Zhuojin Pipe Fitting Co., LtdWelding Speed Effects on Microstructure and Mechanical Properties of AZ31 Magnesium Alloy TIG Welds
Literature Overview
Yang Yongchun's study, published in Hot Working Technology (2015, Vol. 44, No. 15, pp. 201-202), examines the influence of TIG welding speed on the microstructure and mechanical properties of AZ31 magnesium alloy welds. AZ31 is one of the most widely used wrought magnesium alloys, valued for its excellent combination of light weight, good formability, and reasonable corrosion resistance. However, magnesium alloys are notoriously difficult to weld due to their high thermal conductivity, low melting point, and susceptibility to porosity and cracking. This study focuses on 5 mm thick AZ31 plates and investigates how welding speed affects heat input, weld geometry, microstructure, and tensile strength.
Core Technical Findings
The study reveals a clear relationship between welding speed, heat input, and weld quality. As welding speed decreases, heat input increases, leading to greater weld penetration and wider bead width. However, this relationship is not linear in its effects on mechanical properties. There exists an optimal welding speed range where the weld achieves the best balance between penetration, microstructure refinement, and tensile strength.
| Welding Speed | Heat Input (J/mm) | Weld Penetration | Weld Width | Tensile Strength | Microstructure Character |
|---|---|---|---|---|---|
| High (>100 mm/min) | Low | Shallow | Narrow | Lower | Fine grain, good |
| Medium (70-100 mm/min) | Moderate | Adequate | Moderate | Optimal | Balanced grain size |
| Low (<70 mm/min) | High | Deep | Wide | Reduced | Coarse grain, large beta phases |
The critical finding is that when welding speed drops below approximately 70 mm/min, the excessive heat input causes grain coarsening in the fusion zone and increases both the size and volume fraction of the second phase (beta-Mg17Al12). This degradation in microstructure directly reduces the tensile strength of the weld joint. The beta-Mg17Al12 phase, while contributing to the precipitation hardening of AZ31 in its tempered state, becomes detrimental when it forms in coarse, irregular configurations within the weld metal due to excessive thermal cycling.
Process Analysis
The heat input in TIG welding is directly proportional to current and inversely proportional to travel speed, following the relationship Q = 0.24 × I × V / v, where Q is heat input, I is current, V is voltage, and v is travel speed. For AZ31 magnesium alloy, the recommended heat input range for TIG welding is typically 0.5-1.5 J/mm, depending on plate thickness and joint configuration. The study demonstrates that maintaining a welding speed above 70 mm/min is essential to keep heat input within this beneficial range.
The microstructural evolution during TIG welding of AZ31 involves several key transformations. During welding, the weld pool solidifies rapidly, typically producing an equiaxed dendritic structure. The cooling rate, which is governed primarily by welding speed, determines the grain size and the morphology of the Mg17Al12 precipitates. Faster welding speeds produce finer grains and smaller precipitates, which generally contribute to better mechanical properties through Hall-Petch strengthening and more uniform stress distribution.
Engineering Practice Considerations
For magnesium alloy welding in practice, the selection of welding speed must be balanced against other process requirements such as joint fit-up tolerance, gas shielding coverage, and operator comfort. In automated TIG welding of magnesium alloys, speed control is precise and repeatable, making it easier to maintain optimal parameters. In manual TIG welding, the operator must develop a keen sense of the relationship between travel speed and weld pool appearance, as visual indicators such as weld pool width, bead profile, and surface ripples provide real-time feedback on whether the speed is appropriate.
The study also implicitly highlights the importance of shielding gas selection and flow rate for magnesium alloy TIG welding. The high reactivity of magnesium at elevated temperatures necessitates robust shielding, typically with pure argon at flow rates of 10-15 L/min, and in some cases with a back-purge or gas cup attachment to protect the weld pool from atmospheric contamination.
Study Insights and Implications
This paper provides clear guidance for welding engineers working with AZ31 magnesium alloy, establishing that welding speed is not merely a productivity parameter but a critical quality variable that directly governs weld microstructure and mechanical integrity. The identification of the 70 mm/min threshold as a practical lower limit for maintaining acceptable weld quality is particularly useful for setting up welding procedure specifications and operator training criteria. The findings reinforce the broader principle in welding metallurgy that heat input must be minimized where possible, and that welding speed is often the most effective lever for controlling heat input in TIG welding processes. For magnesium alloy applications in aerospace and automotive industries, where weight reduction is paramount, the ability to achieve high-strength welds through optimized welding speed selection is essential for realizing the full benefits of this lightweight material.