Characterisation and fatigue of friction stir welding

Previous attempts on the characterisation of Friction Stir Welding (FSW) based on microstructures, hardness, and residual stress distribution have been reviewed. The role of these parameters on fatigue damage of FSW is then discussed. Relevant conclusions have been drawn to demonstrate the current issues and the future research potential of these joints

Characterization of shot peened 2024-T351 aluminum alloy

Specimens of 2024-T351 aluminium alloy under different three shot peening intensities were studied. The modifications of the surface layers of the shot peened specimens were investigated through microhardness, surface microstructure and residual stress relaxation after the first and second load cycles under two cyclic loads. No significant changes in microstructure after the three shot peeing intensities were observed with respect to untreated specimens. Rapid residual stress relaxation was observed in specimens after the first cycle. Relaxation of residual stresses occurred within first loading cycles were increased with increasing loading stress amplitude and due to quasi-static relaxation effects

Prediction of residual stress relaxation of shot peened 2024-T351 aluminum alloy: part 1

It is important to account for residual stress relaxation phenomenon in the design of the component. Specimens of 2024-T351 aluminium alloy were used in this study. The specimens were shot peened under three different shot peening intensities. Cyclic tests for two load magnitudes were performed for 1, 2, 10, 1000 and 10000 cycles. Residual stresses, microhardness and the cold work percentage were measured at initial state and after each loading cycle for the three shot peening intensities and for the two loads. The study revealed that most of the drop in the residual stress, microhardness and cold work happened in the first cycle are dependent on the applied load

Modelling of residual stress relaxation: a review

Compressive residual stress, induced by mechanical surface treatment, may relax during component operation life, due to thermal or mechanical mechanism. Fatigue life prediction for the components which have residual stress will be misled and inaccurately predicted the phenomenon of residual stress relaxation is not considered. Despite putting an effort on incorporating the residual stress relaxation, the issues remain concerned with the technical challenge of measuring and quantifying the magnitude of residual stress relaxation as well as redistribution during the loading cycling itself. In this paper, the residual stress relaxation and its models were reviewed and discussed to picture the best knowledge related to this topic, i.e. whether relaxation is a cause or an effect

Residual stress relaxation and surface hardness of a 2024-t351 aluminium alloy

For design it is generally important to consider the residual stress relaxation. In the study for this contribution, 2024 T351 Aluminium alloy specimens were shot peened at three different shot peening intensities, followed by fatigue tests for two loads. Fatigue tests were divided into two stages. The residual stresses and micro-hardness were measured at initial and after each cyclic load for the three shot peening intensities and the two aforementioned sets of loads. The results showed that the residual stresses and micro-hardness of the specimens were decreased. Moreover, the relaxation depended on the fatigue load amplitude. Residual stress relaxation reached 54% of the initial residual stress while the micro-hardness relaxation reached 39% of the initial micro-hardness. Most of the residual stress relaxation occurred during the first cycle. The relaxation of the initial residual stress is severe when there is low shot peening intensity and high applied load, and the reduction of the micro-hardness is depending on the residual stress relaxation

Micro-hardness and residual stress relaxation of 2024 T351 aluminum alloy

The residual stress relaxation can be divided into two stages: The first cycle relaxation and the following cycles. In both stages, residual stress relaxed considerably from the initial state. The aim of this study is to investigate the residual stress relaxation and microhardness reduction after first and second cyclic load. A 2024 T351 aluminum alloy specimens were shot peened into three shot peening intensities. The fatigue test for first and second cyclic loads of two loads 15.5 kN and 30 kN was performed. The initial residual stress and residual stress after the first and second cycle stress was measured for the three shot peening intensities using X-ray diffraction. Microhardness test was performed for each specimen. The results showed that the residual stress relaxation for first cycle was reached more than 40% of the initial residual stress and it depends on the load amplitude, and microhardness decreased for the first cycle reached 22% and also it depended on load amplitude

Modelling of residual stress relaxation : a review

Compressive residual stress, induced by mechanical surface treatment, may relax during component operation life, due to thermal or mechanical mechanism. Fatigue life prediction for the components which have residual stress will be misled and inaccurately predicted the phenomenon of residual stress relaxation is not considered. Despite putting an effort on incorporating the residual stress relaxation, the issues remain concerned with the technical challenge of measuring and quantifying the magnitude of residual stress relaxation as well as redistribution during the loading cycling itself. In this paper, the residual stress relaxation and its models were reviewed and discussed to picture the best knowledge related to this topic, i.e. whether relaxation is a cause or an effect

Fatigue damage of 2024-T351 aluminium alloy friction stir welding joints. Part 2: fatigue damage

The characterisation of micro and macro mechanics in 2024-T351 (Al Alloy) FSW joints was conducted to identify the critical regimes for natural fatigue crack initiation in 2024-T351 Al Alloy FSW welded joints and was presented in Part 1. In this Part, the fatigue tests were performed. Scanning electron microscopy analysis on fracture surfaces revealed that natural crack initiates from multiple sites and is propagated through different regimes causing coalescence. Replicas of crack images confirmed that multiple cracks coalesce. The natural fatigue initiation sites which were found begin from subsurface defects rather than form a free surface. For a different applied stress level, the initiation sites were changed from one regime to another. The number of cracks observed reduces as the applied stress drops. The fatigue limit of this welded joint was governed by a coalescence of the cracks rather than by the propagation

Relaxation of compressive residual stress. Part 1: relaxation of stage I

In this study, 2024-T351 aluminum alloy specimens were shot peened into three shot peening intensities condition to induce compressive residual stresses (RSs). Fatigue test was performed for the first and second cyclic load. Initial RSs at initial condition and after first and second cycle of fatigue loading were measured using X-ray diffraction method. Relaxation for first cycle was found to reach over 40% of initial RS and it depended on load amplitude