Additive manufacturing is transforming how complex metal parts are made, offering efficiency and design flexibility. However, many of these parts suffer from hidden defects and weaknesses that reduce their durability and performance in demanding applications such as aerospace and defense. This Engineering Research Initiation (ERI) award seeks to address this challenge by developing a novel post-processing method to improve the strength and reliability of 3D-printed metal parts by refining their surface structure and reducing internal defects. This award aligns with national needs of enhancing advanced manufacturing capabilities and preparing a future workforce. Students are expected to gain hands-on experience in emerging technologies through new courses, summer outreach programs, and research opportunities, making them better prepared for the workforce. Research enabled by this award investigates the thermomechanical mechanisms of vibration-assisted friction stir surfacing (VAFSS) as a post-treatment for metal parts produced via laser powder bed fusion (LPBF). Despite current post-processing techniques like hot isostatic pressing (HIP) and heat treatment (HT), surface-level defects and residual stress often remain in LPBF-built components, limiting their fatigue performance. The research seeks to establish the relationships between VAFSS parameters, surface layer microstructure, and fatigue strength. Using both experimental and simulation approaches, the study aims to e