| Because of the advantages of high welding strength,no vibration,no contact or particles,small heat affected area,no contact and high efficiency,laser transmission welding(LTW)meets the demand of high-precision manufacturing,and gradually becomes the mainstream plastic processing technology.Laser transmission welding is a complex process of interaction and coupling of thermodynamic,mechanics and polymer rheology.During welding,polymer can obtain higher laser absorption rate by adding laser absorbing pigment.Compared with other metals,carbon black(CB),Clearweld and so on,copper has better thermal conductivity and higher weldability.Therefore,this research selects copper film instead of dye as absorbent.As the temperature of HAZ changes dramatically and the distribution of HAZ is not uniform,the thermal deformation of the weldment will lead to the residual stress at the weldment joint which is not conducive to the structural rigidity and bearing capacity.Therefore,it is of great practical significance to study the influence of welding quality on the parameters of laser transmission welding process to reduce the welding residual stress and improve the welding strength.In this paper,polycarbonate(PC)was taken as the research object,and copper film was selected as the absorbent to analyze the weld morphology and formation principle of laser transmission welding of PC/Cu/PC.The small hole method was used to measure the residual stress at the weld center of the weldment and study the distribution of the residual stress.Response surface methodology was used to study the effects of welding power,welding speed,width of copper film on the welding strength,residual stress and weld morphology.Then process parameters were optimized.The three-dimensional finite element model including copper film of thermal-fluid coupled field and thermo-solid coupled field was established.The temperature field,flow field and stress distribution were discussed.The detailed research contents are as follows:Firstly,the experimental materials were prepared.The transmittance,reflectivity,thermal expansion coefficient and other physical properties of PC and Cu films was measured.The experiments of laser transmission welding of PC/Cu/PC were carried out.The weldability,weld morphology and related mechanism were studied,and the characteristics and formation reasons of physical phenomena such as bubble,tensile section morphology and thermal expansion were analyzed.The release coefficients of small hole method and layer by layer drilling method were confirmed by finite element method.The residual stress at the center of the plate and the different positions of the plate was measured by the small hole method.The residual stress of different thickness of weldment was measured by layer by layer drilling method,so as to explore the distribution of residual stress.Secondly,the welding parameters of PC/Cu/PC of laser transmission welding were determined by single factor.In this process,the effects of process parameters(welding power,welding speed,copper film width)on welding strength,residual stress and weld micro morphology were studied.Based on the statistics of the deformation depth and area of copper film,the influence of the deformation of copper film on the welding strength and residual stress was studied.The mathematical model of welding process parameters,welding strength and residual stress was established by RSM,and the optimal process parameters were obtained by cuckoo algorithm.Thirdly,based on the unified momentum equation,the fluid-solid coupling finite element model of laser transmission welding of polycarbonate based on the copper film interlayer was established.Considering the phenomenon of melting and solidification in polymer welding,the phenomenon of coexistence and interaction of solid,liquid and solid-liquid(or mush)mixture in the welding process was reproduced by simulation.The numerical value and distribution of temperature field,flow field and thermal stress of PC/Cu/PC of laser transmission welding was studied.The temperature field,flow field,numerical value and distribution of thermal stress of laser transmission welding were studied.The influence of material transformation on thermal stress distribution was analyzed.Finally,a thermo-mechanical coupling model was developed to evaluate the thermal stress and residual stress distribution of laser welding of PC/Cu/PC.According to the heat transfer conditions,the distribution of residual stress was calculated,and the influence of time dependence of thermal parameters on the distribution of residual stress was emphasized.Based on the thermos-viscoelastic constitutive equation,the thermo viscoelastic parameters were introduced by COMSOL.Then the simulation results were discussed and compared with the experimental results.The research in this paper provides a valuable reference for the research of laser transmission welding of metal absorbers.The study of residual stress is helpful to develop process parameters to achieve high quality welding.The simulation has theoretical significance and reference value for studying the thermal stress of laser transmission welding. |
