Effect of duty cycle on microstructure and mechanical properties of pulsed GTAW lap joint of Invar

Abstract 0.7 mm and 1.0 mm Invar sheets in vertical position were lap-jointed by pulsed gas tungsten arc welding (pulsed GTAW). The macro morphology, microstructure and mechanical properties were studied. The feature size La (the largest weld width of lower sheet), P (the largest weld penetration of lower sheet) and R (the shortest distance from the cutting-edge of the uncombined sheets to the weld surface) are directly proportional to duty cycle. The fusion zone (FZ) is completely composed of columnar grains in which two forms of substructure are observed. Series of layer lines, which spread roughly parallel to the partially melted zone (PMZ), periodically appear in the FZ. No apparent precipitation, phase transformation and welding defects such as hot cracks are found in heat affected zone (HAZ) and FZ. Microhardness tests reveal that the microhardness of HAZ can reach more than 85% of base metal (BM), and the microhardness of FZ can reach more than 90% of BM. The effective bonding area has dominant influence on the tensile shear force in this study and the maximum tensile shear force is 4425N.