High-Intensity Ultrasound Used for Grain Structure Control with Titanium Alloy Ti-6Al-4V

January 11, 2020 by Bridget O'Neal 3D Printing Metal 3D Printing

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As 3D printing in metal continues to lead the next industrial revolution, engineers from Australia are taking research one step further in examining improved methods for improving mechanical properties. Choosing a titanium alloy to create samples for experimentation and testing, the authors outlined their findings in the recently published ‘Grain structure control during metal 3D printing by high-intensity ultrasound.’

Samples consisted of 10 mm × 10 mm × 10 mm cubes for microstructural examination and 24 mm × 8 mm × 10 mm (length, width, and height) blocks for tensile testing. Without transforming the alloy at all, the researchers employ an additive manufacturing solidification-control solution to print metallic alloys with equiaxed grain structures, to prevent property anisotropy and the consequent decrease in mechanical performance. In this study, titanium alloy Ti-6Al-4V is used.

Reigning as the ‘benchmark alloy’ of titanium, and the subject of many studies, titanium alloy Ti-6Al-4V is used as a comparison for quality in metal AM processes.

“However, Ti-6Al-4V fabricated by different fusion-based AM processes exhibits a strong columnar grain structure. The columnar prior-β grains in AM-fabricated Ti-6Al-4V feature the strong <001> orientation along the build direction. This gives rise to a β → α transformation texture, which is an important concern for AM qualiﬁcation because of the resulting anisotropy of mechanical properties,” explain the researchers.

“In addition, the coarse columnar prior-β grains may further degrade the strength of Ti-6Al-4V according to the Hall–Petch relationship established for lamellar α–β Ti-6Al-4V21–23 (exceptions can exist).”

High-intensity ultrasound during metal AM. Cross-sectional schematic showing metal AM by laser-based DED onto an ultrasound sonotrode vibrated at 20 kHz. The formation of acoustic cavitation and streaming in the liquid metal by high-intensity ultrasound can vigorously agitate the melt during solidiﬁcation, thereby promoting signiﬁcant structural modiﬁcation or reﬁnement.

While it can be difficult in metal 3D printing to find a strong and stable nucleant alloy, Ti-6Al-4V is exactly that.

a, b Optical microscopy images of the samples without (a) and with (b) ultrasound. c, d Polarized light microscopy images showing large columnar grains (c) and ﬁne equiaxed grains (d). e, f Histograms of the prior-β grain size (e) and prior-β grain aspect ratio (f) for the samples without and with ultrasound measured from traced prior-β grain images (see Supplementary Fig. 1). The prior-β grain boundaries in c and d are traced in white. Scale bars, 1 mm.

Samples were prepared using Ti-6Al-4V both with and without high-intensity ultrasound. The resulting microstructural analysis showed significant differences between the samples, as those without ultrasound displayed columnar prior-β grains of several millimeters in length and ~0.5 mm in width traversing multiple deposited layers (as the researchers anticipated), while the samples used with ultrasound indicated ﬁne (~100 µm), equiaxed prior-β grains. After scanning all the samples with scanning electron microscopy (SEM), the researchers noted a basketweave-like α–β microstructure inside the prior-β grains.

Ultimately, the study showed that the use of ultrasound during the AM process with Ti-6Al-4V allows a fully equiaxed structure to form, resulting in:

Improvement in microstructural homogeneity
Significant reduction in the prior-β grain size
Substantial weakening of the solidification texture

“Assessment of the ultrasonic conditions reveals that the selection of the ultrasound transducer element can be an important practical consideration for structural reﬁnement of large-volume AM-fabricated parts and the use of a magnetostrictive transducer is recommended,” concluded the researchers.

“To assess the generality of our approach, the ultrasonic grain reﬁnement method is successfully applied to the AM of Inconel 625, including the creation of an alternating columnar/equiaxed/ columnar Inconel 625 grain structure along the build height by simply switching on and off the ultrasound during AM. We expect that this technique can be extended to the AM of other metallic materials.”

Microstructure characterization of the AM-fabricated Ti-6Al-4V with and without high-intensity ultrasound. a–d SEM images showing the α–β structure inside the prior-β grains of the samples without (a, c) and with (b, d) ultrasound. e, f Histograms of the α-lath thickness of the samples without (e) and with (f) ultrasound. The prior-β grain boundaries in a and b are traced in white. Scale bars, 50 μm in a, b and 5 μm in c, d.

As metal 3D printing grows in popularity–and industrial users begin to rely more on AM processes, researchers are studying titanium powder as it shows potential for so many applications; for instance, in the fabrication of medical implants in critical surgeries, assistance in bone regeneration, and more.

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[Source / Images: ‘Grain structure control during metal 3D printing by high-intensity ultrasound’]