Microstructure and Properties of AZ91D Magnesium Alloy TIG Weld Joints

Literature Overview

This paper by Xu Peiquan and colleagues, published in Thermal Processing Technology in 2011 (Volume 40, Issue 15, pp. 120-121), investigates the microstructure and mechanical properties of AZ91D magnesium alloy weld joints produced by TIG welding. The research was supported by the Shanghai Natural Science Foundation (10ZR1412900) and the Shanghai Municipal Education Commission Key Discipline Project (J51402). The study is notable for demonstrating that AZ91D magnesium alloy plates of 3.4 mm thickness can be successfully welded without filler metal and without groove preparation under specific TIG welding conditions.

Welding Parameters and Process Conditions

The welding parameters used in this study are summarized below:

The achievement of a full-penetration weld in 3.4 mm thick AZ91D magnesium alloy without filler metal and without groove preparation is a significant result. This demonstrates that the TIG welding process can be effectively applied to magnesium alloy welding under appropriate conditions, which is important for reducing production costs and simplifying the manufacturing process.

The welding current of 152 A at a travel speed of 6.5 mm/s results in a heat input of approximately 11.2 kJ/mm, which is a relatively high heat input for magnesium alloy welding. This high heat input is necessary to achieve full penetration of the 3.4 mm thick plate without filler metal, but it also introduces challenges related to grain growth and potential cracking.

Microstructure Analysis

The weld joint microstructure was analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results reveal the following microstructural features:

The weld metal microstructure consists primarily of the α-Mg matrix phase, which is the solid solution of aluminum in magnesium. The white blocky phase identified as the (α + Mg17Al12) eutectic appears at grain boundaries and interdendritic regions, where it forms during the final stages of solidification. The gray blocky phase is likely the β-phase (Mg17Al12), which is a hard and brittle intermetallic compound that forms when the local aluminum concentration exceeds the solubility limit.

The distribution of these phases is critical to the mechanical properties of the weld joint. The α-Mg matrix provides ductility and toughness, while the Mg17Al12 phase provides strength but reduces ductility. The morphology and distribution of the Mg17Al12 phase are particularly important, as continuous networks of this phase at grain boundaries can significantly reduce fracture toughness and promote intergranular cracking.

Mechanical Property Analysis

The microhardness distribution across the weld joint was measured using Vickers hardness testing with a 0.2 kgf load. The results show:

The weld zone exhibits a higher hardness than the base metal, which is attributed to the presence of the Mg17Al12 intermetallic phase and the refinement of the grain structure due to the rapid solidification of the weld pool. The hardness transition from the weld zone to the base metal is described as smooth, indicating a gradual change in microstructure and composition across the joint. This smooth transition is favorable for mechanical performance, as it minimizes stress concentrations at the weld-to-base-metal interface.

The absence of a soft zone in the HAZ is particularly noteworthy. In many aluminum alloy welds, a softened zone adjacent to the weld can significantly reduce the joint strength. The fact that the AZ91D weld joint does not exhibit this softening suggests that the heat input was controlled sufficiently to avoid excessive grain growth in the HAZ.

Engineering Practice Considerations

For engineers working with magnesium alloy components, this study provides several important insights:

1. Feasibility of autogenous welding: The successful welding of 3.4 mm AZ91D without filler metal demonstrates that autogenous TIG welding is feasible for magnesium alloy applications. This simplifies the manufacturing process and reduces material costs, as no filler metal is required.
2. Shielding gas requirements: The use of 20 L/min argon shielding gas is critical for magnesium alloy welding. Magnesium is highly reactive and forms oxides readily at elevated temperatures. Adequate shielding is essential to prevent oxidation and porosity in the weld metal. The shielding gas flow rate must be carefully controlled to ensure complete coverage of the weld pool without causing turbulence that could entrain atmospheric gases.
3. Heat input management: The high heat input required for full penetration of 3.4 mm plate introduces challenges related to grain growth and cracking. For thicker plates or higher-strength requirements, the heat input may need to be further increased, which could exacerbate these issues. Alternative approaches such as multi-pass welding with filler metal may be more appropriate for thicker sections.
4. Post-weld treatment: The weld joint may benefit from post-weld heat treatment to homogenize the microstructure and relieve residual stresses. However, heat treatment of magnesium alloys requires careful control of temperature and time to avoid overaging or grain growth.

Key Questions and Reflections

A critical question that arises from this study is whether the weld joint properties are sufficient for structural applications. While the hardness of the weld zone is higher than the base metal, the ductility and toughness of the joint are not reported. For structural applications, ductility and toughness are often more critical than hardness, and the presence of the brittle Mg17Al12 phase in the weld metal could compromise these properties.

Another important consideration is the effect of welding on the corrosion resistance of the AZ91D alloy. Magnesium alloys are inherently susceptible to corrosion, and the welding process can exacerbate this susceptibility by creating microstructural heterogeneity and residual stresses. The weld joint may require protective coatings or other corrosion mitigation measures for long-term service in corrosive environments.

The study also raises questions about the scalability of this welding approach. The successful welding of 3.4 mm plate without filler metal is a significant achievement, but the applicability to thicker sections or more complex geometries is not addressed. For industrial applications, the welding procedure must be qualified for the specific thickness range and geometry of the intended application.

Study Insights and Implications

This study demonstrates that TIG welding can be successfully applied to AZ91D magnesium alloy under appropriate conditions, achieving full penetration of 3.4 mm plate without filler metal. The microstructural analysis provides valuable insights into the phase distribution and composition of the weld joint, and the hardness measurements confirm that the weld zone is harder than the base metal with a smooth hardness transition.

For the magnesium alloy industry, this research contributes to the understanding of welding process parameters and their effect on weld joint properties. The ability to weld magnesium alloy without filler metal simplifies the manufacturing process and reduces costs, which is particularly important for mass production applications such as automotive components and aerospace structures.

However, further research is needed to address the limitations identified in this study, including the evaluation of ductility and toughness properties, the effect of welding on corrosion resistance, and the scalability of the welding approach to thicker sections and more complex geometries. The development of welding procedures for magnesium alloy applications requires a comprehensive understanding of the metallurgical effects of the welding process and the ability to control these effects to achieve the required mechanical properties.