| In the automotive,aerospace,medical and other fields,due to thermoplastic polymers have the advantages of light weight,high strength,easy forming,low cost,corrosion resistance and so on,which are replacing steel and cast iron gradually.Subsequently,the connection problem of thermoplastic polymer is brought forward.Laser transmission spot welding technology has many advantages,which is one of the most promising welding methods.However,the lack of laser transmission spot welding research,resulting in the lack of knowledge of laser transmission spot welding technology,limiting the laser transmission spot welding technology for industrial applications.As the thermoplastic polymer PMMA is widely used in automotive and other industries.In this paper,the laser transmission spot welding of Polymethyl methacrylate(PMMA)is studied from the aspects of welding mechanism,process optimization and numerical simulation.Firstly,the mechanism and failure form of laser transmission spot PMMA are studied.Three parameter variables,namely peak voltages,defocusing distances and the welding type(type I(pulse frequency and the duration is 25 Hz,0.6 s)and type II(pulse frequency and the duration is 5Hz,3s))are used for analysis.Laser transmission spot welding experiment of type I produces a lot of small bubbles and some large bubbles,so that the upper and lower materials to achieve micro-anchor,to improve the welding strength.However,experiment of type II produces a small number of bubbles and which are small bubbles.The above indicates that the bubble on the connection performance have a certain impact.The tensile failure of type I is the fracture at the substrate along the solder joint,and the welding strength of type I is higher than type II.By analyzing the influence of process parameters on the spot welding pool,the connection strength of PMMA has a certain influence on the weld diameter/depth ratio.In the case of ensuring that the peak voltage and defocusing distance are the same,the weld diameter / depth ratio of type I is significantly larger than type II and the welding effect is better.Secondly,the effect of process parameters(peak voltage,pulse frequency,stand-off distance,exposure)on the tensile loading and weld diameter are studied by single factor method,the reasonable range of process parameters is obtained.Based on the response surface method,the mathematical model of the tensile loading and weld diameter of PMMA are established by using Design-Expert V8 software.The interaction influence of process parameters on the tensile loading and weld diameter are studied by variance analysis.Then the model prediction results are compared with the experimental results to verify the reliability of the model.The satisfaction parameter is used to optimize the analysis and the combinations of the highest satisfaction parameters are obtained.Finally,based on the finite element method,the temperature field of laser transmission spot welding PMMA is simulated and the programming of the dynamic loading process of pyrogen model is carried out by APDL language.The temperature field distribution and node temperature variation of the type I and type II are obtained by POST1 and POST26,the welding rules and pool morphology of the two welding types are compared and analyzed.By analyzing the simulation results and experimental results at different peak voltages,it is concluded that the simulated pool shapes of type I and type II are similar to that of experiment.The simulated values of weld depth and weld diameter have similar trends with experimental values.It is proved that the temperature field model is feasible.The research of this dissertation has laid a theoretical foundation for the study of laser transmission spot welding of thermoplastic polymer.It also provides practical technical reference for PMMA laser transmission spot welding. |
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