TIG Welding Process and Microstructure Properties of AZ31 Magnesium Alloy

Literature Overview

This 2016 study published in Special Casting and Nonferrous Alloys investigates the TIG welding process parameters and their effects on the microstructure and mechanical properties of AZ31 magnesium alloy. The research was conducted jointly by Changchun Vocational and Technical College and Zhejiang University, supported by the Jilin Province Science and Technology Plan Key Project (2014JLKF0126).

Core Technical Content

AZ31 (Mg-3Al-1Zn) is one of the most widely used wrought magnesium alloys, valued for its excellent combination of strength, formability, and corrosion resistance. However, magnesium alloys present unique welding challenges due to their low melting point (650°C), high thermal conductivity, high vapor pressure, and susceptibility to oxidation and cracking.

Typical TIG Welding Parameters for AZ31

Parameter	Recommended Range	Effect on Weld Quality
Arc current (A)	100–200	Penetration depth, HAZ width
Arc voltage (V)	14–20	Heat input, bead width
Travel speed (mm/min)	200–500	Heat input, solidification rate
Shielding gas (Ar) flow (L/min)	12–20	Oxidation prevention
Tungsten electrode diameter (mm)	2.4–3.2	Current capacity, arc stability
Joint gap (mm)	0.1–0.5	Penetration, distortion
Preheating temperature (°C)	150–250	Cracking prevention

The heat input during TIG welding of AZ31 typically ranges from 0.5 to 2.0 kJ/mm, significantly lower than for steel welding due to the lower melting temperature and higher thermal diffusivity of magnesium alloys.

Microstructural Evolution

The microstructure of AZ31 welds exhibits distinct zones with different characteristics:

Weld Metal Microstructure

The weld metal solidifies from a magnesium-rich melt through a complex solidification sequence. The primary phase is alpha-Mg (hexagonal close-packed), with secondary phases including beta-Mg17Al12 and gamma-MgZn2 depending on the local composition and cooling rate.

Zone	Primary Phase	Secondary Phase	Grain Size (μm)	UTS (MPa)
Weld center	α-Mg equiaxed	β-Mg17Al12 (dendritic)	15–40	140–180
Weld edge	α-Mg columnar	β-Mg17Al12 + γ-MgZn2	20–50	160–200
HAZ (coarse grain)	α-Mg recrystallized	β-Mg17Al12 (grain boundary)	80–200	120–160
HAZ (fine grain)	α-Mg partially recrystallized	β-Mg17Al12 (dendrite)	30–80	180–220
Base metal	α-Mg equiaxed	β-Mg17Al12 (dendrite)	50–150	220–260

The beta phase (Mg17Al12) distribution is critical for mechanical properties. In the weld metal, the beta phase forms as interdendritic networks during solidification, creating a brittle network that reduces ductility. In the HAZ, the beta phase precipitates along grain boundaries during heat treatment or slow cooling, which can lead to intergranular cracking.

Mechanical Properties and Fracture Behavior

The mechanical properties of AZ31 TIG welds show characteristic degradation compared to the base metal:

Ultimate tensile strength: 60–80% of base metal value (typically 140–200 MPa vs. 220–260 MPa for base metal).
Yield strength: 50–70% of base metal value.
Elongation: 30–50% of base metal value (typically 3–8% vs. 10–15% for base metal).

The fracture typically initiates in the HAZ coarse grain zone or at the weld/HAZ interface, where the beta phase network provides preferential crack paths. Fractographic analysis reveals a mixed mode of intergranular and transgranular fracture, with the proportion depending on the welding parameters and post-weld treatment.

Process Optimization Strategy

A systematic approach to optimizing AZ31 TIG welding involves:

Preheating: Applying 150–250°C preheat to reduce thermal gradients and prevent hot cracking. The preheat temperature should be maintained above the solution treatment temperature (approximately 200°C) to dissolve existing beta phase and promote homogeneous solidification.
Shielding: Using pure argon (99.99%) with high flow rates (15–20 L/min) and appropriate backing gas protection. Magnesium oxidizes rapidly in air, and the oxide scale (MgO) has a melting point of 2852°C, forming refractory inclusions in the weld.
Post-weld heat treatment: Solution treatment at 400–420°C followed by artificial aging at 150–180°C can homogenize the microstructure and improve mechanical properties. This treatment dissolves the interdendritic beta phase and redistributes it as fine precipitates.
Parameter control: Maintaining consistent arc length (2–3 mm), travel speed (300–400 mm/min), and current (150–180 A) to ensure uniform heat input and minimize microstructural variation along the weld length.

Common Defects and Countermeasures

Defect Type	Root Cause	Prevention
Hot cracking	Low melting point eutectics at grain boundaries	Preheating, reduced heat input, filler metal selection
Porosity	Hydrogen pickup from atmosphere or contaminants	Improved shielding, clean base metal, dry filler
Undercut	Excessive arc force at weld edges	Reduced current, optimized torch angle
Oxidation	Insufficient shielding, high arc temperature	Increased gas flow, back purge, shorter arc length
Distortion	High thermal expansion, low modulus	Fixturing, low heat input, symmetric welding

Study Insights and Outlook

The TIG welding of AZ31 magnesium alloy requires careful balancing of competing requirements: sufficient heat input for complete fusion versus limited heat input to prevent excessive grain growth and phase coarsening. The study demonstrates that through systematic parameter optimization and appropriate post-weld treatment, acceptable weld quality can be achieved for structural applications.

The key challenge remains the brittleness of the beta phase network in the weld metal and HAZ. Future research should focus on developing flux-assisted or hybrid welding processes that can achieve more uniform microstructures, as well as advanced filler metals designed to minimize brittle phase formation. The application of AZ31 welds in automotive lightweighting, aerospace components, and 3C electronics requires continued improvement in weld reliability and fatigue performance.