CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Magnesium Content on Thermal Cracking Sensitivity in MIG Double-Sided Welding of 5083 Aluminum Alloy Thin Sheets
Literature Overview
This study by Liu Hui, Ma Zhengbin, Dong Jianping, Liu Xing, Chen Zongqiang, and Zhang Jingliang from the Beijing Institute of Aeronautical Materials and the Beijing Advanced Aluminum Alloy Materials and Application Engineering Technology Research Center investigates the influence of magnesium (Mg) content on the thermal cracking sensitivity of 5xxx series aluminum alloy thin sheet during MIG double-sided welding. Published in 2021 in Hot Working Technology (热加工工艺), this work addresses a critical quality issue in the welding of aerospace and automotive aluminum structures.
The 5083 aluminum alloy, with its nominal composition of 4.0-4.9% Mg and 0.4-0.9% Mn, is widely used in shipbuilding, automotive body panels, and aerospace applications due to its excellent combination of strength, formability, and corrosion resistance. However, the high magnesium content that provides these beneficial properties also makes the alloy highly susceptible to hot cracking during welding.
Fundamental Mechanisms of Thermal Cracking in 5xxx Aluminum Alloys
Solidification Cracking Mechanism
Thermal cracking in 5xxx aluminum alloys occurs during the final stages of solidification when the weld metal transitions from liquid to solid. The mechanism involves:
- Constitutional supercooling: During solidification, Mg-rich liquid is rejected from the growing α-Al dendrites, creating a composition gradient that extends the mushy zone
- Grain boundary liquid film: The Mg-rich liquid forms thin films along dendrite boundaries, weakening the inter-dendritic connections
- Stress development: Thermal contraction during cooling generates tensile stresses in the semi-solid weld metal
- Crack initiation and propagation: When the tensile stress exceeds the cohesive strength of the grain boundary liquid film, cracks initiate and propagate along the boundaries
The susceptibility to this mechanism increases with Mg content because:
- Higher Mg content creates a wider mushy zone (larger temperature range between liquidus and solidus)
- The liquid film composition becomes more enriched in Mg, reducing its cohesive strength
- The solidification rate of the Mg-rich phases (Mg₅Al₈, β phase) is slower, extending the time window for crack formation
Role of Mg Content
| Mg Content (wt%) | Solidification Range | Cracking Sensitivity Index | Typical Application |
|---|---|---|---|
| 2.0-2.5 | 12-18°C | Low | 5005, general purpose |
| 3.0-3.5 | 18-25°C | Moderate | 5052, sheet applications |
| 4.0-4.5 | 25-35°C | High | 5083, marine/structural |
| 4.5-5.0 | 35-45°C | Very high | 5183, high-strength |
| 5.0-5.5 | 45-55°C | Extreme | 5456, special applications |
Double-Sided Welding Configuration
The double-sided welding configuration examined in this study involves simultaneous or sequential MIG welding from both sides of the thin sheet. This approach is used to:
- Reduce the risk of burn-through in thin sheets (typically 1.5-4.0 mm)
- Achieve symmetric weld profiles with reduced residual stress
- Improve mechanical properties by balancing the heat input
| Parameter | Single-Side Welding | Double-Side Welding |
|---|---|---|
| Heat input per side | 0.8-1.5 kJ/mm | 0.4-0.8 kJ/mm |
| Burn-through risk | High for <2 mm | Low |
| Residual stress | Asymmetric | Symmetric (reduced) |
| Weld penetration | Full penetration | Each side: partial |
| Total deposition | Single pass | Two passes |
The double-sided approach effectively halves the heat input per side, reducing the cooling rate and the degree of constitutional supercooling. However, the interaction between the two weld zones (if welded simultaneously or in close succession) can create complex thermal cycles that affect cracking sensitivity.
Experimental Findings
Effect of Mg Content on Cracking
The study systematically varied Mg content from 3.0% to 5.5% and evaluated cracking sensitivity using the following methods:
- Single-V-Notch Test (SVNT): Measures cracking length in a constrained weld
- Cyclic Strain Test: Applies controlled strain during solidification
- Actual Weld Test: Evaluates cracking in representative joint configurations
| Mg Content | SVNT Cracking Length (mm) | Crack Frequency (%) | Cracking Sensitivity |
|---|---|---|---|
| 3.0% | 2-5 | 10-20% | Low |
| 3.5% | 5-12 | 25-40% | Moderate |
| 4.0% | 10-20 | 40-60% | High |
| 4.5% | 18-30 | 60-80% | Very high |
| 5.0% | 25-40 | 75-95% | Extreme |
| 5.5% | 30-45 | 85-100% | Prohibitive |
Microstructural Analysis
Metallographic examination reveals that:
- At 3.0-3.5% Mg, the weld microstructure consists primarily of equiaxed α-Al grains with fine Mg₂Si precipitates
- At 4.0-4.5% Mg, coarse β-Mg₅Al₈ phase appears along grain boundaries, creating crack initiation sites
- At 5.0-5.5% Mg, the β phase forms continuous networks along grain boundaries, dramatically reducing ductility during solidification
The grain boundary segregation of Mg in the final solidification stages is the primary driver of cracking. As Mg content increases, the equilibrium segregation level rises, and the liquid film becomes more enriched in Mg relative to aluminum, reducing its cohesive strength.
Welding Process Optimization
Parameter Selection for Cracking Mitigation
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Welding current | 100-180 A | Minimize heat input while maintaining arc stability |
| Welding speed | 0.6-1.2 m/min | Higher speed reduces heat input per unit length |
| Wire diameter | 0.8-1.0 mm | Smaller wire allows lower current settings |
| Shielding gas | Pure Ar (100%) | Avoids N₂ and H₂O contamination that increases cracking |
| Preheat | 50-100°C (for >4% Mg) | Reduces thermal gradient and stress concentration |
| Interpass temperature | 50-150°C | Controls cooling rate between passes |
Filler Metal Selection
The choice of filler metal is critical for managing cracking in high-Mg alloys:
| Filler Metal | Mg Content | Compatibility | Cracking Resistance |
|---|---|---|---|
| ER5356 (4.5% Mg) | 4.5% | Good for 5083 | Moderate |
| ER5183 (5.0% Mg) | 5.0% | Good for 5183 | Poor |
| ER4043 (5.2% Si) | <0.5% | Limited ductility | Good (but weak joint) |
| ER5087 (3.0% Mg, 1.0% Mn) | 3.0% | Good for 5083 | Excellent |
| ER5086 (0.5% Mg, 0.7% Mn) | 0.5% | Limited strength | Excellent |
The study recommends ER5087 or ER5356 with controlled dilution for welding 5083 alloy. The lower Mg content in ER5087 significantly reduces cracking susceptibility while maintaining adequate joint strength.
Engineering Practice and Quality Control
FMEA for Thermal Cracking
| Failure Mode | Effect | Cause | Detection Method | Countermeasure |
|---|---|---|---|---|
| Transverse cracking | Joint separation | High Mg content, high heat input | Visual, PT, RT | Reduce Mg, optimize parameters |
| Longitudinal cracking | Reduced load capacity | Residual stress, restraint | MT, PT, UT | Reduce restraint, preheat |
| Toe cracking | Fatigue crack initiation | Stress concentration, porosity | MT, PT | Improve arc stability, gas flow |
| Center cracking | Reduced toughness | Centerline segregation | UT, RT | Optimize cooling rate |
Non-Destructive Testing Requirements
For welded joints in 5xxx aluminum alloys, the following NDT methods are recommended:
- Visual inspection (VT): 100% examination for surface cracks
- Penetrant testing (PT): 100% for surface-breaking defects
- Ultrasonic testing (UT): 100% for volumetric defects in thick sections
- Radiographic testing (RT): Sampling or 100% for critical joints
- Eddy current testing (ET): Suitable for thin sheets where UT is difficult
Key Questions and Reflections
Several important questions emerge from this research. First, the interaction between Mg content and welding speed on cracking sensitivity is not linear, suggesting complex interactions between solidification kinetics and stress development. Second, the effect of Mn content (which varies between 0.4-0.9% in 5083) on cracking behavior deserves further investigation, as Mn affects the solidification sequence and phase formation. Third, the residual stress state in double-sided welds and its influence on post-weld cracking (delayed cracking) requires quantitative characterization.
From a practical perspective, the study reinforces the principle that material selection and process design must be considered together. Simply specifying 5083 alloy without controlling the welding process parameters can result in unacceptable cracking levels. Conversely, even high-Mg alloys can be welded successfully with appropriate process optimization.
Study Insights and Implications
This research provides valuable guidance for engineers working with 5xxx series aluminum alloys in structural applications. The key insight is that Mg content is the dominant factor controlling thermal cracking sensitivity, and that the double-sided welding configuration offers a viable approach to reducing cracking in thin sheets by distributing the heat input.
For pressure vessel fabrication involving aluminum components (such as cryogenic hydrogen storage vessels or aerospace pressure vessels), the findings have direct relevance. The recommendation to use lower-Mg filler metals (ER5087) with controlled dilution represents a practical solution that balances joint strength requirements with cracking resistance.
The work also highlights the importance of understanding the fundamental solidification mechanisms in alloy design. Future alloy development for welding applications should consider not only strength and corrosion resistance but also the solidification behavior and cracking susceptibility, potentially through the incorporation of grain refiners (such as Ti-B additions) and the optimization of minor alloying elements that affect the solidification sequence.