Geometrical effects on residual stresses in 7050-T7451 aluminum alloy rods subject to laser shock peening

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Abstract

Laser shock peening (LSP) is an emerging surface treatment technology for metallic materials, which appears to produce more significant compressive residual stresses than those from the conventional shot peening (SP) for fatigue, corrosion and wear resistance, etc. The finite element method has been applied to simulate the laser shock peening treatment to provide the overall numerical assessment of the characteristic physical processes and transformations. However, the previous researchers mostly focused on metallic specimens with simple geometry, e.g. flat surface. The current work investigates geometrical effects of metallic specimens with curved surface on the residual stress fields produced by LSP process using three-dimensional finite element (3-D FEM) analysis and aluminium alloy rods with a middle scalloped section subject to two-sided laser shock peening. Specimens were numerically studied to determine dynamic and residual stress fields with varying laser parameters and geometrical parameters, e.g. laser power intensity and radius of the middle scalloped section. The results showed that the geometrical effects of the curved target surface greatly influenced residual stress fields.

Introduction

Laser shock peening (LSP) is an innovative surface treatment method, which has been shown to greatly improve the fatigue life of many metallic components. Compared to conventional glass bead peening (GBP), the LSP process introduces a deeper layer of compressive residual stresses—about a millimetre deep than 250 μm deep by GBP (Montross et al., 2002). Moreover, LSP produces very little or no modification of the original surface profile or dimensions of a metallic component in comparison to SP, and has been successfully applied to increase fatigue life, reduce fretting fatigue, enhance resistance to corrosion and increase foreign object damage resistance of compressor blades. It is shown that when LSP conditions are optimal for the material and specimen configuration, a three to four times increase in fatigue life over the as-machined specimens can be achieved. However, if the process parameters are not optimal, the fatigue lives of LSP treated specimens may not reach such an improvement.

Studies of the LSP process have mainly focused on several key issues, which involve in the optimization of the confined interaction modes, the influence of the laser parameters and the relative analytical modelling of the mechanical processes. However during a LSP process, it is very difficult to monitor and investigate dynamic and residual stresses in the target by experimental approaches. Meanwhile, the complexity of the process cannot easily be described using analytical models. Therefore, the finite element method (FEM) can be applied to simulate the LSP process for its exploration and development. Braisted and Brockman (1999) first employed 2-D FEM to investigate mechanical behaviours and predict the residual stresses of Ti-6Al-4V alloy and 35CD4 steel alloy subjected to LSP using a combination of explicit and implicit dynamic analysis algorithms with the FEA package—ABAQUS. Ding and Ye, 2003a, Ding and Ye, 2003b extended and developed their approach from 2-D to 3-D cases to consider different LSP processing on different metal alloys using the Hugoniot elastic limit (HEL) plastic model. Peyre et al. (2003) also applied FEM to simulate the LSP process through 2-D axisymmetric FE models for 12% Cr martensitic stainless steel and 7075 aluminium alloy by applying Johnson–Cook plasticity model for the high strain rate during LSP. Ocana et al. (2003) developed an exact model for the laser-induced time-dependent plasma pressure through fluid dynamics theory for FE modelling.

Although these ABAQUS-based FE simulations provided a relatively close match with the measured residual stresses from experiments, further development is still required to accurately model the LSP process, especially for complicated three-dimensional (3-D) cases. In current study, 3-D finite element modelling technique was further applied to LSP processes using ABAQUS and the main focus of the study was to evaluate the geometrical effects of the scalloped section of aluminium rods subject to LSP with varying laser power intensity.

Section snippets

Basics of laser shock peening

The laser shock peening process is based on producing shock waves using a high power pulsed laser, which involves the generation of confined plasma on the surface of the target material. In most LSP experiments, the laser system is a Q-switched neodymium (Nd)-glass laser with short pulse duration (around 30 ns) focused to produce laser power densities of several GW cm−2 at the target. When an intense pulsed laser impacts the target surface with this confinement, the surface layer is

Design of metal specimens

A series of the hourglass-type specimen geometry was designed for the investigation of effects from variant surface curvature on LSP. The specimens were with continuous radius at their central area, which was scalloped with about 0, 1, 2, 3 and 4 mm depth (h) and 35 mm width (l); the diameter in the central section was 4, 6, 8, 10 and 12 mm as shown in Fig. 2. The scalloped area was the laser shock peened region. Note that the ‘straight surface’ cases of h = 0 mm and d = 12 mm are applied as benchmark

Dynamic stresses

The LSP-induced stress waves have a constant velocity in the same material. The stress waves from the treated surface reach the centre of the rods where they meet the waves from the other side and complex interaction occurs, leading to an increase in amplitude of the compressive stresses and a decrease in amplitude of the tensile stresses. Then the stress waves start their attenuating due to the plastic deformation formed in the specimens. After the shock waves have dispersed the plastic

Summary

In this study, a systematic procedure of finite element modelling and analysis combined the explicit dynamic and implicit static analysis techniques using commercial FEA package—ABAQUS was developed for laser shock peening process of metal alloys. The proposed nonlinear FEA procedure and algorithm were successfully applied to investigate the geometrical effects of the treated surface of AA7050 rods and numerical results showed the existence of curved surface greatly influenced the distribution

Acknowledgements

C. Yang and P.D. Hodgson would like to appreciate the financial support from the Australia Research Council (ARC) through Prof. Hodgson's Australian Federation Fellowship (FF0455846).

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