Research On Short-process Preparation And Annealing Process Of Large-diameter TC4 Titanium Alloy Seamless Pipe

TC4(Ti-6Al-4V)is a typical ?+? two-phase titanium alloy,which has the advantages of high specific strength,low density,good thermal stability and corrosion resistance.It is made into a titanium alloy seamless tube and has broad application prospects in many fields such as marine engineering,naval,aerospace,chemical,automotive and electric power.The traditional TC4 titanium alloy seamless tube manufacturing process is mainly to forging the ingot and then carry out the drilling or cross piercing.Finally,the finished tube with different specifications prepared by rolling,drawing and spinning.It has the disadvantages of long process flow,low yield,and difficulty in obtaining ultra-long tube,which lead to high tube cost and application is greatly limited.Based on the above reasons,this paper proposes a new process for VAR(Vacuum Arc Remlting)+ EB(Electron Beam Cold Hearth Melting)smelting TC4 titanium alloy ingots to directly cross piercing to obtain seamless tube.The process not only can take advantage of EB smelting to obtain high-quality TC4 titanium alloy ingots of different specifications,but also can directly produce seamless tubes by using ingots without a forging process.Therefore,this paper firstly studied the composition uniformity and microstructure of TC4 titanium alloy smelted by VAR+EB and then studied the hot deformation behavior of the ingot,the microstructure evolution and mechanical properties of the rolled and annealed tube.Obtain the following main conclusions:The ingot is alloyed by VAR smelting and then EB smelting.The ingot structure is widmanstatten structure.The Al and V elements in the ingot are evenly distributed,and the content meets the requirements of GB/T 3620.1-2016.The thermal deformation results of TC4 titanium alloy smelted by VAR+EB show that the flow stress of the alloy decreases with the increase of deformation temperature,and the flow stress increases with the increase of strain rate.As the deformation temperature increases,the lath phase ? inside the original ? crystal grains gradually becomes a short rod shape or a strip shape,and there is a small amount of spherical shape.When the temperature continues to rise,a large amount of dynamic recrystallization occurs.As the strain rate increases,dynamic recrystallization becomes less and less,and the increase of strain rate is not conducive to the dynamic recrystallization of TC4 titanium alloy smelted by VAR+EBBased on the DDM model,a thermal processing map with true strain of 0.9 was established.According to the thermal processing map,the suitable hot processing parameters of VAR+EB smelted TC4 titanium alloy were determined.The strain rate was 0.001~0.01 s-1,and the deformation temperature was 900 ~ 950 °C is suitable for the hot processing of the alloy,at this time the microstructure of the TC4 titanium alloy is a small,equiaxed ? phase,this fine equiaxed ? phase is beneficial to the mechanical properties of the material.The microstructure of the seamless tube obtained by direct cross piercing of the ingot is also widmanstatten structure,and the sheet ? is thinner than the as-cast layer ?,and the mechanical properties of the rolled seamless tube are Rp=907 MPa,Rp0.2= 841 MPa,A% = 9.7 %.Annealing the seamless tube at 750~950 °C for 1 h,air cooling.the ? phase of the seamless tube is coarsened and gradually staggered,and the ? grain boundary disappears;when the annealing temperature is 900 °C,the tube The tissue was transformed into a basket structure.When the annealing temperature was raised to 950 °C,the ? phase of the sheet was obviously coarsened,and a short rod-like ? phase appeared.As the annealing temperature increases,the strength of the tubes gradually rises,reaching a maximum at 950 °C;and the elongation gradually decreases,reaching a maximum at 850 °C.According to the comprehensive analysis,the annealing treatment was carried out at 850 °C,and the sample had good toughness(the average tensile strength,yield strength and elongation were 925.5 MPa,844.0 MPa and 13.9 %,respectively),and the mechanical properties were close to TC4 titanium alloy seamless tube prepared by traditional process.In summary,it is feasible to obtain a new seamless process for VAR+EB smelting TC4 titanium alloy ingots by direct cross piercing This new process can realize the preparation of large-diameter TC4 titanium alloy seamless tubes without forging and subsequent processing.Compared with the traditional process,it has the advantages of short process flow,high yield and low production cost.