Research On Rapture Performance And Microalloying Effect Of 7xxx-series Aluminum Alloy

7xxx series aluminum alloys used in key components of airplane are characterized by high specific strength and stiffness,excellent machining and SCC property.This paper reports on the tensile and fatigue fracture performances of 7075 alloy in the T651 temper were studied.The rapture mechanisms of 7075 alloy were concluded on the observation of fractography using scanning microscope.Aiming at promoting fatigue performance of 7xxx series alloy,the effects of addition of yttrium on as-cast microstructure and mechanical property of 7055 aluminum alloy were investigated using optical microscope,scanning electron microscope,X-ray diffraction,etc.The tensile fracture face of 7075-T651 alloy in its short-transverse direction displayed a mix of quasi-cleavage crack and dimple crack.There is no horizontal part on the S-N curve of 7075-T651 and the cycle numbers just increased with the reduction of stress amplitude.The fatigue crack initiations derived from the surface or inner contaminant,and lots of fatigue fringe were generated in the age of crack propagation, meanwhile other fatigue characters such as tyre patterns,fatigue sidestep and secondary crack also can be found on the fracture face.The fatigue failure region showing mixed failure mode of brittle and ductile rapture was similar with the character of tensile fracture.The precipitated phase particles of 7075 alloy have some effects on its fatigue performance.Specifically,the crack initiation resulted from the massive precipitated phase particles,while dispersion of small precipitated phase particles have positive effects on fatigue performance.The micro-alloying research showed that the Al₆Cu₆Y and YAl₃ resulting from addition of yttrium to the 7055 alloy is distributed at the grain boundary.And the 7055 alloy obtains wide-range rate of growth in its toughness with little loss in its tensile strength and hardness with the addition of yttrium,while the as-cast microstructure of 7055 aluminum alloy was fined remarkably and the formation of massive eutectic was restrained.The addition of yttrium narrowed the spacing of second arborescent crystals and skeleton-shape second phase of the 7055 alloy were sphericized and fined,so the elimination of the hard-to-remove eutectic texture reduce the crack initiation field in the subsequent heat treatment.It makes practical sense in promoting the mechanical performance of 7055 alloy.