Technical Studies Using Ultrafine Powders TiH₂ Ultrafine Grained Titanium

Ultrafine grained titanium is an important direction of development,but high manufacturing cost limits its application.Lei Bao,who confirmed the grain size of samples would reduce with the reducing particle size of the raw material powder after the TiH₂ powder sintering.Using this principle,I can prepare ultrafine grained titanium alloys.In this paper,I use superfine TiH₂ powder as raw material to fabricate a fine grain titanium products via repression-sintering and hot isostatic pressing process.Furthermore,a systematic research in raw materials preparation,performance testing,sintering process and other aspects was conducted.First,I use stirred ball mill,planetary ball mill and jet mill to prepare ultrafine TiH₂,the particle size of the powder was found to achieve ultra-fine powder standards through the above three methods,the particle size distribution of the powder prepared by stirred ball milling and planetary ball milling is narrow,but the powder has been seriously polluted,while the purity of TiH₂ powder prepared by jet milling is higher than those prepared by the first two methods,this can be used as raw material for the preparation of fine grain titanium.By differential scanning calorimetry to study the decomposition characteristics of different granularity TiH₂ powder and superfine powder TiH₂ decomposition characteristics at different heating rate.I found TiH₂ endothermic start temperature?decomposition temperature?and the temperature corresponding to the endothermic peaks decreases as particle size decreased.For example,the first endothermic peak temperature corresponding to the particle size of the TiH₂ powder 17,5,1.56?m were 454,444 and 435 ?,corresponding to the second endothermic peak temperatures were 554,541 and 533?,the particle size affects the decomposition characteristics markedly.However,the endothermic start temperature?the temperature corresponding to the endothermic peak and the absorption of heat per unit of time increases as the heating rate increases.Secondly,using easily moved away flap mold,vacuum sintering furnace and the high-vacuum system,I have a comprehensive study of suppression,degreasing and sintering of ultrafine TiH₂.Found suppressing properties of ultrafine TiH₂ is undesirable,it must add molding agent of repression,but its better sintering activity,blank samples sintered at 950?×2h,its relative density can reach more than 95%,sintered at 1250?×2h,the relative density of nearly 100%.Blank sample density has little affection the density of the sintered product.Superfine TiH₂ vulnerable to pollution,beginning decomposition temperature is low,so it should minimize the overlap range of its degreasing temperature range and decomposition temperature range.Must have been fully degreased and use forming agent with a lower boiling point before powder sintering.The study found the powder size,furnace H₂ pressure,heating rate has a significant impact on the quality of blank samples sintered;in dehydrogenating decomposition of the blank sample stage,a lower heating rate and the greater pressure of H₂ should be used to avoid defects such as cracks;After the blank sample completely decomposed,faster heating rate can be used,and open the high vacuum system to pump furnace gas.After 2h the blank sample sintered at 950 ?,the average grain size of the particle to 4.8?m,a tensile strength of 396MPa.With 800? ×1h hot isostatic pressing process of sintered products for reprocessing,the average grain size of the sample was 8.3?m,tensile strength reaches 717 MPa,if you add 0.8wt.%yttrium elements to sample to deoxidation of matrix,the tensile strength up to 894MPa.Hot isostatic pressing eliminates the majority of pores in the products to improve the tensile strength of products,but also the grain size has grown.Finally,the analysis of the microstructure and density of the sintered titanium powder in an atmosphere of H₂,then combined with some rules of the TiH₂ sintering,I reach a conclusion that H₂ in the dehydrogenation stage of the ultrafine powder of TiH₂ will suppress the decomposition,to avoid cracks in the blank sample,in the meantime,it can also inhibit grain growth.After all the powder decomposed,with the increase of sintering temperature,the grain of blank sample is growing up.This study demonstrates scientificity and feasibility of technical program which uses ultra-fine powders of TiH₂ material through suppress+sintering+hot isostatic pressing process to prepare high-strength fine-grained titanium,thus explore a kind of new economic manufacturing method for the preparation of ultrafine grained titanium alloy.