Joining dissimilar metals with superior quality is important to provide tailored, lightweight and cost-efficient components. Expensive and dura-ble materials are exceptionally used where the cheaper material would not withstand the requirements. With laser beam welding, dissimilar metals can already be joined with high precision, low heat input and a customizable mixing degree. Introducing ultrasonic excitation into the weld pool is a promising approach for further improvements like customizing the solidification morphology and avoiding weld defects. The experiments are carried out with round bars of 30 mm diameter made of 1.4301 steel alloy and 2.4856 nickel base alloy. Ultrasonic assisted laser beam butt welding is conducted on rotating specimens with a laser beam power of 7.75 kW and a welding speed of 0.95 m/min. The specimens are evaluated by metallographic cross sections, hardness measurements and energy-dispersive X-ray spectroscopy (EDX). The ultrasound is used to excite an eigenmode of the sample and the weld position is varied at stress- and displacement-nodes. Two different mechanisms of acoustic grain refinement are revealed. Heterogeneous nucleation is fostered in weld seams which are posi-tioned in stress-nodes and the fragmentation of dendrites is fostered in displacement-nodes. The welds’ chemical compositions correspond with the change of solidification morphology.