Influence of High-Intensity Ultrasound on Ti-6Al-4V Microstructure During Laser Powder Bed Fusion Solidification Conditions

Additive manufacturing (AM) has seen rapid growth in recent years as a consequence of its various process attributes. However, the quasi-rapid solidification associated with AM processing often induces microstructural directionality in the build in the form of large, columnar grains oriented in the build direction. Prior work has demonstrated that in situ high-intensity ultrasound can refine the microstructure during directed energy deposition (DED) AM processes. The primary microstructure controlling factors for DED AM processes, such as the solidification rate and temperature gradient, are typically orders of magnitude lower than those experienced during laser powder bed fusion (LPBF) AM processes. In this work, the role of high-intensity ultrasound on the resulting microstructure of Ti-6Al-4V under LPBF solidification conditions is explored. The coordinated effect of laser scanning velocity and ultrasound intensity was characterized through controlled line and area scans with the laser. Subsequent microstructural characterization demonstrated the feasibility of using high-intensity ultrasound for influencing the microstructure during LPBF AM processing. Further development of these techniques to ultrasonically excite AM LPBF process melt pools may serve to reduce or eliminate the highly directional microstructures that are often observed in bulk AM parts.