| The application of multiple materials in automotive is an effective means to realize the lightweight,but the bonding of different materials is an urgent problem to be solved.Polylactic acid(PLA)is a kind of pollution-free plastic material in the process of manufacturing and using.The application of PLA in automobile can not only meet the requirements of automobile lightweight,but also effectively solve the environmental problems.With the popularization and application of PLA and its composite products in automobile,the bonding between PLA and metal plate becomes an urgent problem.This paper takes the aluminum alloy of 5052 and PLA as research objects,and aims at the defects of PLA material,such as low strength,poor aging resistance,and low bonding strength between aluminum alloy and PLA.Based on the research of global scholars in the related fields,the research of this paper is combined with the requirement of project.The mechanism research of aluminum alloy-PLA bonding for automobile multi-materials lightweight was carried out from the aspects of laser treatment of aluminum alloy surface,optimization of laser process parameters in central composite design,basalt fiber(BF)reinforced PLA,temperature aging and finite element simulation and application.The specific research work includes the following aspects:(1)In order to study the influence of the surface characteristics of aluminum alloy on the connection strength of aluminum alloy PLA.On the basis of removing the oxide film of aluminum alloy surface by acid-base solution,the aluminum alloy with micro-morphology was obtained by laser treatment.The relationship between the laser processing parameters(laser power,scanning line width,scanning speed)and the bonding strength of aluminum alloy-PLA was studied from the macroscopic mechanical properties and microscopic morphology.The mechanism of laser treatment of aluminum alloy surface to enhance the bonding strength of aluminum alloy-PLA was analyzed by combining the surface morphology,roughness and the bonding strength of aluminum alloy PLA.The results show that for different laser power,the bonding strength of aluminum alloy PLA increases first and then decreases with the increase of laser power;for different scanning line width,the bonding strength of aluminum alloy PLA decreases with the decrease of scanning line width;for different scanning speed,the bonding strength of aluminum alloy PLA decreases first and then increases with the increase of scanning speed.After laser treatment,the bonding strength of aluminum alloy-PLA is increased by increasing contact area,strengthening coefficient and mechanical engagement.(2)The design of the aluminum-PLA bonding test scheme was completed by the central composite design method.Based on the multiple nonlinear theory,a multiple nonlinear regression model was established between laser power,scanning line width and scanning speed and the bonding strength of the aluminum-PLA.A stepwise regression iterative strategy was used to analyze the effects of laser process parameters and the interaction of process parameters on the bonding strength of aluminum-PLA.An aluminum alloy-PLA bonding test was performed to verify the reliability of the model.The results show that the laser power has the strongest influence on the bonding strength of aluminum-PLA,the scanning line width has the second influence on the bonding strength of aluminum-PLA,and the scanning speed has the weakest influence on the bonding strength of aluminum-PLA.The laser power and scanning line width increase the contact area between the aluminum and PLA,which makes the bonding strength of the aluminum-PLA significantly enhanced.The effect of scanning speed is to change the steam pressure and surface tension so as to affect the bonding strength of aluminum-PLA.The predicted value of the model is basically consistent with the experimental value,and the model can effectively predict the bonding strength of aluminum PLA.Moreover,it is found that the failure position of the aluminum-PLA joint specimen mainly occurs in the PLA itself,and the failure mode is mainly cohesive failure.(3)In order to improve the bonding strength between aluminum alloy and PLA,PLA is reinforced by adding BF to PLA to obtain a BF/PLA composite material.BF/PLA composites with different BF contents were obtained to study the effect of BF content on the BF/PLA composite-aluminum alloy bonding strength.The fracture morphology and regularity of BF/PLA composite with different BF content were analyzed by SEM(scanning electron microscope).The effect of BF content on the crystallization and nucleation of BF/PLA composite was analyzed by DSC(differential scanning calorimetry).It is found that when BF content is 40%,the maximum bonding strength between BF/PLA composite and laser treated aluminum alloy is 59 MPa.When the BF content is 40%,the crystallization temperature is 85℃,and the nucleation crystallization rate increases greatly.The increase of nucleation crystallization rate directly leads to the enhancement of bonding strength between BF/PLA composite and aluminum alloy.(4)The effects of high temperature,low temperature and time on the bonding strength of BF/PLA aluminum alloy were studied by using high temperature 85℃ and low temperature ﹣35℃ as temperature aging conditions and utilizing different aging time intervals and periods.Combined with the crystallization characteristics measured by DSC and the microscopic characteristics obtained by SEM,the change of the bonding strength of BF/PLA aluminum alloy with aging time under different environmental conditions was analyzed,and the fitting prediction was performed in various ways.The results show that the bonding strength of BF/PLA-aluminum decreases with the increase of aging time.The failure mechanism of high temperature aging is mainly that the strength of BF/PLA composite is reduced due to the decomposition.The failure mechanism of low temperature is that the strength is reduced due to the thermal stress caused by the inconsistent expansion coefficient between aluminum alloy and PLA.By comparing the fitting coefficients,it is found that the polynomial function can effectively predict the change of BF/PLA-aluminum alloy bonding strength with aging time.(5)By simplifying the micro-morphology of the bonding between aluminum alloy-PLA,a calculation model was established.And the tie binding and zero thickness cohesive element were used to simulate the bonding strength of aluminum alloy-PLA.Taking the shell of vehicle control unit as a demonstration example,the application of aluminum alloy-PLA bonding technology in lightweight is studied.The BF/PLA aluminum alloy shell is obtained by combining topology optimization,modal and stiffness analysis,furthermore the feasibility of the scheme is verified by experiments.The results show that the bonding strength of the aluminum alloy-PLA is the best,which is consistent with the results of the center composite design analysis and test.The cohesive element with zero thickness can more effectively reflect the real situation of the aluminum alloy-PLA bonding specimen,and the agreement with the test results is higher.The optimal material distribution is obtained through the topological optimization of the vehicle control unit shell.The feasibility of optimization is verified by modal and stiffness analysis.The BF/PLA composite with the best ratio of 40% BF and 60% PLA and the aluminum alloy bonding shell are processed.Through the test,BF/PLA-aluminum alloy bonding shell can meet the use requirements and achieve 8.9% weight reduction effect.The application case proves the possibility of aluminum alloy-PLA bonding technology in the lightweight application of automobile multiple materials,which lays a foundation for the promotion and application of metal plastic bonding technology. |
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