Microstructure and Mechanical Properties of AZ91 Magnesium Alloy TIG Arc Additive Manufacturing

Literature Overview

This study, published in The Chinese Journal of Nonferrous Metals (2022, Vol. 32, No. 3, pp. 692-699) by Ni Cheng, Zhu Keyu, Fan Jikang, Peng Yong, Yang Dongqing, and Wang Kehong from the Key Laboratory of Controlled Arc Intelligent Additive Manufacturing Technology (MIIT) at Nanjing University of Science and Technology and Kunshan Huaheng Welding Co., Ltd., investigates the microstructure and mechanical properties of AZ91 magnesium alloy fabricated via TIG arc additive manufacturing. Funded by the National Natural Science Foundation of China (51805265, 51805266) and Jiangsu Provincial Natural Science Foundation (BK20180472), the research demonstrates the potential of arc additive manufacturing for lightweight structural components in aerospace applications.

Process Parameters and Build Configuration

The study successfully fabricated a single-track, multi-layer thin-walled specimen using TIG arc additive manufacturing. The process parameters and build configuration are critical to understanding the resulting microstructure:

Parameter	Value
Base material	AZ91 Mg alloy
Process	TIG arc additive manufacturing
Build configuration	Single-track, multi-layer thin-wall
Build direction	Vertical
Layer height	Dependent on torch feed and traverse speed

Microstructural Analysis

Grain Morphology

The microstructure analysis reveals equiaxed fine grain structures throughout the specimen, with consistent morphology across the bottom, middle, and top regions. This uniformity is notable given the significant thermal history differences between the first deposited layers (which solidify on the cold substrate) and subsequent layers (which solidify on previously deposited hot material).

Second Phase Distribution

The second phase β-Mg₁₇Al₁₂ precipitates are observed both within grains (intragranular) and at grain boundaries. This distribution pattern is consistent with the solidification behavior of AZ91, where Mg₁₇Al₁₂ forms during the final stages of solidification. The presence of these precipitates in both locations suggests that the cooling rates during additive manufacturing are sufficient to promote both intragranular and grain boundary precipitation.

Region	Grain Morphology	Second Phase	Grain Size Character
Bottom	Equiaxed fine grains	β-Mg₁₇Al₁₂ (intra + boundary)	Fine
Middle	Equiaxed fine grains	β-Mg₁₇Al₁₂ (intra + boundary)	Fine
Top	Equiaxed fine grains	β-Mg₁₇Al₁₂ (intra + boundary)	Fine

Mechanical Properties

The mechanical performance of the additively manufactured AZ91 specimen shows significant improvements compared to conventional die-cast AZ91:

Property	Additive Manufacturing	Die-Cast AZ91 (Reference)	Improvement
Hardness (average)	62 HV	~50 HV	~24%
Tensile strength (middle region)	276 MPa	~230 MPa	20%
Elongation (after fracture)	13.1%	~7.0%	87%

The uniform hardness distribution (62 HV average) across the specimen indicates consistent microstructure throughout the build, which is critical for structural applications. The 20% improvement in tensile strength and the remarkable 87% improvement in elongation compared to die-cast AZ91 suggest that the additive manufacturing process produces a material with superior ductility while maintaining adequate strength.

Metallurgical Mechanism Analysis

The improved mechanical properties can be attributed to several metallurgical factors:

Fine grain structure: The rapid solidification during additive manufacturing produces finer grains than conventional casting, enhancing strength through grain boundary strengthening (Hall-Petch effect).
Reduced porosity: The arc additive manufacturing process, with proper shielding and parameter control, produces denser material with fewer internal defects than die-casting.
Uniform second phase distribution: The β-Mg₁₇Al₁₂ precipitates are finely dispersed, providing effective precipitation strengthening without the coarse interdendritic networks typical of cast materials.
Reduced segregation: The layer-by-layer solidification minimizes macrosegregation, producing more homogeneous composition throughout the build.

Engineering Applications and Implications

The demonstrated capability of TIG arc additive manufacturing for AZ91 magnesium alloy has direct implications for:

Aerospace lightweight structures: Shell components, brackets, and mounting structures where weight reduction is critical.
Custom piping components: Complex-shaped magnesium alloy pipe fittings that would be difficult or expensive to produce by conventional methods.
Repair and retrofit applications: In-situ repair of magnesium alloy components with matched or superior properties.
Rapid prototyping of structural components: Quick iteration of lightweight structural designs for validation testing.

However, several challenges remain for production deployment:

Build speed limitations compared to conventional manufacturing.
Surface finish requirements for structural applications.
Long-term mechanical property stability under cyclic loading.
Standardization and qualification requirements for aerospace applications.

Study Insights and Reflections

This research demonstrates that TIG arc additive manufacturing can produce AZ91 magnesium alloy with mechanical properties superior to conventional die-cast material. The key insight is that the process-induced microstructure, characterized by fine equiaxed grains and uniformly distributed second phases, provides a favorable combination of strength and ductility. The 87% improvement in elongation is particularly significant, as magnesium alloys are typically limited by poor ductility in cast form. This opens new design possibilities for magnesium alloy components in applications where both strength and energy absorption are required. The study represents a meaningful step toward the practical application of arc additive manufacturing for lightweight structural materials, though significant work remains in process optimization, scale-up, and qualification for critical applications.