Improvements in bonding through ultrasonic additive manufacturing of titanium by stabilizing displacive phase transformations

The development of advanced materials to operate in extreme environments (temperature, pressure, strain rate, irradiation, etc.) is essential to meet future energy challenges. In addition to being an advanced solid state bonding technique, ultrasonic additive manufacturing (UAM) can be considered as a type of extreme environment due to the high strain and high strain rate deformation that is created. Understanding the physical processes that occur in this extreme environment can be valuable to creating new desirable microstructures and/or phase changes. Although UAM has demonstrated great success in bonding a variety of materials, the underlying science mechanisms controlling the bonding are not well quantified. We observed crystal structure changes from hexagonal closed packed (HCP) to body centered cubic (BCC) in Ti and Ti alloy specimens occurring within ∼0.5 s following UAM bonding with an estimated peak temperature of ∼400 °C. Extensive interdiffusion of elements (0.2 µm – 2 µm depending on location) occurred that does not conform to thermal equilibrium bulk or grain boundary diffusion. We present evidence that a significant concentration of deformation induced vacancies X_v (between 10⁻⁴ - 10⁻⁶ atomic fraction) was created during UAM, approximately ten orders of magnitude higher than the X_v value of ∼10⁻¹⁵ expected for thermal equilibrium conditions. This caused pronounced metallurgical changes including rapid elemental diffusion, strain induced phase transformation, and bonding. We examined this UAM-induced severe plastic deformation on a variety of materials and performed uncertainty calculations from the measurements.