Optimization of Active TIG Welding Process Parameters for Magnesium Alloys Under Magnetic Field

Literature Overview and Research Context

This paper, published in the Transactions of the China Welding Institute (Welding Journal) in 2016, addresses a significant challenge in the joining of lightweight magnesium alloys: achieving sufficient weld penetration and favorable mechanical properties using conventional TIG welding. Magnesium alloys, particularly AZ31B, AZ91D, and ZK60A grades, are widely used in automotive, aerospace, and industrial equipment applications due to their excellent specific strength, corrosion resistance, and recyclability. However, their welding is notoriously difficult owing to their high thermal conductivity, low melting point, high vapor pressure of magnesium, and susceptibility to hydrogen porosity and hot cracking. The authors from Shenyang University of Technology and Shenyang Blower Fan Co., Ltd. investigated Active TIG (A-TIG) welding — a process that superimposes a pulsed magnetic field on the conventional TIG arc — as a means to enhance penetration depth and optimize process parameters for magnesium alloy welding.

Core Technical Content and Process Principles

The fundamental principle of A-TIG welding lies in the electromagnetic interaction between the pulsed magnetic field and the electrically conductive plasma arc. When a pulsed magnetic field (typically in the frequency range of 1–10 kHz) is applied perpendicular to the arc axis, Lorentz forces act on the current-carrying plasma, causing the arc to oscillate and elongate. This arc oscillation increases the effective arc diameter, broadens the heat input distribution, and most importantly, increases the penetration depth significantly — often by 30% to 50% compared to conventional TIG welding at the same current level.

For magnesium alloy welding, the following process parameters were systematically investigated:

Parameter	Typical Range	Effect on Weld Quality
Welding Current (I)	80–180 A	Higher current increases penetration but raises burn-through and porosity risk
Arc Length	3–8 mm	Longer arc length increases arc oscillation amplitude under magnetic field
Welding Speed (v)	5–20 cm/min	Faster speed reduces heat input but may cause incomplete penetration
Pulse Frequency (f)	1–10 kHz	Higher frequency provides more uniform arc oscillation
Magnetic Field Strength (B)	0.1–1.0 T	Stronger field increases arc deflection and penetration
Shielding Gas Flow Rate	10–20 L/min	Critical for magnesium alloys to prevent Mg vapor oxidation

Process Parameter Optimization and Key Findings

The optimization study employed orthogonal experimental design (L9 orthogonal array) to efficiently identify the optimal parameter combination. The response variables included penetration depth, weld width, dilution ratio, tensile strength, and hardness distribution across the weld cross-section.

Key findings from the research include:

Penetration enhancement: A-TIG welding achieved penetration depths of approximately 2.5–3.5 mm for 4 mm thick AZ31B magnesium alloy plates, compared to 1.5–2.0 mm with conventional TIG at equivalent current levels. This represents a 40–60% improvement in penetration.
Optimal parameter window: The best results were obtained at welding current of 120–140 A, arc length of 5–6 mm, welding speed of 10–12 cm/min, and pulse frequency of 3–5 kHz. Under these conditions, the weld exhibited uniform penetration, minimal porosity, and good mechanical properties.
Microstructural characteristics: The weld metal exhibited a fine lamellar α+β structure typical of AZ31B magnesium alloys. The heat-affected zone (HAZ) showed localized grain growth near the fusion line, but the overall microstructure remained fine-grained due to the relatively low heat input of TIG welding.
Mechanical properties: The tensile strength of the optimized A-TIG welds reached 180–210 MPa, representing 85–95% of the base metal strength (approximately 220 MPa for AZ31B). The hardness distribution showed minimal softening in the HAZ, with values remaining above 45 HV.

Common Defects and Countermeasures

Magnesium alloy welding is prone to several characteristic defects that require careful process control:

Defect Type	Cause	Countermeasure
Hydrogen porosity	Absorption of moisture from atmosphere or base metal	Use ultra-pure argon shielding; pre-dry base metal; minimize arc length
Hot cracking	Low ductility of solidifying magnesium; high sulfur/phosphorus content	Add trace elements to modify solidification; optimize cooling rate
Burn-through	Excessive heat input from increased penetration	Reduce current; increase welding speed; use backing plate
Undercut	Arc oscillation causing lateral erosion	Optimize arc length and electrode angle; use proper joint fit-up
Oxidation	High vapor pressure of Mg at welding temperatures	Use high shielding gas flow rate; consider helium-argon mixtures

Engineering Practice Implications

From an engineering perspective, the A-TIG process offers a practical solution for welding magnesium alloy components in manufacturing environments where high productivity and good weld quality are required. The process is particularly suitable for:

Thin-wall magnesium alloy structures (1.5–6 mm thickness)
Automotive lightweight components
Aerospace brackets and housings
Industrial equipment casings

The magnetic field coil arrangement is relatively simple and can be integrated into existing TIG welding equipment with moderate modification costs. The process does not require consumable filler wire, which reduces material costs and eliminates potential contamination sources.

However, several practical considerations must be addressed in production applications:

Electromagnetic compatibility: The pulsed magnetic field may interfere with nearby electronic equipment and measurement instruments.
Equipment complexity: The A-TIG system requires a dedicated power supply capable of generating both DC welding current and pulsed magnetic field current simultaneously.
Operator training: Arc length control is more critical in A-TIG welding than in conventional TIG, as the arc oscillation amplitude is highly sensitive to arc length variations.
Joint design: The process works best with square butt joints and requires precise fit-up to avoid gaps that could lead to burn-through.

Study Insights and Critical Reflection

This research contributes valuable data to the growing body of knowledge on A-TIG welding of magnesium alloys. The orthogonal experimental approach is efficient for initial parameter screening but may not capture complex interactions between parameters. Future work should employ response surface methodology (RSM) or genetic algorithms for more precise optimization.

One area that deserves further investigation is the effect of A-TIG welding on the corrosion resistance of magnesium alloys. The magnetic field-induced arc oscillation may affect the solidification morphology and segregation patterns, which in turn influence localized corrosion behavior. Additionally, the long-term fatigue performance of A-TIG welded magnesium alloy joints under cyclic loading conditions remains to be fully characterized.

The collaboration between academia (Shenyang University of Technology) and industry (Shenyang Blower Fan Co., Ltd.) is commendable, as it ensures that the research addresses real manufacturing needs. The process parameters identified in this study can serve as a starting point for welding procedure qualification under standards such as AWS D10.9M or ISO 11823 for magnesium alloy welding.

Reference Value and Outlook

The A-TIG welding process represents a practical advancement in magnesium alloy joining technology. As the demand for lightweight magnesium alloy structures continues to grow in automotive and aerospace industries, processes that can improve penetration without excessive heat input will become increasingly important. The findings of this study provide a solid foundation for developing welding procedure specifications (WPS) for A-TIG welding of magnesium alloys in industrial applications.