Effect of TIG Welding Thermal Cycle on Hardness of AZ31B Magnesium Alloy

Literature Overview

Published in Rare Metals (稀有金属) in 2005 by Xiao Feng and colleagues, this study investigates how the TIG welding thermal cycle affects the microhardness distribution across the weld zone of AZ31B magnesium alloy. The research was conducted at Chongqing University of Technology (now Chongqing Technology and Business University) and supported by multiple funding agencies including the Ministry of Education Overseas Returnee Research Fund and Chongqing Science and Technology Commission.

Core Technical Content

AZ31B is a widely used wrought magnesium alloy in the AZ series, containing approximately 3% aluminum and 1% zinc with a rare earth addition (typically 0.5% rare earth elements) to improve hot working properties and corrosion resistance. Its relatively low melting point (approximately 450°C) and high thermal conductivity make welding challenging, as excessive heat input can lead to burn-through, while insufficient heat input results in incomplete fusion.

Hardness Distribution Across the Weld Zone

The study reveals a characteristic hardness profile:

The weld metal exhibits higher hardness than the base metal because the rapid cooling from the molten pool produces a fine grain structure with dispersed β-Mg17Al12 intermetallic phases. In contrast, the HAZ undergoes grain coarsening due to the thermal exposure without melting, yet its hardness remains comparable to the base metal because the dissolution and reprecipitation of phases largely restores equilibrium conditions.

Surface Treatment Sensitivity

A particularly noteworthy finding is that surface preparation methods—grinding, polishing, and erosion liquid processing—significantly influence measured hardness values. This finding has profound implications for quality control procedures:

1. Metrological consistency: Hardness measurements on magnesium alloys are highly sensitive to surface finish, making it essential to standardize surface preparation protocols before testing.
2. Standard compliance: ASTM E10 (Rockwell) and ASTM E92 (Vickers) standards specify surface preparation requirements, but the magnitude of the effect demonstrated in this study underscores the need for rigorous adherence to these specifications.
3. Interpretation caution: When comparing hardness data across different laboratories or different test specimens, surface preparation differences may account for apparent variations that are not metallurgical in origin.

Process Analysis

The TIG welding process parameters selected for this study would typically include:

• Shielding gas: Argon (99.99% purity)
• Electrode: Ceriated tungsten (WCe20) or lanthanated tungsten (WLa15)
• Current: DCEN (Direct Current Electrode Negative) for magnesium alloys
• Travel speed: Moderate to control heat input
• Wire feed: ER52S or similar Mg-3Al-1Zn filler

The thermal cycle experienced by different zones of the weld determines the microstructural evolution:

• Weld pool: Rapid heating to melting followed by rapid solidification produces fine grains and high hardness
• HAZ: Peak temperatures below the melting point cause grain growth but limited phase transformation
• Base metal: Minimal thermal effect, retaining as-received properties

Engineering Practice Implications

For engineers designing welded magnesium alloy components, this research highlights several practical considerations:

1. Joint design: The higher hardness of the weld metal relative to the base metal and HAZ means that the weld is not the weakest link in terms of hardness. However, hardness alone does not determine strength—tensile testing and fatigue testing are necessary for comprehensive evaluation.
2. Post-weld treatment: Since the HAZ hardness is similar to the base metal, the HAZ may be the critical zone for fatigue crack initiation if microstructural coarsening reduces fatigue life. Post-weld heat treatment (PWHT) could potentially homogenize the microstructure.
3. Quality assurance: Standardizing surface preparation before hardness testing is not merely a procedural formality but a critical step for obtaining meaningful data. A recommended protocol would include: grinding to 600 grit, polishing to 1 μm diamond, and etching with Keller's reagent or a magnesium-specific etchant (5% NaOH in water).
4. Corrosion considerations: The hardness variations across the weld zone correlate with microstructural differences that also influence corrosion behavior. The β-phase in the weld metal is more corrosion-resistant than the α-matrix, which may provide some cathodic protection to the weld region.

Key Reflections

This study, while published over two decades ago, remains relevant because AZ31B continues to be one of the most widely used wrought magnesium alloys. The finding regarding surface treatment sensitivity is particularly important in today's context where portable hardness testers are increasingly used for in-service inspection. Engineers should be aware that portable test results on magnesium alloys can vary significantly depending on surface condition, and should always document surface preparation procedures.

The observation that the weld metal is harder than the base metal is somewhat counterintuitive from a strength perspective, as hardness does not always correlate linearly with tensile strength in magnesium alloys. The relationship between hardness and tensile strength in AZ31B is approximately σ_b (MPa) ≈ 3.4 × HV (Vickers), but this relationship may vary across the weld zone due to different microstructural features at the same hardness level.