Mixed Inconel Alloy 718 Inertia Welds for Rotating Applications: Microstructures and Mechanical Properties

Abstract

IN718 is the work-horse nickel-iron-chromium alloy for a variety of parts for aero-engine applications such as disks, shafts, blades, vanes, casings and fasteners due to a good combination of relevant mechanical properties, good corrosion resistance, easy fabricability and reasonable cost. The conditions in aero-engines often require joining rotating parts which are made out of different materials, for example due to temperature limitations of one material. Besides the possibility of bolting the parts together, welding can lead to some advantages and is used in the compressor and high-pressure turbine section of aero-engines. For joining nickel-super-alloys and especially different nickel-alloys inertia welding represents often the only available possibility. The following work summarizes investigations of inertia welding IN718 with alloys INCOLOY® alloy 909, UDIMET® 720LI, Rene® 88DT and itself. The influence of various post-weld heat-treatments on the microstructure and some relevant mechanical properties as tensile and low cycle fatigue properties is described. The focus in this work is on the IN718 part of the investigated weld. Introduction Inconel 718, or short IN718, was developed in the 1950s for gas turbine applications and is still today the work-horse nickel-iron-chromium alloy for a variety of parts for aero-engine applications such as disks, shafts, blades, vanes, casings and fasteners. IN718 obtains its desired properties through a heat-treatment, which leads primarily to the precipitation of gamma-prime (γ’) and gamma-double-prime (γ’’) for strengthening, and delta (δ) phase for grain size control [1]. The standard processing, as used for the IN718 material in this work, involves forging, solutionizing in the temperature range 955°C to 985°C and two step aging; 720°C, 8h, cooling to 620°C with 50°C/h and holding at 620°C for another 8h followed by air cooling. Parts for rotating applications in aero-engines such as disks or BLISKs are normally combined to a bigger component such as a rotor-drum as shown in figure 1. Depending on the temperatures and/or loads within the component this can necessitate joining dissimilar materials. Material combinations realized at MTU Aero Engines for such applications are among others IN718 with IN718, UDIMET® 720LI (U 720LI), and Rene® 88DT (Rene88DT). Furthermore, design 649 Superalloys 718, 625, 706 and Derivatives 2005 Edited by E.A. Loria TMS (The Minerals, Metals & Materials Society), 2005 limitations can lead to the need of dual material parts, as seen in the tieshaft of one of MTU’s latest high pressure compressor designs. Within this part INCOLOY® alloy 909 (Incoloy909) and IN718 are combined to meet the requirements for strength and thermal expansion [2]. Often the only possible joining method is inertia friction welding due to material selection, design and/or space considerations. Inertia friction welding (inertia welding) is a pressure welding process allowing high strength joints being produced with high reproducibility. The necessary weld-heat is produced by friction and the bonding results from complex plastic deformation below the melting point. Welding of dissimilar materials is possible and the strength of these joints reaches or even exceeds that of the base materials being bonded. In the application area of rotating components for aero-engines, inertia welding is a common practice to join titanium and nickel alloys [3]. Figure 1. Example of an inertia welded rotor-drum consisting of 5 single parts The following work summarizes investigations of inertia welding IN718 with alloys Incoloy909, U 720LI, Rene88DT and itself. The influence of various post-weld heat-treatments on the microstructure and hardness and some relevant mechanical properties as tensile and low cycle fatigue properties are described. The focus in this work is on the IN718 part of the investigated welds.