Research On The Effect Of Contact Pressure On Torsional Fretting Fatigue Properties Of A 7075 Aluminum Alloy

Fretting widely exists in modern industry.Due to the complex working environment,fretting wear,fretting fatigue and fretting corrosion take place in some mechanical components,which decrease their life greatly and reduce their service time,sometimes even cause a great threat to public security.As a new cross-subject,fretting involves machinery,materials,chemistry,and so on.Scholars domestic and overseas have studied its mechanism one after another in over a hundred years and made remarkable achievements,but as its extremely complicated mechanism and process,there are still some divergences in concept and mechanism.Therefore it's necessary to invest in more research on fretting.7075 aluminum alloy is researched on a servo hydraulic fatigue-testing machine with a self-designed pneumatic device which provides normal loads in this paper,the torque was fixed at 72.5N·m,and changed the contact pressure in the test,and the influence of contact pressure on torsional fretting fatigue of 7075 aluminum alloy is investigated with scanning electron microscopy?SEM?,electron probe micro-analyzer?EPMA?,X-ray photoelectron spectroscopy?XPS?and white light interferometer?WLI?is used to analyz e the damage area.The conclusions are as follows:?1?As the contact pressure increases,the TFF life of 7075 aluminum alloy decreases at first and then reaches the lowest point at 300N,while afterward the TFF life increases.On the whole,The P-N curves shows a C shape.The process of contact pressure increase can be divided into three regimes,they are slip regime?SR?,mixed fretting regime MFR,and partial slip regime PSR.Fretting runs from SR to MFR,and finally enters into PSR with increasing the contact pressure.?2?The failure mode of torsional fretting fatigue of 7075 aluminum alloy is fracture at an angle of 45°,the fatigue fracture can be divided into three regions,which are the crack source region,the crack propagation region and the instantaneous fracture region respectively.The contact pressure can impact the crack source region obviously,the depth of it increase with contact pressure.By contrast,the crack propagation region and the instantaneous fracture region is almost impervious.?3?The finite element results show that the distribution of Mises stress in contact region is elliptic,as the contact pressure increases,there are stress concentration appear near the fretting scars,and the stress concentration in the edge and upside is much more severe than that in the center zone and bottom.The location of the stress concentration is conform to the crack location in test,which prove that the crack is caused by stress concentration.?4?TFF damage degree increases with contact pressure,the characteristic of damage is similar at different contact pressures,that means the damage area is basically bilateral symmetry,the loading side is more serious than the fixed side and there is ploughing phenomenon in the fretting scars.The change of ploughing phenomenon causes the change of friction coefficient,when it is stable,the friction coefficient decrease with contact pressure.?5?The oxidation degree of damage zone is positive correlated with the contact pressure,and the main oxidative products are Al₂O₃,meanwhile slight Al?OH?₃ and pure aluminum exist.