Research On Electrolytic Machining Law Of Developed Surface Of Elliptical Piston Pin Hole

With the implementation of national VI emission standard,the piston,as one of the core parts of the engine,is bearing increasingly severe mechanical and thermal loads,the traditional aluminum silicon piston is gradually replaced by forged steel piston due to its poor bearing capacity.The pin hole shape of "bell mouth+ellipse" can effectively reduce the stress concentration and is an effective means to improve the bearing capacity of the pin seat.However,the structure of special-shaped forged steel piston pin hole is complex,and tool wear and chatter are easy to occur in the machining process,which affects the machining accuracy and production efficiency,non contact electrochemical machining is an effective way to avoid the above defects.Since the existing researchers have explored the electrochemical forming law of the "bell mouth" shape,this paper will focus on the research on the cross-section shape of the "ellipse" pin hole,so as to provide the basis for the final precision forming of the "ellipse+bell mouth" pin hole shape.In order to study the basic rule of ellipse forming under the condition of variable gap,this thesis presents a research scheme that expands the cathode ellipse surface into a sinusoidal surface in the rectangular coordinate system,and the electrode thickness is the pin hole length.The initial machining gap is adjusted by adjusting the distance between the fixed cathode and the machined plane.Through theoretical analysis,numerical simulation and experimental research,the key technologies of ECM for the expanded surface of standard elliptical hole are put forward and verified,in order to provide research direction for the subsequent research on forming cathode to complete the hole enlarging process of elliptical piston pin hole.The main research contents are as follows:Firstly,the polarization test and electrochemical impedance spectroscopy analysis of workpiece material 38MnVS6 in NaNO3 electrolyte with different concentrations were carried out by using potentiodynamic scanning and impedance frequency scanning methods respectively.The electrochemical characteristics of workpiece material were studied to provide a reference for the preliminary rational selection of electrolyte concentration range.Moreover,the electrochemical machining test system of profiling surface is built,which proves the reliability of the system.In the aspect of numerical simulation,this thesis establishes the cathode simulation model of the developed surface of standard elliptical hole,explores the electric field distribution law of electrochemical machining of profiling surface with the help of COMSOL Multiphysics simulation software,predicts the theoretical cathode surface profile through the transient change of anode surface profile,and determines the scheme that the developed surface of standard elliptical hole with ovality G=0.04mm is used as the cathode profiling surface,which provides a basis for the preliminary design of cathode.Based on the ANSYS Workbench simulation platform,the electrolyte pressure and velocity distribution in the gap basin of profiling surface electrochemical machining are studied.By optimizing the electrolyte inlet pressure,the electrolyte in the gap basin can meet the minimum velocity requirements,so as to achieve the purpose of uniform flow field.On the basis of numerical simulation,the design and manufacture of the primary cathode for profile surface ECM are completed,and a reasonable special fixture for cathode is designed according to the characteristics of fixed cathode ECM.The test platform was used to carry out the profile surface ECM test.The effects of electrolyte concentration,machining voltage,electrolyte pressure and initial machining gap on the profile surface ECM forming law were explored through single factor test.Through four factors and four levels orthogonal test,the best combination of process parameters with electrolyte concentration percentage of 14%,machining voltage of 4.5V,electrolyte pressure of 0.25MPa and initial machining gap of 0.15mm is obtained.The cathode correction is completed based on machining error compensation,and the machined surface roughness is less than Ra0.8 μm.And the forming accuracy of anode profile is 1.2μm higher than that before correction.The feasibility of the research scheme is proved.