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.