Structural evolution and weld formation during ultrasonic welding of nanocrystalline alloys

Nanocrystalline metals have high strengths and high wear resistances, making them ideal for use in spaceflight applications. However, they also exhibit rapid grain growth at elevated temperatures, which makes it difficult to preserve their structure and unique properties when they are joined with fusion-based welding techniques. Motivated by this problem we have attempted to bond nanocrystalline feedstock using a low-temperature solid-phase welding process termed ultrasonic welding. In ultrasonic welding the workpieces are clamped between a stationary anvil and a sonotrode that oscillates at ultrasonic frequencies transverse to the loading axis. This shearing motion creates a metallurgical bond by disrupting the surface oxides at the weld interface. Although ultrasonic welding is a solid-phase joining process, the ultrasonic vibrations initiate gross slip at the weld interface which causes frictional heating. In this work we analyze the effects of this frictional heating on the structure and strength of ultrasonic welds between nanocrystalline foils. By combining a frictional heating model with classical models of grain growth and junction growth, we relate the process variables to the instantaneous grain size and the real area of contact inside the weld nugget. We then identify parameter sets that can suppress grain growth while still giving strong welds.