Research On Effect Of Laser Shock Processing On Fatigue Performance Of TC4 Titanium Alloy Single-side Laser Modification Welding Joints

TC4 titanium alloy was commonly used in the airline industry as an structural material due to its great comprehensive mechanical properties,and laser welding technology has significant advantages in terms of titanium alloy plate connection.The heat source of laser welding which used high energy density laser beam,not only can realize highspeed welding of titanium alloy,but also reduce the absorption of harmful gas during welding process,and low heat input can reduce the damage to materials and reduce the deformation after welding.On the other hand the welding undercut of the titanium alloy laser welding without filler wire was often formed after welding,and fatigue cracks was easily initiated in the stress concentration place of welding undercut under cyclic stress for a long time,which caused thepremature rupture of welded joint.So the study on the effect of laser shock processing on fatigue performance of TC4 titanium alloy single-side laser modification welding joints has important engineering significance.In this paper,the effect of different shock order on the fatigue properties of TC4 titanium alloy single-side laser modification welding joints was studied.Firstly,the relationship between the peak pressure of the shock wave and the power density of the pulsed laser beam was deduced in the process of laser shock processing,and the formation mechanism of the residual stress after laser shock processing and the pressure range of the best strengthening effect were analyzed theoretically,which made a guidance for the determination of laser parameters.The microstructures,microhardness,surface morphology,roughness and residual stress distribution of the welded joints before and after laser shock processing were observed.The experimental results show that a series of changes of TC4 titanium alloy single-side laser modification welding joints appeared after laser shock processing.The grain size of the needle-like martensite is shortened,and the microhardness is also improved with the grain refinement,which is helpful to suppress the initiation of the early fatigue crack at the undercut.Besides the high amplitude residual compressive stress is introduced on the back of the weld after two different strengthening processes,and the residual compressive stress formed by the strengthening process B at the back of the weld is 1.8 times of that by the strengthening process A which is more conducive to improve the threshold of fatigue crack initiation at the stress concentration of the undercut on the back of the weld,and suppress the expansion of the fatigue crack.On the other hand,the laser shock processing has no obvious effect on the contour of the undercut on the back of the weld and the surface roughness of the weld,that is to say the laser shock processing can not improve the stress concentration at the back of the weld.Finally,the median fatigue life of the three groups of samples under the same stress level was compared with the tensile-tensile fatigue tests of the unenhanced samples and two different enhanced process samples.The fatigue crack initiation,expansion and transient breaking were observed under optical microscope and scanning electron microscope.The experimental results show that the median fatigue life of TC4 titanium alloy single-side laser modification welding joints can be significantly improved by laser shock processing,and the fatigue life of strengthening process A sample and strengthening process B sample is improved by 1.68~4.17 times and 3.61~9.56 times compared with unenhanced sample respectively,indicating that double-sided reinforced titanium alloy laser welding joint which the first impact is on the back of the weld has better reinforcement.It is found that the fatigue crack initiation source of TC4 titanium alloy single-side laser modification welding joint is located at the undercut of the back of the weld,and the crack initiation is suppressed after laser shock processing,and the crack initiatied from point source.The fatigue step path in the extension process becomes more tortuous,while the secondary crack increases and consumes more strain energy.It can be found that the fatigue crack growth rate of the specimen is slowed down at all stages compared with the unenhanced specimen,and the inhibitory effect is most significant in the early stage of fatigue crack propagation.The greater the residual compressive stress value on the back of the welding joint,the more significant the inhibition of crack propagation.The three groups of specimens in the final fault zone showed ductile rupture characteristics.The size of the dimple was not uniform and was increased after laser shock processing,which indicating that the plasticity of the material after the double-sided strengthening is improved.