Springback Behavior And Models Of Copper Alloy And Aluminum Alloy Under Large Plastic Compression Deformation

Springback is a common phenomenon in the metal plastic forming process.The geometric and dimensional deviation of the formed parts caused by it is one of the important quality problems faced in the forming process.Effective control of the geometric shape and dimensional accuracy of the formed parts is an important prerequisite for achieving precision plastic forming.Therefore,springback behavior and springback model have always been the most concerned and solved issues in precision plastic forming.People have paid extensive attention and conducted a lot of researches on the springback issue in stamping of sheet metal.As for the springback issue in bulk forming such as forging,extrusion and rolling.it is believed traditionally that,there will be a certain margin after the large formed components are formed.Subsequent machining can eliminate the shape and size deviations of the formed parts caused by the springback effect,etc.,resulting in the springback problem in bulk forming being usually ignored.With the development of production technology and the demand for product diversity,the geometry of formed parts has become increasingly complex,placing higher requirements on forming accuracy and precision forming processes.In particular,for the formed parts with complex geometries that are difficult to machine,it is expected to directly obtain precision formed parts that meet accuracy requirements through a single/multi-pass forming process.This requires that the effect of deformed materials’springback on its geometric dimensional accuracy must be considered as designing process and die.Therefore.studying the springback behavior of bulk forming and establishing an accurate springback model are necessary prerequisites for accurately analyzing the springback problem in the precision bulk forming process;it has important theoretical significance and engineering application value for realizing the optimization of the forming process and die shape as well as the effective compensation of springback,and then solving a series of problems caused by springback.In order to solve the springback issue during the forming process,many studies have been carried out on the springback behavior during stamping forming,and various unloading and springback models have been proposed.However,there are obvious differences in the forming process,deformation amount,and macro-micro behavior between bulk forming and stamping forming.The bulk forming is mainly dominated by large plastic compression deformation,and the geometric shape changes drastically during the forming process,while stamping forming is dominated by tensile deformation,and only the shape of the plate surface changes during the forming process,and the deformation in the wall thickness direction is very small or basically non-existent.Meanwhile,the deformation amount of the bulk-formed part is much greater than the deformation amount of the stamped part before local instability occurs.Furthermore,there is asymmetry in the micro and macro deformation behaviors between compression and tensile deformation.These all lead to obvious difference in springback behavior between bulk forming and stamping forming.The model established in the existing research for analyzing the springback in stamping cannot be applied to analyze the springback in bulk forming process.Therefore,there is an urgent need to study the springback behavior of bulk forming and establish springback models suitable for bulk forming.There are multiple forming processes in the bulk forming process,different loading histories and deformation conditions inevitably lead to different springback behaviors.The chord modulus and its stress-strain response during unloading process are important methods to characterize springback behavior.The strain,strain rate and deformation temperature are the main parameters that affect the mechanical response of materials.Therefore,establishing accurate chord modulus models and unloading models related to strain,strain rate and deformation temperature are important basic conditions for revealing deformation parametersdependent springback behavior under bulk forming and conducting numerical simulation analysis of springback.Meanwhile,elucidating the physical mechanism of the unloading process is of great significance for deeply understanding the causes of springback and effectively regulating springback behavior.Establishing an accurate physical model based on the unloading physical mechanism is the theoretical basis for revealing the relationship between deformation parameters-microstructure parameters-springback amount,thereby optimizing the forming process parameters and achieving the control of microstructure and forming accuracy of deformed materials.At present,research works on springback behavior and its prediction model mainly focuses on springback problems in stamping forming process of sheet metals.However,the springback problems in precision plastic bulk forming still lack relevant systematic and in-depth research.The following key scientific issues need to be studied and solved urgently:Revealing the springback macroscopic behavior during monotonic and cyclic multi-pass large plastic compression cold,warm and hot deformation;Analyzing the effects of key deformation process parameters(strain,strain rate,and deformation temperature)on unloading chord modulus and nonlinear unloading stress-strain response during large plastic compression deformation process;Establishing strain-,strain rate-and temperature-dependent unloading chord modulus model as well as nonlinear unloading and reloading macroscopic constitutive model,which are suitable for bulk forming;Elucidating the intrinsic mechanism of the complex unloading and springback behavior after large plastic compression deformation and establishing physicallybased unloading model;Studying and establishing prediction model of saturated springback in large plastic compression deformation considering anisotropy.Both copper alloy and aluminum alloy are face-centered cubic crystal structure materials.Copper alloy has good cold forming plasticity and is often used for cold bulk forming;while aluminum alloy has good hot forming plasticity and is often used for hot bulk forming.In this paper,copper alloys and aluminum alloys are mainly taken as research objects.In view of the above-mentioned problems in the springback behavior and springback model of large plastic compression deformation,an in-depth and systematic studies on the springback behaviors and springback models of copper alloys and aluminum alloys under the cold and hot large plastic compression deformation are carried out.The main research work and results obtained are as follows:(1)The springback behaviors of copper alloy and aluminum alloy under large plastic single-/multi-pass cyclic cold,hot compression deformations were studied.The variation laws of the instantaneous tangent modulus and chord modulus with strain and stress in unloading were analyzed.The differences in springback behavior after compression at different deformation stages and the effect of thermo-mechanical coupling effect on the springback behavior after large plastic hot compression deformation were revealed.The effects of strain hardening,dynamic softening and static softening on the springback behavior during compression deformation process were expounded.The study results indicate that,the unloading stress-strain relationship after large plastic cold and hot compression deformation is nonlinear.Under different deformation stages,there are differences in the springback behavior of cold compression deformation,while the springback behavior of hot compression deformation is the same.The unloading springback strain εu increases with increase of strain amount and strain rate,and decreases with increase of deformation temperature.Springback after compression deformation has a time-dependent effect,which is affected by strain,strain rate and deformation temperature.The increase of strain and strain rate intensify the timedependent effect of springback,and the increase of deformation temperature weaken the timedependent effect of springback.(2)The variation laws of unloading chord modulus with strain,strain rate and deformation temperature during cold and hot compression deformation process were studied.The contributions of strain,strain rate and deformation temperature on the unloading chord modulus and nonlinear unloading process were analyzed and then quantified.It is found from study results that,during the large plastic compression deformation,the unloading chord modulus begins to stabilize at a relatively large strain,and the maximum decrease rate of the unloading chord modulus exceeded 90%,which was much greater than the value during tensile deformation process.The unloaded chord modulus showed different changes in different deformation stages.In the hardening stage,its decreasing rate was relatively fast;in the softening stage,its decreasing rate gradually slowed down.The study results indicate that,the effect of strain and strain rate on the unloaded chord modulus can be quantified using an equation in the form of a power exponent.The effect of deformation temperature on the unloaded chord modulus can be quantified using an equation in exponential form,which provides a premise basis for accurately modeling the strain,strain rate,and temperaturedependent chord modulus.(3)For cold compression deformation,unloading chord modulus model and instantaneous modulus model in the form of power exponent for describing unloading process and predicting springback strain during large plastic compression deformation were established.For hot compression deformation,taking the effect of thermo-mechanical coupling effect on unloading chord modulus into consideration,a strain,strain rate and temperature-dependent unloading chord modulus model was established;the effects of deformation temperature,strain rate and cumulative plastic strain on the instantaneous tangent modulus at different stress levels during unloading were also considered,a nonlinear unloading model was established for describing the deformation parameters-dependent unloading process and predicting springback strain after hot compression deformation.For multi-pass cycle cold and hot compression deformation,the nonlinear symmetric relationship between unloading and reloading is studied.Based on the unloading model established above,a deformation parameters-dependent unloading-reloading model was established.The study results indicate that,the chord modulus model and unloading model established in this paper can more accurately describe strain,strain rate,and temperaturedependent unloading process after large plastic compression deformation,which has higher accuracy in predicting the springback strain after large plastic deformation.The nonlinear symmetry axes of unloading-reloading circuits during multi-pass cold and hot compression deformation satisfy the relationship expressed by polynomial equation and Gompertz equation,respectively.(4)The dislocation-associated behaviors and dislocation density evolution law during and after the cold and hot compression deformation were studied.The physical mechanism of springback in unloading after compression deformation was clarified.The reverse slip mechanism of reversible dislocation and the softening mechanism of static recovery and static recrystallization were comprehensively considered,a unified physically-based model for describing the dislocation density evolution in unloading after cold and hot deformation was proposed.Meanwhile,an anelastic strain model was established by considering the effects of strain hardening,dynamic recovery and dynamic recrystallization on the reversible dislocation density evolution.The study results indicate that,the reverse slip of reversible mobile dislocations driven by back force is the main mechanism of unloading after cold deformation,and this reverse slip causes the nonlinearity of unloading after cold deformation;while the nonlinearity of unloading after hot deformation is affected by the combined effects of reverse slip motion of reversible dislocations,static recovery and static recrystallization.For the cold compression deformation process,the strain hardening and dynamic recovery during the loading process are the main factors affecting the anelasticity;while for the hot compression deformation process,the anelasticity is not only affected by the strain hardening and dynamic recovery during the loading process,but also by the dynamic recrystallization.(5)The effects of macroscopic variables(strain,load,and potential energy,etc.)and microscopic variables(dislocation density,grain size)on saturated springback was analyzed and then quantified from macroscopic and microscopic perspectives.By considering the effect of change of anisotropy coefficient,a prediction model of anisotropic saturated springback was established.It is found from study results that,strain and anisotropy coefficient are key factors affecting saturated springback strain.The three-term Gaussian anisotropic springback prediction model established in this paper can describe the effect of anisotropic changes in deformed materials on saturated springback strain,and accurately predict the saturation springback strain during the bulk forming process,which has a high prediction accuracy.