Numerical Simulation And Experimental Study Of TA15Titanium Alloy Post Welding Local Laser Heat Treatment

Titanium alloy welded structure has been widely applied in many fields, but inthe welding process, there is a huge temperature gradient between the weld pool andthe base metal, plus a titanium alloy with low thermal conductivit y, high heatcapacity, high melting point, resulting in a titanium alloy welded joints fittingscoarse microstructure and high welding residual stress and produce a certaindeformation. To solve this problem, select the TA15titanium alloy plate weldmentsas the object of study. Using the methods of simulation combined with experimentattempts to reduce welding residual stress and the deformation during the weldingby local laser heat treatment, and discuss the affect of laser heat treatment onmicrostructure.At first, simulate the welding process, and analysis the temperature field andstress field of welding. An appropriate model was established by finite elementanalysis software ABAQUS. The material parameters used in the simulationdepended on temperature. The convective and radiative heat transfer was considered,and ignored the effect of the stress field on the temperature field. A sequentialcoupling method was used in the analyses of stress field. Select the double ellipsoidheat source model as the welding heat source model. The maximum temperature ofthe heat source center in the welding process was2600℃, and cooling rate fasterthan the heating rate. Weld centerline residual stress mainly showed as longitudinaltensile stress, the value was about870MPa. In the heat affected zone, the stressdecreased rapidly, and showed as compressive stresses, then, the stress valuebecame stabilized. The deformation of welding mainly showed as warping facingthe welding heat source. The Y-direction displacement of the free end was0.15mm.Set the simulation results of welding process as the initial state of the post weldlocal laser heat treatment process. Examine the impact of the laser heat treatment onwelding residual stress and deformation. Choice the Gaussian heat source model asthe laser heat treatment source, loading on the lower surface of the welding process.draw up10laser heat treatment process to be simulated. the highest temperature ofthe heated surfaces was between760~1580℃，belowing the melting point. Thecooling process of non-heated surfaces was behind the heated surfaces. The size andshape of the phase transition area in the heating process was predicted. The bestprocess of improve the welded joints were P320D6V10and P450D6V10. Thelongitudinal tensile stress of the heated surfaces could be reduced to about600MPafrom870MPa after laser heat treatment. The Y-direction displacement of the free end was-0.16mm and-0.17mm, basic correction of welding deformation.The experiments of local laser heat treatment were conducted. The originalwidmanstatten of weld zone became into martensite after laser heat treatment.Martensite size along the thickness direction from the surface gradually becamesmall. A clear boundary was between small martensite widmanstatten. The basemetal also presents the characteristics of the martensite from the surface along thethickness direction gradually small. Transmission analysis found that the originallength of the weld zone and straight widmanstatten into the smaller orientationconfusion martensite, the dislocation density has also increased. After lasertreatment and found a large number of α’ phase and a small amount β phase, α’phase and the size difference, some of the large size between α’ phase mixed withsmall size α’ phase. Microhardness of the weld zone from the surface360HVgradually decreased to the original widmanstatten330HV, the microhardness of thebase metal from around the surface of340HV gradually reduced to about the basemetal310HV.