Microstructure Evolution And Control Of TC4 Titanium Alloy Via CMT Arc Additive Manufacturing

Titanium alloy additive manufacturing technology is widely used in the transportation,aerospace and petrochemical industries.However,the columnar primaryβgrains caused by heat input accumulation affects its application in titanium alloy via wire and arc additive manufacturing.Thus,the formability,microstructure evolution and control of the titanium alloy components via wire and arc additive manufacturing has important theoretical significance and practical application value.First of all,the optimization test of formability parameters was carried out in the wire and arc additive manufacturing.TC4 titanium alloy deposited wall was obtained.The effects of different CMT（Cold Metal Transfer,CMT for short）modes on the thermal cycle and microstructure evolution of the TC4 titanium alloy deposited wall were investigated.The current and voltage waveform,arc shape and heat input were studied in order to ensure the stability of the additive manufacturing process.Secondly,the microstructure and properties of TC4 titanium alloy deposited wall were improved by refining the prior-βgrains via adding alloying elements.After the addition of trace Nb powder,the（β-Ti,Nb）phases were formed under the conditions of the temperature range of T_diss–T_βand Rc≤25°C/s.The solid-solution strengthening played the dominant role in strengthening the specimens.After the addition of trace Mo Si₂ powder,trace（β-Ti,Mo）phase and Si O₂ particles were obtained and partly prior-β→α’phase transformation was suppressed.Then,the effects of ultrasonic peening treatment（UPT）on grain refinement of titanium alloy deposited wall were studied.This method shows no apparent plastic deformation on the surface appearance,but has a great improvement in grain refinement.The ratio of length to width of the columnar prior-βphase changes from15.6 to 6.2,and the average length of the secondaryαphase changes from 33μm to14μm.The temperature range of the reaction isα’dissolution temperature–liquidus temperature(T_diss–T_L).After UPT,the anisotropic percentages of the specimen decrease from 6%to 0.8%.Finally,in order to improve the microstructure inhomogeneity and reduce the amount ofα’martensite,the effects of heat treatment on the microstructure evolution of titanium alloy deposited wall were studied.The effects of holding time and heat treatment temperature on the microstructure and mechanical properties were studied.The mechanism of microstructure evolution of TC4 titanium alloy after heat treatment was discussed.Ultra-precision machining was used to improve the forming precision of the specimens.The machinability of the specimens with different microstructures was analyzed.The experimental results show that a well-formed TC4 titanium alloy deposited wall can be obtained by using CMT wire and arc additive manufacturing.The addition of alloying elements such as Nb and Mo Si₂,can acted heterogeneous nuclei to refine prior-βgrains.UPT in three directions can refine the columnar prior-βand the secondaryαgrains by mechanical effects.Post heat treatment can improve the inhomogeneity in microstructure and reduce the amount ofα’martensite.The methods proposed in this work can control the microstructure and improve their service properties of TC4 titanium alloy deposited wall.The results of the mechanism study provide a theoretical basis for further improving the microstructure and properties of the TC4 titanium alloy deposited wall.