Grain size reduction in weld fusion zones confers the advantages of an increased resistance to solidification cracking and an improvement in mechanical properties. Oscillation of the welding arc through an imposed alternating magnetic field is one of several approaches to modify weld solidification structures. In this study, gas tungsten arc welds were produced in two high strength, age hardenable aluminum alloys with and without an external magnetic field. Metallographic characterization revealed the degree of structural refinement produced by magnetic arc oscillation. The decrease in grain size was found to increase tensile elongation, while the effect on strength and age hardening response was only meager. The improvement in ductility was partially maintained in the peak aged condition also.