Subsurface Cooling Rates and Microstructural Response during Laser Based Metal Additive Manufacturing

• Published in: Scientific reports Vol. 10; no. 1; p. 1981
• Main Authors: Thampy, Vivek, Fong, Anthony Y., Calta, Nicholas P., Wang, Jenny, Martin, Aiden A., Depond, Philip J., Kiss, Andrew M., Guss, Gabe, Xing, Qingfeng, Ott, Ryan T., van Buuren, Anthony, Toney, Michael F., Weker, Johanna Nelson, Kramer, Matthew J., Matthews, Manyalibo J., Tassone, Christopher J., Stone, Kevin H.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 06.02.2020; Nature Publishing Group
• Subjects: 639/301/1023/1026; 639/301/930/12; Additive manufacturing; Characterization and analytical techniques; Contraction; Cooling; Humanities and Social Sciences; Lasers; MATERIALS SCIENCE; Metals and alloys; multidisciplinary; Science; Science (multidisciplinary); Titanium; Titanium alloys; X-ray diffraction
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/s41598-020-58598-z

Laser powder bed fusion (LPBF) is a method of additive manufacturing characterized by the rapid scanning of a high powered laser over a thin bed of metallic powder to create a single layer, which may then be built upon to form larger structures. Much of the melting, resolidification, and subsequent cooling take place at much higher rates and with much higher thermal gradients than in traditional metallurgical processes, with much of this occurring below the surface. We have used in situ high speed X-ray diffraction to extract subsurface cooling rates following resolidification from the melt and above the β -transus in titanium alloy Ti-6Al-4V. We observe an inverse relationship with laser power and bulk cooling rates. The measured cooling rates are seen to correlate to the level of residual strain borne by the minority β -Ti phase with increased strain at slower cooling rates. The α -Ti phase shows a lattice contraction which is invariant with cooling rate. We also observe a broadening of the diffraction peaks which is greater for the β -Ti phase at slower cooling rates and a change in the relative phase fraction following LPBF. These results provide a direct measure of the subsurface thermal history and demonstrate its importance to the ultimate quality of additively manufactured materials.