloy, several practical implications emerge from this study. First, the welding current should be carefully controlled to minimize heat input while ensuring adequate penetration. The optimal current of 90 A for 2.2 mm thick sheets represents a relatively low heat input, which is consistent with the general principle of using low heat input for magnesium alloy welding. Second, the weld zone is the critical region for failure, and post-weld heat treatment may be necessary to improve the mechanical properties of the weld zone. Solution treatment followed by aging can dissolve coarse Mg17Al12 phases and re-precipitate fine particles, improving both strength and toughness.

Third, the high hardness of the weld zone combined with the quasi-cleavage fracture mode raises concerns about the fatigue resistance of the welded joint. In cyclic loading applications, the hard and brittle weld zone may be prone to fatigue crack initiation. Engineers should consider applying fatigue assessment procedures specific to magnesium alloy welds, such as those outlined in EN 1999-1-2 or DNV standards for aluminum structures, adapted for magnesium alloys.

Fourth, the use of appropriate filler metal is critical. The study does not explicitly discuss filler metal selection, but in practice, AZ91 or AZ61 filler wire is commonly used for welding AZ71, as they provide good compatibility and help minimize hot cracking susceptibility.

Study Insights and Reflections

This study provides fundamental data on the weldability of AZ71 magnesium alloy, but several aspects warrant further investigation. The study does not report on the effect of welding speed, gas flow rate, or electrode diameter on weld quality. These parameters, along with the welding current, significantly influence the thermal cycle and the resulting microstructure. Additionally, the study does not address the issue of porosity, which is a common defect in magnesium alloy welds due to hydrogen absorption from the atmosphere and from surface oxides. In practical welding operations, controlling porosity requires careful management of gas shielding, surface preparation, and welding atmosphere.

The finding that the HAZ has the lowest hardness is somewhat counterintuitive and warrants careful interpretation. While the HAZ hardness is lower, the weld zone hardness is higher. In terms of strength, the weld zone is the weakest region. This apparent contradiction is resolved by recognizing that hardness and strength are not always directly correlated in welded joints, particularly when microstructural differences are significant. The weld zone may have higher hardness due to fine precipitates but lower strength due to the presence of coarse grain boundaries and intermetallic phases that facilitate crack initiation.