The manufacture of large civil aircraft is a significant issue in current science and technologyfield in China. In the manufacture of large aircraft, fuselage siding connection is a very importantpart of the manufacturing process. Laser welding technology, has become a new connectiontechnology in the aviation industry due to its high depth-width ratio, high concentration of energydensity and small welding thermal deformation.2024aluminum alloy, due to its excellentcomprehensive mechanical properties, is most widely used in the aviation industry. The mainapplication backgrounds of this paper are the fuselage weld panels used in large civil aircrafts, andits research subjects are high power CO2laser welding of2024aluminum sheet. Under thebackground of the manufacture of large aircraft, this paper studied the weldability of2024aluminum alloy systematically using high power CO2laser with3different metheds—autogenous,laser-TIG hybrid welding and laser-TIG welding with filler wire. The microstructure, hot crackingtendency and the mechanical properties of the joints were also analyzed in depth.First, the weld formability of2024aluminum alloy was studied. The influence of laser power,weld speed, and gas flow rate on weld formability was studied by autogenous laser welding. Sincealuminum alloy has a very high reflection rate to laser, both weld formability and stability of thewelding process were not good. In order to improve weld formability, TIG arc was added tocouple with the laser energy to enhance the absorption rate of the material to laser energy. It wasfound that the absorption of the aluminum alloy to laser energy became more even throughcomparing the weld formation and technology curve under the same heat input. No energyabsorption rate mutation and threshold phenomenon were found with more continuous weldformation and better weld quality, indicating that adding TIG arc did improve material’sabsorption rate to laser energy. To further improve weld formation and reduce hot tearing tendency,filler wire was added and nice weld joint was obtained by using optimized process parameters.Fishbone cracking test was used to evaluate the hot cracking tendency of2024aluminumalloy under different technological conditions. Results showed that high heat input and weldingspeed would increase the hot cracking tendency. The test also compared hot cracking tendencyamong autogenous, hybrid and hybrid with filler wire laser welding under the same heat inputconditions. It was found that hot cracking tendency of hybrid welding was significantly greaterthan autogenous laser welding and hot cracking could be well healed by adding4043filler wire. By observing the surface morphology of hot crack, hot crack formation mechanism under threetechnological conditions was discussed based on solidification cracking model. Autogenous laserwelding belongs to the B-type cracking model accompanied with several transgranular cracking;laser hybrid welding belongs to the B-type cracking model characterized by intergranular liquidmembrane separation; the laser hybrid welding with filler wire belongs to the A-type crackingmodel that has healing effect.The microstructure of weld joint under three technological conditions was observed. Theweld microstructure is primarily composed of massive matrix of α-Al and a large number ofprecipitates distributed on it including the S-phase (CuMgAl2) and the-phase (CuAl2). There isa certain amount of Mg2Si on the joint of laser hybrid welding with filler wire.Under the scanningelectron microscope it can be observed that most of the precipitates in autogenous laser weld jointare of granular dispersion, while the precipitates in hybrid weld joint are of network-likecontinuous distribution in grain boundaries. The EDS scanning results showed that in the hybridweld joint with filler wire, alloy elements segregate in the grain boundaries, thus reducingintragranular alloy elements significantlyMicrohardness test results showed that hardness of the joint is significantly lower than thebase material due to the impact of high temperature melting in the laser welding process and thereis a"the softening zone "in heat affected zone. Comparing the hardness value of laser, laser hybrid,laser hybrid welding with filler wire under the same heat input, it is found that laser hybridwelding joint obtain the highest average hardness value: about110HV; autogenous laser weldingfollowed with98HV; laser hybrid weld joint holds the lowest average hardness values: only90HV.Tensile mechanical tests were conducted for the the autogenous, hybrid and hybrid with filler wireweld joint. Results showed that laser hybrid weld joint with filler wire has the highest value of316.32Mpa; autogenous laser weld joint has the lowest value of235.15Mpa. Both are muchsmaller than the strength of the base material. The fracture morphology observation by scanningelectron microscope showed that autogenous laser weld joints fracture is a ductile-brittle mixedfracture. As for laser hybrid and laser hybrid weld with filler wire, Besides the ductile-brittlemixed fracture characteristics, they also have intergranular fracture morphology. |