Microstructure Characteristics of AZ31 Mg and 5A05 Al Dissimilar Metal Joints by Pulsed Cold Arc MIG Welding

Research Background and Significance

This 2023 study from the North University of China investigates the welding of dissimilar magnesium-aluminum alloys, specifically joining AZ31 magnesium alloy to 5A05 aluminum alloy using pulsed cold arc MIG (GMAW) welding. The research was funded by the Shanxi Provincial Natural Science Foundation (201901D111174) and the Shanxi Provincial Higher Education Science and Technology Innovation Project (2019L0568). Dissimilar Mg-Al joints are of significant interest in aerospace and automotive industries due to the potential for weight reduction, but they present formidable challenges related to intermetallic compound (IMC) formation, porosity, and poor bonding at the interface.

The use of pulsed cold arc MIG welding—a variant of pulsed GMAW that employs a cold metal transfer (CMT) or similar low-heat-input technique—was selected to minimize the thermal input and, consequently, limit the formation of brittle intermetallic phases at the weld interface.

Welding Process Parameters and Experimental Setup

The researchers employed a pulsed MIG welding process with specific parameters optimized for the dissimilar joint. The key process parameters are summarized below.

The pulsed welding mode was selected because it allows precise control over the heat input per droplet transfer cycle. During the pulse phase, a high current momentarily detaches a single droplet from the wire tip, while the background current maintains the arc without adding significant heat. This results in a significantly lower total heat input compared to conventional continuous MIG welding, which is critical for suppressing excessive IMC formation in dissimilar Mg-Al welds.

Microstructural Analysis of the Weld Interface

Metallographic examination of the weld cross-sections revealed a complex microstructural evolution across the joint. The AZ31 side of the weld exhibited a typical magnesium alloy microstructure with equiaxed alpha-Mg grains and secondary Mg17Al12 intermetallic phases distributed along grain boundaries. The 5A05 side showed a refined aluminum grain structure with dispersed Mg2Si and Al6Mn intermetallic particles.

At the weld interface, a reaction layer formed consisting primarily of Mg2Al3 and Mg5Al8 intermetallic compounds. The thickness of this IMC layer was found to be strongly dependent on the welding parameters. At lower heat inputs (pulse current below 200 A, travel speed above 300 mm/min), the IMC layer was thin, typically less than 10 micrometers, and exhibited a relatively uniform morphology. At higher heat inputs, the IMC layer thickened to over 50 micrometers and developed a branched, needle-like morphology that significantly reduced the joint strength.

The formation of Mg2Al3 was favored at lower temperatures and shorter reaction times, while Mg5Al8 formed preferentially at higher temperatures and longer dwell times. The transition from Mg2Al3 to Mg5Al8 was observed at the interface between the weld metal and the heat-affected zones, indicating a gradient in the local thermal history.

Porosity and Defect Analysis

Porosity was identified as a major defect in the dissimilar Mg-Al welds. Two types of porosity were observed: gas porosity, resulting from hydrogen dissolution in the molten weld pool, and shrinkage porosity, caused by inadequate feeding of the molten pool during solidification. The hydrogen porosity was more prevalent on the magnesium alloy side, where the solubility of hydrogen in liquid Mg is significantly higher than in solid Mg, leading to rapid gas evolution during solidification.

The use of hydrogen-containing shielding gas (Ar-2%H2) was found to be detrimental in this application, as it increased the hydrogen pickup in the weld pool and exacerbated the porosity. Pure argon shielding was recommended for all Mg-Al dissimilar welds to minimize hydrogen-related defects.

Engineering Practice and Process Recommendations

For engineering applications involving dissimilar Mg-Al joints, the following recommendations emerge from this study:

1. The welding heat input should be kept below 1.0 kJ/mm to limit IMC layer thickness to below 15 micrometers, ensuring adequate joint strength and ductility.
2. Pulse frequency should be optimized to achieve a single droplet transfer per pulse cycle, ensuring stable and consistent heat input.
3. Pure argon shielding gas should be used to minimize hydrogen porosity.
4. The filler wire selection should consider the dilution ratio; using a filler wire with a composition closer to the weaker alloy (AZ31) can help reduce the formation of brittle intermetallics.
5. Post-weld heat treatment, such as solution treatment and aging, may improve the joint properties by dissolving some of the brittle intermetallic phases and redistributing the precipitates.

Key Reflections

This study underscores the fundamental challenge of joining dissimilar metals with large differences in melting point and thermal expansion coefficient. The Mg-Al system is particularly challenging due to the formation of multiple brittle intermetallic phases that are thermodynamically stable across a wide temperature range. The pulsed cold arc MIG approach offers a promising pathway to mitigate these issues by controlling the thermal input, but the results also demonstrate that complete elimination of intermetallics is not achievable. Engineers designing Mg-Al joints must accept a certain degree of IMC formation and focus instead on optimizing the IMC morphology and thickness to achieve acceptable mechanical performance. The findings are directly applicable to the design of lightweight structures in aerospace and automotive applications where weight reduction is a primary design driver.