Numerical Simulation And Technology Foundation In Laser Bending Of Stainless Steel-Carbon Steel Laminated Plate

A new corrugated bulkhead made of stainless steel-carbon steel laminated plate(SCLP)determines the carrying capacity,performance and safety of the equipments such as flying machine,deep-see scuba machine.Laser bending is an effective forming process which is suitable for the parts of plate and shell such as bulkhead.The SCLP has joint surfaces with different materials,and there are sharp changes of material properties along the thickness direction of the transition layers.In order to ensure high forming accuracy,it is necessary to accurately predict bending angle of SCLP,so that provides basic research for the carrying capacity of those equipments.According to different material properties of each layer of SCLP,the multi-layer analytical model of laser bending angle is established based on temperature gradient mechanism.During heating,the depth of the plastic zone s is defined and calculated by introducing the critical temperature of the plastic zone.Based on thermoelastic-plastic finite element theory,the finite element model of SCLP considering the equivalent properties of the transition layer is established to predict bending angle during laser bending.The energy compensation method and the line-shape spot simultaneous heating method are proposed to reduce the warping deformation and improve forming accuracy of SCLP.The research achievements will enrich laminate bending theory and promote applications of SCLP for high-end carrier.The main research contents are as follows:(1)The principle of laser bending and an multi-layer analytical model in laser bending of SCLP considering the depth of plastic zoneConsidering the different material properties of each layer,the temperature distribution equation along the thickness direction of the SCLP is established by piecewise function.Then,based on temperature gradient mechanism,the depth of the plastic zone is defined by the critical temperature of the plastic zone.Moreover,by fitting the yield strength curves of stainless steel and carbon steel,the average compressive stress of the plastic zone is calculated through the integral method.Based on the assumptions that the plastic deformation is generated during laser heating and the elastic deformation is generated during cooling,the multi-layer analytical model is established by optimizing the calculation of the plastic depth and the average compressive stress in the plastic zone.This analytical model quantifies the principle of laser bending,which provides a theoretical basis for numerical simulation.Compared with the experimental data,the average error of the model is 9.95%.The calculation accuracy is better than other analytical model.(2)Finite element model of SCLP considering transition layer during laser bendingIn order to describe the finite element model of the SCLP accurately,it is of great significance to determine material properties of the transition layer.The transition layer is formed between the stainless steel layer and carbon steel layer by the interdiffusion of elements due to the concentration gradient.The electron probe experiment is used to precisely measure the thickness and the element distribution of the transition layer.An equivalent method is adopted to calculate the equivalent material properties of transition layer.Based on the heat conduction and elastoplastic finite element method,a laser bending model of SCLP is established by ANSYS software.The experimental verification with same process parameters shows that the maximum of bending angle errors is 3.74%,which is lower than the maximum 4.93%of errors calculated by the mean value method.The equivalent properties of the transition layer provides a more accurate simulation environment for laser bending mechanism analysis.(3)Simulation and forming law in laser bending of SCLPCompared with a single-layer stainless steel plate(SLSP),considering the influence of the transition layers and carbon steel layer in the model of the SCLP on the bending angle,the transfer law of the temperature field,stress field and strain field in the thickness direction has been studied during laser bending process.The simulation results show that the temperature of the SCLP in the thickness direction is lower than that of the SLSP due to interfacial thermal resistance of the interface and fast heat conduction of the carbon steel layer,resulting in a smaller depth of the plastic zone of the SCLP.Affected by the temperature distribution and materia properties of carbon steel,the plastic stress and strain of the SCLP in the plastic zone are smaller than those of the SLSP,leading to a smaller bending angle of the SCLP.When the laser power is 140 W,the scanning speed is 400 mm/min,the defocus distance is 10 mm,and the scanning time is 1,the bending angle of the SCLP is 1.336°,which is smaller than the bending angle 1.760° of the SLSP.In addition,the conforming bending method of the SCLP based on the superposition of laser and convection is proposed for abrupt-changed stress of the interface to improve the bonding quality.(4)The analysis and correction method of warpage deformation for SCLPAffected by initial temperature and edge effect,the highest temperature distribution on the scanning line is uneven.Through the combined action of heat conduction and heat loss,heat accumulation phenomenon appears in the middle area of the scanning line.The uneven heat distribution on the scanning line is the main cause of warping deformation.It can be seen from stress field that warping force within the plate region and constraint counterforce at the edges of the plate region are produced,and warping deformation matches both the magnitude and direction.Aiming at the warping deformation caused by uneven temperature,the local energy compensation method and the line-shape spot simultaneous heating method are proposed to reduce the warping deformation by over 30%,which provides the theoretical basis for laser bending of high efficiency,accuracy and performance.This thesis focuses on the forming accuracy of SCLP during laser bending.Study includes the establishment of the analytical model of bending angle and the finite element model considering the transition layer,and the presentation of the local energy compensation method and the line-shape spot simultaneous heating method.On these bases,the high-precision 45° groove type sample is processed to realize the high-precision forming of SCLP during laser bending.