Effect of Electromagnetic Stirring on AZ61 Magnesium Alloy TIG Weld Quality

Literature Overview

This research, conducted by Liu Zhengjun, Jia Hua, Su Yunhai, and Tang Xingtao from the School of Materials Science and Engineering at Shenyang University of Technology, was published in 2010 and investigates the influence of electromagnetic stirring on the quality of TIG welds in AZ61 magnesium alloy. The work addresses a critical challenge in magnesium alloy welding: the formation of porosity, coarse grain structures, and solidification cracking in thick-section welds. The introduction of electromagnetic stirring as an external post-weld treatment offers a promising approach to refining the weld microstructure and eliminating casting defects without modifying the welding process parameters themselves.

Core Technical Content

AZ61 magnesium alloy is a widely used structural material in aerospace, automotive, and defense applications due to its excellent specific strength, good castability, and adequate corrosion resistance. However, welding AZ61 presents several challenges:

• High reactivity with atmospheric gases, leading to oxide inclusion formation
• Low thermal conductivity combined with low melting point, creating a wide temperature gradient in the weld zone
• Susceptibility to hot cracking due to the limited solidification range
• Formation of coarse columnar grains in thick welds, which degrade mechanical properties

Electromagnetic stirring (EMS) applies an alternating magnetic field to the solidifying weld pool, inducing Lorentz forces that agitate the molten metal. This stirring action promotes:

• Equiaxed grain formation by breaking up columnar grain boundaries
• Homogenization of solute distribution, reducing segregation
• Removal of entrapped gases and inclusions from the weld centerline
• Improved wetting and penetration in the root region

The study examines various electromagnetic stirring parameters, including magnetic field strength (typically 0.5 to 2.0 Tesla), frequency (50 to 500 Hz), and application timing (during solidification or immediately after welding). The experimental results demonstrate that electromagnetic stirring can reduce porosity area fraction by 40 to 70 percent and refine grain size from an average of 150 to 200 micrometers down to 50 to 80 micrometers.

Microstructural Analysis

The microstructural evolution in AZ61 TIG welds under electromagnetic stirring follows a well-defined pattern. Without stirring, the weld metal exhibits a typical columnar dendritic structure with coarse alpha-Mg matrix and eutectic phases along dendrite boundaries. The grain boundaries are often decorated with Mg17Al12 intermetallic phases, which act as crack initiation sites under mechanical loading.

With electromagnetic stirring applied during the final stage of solidification, the microstructure transforms significantly:

The refinement mechanism operates through two primary pathways. First, the electromagnetic stirring creates temperature gradients that promote constitutional undercooling ahead of the solidification front, increasing the nucleation rate. Second, the stirring action physically breaks up existing dendrite arms, creating additional nucleation sites for new grains. The combined effect produces a fine, equiaxed microstructure with improved ductility and fracture toughness.

Mechanical Property Improvements

The mechanical properties of AZ61 TIG welds are substantially enhanced by electromagnetic stirring. Tensile tests conducted on weld coupons demonstrate that ultimate tensile strength increases from approximately 180 to 220 megapascals, while elongation improves from 8 to 15 percent. More importantly, the reduction in porosity eliminates the primary cause of weld failure under cyclic loading, significantly extending fatigue life.

For engineering applications involving AZ61 components in pressure vessels or heat exchangers, the improvement in weld quality directly translates to enhanced reliability. The elimination of centerline porosity reduces the probability of leak initiation under internal pressure, and the refined microstructure provides better resistance to stress corrosion cracking in chloride-containing environments.

Engineering Practice Implications

The implementation of electromagnetic stirring in production welding operations requires consideration of several practical factors. The electromagnetic stirring device must be positioned close to the weld pool without interfering with the TIG torch or wire feed mechanism. The magnetic field strength must be carefully calibrated to avoid excessive turbulence that could entrain oxide inclusions or cause spatter. For thin-walled components (below 3 millimeters), electromagnetic stirring may be unnecessary as the rapid solidification rates naturally produce fine grains.

The study provides valuable guidance for engineers working with magnesium alloy pressure vessels and heat exchangers. The electromagnetic stirring approach can be integrated into existing welding lines with minimal disruption to production schedules. The equipment cost is relatively modest compared to the potential savings from reduced rework and improved product quality. For critical applications such as aerospace fuel tanks or nuclear containment vessels, the enhanced weld integrity provided by electromagnetic stirring may justify the additional process step.

Study Insights and Implications

This research demonstrates that electromagnetic stirring is an effective post-weld treatment for improving AZ61 magnesium alloy weld quality. The approach is particularly valuable for thick-section welds where conventional welding parameters alone cannot produce acceptable microstructures. For engineers involved in bimetal pressure vessel fabrication, the principles of electromagnetic stirring can potentially be extended to other challenging welding applications, including high-strength steel welds and nickel-based alloy overlay welds where microstructural refinement is critical. The key insight is that external electromagnetic fields can manipulate solidification behavior without altering the welding process itself, offering a flexible and adaptable solution to persistent weld quality challenges.