Simultaneously enhanced strength and ductility of TIG welds in Inconel 718 super-alloy via ultrasonic pulse current

This paper presents a novel self-induced ultrasonic arc welding (U-TIG) with high-efficient, which improves tensile strength and ductility of TIG welds in Inconel 718 super-alloy simultaneously. The effects of ultrasonic frequencies on arc shape, weld forming and weld penetration characteristics were systematically studied by high-speed camera and optical microscope. The characteristics of grains and precipitated phases in the weld metal were quantitatively analyzed by backscattered electron microscope, electron backscatter diffraction. The evolution models of grains refinement and reduction-dispersion with Laves phase were constructed based on molecular dynamics model and numerical simulation. The evolution diagrams of microstructures in tensile stretching revealed the mechanism of enhancing strength and ductility. Ultrasonic pulse current not only reduces and refines Laves phase, respectively by 81.8% and 52.2%, but refines grains (almost 33%) with higher orientation. The excellent distribution of the finest grains and Laves phase with dispersion is corresponding to ultrasonic frequency nearly 80 kHz, which benefits ductility enhancement of welds from 311% to 410%. Grains refinement leads to the increasing of grains boundary length and zigzag, meanwhile, the morphology of Laves phase changes from long-strip shape to refined pebble one, which is responsible for tensile strength improvement, closing to base metal of 92%.