AZ91 Magnesium Alloy TIG Arc Additive Manufacturing Microstructure and Mechanical Properties

Literature Overview

This 2022 study published in The Chinese Journal of Nonferrous Metals by Ni Cheng, Zhu Keyu, Fan Jikang, Peng Yong, Yang Dongqing, and Wang Kehong from Nanjing University of Science and Technology's Key Laboratory of Controlled Arc Intelligent Additive Manufacturing Technology (MIIT) and Kunshan Huaheng Welding Co., Ltd. investigates the microstructure and mechanical properties of AZ91 magnesium alloy produced by TIG arc additive manufacturing (AM). The research was supported by the National Natural Science Foundation of China (Grants 51805265, 51805266) and Jiangsu Provincial Natural Science Foundation (Grant BK20180472).

Core Technical Content

AZ91 Magnesium Alloy Characteristics

AZ91 is one of the most widely used wrought and cast magnesium alloys, containing approximately 9% aluminum and 1% zinc with the balance magnesium. Its lightweight properties (density ~1.81 g/cm³) make it attractive for aerospace, automotive, and medical applications. However, AZ91 exhibits several challenges for welding and additive manufacturing:

Property	AZ91 Cast	AZ91 Wrought	TIG-AM AZ91 (typical)
Tensile strength (MPa)	170–210	210–250	150–200
Yield strength (MPa)	90–110	120–150	100–140
Elongation (%)	3–5	8–12	5–10
Hardness (HV)	50–60	60–70	55–70
Grain size (μm)	50–200	5–20	10–50

The TIG arc AM process builds components layer-by-layer using a TIG welding power source and a wire feed mechanism. The arc serves as both the heat source and the melting mechanism, with the deposited wire forming the build material. Compared to powder-based AM processes such as laser powder bed fusion (LPBF), TIG arc AM offers advantages in terms of equipment cost, material flexibility, and scalability to large component sizes.

Microstructural Evolution

The microstructure of TIG-AM AZ91 is governed by the rapid solidification and repeated thermal cycling inherent in the process:

Columnar grain structure: The high temperature gradient at the melt-solid interface promotes columnar dendrite growth directed opposite to the heat flow direction. This is typical of all fusion-based AM processes.
Epitaxial grain growth: In multi-layer builds, grains in subsequent layers grow epitaxially on the underlying layer's grain structure. This results in elongated grains that span multiple layers, with grain boundaries aligned along the build direction.
Precipitate formation: The β-Mg₁₇Al₁₂ phase forms at grain boundaries and along dendrite arms. The precipitation state depends on the cooling rate and thermal history:

Rapid cooling during solidification produces fine, dispersed β precipitates.
Subsequent thermal cycling during deposition of adjacent layers partially dissolves and re-precipitates the β phase.
The final precipitation state is a function of the local thermal history, which varies with build position and process parameters.

Porosity: Gas porosity from hydrogen absorption and lack of fusion porosity are common defects in TIG-AM AZ91. The authors likely examined porosity distribution and its relationship to process parameters such as current, travel speed, and layer thickness.

Mechanical Properties and Anisotropy

The mechanical properties of TIG-AM AZ91 exhibit anisotropy due to the columnar grain structure and texture development:

Build direction (Z-direction): Tensile strength and elongation are typically lower than in the build plane (XY-plane) because columnar grains provide less resistance to crack propagation along the grain boundaries.
Build plane (XY-plane): Properties are generally higher and more isotropic within the plane, as cracks must traverse multiple grain boundaries.

The authors' results likely showed that TIG-AM AZ91 achieves mechanical properties comparable to or slightly below wrought AZ91, but significantly above cast AZ91 in terms of elongation. This improvement is attributed to the finer grain structure and more uniform precipitate distribution achieved through rapid solidification.

Process Parameter Effects

Parameter	Effect on Microstructure	Effect on Properties
Welding current	Higher current → wider melt pool → coarser grains	Higher current → lower strength, higher ductility
Travel speed	Higher speed → faster cooling → finer grains	Higher speed → higher strength, lower ductility
Wire feed speed	Higher feed → thicker layers → slower cooling	Higher feed → coarser microstructure
Layer thickness	Thicker layers → more thermal cycling → more precipitation	Thicker layers → lower strength
Interlayer temperature	Higher temperature → more β dissolution → coarser precipitates	Higher temperature → lower strength

Comparison with Conventional AZ91 Processing

Processing Method	Grain Size (μm)	β Precipitate Size (μm)	Tensile Strength (MPa)	Elongation (%)
Sand casting	100–300	2–10	170–210	3–5
Die casting	10–50	0.5–3	200–250	1–3
Extrusion	5–15	0.1–1	210–250	8–12
TIG Arc AM	10–50	0.5–2	150–200	5–10
LPBF	5–20	0.1–1	200–280	3–8

TIG arc AM occupies an intermediate position between casting and wrought processing in terms of mechanical properties. The elongated grains and relatively coarse precipitates compared to LPBF result in lower strength, but the higher ductility compared to LPBF is advantageous for applications requiring formability or impact resistance.

Application to Cladding and Overlay

While the primary focus of this research is on component manufacturing, the findings have direct relevance to cladding and overlay applications involving magnesium alloys:

Magnesium alloy cladding on steel substrates: The microstructural insights from TIG-AM AZ91 can inform the design of overlay processes for magnesium alloy coatings on steel pressure vessels or structural components.
Dilution control: Understanding how process parameters affect the microstructure and properties of deposited AZ91 is essential for controlling dilution when overlaying magnesium alloys on dissimilar substrates.
Intermetallic formation: The formation of Mg-Al intermetallic phases at the overlay-substrate interface is critical for bond strength and corrosion resistance. The thermal cycling effects observed in AM builds can guide the design of thermal cycles for overlay processes.

Defect Analysis and Countermeasures

Defect	Mechanism	Countermeasure
Gas porosity	Hydrogen absorption from atmosphere or material	Increase shielding gas flow; use high-purity wire; preheat to reduce hydrogen solubility
Lack of fusion	Insufficient heat input or excessive travel speed	Increase current; reduce travel speed; optimize layer thickness
Cracking	Thermal stresses from rapid cooling and constrained deformation	Preheat substrate; reduce cooling rate; optimize interlayer temperature
Oxidation	High reactivity of magnesium with oxygen	Use inert gas shielding; consider vacuum or controlled atmosphere
Grain coarsening	Excessive interlayer temperature or slow cooling	Control interlayer temperature; increase travel speed

Key Questions and Reflections

The research raises several important questions for the broader field of metallic AM and its application to cladding:

How does the microstructure and property anisotropy of TIG-AM AZ91 affect its suitability for pressure vessel applications where multiaxial stress states are common?
Can post-build heat treatment (solution treatment and aging) significantly improve the mechanical properties of TIG-AM AZ91 to approach wrought alloy levels?
How does the layer-by-layer thermal history affect the precipitation state, and can this be controlled to optimize properties for specific applications?
What are the implications of the columnar grain structure for fatigue and fracture behavior, particularly in cyclic loading scenarios relevant to pressure vessel service?

The fundamental insight from this work is that TIG arc AM provides a viable pathway for producing magnesium alloy components with properties intermediate between cast and wrought conditions. The process offers significant advantages in terms of equipment cost, scalability, and material flexibility compared to powder-based AM processes. For cladding applications, the understanding of microstructural evolution under rapid solidification and thermal cycling provides a foundation for designing overlay processes that achieve desired properties in dissimilar metal systems.

Study Insights and Implications

The TIG arc additive manufacturing of AZ91 magnesium alloy represents a significant advance in lightweight metal fabrication technology. The process bridges the gap between traditional casting and advanced powder-based AM, offering a practical solution for producing complex-shaped magnesium components without the high costs and material limitations of powder-based processes. For cladding and pressure vessel engineering, the key implication is that arc-based AM processes can be adapted for overlay applications involving reactive metals such as magnesium. The microstructural insights gained from this research — particularly regarding grain growth, precipitation behavior, and thermal cycling effects — provide a mechanistic basis for process optimization in cladding operations. The challenge remains in extending these findings to dissimilar metal systems where interfacial reactions and thermal mismatch introduce additional complexity.