| The T-type joint welding technology of Dual Laser-Beam Bilateral Synchronous Welding(DLBSW) is a new process for joining fuselage skin-stringer in airplane industry. Compared withconventional riveting process, the manufacturing of the fuselage can be greatly simplified usingDLBSW technology. Meanwhile, the DLBSW process is successful to avoid breaking the integrity ofthe bottom skin which has proved to be one of the most effective methods for joining airplanestructures of aluminum alloys. However, due to the characteristics of aluminum alloys and theparticularity of DLBSW technology for T-structure, research on the stability of the keyhole, thebehavior of the molten pool and the metallurgy mechanism specific to the new technological is still inthe preliminary stage in civil aircraft industry. Aiming at the DLBSW technology for T-structure ofAluminium alloys to be adopted in large passenger aircraft, this thesis will do the research on thesethree aspects mentioned above.First of all, force conditions of the particular keyhole developed during the DLBSW process forT-Structure were studied. The magnitude and orientation of the steam pressure, surface tension andgravity and so on were investigated, mechanics models for different sections of the keyhole wereestablished. On the basis of welding process parameters, pressure gradient acting on the keyhole wallwas calculated and the travelling speeds of the keyhole wall relative to the workpiece were figured upby means of Navier-Stokes Equations. The travelling speeds were then analyzed comparatively withthe moving speed of the laser-beam, and the stability of the keyhole was discussed.Secondly, a3D model which is suitable for the DLBSW process for T-structure of Aluminiumalloys on the problems of solid/liquid phase transformation for welding pool was established bycombining fluid flow dynamical equations and porous medial flow equations. The latent heat of phasechange was considered as common source term, and the welding heat sources were dealed asadditional source terms. The fluid flow, temperature field and liquid fraction field were calculated, theeffects of buoyancy and surface tension on the behavior of the molten pool were studied. The resultproved that the calculated weld shape was agreed well with the transverse macrosection of aDLBSW joint, Marangoni effect plays a leading role in driving the flow of the molten pool and theheat transfer in the molten pool. Due to the negative surface tension gradient of aluminum alloy, thehigher the temperature is, the faster the molten metal flows within the pool.At last, on the basis of the technology and relation theory of DLBSW technology for T-structure of Aluminium alloys, macroscopical shape, microstructures, composition distribution, microelementcontent of the welded joint were analyzed by metallographic and Scanning Electron Microscope(SEM). The results indicated that a satisfied weld appearance was obtained under the optimizedprocess parameters, the surface of observed joint is smooth free from appearance defects. Thesolidification structure of the joint was characterized and it was found that the crystallizingmorphology from the fusion line to the weld center was: coarse columnar crystals, cellular crystal,crystiform dentrite, dentrite and equiaxed grains. The fusion area was divided into top-fusion area andbottom-fusion area and their structure and properties were different. There were no obvious meltinglosses of alloying element Cu, Mg, Si in the weld observed by line scanning, but there was a seriousgravitational segregation of Si. Meanwhile, two different porosities existed in typical T-joint:metallurgical porosity and process porosity, and their induced mechanisms and avoided measureswere discussed.To sum up, researches on the stability of the keyhole, the behavior of the molten pool and themetallurgy mechanism during DLBSW process could establish a theoretical foundation for thestability and continuity of welding process for T-structure of airplane panel. |
PDF Full Text Request