Microstructure and Mechanical Properties of LZ91 Magnesium-Lithium Alloy TIG Welded Joints

Literature Overview

This study, published in Light Alloy Fabrication Technology (Vol. 45, No. 1, 2017, pp. 61–68) by Liu Yang, Liu Xuehe, Xiao Yang, Xie Haitao, and Guo Xiaoguang from the Zhengzhou Light Metal Research Institute and Yanshan University, investigates the TIG welding behavior of 2 mm thick LZ91 Mg-Li alloy plate. Funded by the National Natural Science Foundation of China (Grant No. 51371161) and the National International Science and Technology Cooperation Program (2015DFR0020), this research addresses a critical material challenge in lightweight structural applications where Mg-Li alloys offer superior specific strength but present significant welding difficulties.

Material Background: LZ91 Magnesium-Lithium Alloy

LZ91 is a wrought Mg-Li alloy containing approximately 9% lithium and 1% zinc. The lithium addition reduces the density from ~1.75 g/cm³ (conventional Mg alloys) to ~1.35 g/cm³, providing a 23% density reduction with only a modest strength loss. This makes LZ91 attractive for aerospace, automotive, and defense applications where weight savings are critical.

The β phase in LZ91 is an intermetallic compound with a body-centered cubic structure, which forms during solidification and significantly influences the weld microstructure. The α phase is the magnesium-rich solid solution matrix.

Welding Process and Parameters

The study employed AC TIG welding, which is preferred for magnesium alloys because the AC cycle alternates between electrode-positive and electrode-negative half-cycles. The electrode-negative half-cycle provides deep penetration, while the electrode-positive half-cycle provides cathodic cleaning that removes the oxide layer from the workpiece surface.

Microstructural Analysis

The welded joint consists of three distinct zones, each with characteristic microstructures:

Weld Metal Zone (~8 mm width)

The weld metal exhibits a cellular dendritic structure where α-phase dendrites are randomly and uniformly distributed within the β-phase matrix. This microstructure results from the solidification sequence:

1. The β phase solidifies first due to its lower melting point in the Mg-Li system.
2. The α phase precipitates from the remaining liquid as temperature decreases.
3. The rapid cooling rate produces fine cellular dendrites.

Heat-Affected Zone (~6 mm width)

The HAZ shows a gradient in microstructure with distance from the fusion line:

• Near fusion line: Coarse equiaxed β grains with needle-like or strip-shaped α phase. The α phase content is lower near the fusion line.
• Far from fusion line: Finer β grains with higher α phase content, approaching the base metal structure.

The grain size of β phase decreases with increasing distance from the fusion line, following an inverse relationship. This is because the peak temperature decreases with distance, resulting in less grain growth and less phase transformation.

Base Metal Zone

The base metal retains its typical rolled and annealed microstructure, with fine equiaxed grains of both α and β phases.

Mechanical Properties

Mechanical testing revealed the following results:

The tensile strength of the weld metal reaches 95.6% of the base metal strength, which is acceptable for most structural applications. However, the elongation drops to 77.5% of the base metal value, indicating reduced ductility in the weld zone. The fracture occurs in the weld metal, which is the weakest region.

Fracture Mode Analysis

An important finding is the change in fracture mode: the base metal exhibits cleavage fracture (brittle), while the weld metal exhibits ductile fracture. This counterintuitive result is attributed to:

• The base metal's rolled annealed microstructure contains stress concentrators that promote cleavage.
• The weld metal's cellular dendritic structure provides more crack-resistant morphology.
• The HAZ's coarse β grains may be the actual weakest region for crack initiation, but the crack propagates through the weld metal.

Engineering Practice Implications

For engineers working with Mg-Li alloys, this study provides several critical insights:

1. Weld strength adequacy: The 95.6% strength ratio meets typical code requirements (≥80% of base metal), making LZ91 weldable for structural applications.
2. Ductility concern: The 77.5% elongation ratio may be problematic in applications requiring high strain capacity. Post-weld heat treatment (PWHT) could improve ductility by homogenizing the microstructure.
3. HAZ vulnerability: The HAZ has the highest hardness, which correlates with lower ductility and potential susceptibility to stress corrosion cracking. The coarse β grains near the fusion line are particularly concerning.
4. Process sensitivity: The wide weld and HAZ zones (8 mm and 6 mm respectively for 2 mm plate) indicate significant heat input, which is typical for Mg alloys due to their high thermal conductivity. Process parameter optimization is essential.

FMEA for Mg-Li Alloy Welding

Study Insights and Reflections

This study demonstrates that LZ91 Mg-Li alloy can be successfully welded by AC TIG with acceptable mechanical properties. The 95.6% strength ratio is encouraging, but the ductility reduction warrants attention in design-critical applications. The microstructural gradient across the HAZ—with β grain size inversely proportional to distance from the fusion line—provides a clear understanding of the thermal effects on phase transformation.

The finding that the fracture mode changes from cleavage (base metal) to ductile (weld metal) is particularly interesting and suggests that the welding process actually improves the local fracture resistance of the weld metal, even though the overall joint strength is reduced. This has implications for fatigue and impact performance, which were not investigated in this study but would be valuable for comprehensive evaluation.

For pipe and fitting manufacturing using Mg-Li alloys—a niche but growing application in aerospace fuel systems and cryogenic applications—this study provides the fundamental welding data needed for process qualification. The wide HAZ (6 mm for 2 mm plate) means that even thin-walled pipe will have a significant proportion of altered microstructure, which must be considered in design and inspection.

This research establishes the feasibility of TIG welding for LZ91 Mg-Li alloy and provides the microstructural and mechanical data necessary for engineering qualification, with clear recommendations for process optimization and post-weld treatment to further improve joint performance.