Residual Stresses in Linear Friction Welded IMI550

Abstract

Residual stresses in linear friction welded (LFW) IMI550 have been determined by means of high energy synchrotron X-ray diffraction. Specimens were studied in the as-welded, and three different post weld heat treated conditions. Mid-thickness-mid-width line scans were recorded across the weld line in the three principal directions of the welds. For calculating the nominal residual strain the “stress-free” lattice spacing was taken from far field measurements. The separation of a chemical induced and strain induced shift of the reflection was carried out by determining the variation of the d-spacing across the weld line. This was done on thin slices cut from the centre region of a LFW sample using the biaxial sin²Ψ-technique on a collimated laboratory X-ray source. Results show large tensile stresses close to the weld line for the as-welded sample and the sample post weld heat treated at the aging temperature typical of IMI550. Moderate residual stresses were observed after increasing the post weld heat treatment (PWHT) temperature by 100 or 150°C. All conditions displayed a significant hydrostatic tensile stress field component, which is generally difficult to relieve. This work demonstrates that residual strain measurements have to be carried out using a high spatial resolution in order to resolve the sharp strain gradients apparent in LFW Titanium alloys.