Monitoring and repair of defects in ultrasonic additive manufacturing

Ultrasonic additive manufacturing (UAM) involves ultrasonic welding of similar or dissimilar metal foils on top of a base substrate. UAM can produce solid consolidated structures under optimal processing conditions. However, inter-layer defects such as delamination/kissing bonds (type 1) and inter-track (type 2) defects are common. The authors previously developed an ultrasonic nondestructive evaluation (NDE) monitoring methodology to quantify layer-bonding stiffness modeled as an interfacial spring. In this study, ultrasonic NDE is used to monitor the evolution of type 1 defects in a UAM component divided into two zones. The first represents the base/build interface comprising of the first few layers on the base substrate, and the second region represents the bulk of the UAM stack. A mechanism for the formation and evolution of type 1 defects was proposed based on NDE and optical examination. Type 2 defects are often more catastrophic and are challenging to repair. In the present work, a novel solid-state repair technique using friction stir processing (FSP) was used to repair typical UAM defects. The use of FSP ensures that the microstructural advantages of UAM are retained while improving the part quality. Two modes of FSP were designed—FSP from above for repair of inter-track (type 2) defects and FSP from below the base for the repair of base/build (type 1a) defects. The results of this study pave the way towards the development of an integrated solid-state additive manufacturing system with UAM as the primary bonding mechanism and FSP as an enhancement and repair tool.