Pore Effects on Solidification Crack Sensitivity of MIG Welded Aluminum-Magnesium Alloys

Literature Overview

This study, authored by Chen Yu, Zhang Tianli, Yu Hang, Chen Haoxin, Wu Wen, and Wang Weiguang from Shanghai University of Engineering Science, investigates the influence of porosity on the solidification crack sensitivity of MIG welded aluminum-magnesium alloy welds. Published in 2021 in the Physical Testing and Chemical Analysis journal, this research was supported by the National Natural Science Foundation of China (51804196).

The research addresses a critical quality issue in aluminum-magnesium alloy welding, where porosity and solidification cracking often coexist and interact in complex ways. Understanding this interaction is essential for developing welding process parameters that produce crack-free welds with minimal porosity in applications ranging from marine structures to aerospace components.

Core Technical Content

Aluminum-magnesium alloys, particularly the 5xxx series such as 5083, 5086, and 5456, are widely used in marine, automotive, and aerospace applications due to their excellent corrosion resistance and good weldability. However, these alloys are susceptible to solidification cracking during welding, particularly when the magnesium content exceeds 4%. The cracking susceptibility is influenced by the solidification rate, thermal gradient, and the presence of porosity.

The study systematically varies the welding parameters to produce welds with different levels of porosity, and then evaluates the solidification crack sensitivity through macroscopic and microscopic examination of the weld cross-section. The porosity is characterized by its volume fraction, size distribution, and location within the weld cross-section.

Welding Parameters and Porosity Control

Interaction Mechanism Between Porosity and Solidification Cracking

The study identifies a complex interaction mechanism between porosity and solidification cracking. Porosity acts as a stress concentrator that initiates and propagates cracks during the solidification process. The hydrogen gas that forms the pores is absorbed from moisture in the shielding gas, base metal surface, or wire coating, and is trapped in the solidifying weld metal as the solubility of hydrogen decreases with decreasing temperature.

The critical finding of this study is that porosity does not merely coexist with solidification cracks but actively promotes their formation and growth. The pores create localized regions of reduced cross-sectional area in the solidifying weld, which increases the tensile stress during shrinkage. This elevated stress exceeds the solidification cracking threshold at lower overall strain levels than would be the case in a pore-free weld.

The study also reveals that the location of porosity within the weld cross-section significantly affects the cracking behavior. Pores located near the weld centerline, where the solidification rate is highest and the thermal gradient is most severe, are most effective at promoting crack initiation. Pores located near the weld toes, where the thermal gradient is lower, have a reduced effect on cracking sensitivity.

Crack Sensitivity Quantification

Metallurgical Analysis and Microstructure

The microstructural analysis of the weld cross-sections reveals that the solidification microstructure consists of a primary α-Al dendrite structure with interdendritic eutectic phases. The magnesium content of the alloy determines the morphology of the interdendritic phases, with higher magnesium content producing more continuous eutectic networks that are more susceptible to cracking.

The porosity is distributed primarily in the interdendritic regions, where the hydrogen gas is trapped during the final stages of solidification. The pores are typically spherical or near-spherical in shape, with diameters ranging from 50 μm to 500 μm. The largest pores are located near the weld centerline, where the solidification rate is highest and the gas entrapment is most severe.

The crack initiation sites are consistently located at or near the pore boundaries, confirming the stress concentration mechanism identified in the study. The cracks propagate preferentially along the interdendritic eutectic phases, which have lower melting points and are more susceptible to solidification cracking than the primary dendrite structure.

Engineering Practice and Quality Control

For production welding of aluminum-magnesium alloys, the findings of this study have direct implications for process parameter selection and quality control. The study recommends the following strategies to minimize porosity and reduce solidification crack sensitivity:

First, the shielding gas flow rate must be optimized to prevent both gas contamination and turbulence. A flow rate of 12–15 L/min is recommended for most MIG welding applications, with adjustments based on joint geometry and travel speed.

Second, the wire feed rate and travel speed must be matched to ensure complete joint filling without excessive heat input. The study recommends a heat input range of 0.8–1.5 kJ/mm for typical aluminum-magnesium alloy welding, with adjustments based on plate thickness and joint configuration.

Third, the base metal and wire surfaces must be thoroughly cleaned to remove moisture, oxide films, and organic contaminants that contribute to hydrogen absorption. Mechanical grinding followed by solvent cleaning is the recommended surface preparation procedure.

Fourth, the welding sequence must be designed to minimize restraint and reduce the tensile stress during solidification. Multi-pass welding with appropriate pass sequencing can significantly reduce cracking susceptibility by allowing stress relief between passes.

Key Questions and Reflections

The study raises several important questions that require further investigation. First, the effect of porosity on the fatigue resistance of aluminum-magnesium alloy welds has not been evaluated, despite the fact that many applications are subject to cyclic loading. Pores act as fatigue crack initiation sites, and their presence can significantly reduce the fatigue life of the weld.

Second, the study does not address the effect of welding position on the porosity-crack interaction. In vertical and overhead welding positions, the metal flow patterns are different from those in flat position welding, which can affect both porosity formation and crack propagation.

Third, the study focuses on conventional MIG welding, but the findings may not be directly applicable to advanced welding processes such as pulsed MIG, hybrid laser-MIG, or cold wire MIG. These processes have different heat input characteristics and metal transfer mechanisms that can alter the porosity-crack interaction.

Study Insights and Implications

The most significant finding of this study is the demonstration that porosity actively promotes solidification cracking in aluminum-magnesium alloy welds, rather than merely coexisting with it. This finding has important implications for welding process development and quality control, as it suggests that porosity reduction strategies can simultaneously reduce cracking susceptibility.

For engineers working on aluminum alloy pressure vessels, heat exchangers, and structural components, the porosity-crack interaction must be considered in the welding procedure specification and qualification process. The study provides a quantitative framework for evaluating the cracking risk associated with different porosity levels, which can be used to set acceptance criteria for porosity in welded joints.

The study also highlights the importance of surface preparation and shielding gas control in aluminum alloy welding. The hydrogen absorption that causes porosity is often underestimated in practice, and the study emphasizes that even small amounts of moisture can significantly increase the porosity level and cracking susceptibility of the weld.