In Situ Micro-mechanical Testing of a PM Nickel-Base Superalloy Weld
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INTRODUCTION
NICKEL-BASED superalloys are an unusual class of metallic materials with exceptional combination of high-temperature strength, toughness, and resistance to degradation in corrosive or oxidizing environments. These materials are widely used in situation requiring superior strength at high temperatures and consequently find extensive application in the hot sections of gas turbine and rocket engines and nuclear reactors. This high temperature strength is usually provided by a distribution of c¢ (Ni3(Al,Ti)) precipitates which hinder dislocation movement. The need for jet engines to burn fuel more effectively at higher temperatures requires the development of nickel-based superalloys containing increasing amounts of the main strengthening, stable, ordered L12 intermetallic (Ni3(Al,Ti)) c¢ phase. Hence, in the last 1 to 2 decades, new polycrystalline nickel-base superalloys have been developed for disk applications operating at high temperatures, containing a significantly higher volume fraction of c¢ than previous superalloys (Waspaloy, IN718, Udimet 720, etc.). These new alloys (RR1000, Rene´ 104, LSHR, Astroloy, Alloy 720Li, N18 etc.) have a c¢ volume fraction of close to 50 pct. However, these alloys are very difficult to fusion weld and are prone to heat-affected zone (HAZ) liquation cracking as solidification takes during welding. In order to produce welds with good joint integrity, inertia friction welding (IFW), a nominally solid state welding process, is always used to join gas turbine parts made from these K.M. OLUWASEGUN, J.O. OLAWALE, M.D. SHITTU, O.O. IGE, Lecturers I, and G.M. OYATOGUN, Senior Lecturer, are with the Department of Materials Science and Engineering, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria. Contact e-mail: [email protected] B.O. MALOMO, Lecturer I, is with the Department of Mechanical Engineering, Obafemi Awolowo University. Manuscript submitted February 6, 2013. Article published online July 22, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys, based on the premise that the joining occurs below the melting point of the bulk material. Furthermore, it is recognized that inertial friction welding is better suited to mass production than electron beam welding since it does not require a vacuum during joining process.[1] But due to the extreme thermo-mechanical history, the microstructure is heavily modified in the HAZ of a joint. Various studies of inertia friction welded nickel-based superalloys in the as-welded condition have shown two different types of hardness profiles in the HAZ. Alloys such as Waspaloy (25 pct c¢) and Inconel 718 (25 pct c¢/c¢¢) generally exhibit a pronounced hardness drop, which is attributed to the absence of c¢ in this region after welding while, alloys like N18, Astroloy, Alloy 720Li and RR1000, which contain about 50 pct c¢, show a hardness peak in the HAZ.[2,3] Also, it has been reported that macro-tensile properties varies along bond line zone (BLZ) of these nickel-base alloys.[4] Recently microstructural variations between the bo
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