Understanding of Humping Based on Conservation of Volume Flow

A generalised view on the causes leading to humping during high-speed seam welding is proposed. It is based on the fact that, whenever the melt velocity in a welding bead exceeds considerably the welding speed, the weld groove cannot be completely filled by melt at any location and any instant. As a consequence of solidification from the walls of the weld groove, the shape of the free cross section for the melt flow is changed in such a way that the rest melt stream experiences an upwards deflection which initiates the formation of a hump. Conservation of volume in an incompressible fluid requires that in the solidified seam regions of smaller and larger cross sections compared to that of the groove must exist. The initiation of humping is discussed for different situations in laser beam welding characterized by the aspect ratio welding depth over spot diameter. As this figure is typical for the geometry of the melt pool/weld groove, it appears as a meaningful parameter for correlating the onset of humping.