Research On Forming Quality And Optimization Design Of Internal Thread Of High Strength Steel Based On Cold Extrusion

Marine diesel engine is the heart of ship power system which may affect the performance of ship.The improvement of diesel engine performance is related to the development of the whole shipbuilding industry.Threaded connection is commonly used on the connecting rod of diesel engine,which will bear large radial,axial and shear loads when the diesel engine is running.So the quality of thread is an important factor affecting the running stability and service life of diesel engine.The forming quality of thread is closely related to the processing mode,process parameters,tool structure and other factors.Understanding the influence of various factors on thread forming and putting forward the processing technology of high quality thread have always been the research focus of researchers and engineers.In this thesis,the connecting rod of diesel engine is taken as the research object.In order to improve the quality of thread forming,the selection of thread processing method,the optimization of process parameters and the optimization of tap structure parameters were studied by means of numerical simulation and experiment.The optimal design scheme of thread machining was put forward on the basis of deeply understanding the influence of different factors on thread forming,which provides a reference for the thread production and tool selection of enterprises.The specific research content of this thesis is divided into the following parts:（1）The forming processes of extruded thread and cutting thread were numerically simulated by rigid plastic finite element analysis software DEFORM-3D,and the thread tooth profiles under two processing methods were obtained.The forming mechanism of the two kinds of threads was analyzed.The results show that the cutting thread is formed by cutting the workpiece material through the cutting teeth of tap.But the extrusion thread is formed by the extrusion of the tapered teeth,which makes the metal material flow along the teeth and gradually fills the grooves between the teeth.The variation curves of torque and temperature in the two processing methods were obtained through the post-processing function of the software and then compared with the experimental curves to verify the accuracy of numerical simulation.（2）In order to determine the appropriate mode of thread processing,the machining experiments of thread were carried out by cutting and extrusion respectively.The test results show that the material utilization rate is higher under the extrusion mode,and there is no metal chip in the machining process.The quality of the two kinds of threads was compared from the perspectives of hardness,hardened layer and surface microstructure.It is found that compared with the cut thread,the hardness at tooth root of the extruded thread increased by71.06HV_0.2,the depth of hardened layer increased by 0.3mm.The hardness at tooth side increases by 48.73HV_0.2,the depth of hardened layer increases by 0.2mm.The hardness at tooth top increases by 12.75HV_0.2,the depth of hardened layer increases by 0.15mm.（3）Considering that the extrusion torque and extrusion temperature may affect thread quality in the machining process,in order to improve the forming quality of extruded thread,the finite element analysis and experimental research were combined to study the effects of process parameters such as bottom hole diameter of workpiece,machine tool speed and lubricating medium on extrusion torque and temperature.For the purpose of reducing the extrusion torque and temperature,the process parameters were optimized by orthogonal design.The maximum torque and the maximum temperature respectively decreased by 15.06%and 12.63%after optimization.The hardness of the thread root,tooth side,and tooth top areas increased by 20.65HV_0.2,13.95HV_0.2 and 7.15HV_0.2,respectively.And the depth of hardened layer is increased by 0.05mm.（4）In order to further improve the forming quality of thread,the effects of the number of edges,the amount of shovel back,the extrusion cone,the calibration part and the extrusion cone angle on the torque and temperature were analyzed by numerical simulation.With the aim of reducing torque and temperature,the structural parameters of extrusion tap with the lowest torque and temperature were obtained through orthogonal optimization.The maximum torque and the maximum temperature respectively reduced by 15.67%and 15.96%after optimization.The hardness of the thread root,tooth side,and tooth top areas increased by14.31HV_0.2,8.83HV_0.2 and 4.89HV_0.2,respectively.And the depth of hardened layer is increased by 0.05mm.