There are many reasons for influencing the laser beam induced weld pool. The manipulation of the dynamics, the solidification, the resulting grain size and in the end as a result of the aforementioned influencing the mechanical characteristics of the weld is the aim of different attempts to gain an impact on the melt. Aluminum alloys tend to porosity formation because of different solubility of hydrogen in solid and liquid state. For reliable welds the porosity has to be limited. An ultrasound excitation is one possibility to allow a fast degassing, especially for the considered round bars, for which no welding through is possible for geometric reasons. The presented research shows the influence of the ultrasonic amplitude on the microstructure of laser welded round bars of the aluminum alloy AA6082-T6. Furthermore, the position of the weld pool in the vibration distribution is varied and the influence evaluated. Metallographic cross sections show in analyses the resulting weld characteristic and the microstructure of the weld metal. The grain size and the grain orientation is evaluated for the different ultrasound parameters. Additionally, the summed porosity area is compared to acquire knowledge about the correlation between ultra-sound excitation (with regard to vibration amplitude and position in the vibration distribution) and pore formation.