Site-Dependent Tension Properties of Inertia Friction-Welded Joints Made From Dissimilar Ni-based Superalloys

Volume Site-Dependent Tension Properties of Inertia Friction-Welded Joints Made From Dissimilar Ni-based Superalloys O.N. Senkov 0 D.W. Mahaffey 0 S.L. Semiatin 0 C. Woodward 0 0 O.N. Senkov , D.W. Mahaffey, S.L. Semiatin, and C. Woodward , Air Force Research Laboratory , Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433. Contact , USA Microstructure, tensile properties, and fracture behavior of the inertia friction weld joints of dissimilar superalloys, cast Mar-M247 and wrought LSHR, were studied to assess the weld quality. Tensile tests were conducted at 23 and 704 C on the samples containing different areas of the weld interface of the same welded material. The stress-strain curves were registered at different axial distances from the weld interface. In all tested samples, plastic deformation was localized on Mar-M247 side, outside the heat-affected zone (HAZ), and the resistance to plastic deformation of Mar-M247 increased with a decrease in the distance from the weld interface inside HAZ. Only elastic deformation occurred on the LSHR side. Fracture occurred on the Mar-M247 side, outside HAZ, or at the weld interface. In the latter case, welding defects in the form of clusters of nanometer-sized oxide and carbide particles were observed at the fracture surfaces. These results revealed that the IFW process is capable of producing the weld joints between Mar-M247 and LSHR with the fracture strength higher than that of Mar-M247. However, optimization of the IFW processing parameters is required to minimize clustering of oxide/carbide particles at the weld interface in this alloy pair. inertia friction welding; LSHR; Mar-M247; microstructure; Ni superalloy; tension properties; welding defects 1. Introduction Nickel-based superalloys are among the most important structural materials for use in high temperature applications (Ref 1). Fine-grained wrought and/or powder metallurgy (PM) superalloys, such as IN100 or LSHR, possess high strength at temperatures £ 700-760 C, but have insufficient creep resistance at higher temperatures. Single-crystal or coarse-grained cast superalloys, such as CMSX-10 or Mar-M247, have outstanding creep resistance at temperatures £1000 C, but have almost half the tensile strength of the PM superalloys below 700-800 C (Ref 2, 3). Several advanced applications require different sections of a single structure to operate in very different temperature and loading conditions. An attractive way to fulfill this requirement is to join different superalloys in one structure (Ref 4). In such a structure, sections operating under high loading conditions, but at lower temperatures, are made of a PM/wrought alloy, while sections requiring outstanding creep resistance at higher temperatures are made of a cast alloy. Solid-state friction welding processes, including inertia friction welding (IFW), are considered suitable for joining Ni-based superalloys, as solidification-related defects inherent to fusion welding can be avoided (Ref 5-7). Indeed, the IFW process was successfully used to join wrought superalloys such as IN718, RR1000, and 720Li (Ref 7-10). Unfortunately, no reports are yet available on using the IFW process to weld cast superalloys, although a partially successful attempt was made to join these hard-to-weld alloys by linear friction welding (Ref 11). Only two publications report friction welding of cast superalloys to PM/wrought superalloys (Ref 12, 13). IFW was recently used to join a cast Mar-M247 to a forged PM LSHR, and the microstructure, chemical composition, and microhardness of the welded material were determined and correlated to the welding parameters (Ref 13). These alloys are considered as candidate superalloys for an advanced hybrid turbine disk (Ref 4). Tensile strength of both alloys is mainly controlled by c¢ particles and also enhanced by solid solution and grain boundaries. Having much finer c grains and c¢ precipitates, LSHR has almost twice the strength of Mar-M247 at temperatures below 700 C. However, the strength of LSHR rapidly decreases at higher temperatures and above 9001000 C Mar-M247 becomes stronger than LSHR (Ref 4, 14-16). Considerable high temperature softening of LSHR, relative to Mar-M247, is caused by a lower c¢ solvus temperature of LSHR, TLS_S = 1157 C (Ref 17), than that of Mar-M247, TM_S = 1225 C (Ref 18) and, at low strain rates, by grain-boundary sliding in the fine-grained LSHR. As a result, during the IFW process, which occurred above TLS_S but below TM_S (Ref 13), LSHR was softer than Mar-M247 and showed extensive local upset at the joint surface under the applied compression force. Extensive radial flow of the joining material flushes oxides and other contaminants from the joining surfaces toward a flash beyond the original outer diameter and is considered to be beneficial for producing sound metallurgical bonding during IFW (Ref 6, 19). Unfortunately, sluggish radial plastic flow of Mar-M247 du (...truncated)