Superior tensile and fatigue properties of Incoloy 901 repair welds produced by direct energy deposition

Abstract

A comparative study of the microstructure and mechanical properties (tensile and high-cycle-fatigue) of Incoloy 901 repair welds produced via direct-energy-deposition (DED) and conventional tungsten-inert-gas (TIG) welding was conducted. The initial microstructure of the cast Incoloy 901 substrate exhibited a γ-matrix (an average grain size of ∼235 μm) and spherical γ′ particles (a size of <20 nm). Most of the grain boundaries were not decorated with precipitates, and bulky Mo-rich and Ti-rich MC carbides were precipitated in the γ-matrix. Following DED, the fusion zone exhibited finer dendritic grains and a lower degree of microsegregation than the TIG repair coupon; this resulted from the higher cooling rate associated with lower heat input than TIG. In addition, the width of heat-affected zone (HAZ) was much narrower in the DED repair coupon (0.23 mm) than that in the TIG repair coupon (2.5 mm) in which the dissolution of γ′ precipitates occurred during TIG weld thermal cycles. This microstructural characteristic resulted in different hardness traverses revealing overmatching and undermatching in the DED and TIG repair coupons, respectively. The DED repair coupons exhibited superior tensile and high-cycle-fatigue properties to the TIG repair coupons. The wide HAZ of the TIG repair coupon was preferentially fractured by tensile and cyclic loading because of the easy gliding of the dislocations. Meanwhile, the DED repair coupon exhibited refined slip bands, which facilitated homogeneous deformation, leading to base-metal fracture. These results demonstrate the potential of the DED repair process to achieve excellent mechanical properties comparable to those of the base metal.