Microstructure And Mechanical Properties Of BT20Titanium Alloy Casted In Graphite Mold And Ceramic Shell Mold

The graphite mold and ceramic shell mold were used to produced the castingBT20titanium castings in different conditions of pouring in this thesis, in the α+βtwo-phase region and β single-phase region, the as-cast microstructure was treatedwith different cooling rates. The as-cast and heat treated microstructure of twokinds of molds were analysed as a comparison，tensile mechanical properties testwere measured.With the pouring temperature rises,the spacing of α lamellars increases,anddifferent directions of α cluster decreases; With the casting wall thickness increases,α-phase morphology also appeared the same variation. Using graphite mold castingwith low-temperature，α lamellar spacing is minimum of1μm, due to the coolingspeed. The intragranular α-phase with more orientation and intertwined shows abasket-like features. In the casting microstructure by pouring of ceramic shell mold,There are many α-phase which have same orientation between two large α lamellar,The α-phase was short rod morphology with small length, and larger width. thethick part of the casting, whose α lamellar spacing reached a maximum3.16μm, ispoured at high temperatures by ceramic shell mold.The BT20titanium alloy casted by graphite mold at low temperature,have ahigh tensile strength of986MPa and yield strength of915MPa, elongation4.23%,inthe same conditions as castings of ceramic shell, although lower tensile strength of889MPa and yield strength of807MPa, but with higher elongation is reached5.71%; the mechanical properties of castings decreases as pouring temperature andwall thickness reducing.Different processes of heat treatment were used for the two kinds of castings.When air-cooled and quenched from the two-phase region,with temperatureincreasing, α lamellar length decreased and width increased,which likes short rodor equiaxed, and the content of β transformation structure increases. When furnacecooled from the two-phase region, with the temperature increasing, the lamellarspacing of α lamellar increased,and the β-phase content is almost no change.Thereisn`t acicular α phase precipitating in the microstructure,when air-cooled at970℃,β transformation organization begins to precipitate a small acicular α-phase,andwith the heat treatment temperature increasing, acicular α phase content increases.To the sample of graphite casting and ceramic shell mold casting which pured inhigher temperature,1080℃is in β single phase region, the microstructure is abasketweave when air-cooled form this temperature, which were mainly composed of the acicular α phase and β residues phase.While casting at a lower temperaturewith ceramic shell mold, a small amount of equiaxed primary α phase were foundin the microstructure. When quenched form two phase region, the microstructure ofBT20titanium did not precipitate the martensite, but a rod-shaped α-phase isfound on the matrix,whose width is about2μm.When furnace cooled from lower temperature of the two-phase region, themechanical properties of castings have little change. With temperature increasing,mechanical properties of sample is decreasing. Air-cooled and quenched from twophase region,the influence of mechanical properties is similar.With temperatureincreasing,the tensile strength and yield strength increased and then decreased,andelongation decreased with temperature increasing. Heat treatment have greatereffection to ceramic shell mold casting, after air-cooled or quenched, tensilestrength and yield strength were significantly improved, and elongation reducedslightly. Air-cooled from β phase region, the casting get a higher tensile strength,yield strength and elongation,with the best mechanical properties.