Microstructure and Mechanical Properties of TIG and LBW Welded Mg-Li Alloy

Literature Overview

This study by Jia-wei Lu, Hong-jie Liu, and colleagues from Shanghai Jiao Tong University, Beijing Institute of Electronic System Engineering, and Shanghai Cloud-Manu 3D Technology, published in the Transactions of Nonferrous Metals Society of China (2025, Vol. 35, No. 12, pp. 4084–4100), investigates the weldability of the Mg-8Li-3Al-2Zn-0.5Y (LAZ832-0.5Y) magnesium-lithium alloy using both TIG and laser beam welding (LBW). The research examines as-cast and as-forged base materials, evaluates microstructure and mechanical properties, and investigates the effect of solid solution treatment at 350°C for 4 hours on weld joint strength. The work is supported by multiple national defense and scientific research funding programs, indicating its relevance to aerospace and defense applications.

Material Background and Weldability Challenges

The LAZ832-0.5Y alloy belongs to the AZ series of Mg-Li alloys, which are of significant interest for aerospace structural applications due to their low density (approximately 1.35–1.45 g/cm³) and good combination of strength and formability. The addition of 0.5Y (yttrium) is intended to improve creep resistance and high-temperature performance through the formation of fine dispersoids. However, Mg-Li alloys present unique welding challenges:

• Low melting point (approximately 450°C for this composition) requires careful thermal management to avoid excessive HAZ softening.
• High chemical reactivity with atmospheric oxygen and nitrogen demands robust shielding gas protection.
• The presence of lithium affects solidification behavior, with Mg-Li systems exhibiting a peritectic reaction that can lead to hot cracking if not properly controlled.
• The Al₂Y intermetallic phase and AlLi particles serve as strengthening precipitates but may dissolve or coarsen during welding, affecting post-weld strength.

Welding Process Parameters and Configuration

The study employed two distinct welding processes with carefully optimized parameters:

The choice of as-cast material for TIG and as-forged material for LBW reflects the different thermal input characteristics of the two processes. TIG welding, with its higher thermal input and slower cooling rates, is more suitable for the as-cast microstructure which may contain casting defects and larger grain sizes. LBW, with its high energy density and rapid solidification, benefits from the finer, more uniform microstructure of the forged material.

The double-side welding configuration for LBW (2.1 kW from one side, 2.0 kW from the opposite side) is a notable process innovation. This approach ensures complete penetration through the plate thickness while minimizing the risk of hot cracking by distributing the thermal input more evenly. It also reduces the cooling rate asymmetry that would occur with single-side welding, potentially improving weld integrity.

Microstructural Characteristics

The microstructural analysis of the as-welded joints revealed significant differences between TIG and LBW welds:

TIG Weld Joint:

• Fusion zone (FZ): Coarse columnar grains with dendritic substructure, characteristic of the relatively slow cooling rates in TIG welding.
• Heat affected zone (HAZ): Partial dissolution of strengthening precipitates, with some grain growth at the HAZ boundary.
• Base metal (BM): Retained original as-cast microstructure with Al₂Y phases and AlLi particles.

LBW Weld Joint:

• Fusion zone (FZ): Fine equiaxed grains with significantly reduced grain size compared to TIG, attributable to the rapid solidification rates (potentially exceeding 10³ K/s) in laser welding.
• Heat affected zone (HAZ): Narrower affected zone with less precipitate dissolution due to the shorter time at elevated temperatures.
• Base metal (BM): Retained original as-forged microstructure.

The microstructural differences directly explain the mechanical property disparity. The coarser grain structure in the TIG weld reduces strength through the Hall-Petch mechanism, while the finer grains in the LBW weld provide greater strength.

Solid Solution Treatment Effects

The solid solution treatment at 350°C for 4 hours was applied to both TIG and LBW weld joints to evaluate post-weld strengthening potential. The results were instructive:

The significantly greater strengthening effect in the TIG joint after solid solution treatment is attributed to the dissolution of the smaller white AlLi particles into the matrix, which provides solid solution strengthening. In the TIG weld, the coarser microstructure and higher volume fraction of dissolved AlLi particles available for solid solution strengthening resulted in a more dramatic property improvement. In contrast, the LBW weld's already fine microstructure and potentially different precipitate distribution limited the additional strengthening benefit from solid solution treatment.

The finding that α-Mg grain growth in the FZ, HAZ, and BM was insignificant after solid solution treatment is encouraging. This indicates that the treatment temperature and duration were selected conservatively to avoid grain coarsening, which would offset the strengthening benefits.

Mechanical Property Analysis and Engineering Implications

The yield strength of the TIG joint after solid solution treatment (188 MPa) exceeded that of the LBW joint (160 MPa), reversing the as-welded hierarchy. This outcome highlights the importance of considering the entire processing sequence—welding followed by post-weld heat treatment—rather than evaluating welding processes in isolation. For applications where post-weld heat treatment is part of the manufacturing sequence, TIG welding may actually produce superior final properties.

The elongation values, while not explicitly reported in the abstract, would be critical for structural applications. Mg-Li alloys generally exhibit limited ductility, and the welding process tends to further reduce ductility through grain coarsening and precipitate dissolution. The solid solution treatment, by dissolving brittle intermetallic particles, may improve ductility in addition to strength.

Study Insights and Reflections

This research makes a valuable contribution to the understanding of Mg-Li alloy weldability by systematically comparing two fundamentally different welding processes. The key insight is that the "best" welding process depends on the complete processing sequence, including post-weld heat treatment. TIG welding, often considered inferior to laser welding for thin-section Mg alloys due to its higher thermal input, can actually produce better final properties when followed by appropriate solid solution treatment.

The double-side LBW configuration is a practical innovation worth further development. The use of slightly different powers from each side (2.1 kW and 2.0 kW) suggests careful optimization of the thermal balance to achieve full penetration without excessive HAZ softening. This approach could be adapted for other Mg-Li alloy compositions and thicknesses.

One limitation of the study is the absence of fatigue and creep property data, which are critical for aerospace applications of Mg-Li alloys. The solid solution treatment, while improving tensile properties, may affect fatigue crack initiation resistance and high-temperature creep behavior. Additionally, the long-term stability of the solid solution strengthened condition (i.e., susceptibility to age softening) should be evaluated for service applications.

Reference Value and Outlook

This work provides essential data for the selection of welding processes and post-weld heat treatment parameters for Mg-Li alloys in aerospace applications. The demonstration that TIG welding followed by solid solution treatment can achieve yield strengths exceeding those of laser-welded joints challenges conventional assumptions about the superiority of laser welding for lightweight structural alloys. Future research should extend to fatigue, creep, and corrosion performance of the heat-treated weld joints, as well as to thicker sections and complex joint geometries. The findings also suggest that for production environments where laser welding equipment is unavailable or cost-prohibitive, TIG welding with appropriate post-weld treatment represents a viable alternative for Mg-Li alloy fabrication.