Reduction of ductility-dip cracking susceptibility by ultrasonic-assisted GTAW

The ability to reduce ductility-dip cracking (DDC) susceptibility in nickel filler metals is one of the keys to their reliable applications in nuclear industry. Here, it is demonstrated that improvement in DDC susceptibility can be attained for nickel filler metal 52M (FM-52M) through the superimposition of ultrasonic field during gas tungsten arc welding (GTAW). A concept of Detrimental Grain Boundary Length (DGBL) has been proposed to elucidate DDC features in two kinds of samples: without ultrasound and with 20 kHz ultrasonic-assisted GTAW. Electron backscattered diffraction, thermo-mechanical simulation tests, scanning electron microscopy and optical microscopy were utilised to quantitatively analyse the microstructure and the cracking features in FM-52M. The results reveal that the external ultrasonic field can refine the grain of FM-52M welds, and can further decrease the DGBL from 2890 mu m to 1700 mu m. This decrement of similar to 41.2% in DGBL arises from the violent stirring induced by ultrasonic cavitation. The suppressed DGBL, under ultrasonic circumstance, shortens the potential path for crack propagation, and improves the DDC resistance in FM-52M. (C) 2016 Elsevier B.V. All rights reserved.