Zhuojin Pipe Fitting Co., LtdOscillographic Impact Properties of AZ31 Magnesium Alloy and Its TIG Welded Joints
Literature Overview
The paper by Zhang Lan, Wang Wenxian, Zhang Hongxia, and Su Juan from Taiyuan University of Technology, published in Rare Metal Materials and Engineering (2009, Vol. 38, No. A3, pp. 175–180), presents a comprehensive study of the oscillographic impact properties of AZ31 magnesium alloy base metal and its TIG welded joints. Funded by the National Natural Science Foundation of China (Grant No. 50675148), this research provides critical data on the dynamic fracture behavior of magnesium alloy weldments, which is essential for the safe application of magnesium alloys in structural and pressure-containing applications.
Technical Background
AZ31 magnesium alloy (containing approximately 3.0 wt% Al and 1.0 wt% Zn) is one of the most widely used wrought magnesium alloys due to its good combination of strength, formability, and corrosion resistance. However, magnesium alloys exhibit notably lower ductility and toughness compared to aluminum and steel alloys, making their fracture behavior under dynamic loading conditions a critical concern for structural applications. The oscillographic impact test provides detailed information about the energy absorption mechanism during fracture, distinguishing between energy consumed in crack formation and energy absorbed during crack propagation.
Oscillographic Impact Testing Methodology
The oscillographic impact test records the force-displacement relationship during the Charpy impact test with high temporal resolution, enabling the separation of total absorbed energy (Wt) into crack formation energy (Wi) and crack propagation energy (Wp). The ratio Wi/Wt provides insight into the fracture mechanism:
- High Wi/Wt ratio (>50%): Energy is predominantly consumed in crack initiation, indicating high resistance to crack formation.
- Low Wi/Wt ratio (<20%): Energy is predominantly consumed in crack propagation, indicating that once a crack initiates, it propagates relatively easily with the remaining energy.
Key Test Results
Base Metal Properties
| Condition | Notch Type | Wt (J/cm²) | Wi/Wt (%) | Fracture Character |
|---|---|---|---|---|
| AZ31 base metal | Unnotched | 50.9 | 62.0 | Ductile, extensive plastic deformation |
| AZ31 base metal | Charpy V-notch | 8.7 | 53.8 | Semi-ductile, mixed mode |
The dramatic reduction in impact energy from 50.9 to 8.7 J/cm² upon introduction of a Charpy V-notch demonstrates the extreme notch sensitivity of AZ31 magnesium alloy. The high Wi/Wt ratio of 53.8% for the notched base metal indicates that the material consumes a significant proportion of available energy in crack formation, suggesting relatively good resistance to crack initiation despite the low absolute toughness.
TIG Welded Joint Properties
| Location | Wt (J) | Wi/Wt (%) | Relative Toughness |
|---|---|---|---|
| Weld center | 4.9 | 16.3 | Lowest |
| Weld center | 4.5 | 15.6 | Lowest |
| HAZ | 3.9 | 15.4 | Lowest |
| HAZ | 4.4 | 15.9 | Lowest |
The TIG welded joint exhibits significantly lower impact energy (3.9–4.9 J) compared to the notched base metal (8.7 J/cm²), and the Wi/Wt ratio drops dramatically to 15.4–16.3%. This indicates that in the welded joint, once a crack initiates, it propagates with minimal additional energy consumption—a highly undesirable fracture behavior for structural applications.
Microstructural Analysis and Fracture Mechanism
Base Metal Microstructure
AZ31 base metal typically exhibits a polycrystalline microstructure consisting of:
- Magnesium matrix with hexagonal close-packed (HCP) crystal structure
- β-phase (Mg₁₇Al₁₂) precipitates at grain boundaries
- Limited slip systems due to HCP crystal structure (only basal slip at room temperature)
The HCP crystal structure of magnesium limits the available slip systems to three basal systems at room temperature, severely constraining plastic deformation capacity. This fundamental crystallographic limitation contributes to the low ductility and notch sensitivity observed in the impact test results.
Welded Joint Microstructure
| Zone | Microstructure | Grain Size | Phase Distribution |
|---|---|---|---|
| Weld center | Columnar dendritic, fine β-phase | Fine (<20 μm) | Continuous β-phase network at dendrite boundaries |
| HAZ | Partially recrystallized, mixed grain sizes | Variable (20–80 μm) | β-phase at original grain boundaries |
| Base metal | Equiaxed, coarse | Coarse (50–150 μm) | Discrete β-phase particles |
The columnar dendritic structure in the weld center, combined with the continuous β-phase (Mg₁₇Al₁₂) network at dendrite boundaries, creates a microstructure that is particularly susceptible to intergranular crack propagation. The brittle intermetallic phase provides a preferential crack path with minimal energy absorption.
Fracture Surface Morphology
The fractographic analysis reveals distinct fracture characteristics:
- Base metal (notched): Mixed mode fracture with evidence of both microvoid coalescence and cleavage, consistent with the moderate Wi/Wt ratio.
- Weld center: Predominantly intergranular fracture along dendrite boundaries, with minimal plastic deformation features. The continuous β-phase network acts as a crack guide.
- HAZ: Mixed intergranular and transgranular fracture, with some evidence of grain boundary decohesion. The partially recrystallized microstructure provides some resistance to crack propagation.
TIG Welding Process Parameters and Their Effects
| Parameter | Typical Value | Effect on Impact Properties |
|---|---|---|
| Welding current | 80–150 A | Higher current increases weld width, may reduce dilution ratio |
| Travel speed | 150–300 mm/min | Faster speed reduces heat input, promotes finer microstructure |
| Shielding gas | Ar + 2–5% H₂ | Hydrogen addition improves wetting but may affect porosity |
| Filler wire | AZ31 or AZ91 | Matching filler maintains composition; higher Al filler increases β-phase |
| Joint preparation | V-groove, 60° included angle | Adequate root penetration critical for joint integrity |
| Preheating | 100–200°C | Reduces thermal stress, minimizes cold cracking |
Engineering Implications for Pressure Vessel and Piping Applications
The findings of this research have direct implications for the use of magnesium alloys in pressure-containing applications:
- Notch sensitivity: The extreme notch sensitivity of AZ31 (Wt reduction from 50.9 to 8.7 J/cm²) means that any geometric discontinuity, surface defect, or stress concentration in magnesium alloy pressure vessels or piping must be designed to minimize stress concentration factors.
- Welded joint vulnerability: The welded joint's low Wi/Wt ratio (15–16%) indicates that crack propagation resistance is very poor. Once a crack initiates in a magnesium alloy weld, it will propagate rapidly with minimal additional energy input. This is a critical safety concern for pressure-containing applications.
- Inspection requirements: Given the fracture characteristics, non-destructive testing of magnesium alloy welds must be particularly thorough, with emphasis on detecting even small surface-breaking defects that could serve as crack initiation sites.
- Design considerations: The low absolute impact energy values (3.9–4.9 J) suggest that AZ31 welded joints may not meet impact toughness requirements for many pressure vessel codes, potentially limiting their application to low-pressure or non-critical service.
Comparison with Other Magnesium Alloy Welded Joints
| Alloy | Base Metal Wt (J/cm², notched) | Weld Wt (J) | Wi/Wt (%) | Relative Assessment |
|---|---|---|---|---|
| AZ31 | 8.7 | 3.9–4.9 | 15–16 | Very low toughness, poor crack propagation resistance |
| AZ91 | ~12–15 | ~5–7 | ~20–25 | Slightly better but still limited |
| AZ61 | ~10–13 | ~4–6 | ~18–22 | Similar to AZ31 |
| WE43 | ~15–20 | ~6–9 | ~25–30 | Better due to Zr and rare earth additions |
The relatively poor impact properties of AZ31 welded joints compared to other magnesium alloys suggest that alloy selection is a critical design parameter for applications requiring dynamic load resistance.
Study Insights and Reflections
This research provides essential quantitative data on the dynamic fracture behavior of AZ31 magnesium alloy welded joints, filling an important gap in the literature on lightweight structural materials. The dramatic difference in Wi/Wt between base metal (53.8%) and welded joint (15–16%) is particularly striking and highlights the severe degradation of fracture behavior introduced by the welding process.
The findings underscore a fundamental challenge in magnesium alloy welding: the solidification microstructure produced by TIG welding (columnar dendrites with continuous intermetallic networks) is inherently less fracture-resistant than the wrought microstructure of the base metal. This is not merely a matter of absolute toughness values but reflects a fundamental change in fracture mechanism from predominantly crack formation (base metal) to predominantly crack propagation (welded joint).
For engineers considering magnesium alloys in piping or pressure vessel applications, these results suggest that:
- AZ31 may be suitable for non-critical, low-pressure applications where dynamic loading is minimal.
- Welded joints should be designed with generous fillet radii and smooth transitions to minimize stress concentrations.
- Strict quality control of weld surfaces is essential to prevent crack initiation sites.
- Alternative alloys with better weldability and toughness (such as WE-series or ZE-series) should be considered for critical applications.
In conclusion, this paper provides critical fracture mechanics data that should inform the design, qualification, and inspection of AZ31 magnesium alloy welded structures, particularly in applications where impact or dynamic loading may be encountered. The clear demonstration of reduced crack propagation resistance in welded joints serves as an important cautionary finding for engineers evaluating magnesium alloys for structural service.