| TA15 titanium alloy,whose nominal composition is Ti-6.5Al-2Zr-1Mo-1V,was successfully developed by the Soviet Union in 1964.The main strengthening mechanism is solid solution strengthening of ? stable element Al.Adding neutral elements Zr and ? stable elements Mo and V to improve process performance.TA15 titanium alloy belongs to nearly ?-type titanium alloy.It not only has good thermal strength and weldability of ?-type titanium alloy and process plasticity close to?-?-type titanium alloy,but also has excellent characteristics such as medium room temperature and high temperature strength,low density and corrosion resistance,which are widely used in aerospace components and engine structures.With the development of aerospace parts towards lightweight,lightweight structure with excellent mechanical properties will be the inevitable direction of TA15 application in the future.Due to its unique forming method,selective laser melting(SLM)can form complex structures by stacking metal powders layer by layer,and can integrate complex structures that are difficult to form by traditional processing methods.SLM technology is gradually applied to the forming of metal parts with complex structures.Based on the current development trend and demand,this paper studies the process and heat treatment process of SLM forming TA15,and obtains the optimal parameters by characterizing the forming quality such as morphology,microstructure,mechanical properties and density of heat treatment state.Finally,TA15 is applied to lightweight lattice structure design.Several three-period minimal surface structures(TPMS)are designed and compared with traditional body-centered cubic(BCC)and face-centered cubic(FCC)rod structures.Firstly,the forming process parameters such as laser power,scanning speed,scanning distance and powder layer thickness are studied.The optimal process parameters of SLM forming TA15 are determined by detecting the microstructure and mechanical properties of the formed samples.The formed TA15 samples are composed of acicular ? ' martensite.The effects of various parameters on the density of formed samples are discussed,and the laser energy density is introduced to explain the mechanism of defects.The optimal process parameters are: laser power 290 W,scanning speed 1400mm/s,powder layer thickness 0.03 mm.The tensile strength of the sample formed with this process parameter can reach 1280 MPa.In view of the problems that TA15 is prone to warping deformation and large residual stress during SLM forming,the microstructure of TA15 is regulated by heat treatment to eliminate its internal stress and increase its plasticity.The TA15 sample was treated at 800 °C,850 °C,900 °C,950 °C,1000 °C,1050 °C and the holding time of 1 h,2 h and 3 h.Heat treatment will transform ?' martensite into lath ?-phase and ?-phase martensite in TA15,and the grain size will increase with the increase of holding time and annealing temperature.The compression test of annealed TA15 sample was carried out,and the optimal annealing parameters were obtained as follows: holding at850 °C for 2 h,and cooling with the furnace.Under this annealing parameter,the tensile strength of TA15 sample was 1137 MPa,and the tensile rate was 13.4 %.The tensile strength and tensile rate had good matching,which exceeded the forging standard.Finally,the lightweight structural design of TA15 is carried out in this paper,and the lattice structure with excellent mechanical properties is designed to achieve lightweight.The difference of stress distribution and tensile strength between TPMS structure and rod-like point structure was compared by compression test and static compression finite element simulation.The internal relationship between the mechanical properties of TPMS structure and its model characteristics was analyzed,and its failure mechanism was analyzed.The preliminary mathematical relationship between the two was summarized,which laid a certain research foundation for the application of lightweight structures. |
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