The reasons behind such prior β grain refinement effect, when utilising higher He contents
in the gas shielding mixtures and therefore larger arc powers, might be attributed to a variety
of different mechanisms: 1) higher T gradients promote intensive force convection and mass
transport, breaking primary columnar grains near the S/L interface or promoting a globular
structure by spherical growth; 2) fine solid nuclei will work as “seeds” or nucleation sites
within the liquid and therefore decrease the energy barrier for heterogeneous nucleation; 3)
the CMT dip transfer mechanism induces a stirring effect reducing energy barrier for
homogeneous nucleation; 4) augment of the cooling rate due to the larger specific weld bead
contact areas.