| Due to the characteristics of structural functionalization and lightweight,such as high heat resistance of titanium alloy and good formability of aluminum alloy,the welding component of Ti/Al dissimilar materials shows a wide perspective application and urgent need in aerospace field.New generation of large thrust-to-weight ratio aero engines and large thrust liquid launch rocket engines need to break through the problem of reliable welding of medium thickness Ti/Al structure.In recent years,researchers at home and abroad have carried out a great number of studies on the interfacial theories and structural property control methods for the laser welding-brazing of Ti/Al dissimilar materials.These studies are mainly focused on the connection of 1?2mm thick plate by using laser thermal conduction welding method.To solve the problem of IMC control at the interface of brazing joint,the TC4 and 6061 Al alloys with the thickness of 5 mm are taken as the research objects in this paper.Based on the temperature field control capabilities of oscillation laser welding and vacuum laser welding as well as the facilitation outcomes of surface microstructure on wetting and spreading,a new laser deep penetration welding-brazing method with the control of microstructure and properties was proposed.The core ideas are as follows:the peak temperature of the Ti/Al interface and the temperature difference in the thickness direction were effectively reduced by oscillating laser and vacuum laser welding,so as to realize the thickness and uniformity control of IMC.The textured TC4 surface prepared by nanosecond laser is applied to improve the wetting characteristics and form the pinning strengthened structure.The strength of the TC4/6061 joint connected via laser welding-brazing was simultaneously determined by the fusion zone and brazing joint.The forming quality,microstructure and mechanical properties of the joints welded by the different processes including single laser,oscillation laser and vacuum laser welding were systematically studied.The results show that,compared with traditional laser welding-brazing method,both oscillation laser welding and vacuum laser welding were able to improve the joint formation,significantly reduce blowhole defects in the fusion zone,promote the grain refinement of the fusion zone,and improve the joint strength.The crystallization behavior of fusion zone was analyzed via EBSD.After the energy fields were adjusted using oscillating and vacuum laser beams,the average grain sizes decreased from the 27.7?m in laser welding-brazing to 7.2?m and 14.7?m,respectively.Futhermore,the tensile strengths of joints increased to173 MPa and 181 MPa,respectively.After the adjustment of energy fields,quasi-cleavage fractures formed at the interface between the IMC layer and Al alloy near the brazing joint.Based on the optimization of the TC4/6061 fusion zone,the composition,morphology,distribution characterization and control of the IMC layer at the brazing interface were studied.The IMC layer was analyzed via SEM,EDS and TEM.The results show that the interfacial IMC layer was composed of nanoscale-layered Ti Al diffusion layer and continuous serrated Ti Al3 reaction layer growing on the diffusion layer.The average thicknesses of IMC layer under the control of oscillation laser temperature field was 1.1?m,and maximum thickness difference between different positions was 0.7?m.As for that of vacuum laser temperature field,the average thicknesses and maximum thickness difference of IMC layer were1.0?m and 0.2?m.Both methods were proven capable of controlling the size and distribution of the IMC layer at the interface of moderately-thick plates.Then,the fracture behavior in the TC4/6061 micro-area was analyzed via in-situ TEM stretching.The results show that,during the loading of the in-situ TEM stretching on the IMC layer,cracks tend to initiate at either the Al or Ti side near the IMC layer,before propagating and forming a fracture.What's more,when the interfacial temperature field was properly controlled,the IMC layer would lose its deficiency as a weak position for the TC4/6061 joint.Subsequently,a multiphysical field heat-flux coupling model was established,and the corresponding interface temperature field and thermal cycle at different locations of single laser,oscillatinglaser and vacuum laser welding was obtained.The maximum peak temperatures of the Ti/Al interface obtained by the three methods were 1520 K,1346 K and 1222 K,respectively,and the temperature differences along the thickness direction were 479K,311 K and 99K,respectively.Both methods could effectively reduce the peak temperature at the top of the Ti/Al interface and the temperature gradient along the thickness direction,thereby effectively controlling the IMC layer at the interface of the moderately-thick TC4/6061 plate.Combined with the size and distribution characteristics of IMC layer,the regulation mechanism of temperature field on IMC layer was revealed,namely the peak temperature determined the thickness of IMC layer,and the temperature gradient along the thickness determined the distribution uniformity of IMC layer.In laser welding-brazing process,the wetting and spreading capabilities of the brazing filler metal on the surface of the base metal is the key factor that decides the joining quality of the welded joint.In order to promote the wetting and spreading of 6061 Al alloy on TC4 surface,and further strengthen the TC4/6061 dissimilar metal joint connected using deep penetration welding-brazing,this paper adopted the nanosecond laser to form groove microstructures on the TC4 surface to explore the microstructural processing mechanism of nanosecond laser beams on TC4surfaces.The sizes,surface micro-nano morphology and phases of the grooves were calibrated using SEM,AFM,XRD and other methods.Following these,the wetting and spreading characteristics of textured surfaces were tested using a high temperature vacuum brazing furnace.The optimal process parameters were as follows:pulse energy(2.33m J),vibroscope scanning speed(450mm/s)and scan count(10).The groove size was 163?m wide and 107?m deep.No new phase was formed on the surface.The phases were still?-Ti and?-Ti,which were the typical phases of TC4.At this time,the wetting and spreading speed of 6061 aluminum alloy on TC4 surface was the fastest,and the minimum high temperature wetting angle was 1.8°.On the basis of defining the optimal microstructure size and process,the joint strengthening under the control of microstructured TC4/6061 multi-energy field was finally realized.The molten aluminum alloy was found to be well wetted and spread on the TC4 surface and was able to completely fill the grooves on the TC4 surface.The pinned structure was hence successfully strengthened,increasing the tensile strength of the joint to 231 MPa. |
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