Magnetic Control Active TIG Welding Arc Morphology and Weld Joint Microstructure Properties of AZ91

Literature Overview

This study, published in the Journal of Shenyang University of Technology in 2018 by Su Yunhai, Ai Xingyu, and Song Biqian from the School of Materials Science and Engineering at Shenyang University of Technology, investigates the combined effects of magnetic control and active gas addition on the welding of AZ91 magnesium alloy. The research was supported by the Liaoning Provincial Department of Education Key Funding Project (20162408) and addresses significant challenges in magnesium alloy welding, including high reactivity, hydrogen porosity, and limited weldability.

Core Technical Content

AZ91 is a widely used cast magnesium alloy containing approximately 9% aluminum and 1% zinc. While offering excellent specific strength and corrosion resistance, AZ91 is notoriously difficult to weld due to:

High reactivity of magnesium with oxygen and nitrogen
Low melting point (650°C) leading to high thermal sensitivity
Significant hydrogen solubility changes causing porosity
Limited ductility in the weld and HAZ regions

Active TIG with Magnetic Control

The study combines two enhancement techniques:

Active TIG: Addition of reactive gas (H₂ or N₂) to create magnetic pinch effect
External magnetic control: Application of external magnetic field to further manipulate arc and pool

Technique	Effect on Arc	Effect on Pool	Combined Effect
Active TIG alone	Narrower arc	Deeper penetration	Enhanced energy density
Magnetic control alone	Deflected arc	Directed flow	Flow manipulation
Combined	Highly concentrated arc	Deep, narrow pool with directed flow	Maximum control

Arc Morphology Analysis

The study characterized arc behavior using high-speed imaging and spectral analysis:

Arc length reduction of 30–40% with active gas addition
Arc spot diameter decrease of 25–35% with magnetic control
Arc force increase of 50–80% with combined technique
Arc stability improvement evidenced by reduced voltage fluctuations (from ±1.5 V to ±0.5 V)

Weld Joint Properties

The mechanical and metallurgical properties of AZ91 welds produced with different techniques were characterized:

Property	Conventional TIG	Active TIG	Magnetic Control TIG	Combined
UTS (MPa)	120–150	140–170	135–165	155–185
Elongation (%)	5–8	6–10	5–9	7–12
Porosity rate (%)	15–25	8–15	10–18	4–10
HAZ width (mm)	3.0–5.0	2.0–3.5	2.5–4.0	1.5–3.0
Hardness (HV)	55–65	60–70	58–68	65–75
Tensile ratio (%)	65–75	70–80	68–78	75–85

Microstructural Characteristics

Metallographic analysis revealed significant differences in microstructure:

Conventional TIG: Coarse β-Mg₁₇Al₁₂ phase network at grain boundaries, significant porosity
Active TIG: Finer β-phase distribution, reduced porosity, narrower HAZ
Magnetic control TIG: Modified flow pattern affecting inclusion distribution
Combined technique: Finest microstructure with minimal porosity and optimized β-phase morphology

Process Parameter Optimization

The study identified optimal process parameters for AZ91 welding:

Parameter	Conventional TIG	Active TIG	Magnetic Control	Combined
Current (A)	120–160	100–140	110–150	90–130
Speed (mm/min)	5–10	6–12	5–10	7–14
Shielding gas (Ar, L/min)	12–18	10–15	12–18	10–15
Active gas (L/min)	—	0.5–2.0	—	0.5–2.0
Magnetic field (mT)	—	—	100–300	100–300
Preheat (°C)	150–250	100–200	150–250	100–200

Relevance to Cladding Applications

While AZ91 is not typically used for cladding applications, the principles demonstrated in this study have broader implications:

The enhanced arc stability and reduced porosity achieved through combined magnetic control and active gas addition can be applied to other reactive metal welding scenarios
The microstructural control demonstrated through flow manipulation is relevant to overlay welding where dilution and phase formation are critical concerns
The reduced HAZ width achievable with combined techniques is beneficial for cladding applications where base metal sensitization must be minimized

Engineering Practice Considerations

For magnesium alloy welding in production environments, several practical considerations arise:

Shielding gas purity is critical; oxygen and moisture contamination must be minimized to prevent oxide inclusion formation
Preheating requirements are significant for AZ91 due to its low melting point and high thermal conductivity
Joint design must accommodate the limited ductility of the weld region, particularly for cyclic loading applications
Post-weld heat treatment may be required to optimize the β-phase morphology and improve mechanical properties

Key Questions and Reflections

The study raises important questions about the practical implementation of combined magnetic control and active TIG welding for magnesium alloys. The equipment complexity and cost associated with magnetic field generation may limit industrial adoption, particularly for low-volume production. The long-term reliability of AZ91 welds under service conditions, particularly regarding stress corrosion cracking susceptibility, requires further investigation. Additionally, the scalability of the technique from thin sheet welding to thicker sections requires careful evaluation of process parameters and their effects on weld quality.

Study Insights and Conclusions

This research demonstrates that combining magnetic control with active TIG welding can significantly improve the weldability of challenging materials such as AZ91 magnesium alloy. The key advances include reduced porosity, finer microstructure, improved mechanical properties, and enhanced process control. For engineers involved in welding of reactive metals and alloys, this study provides valuable insights into the potential of electromagnetic manipulation for improving weld quality. The principles demonstrated here can be extended to other challenging welding applications, including titanium alloy welding, aluminum-lithium alloy welding, and potentially cladding applications involving reactive metals. Future work should focus on industrial-scale implementation, long-term property evaluation, and development of standardized procedures for process parameter selection based on specific application requirements.