| An effective life extension technology for 7xxx series aluminium alloys by laser shock peening (Final Report) (2008) | |||||||||
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| Fatigue life is one of the critical properties of aircraft components. The general approach to enhancing fatigue life is to modify the microstructure and/or impose compressive residual stress in the surface/subsurface regions of components. Laser shock peening (LSP) is an effective life extension technology because of the deeper compressive residual stresses developed and an improved surface finish relative to conventional shot peening. The DSTO research program into LSP was designed to: (a) quantify the fatigue life improvement achieved by LSP when compared to conventional glass bead peening (GBP) in typical high strength aircraft aluminium alloys and (b) identify any possible technological risks associated with the use of LSP for fatigue life extension of fatiguecritical aluminium alloy aircraft structure. This is the final report of the DSTO feasibility study of using LSP on AA7xxx series aluminium alloys. It summarises the state-of-the-art of LSP including its benefits, limitations and risks. The report presents the comprehensive results of the DSTO research program over the past few years. It is shown that when LSP conditions are optimal for the material and specimen configuration, a 3-4 times increase in fatigue life, over specimens in the as-machined condition, could be achieved. However, the experimental results also showed that fatigue life in LSP-treated AA7050 specimens was reduced if the laser power density was too high (> 3 GW/cm2). The main causes of the reduction of fatigue life in one batch of AA7050 were the formation of internal cracking and surface damage (or ‘burnt’ spots) caused by the laser beam. For another batch of AA7050 Al alloy specimens the fatigue lives were also reduced for laser power density > 3 GW/cm2, however, the causes for fatigue life loss were unclear. It is believed, however, that it may be the result of tensile residual stresses at the surface, combined with shallow compressive residual stress at the sub-surface due to “over-peening” at the higher laser power density. Numerical models were developed to understand the LSP process and these models were verified by experiment. Using these models, the critical conditions for the formation of the internal cracking were identified. The effect of specimen geometry on dynamic stresses during LSP processing was also investigated through refinement of the numerical models. As a result, the full benefits of LSP may be realised. An analysis of the cost effectiveness of LSP technology showed that it involves high capital and running costs. LSP has also been shown elsewhere to be a very effective life enhancement/extension technology, particularly for high-value aircraft components, because of increased inspection intervals and reduced maintenance costs. These advantages must be considered, however, with the possibility that the fatigue lives could in fact be shortened. This could occur if the processing parameters are not carefully selected or if there is some other irregularity in the process such as that which caused the fatigue lives to be shortened by the surface ‘burning’.. This report summarises the state-of-the-art, benefits, limitations and risks of laser shock peening (LSP). The experimental results showed that LSP technology can effectively extend fatigue life for AA7075 Al alloy. However, the fatigue lives in LSP treated AA7050 specimens are reduced when the laser power density exceeds 3 GW/cm2. The cause for the reduction of the fatigue life was determined to be internal cracking, surface “burnt” defects, or “over-peening” due to inappropriate LSP processing. Numerical analysis has been carried out to investigate the effect of laser power density on both dynamic and residual stresses. The optimum process conditions for LSP were established, and the cost-effectiveness of the technique has been analysed. The DSTO investigation showed that LSP is an effective fatigue life extension technology, although it has a high capital investment and running costs.. DGTA | |||||||||
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