Study On The Cold Precision Forging Process Of Huck Rivet Applied To Heavy Vehicle

Huck pull-core rivets are divided into two parts: rivet sleeve and rivet rod,After riveting,it can form a 100% interference fit.Permanent fastening force and high shear resistance are widely used in the connection of railway freight car body.Due to the harsh working environment of the rivets,they are subjected to alternating loads,shocks and vibrations.The stress conditions are extremely complex,which places higher demands on the physical and mechanical properties of the rivets.Cold precision forging technology has the advantages of high utilization rate of materials,high production efficiency,good quality and high precision of parts after forming,and is gradually applied to the production mode of such parts.In this paper,numerical simulation and experimental verification are combined to study the cold precision forging process of Huck pull-core rivets.First of all,the structural characteristics of Huck pull-core rivets and the defects of cold precision forging of rivets are analyzed.The key theories of cold precision forging forming analysis are introduced in detail,such as the effective stress,the effective strain,small deformation geometric equation and friction model in numerical simulation.On this basis,the process design of cold extrusion forming is introduced,the key parameters are calculated in detail,two kinds of four stations process forming schemes of rivet sleeve are preliminarily designed,and the characteristics of the two forming schemes are analyzed to lay a foundation for the follow-up work.Secondly,the flow rules of the internal particles during the metal reverse extrusion are analyzed,and the first step of the rivet sleeve forming scheme,namely the deep hole forming step,is mainly studied.Using the point tracking function of DEFORM-3D,the tracking points were selected in the radial area of the rigid translation area and the innermost circumferential area.The radial offset of the selected point as the evaluation standard,the influence of the four convex die structures with the flat base conical punch,pointed top conical punch,double cone punch and flat base punch on the concentricity of deep holes was studied.At the same time,comparing the stroke load and the stress of the punch after forming,the comprehensive results show that the double-cone punch can help improve the concentricity of the part under the condition of lower forming load.Then,the most critical step of the rivet sleeve forming scheme,that is,the forming of the head at step 3,was mainly researched.The die structure was optimized.A 0.2mm diameter vent hole was reserved at the fillet of the top of the rivet sleeve and the joint between the rod and the head,respectively,in order to solve the problem of insufficient filling.The finite element model is established respectively,and DEFORM-3D is used for numerical simulation to compare the effective stress,the effective strain,stroke load and metal flow direction in the forming process.The comprehensive results show that the equivalent stress and strain values of the end section of process two are 15% and 16% higher than that of process one respectively,and the metal flow is disordered,so the conclusion of process one is more reasonable.Finally,based on the cold rolling forming mechanism of external thread and spline,the key parameters of the cold rolling forming die for the ring groove on the rivet rod are determined.The cold rolling process is simulated by DEFORM-3D,and the effective strain distribution and metal flow during the forming process are observed.Based on the evaluation method of strain uniformity based on cumulative distribution probability and orthogonal test,the effects of bite angle,rolling speed and friction coefficient on the variation uniformity are studied.Based on the analysis of range and variance,the optimal combination of process parameters is obtained,which is biting angle is 2 °,rubbing speed is 200 mm/s and friction coefficient is 0.4.On this basis,the parts with full groove shape,smooth surface,no crack,fold and other defects are obtained.Taking the optimal combination of process parameters as an example,the equivalent stress distribution and metal flow in cold rolling of nail bar are studied.