Comparative Microstructure Study of AZ71 and AZ71E Magnesium Alloy TIG Welds

Literature Overview

Published in Hot Working Technology (Vol. 40, Issue 3, 2011, pp. 134–135), this study by researchers from Chongqing University's School of Materials Science and Engineering compares the weld zone microstructure of AZ71 and AZ71E magnesium alloys after TIG welding without filler wire. The research was funded by the Chongqing Science and Technology Project (Cstc2009AB4010) and provides valuable insights into the effect of rare earth addition on weld microstructure evolution in magnesium alloys.

Material System and Welding Context

AZ71 and AZ71E are wrought magnesium alloys with approximately 7% aluminum content. The primary difference between these grades is the rare earth addition:

• AZ71: Contains mixed rare earths (primarily lanthanum-rich) with total RE content of approximately 0.5–1.0%
• AZ71E: Contains cerium-rich mixed rare earths with total RE content of approximately 0.5–1.0%

The "E" designation indicates a cerium-rich rare earth variant. Both alloys are used in lightweight structural applications including automotive components, aerospace brackets, and electronic housing. TIG welding without filler wire (autogenous welding) is commonly employed for thin sheet applications where minimizing dilution and maintaining composition uniformity is important.

Microstructural Findings

The comparative metallographic analysis revealed a critical finding:

The unexpected coarsening in the AZ71E weld zone is attributed to the formation of the Al11RE3 intermetallic phase. During solidification, rare earth elements preferentially combine with aluminum to form this new phase, consuming a portion of the available aluminum. Since aluminum is the primary grain refiner in magnesium alloys (acting as a heterogeneous nucleation site for α-Mg grains), the depletion of aluminum reduces the grain refinement effect, resulting in coarser grain structures.

Mechanism Analysis

The microstructural evolution can be understood through the following thermodynamic and kinetic sequence:

1. Melting: Both alloys melt to form a homogeneous liquid pool with dissolved Al and RE elements
2. Nucleation: As solidification begins, Al-rich zones provide heterogeneous nucleation sites for α-Mg grains
3. Phase formation: In AZ71E, RE elements (particularly Ce) react with Al to form Al11RE3 according to: 11Al + 3RE → Al11RE3
4. Al depletion: The formation of Al11RE3 reduces the effective aluminum concentration available for grain refinement
5. Coarsening: With insufficient aluminum nucleation sites, α-Mg grains grow coarsely, forming cellular dendritic structures

This mechanism highlights a fundamental trade-off in rare earth-modified magnesium alloys: while rare earths improve high-temperature strength and corrosion resistance of the base material, they can adversely affect weld microstructure by competing with aluminum for grain refinement functions.

Engineering Implications for Magnesium Alloy Welding

The findings have several important implications for magnesium alloy welding practice:

• Filler wire selection: For AZ71E welding, using a filler wire with elevated aluminum content (such as AZ91 or custom compositions) can compensate for the aluminum depletion caused by Al11RE3 formation
• Post-weld heat treatment: Solution treatment followed by aging can partially refine the weld microstructure, though the fundamental aluminum depletion issue persists
• Welding process selection: Friction stir welding (FSW) may be preferred for AZ71E applications since it avoids melting and the associated phase formation issues
• Joint design: For critical structural applications, the coarser weld microstructure of AZ71E should be accounted for in fatigue design calculations

Study Insights and Reflections

This comparative study reveals an important materials science principle: the interaction between alloying elements during welding can produce unexpected microstructural outcomes that are not predictable from base material properties alone. The Al11RE3 formation mechanism identified in this work has broader implications for the design of rare earth-modified magnesium alloys intended for welded fabrication. For engineers selecting magnesium alloy grades for welded structures, the weldability behavior—not just the base material properties—must be considered. The study underscores the importance of alloy design for weldability: future rare earth-modified magnesium alloys should be developed with consideration of their welding behavior, potentially incorporating elements that maintain grain refinement capability even after intermetallic phase formation during solidification.