Microstructure and Mechanical Properties of AZ31 Magnesium Alloy AC-TIG Weldments at Different Welding Currents

Literature Overview

This paper, published in Materials in Mechanical Engineering (2011, Vol. 35, No. 5, pp. 47-50) by Zhou Hai, Ding Chenggang, Hu Fei, and Quan Gaofeng from Dalian Jiaotong University, investigates the influence of welding current on the microstructure and mechanical properties of AZ31 magnesium alloy weldments produced using alternating current tungsten inert gas (AC-TIG) welding. The study examines 7.4 mm thick AZ31 plates welded at varying current levels, providing a systematic analysis of how process parameters affect weld quality in this lightweight structural material.

Core Technical Content

Material and Process Context

AZ31 is a widely used magnesium alloy consisting of approximately 3% aluminum, 1% zinc, and the balance magnesium. It is valued for its excellent combination of lightweight properties, good corrosion resistance, and reasonable mechanical strength. However, magnesium alloys are notoriously difficult to weld due to several inherent challenges:

• High reactivity with oxygen and nitrogen, leading to oxidation and nitride formation
• Low melting point (650°C), requiring careful heat input control
• High thermal conductivity, which can lead to incomplete fusion
• Susceptibility to hot cracking during solidification
• Limited weld pool stability in conventional DC-TIG welding

The selection of AC-TIG welding for this study is significant. AC-TIG provides several advantages over DC-TIG for magnesium alloys: the AC cycle includes a cathodic (electrode-positive) half-cycle that provides cathodic cleaning action, removing the native oxide layer from the base metal; the anodic half-cycle provides arc stability and heat input to the base metal. This dual action makes AC-TIG particularly suitable for welding reactive metals like magnesium and titanium.

Experimental Design

The authors conducted welding experiments at multiple current levels on 7.4 mm thick AZ31 plates. The welding parameters were systematically varied, with welding current as the primary variable. Characterization methods included optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), tensile testing, and microhardness testing. This comprehensive characterization approach provides a thorough understanding of the structure-property relationships in the weldments.

Microstructural Analysis

Weld Metal Microstructure

The weld metal microstructure consists of a fine equiaxed alpha-Mg (α-Mg) matrix with fine beta phase (β-Al12Mg17) precipitates along grain boundaries. The presence of the beta phase is a direct consequence of the aluminum content in the AZ31 composition. During solidification, the aluminum-rich liquid segregates to the grain boundaries, forming the intermetallic beta phase upon cooling. The fine distribution of this phase is beneficial for mechanical properties, as it provides grain boundary strengthening without the brittleness associated with coarse precipitate networks.

As welding current increases, both the weld metal and HAZ grain sizes increase. This is a direct consequence of higher heat input, which increases the solidification rate and reduces the nucleation density. Higher temperatures also promote grain growth in the HAZ, leading to a coarser microstructure. The relationship between heat input and grain size follows the expected trends described by the Jackson-Hunt theory of solidification, where grain size is inversely proportional to the solidification rate.

Heat-Affected Zone Microstructure

The HAZ exhibits an overheated microstructure characterized by grain coarsening. The peak temperature in the HAZ exceeds the recrystallization temperature of the base metal, leading to grain growth. The extent of grain coarsening increases with welding current, as higher currents produce higher peak temperatures and longer exposure times at elevated temperatures. This HAZ coarsening is a critical factor in the mechanical performance of the weldment, as the HAZ is often the weakest region in magnesium alloy welds.

Mechanical Properties

Tensile Performance

The tensile properties of the weldments show a non-monotonic relationship with welding current. As current increases, the tensile strength initially increases and then decreases. The optimal current is 190 A, at which the weldment achieves a tensile strength equal to 88% of the base metal strength. This optimal current represents a balance between adequate penetration and limited heat input.

The initial increase in tensile strength with current is attributed to improved weld fusion and reduced defects such as incomplete fusion and porosity. As current increases beyond the optimal value, the tensile strength decreases due to excessive grain coarsening in both the weld metal and HAZ, which reduces the material's resistance to deformation and fracture.

All weldments fractured in the HAZ, which is the expected failure mode for magnesium alloy welds. The fracture morphology exhibits a mixed ductile-brittle character, with features of both dimpled rupture (indicating ductile fracture) and cleavage-like facets (indicating brittle fracture). This mixed fracture mode is characteristic of magnesium alloys, which have limited ductility at room temperature due to their hexagonal close-packed (HCP) crystal structure and limited number of active slip systems.

Microhardness Distribution

The microhardness profile across the weld cross-section exhibits a typical "W" shape, with the following characteristics:

The elevated hardness of the weld metal is attributed to the fine equiaxed grain structure and the presence of grain boundary beta phase precipitates, which provide solid solution and precipitation strengthening. The HAZ softening is caused by grain coarsening and the dissolution of fine precipitates during the welding thermal cycle. The narrowness of the HAZ softening zone is beneficial, as it limits the volume of material with reduced strength.

Engineering Practice Integration

Process Optimization

For engineers welding AZ31 magnesium alloy, the following process recommendations are derived from this study:

1. Optimal current range: For 7.4 mm thick AZ31 plates, a welding current of approximately 190 A provides the best balance of tensile strength and weld quality. This corresponds to a heat input of approximately 15-18 kJ/mm, depending on travel speed.
2. Travel speed adjustment: To maintain consistent weld quality across different thicknesses, travel speed should be adjusted to maintain a constant heat input per unit length. For thicker plates, increased current and travel speed should be used proportionally.
3. Shielding gas: High-purity argon (99.99% minimum) should be used as the shielding gas to minimize oxidation. A back-of-bead gas shield is recommended to protect the weld root from atmospheric contamination.
4. Base metal preparation: Thorough cleaning of the base metal is essential to remove oxide films and contaminants. Mechanical cleaning followed by chemical etching is recommended for optimal results.

Defect Analysis and Prevention

Study Insights and Reflections

This study provides a clear demonstration of the trade-offs inherent in welding magnesium alloys. The non-monotonic relationship between welding current and tensile strength highlights the importance of optimizing process parameters rather than simply maximizing penetration. The finding that the optimal current for 7.4 mm AZ31 is 190 A, achieving 88% of base metal strength, is a practical result that can be directly applied to production welding.

The mixed ductile-brittle fracture mode observed in all weldments is a fundamental limitation of magnesium alloy welding. The HCP crystal structure of magnesium provides limited slip systems at room temperature, resulting in inherently limited ductility. While the AC-TIG process produces acceptable weld quality, further improvements in ductility would require either post-weld heat treatment to optimize precipitate distribution or the development of magnesium alloys with enhanced formability.

The "W" shaped microhardness profile is a useful quality indicator for magnesium alloy welds. Engineers can use microhardness mapping as a rapid assessment tool to verify that the weld metal is properly refined and that the HAZ softening zone is acceptably narrow. Deviations from the expected profile can indicate process parameter drift or base metal variability.

One area not addressed in this study is the corrosion resistance of the weldments. Magnesium alloys are susceptible to corrosion, and the welding process can alter the corrosion behavior of the weld metal and HAZ. The beta phase (Al12Mg17) is anodic relative to the alpha-Mg matrix, creating a galvanic couple that can accelerate corrosion. Future work should evaluate the corrosion performance of AC-TIG weldments in relevant service environments to ensure long-term durability.

Reference Value and Outlook

This study provides valuable engineering data for the AC-TIG welding of AZ31 magnesium alloy, particularly the identification of an optimal current range that maximizes tensile strength. The comprehensive characterization of microstructure, mechanical properties, and hardness distribution offers a solid foundation for process development in magnesium alloy welding applications. For engineers working on lightweight structural components in automotive, aerospace, and transportation industries, this work provides practical guidance on achieving acceptable weld quality in AZ31. Future research should extend to corrosion performance, fatigue behavior, and the application of advanced welding techniques such as laser welding and friction stir welding to further improve the properties of magnesium alloy weldments.