ISIS Science Highlights. Residual stresses in inertia friction welded aeroengine materials.

1. ISIS2003 Science Highlights
Residual stresses in inertia friction welded
aeroengine materials
M Preuss, P J Withers (UMIST/University of Manchester), J W L Pang (Oak Ridge National Laboratory, USA)
and GJ Baxter (Rolls-Royce plc)
In many advanced engineering applications the ability to weld
components reliably, reproducibly and with high joint efficiencies is
a key technology. As materials improve, the challenges of welding
become ever more demanding and, for the new generation of high
performance, high temperature alloys, friction-based solid state
welding techniques are fast becoming the industrial method of
choice.
The structural integrity of a weld is defined by
the mechanical properties and the residual stresses Fig. 1: The full scale disc to disc inertia friction welder,
based at the Rolls-Royce compressor rotor facility near
apparent in the component. It is therefore important
Derby, is expected to be implemented in the production
to characterise the residual stresses generated during
line in the year 2005.
friction welding and the degree of stress relief during
measurements were undertaken on the ENGIN
post weld heat treatment before such a component
diffractometer. With detectors placed at a given
can be put into commercial application.
angle, time of flight (TOF) measurements mean that
Rolls-Royce has committed itself to
the whole diffraction profile can be recorded as a
establishing inertia and linear friction welding as its
function of time. The presence of two detector
major welding tools for joining the new generation
banks, ±90° apart, means that the strains along two
of advanced high temperature alloys. In inertia
perpendicular sample directions can be measured
friction welding, one of the work pieces is
simultaneously. The sampling gauge volume is
connected to a flywheel and the other is restrained
defined by the intersection of the incident and
from rotating. The flywheel is accelerated to a
diffracted beams and the lattice parameter, a, of the
predetermined rotational speed, then disengaged
and the work pieces are forced together. The lattice planes with the plane normal parallel to the
kinetic energy stored in the rotating flywheel is scattering vector, was determined by Rietveld
dissipated as heat through friction at the weld refinement of the spectra. The residual strains were
interface. In this way, it is possible to join new calculated by relating the lattice parameter of each
nickel-base superalloys like RR1000 and Alloy 720LI, measurement point to the stress-free lattice
which are not weldable by electron beam welding parameter of the parent material measured using
(the current welding technique used by Rolls-Royce). the engineering strain equation.
Furthermore, friction methods can form dissimilar When calculating strain and stress in this way,
joints not possible by other means, such as two it is important to determine the stress-free lattice
different Ni-base superalloys or high temperature parameter with a high accuracy (usually done in a far
steel against nickel-base superalloy. Rolls-Royce is field region). Since the material of the heat affected
planning to employ this process in the very near zone was exposed to high temperatures during
future to join high pressure compressor drums, friction welding, each phase in a multiphase material
turbine discs and shafts (fig. 1). can be expected to exhibit a chemical variation
During an EPSRC/Royce-Rolls funded project, across the weld line (the element partitioning effect)
residual stresses in inertia friction welds of RR1000 resulting in a variation of the stress-free lattice
(high γ’ nickel-base superalloy, provided by Rolls- parameter. If this chemically related lattice parameter
variation is not taken into account, significant
Royce plc) with an outer diameter of 143 mm and a
pseudo-strains/stresses are calculated leading to, in
wall thickness 8 mm were studied. Measurements
this case, an overestimation of the residual tensile
were carried out for the as-welded, a conventional
stresses close to the weld line. The characterisation
and a modified post weld heat treated (PWHT)
of the stress-free lattice parameter variation in the
condition (50 °C above the conventional PWHT). All

2. ISIS2003 Science Highlights Residual stresses in inertia friction welded aeroengine materials
(a) (b) (c)
-2
1200
0
-1
0
70
800
60
0
00
40
400
600
10
R (mm)
30
0
80
0
40
1000
0
0
700
0
0
80
60
0 30
1
50 0 0
400
30
0
80
600
0
0
600 20
40
2
0 1 2 3 4 50 1 2 3 4 50 1 2 3 4 5
z (mm)
1400
1200
Fig. 2: Contour plots of the hoop stress fields (in MPa) in an inertia friction weld measured in the (a) as-
1000
welded, (b) conventional and (c) modified/new PWHT condition. Extremely large residual tensile stresses are 800
observed at the weld line (z = 0 mm) and close to the inner diameter of the tubular weld (R = -2.5) in the as- 600
welded condition. After conventional PWHT, the tensile stresses still reach 900 MPa, which is not acceptable 400
in a turbine disc component. Increasing the temperature of the PWHT by 50 °C reduces the tensile stresses 200
0
sufficiently below 500 MPa while maintaining an appropriate microstructure.
heat affected zone is therefore an important part of The data collected on the ENGIN
residual stress measurements. diffractometer indicate that the largest detrimental
The change of stress-free lattice parameter was tensile stresses were generally observed in the hoop
determined on thin slices cut from the weld using the direction, at the weld line and close to the inner
biaxial sin2ψ method in conjunction with a collimated diameter. During friction welding, stresses were
X-ray machine and the forced stress balance model. generated in the range of 1500 MPa (fig. 2a). The
In order to calculate the residual stress fields in the conventional PWHT relieved the residual stresses
welds, it is necessary to measure strain in the three only to a limited extent with tensile stresses in the
principal directions (hoop, axial and radial) of the range of 1000 MPa at the weld line and close to the
tubular weld. The lattice spacing was mapped out inner diameter (fig. 2b). Figure 2(c) shows that the
over a plane at a specific hoop location between the modified PWHT was significantly more effective in
weld line and up to 8 mm away from it. Due to the reducing the stresses to a maximum of about 400
relatively large neutron absorption coefficient of MPa. Together with metallurgical studies of the
nickel, a neutron path length of 10 mm would reduce welds demonstrating that this PWHT does not
the diffracted beam intensity by ≈80%. In order to compromise the microstructure and mechanical
minimise the path length and facilitate the hoop properties of the material, a new PWHT for inertia
strain measurements, a small window of 12x12 mm friction welded RR1000 has been suggested.
was electro-discharge machined from the weld region
of the welds at a position distant from the neutron
measurement location.
Contact: Dr M Preuss.
Tel: +44 (0)161 200 3601. Email: michael.preuss@umist.ac.uk
Further reading: M Preuss et al., Met. & Mat. Trans. 33, 3227 (2002).
www.isis.rl.ac.uk