Plates of Al 6063-T6 (6.35mm thick) were welded using an ER-5356 filler wire. The aim of the experiments was to assess the effects that yield the induction of an axial magnetic field (AMF) during gas metal arc (GMA) welding on the grain structure of the weld metal and on the mechanical properties of the welded joint. Magnetic fields between 0 to 15mT were induced by using a coil which was fed with different current intensities by an external power source. Plots of the grain size distribution showed that applying low magnetic fields homogenizes the grain structure of the weld metal and suppresses the typical columnar-dendritic growth from partially melted grains at the fusion line. The mechanisms involved in these phenomena are discussed with the aid of finite element analysis. Transverse microhardness profiles of the welds revealed a reduction in the loss of hardening in the heat affected zone (HAZ). The loss of hardening after fusion welding in heat treatable aluminum alloys is known as overaging and it is the result of coarsening and transformation of the strengthening phase . It is thought that an electromagnetic interaction between the external magnetic field applied and the inherent magnetic field generated by the direct current of the welding process alters the diffusion process delaying over aging kinetics in the HAZ.