Study On Contact And Friction In Warm Forming Of Aluminum Alloy Sheet

With the issue of energy and environment becoming increasingly acute,the automobile manufacturers are expediting the development of lightweight vehicles.One of the major tasks in the development of lightweight vehicles is to develop lightweight auto body panels.Aluminum alloy sheet has been applied to produce auto body panels because of its high strength and low specific gravity.However,aluminum alloy sheet is more difficult to form than steel sheet due to its lower plasticity.It is necessary to adopt warm forming process to form aluminum alloy sheet.During the warm forming process, lubrication and friction status are the key factors which influence the forming quality of aluminum alloy sheet.So far,the lubrication and friction status during aluminum alloy sheet warm forming are still the technologies which need further investigation.Supported by Technology Consortium of GM and Chinese Institutes for Materials and Tribology,the author has studied the contact and friction between sheet's and die's surface in aluminum alloy sheet warm forming in both theoretical and experimental approach,and established the contact model based on fractal theory and the friction models for different lubrication status.The friction coefficients in different forming areas are measured by experiments.In order to study the laws of the contact and friction behavior,the topography data of aluminum alloy sheet surface should be obtained first so that the topography model can be set up.The author obtained the topography data of the aluminum alloy sheet 5182 surface and the die surface by means of AF-LI Synthesis Measuring Profilometer.The surface topography model of aluminum alloy sheet based on fractal theory has been established. The model characterizes and identifies the sheet surface with fractal parameters of scale-independence.The prediction results of topography using fractal based model are not affected by the resolution of measuring instruments and the length of sample.Hence, the model is more reasonable and effective than those based on statistical analysis.Then,the elastic-plastic contact model between aluminum alloy sheet surface and die surface has been established based on the sheet surface topography model.The contact model,which treats the die as rigid body,gives the laws of the interface's dimension distribution and the calculational formulas of the real contact area under different normal contact loads.Lubrication and friction status are key factors which influence the forming quality of aluminum alloy sheet during the warm forming process.The author has analysed the lubrication and friction status in different forming areas during aluminum alloy sheet warm forming,and drew the conclusion that the lubrication status between the sheet surface and die surface is boundary lubrication status or mixed lubrication status.The calculational formulas of the friction coefficient are given by the author.Together with the real contact area calculated by the contact model,the friction coefficient under different normal contact loads can be calculated.In order to test and investigate the dynamic variation of friction in experimental approach,new friction coefficient measurement method and devices have been developed to measure the friction coefficient at punch corner,die corner and drawbead in aluminum alloy sheet warm forming.The method overcomes the disadvantage of existing measurement method.And the feature of the devices is that the real warm forming process of the sheet can be modeled during the measurement.The aluminum alloy sheet sample with wedge-shape is adopted to get the circumferential compressive stress exerted by the cuneal groove.Therefore,more accurate measuring results can be obtained.Based on the developed experimental systems,the author measured the friction coefficients at punch,die comer and drawbead during aluminum alloy sheet 5182 warm forming.The variation of friction coefficient with the punch's movement is determined. The rationality of the academic models is proved by comparison between the calculated results with the experimental data.