  Internal flow behaviour and melt-pool surface oscillations during arc welding
are complex and not yet fully understood. In the present work, high-fidelity
numerical simulations are employed to describe the effects of welding position,
sulphur concentration (60-300 ppm) and travel speed (1.25-5 mm/s) on molten
metal flow dynamics in fully-penetrated melt-pools. A wavelet transform is
implemented to obtain time-resolved frequency spectra of the oscillation
signals, which overcomes the shortcomings of the Fourier transform in rendering
time resolution of the frequency spectra. Comparing the results of the present
numerical calculations with available analytical and experimental datasets, the
robustness of the proposed approach in predicting melt-pool oscillations is
demonstrated. The results reveal that changes in the surface morphology of the
pool resulting from a change in welding position alter the spatial distribution
of arc forces and power-density applied to the molten material, and in turn
affect flow patterns in the pool. Under similar welding conditions, changing
the sulphur concentration affects the Marangoni flow pattern, and increasing
the travel speed decreases the size of the pool and increases the offset
between top and bottom melt-pool surfaces, affecting the flow structures
(vortex formation) on the surface. Variations in the internal flow pattern
affect the evolution of melt-pool shape and its surface oscillations.
