Effect of Welding Current on Microstructure of TIG Welded Extruded AZ71 Magnesium Alloy

Literature Overview

The paper by Wu Jingting and colleagues from Chongqing University, published in Hot Working Technology (2010, Vol. 39, No. 23, pp. 156-158), investigates the influence of welding current on the microstructure of TIG welded extruded AZ71 magnesium alloy plates. The study examines 2.2 mm thick extruded AZ71 sheets welded at currents of 70 A, 80 A, and 90 A using AC TIG welding, with microstructural characterization performed using optical microscopy, SEM, and EDS analysis.

Material Background and Welding Challenges

AZ71 magnesium alloy (Mg-7Al-1Zn) is a high-strength wrought magnesium alloy with yield strength exceeding 200 MPa and tensile strength of approximately 250-280 MPa in the extruded condition. Its excellent specific strength makes it attractive for lightweight structural applications in automotive, aerospace, and consumer electronics. However, magnesium alloy welding presents unique challenges:

• Low melting point (650°C) requiring reduced heat input to avoid excessive HAZ softening
• High vapor pressure leading to significant magnesium evaporation during welding, causing porosity
• High chemical reactivity requiring rigorous shielding to prevent oxide inclusion and hydrogen porosity
• Limited solid solubility of aluminum in magnesium (maximum 17 wt% at 451°C, decreasing to near zero at room temperature), which means the equilibrium microstructure differs significantly from the as-cast condition
• Grain coarsening sensitivity due to the limited recrystallization temperature range

The extruded condition of AZ71 is particularly important because it provides a refined, elongated grain structure with precipitate dispersion that contributes significantly to strength. Welding disrupts this microstructure through the thermal cycle.

Experimental Methodology

The welding parameters were varied systematically while maintaining other conditions constant:

The microstructural analysis included optical metallography for general morphology, SEM for detailed phase identification, and EDS for compositional mapping of phases.

Microstructural Findings

Weld Metal Microstructure

At 90 A, the weld metal exhibited good surface formation with no macroscopic defects and full penetration. However, the microstructural analysis revealed significant current-dependent variations:

• 70 A: Fine dendritic structure with relatively small inter-dendritic spacing, suggesting higher cooling rates
• 80 A: Moderate dendritic coarsening with increased inter-dendritic spacing
• 90 A: Coarse dendritic structure with significant inter-dendritic spacing and evidence of grain coarsening

The progression from fine to coarse dendritic structure with increasing current is directly related to the increased heat input, which reduces the cooling rate at the weld centerline.

Heat-Affected Zone (HAZ) Microstructure

The HAZ width increased with welding current:

The HAZ in AZ71 is particularly sensitive to thermal exposure because the equilibrium β-Mg₁₇Al₁₂ phase (which provides significant strengthening through precipitation) dissolves at temperatures above approximately 450°C. Once dissolved, the β-phase cannot fully re-precipitate during normal cooling rates, leading to strength loss in the HAZ.

Grain Boundary β-Phase Evolution

A particularly important finding is the progressive increase in β-Mg₁₇Al₁₂ phase along grain boundaries in the weld metal with increasing current. At 70 A, the β-phase is sparse and discontinuous along grain boundaries. At 90 A, the β-phase forms continuous networks along grain boundaries with altered morphology (from discrete particles to interconnected films).

This β-phase redistribution has significant implications for mechanical properties:

• Continuous grain boundary β-phase networks can act as crack initiation sites, reducing fracture toughness
• The morphology change from discrete particles to films indicates different precipitation kinetics under different thermal cycles
• EDS analysis confirmed the composition of the β-phase as Mg₁₇Al₁₂ with minor Zn substitution

Mechanical Property Implications

Although the paper focuses on microstructure, the findings have clear mechanical property implications:

1. Tensile strength: Expected to decrease with increasing current due to weld metal coarsening and HAZ softening
2. Fracture toughness: Expected to decrease due to continuous grain boundary β-phase networks at high currents
3. Corrosion resistance: Continuous β-phase at grain boundaries creates galvanic couples that accelerate intergranular corrosion
4. Creep resistance: HAZ grain coarsening reduces creep strength, which is critical for elevated-temperature applications

Engineering Practice Recommendations

Based on the findings, the following recommendations emerge for AZ71 magnesium alloy TIG welding:

1. Minimize welding current: Use the lowest current that achieves full penetration. For 2.2 mm extruded AZ71, 70-80 A appears to be the optimal range.
2. Maximize travel speed: Higher travel speed reduces heat input per unit length, limiting HAZ width and grain coarsening.
3. Consider pulsed TIG: Pulsed current allows independent control of penetration (peak current) and heat input (average current), potentially achieving full penetration with lower total heat input.
4. Post-weld heat treatment: Aging treatment at 175-190°C for 4-8 hours can partially restore β-phase precipitation in the HAZ and improve mechanical properties.
5. Shielding optimization: Enhanced trailing shield coverage is essential to prevent oxide inclusion in the cooling weld, which is particularly critical for magnesium alloys.

Study Insights and Reflections

This research clearly demonstrates that welding current is a critical process parameter for AZ71 magnesium alloy TIG welding, directly governing both weld metal and HAZ microstructure. The progressive coarsening and β-phase redistribution observed with increasing current highlights the fundamental trade-off between penetration capability and microstructural quality.

The finding that β-Mg₁₇Al₁₂ phase morphology changes with current (from discrete particles to continuous films) is particularly significant from a materials science perspective. It suggests that the precipitation kinetics in the weld metal are governed not only by composition but also by the cooling rate and thermal history, which are directly controlled by welding parameters.

For engineering practice, this study reinforces the principle that magnesium alloy welding should be approached with minimum heat input philosophy. Unlike steel or aluminum welding where adequate heat input is necessary for penetration, magnesium alloy welding requires careful optimization to achieve penetration without excessive thermal damage. The 2.2 mm thickness studied here is relatively thin, and the challenges become more pronounced for thicker sections where higher currents (and consequently more microstructural degradation) are required for penetration.

The work also highlights an important limitation of conventional AC TIG for magnesium alloys: the inability to independently control penetration and heat input. Advanced processes such as pulsed TIG, laser welding, or electron beam welding may offer better solutions for thick-section magnesium alloy fabrication, though these were not investigated in this study. The research provides a solid baseline understanding against which more advanced welding technologies can be evaluated for magnesium alloy applications.