Inertia Friction Welding Dissimilar Nickel-Based Superalloys Alloy 720Li to IN718 Z.W. HUANG, H.Y. LI, M. PREUSS, M. KARADGE, P. BOWEN, S. BRAY, and G. BAXTER This article describes a comprehensive microstructural characterization of an inertia friction welded joint between nickel-based superalloys 720Li and IN718. The investigation has been carried out on both as-welded and postweld heat-treated conditions. The detailed metallo- graphic analysis has enabled the relation of hardness profiles across inertia-welded alloy 720Li to IN718 and morphological changes of the precipitates present. The work demonstrates that inertia friction welding (IFW) 720Li to IN718 results in a weld free of micropores and micro- cracks and no significant chemical migration across the weld line. However, substantial dif- ferences in terms of grain structure and precipitation phase distribution variations are observed on each side of the dissimilar weld. The high c¢ volume fraction alloy 720Li exhibits a wider heat-affected zone than the mainly c¢¢ strengthened IN718. Alloy 720Li displays only a small hardness trough near the weld line in the as-welded condition due to the depletion of c¢, while c†- strengthened IN718 shows a soft precipitation-free weld region. Postweld heat treatment (PWHT) of the dissimilar weld at 760 °C, a typical annealing temperature for alloy 720Li, results in an overmatch of the heat-affected zone in both sides of the weld. The comparison of the as-welded and postweld heat-treated condition also reveals that IN718 is in an overaged condition after the stress relief treatment. DOI: 10.1007/s11661-007-9194-6 Ó The Minerals, Metals & Materials Society and ASM International 2007 I. INTRODUCTION INERTIA friction welding (IFW) is a solid-state welding process, which allows joining nickel-base super- alloys with a high c¢ volume fraction or different types of nickel-base superalloys. During IFW, one part is attached to a rotating flywheel, while the second, nonrotating part is forced into contact with the rotating one under hydraulic pressure. The kinetic energy stored in the rotating flywheel is then transformed into heat at the interface of the two parts. In this way, a sufficiently high temperature is produced at the interface, which, together with the torsional forces and axial pressure, results in material being ejected, and a bond is formed. The IFW is considered an attractive manufacturing process because of its suitability for mass production. More importantly, unlike electron beam welding, laser welding, and other welding technologies, the IFW process is a solid-state joining process. As a result, problems related to liquation such as occurrence of microfissures, [1,2,3] precipitation of detrimental phases, intergranular hot cracking, [4–7] and porosity [8] can be either avoided or diminished. As a consequence, high c¢ volume fraction nickel-base superalloys, considered to be unweldable by fusion welding techniques, can be joined by IFW. The IFW is widely used in the automotive and power generation industry. [9,10] In the aeroengine indus- try, it is mainly used to join high-temperature materials such as titanium and nickel-base superalloys. [11,12] The IFW of two different types of nickel-base superalloys, namely, alloy 720Li to IN718, has been undertaken in this study. Such dissimilar joints are highly attractive for the aeroengine industry as they result in significant weight reductions while maintaining the mechanical properties and required temperature capability. The aim of this study is to improve the understanding of inertia friction joining two types of high-temperature alloys by mapping microhardness and microstructure variations across the weld region of the dissimilar material combination. Previous microstructural characterization of similar friction-welded nickel-base superalloys has demon- strated that, due to the thermal history and heavy plastic deformation in the near weld region, a dramatic variation of the microstructure is observed, resulting in partial or full dissolution of c¢ precipitates in this region. [12,13] Dissolution of c† in this region is also expected, because the temperature reached near the weld line is in the range of the forging temperature. [9] Studies focusing on the residual stress characterization of such inertia friction welds have highlighted the high levels of tensile stresses observed in the weld region and the importance of developing an appropriate stress relief temperature. [14,15] The present work is one of the first Z.W. HUANG, Senior Research Fellow, H.Y. LI, Research Fellow, and P. BOWEN, Professor, are with the Department of Metallurgy and Materials, School of Engineering, The University of Birmingham, Birmingham B15 2TT, United Kingdom. Contact e-mail: z.w.huang@ bham.ac.uk M. PREUSS, Lecturer in Materials Performance, and M. KARADGE, Research Fellow, are with the School of Materials, University of Manchester, Manchester, M1 7HS, United King- dom. S. BRAY, Friction Welding Sprcialist and G. BAXTER, Intertia Welding Sprcialist, are with Rolls-Royce plc., Elton Road, Derby DE24 8BJ, United Kingdom. Manuscript submitted June 15, 2006. Article published online June 26, 2007. 1608—VOLUME 38A, JULY 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A