MIG Welding of AZ91D Magnesium Alloy Using Pure Aluminum Wire

Literature Overview

This paper, published in the journal Casting Technology (Zhuzao Jishu), Volume 39, Issue 2, 2018, by Mao Jin, Hou Jibo, Xu Kai, and Liu Yaxin from the North University of China, investigates the welding processability and joint properties of AZ91D magnesium alloy using pure aluminum wire as the filler metal in MIG welding. The study identifies the formation of extensive α-Mg + β-Mg17Al12 eutectic structure in the weld metal as the primary cause of poor joint properties and highlights several process-related defects including narrow welding parameter window, severe spatter, sagging, and insufficient weld reinforcement.

Metallurgical Analysis of the Weld Joint

The use of pure aluminum wire as filler metal for AZ91D magnesium alloy welding creates a significant compositional mismatch. AZ91D is a wrought magnesium alloy containing approximately 9 wt% aluminum and 1 wt% zinc, with the aluminum primarily present as β-Mg17Al12 intermetallic particles. When pure aluminum wire is used, the resulting weld metal composition is dominated by magnesium from the base metal and aluminum from the filler, with a very low zinc content.

The formation of the α-Mg + β-Mg17Al12 eutectic structure in the weld metal is a direct consequence of the solidification behavior of the Mg-Al binary system. As the molten weld pool cools, primary α-Mg dendrites form first, followed by the eutectic reaction at the final stage of solidification, producing a network of brittle Mg17Al12 particles along the interdendritic regions. This eutectic structure is inherently brittle and has poor mechanical properties compared to the wrought base metal, where the β-phase particles are finely dispersed and can be further refined through heat treatment.

Compositional and Microstructural Comparison

Process Defect Analysis

The study identifies four major process-related defects that limit the practical applicability of pure aluminum wire for AZ91D MIG welding. Each of these defects warrants detailed examination from an engineering perspective.

Defect 1: Narrow Welding Parameter Window

The narrow welding parameter window is a critical practical limitation. In MIG welding of magnesium alloys, the heat input must be carefully controlled to avoid both under-penetration (insufficient fusion) and excessive heat input (burn-through, excessive HAZ softening, and porosity). The narrow window for pure aluminum wire is likely exacerbated by the high melting point of aluminum compared to magnesium, which creates challenges in achieving stable arc characteristics and consistent metal transfer. In practice, this means that minor variations in material thickness, fit-up, or environmental conditions can lead to weld quality degradation, making the process difficult to control in production environments.

Defect 2: Severe Spatter

Spatter in magnesium alloy MIG welding is attributed to several factors: the low surface tension of molten magnesium, the high vapor pressure of magnesium at welding temperatures, and the interaction between the arc plasma and the molten pool. Pure aluminum wire may exacerbate spatter due to the difference in surface tension and vapor pressure between aluminum and magnesium. Spatter not only reduces welding efficiency and increases wire consumption but also creates surface defects that can serve as corrosion initiation sites.

Defect 3: Sagging and Insufficient Reinforcement

The sagging phenomenon, particularly in horizontal or overhead welding positions, is related to the low melting point and low viscosity of molten magnesium. The insufficient weld reinforcement indicates that the deposited metal volume is inadequate to compensate for the weld preparation and ensure proper joint geometry. This is particularly problematic for structural applications where the weld must provide continuity and load-bearing capacity equivalent to the base metal.

Defect 4: Poor Joint Mechanical Properties

The combination of the brittle eutectic structure, coarse grain morphology, and the presence of processing defects results in poor mechanical properties. The weld metal likely exhibits significantly lower tensile strength and ductility compared to the base metal, and the joint may fail in the weld region under both static and fatigue loading.

Process Improvement Strategies

Based on the identified defects, several improvement strategies can be proposed for engineering practice. First, the use of a filler metal with a composition closer to AZ91D, such as an Al-Zn-Mg alloy wire, could reduce the formation of the brittle eutectic structure and improve the mechanical properties of the weld metal. Second, the welding parameters should be optimized through systematic trial to establish a wider acceptable parameter window, potentially through the use of pulsed current to control heat input and metal transfer. Third, the use of appropriate shielding gas mixtures, such as argon with small amounts of helium or hydrogen, could improve arc stability and reduce spatter. Fourth, for critical applications, post-weld heat treatment to dissolve and re-precipitate the β-phase in a controlled manner could significantly improve the mechanical properties of the joint.

Engineering Practice Implications

For engineers considering magnesium alloy welding in manufacturing, this study serves as an important cautionary reference. The use of pure aluminum wire for AZ91D welding is not recommended for structural applications due to the fundamental metallurgical incompatibility and the resulting poor joint properties. The narrow process window and multiple defect modes make this approach impractical for high-volume production where process stability and consistency are essential. Engineers should instead consider alternative filler metals, welding processes, or joint design strategies that can produce joints with properties approaching those of the base metal.

Summary

This study clearly demonstrates that the use of pure aluminum wire for MIG welding of AZ91D magnesium alloy results in weld joints with extensive brittle eutectic structure, narrow processability, and multiple process defects. The fundamental metallurgical issue of eutectic formation in the weld metal cannot be resolved through parameter optimization alone, and alternative filler metals or welding processes should be considered for practical applications. This work underscores the importance of matching filler metal composition to base metal composition in magnesium alloy welding to achieve acceptable joint properties.