Friction stir welds with enhanced fatigue strength and life via laser peening

icaf2023 Tracking Number 144

icaf2023 poster

Friction Stir Welding (FSW) is being studied as a viable solid-state joining process for aircraft fuselage structures. As with any other high-temperature joining process, FSW will create variably distributed residual stress fields that adversely affects the dynamic performance of the joint. The FSW process may also introduce defects that would act as sites for fatigue crack initiation during dynamic testing. Laser Peening (LP) has been demonstrated as an effective tool to improve overall fatigue life in aerospace aluminium alloys due to the introduction of a through-thickness compressive residual stress state. This could also act as a deterrent for crack initiation from a pre-existing defect and potentially recover the fatigue life of a FSW joint. The objective of this study is to understand and quantify the influence of LoP defect depth and profile on the fatigue life of peened and unpeened FSW joints. To investigate this, controlled LoP defects of 400 µm depth have been introduced during FSW. Fatigue testing revealed significant reduction of fatigue life with considerable scatter in fatigue performance. This not only shows that 400 µm defects are providing a higher level of stress concentration, resulting in faster crack initiation but also that the defect geometry is critical. Scanning electron microscopy and optical microscopy were carried out to accurately characterize the impact of defect geometry on crack initiation. Preliminary results have shown that crack initiation does not seem to be influenced only by the defect length, as the crack initiation did not coincide with the sites where the LoP depth was the highest. The influence of defect geometry in the crack initiation behavior is currently being investigated using synchrotron X-ray tomography as an imaging technique to reconstruct the LoP defect profile in peened and unpeened samples. This could allow to investigate changes in the defect profile through the width, as well as to calculate the stress intensity factor along the weld, using finite element modelling. Understanding of the underpinning crack initiation and propagation mechanism would help in designing the peening process to ensure a compressive stress state to delay fatigue crack propagation from LoP defects or similar discontinuities.