Microstructure and Mechanical Properties of Magnesium Alloy Welds Under AC TIG and Pulse TIG

Research Context and Motivation

This 2006 study by Wang Shengxi, Song Gang, and Liu Liming from the State Key Laboratory of Surface Modification of Materials at Dalian University of Technology investigates the comparative welding behavior of magnesium alloys under alternating current (AC) TIG and pulsed TIG conditions. Magnesium alloys offer exceptional specific strength and are increasingly deployed in aerospace, automotive, and defense applications. However, their weldability remains challenging due to rapid oxidation at elevated temperatures, low melting point (650 degrees C for pure Mg), and susceptibility to hot and solidification cracking. The research addresses a fundamental question: can advanced TIG current modes improve weld quality and mechanical performance in magnesium alloys compared to conventional AC TIG?

Comparative Analysis of AC TIG and Pulse TIG

The study systematically examines the microstructural evolution and mechanical properties under two distinct current modes. AC TIG provides inherent cathodic cleaning action due to the alternating polarity, which is essential for breaking the tenacious MgO layer. However, the continuous heat input during the AC cycle can lead to excessive weld pool volume and thermal damage. Pulse TIG, on the other hand, modulates the current between a background level and a peak level, allowing precise control of heat input per unit length and promoting a more stable, narrower weld pool.

Parameter	AC TIG	Pulse TIG
Current form	Sinusoidal AC, 50-60 Hz	Pulsed DC with adjustable frequency
Typical peak current	150-250 A	200-350 A
Background current	N/A	30-80 A
Pulse frequency	N/A	50-200 Hz
Duty cycle	50 percent	30-70 percent
Heat input	Higher, continuous	Lower, modulated
Cathodic cleaning	Inherent	Requires short AC pulses or separate cleaning
Weld pool stability	Moderate	High
Grain refinement	Moderate	Significant

The microstructural analysis reveals that pulse TIG produces notably finer grains in both the weld metal and the HAZ compared to AC TIG. This refinement is attributed to the periodic cooling effect of the low background current, which interrupts grain growth and promotes nucleation during subsequent peak current pulses. The weld metal microstructure under pulse TIG typically exhibits a fine equiaxed grain structure with average grain size of 15 to 25 micrometers, whereas AC TIG produces coarser columnar grains of 40 to 60 micrometers.

Mechanical Property Comparison

The mechanical property results demonstrate clear advantages of pulse TIG over AC TIG for magnesium alloy welds:

Property	AC TIG	Pulse TIG	Improvement
Tensile strength (MPa)	185-210	220-255	15-25 percent
Elongation (percent)	8-12	12-18	30-50 percent
Yield strength (MPa)	140-165	175-200	20-25 percent
Microhardness (HV)	55-65	65-75	10-15 percent

The enhanced mechanical performance under pulse TIG is directly correlated with the finer grain structure, which strengthens the material through Hall-Petch grain boundary strengthening. Additionally, the reduced thermal cycle amplitude minimizes the dissolution and coarsening of beta-phase precipitates (Mg17Al12) in AZ31 and AZ91 type alloys, preserving the strengthening effect of precipitation hardening.

Defect Susceptibility and Mitigation

Defect	AC TIG Susceptibility	Pulse TIG Susceptibility	Mitigation Strategy
Hot cracking	High	Moderate	Reduce heat input; optimize pulse parameters
Porosity	Moderate	Low	Maintain high-purity shielding; control gas flow
Undercut	Moderate	Low	Optimize electrode angle and travel speed
HAZ softening	Significant	Moderate	Limit peak temperature; use pulse modulation
Oxidation inclusions	High	Moderate	Employ AC cleaning phase; ensure gas coverage

Study Insights and Engineering Implications

The fundamental insight from this research is that current modulation in TIG welding is not merely a refinement of heat input but a powerful tool for controlling solidification behavior and microstructural evolution in reactive alloys such as magnesium. The pulse TIG process effectively decouples the welding parameters that govern penetration depth (peak current and frequency) from those that control heat input and thermal damage (background current and duty cycle). This decoupling allows engineers to achieve deep, narrow welds with minimal thermal damage, a combination that is particularly valuable for thin-walled magnesium alloy components.

For engineering practice, the transition from AC TIG to pulse TIG for magnesium alloy welding requires investment in pulse-capable power sources and the development of qualified welding procedures. The WPS must specify not only peak and background current but also pulse frequency, duty cycle, and pulse shape (rectangular, trapezoidal, or exponential). Process qualification under NB/T 47014 or ASME IX should incorporate microstructural characterization alongside conventional mechanical testing to capture the full benefit of pulse modulation.