Microstructure and Properties of AZ91B Magnesium Alloy TIG Weld Joints

Literature Overview

This paper by Xu Jinfeng and Zhai Qiuya from Xi'an University of Technology, published in Special Casting & Nonferrous Alloys in 2004, examines the microstructure and mechanical properties of TIG-welded AZ91B magnesium alloy joints. AZ91B is one of the most widely used wrought magnesium alloys, containing approximately 9% aluminum and 1% zinc, with a base microstructure of α-Mg dendrites with eutectic (α + Mg17Al12) interdendritic phases. The study compares two filler metals—AZ61 and AZ91 wire—and evaluates their effects on weld joint quality.

The research addresses a critical practical problem in magnesium alloy welding: the selection of appropriate filler metal to minimize hot cracking while maintaining compositional compatibility with the base metal.

Core Technical Findings

The study identifies distinct microstructural zones in the AZ91B TIG weld joint, each with characteristic features:

Zone	Microstructure Description	Key Features	Implications
Weld Metal	Well-developed α dendrites with snowflake morphology; interdendritic (α + Mg17Al12) eutectic	High eutectic content due to Mg evaporation; increased Al content	Higher hot crack susceptibility
Fusion Zone	Fine equiaxed α-Mg grains with interdendritic eutectic	Refined grain structure; moderate eutectic content	Good mechanical properties
HAZ	Coarse α-Mg dendrites with (α + Mg17Al12) eutectic between dendrites	Similar to base metal but with coarsened dendrites	Potential for reduced ductility
Base Metal	Standard AZ91B microstructure: α-Mg dendrites with eutectic interdendritic phases	Reference condition	Baseline properties

The key findings regarding filler metal selection are:

AZ61 filler wire produces weld joints with better hot crack resistance and better compositional consistency with the base metal.
AZ91 filler wire produces weld joints with higher eutectic content due to greater Mg evaporation during welding.
Both filler metals produce sound joints with good fusion and dense weld metal.
Mg evaporation during welding leads to relative Al enrichment in the weld metal, increasing eutectic content and hot crack susceptibility.

Interpretation of Technical Points

Magnesium Evaporation Mechanism

The fundamental challenge in magnesium alloy welding is the high vapor pressure of magnesium at welding temperatures. During TIG welding of AZ91B alloy:

The arc temperature exceeds 6000 K at the cathode spot
Local temperatures at the weld pool surface can reach 1000-1200 °C
Magnesium has a boiling point of 1091 °C, making evaporation inevitable
The rate of Mg evaporation depends on arc power density, shielding gas flow, and weld pool geometry

This evaporation causes:

Relative enrichment of Al and Zn in the weld pool
Shift of the local composition toward the eutectic point
Increased fraction of low-melting-point (α + Mg17Al12) eutectic
Enhanced hot crack susceptibility due to the increased liquid film at grain boundaries during solidification

Filler Metal Selection Rationale

The superiority of AZ61 over AZ91 filler metal for AZ91B base metal can be understood through the following analysis:

Parameter	AZ61 Filler	AZ91 Filler	Base Metal (AZ91B)
Al content (wt%)	6.0-7.0	8.5-9.5	8.5-9.5
Zn content (wt%)	0.2-0.7	0.8-1.3	0.8-1.3
Mg evaporation effect	Less severe (lower initial Al)	More severe	Reference
Weld Al content after welding	Closer to base metal	Higher than base metal	Reference
Eutectic content in weld	Lower	Higher	Reference
Hot crack resistance	Better	Worse	Reference

The lower initial Al content in AZ61 wire compensates for the relative Al enrichment caused by Mg evaporation, resulting in a weld metal composition that more closely matches the base metal. This compositional matching reduces the eutectic fraction and improves hot crack resistance.

Connection with Engineering Practice

Although magnesium alloys are not commonly used for structural pipe applications, the welding principles established in this research have broader relevance:

Lightweight structural applications: Magnesium alloy components are used in aerospace, automotive, and defense industries where weight reduction is critical. Understanding weld joint microstructure is essential for these applications.
Joining dissimilar metals: The concept of filler metal selection to compensate for elemental evaporation applies to other welding challenges, such as welding aluminum alloys where zinc and magnesium evaporation occurs.
Welding process development: The systematic comparison of filler metals provides a methodology that can be applied to other alloy systems.

In the context of pipe manufacturing, the relevance of this research extends to:

Welding of magnesium alloy components in chemical processing equipment
Joining of magnesium alloy pipe fittings in specialized applications
Understanding the fundamental mechanisms of hot cracking in lightweight alloy welding

Key Questions and Reflections

Several important questions emerge from this research:

Shielding gas effects: The study does not examine the influence of shielding gas composition (pure Ar vs. Ar/He mixtures) on Mg evaporation and weld quality. How does shielding gas selection interact with filler metal selection?
Welding parameters: The study does not specify welding current, voltage, or travel speed. How do these parameters affect Mg evaporation rates and weld pool composition?
Mechanical properties: While microstructure is described in detail, quantitative mechanical property data (tensile strength, elongation, impact toughness) for the weld joints would strengthen the conclusions.
Corrosion resistance: Magnesium alloys are highly susceptible to corrosion. How does the weld joint microstructure affect corrosion resistance, particularly at the weld/HAZ interface?
Long-term stability: Does the eutectic phase distribution remain stable under service conditions, or does it undergo coarsening or phase transformation?

Study Insights and Implications

The most significant practical insight from this research is the clear recommendation to use AZ61 filler wire for TIG welding of AZ91B magnesium alloy, rather than the compositionally matching AZ91 wire. This counterintuitive finding—that a compositionally dissimilar filler produces better results—highlights the importance of understanding the dynamic changes in weld pool composition during welding, rather than relying solely on base metal composition matching.

The research also demonstrates the importance of considering elemental evaporation as a primary factor in weld metal composition prediction. For any alloy system where volatile elements are present (magnesium, zinc, aluminum, silicon), the final weld metal composition will differ significantly from a simple mass balance calculation that ignores evaporation losses.

For engineers developing welding procedures for magnesium alloys or other alloys containing volatile elements, this study reinforces the need for:

Systematic evaluation of filler metal options beyond simple compositional matching
Consideration of elemental evaporation during welding
Post-weld compositional analysis to verify actual weld metal chemistry
Hot crack resistance testing as a critical qualification criterion

The methodology of microstructural analysis across the entire weld joint (weld metal, fusion zone, HAZ, base metal) provides a template for comprehensive weld quality evaluation in any alloy system.