CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Magnetic Field Frequency on TIG Welding of AZ91 Magnesium Alloy
Literature Overview
This 2014 study by Qi Xiuling, Liu Zhengjun, Su Yunhai, and Zhao Fudong from Shenyang University of Technology and Liaoning Technical University investigates the influence of applied magnetic field frequency on the TIG welding process of AZ91 magnesium alloy. The research was published in the journal of Ordnance Materials Science and Engineering. AZ91 is one of the most widely used wrought and cast magnesium alloys, valued for its excellent specific strength, but notoriously difficult to weld due to its low melting point, high reactivity, and susceptibility to hot cracking.
Core Technical Points
The application of an external magnetic field during TIG welding is an electromagnetic stirring technique that influences the weld pool fluid flow, heat transfer, and solidification behavior. The magnetic field interacts with the electric current flowing through the weld pool, generating Lorentz forces that drive fluid motion. The frequency of the applied magnetic field determines the temporal characteristics of this stirring effect and, consequently, the resulting weld microstructure and properties.
AZ91 magnesium alloy (9 wt% aluminum, 1 wt% zinc, balance magnesium) is particularly challenging to weld because of several factors: the low melting point (621 °C) leads to a large weld pool with high fluidity; the high thermal conductivity causes rapid heat dissipation; the strong reactivity with atmospheric oxygen and nitrogen leads to oxidation and nitride formation; and the low solid solubility of aluminum in magnesium promotes the formation of Mg17Al12 intermetallic at grain boundaries, which is the primary cause of hot cracking.
The magnetic field frequency affects the weld pool in several ways. At low frequencies (1-10 Hz), the magnetic stirring is quasi-steady and produces large-scale convective flow that can significantly alter the weld pool shape and solidification pattern. At medium frequencies (10-100 Hz), the stirring effect is more localized and can promote grain refinement. At high frequencies (100-1000 Hz), the skin effect limits the penetration of the magnetic field into the weld pool, reducing the stirring effectiveness.
Magnetic Field Parameters and Effects
| Magnetic Field Frequency (Hz) | Stirring Intensity | Weld Pool Shape | Grain Size (μm) | Cracking Tendency |
|---|---|---|---|---|
| 0 (no field) | None | Wide, shallow | 120-180 | High |
| 5 | Low | Moderately refined | 80-120 | Moderate |
| 20 | Medium | Narrow, deeper | 50-80 | Low |
| 50 | High | Significantly refined | 30-50 | Very low |
| 200 | Moderate (skin effect) | Moderately refined | 60-100 | Low |
Microstructure and Cracking Analysis
The application of a magnetic field at an optimal frequency (typically 20-50 Hz for AZ91 TIG welding) produces several beneficial effects on the weld microstructure. The electromagnetic stirring promotes the detachment of dendrite arms from the weld pool boundary, increasing the nucleation site density and promoting equiaxed grain growth. This refinement is particularly effective at reducing the primary dendrite arm spacing, which in turn reduces the interdendritic spacing where Mg17Al12 intermetallic segregates.
The hot cracking susceptibility of AZ91 welds is primarily governed by the solidification range and the interdendritic connectivity of the Mg17Al12 phase. When the magnetic field frequency is optimized, the reduced interdendritic spacing and the more uniform distribution of Mg17Al12 particles decrease the cracking susceptibility. The magnetic stirring also helps to homogenize the composition of the weld pool, reducing macrosegregation and the associated cracking risks.
However, the magnetic field can also have detrimental effects if not properly controlled. Excessive stirring can increase the weld pool turbulence, which may entrain oxide inclusions and increase porosity. The magnetic field can also deflect the arc, which may cause arc instability and poor weld bead appearance. These effects must be carefully managed through proper shielding gas flow and electrode positioning.
Engineering Practice Considerations
For practical implementation of magnetic field-assisted TIG welding of AZ91 magnesium alloy, the following considerations are important:
| Process Parameter | Recommended Value | Rationale |
|---|---|---|
| Base current | 100-150 A | Adequate penetration for AZ91 |
| Travel speed | 5-10 mm/min | Balances heat input and cooling rate |
| Shielding gas | 100% Ar or Ar/He mix | Prevents oxidation of Mg weld pool |
| Magnetic field strength | 0.1-0.5 T | Sufficient for stirring without arc deflection |
| Magnetic field frequency | 20-50 Hz | Optimal grain refinement without excessive turbulence |
| Electrode type | Pure tungsten, 2.4-3.2 mm | Good arc stability and electrode life |
The equipment requirements for magnetic field-assisted TIG welding include a controlled magnetic field generator (typically a coil or permanent magnet array), a frequency control unit, and appropriate shielding to prevent interference with the welding power source. The cost of the magnetic field equipment is modest compared to the value of producing high-quality magnesium alloy welds, making this technique economically viable for high-value applications such as aerospace components, automotive structural parts, and electronic housings.
Study Insights and Reflections
This research demonstrates that electromagnetic stirring is a promising approach to improving the weldability of magnesium alloys without requiring changes to the alloy composition or filler metal selection. The frequency-dependent behavior of the magnetic field effect provides engineers with a tunable parameter for optimizing the weld microstructure and properties. The technique is particularly attractive for AZ91 alloy because it addresses the hot cracking problem, which is the primary barrier to the widespread use of magnesium alloy weldments. Future work should focus on scaling the technique to thicker sections, investigating the effect on fatigue properties, and developing standardized welding procedures that incorporate magnetic field parameters.