Investigation on AA7075 Strips Subjected to Cryorolling and Cryogenic Treatment and Their Fatigue Crack Propagation Performance

doi:10.3901/JME.260175

Abstract

Abstract: 7000 series aluminum (Al) alloys are often used in load-bearing components such as aircraft frames, stringers, and the underframes and traction beams of high-speed rails due to their low density and high strength. Not only are high requirements imposed on the strength of 7000 series Al alloys, but they are also required to have good fatigue crack propagation (FCP) performance. Taking AA7075 as object, cryorolling (CR), corresponding artificial aging treatments and deep cryogenic treatment (DCT) were conducted. It is found that although peak aging can significantly improve the strength of AA7075, attention should be paid to its potential negative impact on the FCP performance. Because η' phases are generated during peak aging, which cannot be shorn by dislocations, dislocations accumulate at the crack tip, accelerating crack propagation. By contrast, DCT scarcely affects strength yet distinctly retards FCP. The yield strength (YS), ultimate tensile strength (UTS), and fracture elongation (FE) of the sample subjected to peak aging followed by DCT after CR (CR-AT-DCT) are 571 ± 0.8 MPa, 612 ± 3 MPa, and 11.2 ± 0.24%. The Paris fitting index m of the sample subjected to peak aging followed by DCT after CR is 7.4% smaller than that of the sample that only undergoes peak aging after CR during the stable FCP stage. DCT promotes 13.8% reduction in the width of the PFZs, making it difficult for fatigue cracks to propagate along the rolling direction of the grain boundaries and thereby slowing the FCP rate. A systematic investigation correlating microstructure with FCP performance demonstrates that the CR-AT-DCT hybrid route furnishes a new process for lightweight design.

Key words: AA7075, deep cryogenic treatment, fatigue crack propagation rate, microstructural evolution

CLC Number:

TG339

SHI Nannan, GAO Haitao, TANG Leqian, KONG Charlie, YU Hailiang. Investigation on AA7075 Strips Subjected to Cryorolling and Cryogenic Treatment and Their Fatigue Crack Propagation Performance[J]. Journal of Mechanical Engineering, 2025, 62(6): 47-64.

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