Hydrogen-assisted Powder Metallurgy And Mechanical Enhancement Mechanism Of TC16 Titanium Alloy

Titanium alloys have excellent properties such as high strength-to-weight ratio,good corrosion resistance and high biocompatibility,which have been widely used in engineering fields such as aerospace,marine and biomedical industry.Among them,theα+βtype Ti-3Al-5Mo-4.5V（TC16）alloy has become an important structural material in the aerospace industry due to its excellent cold formability and high strength and ductility.However,the relatively high production cost limits the large-scale development and application of TC16 titanium alloy.Due to the presence of molybdenum（Mo）,which is a refractory and low diffusion coefficient metal,in TC16 alloy,defects of segregation and void points may occur during the ingot metallurgy process（IM）.Although the microstructure and mechanical properties can be improved through the subsequent heat treatment,the complex thermal process causes serious waste of energy and materials.Developing more efficient technology for TC16 titanium alloy preparation has become an urgent need for high-quality aerospace metal materials.The titanium hydride based powder metallurgy（TiH₂-BEPM）is an efficient and low-cost titanium alloy production technology,which has gradually become an alternative method to IM in recent years.In this thesis,we firstly studied the powder pressing and sintering process of preparing high-density pure titanium（Ti）based on TiH₂,demonstrating the dense advantages of highly brittle TiH₂ powders that fragment and rearrange in the pressing stage,and phase transition during sintering can accelerate volume shrinkage.For TC16 titanium alloy,the morphology and properties of the master alloy（MA）are significantly different from TiH₂powder,as well as the presence of the low diffusion coefficient metal of Mo,creat a large amount of pores during the powder pressing and sintering process,which bringing challenges for the preparation of high-density TC16 alloy by TiH₂-BEPM process.Based on the dense advantage of the high brittle TiH₂ powder and dehydrogenation phase transition,this thesis innovatively proposes a pre-hydrogenation process for adding Ti in MA（MAPH）,successfully modifying MA to contain H brittle 7.5Ti MA-H_x powder,and preparing high-density TC16titanium alloy with good performance.On this basis,the effect of thermal processing on the microstructure and properties of sintered TC16 alloy was further explored,revealing the intrinsic relationship of process-microstructure-properties,and exploring a new path for the low-cost preparation of high-performance TC16 titanium alloy.The main conclusions of this thesis are as follows:（1）The powder metallurgical process for the preparation of pure Ti from TiH₂ was studied,revealing the compaction characteristics and the sintering dehydrogenation mechanism of TiH₂ powder.Compared with ductile HDH Ti powder,the brittle TiH₂ powder can be crushed into small particles with the formation of strong mechanical interlocks between them during pressing,which enables the higher relative density of green compacts.The tightly interconnected TiH₂ particles are advantageous for material transport during sintering,promoting the formation of sintering necks and the healing of pores.During sintering,H atoms are released from TiH₂,and the lattice structure changes from theδ-FCC structure of saturated TiH₂ to theβ-BCC structure of unsaturated TiH_x,further evolving into theα-HCP structure of fully dehydrogenated Ti,causing a sharp change in the lattice volume and introducing a large number of vacancies and dislocations,promoting the synergistic shrinkage of the powder and densification of the samples.In addition,the H atoms released from TiH₂ have the effect of cleaning the powder oxide layer,resulting in much lower oxygen content in sintered material than that from HDH Ti.Pure titanium sintered using TiH₂ powder with a particle size of 0-45μm has the density of 99.1%,the tensile strength of 516.1 MPa,and an elongation of 26.8%.（2）The high density TC16 alloy preparation technology using the Master Alloy Pre-Hydrogenation（MAPH）method was developed,and the key mechanism for improving density and properties using the MAPH method was elucidated.Considering the difficulties in hydrogenating high hardness MA and the challenge in preparing high-density TC16titanium alloy using the TiH₂-BEPM process,this study proposes adding a certain proportion of Ti to MA and hydrogenating it to modify the high-hardness MA powder into a high-brittleness,irregular-shaped,H-containing powder（7.5Ti MA-H_x）,which exhibits similar compression-sintering behavior as TiH₂ powder.The mechanical interlocking structure formed between the irregular-shaped,brittle 7.5Ti MA-H_x and TiH₂ particles eliminates the incompatible interface between MA and TiH₂ powder in traditional powder metallurgy processes.In addition,both 7.5Ti MA-H_x and TiH₂ particles can simultaneously dehydrogenate and shrink to reduce the mismatch in powder volume changes.The phase transition of7.5Ti MA-H_x during dehydrogenation can accelerate solid-state diffusion by increasing the lattice defect density,leading to faster homogenization of alloy elements in 7.5Ti MA-H_x than in MA.Moreover,the Ti melted into MA can reduce the unbalanced mass transfer of Ti matrix to the MA and the Kirkendall voids caused by the element Mo.Ultimately,the TC16 alloy sintered using MAPH method achieved a high relative density of 99.2%and exhibited good tensile strength（1004.6 MPa）and elongation（15.8%）.（3）The thermal processing technology of TC16 titanium alloy prepared by powder metallurgy sintering was proposed,and the mechanism of thermal processing on the microstructure and mechanical properties of titanium alloy was elucidated.There are problems such as high porosity,coarse grains,and poor mechanical properties in the sintered TC16titanium alloy with large size.When the deformation reaches 90%,the residual pores are basically eliminated,accompanied byαgrain refinement and dynamic recrystallization.During annealing,theαphase transforms from low angle to high angle boundaries through grain boundary migration,and static recrystallization occurs with equiaxedαgrains.Equiaxedα-phase is conducive to improving the plasticity of the material,and the elongation of TC16alloys annealed at 800-860 ^oC is above 20%.During aging,secondaryα-phase precipitates and coarsens with increasing temperature.The smallαs phase can effectively hinder dislocation slip by generating a large number ofα/βinterfaces with theβmatrix,which plays a decisive role in improving the strength of the TC16 alloy.After aging at 520°C,the TC16alloy has a tensile strength of 1424.6 MPa,an elongation of 7.7%,and a hardness of 380.2 HV.Compared with the sintered TC16 alloy,the strength,plasticity and hardness have been improved by 60.7%,40%and 35.7%,respectively,achieving the transformation to a high-strength titanium alloy with excellent properties.In summary,this thesis conducted systematic research on the preparation of high-performance TC16 titanium alloy.High-density TC16 titanium alloy was prepared by modifying MA into brittle 7.5Ti MA-H_x powder.By controlling the microstructure of the TC16alloy through heat treatment,the influence mechanism among process,microstructure and properties was established,and high-strength TC16 titanium alloy was obtained.The complete process developed in this thesis provides a favorable theoretical basis and technical support for the powder metallurgy method to prepare high-performance titanium alloys,which containing high-proportion master alloy and difficult-to-diffuse metal elements.