Study On The Microstructure And Properties Of TC4 Titanium Alloy Laser Self Fusion Welded Joint And Post Weld Ultrasonic Impact Strengthening

TC4 titanium alloy is a dual phase titanium alloy that combinesαType andβexcellent properties of type titanium alloys.It is often widely used as a heat and high-pressure resistant material in aerospace and other fields.Laser welding has many advantages over traditional welding technologies,such as high energy density,narrow weld bead,and ease of automation,and is suitable for welding sheet titanium alloy.However,TC4 titanium alloy requires strict inert gas protection during the welding process to prevent joint contamination by air.Ultrasonic impact,as a surface strengthening technique,is often used to strengthen welded joints,effectively reducing residual stress and improving mechanical properties of joints.In this thesis,laser self-fluxing flat plate butt welding experiments were conducted on 3mm thick TC4 titanium alloy.A front protection device suitable for laser self-fusion welding of titanium alloy flat plates was designed,and gas shielding process tests were conducted.The optimization of process parameters for laser self-fusion welding of 3mm thick TC4 titanium alloy was emphasized,and its microstructure and properties were analyzed.Subsequently,ultrasonic impact strengthening tests were conducted on the joint to study the effects of ultrasonic impact strengthening on the microstructure and properties of the joint and on the residual stress of the joint.The protective effect of the front gas protection device designed in this test is good,and the optimal protective process parameters of the protection device is:the advance ventilation time is 1.5 min,and the protective gas flow rate is 25 L/min.Comparing the protection device designed by this experiment with the two sets of connectors directly protected by the nozzle,it was found that the surface of the joint protected by the protective device is silver white,without obvious defects.The tensile strength and elongation after fracture of the joint are higher than those of the joint directly protected by the nozzle;The joint directly protected by the nozzle is polluted by air,resulting in defects such as collapse,porosity,shrinkage in the middle of the weld,and coarse microstructure in the center of the weld.The oxides and inclusions present in the center of the weld lead to an increase in hardness in various areas of the joint,a serious decrease in elongation after fracture,and a decrease in mechanical properties.Select the tensile strength and elongation after fracture of the joint as indicators to conduct a range analysis on the results of the orthogonal test,and the results show that:The change of laser power has no significant impact on the two experimental analysis indicators.The optimal process parameters is:P=2.5k W,v=20mm/s,Δf=0mm.The center of the weld is thickβcylindrical crystals and acicular martensite are distributed in a basket like network.The heat effect is divided into coarse grained regions near the center of the weld and fine-grained regions near the base metal.The tensile strength and elongation after fracture of the joint under the optimal process parameters are 1010MPa and 9.82%,respectively.The average micro-hardness of the weld center is 360HV_0.5,higher than that of the heat affected zone and base metal.After ultrasonic impact with a coverage of 200%,the average roughness Ra increased from 0.811μm reduced to 0.26μm.Plastic flow occurs in the columnar crystals within the thickness range of 0～1.5mm in the weld center,and the columnar crystals are elongated.When the impact coverage is 300%,the columnar crystal size decreases by 33.60%compared to the original state.The impact area surface produces a thickness of 10～30μm unequal plastic deformation.Inside the surface columnar crystalαphase appears broken and bent.The average micro-hardness of the central surface layer of the weld increases with the increase of impact coverage.The microhardness value increased by up to 5.40%.The maximum impact depth of ultrasonic impact on hardness reaches 1.2mm.The TC4 titanium alloy laser welded joint is in a tensile stress state.Ultrasonic impact introduces residual compressive stress,which increases with the increase of impact coverage.The maximum residual compressive stress introduced at a coverage of 300%is-433MPa.Double sided impact is more effective in reducing residual stress than single sided impact.