| TC4 titanium alloy is widely used in aerospace,chemical industry and medical fields due to its excellent corrosion resistance,high specific strength and heat resistance.The Laser Solid Forming(LSF)technology can realize near net forming of the relatively complicated structure of the titanium alloy structural parts.This processing method not only shortens the leading time,reduce production costs,but also has the ability to manufacture the complex parts.However,the LSF TC4 titanium alloy exhibits obvious characteristics of high strength and low plasticity,which seriously affects its use as structural parts.Besides,the price of V element is expensive,resulting in a higher cost of the alloy.Thus,it is necessary to improve the alloy composition and refine the ? grain to optimize the mechanical properties of the material and eliminate the anisotropy of the material.Based on the above analysis,this study used Mo element instead of V element in TC4.By studying LSF Ti-6Al-xMo(x = 2,3,4)alloy,based on the optimization of alloy composition,the microstructure and mechanical properties of the alloy were further optimized.The main research results obtained in this thesis are as follows:1.The effect of Mo content on the microstructure and mechanical properties of LSF Ti-6Al-xMo(x = 2,3,4)alloys were investigated.When the content of Mo in the alloy is increased from 2wt% to 4wt%,the size of the prior ? columnar grains of the LSF Ti-6Al-xMo alloy is slightly reduced,the thickness of the equiaxed grains layer in the top regionis gradually increased,and the size of the ? phase is gradually reduced.In addition,the microhardness of the deposited sample gradually increases as the Mo content increases.The comprehensive properties of LSF Ti-6Al-3Mo alloy are optimized among the three alloys with a tensile strength of 982 MPa and an elongation of 9.2%2.LSF Ti-6Al-3Mo alloy is the research object,based on process control,the properties of raw material powder and the piror ? grain refinement mechanism are studied.Compared with other lasers,fiber lasers have higher deposition efficiency when depositing Ti-6Al-3Mo alloy.The prior ? grain morphology of the deposited sample changed from coarse columnar grains to equiaxed grains with an aspect ratio of about 2.5.After the powder is bonded and coated,the prior ? grain morphology of the alloy sample deposited by the fiber laser is furtherconverted into the fine equiaxed grains with the size of ~ 100 ?m.This is caused by the un-melted Mo particles which acts as heterogeneous nucleation in front of the liquid-solid interface of the molten pool.3.The macrostructure and intragranular ? phase orientation distribution of LSF Ti-6Al-3Mo alloys with different grain morphology were investigated.While the prior ?grains change from coarse columnar grains to fine equiaxed grains,the texture disappears in the <100> direction,and the grain size of ? phase decreases slightly.The ? phase of the Ti-6Al-3Mo alloy sample has obvious variant selection characteristics.However,with the refinement of ? grains,the variation selection phenomenon is gradually weakened,resulting in no significant change in the orientation difference of ? variant.4.The room temperature tensile properties and tensile fractures of LSF Ti-6Al-3Mo alloy were investigated.When the prior ? grain morphology changes from coarse columnar grains to fine equiaxed grains,the tensile strength of the deposited samples increases from982 MPa to 1082 MPa,and the yield strength increases from 840 MPa to 922 MPa.The Ti-6Al-3Mo alloy with equiaxed grains has excellent comprehensive properties,and its elongation reaches a maximum of 9.8%. |
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