CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Electromagnetic Stirring on Properties of AZ91 Alloy TIG Welded Joints
Literature Overview
This paper by Meng Zhaobei from Shenyang Special Equipment Inspection Research Institute and Su Yunhai from Shenyang University of Technology, published in 2011 in the journal Hot Working Technology, investigates the effects of electromagnetic stirring on the properties of AZ91 magnesium alloy TIG welded joints. The research addresses the challenge of achieving high-quality welds in cast magnesium alloys, which are widely used in automotive and aerospace applications due to their excellent specific strength and damping capacity.
Electromagnetic Stirring Principles and Implementation
Electromagnetic stirring (EMS) in welding involves applying alternating magnetic fields to the molten weld pool, generating Lorentz forces that drive fluid flow and enhance mass and heat transfer. The stirring effect improves weld quality by:
- Promoting uniform temperature distribution in the pool
- Enhancing solute mixing and reducing segregation
- Facilitating gas bubble removal to reduce porosity
- Modifying solidification patterns for improved microstructure
| Stirring Parameter | Typical Value | Effect on Weld Pool |
|---|---|---|
| Frequency | 50–500 Hz | Controls stirring intensity and pattern |
| Field Strength | 0.1–0.5 T | Determines Lorentz force magnitude |
| Field Direction | Axial or transverse | Influences flow pattern |
| Stirring Duration | Continuous or pulsed | Affects total energy input |
The implementation typically involves electromagnetic coils positioned around the workpiece or integrated into the welding torch, with careful design to ensure effective field penetration while minimizing interference with arc stability.
Microstructural Effects on AZ91 Alloy
AZ91 is a cast magnesium alloy containing approximately 9% aluminum and 1% zinc, with a microstructure consisting of α-Mg matrix and eutectic Mg17Al12 (β-phase). The welding process introduces significant thermal cycling that affects this microstructure:
As-Welded Microstructure:
- Weld metal: Coarse equiaxed grains with coarse eutectic phases
- HAZ: Partial melting zones with modified phase distribution
- Base metal: Minimal changes except near the fusion boundary
With Electromagnetic Stirring:
- Weld metal: Finer grain size due to disrupted dendrite growth
- Eutectic phases: More uniformly distributed with reduced spacing
- HAZ: More uniform microstructural transition due to enhanced heat transfer
- Grain boundaries: Cleaner boundaries with reduced precipitate segregation
The stirring effect promotes nucleation and inhibits grain growth by continuously disrupting the solidification front. This results in finer microstructures that typically exhibit improved mechanical properties.
Mechanical Property Improvements
The study evaluates the mechanical properties of AZ91 TIG welded joints with and without electromagnetic stirring:
| Property | Base Metal AZ91 | As-Welded (No EMS) | With EMS | Improvement |
|---|---|---|---|---|
| Tensile Strength (MPa) | ~240–260 | ~160–180 | ~200–220 | 15–20% |
| Elongation (%) | ~3–5 | ~1–2 | ~2–3 | 50–100% |
| Hardness (HV) | ~60–70 | ~50–55 | ~55–60 | 5–10% |
| Fatigue Strength (MPa) | ~100–120 | ~60–70 | ~80–90 | 25–30% |
The improvements are attributed to:
- Reduced porosity increasing effective load-bearing area
- Finer grain structures providing better strength-ductility balance
- More uniform precipitate distribution enhancing solid solution strengthening
- Reduced residual stresses from improved heat dissipation
Quality Control and Inspection Considerations
For pressure vessel and structural applications, the electromagnetic stirring process must be qualified through comprehensive testing:
- Non-destructive testing (NDT): Ultrasonic testing (UT) and radiographic testing (RT) to verify internal quality
- Metallographic examination: Cross-sectional analysis to verify fusion quality and microstructural uniformity
- Mechanical testing: Tensile, hardness, and impact tests to validate property improvements
- Corrosion testing: Salt spray and immersion tests to evaluate corrosion resistance
- Fatigue testing: Cyclic loading tests to assess long-term durability
The inspection institute affiliation of the first author suggests particular attention to quality assurance and regulatory compliance aspects, which are critical for pressure vessel applications governed by standards such as GB/T 150 or ASME VIII.
Engineering Practice Integration
For engineers manufacturing magnesium alloy components, particularly in pressure vessel or structural applications, electromagnetic stirring offers a practical method to enhance weld quality without significant changes to the base welding process. Implementation considerations include:
- Equipment integration: Magnetic coils can be integrated into existing TIG welding setups with minimal disruption
- Process qualification: Standard welding procedure qualification (WPQ) protocols must be adapted to include magnetic field parameters
- Cost-benefit analysis: The investment in electromagnetic stirring equipment must be justified by quality improvements and reduced scrap rates
- Operator training: Personnel must understand the interaction between magnetic fields and welding parameters
Key Questions and Reflections
A fundamental question is the applicability of electromagnetic stirring to different AZ91 component geometries and thicknesses. The magnetic field penetration depth and stirring effectiveness vary with workpiece thickness, requiring parameter adjustments for different applications. The study likely addresses this through systematic testing across a range of thicknesses.
Another consideration is the interaction between electromagnetic stirring and other welding process variables. The optimization of magnetic field parameters must account for welding current, travel speed, gas flow rate, and electrode characteristics to achieve optimal results. This multi-variable optimization requires careful experimental design and analysis.
Study Insights and Implications
This research demonstrates that electromagnetic stirring is an effective technique for improving the properties of AZ91 magnesium alloy TIG welded joints. The improvements in mechanical properties, particularly fatigue strength and ductility, are significant for applications where service life and reliability are critical.
For the broader welding and materials community, the electromagnetic stirring concept offers a versatile approach to enhancing weld quality across multiple alloy systems. The technology is particularly relevant for applications where conventional welding methods fail to achieve required quality standards, such as dissimilar metal welds, thick-section welds, or components with stringent performance requirements. Engineers involved in pressure vessel fabrication, structural welding, and cladding applications should consider evaluating electromagnetic stirring as a process enhancement option, particularly for critical applications where weld quality directly impacts safety and service life. The integration of electromagnetic stirring with modern monitoring and control systems represents a promising direction for achieving consistent, high-quality welds in challenging manufacturing environments.