Densification Process Analysis And Mechanical Properties Of TC4 Alloy Synthesized By Blended Element Method

Titanium alloys have low-cost and excellent properties,so they were widely used in aerospace,biomedicine,industrial production and other fields.Among them,Ti-6Al-4V（TC4）alloy has been widely used because of its high strength,high temperature resistance,corrosion resistance and good machinability.Powder metallurgy blended element method（BEPM）has attracted more and more attention in research and production because it can significantly reduce the cost of titanium alloy.However,there are few research on the compaction and sintering densification mechanism of titanium and titanium alloy powder.In particular,there is a lack of analysis on the dehydrogenation mechanism of titanium hydride,the homogenization law of alloy elements and the relationship between microstructure evolution and properties of as-sintered samples.Therefore,there are great challenges in the initial powder selection,preparation conditions and subsequent processing scheme of high-performance titanium and titanium alloy products.Taking pure titanium and TC4 alloy as the research object,the effects of powder type,pressing conditions,impurity content and element diffusion on the mechanical properties of as-sintered samples were investigated,and the laws of powder densification and dehydrogenation of titanium hydride powder were obtained.On this basis,the effects of rolling deformation and heat treatment on the microstructure and properties of TC4 alloy as-sintered samples were further explored,and the influence mechanism of microstructure evolution on properties was revealed.The main research contents and conclusions of this thesis are as follows:（1）The characteristics of pure titanium powder and the green compacting mechanism were explored.In the densification process of green powder,the densification of hydrogenation-dehydrogenation titanium powder（HDH Ti）is the volume shrinkage caused by powder plastic deformation,and the densification mechanism of titanium hydride（TiH₂）powder is the filling of pores by powder crushing.The compacting behavior of TiH₂ powder mainly includes the rearrangement and fragmentation of particles in the early stage of pressing,slight plastic deformation in the middle stage,and the rearrangement of particle fragments filling pores in the later stage.This compacting characteristic provides a higher relative density for the green compacts.The compacting equations have very high fitting degree to the compacting data of pure titanium powder（R²>0.99）.The linear compacting equations can distinguish different particle size distributions in the same type of powder by fitting parameters.The nonlinear compacting equations can analyze the contribution of powder densification mechanism through parameter calculation.The fitting parameters obtained from Gerdemann and Jablonski equation are more in line with the objective mechanism of powder densification.（2）The sintering densification mechanism of pure titanium powder and the dehydrogenation mechanism of TiH₂ powder were studied and revealed.The sintering performance of powder green compacts is greatly affected by the specific surface area and contact area of powder particles.Therefore,higher sintering density can be obtained by using finer powder and higher pressing pressure.The TiH₂ powder as-sintered samples have higher density and lower impurity content,so they have more balanced mechanical properties.This advantage is mainly due to the high relative density of green compacts made of brittle TiH₂ powder and the high sintering capacity provided by powder dehydrogenation.The main phase transformation of TiH₂ powder during dehydrogenation isδ-TiH₂（FCC）→β（BCC）+δ-TiH_x（BCT）→α-Ti（HCP）.（3）The effect of element homogenization on the mechanical properties of TC4alloy as-sintered samples was studied.In the process of densification and homogenization,alloy element diffusion and powder densification are a dynamic overlapping process.Compared with Al element in Al-V alloy,Al element in Ti-Al alloy diffused preferentially and diffused faster.The hydrogen produced in the dehydrogenation process of TiH₂ based alloy can“clean”and improve the activity of the master alloy,promote the process of element diffusion and improve the properties of the sample.Reasonable sintering density and oxygen content can obtain the optimal combination of mechanical properties,in this study,TC4 alloy synthesized from 0-125μm TiH₂ powder and 0-26μm single master alloy has obtained the optimal mechanical properties with tensile strength of 1000 MPa,yield strength of 906 MPa and elongation of 17.83%.（4）The effects of rolling deformation and heat treatment on the microstructure evolution and properties of TC4 alloy as-sintered samples were investigated.After rolling deformation of TC4 alloy as-sintered samples,the grains are broken and accompanied by dynamic recrystallization grains,and the texture of{10（?）0}cylinder in RD orientation was formed.The equiaxedαgrain structure was obtained by recrystallization annealing of as-rolled TC4 samples.The as-annealed samples are subjected to solution and aging treatment,and the samples underwent a phase transformation process ofα_p+β→α_p+α’+α"→α_p+β_T（α_s+β）,and formed a microstructure of equiaxedαgrains coexisting with needle-like or rod-likeβ_T,which had the best comprehensive mechanical properties（tensile strength of 1269 MPa,yield strength of 1248 MPa,and elongation of 16.05%）.In summary,based on TiH₂ and HDH Ti powders,the powder characteristics,compacting mechanism,element homogenization and mechanical properties of pure titanium and TC4 alloy were systematically studied in this thesis.The dehydrogenation mechanism of TiH₂ powder was clarified,and the rules of powder densification,element homogenization and microstructure evolution were obtained,which provided an effective method and theoretical basis for the synthesis and mechanism analysis of titanium alloy powder metallurgy.