Abstract
Non-standard processing routes for the manufacture of industrial scale Ti834 billet have been undertaken to investigate their effect on macrozones in final forged product. Microstructure, texture and dwell fatigue fracture surfaces were characterised from forged disc samples fabricated from these new billets. All processing routes showed a bimodal microstructure consisting of 25pct of primary alpha grains in a transformed beta matrix. Texture analysis has revealed variations in the presence and size of macrozones with relatively weak textures. Quasi-cleavage facets were present in all dwell fatigue samples although the fatigue life was doubled for the sample whose thermomechanical processing has the highest imposed strain.
Highlights
Near-α titanium alloys have their main application in rotating compressor discs in the high-pressure section of jet turbine engines where a balance of good fatigue and creep properties at high temperatures is required [1]
Representative micrographs from all three processing routes show a bimodal microstructure on the final component with primary alpha grains and secondary alpha colonies, Figure 1
The coarser and more equiaxed microstructure belongs to the compressor disc sample fabricated from the smaller billet diameter (200T)
Summary
Near-α titanium alloys have their main application in rotating compressor discs in the high-pressure section of jet turbine engines where a balance of good fatigue and creep properties at high temperatures is required [1]. Its excellent fatigue and creep performance capability comes from a bimodal microstructure consisting of primary alpha grains (αp) and secondary alpha colonies (αs). The development of this microstructure consists of several steps within two processing stages: 1. In primary forging the first step is above the beta transus (Tβ) temperature to break down the as-cast microstructure of large β-grains This is followed by sub Tβ deformation and a heat treatment to promote recrystallization to reduce β-grain size, which upon cooling produces a microstructure consisting of αs, whose size is directly impacted by the parent β-grain size. The secondary forging, takes place only sub Tβ, aiming to get the near final shape and the optimum volume fraction of αp grains required for the component’s application
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