AbstractGrain refinement was reported to improve the mechanical properties and reduce the hot crack susceptibility of aluminum welds. Some of today's filler materials already contain grain refiners. However, their effectiveness will strongly depend on the boundary conditions of welding. The aim of this study is thus to determine the minimum content of Ti/B grain refiner, which is needed to achieve a small grain size with 100% globular grains of the weld metal for different welding conditions, particularly with respect to high welding speed and cooling rate by laser welding. Controlled amounts of TiborTM grain refiner (containing Ti and B in a ratio of 5:1) were introduced into the molten pool of AA 5083 by pre-deposited cast inserts under different welding conditions by laser welding. The results show that, despite the high cooling rate and great melt overheating, the laser weld could be grain refined to a mean grain size at ca. 22μm. The minimal required Tibor™ concentration for complete grain refinement increases with welding speed. The WDX analysis has confirmed that the titanium aluminides are the nucleation site for equiaxed grains.