Control of fine grain structures and strengthening-toughening mechanisms in magnesium alloys fabricated by wire-arc directed energy deposition

Abstract

Wire-arc directed energy deposition (WA-DED) has attracted considerable attention for the fabrication of magnesium (Mg) alloys due to its high efficiency, low cost, and rapid prototyping capability for complex components. However, the inherent rapid solidification and complex thermal cycling associated with WA-DED often result in coarse columnar grains and pronounced mechanical anisotropy, which severely limiting its application potential. In this study, a novel spiral oscillation (SO) strategy was implemented during WA-DED AZ31 Mg alloy to refine the microstructure, reduce mechanical anisotropy, and achieve a strength-ductility synergy. Specifically, the yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) are increased by 9.7%, 38.1%, and 147%, respectively. These improvements by the SO strategy are primarily attributed to the promotion of columnar-to-equiaxed transformation (CET), a 74.2% reduction in maximum texture intensity, and a more uniform distribution of second-phase particles. Second-phase particles are primarily composed of Al8Mn5 and Al8Mn4Y. This study provides a novel strategy for microstructural control aimed at improving the performance of WA-DED AZ31 Mg alloy components.

1. Introduction

Magnesium alloys represent the lightest class of engineering structural materials, with a density of ~1.74 g·cm^-3, approximately 65% that of aluminum, 40% of titanium, and 25% of steel [1, 2]. Although the absolute tensile strength of Mg alloys is generally lower than that of high-strength steels or certain aluminum alloys, their exceptionally low density results in a remarkably high specific strength and specific stiffness, which can exceed those of some aluminum alloys and steels. Furthermore, Mg alloys have excellent electromagnetic shielding capabilities and biodegradability, making them ideal for lightweight structural applications with significant potential in automotive, military equipment, aerospace and other fields [3-6].

However, conventional fabrication methods for Mg alloys, such as casting, forging, extrusion, and rolling, are often constrained by mold limitations. These processes are susceptible to metallurgical defects like porosity and generally struggle to directly form complex components. Moreover, the inherent hexagonal close-packed (HCP) crystal structure of Mg alloys results in low plasticity and poor formability at room temperature, which further limits the application of Mg alloys [7, 8]. Therefore, there is an urgent need to develop advanced manufacturing techniques capable of integrating complex geometries while mitigating the limitations of traditional processing routes.

Additive manufacturing (AM) is an emerging fabrication technique that builds components layer-by-layer along a predefined path, enabling the rapid prototyping of complex geometries [9]. Unlike traditional subtractive manufacturing methods, AM is not constrained by shape limitations, offering substantial potential for the fabrication of complex Mg alloy components. According to the type of heat source and raw materials, the most commonly used metal AM processes are categorized into three types: laser powder bed fusion (L-PBF), laser directed energy deposition (L-DED), and wire arc additive manufacturing (WAAM, also called wire arc directed energy deposition, WA-DED) [10]. However, the high laser reflectivity of Mg alloys, coupled with the high flammability and explosiveness of Mg alloy powders, introduces significant safety concerns in industrial applications [11]. These safety concerns severely limit the application of L-PBF and L-DED for Mg alloys. In contrast, WA-DED employs Mg alloy wire as raw material and an electric arc as the heat source, effectively eliminating powder-related safety hazards. This intrinsic safety advantage makes WA-DED a highly promising technology for the additive manufacturing of Mg alloys.