Study on Forming Limit of Rolling-spinning Forming of TA15 Alloy Thin-walled Tube Components

Author:Guo Jing

Supervisor:zhan mei gao peng fei

Database:Doctor

Degree Year:2018

Download:13

Pages:134

Size:9924K

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Thin-walled titanium alloy tube components(TWTATCs)are crucial parts for high-end equipments in the fields of aviation and aerospace.Rolling-spinning technology,which combines the technical advantages of rolling and spinning,provides a promising way to break through the forming limit of existing technology and expand the components limit size.Thus,rolling-spinning becomes an advanced plastic forming technology urgently needed to be developed.However,rolling-spinning is a hot forming process with uneven deformation under the effects of multi-fields,multi-parameters,multi-die and multi-passes.In the process,the microstructure and damage evolutions of the material are very complex and interact with each other,and the fracture defects of TWTATCs are easy to occur.These factors make the accurate prediction and control of microstructure and damage evolutions in the rolling-spinning forming process,thus improving the forming limit of this component a challenging task.To this end,a systematic and in-depth investigation on the microstructure and damage evolutions of the rolling-spinning forming process of TWTATCs has been carried out by theoretical modeling,finite element(FE)modeling and experiment.The main contents and results obtained are as follows:The deformation behavior and microstructure evolution during the hot forming of TA15 alloy were investigated using hot tension tests.It is found that the mechanism of microstructure evolution is dynamic recrystallization(DRX),and the occurrence of DRX leads to flow softening behaviors of the flow stress.With the increasing temperature and decreasing strain rate,the flow stress shows temperature and strain rate sensitivities,the discontinuous dynamic recrystallization(DDRX)occurs and the DRX volume fraction increases,which results in more obvious flow softening characteristics.The fracture behaviors and mechanism with the complex microstructure evolution and stress state in the hot forming process of TA15 alloy were analyzed.The results show that the fracture is caused by void evolution.The void nucleation firstly occurs at the interface of the alpha/beta phase or in the transformed beta phase,then grows along the phase interface,showing irregular ellipse shape,and finally coalesce due to the ligament necking.With the increase of DRX volume fraction,the stress concentration at the phase and grain interfaces deceases,thus the void nucleation rate decreases,the growth trend slows down and shows slender ellipse shape,the coalescence interval decreases,and the fracture strain increases in an exponential manner.While for the increase of stress triaxiality,the number of shear band decreases significantly and the number of dimples increases obviously,which indicates that the mechanism of ductile fracture changes from shear dominated to dimple dominated.Besides this,the fracture strain increases firstly and then decreases with the increasing stress triaxiality.Based on the microstructure characteristics,a physical property-based model was firstly established to describe the microstructure evolution.In this model,the dislocation density was used as the internal variable.The temperature and strain effects were considered in the critical strain of DRX and Hall-Petch coefficient.Then,based on the GTN model,the damage model considering the microstructure and stress state was established.In this damage model,the exponential functions were adopted to describe the variation of void nucleation strain,growth coefficient and critical void volume fraction with the beta phase and DRX volume fraction,and the stress state parameter was introduced to describe the shearing effect in void evolution.Finally,a unified model considering microstructure and damage evolution for the hot forming of TA15 titanium alloy was established by calculating the initial flow stress using microstructure mode,and correcting the stress using damage model.The parameters in the unified model were obtained by genetic and inverse algorithm.The model realizes the unified prediction of microstructure and damage evolutions in the hot forming of TA15 titanium alloy.A three-dimensional FE model for TWTATCs of TA15 alloy in the rolling-spinning process was established and validated.Based on this model,the microstructure and damage characteristics in the rolling-spinning processes were revealed.In the rolling process,the minimum volume fraction of beta phase and DRX appears in the outer and core surface respectively,the void volume fraction(VVF)is larger in the inner and outer surface areas of the upper and lower ends of the ring part.Due to the periodic contact of rolls,the beta phase,DRX and VVF increase discontinuously.While for the spinning process,the minimum volume fraction of beta phase and DRX appears in the inner surface,the maximum VVF occurs in the inner surface of spun part.The beta phase,DRX and VVF only increases around the roller contact area.The VVF in the spinning process is much larger than that of the rolling process,which indicates that the fracture is most likely to occur in the outer surface of the spun part in the rolling-spinning process.The effects of key process parameters on the microstructure and damage evolutions in the rolling-spinning process were investigated.Based on these influences,setting the microstructure parameters(volume fraction of beta phase is more than 10%,DRX is more than 25%),the forming limit was determined based on the fracture thinning ratio in the spinning process using step-by-step search method.The influences of process parameters on the forming limit were analyzed and the optimized parameters to improve the forming limit was obtained: initial spinning temperature: 850℃;spinning pass: 4 passes;roller feed rate: 1.5mm/r;roller radius: 25 mm.These parameters lay an important foundation for improving the forming limit for TWTATCs of TA15 alloys in the rolling-spinning process.