Transverse fatigue behaviour and residual stress analyses of double sided FSW aluminium alloy joints

Abstract

Friction stir welding (FSW) since its invention has been attracting relevant interest for joining aluminium alloys. Due to the nature of this process, the materials can be joint without melting. Thanks to this peculiar characteristic, the issues associated with the cooling from liquid phase are avoided or considerably reduced, such as cracking, porosity, and defects. However, as well as other well‐established welding techniques, the FSW process gives rise to formation of residual stress in the welding region and surrounding volume: heat and thermo‐mechanical affected zones. Presence of residual stress in a mechanical component is well‐known to affect its performance, particularly regarding fatigue at high number of cycles. Another aspect that influences the fatigue life is the underlying microstructure. In this work, we firstly study the residual stress field and the underlying microstructural features arising in FSW butt joints and their effect on the fatigue performance of this type of weldments. The evaluation of residual stress field is carried out by means of modern experimental techniques. In the first instance, synchrotron X‐ray powder diffraction was employed for two‐dimensional full field maps of residual stress. Corroboration of these measurements was done by exploiting the capability of focused ion beam and digital image correlation (FIB‐DIC), which is able to deliver pointwise absolute measurement of residual stress. A set of FSW samples were then tested under uniaxial fatigue loading at several loading ranges, in the high cycle fatigue regime, in order to understand whether the severity of loads affects the crack path and life endurance. Fractographic and electron backscattered diffraction (EBSD) analysis then revealed crack nucleation site and propagation mechanisms with the respect of the underlying microstructure. Outcome of these experimental studies is then thoroughly discussed.

Effect of rolling process on fatigue performance of stir zone of AA6061‐T6 doube‐side friction stir welding joint

“…Deng et al 12 assumed that there existed the weakest link on the AS of FSW joint which lead to the inferior fatigue crack initiation threshold in gigacycle fatigue of AA7050 FSW joint. Salvati et al 46 found the adverse effect of tool advance on the fatigue fracture of AA6082‐T6 DS‐FSW joint according to the macro morphology of fracture surface. From these investigations, the fracture of the AS depended on the local crystallographic orientation heterogeneities or the weakest link on the AS of FSW.…”

Residual Stress in Engineering Materials: A Review

“…Various researches on residual stress characterization using synchrotron X-ray-based methods were accomplished by Salvati and teammates. [115][116][117][118][119][120] The residual stress measurement Figure 6. a) Effect of CNF percentage on the curvature of asymmetric ½0 2 =90 2 laminated composites.…”

“…Various researches on residual stress characterization using synchrotron X‐ray‐based methods were accomplished by Salvati and teammates. [ 115–120 ] The residual stress measurement principle in synchrotron XRD (SXRD) is similar to the standard XRD, while SXRD uses the polycrystalline lattice of the material as an atomic strain gauge and relies on accurate measurement of the change in separation of atomic lattice planes in polycrystalline materials due to stress. [ 121 ] The penetration depth in SXRD is much higher than laboratory‐based X‐rays, thus providing high spatial resolution 3D maps of the strain distribution to millimeter depths in engineered components.…”

“…This differential constitutes law was proved to be more suitable than previous integral constitute laws. [ 140 ] Authors [ 117 ] used energy and FE methods to study the influence of poor adhesion between matrix and fiber on the microresidual stresses to determine the effect of some influential parameters on the residual thermal stresses in polymeric composites. In particular, the effects of Poisson's ratio, CTE, and modulus of elasticity were taken into account.…”

An electron backscatter diffraction investigation on microstructural evolution of pearlite wheel steel near rolling contact fatigue crack tip

“…The relationship between the orientation of the grains and the fatigue crack propagation was discussed. Salvati et al 22 studied the transverse fatigue behavior of double‐sided friction stir welding aluminum alloy joints. The fatigue crack nucleation site and the propagation mechanisms were characterized using EBSD.…”