Study On Microstructures And Mechanical Properties Of Laser Welded Joints Of Ti-24Al-15Nb Alloy

Ti-24Al-15Nb?at.%?alloy has high specific strength,good oxidation resistance and creep resistance,making it one of the preferred materials for new aero-engines,and it has broad application prospects in aerospace and other high-tech fields.Subsequently,the welding?jointing?of Ti-24Al-15Nb alloy has become an urgent problem to be solved.The laser welding has the advantages such as high energy density,low heat input and good flexibility.Therefore,it is of great theoretical significance and practical value to study the microstructures and mechanical properties of laser welded joint of Ti-24Al-15Nb alloy.In this paper,the Ti-24Al-15Nb alloy was welded by pulsed laser welding technology.The microstructures and mechanical properties of the laser welded joints,and the effects of Nb filler metal,welding parameters and post-weld heat treatment?PWHT?on the microstructures and mechanical properties of the joints were studied.The results showed that the weld zone?WZ?of the as-welded joint had the characteristics of epitaxial solidification,and the columnar crystals grew perpendicular to the fusion line.Due to the increase of constitutional supercooling,the equiaxed crystals were formed in the middle of the WZ.The Al element has higher vapor pressure and lower boiling point.As the laser power increased,the Al element gasified,and pores were easily generated in the WZ.X-ray diffraction and transmission electron microscopy techniques were used to study the phase composition of the WZ.The results showed that the WZ was composed of a single B2 phase when the laser power was lower,and the WZ was mainly composed of B2 phase and acicular?,₂ martensite when the laser power was higher.According to the difference in the types of phases generated in the heat affected zone?HAZ?,the HAZ can be divided into two zones:HAZ1??₂+B2?and HAZ2??₂+O+B2?.The microhardness of the WZ and the HAZ was higher than that of base metal,and the WZ exhibited the highest microhardness in the laser welded joint.In addition,under the action of tensile stress,the fracture of the joints occurred in the WZ,the mechanical properties were poor,and the tensile strength and elongation were 330MPa and 1.9%,respectively.The Nb filler metal had apparent effects on the microstructures and mechanical properties of the WZ.The results showed that the WZ was mainly composed of?Nb,Ti?solid solution,B2 phase and O phase.Compared with the welded joint without filler metal,the microhardness of the WZ was obviously reduced because there was no hard and brittle needle-like?,₂ martensite in the WZ.The tensile strength and elongation of the welded joints increased significantly,reaching 724 MPa and 5.1%,respectively.Compared with those of joints without filler metal,the joints increased by 119.4%and168.4%,respectively.This is mainly attributed to the strengthening effect of precipitation of O phase and the compatibility of deformation between O phase and B2phase.At the same time,the Nb element also restricted the formation of hard and brittle needle-like?,₂ martensite and pores in the WZ,which help to improve the mechanical properties of welded joints.The effects of welding parameters on mechanical properties of welded joints were studied by orthogonal test.The results showed that the order of the influence of laser welding parameters on the tensile strength and elongation of welded joints was pulse frequency,pulse current,welding speed and pulse width in turn.The optimized welding parameters within the range of test parameters were:pulse current of 68A,welding speed of 200mm/min,pulse frequency of 6HZ and pulse width of 8ms.Under the optimized welding parameters,the tensile strength and elongation of welded joints reached 520 MPa and 4.57%,respectively,and the mechanical properties of the welded joints were obviously improved.The PWHT exerted obvious effects on the microstructures and mechanical properties of the welded joints.The WZ was composed of O+B2 phase.As the PWHT temperature increased from 850°C to 1000°C,the number of O precipitates gradually decreased and the size of O phase gradually increased.With the increase of holding time,the number and size of O precipitates in the WZ increased gradually when the PWHT temperature was 950°C.After the PWHT,the fine needle-like O phase was precipitated from the B2 phase matrix in the HAZ,and the?₂ phase gradually decomposed.Due to hardening effect of the O phase precipitation,the microhardness of welded joints has been enhanced.The PWHT also obviously improves the tensile properties of welded joints.When the PWHT temperature was 950°C and the holding time was 1 h,the tensile strength and elongation of the welded joints were 625 MPa and 5.8%,respectively.The results showed that the excellent mechanical properties of welded joints can be obtained only when the number and size of O phase and B2 phase have a suitable ratio.Based on the results of this study,the microstructures and mechanical properties of Ti-24Al-15Nb laser welded joints can be improved by Nb filler metal,optimized welding parameters and PWHT.Among them,the Nb filler metal is more beneficial to improve the mechanical properties of welded joints.