| Additively manufactured titanium alloy components have been widely used in aerospace,national defense,shipbuilding and other fields,but most titanium alloys have low damage tolerance,and additively manufactured components must have defects such as pores,inclusions,and residual stress,which lead to components that are difficult to meet the needs of practical applications.Metastable β-titanium alloys have extremely high strength and plasticity,but the additive manufacturing of this type of material is not fully studied,and still have theoretical and technical problems.If additive manufacturing can be used to prepare metastable β-titanium alloy components with high damage tolerance that are insensitive to micro-defects,it will have important scientific significance and engineering value.In this paper,four metastable β-titanium alloys of Ti-27Nb-5Zr-2Cr,Ti-27Nb-5Zr-5Cr,Ti-27Nb-5Zr-5Cr-2Fe and Ti-27Nb-5Zr-3.5Mo(wt.%)were designed,and the microstructure and mechanical properties of these four alloys in the laser deposited state and the post-treatment state were studied systematically.Firstly,the laser additive forming process of Ti-Nb-Zr metastable β-titanium alloy was developed.The embrittlement behavior of Ti-Nb-Zr metastable β-titanium alloy in the laser deposited state was systematically studied,and the embrittlement mechanism centered on the formation and evolution of ω phase was clarified.On this basis,an intermittent laser additive forming process was proposed,which effectively reduced the size and volume fraction of the isothermal ω phase.The strength and elongation of the Ti-27Nb-5Zr-2Cr alloy fabricated via the intermittent deposition method were 592±26 MPa and 20±2%,respectively,which were 81% and 333% of the alloy prepared by the continuous deposition method,showing good combination of strength and plasticity.On the basis of the above work,the toughening and plasticizing method of laser deposition Ti-Nb-Zr metastable β-titanium alloy based on short-time heat treatment was further developed.After short-term heat treatment(at 950°C,holding time less than 5 minutes),only a small amount of athermal ω phase existed in the alloy,and the isothermal ω phase in the laser-deposited alloy was effectively eliminated,and the alloy was significantly ductile and plasticized.After heat treatment,the elongation rate of Ti-27Nb-5Zr-3.5Mo alloy was 41±4%,which was 273% of the laser deposited alloy.However,for Ti-27Nb-5Zr-5Cr and Ti-27Nb-5Zr-5Cr-2Fe alloys with highβ stability,the significance of short heat treatment was limited.The effects of post-treatment methods such as hot forging and surface strengthening on the microstructure and properties of Ti-Nb-Zr metastable β-titanium alloys in laser deposited state were studied.After hot forging,the isothermal ω phase in the laser deposited alloy was transformed into an athermal ω phase,deformation twinning were formed in some alloys,and the strength of the alloy was significantly improved.The strength of the hotforged Ti-27Nb-5Zr-5Cr alloy was 863±10MPa,which was 25% higher than that of the laser-deposited alloy.Although twinning-induced plasticity(TWIP)in hot-forged and heattreated alloys made the alloy obtain higher plasticity,it led to early necking of the sample.To solve this problem,a post-treatment Ti-Nb-Zr system based on mechanical shot peening was developed.After the surface shot peening,a gradient structure of nanocrystals and deformation twinning was formed on the surface of the alloy,which significantly suppressed the surface wrinkling caused by deformation twinning,and at the same time improved the strength and plasticity of the alloy.The yield strength of the Ti-27Nb-5Zr-2Cr alloy and elongation increased by 18% and 6%,respectively.The research in this paper provides a reference for the design and development of metastable β-titanium alloys for additive manufacturing,and provides support for the additive manufacturing of high damage tolerant titanium alloy components. |
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