Abstract
Cavitation and acoustic streaming created by in situ high-intensity ultrasound have been exploited for microstructural refinement during directed energy deposition (DED) additive manufacturing (AM) of alloy Ti-6Al-4V. Whether the same ultrasound-driven mechanisms are applicable to powder bed fusion (PBF) of Ti-6Al-4V remains uncertain. The primary factors that control the microstructure during deposition processing are the solidification velocity and temperature gradient, which are orders of magnitude higher for PBF than DED processes. This work examines the role of high-intensity ultrasound on the melt pool and resulting microstructure of Ti-6Al4V under PBF solidification conditions. The effect of the laser scanning velocity and ultrasonic excitation on the melt pool and grain structure characteristics is interrogated through controlled line scans and area scans without powder. Temperature field simulations are used to estimate the solidification velocities and temperature gradients and to compare the processing conditions with previously characterized columnar-to-equiaxed transition diagrams. The acoustic pressures during processing are estimated using a coupled field acoustic-elastic finite element simulation and used in a Keller-Miksis model to assess the likelihood of cavitation vs. cavity size. The microstructure of the samples with or without ultrasonic excitation was characterized, allowing the equivalent grain diameters and aspect ratios to be compared. All samples, including those exposed to scanning velocities where cavitation did not occur, showed a reduction in grain aspect ratio when subjected to ultrasonic excitation, but the effect on equivalent grain diameter was inconclusive. A key finding is that the primary effects of ultrasonic excitation may be attributed to acoustic streaming, rather than cavitation, during PBF processing. Further development of the ultrasonic excitation technique explored may permit tailoring of the microstructure and texture characteristics in bulk AM parts.