| At present,the research,development and application of new energy vehicles are in the development stage,and traditional engine vehicles will remain the mainstream of the market for a long time,especially in the field of commercial vehicles.Engine is the "heart" of the car,and its importance is self-evident.Connecting rod is a key part of the engine.With the engine development trendency for high load,low noise,energy saving and emission reduction,high strength,light weight and low cost will dominate the main direction of connecting rod materials and manufacturing technology in the future.Splitting process of connecting rod is a renovation and innovation to the traditional manufacturing technology and it has been widely used in the automobile industry for the remarkable advantages of high precision and efficiency,simplified process,reduced cost,repeated assembly with high precision,good rigidity of connecting rod components,strong bearing capacity and other excellent performance.Pre-processing a fracture splitting notch is the essential segment in the grooving,splitting and construction bolt,and the three key processes are different from traditional connecting rod production.Pre-processing a high quality and high precision notch for fracture splitting is the necessary precondition to ensure the quality of the splitting process.Nd: YAG pulse laser grooving,with the advantages of fast speed,no tool wear,small heat affected zone,the notch with larger depth/width ratio,is suitable for the splitting connecting rod large-scale production.At present,the researches on the technological parameters,machine tool and other aspects of pulsed laser processing of notches have been made with some research and application results.However,there are still many problems to be solved in the optimization of pulsed laser grooving technology,improving the processing quality and stability,reducing the production cost,etc.The intensive study should be carried out to meet the demand of the development,popularization and application of the splitting connecting rod material and new product.Combined with the theories analysis,emulation and experimental research,the temperature field,stress field and auxiliary gas flow field in Nd: YAG pulse laser grooving were thoroughly studied by heat source model,thermal-mechanical coupling analysis model and auxiliary gas flow field analysis model in this paper.The main research contents and achievements in this paper are as follows.1.Based on the analysis of the physical process and the characteristics of auxiliary gas injection in the pulsed laser processing,the scheme about the thermal-mechanical coupling simulation analysis by finite element method and the numerical computation of auxiliary gas flow field by finite volume method were established for the process of pulsed laser processing of notches.Pulsed laser processing of notches is actually a process in which the material is melted and gasified by pulsed laser beam with high power density,and the auxiliary gas ejected coaxially from the nozzle of the laser cutting head is used to blow away the molten metal to form a fracture splitting notch.Macroscopically,the fracture splitting notch is actually a series of closely spaced blind holes.There is the combined effect of laser and auxiliary gas in the whole process of pulsed laser grooving,not only the typical nonlinear transient heat conduction problem exists,but also the complex change of stress field caused by laser beam movement and sharp temperature change appears accompanied by material nonlinearity and phase transformation.Meanwhile,the dynamic performance of the auxiliary gas ejected from the nozzle changes significantly.The theoretical analysis of material state change,nonlinear heat conduction,thermoplastic theory,the fluid problem governing equation and gas impingement jet process during grooving was carried out,and the theoretical foundation for the subsequent simulation model construction was established.2.With the ANSYS finite element analysis software,the thermal-mechanical coupling finite element simulation model of pulsed laser grooving was established,and a dynamic "keyhole" heat source model for the thermal-mechanical simulation analysis of grooving was constructed.The "keyhole" heat source model could well reflect the laser distribution in depth during Nd: YAG pulse laser grooving,and the model is based on the heat source with the heat effect radius attenuation along the depth,assuming that all the energy provided by laser is concentrated in the revolution body,heat flow density distribution in the interface along the workpiece thickness satisfies Gauss distribution,and all the "live" elements with the temperature higher than the melting point are blown away by high voltage auxiliary gas coaxial with laser beam in a very short time.Under the ideal condition,the process of "killing" the "live" elements whose temperature are higher than the melting point,obtaining the lowest coordinate value of the keyhole and correcting the real-time depth of the rotating body heat source after each load step is completed is realized by the restart technique,"life and death" element technique and APDL language.In addition,the problem of material nonlinearity and phase transformation stress in the process of constructing simulation model was solved technically.3.Finite element simulation of the thermal process during notching C70S6 steel with pulsed laser was carried out.Combining with Nd: YAG pulsed laser grooving test,the temperature distribution change during grooving,the influence of process parameters on the temperature distribution and the geometry size of fracture splitting notch was analyzed.The results show that the formation of keyholes plays an important role in the process of processing blind hole.At the each stage of laser processing,the high-temperature area on the workpiece is completely concentrated around the corresponding blind hole.After the processing is completed,the material temperature at the bottom of the blind hole and the surface of the hole is higher than the melting point of the material,this part of the metal will re-solidify and an important part of the heat-affected forms,that is the re-solidification zone.Peak power,moving speed and pulse width have great influence on the temperature distribution during grooving,and further influence the geometry size of fracture splitting notch.The effectiveness of the "keyhole" heat source and the thermal process analysis was proved by comparing the data obtained by loading the "keyhole" heat source model with the experimental data obtained by pulsed laser grooving.4.Based on the temperature field obtained by the thermal analysis,the finite element simulation of stress field of notching C70S6 steel with pulse laser was carried out to analyze the stress distribution rule and the influence of different process parameters on residual stress of grooving.The results show that the residual stress of the pulsed laser grooving is mainly concentrated in the influence zone around fracture splitting notch,but the stress around the blind hole near the free edge of the workpiece is much smaller than that of other blind holes at the same location due to the stress release.There exists a stress state which satisfies the micro-crack at the bottom of the blind hole and near the hole wall,and the existence of micro-cracks was proved by the grooving test.Micro-cracks and martensite coexist in the notch root,which is beneficial to the emergence of brittle fracture,and then reduce the splitting strength and defects.The stress state in the notch root zone along the direction of the crack initiation was tensile stress state,which is very conducive to the occurrence of crack initiation and the crack extension,but the uneven distribution caused by the free edge stress release would intensify asynchronous crack initiation phenomenon and cause the splitting defects.Therefore,residual stress should be reduced as far as possible within a controllable range;Peak power,moving speed and pulse width have an impact on the grooving residual stress.Among them,peak power and pulse width are positively correlated with residual stress and have the most significant impact,while moving speed is negatively correlated with residual stress.In addition,for the selection of process parameters,the grooving residual stress should be reduced as far as possible within a reasonable range on the premise of satisfying the requirements of the fracture groove geometry,so as to ensure the quality of the subsequent splitting process.5.Based on FLUENT fluid dynamics analysis software and combined with the actual processing,the auxiliary gas flow field numerical analysis model during pulse laser grooving was established,which considered the interplay between the temperature field and flow field,and the depth adaptive heat source model suitable for the flow field analysis.In addition,key problems during modeling such as the choice of turbulence model were solved.6.The auxiliary gas flow field simulation during notching C70S6 steel with pulsed laser was carried out,and the temperature field obtained by the depth adaptive heat source conforms to the temperature distribution characteristics of laser processing,which provides a guarantee for the effectiveness of auxiliary gas flow field analysis.The distribution characteristics of the auxiliary gas flow field were analyzed,the flow state of the auxiliary gas from the nozzle to the keyhole was analyzed,the velocity distribution in the keyhole was observed,and then the gas parameters were optimized.Increasing the inlet pressure could increase the effective range of auxiliary gas in the keyhole,and then improve the capacity of removing slag of auxiliary gas at the bottom of the keyhole,but the utilization rate of auxiliary gas would be reduced.The velocity of the auxiliary gas is not positively correlated with the inlet pressure.The optimal value of inlet pressure obtained through analysis is 0.6mpa,and the effectiveness of optimization parameter was verified by experiments. |
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