Characteristics of microstructures in laser spot welds of a sintered NdFeB permanent magnet under different welding modes

Laser spot welds were made on sintered NdFeB permanent magnets under deep penetration and heat conduction modes using 2kW continuous fiber laser. The microstructures of the welds show that the HAZ(heat affected zones) of both welds are rich of cracks caused by the melting of grain boundary phase, and the fusion zones are both composed by Nd2Fe14B and α-Fe, moreover the nuggets of both welds are composed by submicron ultrafine equiaxed grains of Nd2Fe14B. Differently, the α-Fe columnar dendrites exist in the upper surface of the nugget of deep penetration weld but not on that of heat conduction weld. Besides, Nd-rich phase is found in the periphery of the upper surface of the nugget for the deep penetration spot welding, while it is found in the center of the upper surface of the nugget for the heat conduction spot welding. The reasons that lead to such microstructures are analyzed briefly. © Springer-Verlag London 2013.