Reduction of Laves phase in nickel-alloy welding process under ultrasonic Ampère’s force

Abstract

In order to reduce the brittle Laves phase in nickel-alloy overlay welding from the outset, a numerical model has been established to predict Laves phase formation in ultrasonic field, by considering the molten pool as a liquid electric conductor which is composed of innumerable “electric wires”. The model predictions were verified experimentally. The prepared weldments were characterised using scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and electron probe microanalysing (EPMA). Both simulation and experiments showed that the niobium distribution became more homogenous in nickel matrix and the volume fraction of Laves phase was decreased by over 90%, when 30kHz ultrasound was applied. It has been revealed that the high-frequency current, passing through the tiny “electric wires”, will generate ultrasonic field right inside the molten pool. The inside ultrasonic Ampère’s force is the predominant factor that directly makes the liquid metal vibrated at high frequency. This effect alleviates niobium segregation and makes the nickel dendrites more branching and tortuous in overlay weld, which reduces the amount of Laves phase.