Elsevier

Physica B: Condensed Matter

Volume 325, January 2003, Pages 130-137
Physica B: Condensed Matter

Measurement of strain in a titanium linear friction weld by neutron diffraction

https://doi.org/10.1016/S0921-4526(02)01514-4Get rights and content

Abstract

We report measurements of strain by pulsed neutron diffraction in a titanium linear friction weld as a function of distance from the weld zone. While the average strains in a particular direction follow continuum mechanics expectations, examination of strains in different crystallographic directions highlights the process of grain reorientation undergone during the welding.

Introduction

Titanium alloys are widely used for structural applications, especially where high specific stiffness is required at elevated temperature. The bonding together of components made from dissimilar alloys is of particular interest for some of the modern, high-temperature titanium alloys used in the manufacture of today's jet engines. Industry has shown world-wide acceptance of both the cost effectiveness and high weld quality which can be produced when using conventional rotary friction welding in order to produce joints in circular cross section metallic components. This interest has led to the development of linear friction welding (LFW), by which complex geometry components can be joined, such as jet engine blades to discs.

LFW is particularly appropriate for titanium, due to the element's great affinity for oxygen, nitrogen and hydrogen, which rules out all welding processes in which the molten metal can come into contact with any of these elements and result in embrittlement. The primary fusion welding methods used are those carried out in an inert gas atmosphere (TIG and MIG welding). Friction welding, however, avoids the formation of a liquid phase during the welding process, and can therefore be carried out in air. The surfaces are joined in a plastic condition at hot forming temperatures. The typical defects caused by melting and solidification such as pores, pinholes, shrinkage cracks, segregation, grain coarsening and cast structure are therefore avoided and the risk of gas pickup is low due to the short welding cycles.

However, as with all welding processes, due to the presence of inelastic deformation at elevated temperatures the room temperature weld will have internal or residual stresses varying as a function of position in the welded component. The effect of residual strains on engineering component performance and lifetime has been recognised for some time [1]. One highly successful technique for profiling residual strain gradients in polycrystalline materials is neutron diffraction [2]. Similar to the more common X-ray diffraction, this technique relies on the high penetration depth of neutrons into materials to allow the mapping of changes in lattice parameter as a function of position within the bulk of a component. Its development and applicability to bulk engineering components has largely been driven by its ability to measure a ‘macroscale’ stress field (non-destructively) that can be compared to continuum mechanics calculations. Often neutron diffraction results are used to validate models and thus aid life predictions and the development of failure criteria.

We report here measurements carried out using pulsed neutron diffraction on a Ti LFW between two grades of Ti–6Al–4V, MDG10051R and MDG10050 representing turbine disc and turbine blade material, respectively.

Section snippets

Linear friction welding

The LFW process is a solid phase, machine tool based process. One component is rubbed across the face of a second component, which is rigidly clamped, using a linear reversing motion. This motion is typically of an amplitude of ±1–3mm, at a frequency of 25–75Hz, with a maximum welding force perpendicular to the weld line of ∼150kN. The linear reversing motion as shown in Fig. 1 generates frictional heat and softening of material at the weld interface. The material is mixed in a narrow (∼1mm)

Strain measurement using neutron diffraction

The use of elastic diffraction techniques to determine changes in lattice parameter, or d-spacing, (and thus elastic strains) is well established [3], [4]; components of strain are measured directly from changes in crystal lattice spacing. When illuminated by radiation of wavelength similar to interplanar spacings, crystalline materials diffract this radiation as distinctive Bragg peaks. The angle at which any given peak occurs can be calculated using Bragg's law of diffraction.

For thermal

Measured strains

A Rietveld [5] analysis was carried out on the entire diffraction spectra obtained at each measurement position, using the GSAS software [7]. In this method a proposed crystal structure is used to predict a diffraction pattern which is compared with that measured. The crystal structure is varied in a least-squares way to optimise agreement between measured and predicted diffraction patterns. Due to the poor neutron signal obtained from titanium it was not possible to carry out reliable single

Discussion

The agreement of the strain and stress profiles in the x- and z-directions seems reasonable from continuum mechanics arguments. Flash material is observed to emerge from the weld zone during the welding process in both x- and z-directions, though greater amounts do so in the x (welding) direction. Thus large amounts of high temperature plastic flow are occurring outwards in both x- and z-directions. This results in a large tensile residual stress in both directions upon cooling.

However, closer

Conclusions

We have reported strain measurements using neutron diffraction in a titanium linear friction weld as a function of distance from the weld zone. Mean strains in the two in-weld-plane directions are in close agreement. The weld exhibits a moderate hydrostatic tensile stress in the weld itself, superimposed with a biaxial tensile stress in the in-weld-plane, and small compressive stresses outside the weld. Examination of strains in different crystallographic directions highlighted the process of

Acknowledgements

The measurements were made as part of the RESTAND EU funded project (SMT4-CT97-2200), to define standards in measurement of residual strain using neutron diffraction. Thanks are due to Peter J. Webster for his work defining standard protocols for measurement [10] which were used in this work.

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