Effect of δ-Phase on the Weldability and the Hot Ductility of Alloy 718

Abstract

In the weld repair cycles of alloy 718 the solution temperature is usually selected low enough to avoid grain growth in wrought material. At such low temperatures, however, δ-phase also precipitates and the amount increases with each repair cycle.It is well understood that the overall ductility is reduced by the presence of the δ-phase in alloy 718 and, as such, may reduce the weldability. But it is also reported that the δ-phase may cause incipient melting in grain boundaries at the heat affected zone, HAZ, through a niobium rich melt fed by the local abundance of this element from attached δ−platelets. The purpose of the present work is to shed light on this issue on the one hand through practical weld experiments under heavily constrained circumstances and on the other through high temperature ductility testing in special hot tensile test equipment by which the hot ductility of re-melted material also was possible to examine. Materials with two levels of δ-phase and with two grain sizes were included.Contrary to what was anticipated it was extremely difficult to provoke cracking even in inhomogeneous cast material at the highest δ−levels in heavily constrained TIG-weld experiments. Also the hot ductility test results, measured as the high temperature ductility recovery temperature and range, indicated that there is no or very limited deterioration by the presence of δ-phase. Anomalous results, however, much lower ductility recovery temperatures, were found in samples with large grain size and a small amount of δ-phase. For re-melted samples several hundred degrees lower recovery temperatures were measured.

Dissolution of delta phase in Ni-based superalloy during linear friction welding: integrated multiphysics computational process modelling

“…They identified rapid weld temperature variation and resultant conversion of γ′ and γ″ phases to δ phase as factors responsible for the decreased mechanical properties of the weld. It was found that after multiple long-term exposures to high-temperature cycles, IN718 welds degrade and become difficult to repair by welding because of the accumulation of Nb-rich δ phase in grain boundaries and the depletion of δ phase in adjacent intergranular regions in the IN718 microstructure [14,15]. Mary and Jahazi found that the loss of the δ phase in IN718 weld joint facilitates dynamic recrystallisation and dynamic recovery of γ grains in the weld joint during LFW [16].…”