Influence of Shot Peening Process Variations on the Fatigue Life of Al-Cu-Li Alloys

Author:  B. Graham, G. Weber, H. Diep
Source:  ICSP-13
Doc ID:  2017079
Year of Publication:  2017
Abstract:  
Introduction: Al-Cu-Li alloys, also known as third generation Al-Li alloys, have drawn attention for use in aerospace due to their combination of high strength, toughness, density reduction and corrosion resistance.The reduction in the lithium content to levels such that copper is the primary alloying addition induces microstructural changes that resolve some of the problems encountered with the second generation of Al-Li alloys. Along with good performance properties, aerospace applications require long service lifetimes which typically involves surface anodization for corrosion resistance and primer adhesion along with shot peening for fatigue life enhancement for aluminum structures. Al-Cu-Li alloys are used in the peak aged –T8 temper whereas most conventional Al-Cu alloys are used in the naturally aged –T3 or –T4 tempers. While the peak aged temper provides a number of performance benefits, it reduces the fatigue life of the alloy [1-3]. One of the most effective techniques for extending fatigue life is to induce compressive residual stress at the part surface. [2] The superposition of compressive residual stresses onto the in-service stress profile induced by part loading has the effect of locally reducing the maximum stress and the portion of the stress cycle spent in tension. This affects both crack initiation and fatigue crack growth rate. Shot peening provides compressive residual stresses and also introduces cold working at the surface,both of which are beneficial for improving high cycle fatigue life [3]. However, shot peening also roughens the surface texture which can be detrimental to fatigue life [4] [5]. The anodize process introduces pits onto the aluminum surface, and these pits are frequently the source of fatigue crack initiation [6 7]. Pits or other surface stress concentrations can shorten the time required to initiate cracks. Glass bead peening has been successfully applied to overcome the fatigue life deficit caused by chemical milling [8]. Corrosion fatigue studies have also demonstrated that shot peening can improve the fatigue life for samples with surface pits [9].