Process, microstructure and property relationships in dissimilar nickel base superalloy inertia friction welds

Abstract

The objective of this research was to study the difference in microstructure and high temperature fatigue behaviour between three RR1000 to IN718 inertia friction welds and to provide further understanding of the inertia friction welding process. Between the three welds no significant differences in the weld microstructures were found. Also high temperature fatigue crack growth tests within 0.3 mm of the weld interface, showed no difference in crack growth rate due to the three different sets of welding parameters. The cracks were found to propagate from RR1000 through the weld interface into IN718 passing a 10-30 mm wide zone, allowing higher crack growth rates. Fractographic studies have shown that these higher crack growth rates are caused by a higher tendency to intergranular cracking. In the present welds a semi-solid weld contact layer of 10 - 30 mm thickness developed, being an intermediate alloy of varying composition of the two base materials. In the surrounding material constitutional liquation of NbC particles in IN718 and of primary γ′ in RR1000 occurred. Similar welding process characteristics during the final second of the three welding cycles resulted in the observed similar weld microstructure and high temperature fatigue properties. It was further found that the local fatigue crack growth rate increase occurs in the weld contact layer.