Microstructure Evolution Behaviours And Mechanical Properties Of TA15 Alloy By Isothermal Multi-directional Forging

Titanium alloys are becoming more and more important as structural materials in aerospace because of their low density,high specific strength,excellent corrosion resistance and thermal strength.They are widely used in the manufacture of aircraft fuselage skeleton,skin,landing gear,truss,heat shield and shell.TA15 titanium alloy is a kind of near-alpha titanium alloy with high aluminium equivalent.It has the advantages of both alpha and(alpha+beta)titanium alloys.Its nominal composition is Ti-6Al-2Zr-1Mo-1V.It has medium room temperature and high temperature strength,good thermal stability and weldability.Its long-term working temperature can reach500 C.It is mainly used in aircraft structural parts and engines.It is an important aviation material.Traditional methods are mainly to improve the structure and properties of materials by hot deformation and heat treatment.As a typical large plastic deformation method,multi-direction forging process has the advantages of simplicity and low cost compared with other representative large plastic deformation methods such as equal channel angular extrusion and high pressure torsion.Large dense materials can be prepared by using existing industrial equipment,which is expected to be directly used in industrial production.During the deformation,the material is compressed and elongated with the change of load axis.The grain structure of the alloy is refined and the properties of the material are improved by repeated deformation in three axes.In this paper,the multi-pass forging tests of TA15 titanium alloy were carried out at700?,800?and 900?.The effects of deformation passes on the morphology,grain orientation and grain boundary evolution of TA15 titanium alloy were studied by metallographic observation and EBSD technology at different scales.The results show that the analysis of microstructural evolution during deformation process and the investigation of grain refinement behavior and mechanism provide a new idea for improving TA15 titanium alloy.At the same time,the study of microstructural evolution of heat treatment shear band after multi-direction forging of TA15 titanium alloy and its evolution mechanism are of great significance for improving the formability of TA15 titanium alloy and its practical application.The microhardness,tensile properties at room temperature and vacuum annealing tests at different holding time of MDF deformed TA15 titanium alloy were carried out.The strengthening and toughening mechanism and recrystallization thermal stability of TA15 titanium alloy during multi-direction forging deformation were analyzed according to the evolution characteristics of material microstructures.The results show that the grain size of TA15 titanium alloy is obviously refined after multi-direction forging.Increasing the deformation pass and decreasing the deformation temperature can promote the refinement of the structure.In the process of deformation,the deformation zone structure is easily formed in the central shear zone due to the adiabatic temperature rise effect,which further refines the grain size.The main refining mechanisms in multi-direction forging process are dynamic recrystallization refining,mechanical breakage and deformation zone refining.Aiming at the phenomenon of deformation band formed during MDF deformation of TA15 titanium alloy,combined with metallographic observation and EBSD measurement of medium and high temperature vacuum annealed samples(1p,2h,2p,2h,3p,2h),the effects of deformation passes on the morphology,grain orientation and grain boundary evolution of TA15 titanium alloy were studied.The results show that with the increase of annealing time,the microstructures of the deformed bands of the samples are obviously improved,the equiaxed degree of the microstructures is continuously improved,the grain size is gradually increased,and the increasing trend is gradually slowed down.Compared with the initial sample,the content of large angle grain boundary increases,and the recovery of grains,static recrystallization,partial grain growth and precipitation of alpha phase occur during annealing.