Influence of pre-stretching on the tensile strength, fatigue properties and the in-plane anisotropy in Al-Cu-Li alloy AA2099

Abstract

The influence of pre-stretching prior to artificial aging on microstructure, tensile properties, and fatigue behavior of AA2099-T8 sheets was investigated. The results showed that increased pre-strain level from 1% to 6% resulted in a more uniform distribution and increasing density of the T1 phase. The localized plastic deformation during pre-stretching led to the formation of dense precipitation bands (DPBs) and precipitation free bands (T1-PFBs) in the sheets with the 1% pre-strain. The non-uniform distribution of the T1 phase with T1-PFBs and DPBs resulted in a decrement in elongation and a strong in-plane anisotropy of tensile properties. An appropriate level of pre-strain should be executed aiming to improve elongation and reduce the tensile anisotropy by promoting a uniform distribution of the T1 phase. In-plane anisotropy of yield strength due to the presence of DPBs and T1-PFBs was quantitatively analyzed by the modified Hall-Petch formula. The results of the calculation for the modified model showed good agreement with the experiment. In addition, resistances of fatigue crack propagation (FCP) in the same orientation were gradually decreased with the increment of pre-strain due to the increment in yield strength and more uniform distribution of the T1 phase. For the in-plane anisotropy of FCP resistance, the main reasons were the development of rough fracture surfaces and the transgranular fracture modes in L-T orientation.