Research On The Behavior Of Surface Asperity Flattening At Plastic Deformed Interface In Mixed Lubrication

The plastic deformed interface widely exists in metal forming process, at which the surfacetopography of workpiece and tool, lubricant form a complex system. In this dissertation, in order togain a better understanding of the evolution of workpiece surface topography and to provide atheoretical basis for the control of surface quality of products and the interfacial friction, thebehaviour of asperity flattening of workpiece surface at plastic deformed interface in mixedlubrication regime is studied.Based on mechanical-rheological model and the three dimensional surface parameters definedin ISO25178, the characterization of the changes of surface topography resulting from asperityflattening is investigated. Using mathematical morphology image processing technology, theextraction of closed lubricant pockets features and the three dimensional surface parameters derivedfrom mechanical-rheological model and the assessment of real contact area ratio are achievedeffectively.A finite element model for rough surface contact between tool and workpiece is developed.Theeffects of the contact pressure, bulk plastic strain, interfacial friction and asperity angle on asperityflattening are analysed in detail. The results show that the real contact area ratio increases with thecontact pressure and bulk plastic strain. The contribution of interfacial sliding friction to asperityflattening is not comparable to that of bulk plastic strain as well as contact pressure. The resistanceof asperity to flattening increases with increasing asperity angle, but the increasing rate decreases.The experimental results of compression tests on aluminium samples show the validity of the finiteelement simulation.A series of plane strain compression tests on aluminium1050sheet specimens with differenttypes of texturing topographies are conducted with lubricantion and without lubricantionrespectively. Measurement and alalysis of the surface topagraphy of the specimens are performed toinvestigate the asperity flattening behaviour and variations of surface microstructure for variousloads and bulk strains. Based on the correlation analysis between the evolution of the closedlubricant pockets and the behaviour of asperity flattening, the effect of surface topography on theasperity flattening in static contact process is discussed. By using the three dimensional surfaceparameters defined in ISO25178and derived from mechanical-rheological model respectively, thecharacterization of surface topography is achieved to describe the changes of surface microgeometryinduced by asperity flattening. In order to investigate the asperity flattening behaviour and the changes of surfacemicrostructure as well as the frictional performances at plastic deformed interface in sliding contactprocess, a novel metal forming tribometer is built up and used to perform a series of strip drawingtests.It is found that the coefficient of friction shows a dependence on contact pressure and drawingvelocity when drawing velocity and contact pressure remains constant respectively, which can berelated to the evolution of closed lubricant pockets. The variations of surface microstructure withnominal contact pressure and drawing velocity are characterized by such three dimensional surfaceparameters as arithmetical mean height, the fastest decay autocorrelation length, texture aspect ratio,developed interfacial area ratio and root mean square slope of the surface. Based onmechanical-rheological model, the internal relationship between the lubrication regime, thevariations in the surface microstructure, the load bearing mechanism and the interfacial friction isstudied to reveal the influnce of asperity flattening on the interfacial friction. The applicability of themaximum closed void area ratio for a description of the friction performances in metal formingprocess is analyzed.Considering entrapment and escape of liquid lubricant in closed lubricant pockets at plasticdeformed interface, theoretical models for asperity flattening of workpiece surface with a regulararray of wedges for static contact and sliding contact respectively are developed. The effects ofdifferent contact conditions on surface asperity flattening are analyzed in detail. The validity of thetheoretical models of asperity flattening proposed is verified primarily.An important innovation in this dissertation is to extract the closed lubricant pockets featuresand to relate three dimensional surface parameters and the evolution of closed lubricant pockets tothe behaviour of asperity flattening at plastic deformed interface in mixed lubrication regime. Studyresults will be beneficial to improve theoretically the control of surface quality of products andinterfacial friction.