Research On Strengthening Mechanism And Rule For Local Continual Induction Hardening On 3D Curved Surface

Curved surface structure is widely applied on automobile,spaceflight and ship fields,such as stamping die of the automotive panel,automobile crankshaft and aircraft turbine blade.The huge volume and complicated curved surface structure not only adds the difficulty for machine-forming,but also make the following surface strengthening more difficult.In order to realize the surface strengthening for the easily wear area of curved surface workpiece,a local continual induction hardening strengthening method for 3D curved surface is carried out in this work.The relationship among the electro-magnetic,temperature,stress and phase transformation fields is coupled.In addition,the high nonlinearity with temperature of material properties,complication of structures of inductor and workpiece and feeding process make the theory analysis and experimental verify more difficult.In order to reveal the strengthening mechanism and build the process design theory and method for this process,theory analysis,numerical simulation and experimental analysis were performed for curved surface workpiece.The specific research content as follows:Based on the Maxwell equations and the slot effect of magnetizer,the control equation and boundary condition of local electro-magnetic induction on 3D curved surface was deduced by analytical method.The electro-magnetic field numerical analytic expression of local electro-magnetic induction on 3D curved surface was obtained by finite element method.According to the electro-magnetic field theory model,the maximum eddy current density can reach peak value and balance status faster than magnetic flux density.Based on the Fourier conduction differential equations and moving axes method,the control equation and boundary condition of local dynamic eddy current thermogenesis and heat transfer in the case of curved surface boundary was deduced by analytical method.The temperature field numerical analytic expression of local electro-magnetic induction on 3D curved surface was obtained by finite element method.According to the temperature field model,the temperature field distribution in the cases of different feed paths as well as the effect of process parameter on temperature field were obtained.It can be found that the highest temperature has the delay effect.In addition,the average delay distance was about 3mm in coil effective projection area in the cases of different process parameters,which was on third of projection area length.The range of temperature distribution uniformity was 81%~84% for different feed path and workpiece shape and 74%~82% for different process parameters.Finally,the inductor structure was optimized and the optimum geometry for magnetizer with the aim of the optimum heating performance was obtained.The optimum geometry for magnetizer was 10 mm for width,20 mm for length and more than saturation magnetic flux height for height.In order to realize the phase transformation mechanism during the strengthening process,combined with the contribution of magnetic energy on austenitizing process,the local rapidly austenitizing theory on curved surface was deduced in the case of alternating magnetic field.According to the austenite transformation theory model,the effect of magnetic energy and heating rate on austenite transformation fraction and kinetics parameter was investigated.It can be found that relatively low alternating magnetic field(about 0.3T)can affect austenitizing process after reaching curie point.The effect of alternating magnetic field reach maximum when heating rate was minimum.In addition,the maximum value was about 3%.It can also be found that concave surface structure can always shortened the effect range of alternating magnetic field.In addition,the weaken effect was maximum when inductor feed path was longitudinal.Based on the Maynier model,hardened layer distribution after local continual induction hardening on 3D curved surface was predicted.It was found that compared with longitudinal moving,microstructure and hardness distribution gradient uniformity in surface transition region was higher for horizontal moving;compared with horizontal moving,microstructure and hardness distribution gradient uniformity in depth transition region was higher for longitudinal moving.The experimental prediction model was also built according to the response surface method.The theory prediction model was verified by experiment.Finally,the minimum wear loss reduction ratio of flat,convex and concave surfaces was 69.2%,77.6% and 42.1% respectively after the research of friction and wear performance on hardened layer.The results showed that the strengthening process can improve the friction and wear performance effectively.The strengthening mechanism was revealed and process design theory and method was built.Research results have the significant academic value and actual meaning for improving local surface strengthening technology of curved surface workpiece.