Microstructure and Mechanical Properties of 5356 Aluminum Alloy TIG Arc Additive Manufacturing

Literature Overview

This study by Sun Jiaxiao and colleagues from Hohai University and the Institute of Metal Research, Chinese Academy of Sciences, published in the journal Acta Metallurgica Sinica in 2021, investigates the microstructure and mechanical properties of 5356 aluminum alloy components fabricated using TIG arc additive manufacturing (WAAM). The research was supported by the National Key Research and Development Program of China and Changzhou City Key Research and Development Program. The study provides a comprehensive analysis of the microstructural evolution across the deposition height and its influence on mechanical properties, including hardness, tensile strength, and ductility.

Core Technical Findings

The phase composition of the 5356 aluminum alloy additive manufactured component consists of an α-Al matrix and β (Al₃Mg₂) phase. The microstructure exhibits a distinct evolution with deposition height, transitioning from equiaxed grains in the lower layers to columnar grains in the upper layers as thermal equilibrium is reached. This evolution is driven by the thermal accumulation effect inherent to the additive manufacturing process, where the heat input from successive layers progressively raises the temperature of the deposited material.

Microstructural Evolution with Deposition Height

Microstructural Analysis

The microstructural evolution with deposition height is a critical finding of this study. In the lower layers, the microstructure consists primarily of equiaxed grains, which form due to the relatively low thermal gradient and high cooling rate at the base substrate. As the deposition height increases, the thermal accumulation effect becomes more pronounced, and the microstructure transitions to columnar grains. This transition is attributed to the increasing thermal gradient at the solidification front, which favors directional solidification along the heat flow direction.

The most top layer exhibits a dendritic microstructure with severe Mg segregation. This is because the top layer is the last to be deposited and experiences the highest thermal gradient and cooling rate, promoting dendritic growth and segregation of Mg to the interdendritic regions. In contrast, the mid-lower layers exhibit a more diverse microstructure, including equiaxed grains, columnar grains, and mixed structures, with improved Mg segregation.

The interlayer junctions exhibit fine equiaxed grains, which are finer than the intra-layer microstructure. This is because the interlayer region experiences a rapid cooling rate due to the thermal contact between the new molten layer and the solidified previous layer, promoting nucleation and equiaxed grain formation.

Hardness Distribution

Mechanical Properties

The mechanical properties of the 5356 aluminum alloy additive manufactured component exhibit anisotropy. The transverse tensile properties are superior to the longitudinal tensile properties. This anisotropy is attributed to the accumulation of porosity at the interlayer junctions and the non-uniform microstructure distribution. The porosity at the interlayer junctions acts as stress concentrators, reducing the effective load-bearing area and promoting crack initiation in the longitudinal direction.

The yield strength of the thin-walled component is lower than the theoretical calculated value, which is attributed to the interlayer porosity. The presence of porosity reduces the effective cross-sectional area and creates stress concentrations that initiate premature yielding. The elongation also exhibits anisotropy, with the transverse direction showing higher ductility than the longitudinal direction.

Mechanical Property Comparison

Engineering Practice Implications

The findings of this study have important implications for the engineering application of TIG WAAM for 5356 aluminum alloy. The microstructural evolution with deposition height must be considered in the design and fabrication of components, as it affects the mechanical properties and service performance. The anisotropy in mechanical properties suggests that the orientation of the component relative to the deposition direction should be optimized to align the stronger transverse direction with the primary loading direction.

The interlayer porosity is a critical issue that must be addressed in industrial applications. Strategies to reduce porosity include optimizing the process parameters, such as current, voltage, and travel speed, to minimize gas entrapment; using higher purity shielding gas; and implementing post-deposition heat treatment to close residual pores. The thermal accumulation effect can be managed by incorporating interlayer cooling periods or using a temperature-controlled substrate.

The severe Mg segregation in the top layer is a concern for corrosion resistance and mechanical properties. Post-deposition solution treatment and aging can homogenize the Mg distribution and enhance the mechanical properties through precipitation hardening. The β (Al₃Mg₂) phase, which is the primary strengthening phase in 5356 alloy, can be precipitated through aging to improve the strength and hardness.

Key Questions and Reflections

The study raises several important questions for future research. The role of the thermal history in determining the microstructure and mechanical properties is complex and multifaceted. Can the microstructural evolution be predicted using thermal modeling and solidification simulations? If so, can the process parameters be optimized to achieve a uniform microstructure throughout the component?

The interlayer porosity is a persistent challenge in TIG WAAM of aluminum alloys. Could the use of consumable shielding gas or a different shielding gas composition, such as a helium-argon mixture, reduce porosity by improving the shielding effectiveness and arc stability? Additionally, the use of wire feedstock with different compositions or surface treatments could influence the wetting behavior and porosity formation.

The anisotropy in mechanical properties suggests that the component design must account for the directional dependence of the material properties. How can the component geometry and deposition sequence be optimized to minimize the impact of anisotropy on the service performance?

Study Insights and Outlook

This study provides a comprehensive understanding of the microstructure and mechanical properties of 5356 aluminum alloy TIG WAAM components. The identification of the microstructural evolution with deposition height and the anisotropy in mechanical properties are valuable insights for the design and fabrication of additive manufactured components. The findings highlight the importance of thermal management and process optimization in achieving uniform microstructure and consistent mechanical properties. Future research should focus on thermal modeling, process parameter optimization, and post-deposition heat treatment to enhance the performance of TIG WAAM components for 5356 aluminum alloy. The study underscores the potential of TIG WAAM for aluminum alloy fabrication while identifying the challenges that must be addressed for industrial adoption.