Microstructures And Mechanical Properties Of Fiber Laser Beam Welded Thin Sheet2524High Strength Aluminum Alloy

Al-Cu-Mg2524is a new type aluminum alloy with the highest damage toleranceand considered to be the primary material of choice for the skin of aircraft. Greatlyweight reducing and cost saving may occurred by replacing riveting into laser beamwelding (LBW) of aircraft fuselage panels, because of a high speed, a narrow anddeep weld, a small heat-affected zone and low distortion due to the extremely lowheat input, and possibilities for automation and robotization in LBW. However, untilnow there are few reports about laser beam welding of high strength aluminum2524-T3alloy. Therefore, it is significant to investigate laser beam welding of thealloy. In this paper, thin plate2524-T3high strength aluminum alloy was welded byhigh brightness fiber laser beam welding process, the influences including weldingparameters and surface status of the alloy on hot crack susceptibility were investigated,and the microstructures and mechanical properties in different welding parameters,surface status and filler wire composition were studied, the microstructural features ofwelded metal (WM), bond and heat affected zone (HAZ) of laser welded joint werealso discussed.Autogenously bead-on-plate fiber laser beam welding has been presented toevaluate the weldability of2524alloy. The results show that the fiber laser weldedthin plate2524-T3joint has a very low porosity but a high crack susceptibility. Thecracks occur at high temperature, including longitudinal cracks, transverse cracks andfew liquation cracks. The surface status affects the microsturctures and solidificationcracking susceptibilities siginificantly. The alclad on the surface of AA2524can dilutethe alloying elements in joints, reduce the quantity of eutectic and weaken the healingeffect, increase the solidification cracking susceptibility; And the oxide-film on thesurface of alclad can be involved in welding pool, act as heterogeneous nucleation siteplaying an important role in refining grain size, promote a discontinuous eutecticdistribution and reduce the solidification cracking susceptibility.The hot crack free welded joint is obtained by filling Al-Mg filler wire with rareearth elemete Er and inoculants elemete Zr. The results show that the weld metalcomposed of α (Al) primary phase, α(Al)+S(Al2CuMg)/θ(CuAl2) as dendrites or ingrain boundaries and few S/θ phase or quasicrystal containing Cu, Fe, Mn in thegrain.Adding ER5087/ER5E06brings Al3Zr/Al3Er particles as the heterogeneous nuclei, fining grain and disturbing the growth direction of columnar dendrites; andincreases the content of Mg element in welding pool, promoting the amount of lowmelting point eutectic on grain boundary and reducing the solidification crackingsusceptibility of weld joint.The influences of surface status on the microstructural features, weld beadgeometries and mechanical properties of welded joint have been investigated. Soluteelemetes precipitated and formed into coarse approximately rod-like micronscale S/θparticles, the original constituents and grain boundaries liquated, forming thesolute-depleted zone in the bond, becoming the weakest region in welded joint.Different surface status joints have different weld bead geometries, solute-depletedzone widths in bond and eutectic quantities in weld metal, leading to the differenttensile fracture models and mechanical properties. The alclad on the surface ofAA2524affects the curvity of the boundary between bond and weld metal, resulted atensile crack extending into the columnar dendrite zone in weld metal besidescracking in bond. And the oxide-film on the surface of alclad can increase surfacetension of weld pool, leading a high welding profiled coefficient and enlargeddivorced eutectics percentage, and the tensile fracture surfaces observation showedquasi-cleavage characteristics in the columnar dendrite zone of weld metal; While,removing alclad reduced the depth of sample, increasing the heat input per thicknessand decreasing the welding profiled coefficient, leading to a higher strength andelongation of welded joint, and the tensile cracks always in the bond, showing themicro-porous aggregation toughness fracture characteristics.The microstructures, weld bead geometries and mechanical properties of weldedjoints with two different filler wires has been compared. The results show that thetensile strength and elongation of the joint with5E06filler wire is higher than that ofthe joint filling with5087filler wire. The5E06filler wire affords more heterogeneousnuclei, generating finer weld grains; provide higher Mg element, increasing solute inwelding molten pool, helping to reduce the surface tension of weld pool and increasethe absorbility to laser beam, enlarging the width of weld metel, reducing themaximum width of solute-depleted zone in bond, but boosting divorced eutectics atthe interdendritic/grain boundary in weld metel. The tensile strength coefficient andthe elongation of the joint with5E06filler wire in alclad-removed status can reach85%and5%, respectively.The microstructural development and microhardness property of fiber laser welded Al–Cu–Mg2524-T3in the heat affected zone have been investigated.According to the microstructure features, the heat affected zone of welded joint hasbeen particularly identified into liquation zone, retrogression zone, overaging zoneand secondary aging zone. In the liquation zone, the solute elemetes precipitated intonumerous approximately spherical sub-micronscale S/θ particles, the originalconstituents and grain boundaries liquated, forming the solute-depleted zone; In theretrogression zone, original GPB zone dissolute, solute elemetes segregated andsubsequent a new GPB zone formed; The original GPB zone growed excessively intoshort rod-like nanoscale S phases in the overaging zone, inducing a narrow and smallsoften zone; While in the secondary aging zone, original GPB zone growed into tinyneedle-like S’-phase, strengthening the microhardness.