Research On Characteristics Tension-compression Fretting Fatigue Of Ultrafine-grained 6061 Aluminum Alloy

In the context of increasingly serious energy and environmental problems,aluminum alloy,as a lightweight material,has rapidly entered the aviation,automobile,transportation,construction,machinery manufacturing and other fields.However,fretting fatigue widely exists in connection structures of above-mentioned fields,and fretting fatigue is highly sensitive to aluminum alloy materials,which will greatly reduce the service life of aluminum alloy components.Therefore,people try to use various technologies to enhance the anti-fretting fatigue capability of materials.Severe plastic deformation is a method to prepare ultrafine-grained materials by refining the grain size,it can significantly improve the strength and hardness of the material,has shown great potential in the field of improving the plain fatigue life of materials.In order to make full use of the advantages of high strength and high wear resistance of ultrafine-grained aluminum alloy,it is very urgent to carry out the research on fretting fatigue of ultrafine-grained aluminum alloy to promote its application in the field of fretting fatigue,It can not only understand the ability of anti-fretting fatigue and fretting fatigue failure mechanism of ultrafine-grained aluminum alloy,but also find a breakthrough way for the engineering application of ultrafine-grained materials.Therefore,this paper takes the ultrafine-grained 6061 aluminum alloy prepared by multi-directional forging technology of severe plastic deformation as the research object;Firstly the tension-compression fretting fatigue test controlled by stress was carried out;Then the scanning electron microscope,EDS energy dispersive spectrometer,super three-dimensional microscope system,surface roughness measurement instrument and other analysis equipment are used;Finally,the fretting fatigue behavior and mechanism of ultrafine-grained 6061aluminum alloy were studied.The specific research contents and conclusions include the following aspects:(1)Ultrafine-grained 6061 aluminum alloy was prepared by multi-directional forging technology,and a set of surface contact fretting fatigue device was designed based on tension-compression fatigue testing machine to carry out fretting fatigue test;then,the fretting fatigue life of coarse-grained and ultrafine-grained 6061 aluminum alloy was compared and studied,the fatigue life of ultrafine-grained 6061 aluminum alloy even under the stress amplitude of 280MPa is longer than that of coarse-grained 6061 aluminum alloy under stress amplitude of 200 MPa;Under the normal contact stress with the same growth trend,the fatigue life of ultrafine-grained 6061 aluminum alloy decreases by 42%,which is also smaller than that of coarse grain by 56%;At the same time,the fretting fatigue life of coarse-grained and ultrafine-grained 6061 aluminum alloy decreases first,then reaches the normal contact stress threshold,and then increases slowly.The normal contact stress threshold of ultrafine-grained 6061aluminum alloy is 60 MPa,while that of coarse-grained 6061 aluminum alloy is40 MPa,the results show that the ultrafine-grained grain 6061 aluminum alloy has strong capacity of bearing normal contact stress.Based on the above results,ultrafine-grained 6061 aluminum alloy has good fretting fatigue resistance.(2)By observing the morphology of fretting fatigue crack and fracture surface,the crack propagation angle and fatigue source depth are defined and measured with the help of three-dimensional microscope and surface roughness meter,and the distance between the peak oxygen content and the contact surface was analyzed to explore the fretting fatigue crack propagation law.The fretting fatigue crack of ultrafine-grained 6061 aluminum alloy propagates along a certain angle(that is,the angle with the axial stress direction),and with the increase of normal contact stress,the crack propagation angle decreases from 90°to 34.1°;Secondly,with the increase of normal contact stress,the depth of fretting fatigue source region increases,and the peak value of oxygen content is further away from the contact surface along the direction of fretting fatigue crack propagation,but the change of morphology of crack propagation region and instantaneous fracture region is not obvious.This phenomenon shows that the change of normal contact stress has a significant effect on the initiation of fretting fatigue crack,but has little contribution to the propagation of fretting fatigue crack.(3)The morphology,wear condition and chemical state of fretting fatigue damage region were quantitatively analyzed by introducing width,wear depth and element content of damage region.The results show that the fretting fatigue damage mechanisms mainly include fatigue,adhesive wear and abrasive wear of oxides(such as Al₂O₃).The adhesive wear and abrasive wear of oxides in fretting damage zone increase with the increase of normal contact stress;Under different normal contact stress,the morphology of fretting damage region presents asymmetric distribution in axial direction,and the damage of loading end is more serious than that of fixed end;that is the depth of wear scar and oxygen content of the loading end are greater than those of the fixed end,the reason is that the relative slip amplitude of the fretting damage zone near the loading end is large and the wear is relatively serious.