【作者】 蔡华；

【导师】 肖荣诗；

【作者基本信息】 北京工业大学 ， 光学工程， 2013， 博士

【摘要】 2524铝合金是目前断裂韧性和抗损伤容限最高的新型航空高强高韧Al-Cu-Mg合金，被认为是新型大型飞机的首选蒙皮材料之一。由于节能环保的迫切需要，现代飞机制造趋向轻量化，而焊接整体结构取代传统铆接连接结构是实现机身减重的重要手段。激光焊接具有能量密度高、焊接速度快、热输入小、焊接变形小等优点，而高亮度光纤激光器是未来激光焊接的主流器件，为实现高速、高效、高质量的航空高强铝合金焊接整体连接件提供了新机遇。但迄今为止鲜有关于2524铝合金激光焊接研究的相关报道，因此深入开展2524高强铝合金激光焊接接头的显微组织及力学性能研究十分必要。本文采用光纤激光焊接薄板2524高强铝合金，详细研究了焊接参数和表面状态对合金焊接裂纹敏感性的影响，考察了焊接参数、表面状态以及焊丝成分对接头力学性能和组织的影响，对2524高强铝合金激光焊接接头的焊缝、熔合区以及热影响区的组织演变也进行了深入研究。本文考察2524铝合金光纤激光自熔焊的焊接性。结果表明：薄板2524铝合金光纤激光焊接气孔倾向很小，但裂纹倾向大。焊接裂纹均呈高温开裂特征，裂纹包括纵向裂纹、横向裂纹和少量液化裂纹。表面状态显著影响焊缝组织及合金焊接裂纹倾向。合金表面包铝层的存在稀释焊缝合金元素，减少共晶数量，减弱裂纹“愈合”，结晶裂纹倾向大；包铝层表面氧化膜在焊接时卷入熔池，细化焊缝组织，促进晶界共晶分布不连续，降低结晶裂纹倾向。本文研究了含稀土Er和变质元素Zr的Al-Mg焊丝对焊缝组织的影响，获得了无缺陷的焊接接头。研究表明：焊缝主要由先生α-Al和晶界α(Al)+S(Al2CuMg)/θ(CuAl2)共晶组成，在α-Al晶内存在少量S/θ相和准晶相。添加5087/5E06焊丝引入Zr/Er元素，Al3Zr/Al3Er等成为异质形核核心，细化了晶粒尺寸，打乱了枝晶生长方向；焊接熔池Mg含量增多，提高了晶界低熔点共晶数量，减小2524铝合金激光焊接热裂纹倾向。本文研究了表面状态对熔合区的组织特征、接头成形及力学性能的影响。研究表明：熔合区内溶质元素析出形成粗大棒状S/θ相，而晶内原有析出相及晶界出现液化现象，形成贫溶质区，成为接头最薄弱环节。表面状态影响接头成形和贫溶质区宽度，进而影响接头拉伸断裂模式及力学性能。合金表面包铝层的存在改变熔合区与焊缝的边界线的曲率，拉伸时虽以熔合区起裂但沿焊缝柱状枝晶区扩展，最终沿熔合区扩展断裂；去包铝层状态接头始终沿熔合区发生断裂,呈塑性断裂特征。包铝层表面氧化膜的存在，增加焊接熔池表面张力，增大焊缝成形系数，增大焊缝晶界离异共晶比例，接头强度及延伸率减小，拉伸裂纹在焊缝柱状枝晶区扩展断口呈准解理断裂；去除包铝层减小试样厚度，增大单位厚度热输入，焊缝成形系数更小，最大贫溶质区宽度更小，接头强度及延伸率增大，且断裂始终发生于熔合区，呈塑性断裂特征。本文研究了填充焊丝成分对熔合区的组织特征、接头成形及力学性能的影响。研究表明：填充5E06焊丝的接头强度及延伸率高于填充5087焊丝接头。5E06焊丝微合金元素比例更高，利于提供更多的异质形核核心，进一步细化焊缝晶粒尺寸；Mg元素含量更高，减小焊接熔池表面张力，降低焊缝成形系数，同时增大对激光能量的吸收，增大焊缝熔宽，减小最大贫溶质区宽度，但提高焊缝晶界离异共晶数量。填充5E06焊丝去包铝层状态的接头强度系数达到85%，延伸率达5%。本文揭示了2524-T3激光焊接热影响区组织演化规律，并根据组织特点，将热影响区细分为液化区、回归强化区、过时效软化区及二次时效强化区。结果表明：在液化区，溶质元素析出形成大量亚微米级圆形S/θ相，而晶内原有沉淀相及晶界出现液化，形成贫溶质区；在回归强化区，原有GPB区溶解、溶质元素重新偏聚形成新的GPB区，显微硬度与母材基本相当；在过时效区，原有GPB区过度长大成为短棒状亚纳米尺寸S相，造成较小程度软化；在二次时效强化区，原有GPB区演变成细小的针状S’相，起析出强化，硬度高于母材。更多还原

【Abstract】 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.更多还原