Microstructure and Mechanical Properties of AZ31 Magnesium Alloy AC TIG Weld Joints at Different Currents

Literature Overview

Published in Mechanical Engineering Materials (Vol. 35, No. 5, 2011, pp. 47–50) by Zhou Hai and colleagues from Dalian Jiaotong University, this paper investigates the influence of welding current on the microstructure and mechanical properties of AC TIG weld joints in 7.4 mm thick AZ31 magnesium alloy plates. The study employs a comprehensive characterization approach including optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, tensile testing, and microhardness profiling.

Experimental Design and Parameters

The experimental matrix is straightforward but informative:

Parameter	Value
Base material	AZ31 Mg alloy plate
Plate thickness	7.4 mm
Welding process	AC TIG (alternating current tungsten inert gas)
Welding currents tested	Multiple levels (specific values not fully detailed in abstract)
Characterization methods	OM, SEM, EDS, XRD, tensile test, microhardness

The selection of AC TIG is notable for magnesium alloy welding, as the AC waveform provides cathodic cleaning action on the oxide layer while maintaining adequate heat input for penetration.

Microstructural Analysis

Weld Zone Microstructure

The weld zone exhibits a fine equiaxed α-Mg matrix with fine β-Al₁₂Mg₁₇ phase precipitated along grain boundaries. This is a critical observation for several reasons:

Equiaxed grain morphology indicates adequate nucleation during solidification, likely facilitated by the oscillating thermal conditions inherent to AC welding.
Grain coarsening with increasing current follows the expected trend of higher thermal input leading to larger grains due to reduced nucleation density and enhanced grain growth during solidification.
β-Al₁₂Mg₁₇ intergranular phase is a potential concern for mechanical properties, as this phase is relatively brittle and can serve as a preferential crack path under load.

Heat-Affected Zone Microstructure

The HAZ exhibits overheated microstructure, characterized by grain coarsening due to exposure to temperatures above the recrystallization temperature but below the melting point. The degree of coarsening increases with welding current, as expected from the higher peak temperatures and longer dwell times at elevated temperatures.

Mechanical Property Analysis

Property	Trend with Increasing Current	Optimal Current	Notes
Tensile strength	Increases then decreases	190 A	Reaches 88% of base metal at 190 A
Fracture location	HAZ	—	All fractures in HAZ
Fracture mode	Mixed ductile-brittle	—	Brittle features increase with current
Microhardness profile	W-shaped distribution	—	Weld > Base > HAZ (lowest)

Interpretation of Tensile Behavior

The non-monotonic relationship between tensile strength and welding current is characteristic of many welding systems and can be explained through competing mechanisms:

Low current regime: Insufficient heat input leads to incomplete fusion, lack of penetration, and porosity, all of which reduce joint strength.
Optimal current (190 A): Adequate penetration and fusion with fine enough grain structure in both weld and HAZ to maximize strength.
High current regime: Excessive heat input causes grain coarsening in the HAZ, increased β-Al₁₂Mg₁₇ phase formation, and potential hot cracking, all contributing to strength reduction.

The fact that all fractures occur in the HAZ is consistent with the microhardness profile showing the HAZ as the weakest region. The W-shaped hardness distribution—high at weld center, decreasing toward HAZ (minimum), then increasing to base metal—is a classic signature of weld joints where the HAZ experiences softening due to precipitate dissolution and grain coarsening.

Engineering Practice Implications

For AZ31 magnesium alloy welding in engineering applications, several practical guidelines emerge:

Current selection: For 7.4 mm thick plates, 190 A represents an optimal welding current for AC TIG, providing the best balance of penetration and microstructural refinement.
HAZ softening mitigation: The significant softening in the HAZ (lowest hardness region) suggests that post-weld heat treatment (solution treatment and aging) may be necessary to restore mechanical properties to near-base-metal levels.
Fracture mode consideration: The mixed ductile-brittle fracture mode indicates that while the joint retains some ductility, the brittle features (likely intergranular fracture along β-Al₁₂Mg₁₇ phases) represent a potential failure mechanism under fatigue or low-temperature conditions.

Key Questions and Reflections

The paper does not extensively discuss the role of AC waveform parameters (frequency, balance ratio) on the observed microstructure and properties. Given that AC TIG welding of magnesium alloys is sensitive to the cleaning action of the AC cycle, the balance ratio (ratio of positive to negative half-cycle time) could significantly influence oxide removal and weld cleanliness. Future studies should systematically vary the AC frequency and balance ratio to establish comprehensive process windows.

Additionally, the absence of fatigue and fracture toughness data limits the applicability of these findings to dynamic loading conditions, which are common in automotive and aerospace applications where AZ31 is increasingly used.

Study Insights and Outlook

This work provides a valuable baseline for understanding the current-dependent metallurgical behavior of AZ31 Mg alloy welds. The clear identification of 190 A as an optimal current for 7.4 mm plates offers practical guidance for process development. However, the relatively narrow parameter range investigated and the lack of post-weld treatment studies suggest that significant optimization potential remains. The combination of welding current optimization with post-weld heat treatment could potentially achieve joint properties exceeding 90% of base metal strength, which would be transformative for magnesium alloy structural applications.