A Study of Melt Ratio and Microstructure in Dissimilar Fusion Welds

We studied the microstructure in the Ti/Ni laser welds using optical microscope, scanning electron microscope (SEM) and X-ray diffraction (XRD). The welds are asymmetric with respect to the initial butt joint. Despite its higher melting point, Ti melts more than Ni due to its lower thermal conductivity, but because of the high density of Ni the average composition of the weld is Ti–50at%Ni. Composition changes very steeply near the fusion interfaces in both Ti and Ni with associated microstructural changes. The variation is of much lesser magnitude in the rest of the weld, reflecting a well-mixed melt pool on a macroscopic scale. Growth of base metal grains into the weld pool at the fusion interfaces were found to be severely restricted at both Ti and Ni ends. The Ti fusion interface is marked by a band consisting of Ti2Ni dendrites which grow toward the Ti base metal and stopped base metal grains into weld pool. Layers of intermetallic phases (Ni3Ti, Ni3Ti+TiNi, TiNi) form at the Ni fusion interface. TiNi and Ti2Ni are the major phases that appear in the bulk of the weld. From XRD results show that solid state transformation of TiNi leads to the formation of the R-phase and martensite, which reflect the composition heterogeneity in the weld. Non-equilibrium phase Ni2Ti was also observed in the weld