Gas tungsten arc (GTA) welds on Al-4 wt% Cu alloys were investigated to determine effects of gravitational orientation on the weld solidification behavior. A bead-on-plate welding was performed by varying the relation between the arc translation direction and gravity vector, e. g., parallel-up, parallel-down, and perpendicular orientations. A solidification rate (V-S) was calculated from the measured grain orientation, and a thermal gradient (G(L)) was estimated from the observed weld pool shape following a linear relation. A primary dendrite spacing (lambda(1)) decreased continuously from the s-l boundary to the weld pool surface regardless of the gravitational orientations. Larger lambda(1) for the parallel-up weld was observed near the boundary and surface than that of the perpendicular and parallel-down welds, which is believed to be associated with a smaller G(L) due to larger weld pool dimension and with different solidification morphology. A solidification morphology and orientation in the perpendicular and parallel-up welds was comparable with a loss of columnar directionality near the weld surface and a continuous grain orientation. However, the parallel-down weld exhibited more columnar structure near the surface, which might be associated with the larger G(L) and relatively mild convection flows. Outward convection flows in the parallel-down weld might be inhibited because of its reverse direction with respect to the gravity vector. This resulted in abnormal 'S' shape of the trailing s-l interface and the V-S, which was receded toward the weld pool center. Based on these findings, significant influence of gravitational orientation resulted in the variation on the weld pool shape associated with convection flows, which in turn affected solidification orientation/morphology and the primary dendrite spacing. (C) 2003 Kluwer Academic Publishers.