Microstructural features of friction welded MA 956 superalloy material
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I.
INTRODUCTION
OXIDE dispersion strengthened (ODS) materials have superior mechanical and corrosion resistance at high temperatures compared to conventional superalloys.[1] This readily explains the driving force for the replacement of wrought or cast superalloys by ODS materials during turbine nozzle and turbine blade manufacture. Nickel- and ironbased ODS superalloy materials contain fine dispersions of Y2O3 particles and are normally manufactured using mechanical alloying (MA) techniques.[2] Commercial Ni-based ODS superalloys include INCONEL* MA 753 (containing *INCONEL is a trademark of INCO Alloys International, Inc., Huntington, WV.
20 wt pct Cr, 1.5 wt pct Al, 2.3 wt pct Ti, and 1.3 wt pct Y2O3), MA 754 (containing 20 wt pct Cr, 0.3 wt pct Al, 0.5 wt pct Ti, and 0.6 wt pct Y2O3), MA 755 (containing Fe, 20 wt pct Cr and 0.5 wt pct Y2O3), and MA 6000 (containing 15 wt pct Cr, 4.5 wt pct Al, 2.5 wt pct Ti, 4 wt pct W, 2 wt pct Ta, 2 wt pct Mo, and 1.1 wt pct Y2O3). Commercial Fe-based ODS superalloys include MA 956 (containing 20 wt pct Cr, 4.5 wt pct Al, 0.5 wt pct Ti, and 0.5 wt pct Y2O3) and MA 957 (containing 14 wt pct Cr, 1.0 wt pct Ti, 0.3 wt pct Mo, and 0.25 wt pct Y2O3) and have higher melting points and improved corrosion resistance properties compared to the nickel-based ODS alloys. The mechanical properties produced following joining are a critical feature of component fabrication. The ideal endpoint would be joints having mechanical properties that match those of the ODS base material. However, fusion welding techniques such as gas tungsten arc, electron beam, and laser welding produce joints that have inferior creep rupture strengths at high temperature compared to as-received
C.Y. KANG, Professor, is with the Department of Metallurgical Engineering, Pusan University, Pusan, Korea 609-735. T.H. NORTH, Professor, and D.D. PEROVIC, Associate Professor, are with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada M5S 1A4. Manuscript submitted August 3, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
MA 956 base material, for example. The poor creep rupture strength of fusion welds has been ascribed to agglomeration of oxide dispersoids in the fusion zone and to the formation of fine, dispersoid-free grain solidification structures which favor preferential damage accumulation.[3–10] In this connection, the creep rupture strength of joints produced using transient liquid phase bonding, tested in the longitudinal direction, is also much poorer than as-received MA 956 base material as a result of oxide segregation at the melt zone/base metal interfaces and nucleation of polycrystals in the melted region at the joint centerline.[11,12] Solid-state joining techniques such as friction welding do not depend on melting at the joint interface. The friction welding can be characterized by these distinct stages: stage I, where the substrates are brought into contact at a low applied load and the deformation process is dominated by frictional wear; stage II, where
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