TIG Welding of AZ31 and AZ61 Dissimilar Magnesium Alloys

Literature Overview

This paper by Peng Jian and colleagues from Chongqing University and the Chongqing Institute of Technology, published in Journal of Materials Engineering in 2011, investigates the TIG welding of dissimilar magnesium alloys—AZ31 and AZ61—using both AZ31 and AZ61 filler wires. The research is supported by international science and technology cooperation projects and the Chongqing Science and Technology Commission, and it addresses the challenge of joining dissimilar magnesium alloys, which is relevant for weight-optimized structural designs in automotive and aerospace applications.

Experimental Configuration and Materials

The researchers performed butt welding on thin AZ31 and AZ61 magnesium alloy plates using TIG welding with two different filler wires: AZ31 and AZ61. The microstructure, phase composition, and mechanical properties were characterized using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The AZ31 alloy contains approximately 3% Al and 1% Zn, while AZ61 contains approximately 6% Al and 1% Zn, making the AZ61 alloy stronger but more susceptible to hot cracking.

Microstructural Characterization

Both weld joints exhibited the presence of alpha-Mg and beta-Mg17Al12 phases in the weld zone, which is consistent with the equilibrium phase diagram of the Mg-Al system. The heat-affected zones (HAZ) on both sides showed grain coarsening near the weld, but the AZ31 side exhibited more pronounced grain coarsening compared to the AZ61 side. This asymmetry is attributed to the different thermal properties and thermal conductivities of the two alloys, as well as the different cooling rates experienced by each side.

The use of AZ31 filler wire resulted in a narrower coarse grain zone on the AZ31 side compared to the AZ61 filler wire case. This is because the AZ31 filler wire, having a lower melting point and thermal conductivity, creates a more localized heat input on the AZ31 side, resulting in a narrower HAZ. The AZ61 filler wire, with its higher thermal conductivity and melting point, distributes heat more broadly, leading to a wider HAZ on the AZ31 side.

Mechanical Properties and Fracture Analysis

The mechanical properties show a significant difference between the two filler wire options. The AZ61 filler wire produced a joint with an average UTS of 210 MPa, achieving approximately 84% of the AZ31 base metal strength. In contrast, the AZ31 filler wire produced a joint with a UTS of only 161 MPa, reaching about 64% of the AZ31 base metal strength. The fracture location also differed: with AZ31 filler wire, the fracture occurred closer to the centerline, while with AZ61 filler wire, the fracture occurred away from the centerline.

The higher strength achieved with AZ61 filler wire is attributed to the higher aluminum content in the weld metal, which promotes the formation of more Mg17Al12 precipitates and enhances solid solution strengthening. However, the higher aluminum content also increases the susceptibility to hot cracking, which must be carefully managed through welding parameter optimization.

Engineering Considerations for Dissimilar Magnesium Alloy Welding

When joining dissimilar magnesium alloys, several engineering considerations must be addressed:

1. Thermal mismatch: The different thermal conductivities and coefficients of thermal expansion of AZ31 and AZ61 create asymmetric thermal stresses that can lead to distortion and residual stress.
2. Microstructural asymmetry: The different cooling rates on each side produce asymmetric microstructures, which can result in asymmetric mechanical properties.
3. Corrosion galvanic coupling: The different alloy compositions create a galvanic couple that may accelerate corrosion in the HAZ regions.
4. Filler wire selection: The filler wire composition should be chosen to balance strength, ductility, and crack resistance.

The choice of AZ61 filler wire provides significantly better mechanical performance but requires careful control of welding parameters to prevent hot cracking. The AZ31 filler wire offers better crack resistance but at the expense of joint strength. For applications where strength is critical, AZ61 filler wire with optimized welding parameters is recommended, while AZ31 filler wire may be suitable for non-critical joints where crack resistance is the primary concern.

Study Insights and Reflections

This study provides valuable data for the practical joining of dissimilar magnesium alloys, a topic of growing importance in lightweight structural design. The finding that AZ61 filler wire significantly improves joint strength is a practical and actionable result for engineers designing magnesium alloy structures. The asymmetric microstructure and mechanical properties are inherent challenges in dissimilar alloy welding, and the results highlight the need for careful weld procedure development and qualification. The role of filler wire composition as a design variable is clearly demonstrated, and future work should explore the effects of welding parameters, joint design, and post-weld treatment on joint performance. For production applications, the AZ61 filler wire option should be qualified with appropriate non-destructive testing protocols to ensure consistent weld quality and joint integrity.