Ultrasonic-induced deformation nanostructures in coarse-grained aluminum wires at room temperature

Ultrasonic additive manufacturing (UAM) is now extensively used in advanced materials processing since it applies ultrasonic frequency vibrations locally pressed on the metal parts to make them deformed and/or joined. In the field of electronic packaging, wirebonding is such a kind of type, especially such as ultrasonic wedge bonding, which combines ultrasonic and pressure only, without heating during the bonding process. Through high frequency vibrations, the metal ribbons or wires were interconnected with substrates within mille-seconds. Without ultrasounds, commonly it needs more heat or pressures. One of the most important mechanisms is focused on the effects of ultrasonic-induced deformation structures in the metals. Previously, we have observed the evolution of the textures and grains, but there have not been found any data on the report of nanoscale features of the metal itself. Therefore, in the present study, by using high-resolution transmission electron microscope (HR-TEM) pure coarse-grained aluminum was chosen for investigating the deformation microstructures and mechanism when exposed under ultrasonic vibrations at room temperature. It was found that deformed pure aluminum wire has equiaxial grain shape, and recrystallization grains are evident since there has little defects in them. But within some grains, sub-structures can be found. HR-TEM observation reveals that these sub-structures can be defined as sub-grains with diameter of a few nanometers. Further, it is the first that large stacking faults (SFs) with a width longer than 15 nm and even microtwins can be seen clearly. Their distribution mainly aligns parallelly or with an angle of about 70 degree. Increasing ultrasonic power seems enlarge these SFs. Cyclic vibrations contribute to the formation of these SFs/microtwins.