Simulation Of Hot Deformation Behavior And Evolution Of Microstructure And Properties For Net Structured TiB_w/TA15 Composites

The development of titanium alloy-based composite materials is expected to address the demand for lightweight,high-strength materials with a service temperature range of 600-800℃ in the aerospace industry.Owing to the special spatial distribution of reinforcements,net-structured composites has excellent comprehensive mechanical properties,furthermore,the microstructure control and performance optimization can be achieved by hot processing.However,during the hot deformation process,the evolution of network structure for the reinforcements has an important effect on the performance.In this paper,the Deform-3D finite-element software is used to simulate the upsetting hot-deformation behavior of cylinder,ring and flange parts.Finally,the experiment of ring upsetting hot-deformation is selected to study the relationship between the strain states,the evolution of network structure for reinforcements and the mechanical properties.The hot-deformation simulation results of cylinder upsetting show that the radial flow rate of the material increases with the increase of the reduction or the decrease of the shape factor.Secondly,the volume ratio variations of different deformation zones are calculated quantitatively during the deformation process.It is found that with the increase of the reduction,the volume ratio of the difficult-deformation zone continues to decrease,the volume ratio of easy-deformation zone decreases firstly and then increases,while that of the free-deformation zone increases firstly and then decreases.As the shape factor increases,the volume ratio of the difficult deformation zone continuously increases,and the volume ratio of easy-deformation zone decreases firstly and then increases,while that of the free-deformation zone rises firstly and then drops.The strain dispersion coefficient is utilized to evaluate the uniformity of deformation,i.e.the smaller the strain dispersion coefficient is,the better the deformation uniformity.It is noted that increasing the reduction or reducing the shape factor will reduce the strain dispersion coefficient,resulting in a more uniform strain distribution.In addition,the forming load per unit volume continues to increase by increasing the reduction,while the increase of the shape factor will lead to a reduction in the forming load per unit volume.The material diffluence phenomenon occurs during the hot deformation of the ring compression.The radial flow rate of the material on the diffluence surface is zero,and the materials on both sides flow inward and outward respectively.Increasing the reduction or decreasing the shape factor will lead to an increase in the radial flow rate of the material,and the wall thickness factor mainly affects the radial flow rate of the material inside the diffluence surface,which decreases with the increasing of the wall thickness factor.Secondly,as the reduction increases,the volume ratio of the material inward flow zone and strain dispersion coefficient increase firstly and then decrease.Furthermore,the volume ratio of the material in the internal flow zone is continuously decreased,while the strain dispersion coefficient continues to increase by increasing the shape factor.Meanwhile,increasing the wall thickness coefficient will lead to a decrease in the volume ratio and strain dispersion coefficient for inward flow zone.As the shape factor increases,it is also noted that the forming load per unit volume decreases,while the wall thickness factor has no significant effect on it.In the process of upsetting hot-deformation of flange part,increasing the radius of the fillet and the thickness of the flange will reduce the average effective strain and strain dispersion coefficient.The load-displacement curve can be divided into three stages:the initial load-rising stage,the intermediate load-stabilizing stage and the final load sharply rising stage.With the increase of the fillet radius and the flange thickness,the initial load-rising stage extends,and the intermediate load-stabilizing stage shortens.Furthermore,the forming load per unit volume decreases with increasing the flange thickness.The stress state of upsetting hot-deformation of cylinder,ring and flange parts is analyzed.The results show that the ring upsetting hot-deformation can obtain tensile strain,plane strain and compressive strain.Therefore,the ring upsetting hot-deformation is finally selected to conduct the experimental research.The microstructure of the matrix alloy is a typical bimodal structure after hot compression deformation.The interface of the TiB_w reinforcements/matrix alloy remains clear and well bonded after hot-deformation.The Lode’ s parameter（μσ）can be utilized to estimate the strain type and network structure of reinforcements.When μσ＜0,tensile strain deformation will be generated（dε₁＞0;dε₂＜0;dε₃＜0）;when μσ＝0,plane strain deformation occurs（dε₁＞ 0;dε₂＝0;dε₃＜0）;and when μσ＞0,the strain type will transform to the compressive strain（dε₁＞ 0;dε₂＞0;dε₃＜0）.The material on both sides of the diffluence surface flows inwards and outwards respectively,causing the diffluence phenomenon,which leads to a significant change in the network structure of reinforcements after hot-deformation.The size of the deformed net microstructure along the maximum primary strain direction is always elongates（R1＞R₀）,while along the minimum primary strain direction,it shortens（R₃ ＜R₀）continuously.The variation in the intermediate primary strain direction can be determined by μσ as R₂＞ R₀（μσ＞ 0）follows:R₂=R₀（μσ=0）.The relationships between axial length ratio λ of network R₂ ＜R₀（μσ＜0）structure for reinforcements and strain are established as λ₁₂＝exp（2ε₁+ε₃）,λ₂₃＝exp（-ε₁-2ε₃）,λ₁₃＝exp（ε₁-ε₃）.The tested room-temperature compressive mechanical properties of the ring after hot-deformation show that from the position of the tensile strain to that of the compressive strain,the ultimate compressive strength and plastic strain decrease firstly and then increase,with the minimum values at the position of plane strain.The maximum variations are 115.2 MPa and 11.35%,respectively.Compared with the TiB_w-lean region,the TiB_w-rich region has greater stress,strain concentration and strain energy density distribution,which will promote the initiation and propagation of microcracks in the TiB_w-rich region.Coupled with the passivation effect of the matrix alloy,the propagation of microcracks mainly occurs in the TiB_w-rich region and propagates along the long axis of the network structure for reinforcements.The relationship between the axial length ratio λmax of the network structure for reinforcements in the fracture plane and the compressive mechanical properties is established.As λmax increases,the driving force of microcracks propagation increases.In addition,the reinforcements have relatively weaker inhibition effects on crack propagation due to preferred whisker orientation along the long axis of the network structure and the content reduction of reinforcements.Therefore,the microcracks propagate along the long axis of the reinforcements more easily,leading to a decreased strength and plastic strain.