| Under the development background of energy conservation and emission reduction in the automotive industry with the goal of “double carbon”,multi-material mixing use is the inevitable trend of autobody lightweight.Aluminum-steel hybrid autobody can achieve more comprehensive and balanced lightweight.The autobody closure panels system determines the static perception of the whole vehicle and affects the important dynamic performance.Its structure needs to be manufactured by the special roller Hemming-Joining process,involving multi-step processes such as adhesive gluing,roller hemming and high-temperature curing.These important processes are interrelated,which jointly determine the final geometric accuracy of the closure panels system,reflecting the advanced level of manufacturing technology.Aluminum-steel hybrid closure panels are a“sandwich”multi-material roller Hemming-Joining structure including aluminum alloy outer panel,high-viscosity structural adhesive layer and high-strength steel inner panel.In the roller hemming of the closure panels containing adhesive,the aluminum-steel sheet metal is subjected to the combined action of external complex load and non-Newtonian rheology of internal adhesive layer.In the subsequent long-term high-temperature baking-curing conditions,the multi-state transformation of the mechanical properties of the polymer adhesive layer,the unbalanced constraints of sheet parts,the material and geometric asymmetry of hemming structure make the closure panels system experience complex mismatch deformation.Aluminum-steel closure panels with dissimilar materials have great differences in material physical properties and mechanical properties,which puts forward new challenges to the connection process.Therefore,based on the material properties of high viscosity and one-component thermosetting adhesive layer and aluminum-steel,this paper aims at the multi-step process chain “spreading-hemming-curing” for the manufacturing of body closure panels with dissimilar materials,and forms the shape control and optimization method of roller Hemming-Joining process with adhesive of aluminum-steel autobody closure panels,starting with the two major problems of pressure-viscosity effect and curing effect.By establishing a multi-process overall shape control model that takes into account the influence of the filling state of the adhesive layer in the early hemming gap and compensates the subsequent curing deformation in advance,the optimization of the roller Hemming-Joining process chain model based on pre-compensation is realized,and the manufacturing quality of the roller Hemming-Joining of steel-aluminum body closure panels with adhesive is supported to be improved.The main investigated contents of this paper are as follows:1)Rheological properties and thermal-viscoelastic curing model of adhesive layer under multi-state transformationAiming at the problem that the nonlinear transformation law of the physical and chemical material properties of the adhesive layer in the multi process of“spreading-hemming-curing” is not clear,the squeezing rheological properties and thermo-viscoelastic curing constitutive model of one-component thermosetting adhesive under multi-state changes are studied.The characteristics and rheological model of non-Newtonian fluid in viscous flow adhesive layer are analyzed,the state equation of parallel squeezing laminar flow similar to hydrodynamic lubrication is constructed,and the action law of pressure-viscosity effect is analyzed.Based on polymer reaction kinetics,the curing degree is used to describe the material transition state of adhesive layer under heating condition.The relationship between temperature load,curing degree and gel mechanical behavior is analyzed.The viscoelastic properties of the cured adhesive layer are studied,and its glass transition,stress relaxation and time-temperature equivalent behavior are analyzed combined with experiments.On this foundation,a multi-stage thermo-viscoelastic curing model of adhesive layer under cyclic temperature load is established.The accuracy of the model is verified by the dynamic shear mechanical response experiment of cylindrical adhesive.This lays a foundation for the numerical simulation of hemming with adhesive and curing,and process pre-compensation in subsequent chapters.2)Roller hemming-forming of autobody dissimilar materials sheet and adhesive layer filling law under Pressure-Viscosity effectAiming at the problem of unclear influence mechanism of the squeezing rheological characteristics of adhesive layer on the roller hemming-forming accuracy of heterogeneous thin plates and the filling state of adhesive layer,SPH(Smooth Particle Hydrodynamics)smooth particle method is introduced to simulate the adhesive layer to quantify its flow in the gap of thin plates.The feasibility of the method is verified by comparing with the traditional Euler method.On this foundation,considering the rheological factors of adhesive layer,the SPH-FEM(Finite Element Method)coupling numerical model of aluminum alloy sheet,adhesive layer and steel sheet is established,the roller hemming process of aluminum alloy sheet under the Pressure-Viscosity effect is analyzed,and the coupling mechanism between adhesive layer flow and sheet bending is revealed.The evolution law of forming quality and hemming force under the change of geometric profile and process parameters is studied,and the interaction of different adhesive layer factors on roller hemming target are analyzed.The gap filling rate of adhesive layer is proposed to quantify the filling state,and the influence law of gluing parameters on the filling rate is analyzed.3)Curing deformation law of aluminum-steel roller Hemming-Joining structure under wide range cyclic temperature loadAiming at the problem that the mechanism of uncoordinated deformation and residual internal stress of adhesive layer of steel-aluminum Hemming-Joining structure during high temperature curing is not clear,the transient evolution process of coupling field is simulated and analyzed using finite element method.Based on the multi-stage curing model of adhesive layer,combined with autocatalytic reaction dynamics and elastic-plastic mechanics,the thermal-chemical-structural coupling field of “sandwich” Hemming-Joining structure of heterogeneous materials is numerically simulated.The whole field transient deformation process of aluminum alloy outer plate is verified and analyzed by non-contact dynamic measurement.The generation process of non-coordinated deformation of metal structure under high temperature cyclic load is revealed,and the evolution mechanism of residual internal stress and strain of adhesive layer is clarified.The effects of sheet metal material characteristics,bonding geometric parameters,adhesive layer properties and structural contour shape on the change of structural curing deformation and residual internal stress are analyzed,and the matching optimization of process parameters is carried out to reduce the curing deformation.4)Structural deformation pre-compensation and roller Hemming-Joining shape control optimization for process chainAiming at the lack of integrated deformation control and optimization model of multi-step process,a modeling and deformation pre-compensation method of closure panels roller Hemming-Joining system of heterogeneous materials for the process chain is proposed.An overall shape control optimization model under multiple processes is established,which not only takes into account the influence of the filling state of the adhesive layer in the early rolling gap,but also pre-compensates the subsequent curing deformation.Combined with the compensation method of body sheet structural deformation,based on the node compensation algorithm and the principle of NURBS surface reconstruction,a structural deformation precompensation for the process chain is formed.Through iterative compensation,the deformation correction efficiency and control accuracy of plane straight-edge and plane curved-edge samples are analyzed.For the assembly process of body closure panels,taking the door scale model as an example,according to the process chain precompensation method,the overall deformation pre-compensation accuracy control under complex geometric contour is realized,and the manufacturing dimension deviation is reduced.To sum up,facing the roller Hemming-Joining process chain forming of autobody closure panels manufacturing,this paper reveals the pre-compensation forming mechanism of aluminum-steel structure under the pressure-viscosity-curing effect of structural adhesive layer,and supports to improve the roller Hemming-Joining process quality of body appearance assembly with dissimilar materials.This study supports the technical upgrading of new energy vehicle body lightweight,and also provides a reference for improving the manufacturing process accuracy of heterogeneous materials bonded new energy battery pack and other related fields. |
