| With the growth of national economy and the development of material technology,stainless steel has been widely used in the machinery industry.The remarkable features of stainless steel are its good corrosion resistance and excellent mechanical properties.The parts made of stainless steel not only have good strength,but also have longer service life than those made of other materials.However,forging difficulty and strict process control have led to low production level and instable product quality of domestic stainless steel forgings.In this thesis,11Cr17 martensitic stainless steel transmission rod forgings manufactured by a domestic forge plant are used as the research object.Due to the special rheological properties of 11Cr17 martensitic stainless steel,the service life of the studied forging dies is generally low,which are prone to early failures such as cracking and wear.To reduce the loss of the plant by solving these problems and improving the life of dies,the present work optimizes the forging process parameters through physical test combined with finite element analysis to improve the die failure.The main research contents are as follows:Firstly,Isothermal hot compression test is performed at 800~1100? temperature,0.01s-1~5s-1 strain rate and 50% strain capacity with Gleeble-3800 thermal simulation test machine to obtain the flow stress curve of 11Cr17 martensitic stainless steel.The flow stress equation describing the hot deformation of the material is obtained by fitting.Hot processing map of 11Cr17 stainless steel is established based on the dynamic material model,thus identifying the temperature range and strain rate of optimal hot processing parameters for 11Cr17 stainless steel to be temperature 900~1050? and strain rate 0.05s-1~0.1s-1.Besides,the sites prone to instability failure for the material are obtained.Secondly,based on the structural characteristics of turbocharger transmission rod forgings,a forging process suitable for the product is designed.Through using finite element analysis software Forge,the forging process is simulated and analyzed to obtain the forming load curve,die stress distribution and die wear distribution of transmission rod forging,which are conformed to the die failure in the actual production process.Thirdly,to achieve the most reasonable combination of forging process parameters,a multi-objective orthogonal test is designed to obtain three most influential factors to the composite life index of dies.The nonlinear function mapping relationship between the influencing factors and the die life index is derived by BP neural network training and fitting.Then,the mapping relationship is optimized using genetic algorithm to obtain the final optimal combination of forging process parameters for the 11Cr17 martensitic stainless steel transmission rod parts.Fourthly,actual production test is conducted to prove that the optimal forging process parameters obtained using the above methods can effectively reduce the early failure rate of dies.Besides,the produced transmission rod parts have good forming effect and accurate dimensions without forging defects.In this research,the material property test of 11Cr17 martensitic stainless steel is combined with the finite element simulation and the optimization algorithm to improve the service life of dies effectively by a step-by-step approach from physical test of material to finite element analysis of forging process and then to algorithmic optimization of process.In addition,the present study also provides theoretical guidance and basis for the actual production process of similar materials and products in future. |
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