Analysis of Microstructural Changes Induced by Linear Friction Welding in a Nickel-Base Superalloy
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UCTION
INCONEL 738 is a c¢ strengthened nickel-base superalloy used in the production of hot-section components in aero and land-based gas turbine applications due to its excellent high-temperature strength and remarkable corrosion resistance. Fabrication and repair of servicedamaged turbine components often involve various joining methods. Welding has proven to be an economical way of fabricating components and repairing turbine parts. Unfortunately, IN 738, like other precipitation-hardened nickel-base superalloys that contain a substantial amount of Al and Ti, is very difficult to weld due to its high susceptibility to heat-affected zone (HAZ) cracking during conventional fusion welding processes and strain age cracking during postweld heat treatment.[1–3] The cause of this cracking, which is usually intergranular in nature, was attributed to the liquation of various phases in the alloy, subsequent wetting of the grain boundaries by the liquid, and decohesion along one of the solid-liquid interfaces due to on-cooling tensile stresses.[2–4] In order to avoid liquation and liquation cracking in superalloys, the state-of-the-art trend in joining difficult-to-weld structural materials involves the use of solid-state joining O.T. OLA, Doctoral Student, O.A. OJO, Associate Professor, and M.C. CHATURVEDI, Distinguished Professor Emeritus, are with Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, MB R3T 5V6, Canada. Contact e-mail: [email protected] P. WANJARA, Group Leader, is with the National Research Council Canada, Institute for Aerospace Research, Aerospace Manufacturing Technology Centre, Montre´al, QC H3T 2B2, Canada. Manuscript submitted November 24, 2010. Article published online August 16, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
techniques. Friction welding is regarded as a solid-state joining process that has great potential in the aerospace industry for the joining of superalloys, especially for the fabrication of blade-disk assemblies and the repair of aero-engine components.[5–8] The process has produced successful joints in some structural alloys, and current research efforts are now being directed toward its application for the fabrication and repair of turbine components made of nickel-base superalloys. This present work was initiated in order to investigate the weldability of the difficult-to-weld IN 738 superalloy by linear friction welding, which is a variant of the friction welding processes, and to analyze the microstructural changes induced in the alloy by the welding process. The results are presented and discussed in this article.
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EXPERIMENTAL
Cast IN 738 with a nominal composition of (wt pct) 0.11C, 15.84Cr, 8.5Co, 2.48W, 1.88Mo, 0.92Nb, 0.07Fe, 3.46Al, 3.47Ti, 1.69Ta, 0.04Zr, 0.010B, and balance nickel was received in the form of plates having dimensions of 240 mm 9 60 mm 9 15 mm. Welding test coupons of dimensions 12.8 mm 9 11.1 mm 9 17.7 mm were machined from these plates and were given the standard solution heat treatment (SHT) at 1393 K (1
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