Thermal Relaxation of Residual Stresses in Nickel-Based Superalloy Inertia Friction Welds M. KARADGE, B. GRANT, P.J. WITHERS, G. BAXTER, and M. PREUSS This article describes an experimental study aimed at characterizing the extent of residual stress relaxation during thermal treatment of inertia friction-welded alloy 720Li nickel-based super- alloy welded tubular rings. In the as-welded condition, yield level tensile hoop stresses were found by neutron diffraction in the weld region along with axial bending stresses (tensile toward the inner diameter (ID)/compressive toward the outer). The evolution of these residual stress levels during postweld heat treatment (PWHT) was mapped experimentally over the weld cross section. After 8 hours of PWHT, the axial stresses relaxed by 70 pct, whereas the hoop stresses reduced by only 50 pct. Some scatter of residual stress evolution was found between samples, particularly for the axial stress direction. This was attributed to substandard tooling to grip the rings. The results on subscale samples were transferred to a full-scale aeroengine (650-mm diameter) compressor drum assembly that was postweld heat treated for 8 hours. It was found that the residual stresses, particularly in the axial direction, were noticeably lower in this full- scale weld component compared to the subscale weld heat treated for the same time. The differences seem to be best rationalized by the different standards of jigging used during joining these two types of welds. DOI: 10.1007/s11661-011-0613-3 Ó The Minerals, Metals & Materials Society and ASM International 2011 I. INTRODUCTION INERTIA friction welding (IFW) is a solid-state welding method suitable for joining components that are symmetrical about an axis of rotation, e.g., tubes, shafts, and discs. In IFW, one of the workpieces is connected to a flywheel and the other is restrained from rotating. The flywheel is accelerated to a predetermined rotational speed, then disengaged, and the workpieces are forged together under pressure. [1] The kinetic energy stored in the rotating flywheel is dissipated as heat through friction/plastic work. The temperature rise due to this heat released at the weld interface results in softening of the adjacent alloy. This soft material gets ejected as flash under the forging pressure employed in the IFW process. Eventually, the flywheel stops when the kinetic energy of the flywheel is exhausted, leading to the formation of a solid-state bond at the softened interface, before the axial force (pressure) between the compo- nents is removed. This metallurgical bond between the components is achieved through direct metal to metal contact at the joint interface in the absence of any large scale melting phenomenon. Friction welding thus avoids the problems usually associated with melting and resolidification processes (e.g., strong solute segre- gation, liquation cracking, etc.) and allows for the formation of a narrow heat-affected zone (HAZ) due to the extremely localized nature of heat generation/ plastic deformation. [2] High-performance polycrystalline nickel-base super- alloys employed in new generation aeroengines have a significantly higher volume fraction of c¢ phase than conventional superalloys. These alloys are usually very difficult to weld and are prone to microcracking upon solidification during conventional fusion welding. [3] Friction welding is thus proving to be a very efficient technique to join these high-performance alloys. One such structural alloy is polycrystalline alloy 720Li, which is used primarily for turbine discs having service temperatures in the region 923 K to 973 K (650 °C to 700 °C). [4] Since the IFW process is completed within a few seconds, the material in the HAZ experiences extreme heating and cooling rates. Preuss et al. [5–7] demonstrated that, under these extreme thermomechan- ical conditions, drastic changes can occur in the micro- structure across the weld line, accompanied by very high residual stresses reaching levels approaching the yield strength of the parent alloy (comparing von Mises equivalent stresses with yield stress values from uniaxial tensile tests [8] ). Residual stress is a crucial factor when assessing the integrity of engineering components and welded assem- blies. High residual stress levels can have a dramatic effect on the overall lifetime. Therefore, it is necessary to characterize the nature of residual stresses present and to identify ways to reduce these stresses to acceptable levels without compromising the weld microstructure M. KARADGE, Lead Materials Scientist, formerly with the School of Materials, University of Manchester, Manchester, M1 7HS, United Kingdom, is with General Electric–Global Research, Niskayuna, NY 12309. B. GRANT, Research Fellow, P.J. WITHERS, Professor of Materials, and M. PREUSS, Professor of Metallurgy, are with the School of Materials, University of Manchester. Contact e-mail: michael.preuss@manchester.ac.uk G. BAXTER, Materials Specia- list-Capability Acquisition, is with Rolls-Royce plc, Derby DE24 8BJ, United Kingdom. Manuscript submitted June 8, 2010. Article published online February 1, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 42A, AUGUST 2011—2301