Effect of Welding Current on Microstructure and Properties of AZ31B Magnesium Alloy MIG Welds

Background and Motivation

The paper by Jin Yuting et al., published in Light Alloy Fabrication Technology (2020, Vol. 48, No. 8, pp. 54-60), investigates the effect of welding current on the microstructure and mechanical properties of MIG welds in AZ31B magnesium alloy. This research was conducted at Fujian University of Technology and supported by the Fujian Provincial University Industry-Academia Cooperation Project (2019H6021). AZ31B is one of the most widely used wrought magnesium alloys, known for its excellent combination of strength, corrosion resistance, and formability. Its application in lightweight structural components, particularly in automotive and aerospace industries, makes understanding its weldability critical for enabling further lightweighting of transportation structures.

Experimental Design and Methodology

The experimental matrix focused on welding current as the primary variable, ranging from 70 A to 100 A in increments. AZ61A magnesium alloy filler wire was selected for welding, which contains a higher aluminum content than the AZ31B base metal, providing additional β-Mg17Al12 intermetallic phase formation in the weld zone. The following characterization techniques were employed:

The selection of AZ61A as filler wire is a deliberate metallurgical choice. The higher Al content (approximately 6% vs. 3% in AZ31B) promotes the formation of β-Mg17Al12 intermetallic compounds in the weld metal during solidification. These compounds act as precipitation hardening phases, increasing the strength and hardness of the weld zone. However, excessive formation of brittle intermetallic phases can reduce ductility, making the optimization of welding parameters essential.

Results and Analysis

Weld Geometry Response to Current Variation

Within the 70-100 A range, the weld width increased only slightly with increasing current, while the weld reinforcement (excess height) showed a decreasing trend. This behavior is somewhat counterintuitive, as one might expect both width and reinforcement to increase with higher current. The decrease in reinforcement at higher currents can be attributed to enhanced fluidity of the molten pool, which allows the excess metal to flow laterally rather than building up vertically. This improved fusion between the filler wire and base metal is beneficial for ensuring complete bonding and reducing the risk of lack-of-fusion defects.

Microstructural Evolution

The use of AZ61A filler wire resulted in a weld metal microstructure containing a higher volume fraction of β-Mg17Al12 intermetallic phase compared to what would be obtained with a more composition-matched filler. The β-Mg17Al12 phase forms as plate-like or rod-like precipitates within the α-Mg matrix during solidification. These precipitates act as effective strengthening agents through solid solution strengthening and precipitation hardening mechanisms. The finer and more dispersed the β-phase distribution, the more effective the strengthening, but the more brittle the microstructure becomes.

Mechanical Properties

The optimal welding current of 80 A produced a weld with a tensile strength of 227.5 MPa and an elongation of 5.5%, which meets the requirements specified in GB/T 5156 for magnesium alloy products. While the tensile strength is comparable to the base metal, the elongation shows a significant reduction relative to the AZ31B base metal, which typically exhibits elongation values of 8-12%. This ductility loss is primarily attributed to the formation of brittle β-Mg17Al12 intermetallic phases at grain boundaries and the coarsening of the weld metal microstructure due to the high cooling rates in the thin-section weld.

Engineering Application Considerations

The findings have direct implications for the design and manufacturing of magnesium alloy structural components:

• Non-rigid collision applications: For magnesium alloy components where high plasticity is not required (such as non-critical structural brackets, housing panels, and non-safety-critical frames), MIG welding at 80 A with AZ61A filler wire provides an efficient and reliable joining method.
• Process efficiency: The MIG process offers significantly higher deposition rates compared to TIG welding, making it suitable for production environments where throughput is a priority.
• Filler wire selection: The use of a higher-Al-content filler wire (AZ61A) is a viable strategy for strengthening the weld zone, but it must be balanced against the reduction in ductility. For applications requiring higher toughness, a more composition-matched filler such as AZ31 or AZ91 may be preferred.

Study Insights and Reflections

The study demonstrates that welding current is a critical parameter in controlling both the macroscopic weld geometry and the microscopic structure of magnesium alloy welds. The optimal current of 80 A represents a balance between sufficient heat input for complete fusion and avoiding excessive thermal exposure that leads to grain coarsening and excessive intermetallic phase formation. The use of AZ61A filler wire is a pragmatic approach to improving weld strength, but the trade-off in ductility must be carefully considered in design.

One area that could benefit from further investigation is the effect of welding speed on the microstructure and properties, as the cooling rate is jointly determined by both current and speed. Additionally, post-weld heat treatment (PWHT) could potentially improve the ductility of the weld zone by dissolving and re-precipitating the β-Mg17Al12 phase in a more favorable morphology. For engineering practice, the key recommendation is to use MIG welding at 80 A with AZ61A filler wire for non-critical AZ31B magnesium alloy joints, while ensuring that the design accounts for the reduced ductility of the weld zone.