Microstructure and Mechanical Properties of TIG Welded Joints in AZ61 Magnesium Alloy Thin Sheet

Literature Overview

The paper by Peng Jian, Zhou Chou, and Pan Fusheng from Chongqing University investigates the TIG welding behavior of 3 mm thick AZ61 magnesium alloy sheet using two different filler wires, AZ31 and AZ61, with emphasis on how filler composition influences weld joint microstructure and mechanical performance. Published in the journal "Hot Working Technology" in 2010 (Vol. 39, No. 21, pp. 1-4), this work was supported by the Ministry of Science and Technology International Science and Technology Cooperation Program (2010DFR50010) and the Chongqing Science and Technology Commission Key Project (CSTC; 2009AB4134). The study employs optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and mechanical property testing to characterize the weld appearance, microstructure, precipitated phases, and mechanical behavior of both joints.

Core Technical Findings

The key finding is that both filler wires produce weld joints without obvious defects, but the AZ61 filler wire yields superior macroscopic weld bead morphology compared to AZ31. The weld zone microstructure in both cases consists of fine equiaxed grains with two principal phases: alpha-Mg solid solution and beta-Mg17Al12 intermetallic compound. The heat-affected zone (HAZ) exhibits coarser grain structure in both joints.

Regarding hardness distribution, the AZ31 filler wire joint shows both weld zone and HAZ hardness values lower than the base metal, whereas the AZ61 filler wire joint exhibits higher weld zone hardness than the base metal but lower HAZ hardness. The tensile fracture in both joints occurs in the HAZ coarse grain region (CGHAZ), which is the weakest link in the joint.

Microstructural Analysis and Phase Evolution

The microstructural evolution during TIG welding of AZ61 magnesium alloy follows a predictable thermal cycle pattern. In the weld zone, the rapid solidification from the molten pool produces fine equiaxed grains, which is characteristic of the high cooling rates achieved in TIG welding of thin sheets. The presence of beta-Mg17Al12 phase in the weld zone is attributed to the aluminum content in both filler wires. The AZ61 filler wire, containing approximately 6% aluminum, produces a higher concentration of beta phase in the weld zone compared to AZ31, which contains approximately 3% aluminum.

The HAZ coarsening is a critical concern. The thermal cycle in TIG welding delivers sufficient heat input to the HAZ to promote grain growth and potential over-aging of precipitates. In magnesium alloys, the HAZ is particularly susceptible to grain coarsening because the peak temperature reaches the recrystallization range without reaching full melting. The CGHAZ becomes the weakest region due to the loss of grain boundary strengthening and potential precipitation coarsening.

Phase Distribution Comparison

Engineering Practice Implications

For engineers working with magnesium alloy components, several practical lessons emerge from this study. First, the selection of filler wire composition should consider not only weldability but also the resulting hardness and strength profile of the joint. The AZ61 filler wire, being compositionally matched to the base metal, provides better weld zone hardness retention, which is advantageous for fatigue resistance in service.

Second, the consistent fracture in the HAZ CGHAZ highlights the need for post-weld heat treatment (PWHT) to restore HAZ properties. A T5 temper treatment (solution heat treatment followed by artificial aging) can refine the HAZ microstructure and improve the overall joint strength. Without PWHT, the joint strength is limited by the HAZ rather than the weld zone.

Third, for thin sheet applications (3 mm in this case), the relatively low heat input of TIG welding is advantageous for minimizing the HAZ width and reducing distortion. However, the cooling rate must be carefully controlled to avoid excessive beta phase formation at grain boundaries, which can compromise intergranular corrosion resistance.

Key Questions and Reflections

The most significant question raised by this study is why the HAZ remains the weakest region despite the use of compositionally matched filler wire. This points to a fundamental limitation of TIG welding for magnesium alloys: the thermal cycle inevitably produces a region of coarsened grains that cannot be fully compensated by filler selection. In engineering practice, this means that for critical applications involving AZ61 magnesium alloy, alternative welding processes with lower heat input, such as laser welding or electron beam welding, should be considered to minimize HAZ coarsening.

Another important consideration is the corrosion resistance of the joint. The beta-Mg17Al12 phase is less corrosion resistant than the alpha-Mg matrix, and its distribution at grain boundaries in the weld zone could create galvanic coupling sites. For applications in aggressive environments, such as automotive underbody components or marine structures, post-weld surface treatment and possibly PWHT would be essential.

Study Insights and Practical Recommendations

From my experience in welding engineering, this study provides valuable data for process selection in magnesium alloy fabrication. The recommendation is clear: for AZ61 magnesium alloy thin sheet TIG welding, use AZ61 filler wire for optimal weld zone properties, but always plan for PWHT to address the HAZ weakness. The weld parameters should be optimized to minimize heat input while maintaining adequate penetration, keeping in mind that excessive penetration can widen the HAZ and increase the risk of hot cracking due to the narrow solidification range of AZ61 alloy.

The study also underscores the importance of filler wire selection in determining the overall joint quality. While both fillers produce acceptable joints without major defects, the AZ61 filler provides a more favorable hardness profile in the weld zone, which translates to better resistance against wear and fatigue in service. For production applications, this translates to a direct economic benefit through reduced need for post-weld machining and improved service life.