CLADDING TECHNOLOGY SHANXI CO., LTDMagnetic Field Control of Microstructure and Properties in AZ91 Magnesium Alloy TIG Weld Joints
Literature Overview
This 2013 paper published in "Hot Working Technology" by researchers from Jilin Industry Polytechnic College investigates the application of magnetic field control during TIG welding of AZ91 magnesium alloy. AZ91 is one of the most widely used magnesium alloys in automotive and aerospace applications due to its good combination of strength, corrosion resistance, and castability. However, its weldability is limited by susceptibility to hot cracking, excessive grain coarsening, and poor mechanical properties in the heat-affected zone. The introduction of an external magnetic field during welding represents an innovative approach to controlling solidification microstructure and improving joint properties.
Core Technical Content
The magnetic field effect on welding involves several physical mechanisms:
- Magnetohydrodynamic (MHD) stirring: The interaction between the external magnetic field and the electric current in the arc and weld pool creates Lorentz forces that induce fluid flow in the molten pool. This stirring effect promotes more uniform temperature distribution, reduces columnar grain growth, and promotes equiaxed grain formation.
- Modification of solidification morphology: The enhanced convection in the weld pool changes the thermal gradient and solidification rate, which directly affects grain structure. Equiaxed grains are generally preferred over columnar grains for improved transverse mechanical properties and reduced cracking susceptibility.
- Reduction of solidification cracking: By promoting more uniform solidification and reducing the mushy zone width, magnetic field stirring can decrease the susceptibility to hot cracking.
- Potential effect on phase transformation: In some alloy systems, magnetic fields can influence phase transformation kinetics, although this effect is more pronounced in ferromagnetic materials.
| Parameter | Typical Value |
|---|---|
| Base material | AZ91 (as-cast or extruded) |
| Filler wire | AZ91 or AZ92 |
| Welding current | 80–150 A |
| Arc voltage | 10–14 V |
| Travel speed | 30–60 mm/min |
| Shielding gas | 99.99% Ar or Ar/CO2 mix |
| Gas flow rate | 10–20 L/min |
| Magnetic field type | DC or AC |
| Magnetic field strength | 0.5–3.0 T (or 5000–30000 Gauss) |
| Field orientation | Parallel or perpendicular to weld axis |
| Electrode type | Pure tungsten (WC) |
Interpretation of Technical Points
The AZ91 alloy contains approximately 9% aluminum and 1% rare earth elements, with the primary strengthening phase being Mg17Al12 (β-phase). During TIG welding, the weld zone experiences temperatures well above the solidus (~450°C), leading to complete melting and re-solidification. The HAZ experiences temperatures between 200–450°C, where the β-phase may partially dissolve and re-precipitate, affecting mechanical properties.
The magnetic field stirring effect on the weld pool can be quantified by the magnetic Reynolds number (Rem), which characterizes the relative importance of magnetic field effects versus viscous forces. For typical TIG welding parameters with moderate magnetic field strengths (0.5–3 T), the Rem is in the range of 10⁻³ to 10⁻¹, indicating that magnetic field effects are detectable but not dominant.
The expected microstructural improvements include:
- Grain refinement: Equiaxed grains of 50–100 μm in the weld centerline, compared to columnar grains of 200–500 μm without magnetic field.
- Reduced β-phase segregation: More uniform distribution of Mg17Al12 at grain boundaries, reducing the risk of intergranular cracking.
- Decreased porosity: Enhanced fluid flow can help gas bubbles rise and escape from the weld pool before solidification.
- Narrower HAZ: More uniform heat distribution reduces the width of the severely affected zone.
Process Optimization and Parameter Selection
The effectiveness of magnetic field control depends critically on the relationship between field strength, field orientation, and welding parameters. The following systematic approach is recommended:
| Optimization Variable | Low Value | Optimal Range | High Value |
|---|---|---|---|
| Magnetic field strength | < 0.3 T (negligible effect) | 0.8–2.0 T | > 3.0 T (excessive stirring, instability) |
| Field orientation | Perpendicular (weaker stirring) | Parallel to current flow | Anti-parallel (potential arc deflection) |
| Welding current | < 80 A (incomplete penetration) | 100–130 A | > 150 A (excessive heat input) |
| Travel speed | < 30 mm/min (excessive heat) | 40–55 mm/min | > 60 mm/min (incomplete fusion) |
A key finding from such studies is that the optimal magnetic field strength is not monotonically related to joint quality. Moderate field strengths (around 1.0–1.5 T) typically provide the best balance between grain refinement and weld pool stability. Excessive magnetic field strength can cause arc instability, increased spatter, and irregular weld bead geometry.
Quality Control and Inspection
For AZ91 magnesium alloy welds, the following quality control measures are essential:
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual inspection | Surface quality, bead geometry | No visible cracks, uniform bead profile |
| Radiographic testing (RT) | Internal defects (porosity, cracks) | No cracks; porosity ≤ 20% of weld area |
| Ultrasonic testing (UT) | Volumetric defects | No indications above 6 dB above reference |
| Metallographic examination | Microstructure evaluation | No hot cracks; grain size ≤ 100 μm |
| Hardness traverse | HAZ softening assessment | Minimum hardness ≥ 60 HV |
| Tensile testing | Joint strength | ≥ 80% of base metal tensile strength |
| Bend testing | Ductility assessment | No cracking on convex side |
Engineering Practice Considerations
Implementing magnetic field control in TIG welding requires specialized equipment that may not be readily available in standard welding shops. The following practical considerations apply:
- Equipment requirements: A DC magnetic field can be generated using permanent magnets or electromagnets positioned near the weld zone. The field must be applied to the workpiece rather than the welding torch to avoid arc deflection.
- Safety considerations: Strong magnetic fields can interfere with nearby electronic equipment, magnetic tools, and medical devices. Personnel with pacemakers must not be exposed to fields above 5 mT.
- Cost-benefit analysis: For high-value aerospace or defense components where AZ91 weld quality is critical, the investment in magnetic field equipment may be justified. For lower-value applications, conventional parameter optimization may be sufficient.
- Scalability: Magnetic field control is most effective for thin to moderate section thicknesses (2–10 mm). For thicker sections, the field may not penetrate sufficiently to influence the entire weld pool.
Study Insights and Implications
This research represents an innovative approach to improving weld quality through external field manipulation rather than conventional parameter adjustment. For engineers working with magnesium alloys—which are increasingly used in lightweight structural applications—the magnetic field approach offers a promising avenue for overcoming inherent weldability limitations. The technique is particularly relevant for applications where weld quality directly impacts safety or performance, such as aerospace brackets, automotive structural components, and defense systems. Future research should explore the combination of magnetic field control with other advanced welding techniques, such as pulsed TIG or hybrid laser-arc welding, to further enhance the benefits for magnesium alloy joining.