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

Literature Overview

This 2015 study by Yang Yongchun from Xinjiang Institute of Transportation Vocational and Technical College examines how TIG welding speed influences the microstructure evolution and mechanical properties of AZ31 magnesium alloy welded joints. AZ31 (Mg-3Al-1Zn) is the most widely used wrought magnesium alloy in automotive, aerospace, and consumer electronics applications due to its favorable combination of lightweight properties, corrosion resistance, and formability. The study was published in Thermal Processing Technology, indicating its focus on the interplay between welding thermal input and resulting metallurgical outcomes.

Core Technical Content

The welding speed directly determines the linear heat input (q) of the TIG process, which for a constant current and arc voltage can be expressed as q = I × V / v, where I is current, V is arc voltage, and v is travel speed. The authors investigated welding speeds ranging from 20 mm/min to 120 mm/min at a constant current of 120 A and arc voltage of approximately 18 V, resulting in linear heat inputs from approximately 1.08 kJ/mm down to 0.18 kJ/mm. This range spans from a highly heat-affected condition to a relatively cold welding regime.

Metallographic analysis of the weld cross-sections revealed distinct microstructural zones that vary significantly with welding speed. At low welding speeds (20–40 mm/min), the weld zone exhibits a coarse columnar grain structure with pronounced dendritic morphology, and the HAZ shows extensive grain growth with grain sizes exceeding 50 μm. At moderate speeds (60–80 mm/mm), the microstructure transitions to a finer equiaxed grain structure with reduced dendrite arm spacing. At high speeds (100–120 mm/min), the weld zone becomes narrower with a fine-grained structure, but the reduced heat input may lead to incomplete fusion at the toe and root, particularly for thicker sections.

The mechanical properties exhibit a clear trend with welding speed. Tensile strength of the weld zone increases from approximately 145 MPa at low speed to 175 MPa at moderate speed, then slightly decreases at the highest speeds due to potential incomplete fusion. Elongation follows a similar pattern, peaking at moderate welding speeds. Hardness measurements show that the HAZ hardness decreases with increasing welding speed due to reduced grain growth and less dissolution of precipitates.

Key Technical Parameters and Analysis

The microstructural evolution is governed by two competing mechanisms. At low welding speeds, the prolonged thermal exposure promotes grain growth and precipitate coarsening, which degrades strength and ductility. At high speeds, the rapid solidification and cooling rates produce fine grains but may not allow sufficient time for proper wetting and fusion, leading to geometric defects. The optimal welding speed window of 60–80 mm/min represents a balance between adequate fusion and controlled microstructural refinement.

Engineering Practice Implications

For AZ31 magnesium alloy welding in production environments, the following practical guidelines emerge from this study. First, the welding speed should be optimized for each specific joint geometry and thickness. For sheet materials below 2 mm thickness, speeds of 80–100 mm/min are generally appropriate. For thicker sections above 3 mm, multi-pass welding with interpass temperature control below 150°C is recommended. Second, the shielding gas flow rate must be adjusted to compensate for the increased gas entrainment risk at higher speeds; a minimum of 15 L/min of pure argon is recommended for speeds above 80 mm/min. Third, preheating to 150–200°C is beneficial for reducing solidification cracking susceptibility, particularly for AZ31 alloys with aluminum content above 2.5%.

The study also highlights the importance of post-weld heat treatment. Solution treatment at 415°C for 4 hours followed by water quenching and aging at 175°C for 6 hours can homogenize the microstructure and restore mechanical properties to near-base-metal levels. This is particularly important for applications where the welded joint must meet specific strength requirements, such as in automotive structural components or aerospace brackets.

Study Insights and Reflections

This research provides a systematic understanding of how a single welding parameter (travel speed) influences the complete chain of thermal input, microstructural evolution, and mechanical performance. The findings are directly applicable to manufacturing engineers developing welding procedures for magnesium alloy components. The identification of an optimal speed window of 60–80 mm/min for typical AZ31 sheet thicknesses offers a practical starting point for WPS development. However, it is important to recognize that welding speed interacts with other parameters such as current, electrode diameter, and joint preparation, and therefore the optimal speed must be determined through qualification testing for each specific application. The study reinforces the principle that in magnesium alloy welding, thermal management is paramount, and every parameter adjustment must be evaluated in terms of its effect on the thermal cycle and resulting microstructure.