Grain refinement of Ti5321G alloy created by ultrasonic energy field during laser powder direct energy deposition

Abstract

The ultrasonic energy field (UEF)-induced grain refinement mechanisms in laser powder direct energy deposition-manufactured Ti5321G alloys were systematically investigated in this study. This study focused on the interplay between recrystallization in the high-temperature solid deposition layers and the ultrasonic cavitation-acoustic streaming effects during molten pool solidification. A novel experimental design was developed to decouple these mechanisms by creating four distinct UEF action zones (without UEF-N, with UEF-S, with UEF-L, and with UEF-S + L) within a single-pass multilayer sample. This approach enabled the dual effects of UEF (recrystallization in solidified layers and ultrasonic cavitation-acoustic streaming effects in liquid pools) to be directly compared. The UEF significantly refined the microstructures, reducing the average grain size by 64.2% (from (399.6 ± 28.6) to (143.1 ± 16.1) μm) in the with UEF-S + L zone, while promoting columnar-to-equiaxed transition, with the equiaxed grain probability increasing from 11.1% (without UEF) to 53.8%. The texture intensity was reduced by approximately 52.4% and the mechanical properties were enhanced, achieving a 6.2% increase in yield strength ((702.0 ± 10.6) MPa) and 31.7% improvement in elongation. Crucially, this study revealed the synergistic effect of the dual-action mechanisms of UEF, where recrystallization and cavitation-acoustic streaming collectively enabled non-linear grain refinement. This study provides a strategy for microstructural control in additive manufacturing, eliminating the need for complex post-processing and thereby advancing the industrial application of high-performance titanium components.

1. Introduction

Laser powder direct energy deposition (LP-DED) is an effective additive manufacturing technique that can mold materials layer-by-layer using a geometric model, offering a viable alternative for material processing [1–2]. LP-DED possesses certain benefits such as high material utilization, rapid prototyping, customization capabilities, and the ability to create complicated components. This technique can be employed to manufacture lightweight and complex titanium alloy components [3–4].

However, numerous studies [5–9] have shown that the formation of high thermal gradients due to rapid melting and solidification induces the growth of columnar grains along specific orientations. Thereby, LP-DED induces a robust <001> β texture that is oriented along the building direction in DED-treated β-Ti alloys [10–11]. Consequently, such inhomogeneous microstructures lead to severe anisotropy, which dramatically deteriorates mechanical properties and generates considerable residual stress in the components [12–14], limiting the application and promotion of DED-treated titanium alloys. Therefore, it is critical to enhance the deposited microstructure of titanium alloys to achieve fine equiaxed grains using a straightforward and accessible method for LP-DED-treated titanium alloy materials.

Numerous researchers have made significant efforts to obtain equiaxed rather than coarse columnar grains in additive manufacturing-fabricated components [15–20]. On the one hand, adding alloying elements or compounds can promote constitutional supercooling ahead of the solid–liquid interface or trigger heterogeneous nucleation, leading to the formation of equiaxed grains. Thus, nucleation is promoted, resulting in isotropic mechanical properties within alloys fabricated by additive manufacturing [16,18,21]. However, the addition of alloying elements to alloys can alter their composition and other properties to a certain degree, relative to those of pristine materials. Additionally, this strategy is challenging for alloys with highly accurate compositions [22]. On the other hand, external control processing techniques containing low/high-frequency pulse arcs [23] and electromagnetic stirring [24] have been employed to regulate the grain structures in components fabricated using additive manufacturing; however, these inevitably lead to increased manufacturing expenses and extended production times. Furthermore, the ultrasonic energy field (UEF) reportedly refines the microstructures of materials to achieve fine equiaxed grains, thereby enhancing their mechanical properties. Todaro et al.