Inertia welding nickel-based superalloy: Part I. Metallurgical characterization
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A NEW generation of nickel-based superalloys having a significantly higher volume fraction of ␥ ⬘ then conventional superalloys has been developed to meet the demand for better high-temperature properties. These alloys are usually very difficult to weld and are prone to microcracking as solidification takes place during welding.[1,2] Therefore, inertia welding is a very attractive welding process because it does not involve any melting, provided optimum welding parameters are chosen. Furthermore, it is more suitable for mass production than electron-beam welding.[3] In inertia welding, an axisymmetric workpiece is rotated to a specific speed and then a second stationary workpiece is forced into frictional engagement with the first. The frictional heat is sufficient to soften the two components in the weld region without introducing melting. In some respects, inertia welding resembles a high-speed forging process in which the upset (the extent to which the workpieces are foreshortened) creates the flash but with only the near-weld zone reaching forging temperatures. Because of the extreme thermomechanical history, the microstructure is heavily modified in the heat-affected zone (HAZ) of a joint. It is generally believed that, at the weld line, dynamic recrystallization takes place.[4] Furthermore, because of the steep temperature and plastic-deformation gradients in the HAZ, it is possible that near the weld line a work-hardened zone will be present. It is important to characterize the microstructure in this area because it will M. PREUSS, Research Fellow, and P.J. WITHERS, Professor, are with the Manchester Materials Science Centre, University of Manchester and UMIST, Manchester M1 7HS, United Kingdom. Contact e-mail: michael. [email protected] J.W.L. PANG, Research Fellow, is with the Oak Ridge National Laboratory, Oak Ridge, TN 37830. G.J. BAXTER, Process Metallurgist, is with Rolls-Royce plc., Derby DE24 8BJ, United Kingdom. Manuscript submitted November 19, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
have a significant impact on the mechanical properties of the assembly. This article focuses on the variations in microstructure as a function of position and heat treatment caused by inertia welding RR1000. A companion article[5] describes the concomitant variation of residual stress in the same welds. RR1000 is a recently developed powder-processed, highstrength nickel-based superalloy. It is being considered for high-pressure turbine disks in the next generation of gas turbine aeroengines.[6] It comprises a high volume fraction of primary ␥ ⬘ (1 to 3 m), secondary ␥ ⬘ (50 to 500 nm), and tertiary ␥ ⬘ (5 to 30 nm). In the parent alloy, particles of ␥ ⬘, which remain undissolved during the solution heattreatment stage (primary ␥ ⬘), delineate ␥ -grain boundaries, while intragranular ␥ ⬘ occurs upon cooling in a bimodal distribution (secondary and tertiary ␥ ⬘). Whereas particles of secondary ␥ ⬘ nucleate at an early stage during quenching from the solution temperature, tertiary ␥ ⬘ nucleates at a lower temper
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