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High-temperature Deformation Process and Microstructural Characteristics of TC4 Alloy

Author: WangZhaoJun
Tutor: ChenLiQing
School: Northeastern University
Course: Materials Processing Engineering
Keywords: TC4 alloy thermo-mechanical simulation high temperature deformation microstructure flow stress cooling rate
CLC: V250.2
Type: Master's thesis
Year: 2009
Downloads: 122
Quote: 2
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Abstract


As a kind ofα+βtwo-phase titanium alloy with excellent performance,TC4 alloy has been widely used in manufacturing high-performance aerocraft structural parts.Due to its characteristics of high yield to tensile strength,the main manufacturing method for this alloy is hot working.The mechanical properties of TC4 alloy depend greatly on its microstructure,which caused by processing techniques such as deformation temperature, strain rate,deformation amount,cooling rate and heat treatment after deformation.A reasonable design of continuous hot rolling process is not only to roll the material as shaped, and more importantly,to obtain small and uniform microstructure.In this paper,a practical process of multi-stand tandem hot rolling has been firstly simulated through isothermal compressive deformation in MMS-300 thermo-mechanical simulator.The effect of different hot working conditions and following cooling rate on its microstructures has then been explored.Firstly,the hot deformation behaviors of TC4 alloy at elevated temperatures have been studied based on the strain-stress curves.The influences of processing parameters on flow stress during plastic deformation are also studied.As the other processing parameters are kept the same,the higher the deformation temperature is,the lower the flow stress will be. TC4 alloy is positive strain rate sensitive,i.e.as the other processing parameters maintain the same value,the flow stress increases with increasing strain rate.In dual-pass compression process,the flow stress in two-phase field increases as deformation amount and following cooling rate in single-phase field increases.Secondly,with the aids of optical microscope and scanning electron microscope, deformation mechanism for hot deformation of TC4 alloy has been investigated. Relationship between processing parameters,cooling rate and microstructures during hot deformation are also studied.Recovery and recrystallization become more easily when increasing deformation temperature.Meanwhile,recrystallized grains are more likely to combine together and grow up,which result in coarse grains.The main effect due to different cooling rate is changes of the amount of various precipitatedαphase particles and its distribution.Increasing the subsequent cooling rate will lead to thinnerαrim aroundβ grains and more refined grain structure.The variation of recrystallized grain size follows an inverse parabolic law when strain rate is kept to increase.The main reasons for this phenomenon are recrystallization and temperature rise during deformation process.An additional deformation amount can refine the grain.But,only the deformation amount exceeds a certain value,recrystallization in deformed alloy can occur and lead to grain refinement.The specimen can be divided into three regions during deforming process because of friction and temperature gradient.This inhomogeneous deformation can not be avoided,but can weaken its negative impact by improving the processing environment and modifying the technological design.The relationship between microstructure and hardness of deformed alloy has been finally studied.Additionally,the deformation process leading to the favorite microstructure has been discussed.

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