| Laser Peening(LP)technology produces high amplitude shock wave pressure by laser induced plasma,to implant deeper depth residual compressive stress on the surface of a material,while increasing the surface dislocation density as well as refine the surface grains to improve corrosion resistance,fatigue,anti-hydrogen embrittlement and other mechanical properties of the material.Recently,the combination of LP and traditional Heat Treatment(HT)processes has become one of the developing technology trends.Cryogenic Treatment(CT)refers to improving micro-structures and other mechanical properties of materials under ultra-low temperature environment.The typical aviation aluminum alloy 2024-T351 was used as the research object of this dissertation,and the effects of CT on the micro-structures and properties of 2024-T351 aluminum alloy were investigated.Besides,its influence on LP was also studied.The main contents are as follows:(1)Theoretically,the effects of CT on the residual stress and fatigue properties of LPed 2024-T351 aluminum alloy were investigated.It involves residual stress field induced by LP,effects of residual stress on metal fatigue properties,the stability of residual stress and the mechanism of CT on metals.It was found that the strength,hardness and elastic modulus of the 2024-T351 aluminum alloy were improved due to micro-structural evolution of the second phase,the grains and the dislocations after CT,which affected the residual stress and its stability induced by LP,thereby resulting to further improvement of the fatigue properties of the material.(2)Experimentally,the effects of CT on the micro-structures,mechanical properties and surface residual stress of 2024-T351 aluminum alloy were studied systematically.CT was carried out on 2024-T351 aluminum alloy samples for 2h,4h,6h,8h and 12 h with liquid nitrogen(-196 ?),which was followed by tensile and nano-indentation tests.The micro-structures,second phase state and surface residual stress of the materials before and after CT were also tested and analyzed.It was found that the grains were more perfect,the defects in the material were eliminated and the impurities were partially dissolved after CT.Besides,the size and distribution ofsecond phases were more uniform after CT.The results of tensile and nano-indentation tests at room temperature show that CT can significantly improve the tensile strength,yield strength,nano-hardness and elastic modulus of 2024-T351 aluminum alloy.The results show that the plasticity and fracture toughness of the 2024-T351 aluminum alloy have been improved by CT,which could be proved by the transformation of local cleavage to plastic fracture and the deep reason is the dissolution of impurities as well as the refinement and homogenization of second phases.In addition,the residual stress test showed that CT could eliminate the residual tensile stress on the surface of the2024-T351 aluminum alloy and introduce a small residual compressive stress.(3)LP of 2024-T351 aluminum alloy was carried out to study its effects on the material before and after CT.The main work included metallographic observation,surface residual stress tests,cross-section hardness tests,and the stability of residual stress and fatigue tests were also carried out.The results show that grain refinement is better in the specimen of LP after CT,as the grains were more rounded and the grain boundaries were clearer.The surface residual stress amplitude and hardness 2024-T351 aluminum alloy subjected to LP were increased after CT.The stability of residual stress field of 2024-T351 aluminum alloy induced by LP could be significantly improved after CT,thus effectively enhancing the strengthening effect of LP.The fatigue life of2024-T351 aluminum alloy subjected to either CT or LP could be improved,and the effect of LP after CT proves to be much better. |
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