On the stick-slip dynamics in ultrasonic additive manufacturing

Ultrasonic Additive Manufacturing (UAM) is a promising additive manufacturing process used to build complex structures by joining thin metal films. Layers of metal foil are joined by being compressed under moderate pressure using a rolling ultrasonic horn which vibrates at a high frequency in a direction transverse to the rolling motion. Foil-foil interfacial motion combined with moderate heating and pressure results in pure metal contact which causes atomic bonding. Unfortunately, large scale implementation of the UAM process is impeded by the presence of a range of build heights within which bonding between a new foil and the build feature cannot be initiated. This paper simplifies the UAM process into a lumped-parameter dry-friction oscillator and shows that complex stick-slip motions of the build feature near or above its resonance frequency are responsible for bond degradation. Specifically, as the height of the feature increases and its first modal frequency approaches the excitation frequency of the sonotrode, the relative motions between the tape and the feature become either pure stick or aperiodic. Pure stick responses, where the tape and the feature stick together, prevent interfacial motions necessary for bonding. Aperiodic motions, on the other hand, consist of slip trajectories interrupted by long stick intervals resulting in velocity bursts and nonuniform energy distribution at the interface. The adopted model and associated analysis also indicate that periodic uniform responses essential for bonding are always recovered once the critical build height is surpassed. These findings corroborate previous experimental results that demonstrate bonding is hard to initiate near or slightly below the problematic height but can be reinitiated above it.