Macromechanics study of stable fatigue crack growth in Al–Cu–Li–Mg–Ag alloy

Abstract

This study was made on a fresh variety of Al–Li base alloy to investigate the role of ageing precipitates and microstructure dimensions in the fatigue crack growth resistance. The fatigue crack growth rate was measured in three different states of the material (i.e. base metal in T8 condition, friction stir weld and laser beam weld in full‐aged condition). Metallurgical analysis showed that the base metal in T8 temper is precipitation hardened by an equivalent amount of δ′ (AL3Li), T1 (AI2CuLi) and θ′ (AI2Cu) precipitates. The friction stir weld retained the morphology of strengthening precipitate; however, coarsening of Cu containing precipitates has occurred. On the other hand, laser beam weld showed a different type of CuAl phase morphology, which is characteristic of cast metal. The results of fatigue tests confirmed that fatigue crack growth resistance largely depends on microstructural features, specifically the strengthening phases. The fatigue crack resistance was in the order of base metal > laser beam weldment > friction stir weldment. The CuAl phase played a vital role in the crack closure of the laser beam weldment, thus enhancing the fatigue life as compared with the friction stir weldment, which was evident from the plot between log of da/dN (crack growth in each cycle) and log of ΔK (stress intensity range).

Analysis of the Fatigue Damage Behavior of AW2099-T83 Al-Li Alloy under Strain-Controlled Fatigue

“…It can be observed that the tensile strength is comparable to similar material [18,31] but higher than several other types of Al-Li alloys [3,7,19,24,34] reported in the literature. It has been shown that increase in the Cu and Li contents of the alloy promotes the formation of strengthening precipitates such as Al2CuLi and Al2Cu phases which increase the alloy's strength [13,24,35]. Aging condition, homogenization heat treatment, orientation of testing and precipitate phases have been identified as vital factors which influence the alloy's tensile properties [5,18,21,22,31].…”

“…However, challenges such as prohibitive cost, poor fatigue performance, low fracture toughness, poor corrosion resistance and high anisotropic behavior constrained the wide deployment of Al-Li in the aircraft industry [1,2,7,8]. Perhaps, poor fatigue performance is the larger reason that sundry works on different generations of the alloys were focused on fatigue crack behavior [9][10][11][12][13][14][15]. While there is growing research interest in the tensile behavior of the Al-Li, fatigue studies of the alloy are scarce in the open literature.…”

“…The third generation of Al-Li alloy has, in recent times, attracted renewed research interest because of its comparably high strength [18][19][20][21][22][23]. This generation of Al-Li alloy contains less than 2% Li and has been reported to exhibit better corrosion resistance, fatigue crack growth resistance, amenability to various welding technology and higher mechanical strength and toughness [13,[24][25][26][27]. It was anticipated that the new generation of Al-Li alloy may find its presence in aircraft components such as the fuselage skin, wing, doors, windows, etc.…”

A new expression for crack opening stress determined based on maximum crack opening displacement under tension–compression cyclic loading

“…Recently, Antunes investigated the main numerical parameters affecting PICC. The importance of RICC on fatigue crack propagation has also been investigated; for example, Akhtar proposed that the superior fatigue crack growth resistance of the weldment could be attributed to the occurrence of RICC mechanism. Silva made roughness measurement experiments and concluded that at negative stress ratios, RICC was not a relevant mechanism.…”

Crack closure in friction stir weldment using non‐linear model for fatigue crack propagation

“…In their studies, these researchers have used a range of methods from a simple tensile loaded plate to more complex structural components under both uniaxial and mixed‐mode loading conditions. Huffman, Shi et al, Kujawski and Ellyin, Li et al, Noroozi et al, Song et al, Tong et al, and Akhtar et al are some of the researchers who have studied the fatigue behaviour of cracked components under uniaxial or multiaxial non‐linear loading conditions. In some cases, the effect of the mean stress on the fatigue crack behaviour was also assessed.…”