Reinforced thermoplastic composites are an attractive material solution for many commercial and defense vehicle applications due to their ability to reduce manufacturing cycle time and cost. Additionally, thermoplastic composites have superior toughness and environmental resistance compared to thermoset composites and offer the ability to eliminate or decrease the use of mechanical fasteners at joints by fusion/welding. When welding thermoplastic composite assemblies, each substrate is heated to melt the polymer at the interface of the joint. While heating, the joint is held under pressure until the polymer solidifies and the substrates are consolidated. This forms a unitized structure, with no identifiable interface after the welding is complete. Fusion also offers significant benefits over the bonding process due to the minimal substrate preparation, higher weld properties and its time efficiency. Ultrasonic Welding is one such fusion technology where the interface between two substrates is excited through high frequency vibrations. As part of the technology development, the weld process parameters, interface, and substrates were found to play key roles in the final weld quality. This work discusses the sensitivities and the key factors in thermoplastic weld development along with the manufacturing development for a scaling up process from coupon to sub-element level structural assessment. Strength characterization and weld parameter driven influences on interface quality were also investigated and documented based on findings. © 2024 Soc. for the Advancement of Material and Process Engineering. All rights reserved.