Effect of Welding Speed on Microstructure and Mechanical Properties of AZ31 Magnesium Alloy TIG Weld Joints

Literature Overview

This paper by Yang Yongchun from Xinjiang Transportation Vocational and Technical College was published in Hot Working Technology in 2015 (Vol. 44, No. 15, pp. 201-202). The study investigates the weldability of AZ31 magnesium alloy plates (5 mm thickness) using gas tungsten arc welding (GTAW/TIG), with a specific focus on how welding speed influences the heat input, weld geometry, microstructure evolution, and tensile strength of the resulting joints. Magnesium alloys are increasingly used in lightweight structural applications, but their high reactivity, low melting point, and susceptibility to solidification cracking present significant welding challenges.

Core Technical Findings

The study systematically varied the welding speed while maintaining other welding parameters constant, and examined the resulting changes in heat input, weld bead dimensions, microstructure, and mechanical properties. The key findings are summarized in the table below.

Welding Speed (mm/min)	Heat Input (J/mm)	Weld Penetration Depth	Weld Width	Tensile Strength	Microstructural Features
High speed (>100)	Low	Shallow	Narrow	Lower	Fine grains, limited penetration
Moderate speed (~80)	Moderate	Adequate	Moderate	Optimal	Balanced grain size
70 mm/min	High	Deep	Wide	Reduced	Coarse grains in fusion zone, enlarged β-phase

Heat Input and Weld Geometry

The study confirms the fundamental relationship between welding speed and heat input: as welding speed decreases, the heat input per unit length increases. For AZ31 magnesium alloy, this has direct consequences for weld geometry. Lower welding speeds produce deeper penetration and wider weld beads due to the extended time the molten pool is exposed to the heat source. However, this benefit is not monotonic — beyond a certain threshold, excessive heat input begins to degrade the joint quality.

Microstructural Evolution

The microstructural analysis reveals two critical phenomena that are strongly dependent on welding speed:

Grain coarsening in the fusion zone: At welding speeds of 70 mm/min and below, the prolonged thermal exposure leads to significant grain coarsening in the fusion zone. The equilibrium grain size in the as-welded condition increases substantially, which reduces the yield strength and ductility according to the Hall-Petch relationship.
Enlargement of β-Mg17Al12 second phase: The β-Mg17Al12 intermetallic compound is the primary strengthening phase in AZ31 magnesium alloy. At lower welding speeds, the increased heat input causes the β-phase particles to grow in size and increase in volume fraction. While a moderate amount of β-phase is beneficial for strength, excessive coarsening and volume fraction lead to embrittlement and reduced tensile strength.

Mechanical Property Response

The tensile strength results show a clear non-monotonic trend with welding speed. At high welding speeds, insufficient heat input results in incomplete fusion and reduced weld cross-sectional area, leading to lower joint strength. As welding speed decreases, the joint strength initially improves due to better fusion and adequate penetration. However, when the welding speed drops to 70 mm/min, the joint strength decreases again due to the detrimental microstructural changes described above. This identifies an optimal welding speed window that balances adequate penetration with controlled thermal exposure.

Process Analysis and Engineering Considerations

Welding Parameter Optimization

The study's findings have direct implications for the development of welding procedures for AZ31 magnesium alloy. The following process considerations are critical:

Heat input control: The optimal heat input for 5 mm AZ31 TIG welding should be maintained in a range that ensures full penetration without excessive thermal cycling. Based on the study's findings, a welding speed in the range of 80-100 mm/min appears to be the most favorable for this thickness.
Shielding gas considerations: Magnesium is highly reactive with oxygen and nitrogen at welding temperatures. The use of high-purity argon shielding gas is essential, and the gas flow rate must be sufficient to prevent atmospheric contamination. Back-purging with argon is recommended for root passes to prevent oxide inclusion formation.
Filler metal selection: AZ31 filler wire (matching the base metal composition) is typically used for AZ31 welding. The filler metal composition directly influences the β-phase volume fraction in the weld metal, and therefore the mechanical properties of the joint.
Preheating considerations: Unlike steel welding, preheating is generally not recommended for magnesium alloys, as it increases the risk of oxidation and further promotes grain coarsening. Room-temperature welding with controlled heat input is preferred.

Comparison with Other Magnesium Alloy Welding Studies

The findings of this study are consistent with the broader body of research on magnesium alloy welding. The sensitivity of AZ31 to heat input is well-documented, and the critical role of β-phase morphology in determining joint strength is a recurring theme. The study's identification of 70 mm/min as a threshold speed below which microstructural degradation accelerates provides a practical reference point for procedure development, although the exact threshold may vary with plate thickness, joint configuration, and electrode geometry.

Key Questions and Reflections

Several aspects of this study merit further consideration. The study focuses exclusively on tensile strength as the mechanical performance metric, but for structural magnesium alloy applications, fatigue resistance, corrosion resistance, and fracture toughness are equally important. The effect of welding speed on these properties, particularly the susceptibility to stress corrosion cracking in atmospheric environments, is not addressed. Additionally, the study does not investigate the effect of AC frequency and balance on the weld pool dynamics, which is a critical parameter in TIG welding of aluminum and magnesium alloys where the cathodic cleaning action of AC is essential for oxide removal.

The practical relevance of this study extends to the growing use of magnesium alloys in automotive and aerospace applications, where weight reduction is a primary design driver. The ability to produce sound, high-strength TIG welds in AZ31 and similar alloys is essential for the fabrication of lightweight structural components, and the welding speed optimization presented here provides a foundation for further process development.

Summary

This study demonstrates that welding speed is a critical parameter in the TIG welding of AZ31 magnesium alloy, with a clear optimal range that balances penetration adequacy against microstructural degradation. The identification of 70 mm/min as a threshold speed below which grain coarsening and β-phase coarsening lead to strength reduction provides practical guidance for welding procedure specification. Engineers working with magnesium alloy fabrication should use these findings as a starting point for parameter optimization, while recognizing that additional considerations such as shielding gas quality, filler metal selection, and post-weld heat treatment may further influence the final joint quality. The study contributes to the growing body of knowledge on lightweight alloy welding and supports the expanding use of magnesium alloys in structural applications.