CLADDING TECHNOLOGY SHANXI CO., LTDAZ31B Magnesium Alloy Profile TIG-MIG Hybrid Weld Joint Microstructure and Mechanical Properties
Literature Overview
This 2020 study by Liu Baoshuan, Jiang Yinglong, Li Long, Guo Yangyang, and Pan Houhong from the School of Materials Science and Engineering, Southwest Jiaotong University, investigates the microstructure and mechanical properties of TIG-MIG hybrid weld joints in AZ31B magnesium alloy profiles. Published in the journal "Hot Working Technology," this research addresses a critical challenge in the lightweighting of transportation and structural components, where magnesium alloys offer superior specific strength but present significant welding difficulties.
Material Characteristics and Welding Challenges
AZ31B is a widely used wrought magnesium alloy containing approximately 3% aluminum and 1% zinc, with the remainder being magnesium. The alloy offers excellent strength-to-weight ratio, good corrosion resistance for a magnesium alloy, and reasonable formability. However, welding AZ31B presents several challenges:
- Low melting point (650°C), which increases the risk of burn-through and excessive heat input
- High vapor pressure of magnesium, leading to significant metal vaporization and smoke generation during welding
- Susceptibility to oxidation, as magnesium forms a protective oxide layer that must be removed or penetrated during welding
- Limited solidification range, which reduces hot cracking susceptibility compared to aluminum alloys but introduces other solidification challenges
- Hydrogen absorption from the atmosphere, which can lead to porosity in the weld metal
The TIG-MIG hybrid configuration was investigated as a means to combine the stable, low-spatter arc of TIG welding with the high deposition rate of MIG welding, thereby improving productivity while maintaining weld quality.
Weld Process Configuration
The TIG-MIG hybrid welding configuration used in this study involved a TIG torch positioned ahead of the MIG torch in the direction of travel. The TIG torch provided the primary heat input and ensured stable arc initiation, while the MIG torch deposited the bulk of the weld metal. The following parameters were used:
| Parameter | TIG Torch | MIG Torch | Notes |
|---|---|---|---|
| Current | 100–180 A | 120–200 A | Total current 220–380 A |
| Voltage | 14–18 V | 18–24 V | — |
| Travel speed | 200–400 mm/min | Same as TIG | — |
| Shielding gas | Ar | Ar or Ar+CO2 | Ar preferred for Mg alloys |
| Wire diameter | — | 1.0–1.2 mm | ER51A or similar |
| Tungsten electrode | 3.2 mm | — | WCu or pure tungsten |
The arc-to-arc distance was maintained at 3–6 mm, with the TIG torch leading the MIG torch. This configuration allowed the TIG arc to pre-heat the base metal and create a stable melt pool, into which the MIG wire was deposited.
Microstructural Analysis
The microstructure of the TIG-MIG hybrid weld joint was examined using optical microscopy and scanning electron microscopy (SEM). The weld zone was divided into three regions: the weld metal (WM), the transition zone (TZ), and the heat-affected zone (HAZ).
In the weld metal, the microstructure consisted of dendritic grains with interdendritic precipitates of Mg17Al12 and β-phase (Mg17Al12). The dendrite arm spacing was finer in the hybrid weld compared to single-process welds, indicating a higher cooling rate. This finer microstructure contributed to improved mechanical properties.
In the transition zone, partial melting and re-solidification occurred, resulting in a mixed microstructure of partially melted base metal and weld metal. The width of the transition zone was typically 0.5–1.5 mm, depending on the welding parameters.
In the HAZ, the microstructure consisted of coarse grains with precipitates at grain boundaries. The grain size in the HAZ was significantly larger than in the base metal, indicating grain growth during welding. The HAZ was the weakest region of the weld joint, with reduced hardness and tensile strength compared to the base metal.
Mechanical Properties
The mechanical properties of the TIG-MIG hybrid weld joint were evaluated through tensile testing, hardness testing, and impact testing. The following table summarizes the results:
| Property | Weld Metal | HAZ | Base Metal | Notes |
|---|---|---|---|---|
| Tensile strength (MPa) | 220–260 | 180–220 | 240–280 | HAZ is weakest region |
| Yield strength (MPa) | 150–190 | 120–160 | 170–210 | — |
| Elongation (%) | 10–15 | 8–12 | 12–18 | — |
| Hardness (HV) | 60–80 | 50–70 | 70–90 | — |
| Impact energy (J) | 40–60 | 25–40 | 50–70 | — |
The results indicate that the TIG-MIG hybrid weld joint achieves mechanical properties that are acceptable for many structural applications, with the HAZ being the critical region for design consideration. The weld metal properties are slightly lower than the base metal, which is typical for magnesium alloy welds due to the loss of precipitation hardening during the welding thermal cycle.
Relevance to Cladding and Bimetal Applications
While this study focuses on structural welding of AZ31B magnesium alloy profiles, the findings have implications for engineers working on magnesium alloy cladding and bimetal applications. Magnesium alloys are increasingly being used in lightweight structural components, and the development of reliable welding procedures is essential for their widespread adoption.
For bimetal products involving magnesium alloy cladding, such as magnesium-clad steel for lightweight pressure vessels or magnesium-clad aluminum for aerospace applications, the welding challenges are similar to those identified in this study. The low melting point and high oxidation susceptibility of magnesium require careful process control to ensure adequate bonding and minimize defects.
The TIG-MIG hybrid configuration offers advantages for magnesium alloy cladding operations, including stable arc initiation, low spatter, and high deposition rate. The hybrid approach can be used to deposit thick magnesium overlay layers on steel or aluminum base metals, provided that the dilution and intermetallic formation are carefully controlled.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Porosity | Hydrogen absorption; oxide inclusions | Use dry shielding gas; clean base metal; use flux |
| Burn-through | Excessive heat input; thin section | Reduce current; increase travel speed; use backing plate |
| Oxide inclusions | Incomplete oxide removal | Use appropriate flux; pre-clean base metal |
| Hot cracking | Impurity segregation; restraint | Use appropriate filler metal; reduce restraint |
| Poor bonding | Insufficient heat input; contamination | Increase current; clean base metal thoroughly |
Study Insights and Conclusions
The research by Liu and colleagues provides valuable insights into the microstructure and mechanical properties of TIG-MIG hybrid weld joints in AZ31B magnesium alloy. The findings demonstrate that the hybrid configuration can produce welds with acceptable mechanical properties, provided that the welding parameters are carefully optimized.
For engineers working on cladding and bimetal applications, this research highlights the importance of understanding the microstructural evolution during welding. The microstructure of the weld metal, transition zone, and HAZ directly influences the mechanical properties and service performance of the weld joint. Numerical simulation and experimental validation should be used in conjunction to predict and control the microstructural evolution.
The study also underscores the challenges of welding magnesium alloys, which require careful process control and quality assurance to achieve reliable weld quality. Engineers must be aware of the unique challenges posed by magnesium alloys, including the low melting point, high oxidation susceptibility, and limited solidification range, and must develop welding procedures that address these challenges effectively.
In conclusion, the TIG-MIG hybrid welding process for AZ31B magnesium alloy profiles represents a promising approach for lightweight structural applications, and the findings of this research contribute to the growing body of knowledge on magnesium alloy welding. Engineers working on cladding and bimetal applications should consider the hybrid approach as a viable option for magnesium alloy overlay operations, provided that appropriate process control and quality assurance measures are implemented.