Mechanism of Precipitate Microstructure Affecting Fatigue Behavior of 7020 Aluminum Alloy

Abstract

The effect of different precipitate microstructures obtained by different heat treatments on fatigue behavior of 7020 aluminum alloy was investigated. The fine Guinier Preston I (GPI) zones in the under-aged alloy can be repeatedly sheared by dislocations produced in cyclic loading, making the fatigue crack initiate difficultly and fatigue crack path propagate tortuously. Fatigue strength and fatigue crack propagation resistance of the alloy with shearable precipitates are much higher than those of the alloy with unshearable precipitates. The peak-aged alloy with continuous grain boundary precipitate (GBP) and narrow precipitate free zone (PFZ) is prone to initiate fatigue cracks and reduce fatigue strength. With the growth of unshearable precipitates, the fatigue strength of the alloy firstly increases and then decreases. Precipitates with moderate size in the over-aged alloy improve the roughness-induced crack closure (RICC) effect. Soft matrix with appropriate width between the precipitates can promote the slip reversibility and relax the crack tip stress. The fatigue strength of the moderately over-aged alloy reaches to 122.1 MPa at 107 cycles of loading, and the fatigue crack growth rate (FCGR) is 35.6% slower than that of the peak-aged alloy at ΔK of 10 MPa·m1/2.

Welding of AA6061-T6 Sheets Using High-Strength 4xxx Fillers: Effect of Mg on Mechanical and Fatigue Properties

“…The fatigue properties of aluminum alloy joints play an important role in the safety and reliability of structural assemblies [ 6 ], with up to 90% of engineering constructions failing because of fatigue [ 7 ]. Researchers have found that excess solute Mg can improve the fatigue strength and fatigue life of 6xxx alloys [ 8 , 9 , 10 , 11 , 12 ]. Takahashi [ 9 ] reported that the fatigue strength and fatigue life of 6xxx alloys can be improved by adding excess Mg, so that Mg exists in solid solution in the matrix.…”

“…The effect of β″ precipitates on the fatigue properties of Al-Mg-Si 6xxx alloys has been investigated [ 2 ], in which it was found that heat treatment significantly improves the fatigue strength of AA6061-T6 joints with ER4043 filler owing to the formation of β″ precipitates resulting from Mg via base metal dilution. Many studies have investigated the interactions between dislocations and precipitates [ 11 , 13 , 14 , 15 ]. Notably, because dislocations can shear coherent or semi-coherent precipitates, the number density of precipitates increases via shearing, leading to enhanced resistance against dislocation movement and hence increased fatigue strength.…”

Comparison on the Fatigue Crack Propagation of High‐Strength Aluminum Alloys

“…FCP curves of 7055 and 2219 Al alloys at different aging states, experimental results from refs., [26,27] respectively. a,c) 7055; b,d) 2219.…”

“…Both 7075 and 2024 Al alloys should have the lowest FCP rate at the UA state if the effect of the aging state on FCP rate is explained in terms of dislocation slip reversibility as by previous studies. [25][26][27] However, this is not the case with the experimental results, which indicates that dislocation slip reversibility is not the main factor affecting the FCP rate. It is well known that the reversibility of dislocation slip at different aging states is dominated by microscopic deformation mechanisms.…”

“…[20][21][22] However, the OA state usually exhibits a cross-slip mode due to the repulsive effect of the dislocation loops. [23,24] Most studies [25][26][27] have shown that the slip planarity of the UA state facilitates dislocations to reciprocate at the crack tip compared to the OA state, which effectively reduces the FCP rate of the alloy.…”