Comparative Microstructure Study of TIG Welds in AZ71 and AZ71E Magnesium Alloys

Literature Overview

This paper by Wu Jingting, You Guoqiang, Li Aiting, and Long Siyuan from Chongqing University and the National Engineering Research Center for Magnesium Alloy Materials investigates the microstructural differences in TIG welds of AZ71 and AZ71E magnesium alloys. Published in Hot Working Technology, Volume 40, Issue 3, 2011, pages 134–135, the study was supported by the Chongqing Science and Technology Program (Cstc2009AB4010). The research compares the weld zone microstructures of two magnesium alloy grades with different rare earth content, providing insights into how rare earth elements affect solidification behavior in TIG welding.

Material Background and Alloy Design

AZ71 and AZ71E are aluminum-zinc magnesium alloys designed for different applications. The base composition of AZ71 contains approximately 7% aluminum and 1% zinc, while AZ71E is a modified version with the addition of rare earth elements, specifically a cerium-rich mixed rare earth alloy. The rare earth addition in AZ71E is intended to improve certain properties such as corrosion resistance, high-temperature performance, or castability.

The fundamental difference between these two alloys lies in the rare earth content, which significantly affects the solidification behavior during welding. Rare earth elements have strong interactions with aluminum in magnesium alloys, forming intermetallic compounds such as Al11RE3 that can alter the solidification path and grain morphology.

TIG Welding Process Parameters

The TIG welding was performed on thin sheets of both AZ71 and AZ71E alloys without filler metal. The absence of filler metal means that the weld metal composition is determined entirely by the base metal composition, making the microstructural comparison more directly related to the base metal alloy design.

The welding parameters were selected to achieve full penetration of the thin sheets while maintaining a stable arc. The specific parameters are not detailed in the abstract, but typical TIG welding parameters for thin magnesium alloy sheets would include moderate currents (50–150 A), welding speeds of 2–5 mm/s, and argon shielding gas flow rates of 8–15 L/min.

Microstructural Comparison

The most significant finding of this study is the difference in weld zone microstructure between AZ71 and AZ71E:

The AZ71E weld zone exhibits cellular dendritic crystals rather than the normal dendritic structure seen in AZ71. This cellular morphology is associated with the presence of rare earth elements, which modify the solidification behavior by introducing additional solidification phases and altering the constitutional undercooling distribution.

The critical finding is that the rare earth addition in AZ71E does not result in grain refinement; instead, the grains are coarser than in AZ71. This counterintuitive result is explained by the formation of Al11RE3 intermetallic compounds, which consume aluminum from the melt and reduce the effective aluminum concentration available for grain refinement.

Mechanism of Grain Coarsening in AZ71E

The mechanism behind the grain coarsening in AZ71E welds is fundamentally related to the thermodynamic interactions between rare earth elements and aluminum. When the AZ71E alloy melts during welding, the rare earth elements (primarily cerium) react with aluminum to form Al11RE3 intermetallic compounds. This reaction consumes a portion of the aluminum content, effectively reducing the aluminum concentration in the remaining liquid.

In AZ71 alloy, the aluminum content is critical for grain refinement during solidification. The Mg17Al12 (β) phase that forms at the dendrite boundaries acts as a grain refiner by creating constitutional undercooling ahead of the solidification front. However, when the aluminum concentration is reduced due to Al11RE3 formation, the constitutional undercooling is diminished, and the solidification front becomes more planar, leading to coarser grains.

The cellular dendritic morphology observed in AZ71E is a direct consequence of this reduced aluminum concentration. With less aluminum available to create constitutional undercooling, the solidification front becomes less unstable, resulting in cellular rather than dendritic growth. Cellular structures typically have coarser spacing than dendritic structures under similar solidification conditions.

Engineering Implications for Magnesium Alloy Welding

The findings of this study have important implications for the selection and welding of magnesium alloys. The rare earth addition intended to improve certain properties may inadvertently degrade weldability by promoting coarse grain formation in the weld zone. This is particularly concerning for fatigue and fracture resistance, which are critical properties for structural magnesium alloy components.

For engineering design, the selection between AZ71 and AZ71E should consider the welding requirements. If welding is a critical process step, AZ71 may be preferred for its superior weldability and finer weld zone microstructure. If rare earth properties are essential, AZ71E may be selected, but the welding process must be optimized to mitigate the coarse grain formation.

The welding process parameters can be adjusted to promote grain refinement in AZ71E welds. Higher welding speeds would increase the solidification rate, potentially promoting finer grains. However, this must be balanced against the risk of incomplete penetration. Alternative approaches such as electromagnetic stirring, ultrasonic vibration welding, or the use of grain refiners in the filler metal could be explored.

Key Questions and Critical Reflections

The study focuses on microstructural comparison but does not report on mechanical properties. The coarser grains in AZ71E welds likely result in reduced yield strength and potentially lower fatigue resistance, but quantitative data is needed to assess the practical impact.

The effect of welding parameters on the microstructure is not investigated. Different currents, welding speeds, and shielding gas flows may produce different microstructural outcomes, and the optimal parameters for AZ71E welding may differ from those for AZ71.

The role of the Al11RE3 phase in the weld metal is not fully characterized. The distribution, morphology, and volume fraction of Al11RE3 in the weld zone could provide additional insights into the solidification behavior and potential mechanical property implications.

The study does not address the heat-affected zone microstructure, which is also affected by the rare earth content. The HAZ may exhibit different microstructural changes due to the thermal cycle, and the interaction between the weld zone and HAZ microstructures is important for overall joint performance.

Study Insights and Implications

This research demonstrates that rare earth additions to magnesium alloys can have unintended consequences for weldability. The formation of Al11RE3 compounds consumes aluminum and reduces the effectiveness of aluminum-based grain refinement mechanisms, leading to coarser weld zone grains. This finding is important for alloy designers who must balance competing requirements for castability, corrosion resistance, and weldability.

The cellular dendritic morphology observed in AZ71E welds is a distinctive feature that can be used as a diagnostic indicator of rare earth presence in magnesium alloy welds. This morphological signature could be useful for quality control and material identification in welding inspection.

The research highlights the complexity of welding process-material interactions. A simple alloy modification intended to improve one property can adversely affect another property through subtle changes in solidification behavior. Understanding these interactions requires a deep knowledge of both materials science and welding metallurgy.

Reference Value and Outlook

This paper provides valuable insights into the effect of rare earth elements on magnesium alloy weldability. The microstructural comparison between AZ71 and AZ71E welds offers a clear demonstration of how alloy composition affects solidification behavior during welding. Future research should focus on mechanical property evaluation, welding parameter optimization for rare earth-containing alloys, and the development of welding processes that can produce fine-grained welds in AZ71E and similar alloys. The findings also have implications for the design of new magnesium alloys with improved weldability, suggesting that rare earth content must be carefully balanced with welding requirements.