Residual Stress Characterisation in Forgings for Aero-Engine Application

252 Pages·2013·27.47 MB·English

by  James Rolph

RESIDUAL STRESS CHARACTERISATION IN FORGINGS
FOR AERO-ENGINE APPLICATION

A thesis submitted to The University of Manchester for the degree of
Doctor of Engineering
in Faculty of Engineering and Physical Sciences

2013

James Rolph
School of Materials
Table of contents
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List of Figures
Figure 1: A schematic illustration of type I, II and III residual stress. Equilibrium
occurs over the scale of the component for type I, a single grain for type II and
within a localised field in type III. (1). ................................................................ 22!
Figure 2: Atomic lattice planes as a strain gauge. Q represents the scattering vector,
2" the diffraction angle, and d the inter-planar spacing. Adapted from (18). .. 25!
hkl
Figure 3: Synchrotron x-ray setups; a) 2" scanning, b) low angle transmission and c)
energy dispersive (14). ......................................................................................... 29!
Figure 4: Neutron and x-ray scattering interactions with atomic mass (17) ................ 30!
Figure 5: Definition of gauge volume through primary and secondary slits. (32) ..... 32!
Figure 6: Illustration of the strain gauge placement for a typical hole drilling
measurement ........................................................................................................ 36!
Figure 7: Implementation of the contour method. a) EDM cutting of the component
complete with residual stress profile, b) stress relief at the surface, c)
measurement of the surface profile and calculation of residual stress (41). ........ 38!
Figure 8: Symmetric and anti-symmetric errors generated during the cutting process.
Adapted from (47, 52) .......................................................................................... 41!
Figure 9: Materials employed in a typical aero-engine (63). ....................................... 46!
Figure 10: Development of turbine entry temperature with advances in aero-engine
design and material processing (66). ................................................................... 47!
Figure 11: A schematic illustration of the typical microstructure of a !/!´ Nickel-base
superalloy (68). .................................................................................................... 49!
Figure 12: Variation in microstructure across an RR1000 dual microstructure AE2100
forging (71). ......................................................................................................... 51!
Figure 13: Atomic arrangement in the a) ordered L1 structure (!´) and b) the
2
disordered matrix (!) (63). ................................................................................... 52!
Figure 14: Schematic illustration of the development of APBs as a dislocation enters
an ordered precipitate (73). .................................................................................. 53!
6

Figure 15: The increase in critical resolved shear stress for Nimonic 105 with a !´ V
f
of 51% and 22%. In regime 1, hardening is by weakly coupled pairs, regime 2
by strongly coupled pairs, and in regime 3 by the Orowan mechanism. The
transition from weak to strong coupling is associated with peak strength (75). .. 54!
Figure 16: A schematic illustration of the powder metallurgy technique (78). ........... 57!
Figure 17: Gauge volume definition using a combination of variable and fixed slits. 62!
Figure 18: Layout of ENGIN-X instrument at ISIS, RAL, Oxford, UK (86) .............. 64!
Figure 19: a) Schematic illustration of the in-situ heat treatment setup, b) distribution
of thermo-couples on sub-scale forging for temperature control. ........................ 65!
Figure 20: A time of flight (TOF) spectrum as obtained on Engin-X while measuring
strain in RR1000 Nickel Superalloy. ................................................................... 66!
Figure 21: Layout of the Bruker D8 Discover with GADDs, including laser video
camera alignment system (87). ............................................................................ 68!
Figure 22: A typical sin2# plot used to calculate d from the intercept and gradient of
0
the linear fit using Equation 11. ........................................................................... 69!
Figure 23: Sample clamping for EDM cutting. The entire assembly is submerged in
de-ionised water with the exception of the wire spools. ...................................... 70!
Figure 24: Schematic of the µscan NanoFocus con-focal laser scanner. Note that the
laser remains stationary and the clamped sample moves underneath to scan the
surface. ................................................................................................................. 72!
Figure 25: Surface contour raw data – water quenched and aged for two hours @
760°C. The masked region is visible at approximately z=-550µm .................... 73!
Figure 26: Ways in which the top half can be rotated away from the bottom half, a)
about the x-axis, and b) about the y-axis ............................................................. 74!
Figure 27: Cleaned surface contour data – water quenched and aged for two hours @
760°C. a) isometric view and b) XY planar view. ............................................... 75!
Figure 28: a) Residuals between spline fit and surface contour data and b) spline fit of
the contour (sample: water quenched and aged for two hours @ 760°C) ........... 76!
Figure 29: Distribution of embedded thermo-couples in the sub-scale forging. ......... 78!
7

Figure 30: Cooling data generated by embedded thermo-couples, in this from a water
quench from solution heat treatment. ................................................................... 79!
Figure 31: Heat transfer coefficient (HTC) curves for air, oil and water quenches. ... 80!
Figure 32: The material characterisation process, testing must occur sequentially to
build up material data. Adapted from (94) .......................................................... 81!
Figure 33: 2D axi-symmetric mesh of a sub-scale disc forging with surface regions for
weighting factor indicated. ................................................................................... 82!
Figure 34: Yield surface representation of isotropic and kinematic hardening in a
biaxial stress field. ............................................................................................... 84!
List of Tables
Table 1: Typical attenuation lengths for x-rays and neutrons in common engineering
materials. Adapted from (8) ................................................................................ 30!
Table 2: Chemical compositions (wt%) of a number of nickel-base superalloys. ....... 48!
Table 3: The effect of alloy elements on material properties and performance for
nickel-base superalloys. ....................................................................................... 48!
Table 4: Contour method EDM cutting parameters ..................................................... 71!
Table 5: Chemical composition of the Nickel Two Part etch ...................................... 86!
Abstract
Residual stresses are the stresses which are present within a component without any
external load. They can be introduced through any number of manufacturing
processes and in-service conditions, meaning that they are almost ubiquitous in
engineering components. The characterisation of residual stress is an important field
of research particularly in an engineering context since the effects of residual stress
sum with the loads. As a result, the performance of a component can be greatly
enhanced, or significantly reduced, by the presence of residual stress depending on
the sign of the stress and the applied load.
In this EngD thesis the focus has been on the development of residual stress through
the manufacturing processes of aero-engine forgings, specifically the turbine disc.
The forgings studied were sub-scale geometries of the disc, forged from the nickel-
base superalloy RR1000. The overall aim of this work is to improve the
understanding of the residual stress generation and relaxation through implementation
of advanced experimental characterisation techniques, with a view to improving
current stress predicting process modelling capabilities. With this in mind the work
has focussed on the use of neutron diffraction and the contour method to characterise
residual stress experimentally, while residual stress predictions have been made using
finite element modelling.
The findings of this research indicated that very large residual stresses were generated
as a result of the quenching process, and that these stresses were then relaxed and
redistributed by ageing heat treatments and material removal by machining. The
results obtained through the two experimental techniques exhibited very strong
agreement, indicated a robust experimental process. Comparisons to the finite
element predictions highlighted some issues with the current model; in particular it
was found that the simulation of quenching could be improved by better definition of
the heat transfer at the surface. Furthermore, the level of stress relaxation during
ageing was consistently over predicted in the model. This result is thought to be the
result of an over-prediction of the level primary creep in the alloy. Subsequent
studies will investigate this behaviour further using the newly developed in-situ heat
treatment capabilities which have developed as part of this research.
Declaration
No portion of the work referred to in the thesis has been submitted in support of an
application for another degree or qualification of this or any other university or other
institute of learning
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