Research On The Rapid Solution And Analysis Method For Hot Deformation Behavior Of Alloys And Its Application

Flow stress constitutive model and processing map are the major methods to investigate the hot flow behavior of alloys and the main foundation of guiding development of forging process.At present,researchers around the world have already constructed amount of flow stress constitutive models and processing maps.To a certain material,to rapidly choose a suitable constitutive model and a processing map with highest precise among all the models and to apply them in the development of forging process is extremely important to enhance the quality of the products and lower the cost.Besides,flow behavior of an alloy under different status is also different.Hence,choosing suitable material model in the simulation process can improve the precision and better guide the development of forging process.This dissertation is focusing on the hot flow behavior of 7050 aluminum alloy under different hot deformation statuses and development of the forging process of an aviation forging part.The true stress strain data of forged 7050 aluminum alloy were obtained by hot compression tests then temperatural and frictional corrected.After that,four constitutive models and four processing maps were constructed.Based on that,the rapidly calculation and evaluation procedure of flow stress constitutive model and processing map were developed.This procedure was then used to calculate the constitutive models and processing maps of forged/extruded 7050 aluminum alloy as well as to select the most suitable models after evaluation by using relative coefficient(R),average absolute relative error(AARE).The chosen constitutive models and processing maps were secondarily developed by Fortran language so that they can be used in the simulation.Subsequently,the forging processes of the aviation forging part were designed and optimized based on simulations in the aspects of optimizing billet structure and deformation parameters,which helped to avoid the possible occurrences of folding,fill dissatisfaction,high forming load and flow instability.At last,the optimized billets and deformation paraters were applied in the production test and qualified products were obtained.The main research results are as following:(1)Four different constitutive models for forged 7050 aluminum alloy were constructed and their prediction precisions were compared with each other.The flow stress data of forged 7050 aluminum alloy under the strain rate range of 0.01?10s^-1 and temperature range of 330?480?were obtained by hot compression tests and then temperatural and frictional corrected.It can be found from the results that,temperature rise induced by deformation heat was enhanced with the rising strain rate and decreasing temperature.The effects of temperature rise induced by deformation heat and friction on flow stress are distinct and nonnegligible.On the basis of corrected flow stress data,the modified Johnson-Cook model,modified Zerilli-Armstrong model,strain compensated Arrhenius model and modified Hansel-Spittel model were constructed.As to this alloy,modified Johnson-Cook model has the highest prediction precision.(2)Four types of processing maps were developed for 7050 aluminum alloy.Based on the corrected flow stress data,the strain rate sensitivity index,temperature sensitivity index and strain hardening index were calculated.Then,processing maps based on Kumar-Prasad criterion,Murty-Rao criterion,Gegel criterion and Alexander criterion were established.Comparing the four processing maps with each other,it revealed that,the instable area of Kumar-Prasad processing map was usually larger than that of Murty-Rao processing map,and Gegel processing map was similar to Alexander processing map.After microstructure observation,Murty-Rao processing map was identified to have the highest precision.(3)The calculation processes of the four constitutive models were encoded and optimized using MATLAB language.By using the calculation procedure to traverse all the reference conditions of Johnson-Cook model and Zerilli-Armsstrong model,it can be found that reference condition influenced the prediction preise strongly and optimizing the reference condition could effectively enhance the precise.Furthermore,genetic algorithm was applied to optimize the parameters of three of the four models.The results showed that the precise can be furtherly improved.For example,the AARE-value of Zerilli-Armstrong model for forged 7050 aluminum alloy was reduced from 7.4192%to 6.3292%.Besides,the calculator of processing maps and automatically code generator of subroutine for DEFORM were encoding by MATLAB.Then all the functions of the procedure were integrated and the user interface was designed by MATLAB GUI.The final procedure was tested by using the flow stress data of Ti-6554 alloy and it was proved to be reliable.(4)Because the statuses of material during pre-forging and final forging processes are extruded and forged statuses respectively.The flow stress data of forged/extruded7050 aluminum alloys were imported to the procedure for model calculations.The results revealed that the precisions of the constitutive models of different alloys were different.Among the four constitutive models,modified Johnson-Cook model has the highest precision for forged 7050 aluminum alloy while strain compensated Arrhenius model has the highest precision for extruded 7050 aluminum alloy.After using genetic algorithm to optimize the model,the flow stress subtoutine codes were generated automatically and used in the secondary development of DEFORM,which established the foundation of finite element simulation for the development of forging process for the aviation forging part.The developed flow stress subroutines for DEFORM were used to simulate the deformation processes of the aviation forging part.The simulation tests for optimizing the structures of pre-forging billet and final forging billet were conducted.By analyzing the simulation results,it can be conclueded that,during the final forging process,the filling synchronization can be improved and the forming load can be lowered by heightening the rib.Also,after comparing different shapes of billet for pre-forging process,it can be found that folding and filling dissatisfaction defects can be avoided by using cylinder billet.(5)The relationship function of instability with strain rate and deformation temperature was constructed.The calculation and plotting processes were built into procedure based on MATLAB.By using this procedure,the processing maps for extruded and forged 7050 aluminum alloy were obtained and the instability map were overlapped.Then,the instability judgement functions were constructed.The instability judgement functions were used in the postprocessor to analyze the instability of forging processes.A series simulation tests at different loading speeds and billet temperatures were conducted under the precondition of die temperature of 350?and 400?.After analyzing the results of instability,the reasonable final forging parameters were determined to be in the loading speed range of 0.5?1.9mm/s and billet temperature of360?450?,which corresponds to the forming load range of 16200?26000 tons.And the reasonable pre-forging parameters were determined to be in the loading speed range of0.5?5mm/s and billet temperature of 360?450?,which corresponds to the forming load range of 1250?20400 tons.(6)The whole forging processes were simulated under the forming speed of0.5mm/s,billets temperature of 430?and dies temperature of 450?,as well as considering the temperature reduction during the transportation of billets and dies from heating furnace to press and the installation precedures of dies on the press.The results reveal that,the forming loads of final forging and pre-forging were 18000 tons and14400 tons,respectively.The temperature distributions were in reasonable section and instability,folding defect and fill unsatisfaction were not observed.Finally,the optimized billets and deformation paraters were applied into the production test.The actual forming loads of final forging process and pre-forging process during the production tests were 17100 tons and 13900 tons respectively.The shape,size and property of the product are all qualified,and no defect produced.The products were successfully delivered to the purchaser.