ISIS2003 Science Highlights

The structural integrity of a weld is defined bythe mechanical properties and the residual stressesapparent in the component. It is therefore importantto characterise the residual stresses generated duringfriction welding and the degree of stress relief duringpost weld heat treatment before such a componentcan be put into commercial application.

Rolls-Royce has committed itself toestablishing inertia and linear friction welding as itsmajor welding tools for joining the new generationof advanced high temperature alloys. In inertiafriction welding, one of the work pieces isconnected to a flywheel and the other is restrainedfrom rotating. The flywheel is accelerated to apredetermined rotational speed, then disengagedand the work pieces are forced together. Thekinetic energy stored in the rotating flywheel isdissipated as heat through friction at the weldinterface. In this way, it is possible to join newnickel-base superalloys like RR1000 and Alloy 720LI,which are not weldable by electron beam welding(the current welding technique used by Rolls-Royce).Furthermore, friction methods can form dissimilarjoints not possible by other means, such as twodifferent Ni-base superalloys or high temperaturesteel against nickel-base superalloy. Rolls-Royce isplanning to employ this process in the very nearfuture to join high pressure compressor drums,turbine discs and shafts (fig. 1).

During an EPSRC/Royce-Rolls funded project,residual stresses in inertia friction welds of RR1000(high γ’ nickel-base superalloy, provided by Rolls-Royce plc) with an outer diameter of 143 mm and awall thickness 8 mm were studied. Measurementswere carried out for the as-welded, a conventionaland a modified post weld heat treated (PWHT)condition (50 °C above the conventional PWHT). All

measurements were undertaken on the ENGINdiffractometer. With detectors placed at a givenangle, time of flight (TOF) measurements mean thatthe whole diffraction profile can be recorded as afunction of time. The presence of two detectorbanks, ±90° apart, means that the strains along twoperpendicular sample directions can be measuredsimultaneously. The sampling gauge volume isdefined by the intersection of the incident anddiffracted beams and the lattice parameter, a, of thelattice planes with the plane normal parallel to thescattering vector, was determined by Rietveldrefinement of the spectra. The residual strains werecalculated by relating the lattice parameter of eachmeasurement point to the stress-free latticeparameter of the parent material measured usingthe engineering strain equation.

When calculating strain and stress in this way,it is important to determine the stress-free latticeparameter with a high accuracy (usually done in a farfield region). Since the material of the heat affectedzone was exposed to high temperatures duringfriction welding, each phase in a multiphase materialcan be expected to exhibit a chemical variationacross the weld line (the element partitioning effect)resulting in a variation of the stress-free latticeparameter. If this chemically related lattice parametervariation is not taken into account, significantpseudo-strains/stresses are calculated leading to, inthis case, an overestimation of the residual tensilestresses close to the weld line. The characterisationof the stress-free lattice parameter variation in the

In many advanced engineering applications the ability to weldcomponents reliably, reproducibly and with high joint efficiencies isa key technology. As materials improve, the challenges of weldingbecome ever more demanding and, for the new generation of highperformance, high temperature alloys, friction-based solid statewelding techniques are fast becoming the industrial method ofchoice.

Fig. 1: The full scale disc to disc inertia friction welder,based at the Rolls-Royce compressor rotor facility nearDerby, is expected to be implemented in the productionline in the year 2005.

Residual stresses in inertia friction welded aeroengine materialsM Preuss, P J Withers (UMIST/University of Manchester), J W L Pang (Oak Ridge National Laboratory, USA)and GJ Baxter (Rolls-Royce plc)

Contact: Dr M Preuss. Tel: +44 (0)161 200 3601. Email: michael.preuss@umist.ac.ukFurther reading: M Preuss et al., Met. & Mat. Trans. 33, 3227 (2002).

www.isis.rl.ac.uk

heat affected zone is therefore an important part ofresidual stress measurements.

The change of stress-free lattice parameter wasdetermined on thin slices cut from the weld using thebiaxial sin2ψ method in conjunction with a collimatedX-ray machine and the forced stress balance model.In order to calculate the residual stress fields in thewelds, it is necessary to measure strain in the threeprincipal directions (hoop, axial and radial) of thetubular weld. The lattice spacing was mapped outover a plane at a specific hoop location between theweld line and up to 8 mm away from it. Due to therelatively large neutron absorption coefficient ofnickel, a neutron path length of 10 mm would reducethe diffracted beam intensity by ≈80%. In order tominimise the path length and facilitate the hoopstrain measurements, a small window of 12x12 mmwas electro-discharge machined from the weld regionof the welds at a position distant from the neutronmeasurement location.

The data collected on the ENGINdiffractometer indicate that the largest detrimentaltensile stresses were generally observed in the hoopdirection, at the weld line and close to the innerdiameter. During friction welding, stresses weregenerated in the range of 1500 MPa (fig. 2a). Theconventional PWHT relieved the residual stressesonly to a limited extent with tensile stresses in therange of 1000 MPa at the weld line and close to theinner diameter (fig. 2b). Figure 2(c) shows that themodified PWHT was significantly more effective inreducing the stresses to a maximum of about 400MPa. Together with metallurgical studies of thewelds demonstrating that this PWHT does notcompromise the microstructure and mechanicalproperties of the material, a new PWHT for inertiafriction welded RR1000 has been suggested.

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Residual stresses in inertia friction welded aeroengine materialsISIS2003 Science Highlights

Fig. 2: Contour plots of the hoop stress fields (in MPa) in an inertia friction weld measured in the (a) as-welded, (b) conventional and (c) modified/new PWHT condition. Extremely large residual tensile stresses areobserved at the weld line (z = 0 mm) and close to the inner diameter of the tubular weld (R = -2.5) in the as-welded condition. After conventional PWHT, the tensile stresses still reach 900 MPa, which is not acceptablein a turbine disc component. Increasing the temperature of the PWHT by 50 °C reduces the tensile stressessufficiently below 500 MPa while maintaining an appropriate microstructure.