Effect of Heat Treatment on Microstructure and Properties of AZ91 Magnesium Alloy Welded Joints Under Magnetic Field

Overview of the Study

This paper by Qi Xiuling and colleagues from Shenyang University of Technology investigates the effect of applying an external alternating magnetic field during GTAW welding of 5 mm thick AZ91 magnesium alloy plates, followed by post-weld heat treatment. The authors studied three heat treatment conditions: solution treatment, solution treatment plus aging, and aging alone. The study was supported by the Liaoning Provincial Department of Education and published in the Transactions of the Welding Journal in 2015. The results demonstrate that the magnetic field, combined with appropriate heat treatment, can significantly improve the microstructure and mechanical properties of the welded joint.

Core Technical Findings

The application of an external magnetic field during welding is an unconventional approach that has been explored for its potential to modify the solidification behavior of the weld pool. The magnetic field interacts with the electric currents flowing in the weld pool, generating Lorentz forces that can influence fluid flow, heat transfer, and solidification patterns.

The observation that the magnetic field results in finer grains after solution treatment is a significant finding. The magnetic field influences the solidification process by modifying the fluid flow in the weld pool, which affects the temperature gradient and the growth conditions of the solidifying grains. The Lorentz forces generated by the interaction of the magnetic field with the electric currents in the weld pool can enhance mixing, reduce temperature gradients, and promote the nucleation of new grains.

The effect of the magnetic field on the precipitation behavior during aging is equally important. The finer grain structure produced by the magnetic field provides more grain boundaries, which serve as nucleation sites for precipitates during aging. The result is a higher density of precipitates that are finer, more dispersed, and more uniformly distributed than in joints welded without a magnetic field.

The three heat treatment conditions studied represent different stages of the precipitation hardening process for AZ91 magnesium alloy. Solution treatment dissolves the beta-Mg17Al12 phase into the alpha-Mg matrix, creating a supersaturated solid solution. Aging then allows the precipitation of fine beta particles from the supersaturated solution. The combination of magnetic field and heat treatment leverages the beneficial effects of both approaches to achieve superior properties.

Microstructural Analysis

The microstructure of the AZ91 magnesium alloy welded joint consists of the alpha-Mg matrix with beta-Mg17Al12 precipitates. The morphology, size, and distribution of the beta phase are critical determinants of the mechanical properties and corrosion resistance of the joint.

In the as-welded condition, the weld metal typically exhibits a coarse dendritic structure with the beta phase distributed along the dendrite arms and at the interdendritic regions. The coarse microstructure results from the rapid solidification rates in the weld pool, which favor the growth of large grains over the nucleation of many small grains. The heat-affected zone (HAZ) may exhibit a range of microstructures depending on the peak temperature experienced, from fully recrystallized and solution-treated regions near the weld to partially recrystallized regions further away.

The application of the magnetic field during welding modifies the solidification conditions in the weld pool. The Lorentz forces generated by the interaction of the magnetic field with the electric currents can enhance the fluid flow, which improves heat and mass transfer. This leads to a more uniform temperature distribution and promotes the nucleation of new grains, resulting in a finer microstructure. The finer grains are more responsive to subsequent heat treatment, as they have a higher density of grain boundaries that serve as nucleation sites for precipitates.

The solution treatment dissolves the beta phase and creates a supersaturated alpha-Mg matrix. The subsequent aging allows the precipitation of fine beta particles. The magnetic field's influence on the initial grain structure is preserved through the heat treatment, as the finer grains continue to provide more nucleation sites for precipitation. The result is a higher density of finer precipitates that are more effective at strengthening the matrix.

The aging treatment produces the finest and most dispersed beta phase, with the particles being discontinuous and distributed throughout the matrix. This morphology is favorable for mechanical properties because the fine particles provide a high degree of precipitation strengthening without creating continuous networks that could facilitate crack propagation. The discontinuous nature of the precipitates also reduces the risk of intergranular cracking, which is a concern in magnesium alloys.

Mechanical Properties

The mechanical properties of the AZ91 welded joint are directly related to the microstructure, and the combined effect of the magnetic field and heat treatment results in significant improvements. Hardness, tensile strength, and ductility are all enhanced compared to the as-welded condition and the no-magnetic-field condition.

The hardness improvement is a direct consequence of the increased density of precipitates. The fine beta-Mg17Al12 particles act as obstacles to dislocation motion, increasing the resistance to plastic deformation. The magnetic field's contribution to grain refinement also enhances hardness, as finer grains are harder according to the Hall-Petch relationship.

The tensile strength improvement reflects the combined effects of grain refinement and precipitation strengthening. The finer grains provide a higher yield strength through the Hall-Petch effect, while the precipitates provide additional strengthening through the Orowan mechanism. The ductility improvement is more surprising, as precipitation strengthening typically reduces ductility. However, the finer grain structure and the more uniform distribution of precipitates can improve ductility by promoting more uniform deformation and delaying the onset of necking.

The synergistic effect of the magnetic field and heat treatment is most pronounced in the aging condition, where the finest and most dispersed beta phase is achieved. This condition represents the optimal balance between strength and ductility, making it the most suitable for structural applications where both properties are important.

Engineering Practice Implications

The findings of this study have significant implications for the welding of magnesium alloy components, particularly in applications where high strength and ductility are required. Magnesium alloys are increasingly used in lightweight structural applications due to their low density and good specific strength, but their weldability and the properties of welded joints remain areas of concern.

The application of an external magnetic field during welding is a novel approach that can be implemented with relatively simple equipment. The magnetic field can be generated by coils positioned around the weld zone, and the field strength and frequency can be adjusted to optimize the effect. This approach is non-invasive and does not require changes to the welding parameters or the material composition.

The post-weld heat treatment is a standard practice in magnesium alloy welding, but the study demonstrates that the combination of magnetic field and heat treatment yields superior results. The solution treatment and aging parameters should be optimized for the specific alloy and welding condition, and the magnetic field parameters should be selected to complement the heat treatment.

From a quality control perspective, the magnetic field welding process requires monitoring of the magnetic field strength and frequency to ensure consistent results. The field parameters should be included in the welding procedure specification as essential variables, and their stability should be verified during production. Additionally, the microstructure and mechanical properties of the joint should be verified after heat treatment to ensure that the expected improvements have been achieved.

Study Insights and Reflections

This paper presents an innovative approach to improving the properties of magnesium alloy welded joints through the combined use of an external magnetic field and post-weld heat treatment. The systematic investigation of the magnetic field's effect on the microstructure and properties provides valuable insights into the mechanisms by which the field influences the welding process and the subsequent heat treatment response.

The finding that the magnetic field enhances the effectiveness of heat treatment is particularly significant.