Comparative Analysis of TIG and MIG Welding Microstructures and Properties in AZ31B Magnesium Alloy

Literature Overview

This study by Liu Jinhua et al., published in the Journal of Taiyuan University of Technology (2008, Vol. 39, Issue S1, pp. 1-4), provides a systematic comparison of welding microstructures and mechanical properties between tungsten inert gas (TIG) and metal inert gas (MIG) welding processes applied to AZ31B magnesium alloy. The research was conducted at the School of Materials Science and Engineering, Taiyuan University of Technology, and supported by the National Natural Science Foundation of China (Grant 50675148) and the Taiyuan University Student Innovation Enterprise Fund (Grant 1070107457).

Experimental Configuration

The experiments were performed on 8.0 mm thick AZ31B magnesium alloy plates using two distinct welding configurations:

The characterization methods employed included optical microscopy for microstructure examination, X-ray diffraction for phase identification, and microhardness mapping for hardness distribution assessment across the weld zone.

Microstructural Findings

Phase Composition

A significant finding of this study is that both TIG and MIG weld joints exhibit phase compositions identical to the base material. AZ31B magnesium alloy contains the primary α-Mg matrix phase with β-Mg₁₇Al₁₂ intermetallic compound particles. The absence of new phases in either weld joint indicates that the thermal cycles experienced during both welding processes were insufficient to induce phase transformations beyond those present in the as-received material. This observation is consistent with the relatively low melting temperature of magnesium alloys (651°C for pure Mg) and the limited solubility of aluminum in magnesium.

Heat-Affected Zone Microstructure

The most notable difference between the two processes lies in the distribution of intragranular and intergranular precipitates within the heat-affected zone (HAZ):

• MIG weld HAZ: The intragranular precipitates exhibit a dispersed distribution pattern, while the grain boundary precipitates form a continuous network. This morphology suggests that the higher heat input of MIG welding promoted sufficient thermal energy for precipitation dissolution and subsequent re-precipitation during cooling, resulting in a more uniform but boundary-rich precipitate distribution.
• TIG weld HAZ: The microstructure displays a more heterogeneous precipitate distribution, with both intragranular and intergranular phases present in a less organized pattern. The lower and more localized heat input of TIG welding creates a narrower thermal gradient, resulting in a more complex precipitation behavior within the affected region.

Mechanical Property Comparison

The MIG weld joints exhibited higher microhardness values across both the weld zone and HAZ compared to TIG weld joints. This difference can be attributed to the higher heat input and deeper penetration of MIG welding, which promotes more thorough melting and mixing of the base metal with filler material, resulting in a more homogeneous and potentially strengthened weld microstructure. The dispersed precipitate distribution in the MIG HAZ may also contribute to enhanced solid solution strengthening and precipitation hardening effects.

However, the MIG process produced spatter during welding, which is a significant practical concern for magnesium alloy welding. Spatter not only wastes filler material but can also create surface defects, porosity sources, and safety hazards due to the high reactivity of molten magnesium particles. The superior bead formation of TIG welding, while producing slightly lower hardness, offers better surface quality and more consistent weld geometry.

Process Metallurgy Analysis

The fundamental differences between TIG and MIG welding of AZ31B magnesium alloy stem from the distinct heat input characteristics and arc dynamics of each process. TIG welding employs a non-consumable tungsten electrode with external filler wire feeding, resulting in a stable, narrow arc with relatively low heat input density. MIG welding uses a continuously fed consumable wire electrode, generating a higher current density at the arc root and deeper weld penetration.

For magnesium alloys, the heat input directly influences:

1. Weld pool fluidity and solidification rate — Higher MIG heat input promotes deeper penetration but increases the risk of burn-through in thin sections.
2. Precipitation behavior — The thermal cycle determines the dissolution and re-precipitation sequence of Mg₁₇Al₁₂ particles, affecting both strength and ductility.
3. Porosity susceptibility — Magnesium's high hydrogen solubility in the liquid state and rapid decrease in solubility upon solidification make both processes susceptible to hydrogen porosity, though the higher shielding gas flow rates typically used in MIG may provide better protection.

Engineering Practice Considerations

For engineering applications involving AZ31B magnesium alloy structural components, the selection between TIG and MIG welding should consider the following factors:

• Thin-section welding (≤4 mm): TIG is preferred due to lower heat input and better control over weld geometry.
• Thick-section welding (≥6 mm): MIG offers higher productivity and deeper penetration, though multi-pass welding may be required.
• Surface quality requirements: TIG produces superior surface finish with minimal spatter, making it suitable for cosmetic or corrosion-sensitive applications.
• Production rate considerations: MIG provides higher deposition rates, advantageous for batch production of magnesium alloy assemblies.

The 8.0 mm plate thickness used in this study represents a practical thickness range for magnesium alloy structural applications, such as automotive chassis components, aerospace brackets, and portable equipment housings. The findings suggest that for this thickness range, both processes are viable, with the choice depending on the priority placed on surface quality versus mechanical strength.

Key Reflections

This study provides valuable comparative data for welding process selection in magnesium alloy fabrication. However, the analysis could be further enhanced by including tensile testing data, fracture surface analysis, and corrosion resistance evaluation, as magnesium alloys are particularly susceptible to stress corrosion cracking and pitting corrosion. The observation that MIG welding produces higher hardness but inferior bead formation highlights the inherent trade-off between mechanical performance and process quality that welding engineers must navigate in practice.

Summary

The comparative study of TIG and MIG welding of AZ31B magnesium alloy reveals that while both processes produce weld joints with phase compositions matching the base metal, MIG welding yields higher microhardness values and improved mechanical properties at the cost of spatter formation and less uniform bead geometry. TIG welding offers superior weld appearance and process control but with somewhat lower hardness in the weld zone and HAZ. The precipitate distribution differences between the two processes — dispersed intragranular precipitates in the MIG HAZ versus a more heterogeneous pattern in the TIG HAZ — provide insight into the thermal cycle effects on precipitation behavior. For engineering applications, the selection between TIG and MIG should be guided by the specific requirements of the component, balancing mechanical performance needs against surface quality and production efficiency considerations.