Study On Microstructure And Properties Of High Temperature Titanium Alloy Lbw Joints

With the development of aerospace industry,the titanium alloy which is called "aerospace material" has been widely concerned.Titanium alloy has a series of characteristics such as low density,high strength,good corrosion resistance and heat resistance,non-magnetic,good thermal stability and low linear expansion coefficient,et al.To meet the requirements of advanced aerospace engine,the high-temperature titanium alloy with service temperature higher than 600 ? has become the research hotspot.Ti60 alloy is jointly developed by institute of metals,Chinese academy of sciences and baoti group,it has a service temperature of 600?.At present,the usage amount of titanium alloy in aircraft and aero-engine has become one of the important index to measure its advanced degree.Therefore,this paper used laser beam to weld Ti60 alloy under the protection of argon gas.Study the forming,microstructure characteristics of the joints and the mechanical properties at room temperature(25?)and 600? high temperature.The effect of post-weld heat treatment on microstructure and mechanical properties of laser beam welding joints of Ti60 alloy was studied.Laser beam welding can obtain Ti60 welded joints that are well formed,smooth with silver-white metallic luster and no obvious oxidation phenomenon.Under different welding parameters,the fusion zone of joints are in the shape of "X" or "hourglass".The width of upper,middle and lower fusion zone,the maximum width of heat affected zone and the area of joint FZ all increase with the increase of laser power,all decrease with the increase of welding speed.The uneven distribution of heat in the welding process leads to the joints being divided into three regions:fusion zone(FZ),heat affected zone(HAZ)and base metal(BM).The heat affectedarea can be divided into near-HAZ,middle-HAZ and far-HAZ.The main microstructure in the FZ of laser beam welding joints of Ti60 alloy is acicular?'-martensite.With the increase of linear energy,the density and size of ?'-martensite in the FZ zone of laser beam welding joints of Ti60 alloy also increase,but the change of ?'-martensite will not be obvious when the linear energy increases to a certain extent.The microstructure of near-HAZ is also mainly ?'-martensite,and the size and density of martensite were decreased compared with FZ;the middle-HAZ presents bimodal structure,which is a mixture of ?'-martensite and equiaxed ?;the far-haz has only small equiaxed microstructure where the content of partial betastable elements is relatively high.Microhardness of laser beam welding of joint of Ti60 indicates that the FZ of joints have the highest hardness,the HAZ of joints have the second highest hardness,and the BM of joints have the lowest hardness.The maximum hardness of FZ increases with the increase of welding line energy.When the laser power is 2.0 k W,and the welding speed is 0.020m/s,the tensile tests of laser beam welding joints of Ti60 were carried out at room temperature and 600?high temperature,the results show that the two kinds of state,laser beam welding joints of Ti60 are broken in the base metal,the average tensile strength of 1191 Mpa(25?)respectively and 771 Mpa(600?),the average elongation reached 9.27%(25?)and 12.23%(600?).The fracture form of the joint at room temperature can be considered as a kind of brittle fracture.In the condition of 600? high temperature,ductile fracture happened Ti60 laser welded joint.The paper uses the post-welded heat treatment mechanism of940?/2h,FC+600?/1.5h,FC after welding.Compared with the joints that before post-welded heat treatment,the microstructure of the joints after post-welded heat treatment was more uniform,and the microstructure of FZ and HAZ showed cluster alpha microstructure.The hardness of joint FZ is slightly higher than that of HAZ,and the hardness of BM is the smallest.Compared with the joint FZ before post-welded heat treatment,the hardness of joint FZ is significantly reduced.At room temperature(25?)and 600? high temperature condition,the laser beam welding joints of Ti60 after post-welded heat treatment are broken in the HAZ,the averagetensile strength of 980 Mpa(25?)and 728 Mpa(600 ?),the average elongation reached 10.92%(25?)and 18.27%(600?).Ductile fracture occurred in the joint S in both condition.Post-weld heat treatment can reduce the control strength in a limited range,but the plasticity can be significantly improved.Post-welded heat treatment can effectively improve the comprehensive mechanical properties of joints.