An Assessment of the Effect of Machining and Forging Texture on the Fatigue Performance of Aerospace Titanium Alloys

Safety-critical aerospace jet engine components such as titanium alloy compressor discs are manufactured under very strict standards and regulations. It is essential that the effects of every manufacturing stage is fully understood to ensure the best performance and avoid catastrophic failures. In this study the effects of upstream forging and downstream machining were determined and correlated with the variable fatigue performance of a number of Rolls-Royce Ti-6Al-2Sn-4Zr-6Mo compressor disc forgings used in the intermediate pressure section of a gas turbine aero-engine. A four-point flexural testing approach was used to characterise the fatigue life heterogeneity at different circumferential locations in a Ti-6Al-2Sn-4Zr-6Mo compressor discs. Texture analysis using electron backscattered diffraction linked these fatigue life differences with the heterogeneous nonlinear strain pattern induced during the open die hot forging process at the primary forging stage of the VAR ingot. Moreover, this texture analysis also revealed the alignment of the HCP crystal c-axis in the radial direction parallel to the hoop stress during the close die forging secondary forging. Crack initiation analysis of fatigue test coupons of these regions were dominated with alpha phase that was orientated at an angle of 20-40o with respect to the loading direction. This demonstrated that there is no purely basal cleavage and a component of the applied stress has been transformed into deformation slip to initiate a crack. EBSD analysis was performed to understand the crack propagation mechanisms after identifying different crack path geometries during fractography analysis. Best performing circumferential regions displayed a more convoluted crack path and a lower texture index (MUD), as opposed to the straight cracks in the worst performing regions. This was directly correlated to the texture heterogeneities induced during forging. Moreover, the best performing samples showed crack diversion/arrest when the crack tip encountered regions unfavourably orientated for prismatic slip with respect to the loading direction. A small-scale machining test was developed to recreate the industrial face turning process in small samples, making it more resource efficient to test different machining parameters. This process was used to machine flat coupons, extracted from a second Ti-6Al-2Sn-4Zr-6Mo alloy forged disc, which were subsequently tested in the 4-point bend fatigue approach. This enabled the effects of machining parameters on the surface performance to be directly correlated to the fatigue life. Results showed that faster cutting speeds and metal removal rates did not have any detrimental effect on fatigue life. In fact, machined coupons presented up to three times the fatigue life compared to non-machined ones. However, circumferential variation in fatigue life was still present, suggesting that the inherent forging effects on fatigue life cannot be “erased” through the machining process. A third Ti-6Al-2Sn-4Zr-6Mo disc was machined at Rolls- Royce facilities to evaluate the capabilities of the production machines when machining at higher speeds. Vibrations in the component and mid-range speeds induced chattering which produced distinct surface markings. However, this effect was not seen at the current industry standard or fastest machining speeds. This was attributed to modal frequencies and system vibrations. Finally, force feedback analysis (FFA) has been used to characterise the microstructure of a range of titanium alloys (and heat treatment conditions) through the measured cutting forces. Heterogeneous cutting performance has been linked to the primary forging process, demonstrated in a Ti-54M forged billet, and a closer inspection of the collected signal, allowed grain size measurements and microstructural features reconstruction in ”fingerprint diagrams”. This reconstruction was validated using in a �-forged Ti-17 billet.