The influence of Laves phases on the high-cycle fatigue behavior of laser additive manufactured Inconel 718

Abstract

In this paper, a comparative study of high-cycle fatigue tests (T=650 °C, f=110 Hz, R=0.1, K_t=1) were carried out with wrought Inconel 718 and LAMed Inconel 718. The results show that the influences of the Laves phases on high-cycle fatigue properties are based on the applied stress amplitudes. At a low stress amplitude, most of the Laves phases held their original morphologies. The fatigue cracks stopped in front or detoured around them, which means that the unbroken Laves phases play an important role in hindering crack propagation. In this way, the high-cycle fatigue life of LAMed Inconel 718 was superior to that of wrought Inconel 718. However, at a high stress amplitude almost all of the Laves phases in the crack propagation region splintered into smaller fragments, parts of which separated from the austenite matrix. Microscopic holes or cracks were formed at the interface, which provided passages for the fatigue cracks to propagate. In this case, the Laves phases were harmful, leading to the degradation of fatigue performance in LAMed Inconel 718 compared with wrought Inconel 718.

Introduction

Inconel 718 superalloy is one of the most important materials used in making aero-engine blisk due to its excellent, comprehensive mechanical properties [1], [2], [3]. However, when such an integral structure as a monolithic compressor disk is mis-machined during the manufacturing process or damaged by a foreign object in service, whole-rollscrapping is at risk, which causes massive losses in time and economy. Laser Additive Manufacturing (LAM) technology provides an effective way to restore both the geometrical and mechanical properties of the damaged components.

A large amount of Laves phases are formed at the end of the solidification process due to micro-segregation of the alloying elements (especially high atomic diameter elements, such as niobium, molybdenum, etc.) during the LAM process [4], [5], [6]. It is generally accepted that hard, brittle Laves phases are bad for Inconel 718 [7], [8], [9], [10] (tensile, stress rupture and low-cycle fatigue properties). They are the main nucleation points for the formation of microscopic holes during tensile [7] and persistent testing [8], and they aid in easy crack initiation and propagation during low-cycle fatigue tests [9]. A heat treatment administered to the solution has always been used to eliminate Laves phases in Laser Additive Manufactured (LAMed) Inconel 718 [11], [12], [13], [14]. However, it is not suitable for components repaired by a laser because the high temperature can deteriorate the wrought substrate. Therefore, the Laves phases are left in the microstructure.

High-cycle fatigue failure is one of the main failure modes found in practical industry applications [15]. Therefore, it is important to investigate the high-cycle fatigue behavior of Inconel 718. Ono [16] found that high-cycle fatigue cracks predominantly initiated from coarse Nb-enriched carbides and faceted structures that mainly corresponded to the carbides at cryogenic temperatures. However, no inclusions, such as niobium-enriched MC-type carbides, in the Laves phases or δ phases were observed at the fatigue crack initiation site at room temperature, which meant that these precipitations had nothing to do with the origin of the cracking [17]. Sivaprasad [18] determined that cracks could detour around Laves phases, but he did not deeply study the influence of Laves phases on crack propagation. There have been a number of additional studies on crack propagation in wrought Inconel 718 [19], [20], [21].

As seen above, most research has studied wrought and welded Inconel 718. With respect to LAMed Inconel 718 with the Laves phases remaining in the microstructure, almost no investigations have been carried out. Moreover, Laves phases have excellent high temperature mechanical properties [22]. Therefore, it is necessary to investigate the effect of Laves phases on the properties affecting high-cycle fatigue at high temperature for the better use of LAM technology for laser repairing components. In this paper, A comparative study of high-cycle fatigue tests (T=650 °C, f=110 Hz, R=0.1, Kt=1) were carried out with wrought Inconel 718 and LAMed Inconel 718. The influence of the Laves phases on crack propagation was investigated.