How does surface integrity of nanostructured surfaces induced by severe plastic deformation influence fatigue behaviors of Al alloys with enhanced precipitation?

Abstract

Effects on fatigue behaviors and microstructures of Surface Mechanical Attrition Treatment (SMAT) before or after precipitation aging have been investigated on two hardenable Al alloys. The aluminum alloy with high notch sensitivity (7075) should not be processed by SMAT as the generated low surface integrity is always detrimental to fatigue performances. On less notch sensitive alloys (2024), SMAT before aging formed smaller and denser precipitates, resulting in a high hardened depth, and microstructures more resistant to residual stress relaxation than after conventional shot-peening. SMAT after aging resulted in a significant improvement of fatigue performance with only subsurface crack nucleation sites.

On the high cycle fatigue resistance of austenitic stainless steels with surface gradient microstructures: Effect of load ratio and associated residual stress modification

“…These negative behaviours were again attributed to the easier initiation of the fatigue cracks which was cancelling the improved crack propagation resistance due to the compressive residual stress layer [57,58]. Maurel et al studied the effect of SMAT duration when combined with ageing heat treatment on the fatigue behavior of two aluminium alloys, namely a 2024 and 7075 series [26]. Some samples were aged before SMAT and some after SMAT.…”

“…SMAT or similar treatments were suggested to be efficient ways to enhance the fatigue behavior of many metallic alloys due to the coupled effect of microstructure, hardening gradient, and compressive residual stresses [17][18][19][20][21][22]. However, some manuscripts suggested that SMAT can be detrimental for some materials or if the chosen set of parameters for the SMAT treatment is not optimized [23][24][25][26][27].…”

Effect of Surface Mechanical Attrition Treatment on Torsional Fatigue Properties of a 7075 Aluminum Alloy

“…Rough surface is a negative factor, as it generally causes local stress concentration that promotes crack nucleation from the surface, thereby degrading the fatigue performance of materials [56][57][58]. According to Maurel et al [3], the high notch sensitivity of AA7075 alloy could explain why SMAT cannot improve its high cycle fatigue resistance, compared to other aluminum alloys such as AA2024. Moreover, according to Akiniwa et al [28], the fatigue strength of harder materials is very sensitive to defects.…”

“…Therefore, extensive studies have been conducted to understand the fatigue behavior of high strength aluminum alloys over the past decades. Most of these studies were performed under uniaxial loading [2][3][4][5], but little data could be obtained for torsional fatigue. However, with the increase in structural design requirements and the improvement in testing systems, torsional fatigue behavior of structures is gaining increasing attention, but always constitutes a challenging research subject.…”

“…It is well documented that both high strength and sufficient ductility are important to obtain high fatigue resistance for nanostructured metals and alloys. Unfortunately, the ductility of materials is, in general, severely impaired after severe plastic deformation treatment processes [3]. Therefore, it is necessary to improve the mechanical properties in terms of ductility and toughness [32].…”