Zhuojin Pipe Fitting Co., LtdMicrostructure and Mechanical Properties of Deformed Magnesium Alloy TIG Welded Joints
Literature Overview
This paper, authored by Chu Yajie and colleagues from Southeast University and Nanjing Institute of Technology, was published in the journal Hot Working Technology in 2010. Funded by the Jiangsu Provincial Natural Science Foundation (Grant BK2009354), the study examines the welding behavior of AZ31B deformed magnesium alloy using TIG welding with matching filler wire. The research employs a comprehensive characterization approach including optical microscopy, scanning electron microscopy, microhardness testing, and electron tensile testing to evaluate the weld joint microstructure, elemental distribution, fracture morphology, hardness, and strength.
AZ31B magnesium alloy is one of the most widely used wrought magnesium alloys, valued for its excellent combination of strength, formability, and cost-effectiveness. However, magnesium alloys are notoriously difficult to weld due to their high reactivity with oxygen and nitrogen, low melting point, and susceptibility to hot cracking. Understanding the weld joint characteristics of deformed AZ31B is essential for expanding the application of magnesium alloys in structural and automotive components.
Weld Microstructure Characterization
The weld metal microstructure consists of fine equiaxed grains composed of the alpha-Mg solid solution matrix with a network distribution of beta-Mg17Al12 intermetallic compound particles. This microstructure is distinctly different from the deformed base metal, which exhibits elongated grains with a fibrous texture resulting from the cold rolling or extrusion processing.
| Region | Microstructure | Grain Size | Hardness (HV) |
|---|---|---|---|
| Base metal (BM) | Elongated deformed grains, α-Mg matrix | 50–100 μm | 55–65 |
| Weld metal (WM) | Fine equiaxed grains, α-Mg + β-Mg17Al12 network | 10–30 μm | 70–85 |
| Heat affected zone (HAZ) | Coarsened grains, α-Mg matrix | 100–200 μm | 45–55 |
| Fusion boundary | Transition zone with mixed grain morphology | 30–80 μm | 55–70 |
The formation of fine equiaxed grains in the weld metal is attributed to the rapid cooling rate and the high nucleation density provided by the beta-Mg17Al12 intermetallic particles. These particles act as heterogeneous nucleation sites during solidification, promoting equiaxed grain formation over columnar growth. The network distribution of the intermetallic compound is a consequence of the non-equilibrium solidification conditions and the positive temperature coefficient of the alpha-beta phase boundary in the Mg-Al system.
Elemental Distribution Analysis
The elemental analysis reveals significant compositional changes in the weld metal compared to the base metal. The magnesium content in the weld metal is lower than in the base metal due to magnesium burn-off during the welding process. Magnesium, with its low boiling point (1091°C) and high vapor pressure, readily evaporates from the arc and weld pool, leading to a relative enrichment of aluminum in the weld metal.
| Element | Base Metal (wt%) | Weld Metal (wt%) | Change |
|---|---|---|---|
| Mg | Balance (~96.5) | Reduced (~95.0–95.5) | Burn-off |
| Al | 3.0–3.5 | 3.5–4.0 | Relative enrichment |
| Zn | 0.8–1.2 | 0.8–1.2 | Minimal change |
| Mn | 0.2–0.4 | 0.2–0.4 | Minimal change |
The magnesium burn-off has implications for weld metal composition and, consequently, for the phase composition and mechanical properties. The relative aluminum enrichment increases the volume fraction of the beta-Mg17Al12 intermetallic compound in the weld metal, which contributes to the higher hardness observed in the weld zone.
Mechanical Properties and Fracture Behavior
The hardness distribution across the weld joint is non-uniform, with the highest hardness in the weld metal and the lowest in the heat affected zone. This distribution pattern is characteristic of magnesium alloy welds and is attributed to the different thermal histories experienced by each region.
The weld metal achieves a microhardness of 70–85 HV due to the fine grain structure and the network distribution of the hard beta-Mg17Al12 intermetallic particles. The base metal has a hardness of 55–65 HV, reflecting the work-hardened deformed microstructure. The heat affected zone exhibits the lowest hardness of 45–55 HV because the elevated temperatures during welding cause grain coarsening and partial recovery of the work-hardened microstructure, without sufficient cooling rate to produce a refined structure.
The tensile strength of the weld joint reaches 220 MPa, which is 93% of the base metal strength. This is a notably high joint efficiency for a magnesium alloy weld, indicating that the matching filler metal and the TIG welding process parameters were well selected. The fracture occurs in the heat affected zone rather than in the weld metal, which is consistent with the lower hardness and coarser grain structure of the HAZ.
The fracture morphology analysis reveals a mixed ductile-brittle fracture mode. The ductile features include dimples characteristic of microvoid coalescence, while the brittle features include cleavage facets and intergranular fracture surfaces. This mixed fracture mode is typical of magnesium alloy weld joints, where the competing effects of grain coarsening (which promotes brittle fracture) and intermetallic particle distribution (which can promote either ductile or brittle failure depending on size and spacing) create a complex fracture behavior.
Defect Analysis and Process Considerations
The welding of AZ31B magnesium alloy with TIG welding requires careful attention to several process parameters to avoid common defects:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Porosity | Gas absorption (H2, N2) from atmosphere | High-purity argon shielding, proper gas flow rate |
| Hot cracking | Low solidification range, high Al content | Matching filler metal, reduced travel speed |
| Excessive burn-off | High arc temperature, Mg evaporation | Short arc length, proper electrode preparation |
| Incomplete fusion | Low welding current, high travel speed | Increased current, optimized travel speed |
| HAZ softening | Grain coarsening, recovery | Preheating control, post-weld heat treatment |
The 4.6 mm plate thickness represents a moderate thickness for TIG welding of magnesium alloys. For thicker sections, multi-pass welding or alternative processes such as gas metal arc welding (GMAW) with higher deposition rates may be more appropriate. For thinner sections, careful control of heat input is essential to avoid burn-through.
Key Insights and Reflections
The 93% joint efficiency achieved in this study is encouraging and demonstrates that TIG welding with matching filler metal is a viable process for AZ31B magnesium alloy. However, the fracture in the heat affected zone highlights a fundamental limitation: the HAZ softening caused by grain coarsening and work-hardening recovery is difficult to mitigate through welding process optimization alone. Post-weld heat treatment, such as solution treatment followed by aging, could potentially restore the HAZ strength by precipitating strengthening phases and refining the grain structure.
The elemental burn-off analysis underscores the importance of proper shielding gas coverage. In industrial production environments, drafts and poor gas flow patterns can exacerbate magnesium evaporation, leading to compositional variations and reduced weld quality. Shielding gas flow rate optimization and the use of trailing shields for back-side protection are essential practices.
The mixed ductile-brittle fracture mode observed in the HAZ has implications for fatigue and fracture toughness performance. For applications requiring high fracture resistance, such as automotive crash structures or aerospace components, additional investigation of the fatigue and fracture toughness properties would be necessary.
Reference Value and Outlook
This study provides valuable baseline data for the TIG welding of AZ31B magnesium alloy. The comprehensive characterization approach, combining microstructural, elemental, hardness, and mechanical property analyses, offers a complete picture of the weld joint quality. For engineers working with magnesium alloy components, the findings highlight the critical role of the heat affected zone as the weakest link in the weld joint and suggest that process optimization should focus on minimizing HAZ softening through careful thermal cycle control.