Microstructural characterisation of inertia friction welded RR1000 superalloy

Oluwasegun, Kunle Michael (2012). Microstructural characterisation of inertia friction welded RR1000 superalloy. University of Birmingham. Ph.D.

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Abstract

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 L1\(_2\) intermetallic (Ni\(_3\)(Al, Ti)) \( \gamma\)' phase, with RR1000 being a candidate. Welding of this alloy by conventional methods has been found difficult due to a high susceptibility to heat affected zone (HAZ) liquation cracking. In order to produce welds with good joint integrity, inertia friction welding (IFW), a nominally solid state welding process, has been used to join gas turbine parts made from this alloy, based on the premise that the joining occurs below the melting point of the bulk material. The failure rate, however, is not zero. Detailed microstructural characterisation of the actual weld and of a thermo-mechanically simulated HAZ has revealed for the first time that non-equilibrium constitutional liquation of some strengthening precipitates occurs at the grain boundaries and within the grains of this alloy during IFW, with attendant formation of liquation microvoids within the HAZ. The temperature gradient across the HAZ is predicted to be 1150 \(^o\)C-1286 \(^o\)C. Hafnium-rich oxides were also found to coalesce and become smeared by friction along the weld interface, forming brittle hafnium oxide flakes. However, IFW has been found to be more effective than conventional welding techniques, always producing liquation crack-free welds within 150 \( \mu\)m of the bond line (not common in conventionally welded alloys). Micro tensile testing was used to characterise the local strength within the weld and to rationalise it with the microstructure.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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