Microstructure and Mechanical Properties of Magnesium Alloy TIG Welded Joints

Literature Overview

This 2012 paper by Ren Jingying from Harbin Electric Power Vocational and Technical College presents a detailed analysis of the microstructure and mechanical properties of magnesium alloy welded joints produced by TIG welding. Magnesium alloys are of growing interest in lightweight structural applications, including pressure vessels for hydrogen storage, aerospace components, and automotive applications. Understanding the weld microstructure-property relationships in magnesium alloys is essential for developing reliable welding procedures and ensuring the integrity of magnesium-containing bimetal products.

Technical Background

Magnesium Alloy Classification

Magnesium alloys are classified into several families based on their principal alloying elements:

Alloy Family	Principal Elements	Typical Grades	Key Characteristics
AZ-series	Al, Zn	AZ31, AZ91	Good castability, moderate strength
AM-series	Al, Mn	AM60, AM20	High strength, limited corrosion resistance
AZE-series	Al, Zn, Zr	AZE61	Improved elevated temperature properties
ZE-series	Zn, Zr	ZE10, ZE31	High strength, good corrosion resistance
AE-series	Al, Rare earth	AE42, AE62	Excellent corrosion resistance

Why TIG for Magnesium?

TIG welding is preferred for magnesium alloys because:

Precise heat input control minimizes the narrow melting range effects
Excellent arc stability with appropriate AC or DC+ parameters
Ability to use both AC (for oxide removal) and DC+ (for penetration)
Clean welds achievable with proper shielding
Compatibility with thin to medium thickness sections

Microstructural Analysis

Base Metal Microstructure

The base metal microstructure of magnesium alloys typically consists of:

Alpha phase (α-Mg): Face-centered cubic (FCC) matrix
Second phase particles: β-phase (Mg₁₇Al₁₂) in AZ-series, or other intermetallics depending on composition
Grain size: Typically 20–100 μm depending on processing history

Weld Zone Microstructure

The TIG welded joint exhibits several distinct microstructural zones:

Zone	Microstructure	Characteristics
Fusion zone (WZ)	Columnar grains, dendritic	Rapid solidification, fine grain, possible porosity
Partial melting zone (PMZ)	Mixed grain structure	Partial dissolution of second phases
Heat-affected zone (HAZ)	Grain growth, phase precipitation	Reduced strength, possible cracking susceptibility
Base metal (BM)	Original microstructure	Unchanged properties

Key Microstructural Features

Columnar grain growth: The fusion zone typically exhibits columnar grains growing perpendicular to the fusion boundary, with grain sizes of 50–200 μm
Dendritic solidification: Primary and secondary dendrite arm spacing (SDAS) of 5–20 μm, depending on cooling rate
Second phase redistribution: Dissolution of β-phase during welding followed by re-precipitation during cooling
Porosity formation: Hydrogen porosity is common due to hydrogen pickup from atmosphere or flux contamination

Mechanical Properties Analysis

Tensile Properties

Location	UTS (MPa)	Yield Strength (MPa)	Elongation (%)	Notes
Base metal (AZ31)	205–275	95–125	15–20	As-received condition
Fusion zone	180–250	80–110	10–18	Slightly reduced due to grain coarsening
HAZ	150–200	70–95	8–15	Weakest region, grain growth
Weld joint (overall)	150–220	70–100	8–15	Joint efficiency 70–85%

Hardness Distribution

The hardness profile across the weld joint typically shows:

Peak hardness in the fusion zone due to fine dendritic structure and solid solution strengthening
Minimum hardness in the HAZ due to grain coarsening and second phase dissolution
Recovery to base metal hardness in the unaffected base metal

Fracture Behavior

Fracture analysis of magnesium alloy TIG welds typically reveals:

Fusion zone fracture: Ductile dimple fracture with some intergranular character
HAZ fracture: Mixed mode with significant intergranular component
Base metal fracture: Fully ductile dimple fracture

Welding Process Optimization

Critical Process Parameters

Parameter	Effect on Microstructure	Effect on Properties
Welding current	Grain size, SDAS	Strength, ductility
Travel speed	Cooling rate, grain morphology	Joint efficiency
AC balance ratio	Oxide removal, penetration	Surface quality, porosity
Shielding gas flow	Contamination level	Corrosion resistance, hydrogen content
Filler wire composition	Weld metal composition	Strength, corrosion resistance

Recommended Parameters for AZ31

Parameter	Value	Rationale
Current	100–150 A	Sufficient penetration without excessive heat
Travel speed	200–350 mm/min	Adequate cooling rate for fine grain
AC balance	50:50 to 60:40 (pos:neg)	Balance cleaning and penetration
Shielding gas	Argon, 15–20 L/min	Prevent oxidation and hydrogen pickup
Filler wire	AZ91 or ER50A	Higher strength filler compensates for HAZ weakening
Preheat	None or <100°C	Avoid grain growth in HAZ

Defect Analysis

Common Defects in Magnesium Alloy TIG Welds

Defect	Cause	Detection Method	Countermeasure
Hydrogen porosity	Atmospheric H₂ pickup, flux contamination	RT, UT	Improved shielding, strict cleaning
Hot cracking	Low melting point eutectics, restricted shrinkage	MT, PT	Filler selection, preheat control
Oxide inclusions	Inadequate oxide removal	Visual, MT	AC balance optimization
Tungsten inclusion	Electrode contamination	RT, visual	Electrode preparation, torch angle
Undercut	Excessive current, improper torch angle	Visual, UT	Parameter adjustment

Engineering Implications for Bimetal Applications

Magnesium in Pressure Vessel Service

While magnesium alloys are not yet widely used in pressure vessel applications, they are being investigated for:

Hydrogen storage vessels: Lightweight pressure vessels for compressed hydrogen
Cryogenic applications: Low-temperature hydrogen and natural gas storage
Aerospace pressure systems: Lightweight pressurized containers

Cladding Considerations

For magnesium-containing bimetal products, the following considerations apply:

Dissimilar material welding: Magnesium-to-aluminum or magnesium-to-steel joints require transition layers
Galvanic corrosion: Magnesium is highly anodic relative to most engineering metals, requiring electrical isolation
Heat treatment sensitivity: Post-weld heat treatment must be carefully controlled to avoid grain growth or phase instability
Code compliance: Current pressure vessel codes have limited provisions for magnesium alloys, requiring design-by-analysis approaches

Study Insights and Implications

This paper provides a solid foundation for understanding the weldability of magnesium alloys through microstructure-property correlation. The key finding that the HAZ represents the weakest link in the welded joint has direct implications for pressure vessel design, where the allowable stress must be based on the joint efficiency rather than the base metal properties. For engineers developing welding procedures for magnesium-containing components, this work emphasizes the critical importance of controlling cooling rates through travel speed optimization, the necessity of proper shielding to prevent hydrogen contamination, and the value of filler metal selection in compensating for HAZ property reductions. The work also highlights the need for further research into post-weld heat treatment strategies that can improve HAZ properties without compromising the fusion zone. As magnesium alloys find increasing application in lightweight structural and pressure-retaining components, a deeper understanding of their welding behavior will be essential for safe and economical design.