Study On Double-sided Laser Beam Welding Characteristics And Weld Pool Behavior For Aluminium Alloy Fuselage Panel Structure

Lightweight processing and manufacturing of the large passenger aluminiumalloy aircraft panel structure is the hotspot in the aircraft manufacturing field，which is consist of skin and stringer. The Airbus had achieved the target of weightand costs reduction by using laser beam welding instead of the dominant rivetingtechnique. At present, our country is developing our own large aircraft, after fulltechnical appraisal and economic evaluation; the plan is to apply laser beamwelding technique to manufacture the aircraft panels. However, this innovativetechnology was only mastered by Airbus and still a blank in China. It isindispensable to carry out the related research work to breakthrough and masterthis technique. It will be a substantial theory basis for realizing the lightweightmanufacturing of China-made large passenger aircraft; also can shorten the gapwith Airbus in the fuselage panel manufacturing field.According to this specific requirement, in order to obtain the T-joints whichmeet the design requirements, we studied the double-sided laser beam weldingprocess, and researched the effects of the welding parameters on the weldappearance, porosity defects and mechanical properties in the present paper. Thefiller wire melting dynamics was also analysed. A3D mathematical model ofthermal-fluid coupling of double-sied laser beam welding was developed, with theaid of computational simulating to exactly understand what happens during thewelding process. Finally, we got the theoretical foundation for controllingeffectively the weld appearance and porosity defects.The experimental results showed that weld symmetrical characteristic, weldpenetration and weld seam were three main elements for evaluating the quality ofweld seam. The laser beam attitude was the key influence parameters. The beamdistance determined the symmetrical characteristic of the weld appearance and theuniformity of the microstructure. Reducing beam incident angles could increasethe fusion area between the skin and stringer; also reduce the thermal deformationof the skin panels. The beam incident position on the stringer was conducive toobtain smooth seam angle. Weld X-ray detection results show that increasewelding speed and control beam distance were two most important ways to reduceporosity defects.The characteristic of the double-sided laser beam welding of the T-jointsmade the two laser beam interaction with each other, resulting in the weld poolnear the skin retention high temperature for a long time and cooling speed was high. Therefore, the heat affected zone on the skin side was wider than that on thestringer side, resulting in it became the weakest area of the whole T-joint. Thetranserve, axial, longitudinal tensile tests and transerve pull-push fatigue test werecarried out according its actual stress conditions. The test results show that thetensile properties were decided by the weld penetratin, the tensile strength canreach a high level when the weld penetration was about half of the skin thickness.Its ranserve, axial, longitudinal tensile strengthes can reach87.8%、53.1%and90.8%of the base metal, which meets or surpasses the Airbus’s dates. Theconditioned fatigue strength was about80.7MPa, was30%of its tensile strength.Concerning the issue of welding process stablility under the special condition,we applied high speed photography to analyse the main influence of the filler wiremelting dynamics. The experimental results show that the laser-wire relatedposition was the main affecting factors of the filler wire melting dynamics. Whenlaser-wire partially overlapping, the liquid filler wire could through the liquidbridge between the wire tip and front keyhole smoothly and steadily flow into theweld melton pool, the welding process was stable and the weld appearance waswell. This transfer mode needs two necessary conditions: one is the filler wirecontact with the workpiece, the other one is the contact place located in front ofthe keyhole.A3D mathematical model of thermal-fluid coupling of double-sied laserbeam welding was developed. The numerical simulation results showed that thelaser energy coupling effect was remarkable; resulting in the longest size inside theweld pool located the coupling area and the convective flow was appeared abovethe keyhole. An obvious vortex was appeared under the keyhole. The liquid metalfirstly flowed to the bottom of the pool and then flowed to the surface.The disappearing of the keyhole contact condition resulted in the convectiveflow was completely disappeared and transferred to flow to the bottom of the weldmolten pool. The reducing of the keyhole synchronism resulted in the weld moltenpool was not symmetry, the trailing pool was flow into the leading pool, and twonew vortex flows were appeared between the two keyholes. Improve the weldingspeed resulted in the laser energy coupling effect was reduced, the convective flowwas completely disappeared and transferred to flow to the inside of the keyhole,and liquid metal was totally flow to pool surface. This was beneficial for pores toescape from the weld molten pool. Keep the keyhole linking, Symmetry andimprove welding speed are also good for pores to escape from the weld moltenpool.At last, we applied the double-sided laser beam welding to manufacture the large scale fuselage panel structure, the skin size was2000mm×1000mm. Theweld appearance and mechanical properties met the design requirements, and thewelding process was stable. The welding speed can reach up to12m/min.