CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of LZ91 Magnesium-Lithium Alloy TIG Welded Joints
Literature Overview
This 2017 study published in Light Alloy Fabrication Technology by Liu Yang, Liu Xuhe, Xiao Yang, Xie Haitao, and Guo Xiaoguang from the Metal Materials Research Institute of Zhengzhou Light Metals Research Institute and the School of Materials Science and Engineering at Yanshan University, investigates the TIG welding of LZ91 magnesium-lithium alloy. Supported by the National Natural Science Foundation (51371161) and the National International Science and Technology Cooperation Program (2015DFR0020), this research addresses a material system of growing importance in aerospace and automotive applications due to its exceptional low density.
Core Technical Content
LZ91 is a magnesium-lithium alloy containing approximately 9% lithium and 1% zinc, with a density of approximately 1.35 g/cm³, making it one of the lightest structural alloys in commercial use. The lithium addition transforms the crystal structure from hexagonal close-packed (HCP) to body-centered cubic (BCC) β phase, significantly improving ductility and formability. However, the welding of this alloy presents unique challenges related to its low melting point, high reactivity, and phase instability.
Welding Process Parameters
The TIG welding was performed under high-purity argon shielding with a helium-argon mixture (75% He / 25% Ar) to improve arc stability and heat input efficiency. The welding parameters were optimized to minimize porosity and oxidation while achieving adequate penetration.
| Parameter | Value | Rationale |
|---|---|---|
| Welding current | 100–160 A | Balances penetration and heat input |
| Arc voltage | 14–18 V | Maintains arc stability |
| Travel speed | 100–200 mm/min | Controls heat-affected zone width |
| Shielding gas | 75% He / 25% Ar | Improves arc energy density |
| Gas flow rate | 15–25 L/min | Adequate protection against oxidation |
| Electrode | Pure tungsten, 3.2 mm | Standard TIG electrode |
| Base metal thickness | 3–5 mm | Typical structural application |
Microstructural Evolution
The base metal LZ91 alloy exhibits a dual-phase microstructure consisting of β-Mg(Li) grains with α-Mg precipitates along grain boundaries. The welding process causes significant microstructural changes in both the weld zone and the heat-affected zone.
In the weld zone, the rapid solidification leads to a fine-grained microstructure with a mixture of α-Mg and β-Mg(Li) phases. The cooling rate during solidification is significantly higher than in cast or wrought conditions, resulting in a finer grain size and a higher proportion of the metastable β phase. The weld metal typically exhibits elongated grains parallel to the solidification direction, with the grain size decreasing from the center of the weld toward the fusion boundary.
The heat-affected zone shows a gradient of microstructural changes. The region immediately adjacent to the fusion boundary experiences partial melting and resolidification, producing a fine-grained microstructure similar to the weld metal. Moving away from the fusion boundary, the microstructure transitions through a region of phase transformation where the β phase partially dissolves into the α phase, and finally to the original base metal microstructure. The width of the HAZ is typically 1.5–3 mm depending on the heat input.
| Zone | Grain Size (μm) | Phase Composition | Hardness (HV) |
|---|---|---|---|
| Base metal | 40–60 | α + β (equiaxed) | 45–55 |
| Fine-grained HAZ | 15–25 | α + β (refined) | 50–60 |
| Coarse-grained HAZ | 60–100 | α + β (coarsened) | 40–50 |
| Weld metal | 10–20 | α + β (fine, columnar) | 48–58 |
Mechanical Properties
The tensile strength of the weld joint was measured at 180–210 MPa, representing a joint efficiency of 70–80% relative to the base metal strength of 230–260 MPa. The elongation of the welded joint was 8–12%, compared to 15–20% for the base metal. The reduction in ductility is attributed to the coarser grain structure in the HAZ and the presence of porosity and oxide inclusions in the weld metal.
The hardness distribution across the weld cross-section shows a characteristic profile with a slight increase in the weld metal and fine-grained HAZ, followed by a decrease in the coarse-grained HAZ region. The maximum hardness of approximately 60 HV occurs in the weld metal, while the minimum hardness of approximately 40 HV occurs in the coarse-grained HAZ, representing a 25% reduction from the base metal value.
Defect Analysis and Countermeasures
The primary defects observed in LZ91 TIG welded joints include porosity, oxide inclusion, and lack of fusion. Porosity formation is attributed to hydrogen absorption from the atmosphere and moisture contamination of the shielding gas. The high reactivity of magnesium with oxygen and nitrogen leads to the formation of MgO and Mg3N2 inclusions within the weld metal.
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Gas porosity | Hydrogen absorption, inadequate shielding | Increase gas flow, use dry gas |
| Oxide inclusion | Surface oxidation, poor gas coverage | Pre-cleaning, back-purge with argon |
| Lack of fusion | Low heat input, excessive travel speed | Increase current, reduce travel speed |
| Cracking | Thermal stress, phase transformation | Reduce heat input, post-weld stress relief |
The countermeasures for improving weld quality include thorough surface preparation to remove the native oxide layer, use of a high-purity shielding gas with back-purge protection on the root side, and optimization of the welding parameters to minimize hydrogen absorption while maintaining adequate penetration. The addition of a small amount of zinc to the filler metal can also improve weldability by reducing the surface tension of the molten pool and promoting better wetting.
Engineering Implications
For aerospace and automotive applications where weight reduction is critical, the welding of magnesium-lithium alloys is an enabling technology. The study demonstrates that acceptable welded joints can be achieved with TIG welding using appropriate process parameters and shielding gas conditions. However, the relatively low joint efficiency (70–80%) limits the application to non-critical structural components or requires design modifications to account for the reduced strength.
For pressure vessel applications, the use of magnesium-lithium alloys is currently limited due to the relatively low strength and creep resistance of the material. However, for lightweight cryogenic vessels or low-pressure storage applications, the findings of this study provide a foundation for developing welding procedures specifications. The high reactivity of the alloy requires meticulous attention to surface preparation and gas shielding, which increases manufacturing complexity and cost.
Study Insights and Reflections
The research provides valuable baseline data on the weldability of LZ91 magnesium-lithium alloy, which is still relatively limited in the published literature. The identification of the optimal shielding gas composition (75% He / 25% Ar) and the emphasis on back-purge protection are practical recommendations that can be directly applied in production welding. The relatively low joint efficiency highlights the need for further research on post-weld heat treatment and advanced welding techniques such as friction stir welding to improve the mechanical properties of magnesium-lithium alloy welded joints.