This study investigates the effects of ultrasonic assisted vibration and post weld heat treatment on the microstructure and mechanical properties of metal inert gas (MIG) welded joints in AISI 1020 steel. Ultrasonic vibration significantly refined the grain structure and improved microstructural uniformity, particularly within the thermomechanically affected zone (TMAZ) and heat affected zone (HAZ). When combined with a post weld heat treatment at 650 ℃, the process achieved an optimal balance between tensile strength (464.9 MPa) and ductility (18% elongation). In contrast, heat treatment at 800 ℃ resulted in a higher strength (805.5 MPa) but compromised ductility (10%). Optical microscopy confirmed that ultrasonic vibration effectively suppressed coarse grain growth and mitigated residual stress compared to conventional MIG welding. These results demonstrate that the synergistic application of ultrasonic vibration and a 650 ℃ heat treatment provides a promising route to producing welds with refined microstructure and balanced mechanical performance, well suited for high reliability industrial applications.