Microstructure evolution and mechanical properties of Ti-6Al-4V alloy prepared by dual ultrasonic vibration assisted directional energy deposition

1. Research background
Directed energy deposition (DED), as an efficient and economical technology in the field of additive manufacturing (AM), is widely used in the manufacturing of metal materials. However, its high heating and cooling rates, as well as significant temperature gradients, often lead to rapid solidification, forming cross layer columnar grains and internal defects, seriously affecting the mechanical properties of additive manufacturing components. Especially Ti-6Al-4V alloy, due to its low nucleation rate, is more prone to form coarse columnar grains during the DED process, which limits its application in aerospace and marine engineering fields.

To address this issue, researchers have proposed various methods, including modifying printing strategies, implementing post-processing techniques, adding nucleating agents, and introducing external energy fields. Ultrasonic vibration (UV) technology has been introduced into additive manufacturing processes in recent years due to its successful application in welding and casting fields. It promotes the formation of fine equiaxed crystals through ultrasonic cavitation and acoustic flow effects, thereby significantly improving the mechanical properties of materials. However, existing methods still have limitations, such as the inability to dynamically adjust the fixed position of a single ultrasound source and the difficulty in fully achieving the transition from columnar crystals to equiaxed crystals. Therefore, developing a simple, efficient, and environmentally friendly bidirectional ultrasound assisted additive manufacturing technology has become a current research focus.

Recently, Harbin Engineering University, together with the Key Laboratory of Extreme Manufacturing Technology for Aircraft Engines in Zhejiang Province and Siberian State University of Technology, published a paper in the journal Materials Science and Engineering in the field of materials science A research result titled "Microstructure evolution and mechanical properties of Ti-6Al-4V alloy fabricated by directed energy deposition assisted with dual ultrasonic vibration" was published on A. This article introduces a new method for manufacturing Ti-6Al-4V alloy thin-walled parts by simultaneously and synchronously applying top and bottom ultrasonic vibrations during directional energy deposition. Through this method, coarse columnar primary β grains were successfully transformed into equiaxed grains, and the microstructure and mechanical properties of the material were significantly improved.

2. Paper images

Figure 1 (a) Schematic diagram of dual ultrasound assisted DED process and (b) Configuration and orientation of stretched samples

Figure 2 Deposition microstructure of Ti-6Al-4V alloy, (a, e) sample O, (b, f) sample T, (c, g) sample B, (d, h) sample D

Figure 3 (a) Engineering stress-strain curve, (b) Average yield strength, tensile strength, and elongation, (c) Comparison of the properties of Ti-6Al-4V alloy (sample D) prepared by dual ultrasonic process in this study with those reported in the literature

Figure 4 shows the inverse pole and orientation diagrams of the alpha phase (a, b, c, d) and native beta phase (e, f, g, h) along the construction direction. Sample O (a, e), sample T (b, f), sample B (c, g), and sample D (d, h)

Figure 5 Recrystallization and KAM maps of Ti-6Al-4V alloy after different ultrasonic treatments, (a, e) sample O, (b, f) sample T, (c, g) sample B, (d, h) sample D

Figure 6 (a) IPF of alpha phase for sample O, (b) sample T, (c) sample B and (d) sample D, (e) aspect ratio of alpha phase for sample O, (f) sample T, (g) sample B and (h) dual ultrasound treatment, KAM plot for (i) sample O, (j) sample T, (k) sample B and (l) sample D, BSE images for (m) sample O, (n) sample T, (o) sample B and (p) sample D

Figure 7 TEM image of sample D, (a, b, c) typical microstructure, (d, e, f) "petal shaped" microstructure, (g, h) nanocrystals, (i) SADE image

Figure 8 Tensile fracture morphology, (a) Sample O, (b) Sample D, (c) Enlarged view of the yellow circle area in Figure (a), (d) Enlarged view of the red circle area in Figure (b)

Figure 9 SEM images of the microstructure of the polished cross-section, where (a, c) represents sample O and (b, d) represents sample D

Figure 10 (a) Schematic diagram of a two-dimensional model, illustrating the influence of ultrasound on the displacement field of the sediment layer and the sound pressure distribution inside the melt pool, (b) Sound pressure distribution inside the melt pool only during bottom ultrasound treatment, (c) Sound pressure distribution inside the melt pool during dual ultrasound treatment, (d) Changes in solidification conditions, temperature gradient (G), and solidification rate (R) of the melt pool under different ultrasound loading conditions

3. Key conclusions
(1) Introducing ultrasonic vibration (UV) into the directed energy deposition (DED) process can refine the primary β - grains of Ti-6Al-4V alloy in situ. This method promotes the formation of equiaxed grains, suppresses the epitaxial growth of columnar grains, and ultimately improves the mechanical properties of the material. The equiaxed crystal grains observed in the multi-layered samples after multiple reciprocating sedimentation indicate that dual ultrasonic vibration treatment is more effective than single ultrasonic vibration treatment.

(2) Introducing dual ultrasonic vibration during the DED process promotes the transformation of the growth mode of the alpha phase from parallel arrangement to radial arrangement. This change helps to reduce grain boundary continuity, enhance mechanical interlocking between grains, and thus improve the overall mechanical properties of the material.

(3) Compared with the condition without ultrasonic treatment, the tensile strength (UTS), yield strength (YS), and elongation at break of Ti-6Al-4V alloy manufactured by dual ultrasonic technology were significantly increased by 26.7%, 24.7%, and 104%, respectively. These results indicate that the introduction of ultrasonic vibration effectively reduces the formation of coarse columnar grains in Ti-6Al-4V alloy during additive manufacturing process.

(4) The introduction of ultrasonic vibration changed the solidification conditions in the melt pool and induced the transformation from columnar to equiaxed crystals during the solidification process of Ti-6Al-4V alloy. In addition, the generation of ultrasonic cavitation in the melt pool promotes the refinement of grains and the improvement of microstructure characteristics. In this study, the application of ultrasound conditions promoted the formation of completely equiaxed grains in the melt pool, which is consistent with the experimental results observed through microstructure analysis.

Source: Yangtze River Delta Laser Alliance