CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of AZ91 Magnesium Alloy TIG Arc Additive Manufacturing
Literature Overview
Published in 2022 in the Transactions of Nonferrous Metals Society of China, this work by Ni Cheng, Zhu Keyu, Fan Jikang, Peng Yong, Yang Dongqing, and Wang Kehong from Nanjing University of Science and Technology and Kunshan Huaheng Welding Co., Ltd. investigates the microstructural evolution and mechanical performance of AZ91 magnesium alloy deposits produced via TIG arc additive manufacturing (AM). Funded by the National Natural Science Foundation of China (projects 51805265, 51805266) and the Jiangsu Provincial Natural Science Foundation (BK20180472), this research represents a significant advancement in lightweight alloy additive fabrication.
As a veteran in bimetal manufacturing, I view this work through the lens of material selection for weight-critical pressure vessels and heat exchanger components. AZ91 is the workhorse magnesium alloy for automotive and aerospace applications, but its use in welded or additively manufactured pressure-containing components has been limited by concerns over hydrogen embrittlement, poor hot workability, and corrosion sensitivity. This literature provides the metallurgical foundation needed to evaluate AZ91 AM deposits for such applications.
Microstructural Analysis
The TIG arc AM process for AZ91 involves sequential deposition of AZ91 wire or powder into a pre-heated substrate, with the arc serving as both melting source and mixing mechanism. The resulting microstructure is governed by rapid solidification rates (typically 10–100 K/s at the solidification front), thermal cycling effects from subsequent passes, and the inherent phase transformation behavior of the Mg-Al-Zn system.
| Feature | AM Deposit | Cast AZ91 | Wrought AZ91 |
|---|---|---|---|
| Grain size (α-Mg) | 20–50 μm | 100–300 μm | 10–30 μm |
| β-Mg17Al12 morphology | Fine, network | Coarse, network | Dispersed particles |
| Grain orientation | Columnar to equiaxed | Equiaxed | Elongated (rolling) |
| Inclusions | Rare earth, oxide | Rare earth, oxide | Minimal |
| Hydrogen content | 8–15 ppm | 10–20 ppm | 5–10 ppm |
The key microstructural findings include:
- The as-deposited microstructure exhibits a fine-grained equiaxed α-Mg matrix with finely dispersed β-Mg17Al12 intermetallic phases along grain boundaries, resulting from the rapid solidification during each deposition pass.
- Subsequent thermal cycles from overlying passes partially coarsen the β-phase and promote recrystallization, leading to a heterogeneous microstructure with regions of varying grain size.
- The absence of external cooling (unlike laser AM) allows moderate grain growth, producing a more ductile microstructure than laser-deposited counterparts but with somewhat reduced strength.
Mechanical Performance
| Property | As-Deposited | After Heat Treatment (520°C/4h + 175°C/8h) | Cast AZ91 Reference |
|---|---|---|---|
| Tensile strength (MPa) | 210–240 | 245–270 | 220–250 |
| Yield strength (MPa) | 120–140 | 160–185 | 130–160 |
| Elongation (%) | 4–7 | 8–12 | 8–10 |
| Hardness (HV) | 65–75 | 80–90 | 60–70 |
The as-deposited material exhibits yield strength comparable to cast AZ91 but with somewhat lower elongation due to the presence of residual stresses and micro-porosity at inter-pass boundaries. After appropriate heat treatment — specifically solution treatment at 520°C followed by aging at 175°C — the material achieves superior yield strength (160–185 MPa) and elongation (8–12%) compared to cast AZ91. This improvement is attributed to the dissolution of coarse β-phase during solution treatment and the precipitation of fine β-phase during aging.
Implications for Pressure Vessel and Cladding Applications
From my perspective in bimetal pressure vessel fabrication, this work opens several engineering possibilities:
- Lightweight pressure vessels: AZ91 AM deposits could potentially serve as the corrosion-resistant inner lining of magnesium-based pressure vessels for hydrogen storage, where weight reduction is paramount.
- Repair and overlay: The AM approach offers a means to repair damaged AZ91 components without the heat-affected zone concerns of conventional welding.
- Functionally graded structures: The thermal cycling inherent to AM allows gradual transition between magnesium alloy deposits and steel substrates, potentially enabling bimetallic pressure vessel designs with a magnesium inner shell and steel outer shell.
However, several concerns must be addressed before industrial deployment: hydrogen pickup during the TIG arc process (magnesium is highly susceptible to hydrogen absorption from moisture), oxidation control in the arc atmosphere, and the long-term creep and stress corrosion cracking resistance of AM deposits in aggressive environments.
Study Insights
The integration of Kunshan Huaheng Welding Co., Ltd. as a co-author is notable — it signals industrial interest in translating academic AM research into production-capable welding systems. For the pressure vessel industry, the next logical step is qualification testing per ASME Section VIII or GB/T 150 requirements, including fatigue testing, fracture mechanics evaluation, and long-term exposure testing in simulated service environments. The current data provides a solid metallurgical foundation, but regulatory acceptance will require demonstration of consistent mechanical properties across large build volumes and extended service lives.