| Al-Li alloy, owning to its low density, high specific strength, low-temperature performance andgood corrosion resistance, has been widely used to replace conventional aluminum alloy in the fueltank of aircraft and low-temperature tank of spaceshuttle, which is considered as a preferentialaerospace structural material. In addition, Al-Li alloy has also been applied in some fields such asautomobile, shipbuilding and sports product, etc. Due to Al-Li alloys are usually used as weldstructure in practical application, weld defects such as heat-affected zone (HAZ) softening, porosityand solidification crack will appear when the conventional welding methods such as MIG, TIGwelding are used to weld Al-Li alloys, so the application of Al-Li alloy is limited to a great extent.Because vacuum electron beam welding (EBW) has the features of high energy density, low weldingheat input and good welding zone protection, excellent welded joints with the characteristics of fineweld grain size, small HAZ and high tensile strength can be obtained by the use of EBW. Futhermore,through the numerical simulation analyses of Al-Li alloy electron beam welding, the distribution ofwelding temperature field and stress field can be obtained, and the results of numerical simulation canprovide guidance for welding parameters optimization. Consequently, the electron beam weldingprocedures of1420and2090Al-Li alloys widely used in aerospace field are investigated based on theoptimized welding technology with finite element analyses. The effect of electron beam current,welding speed and scanning patterns on the microstructure and mechanical properties of welded jointsare studied. Meanwhile, the microstructure and mechanical properties of welded joints are analysedunder the condition of both as-welded and post-weld heat treatment (PWHT), and the precipitation ofstrengthening phases in joints after heat treatment is discussed.Results of numerical simulation analyses show that the Al-Li alloy electron beam welding has fastheating rate and cooling rate, and the temperature gradient is great near weldment. The simulationshape of weld fusion line by using combined heat source is well consistent with the actual weld, and itpresents a nail-shaped feature. There are a certain compressive stress in front of the molten pool andthe high value longitudinal tension stress near weldment during welding process. By optimizing thewelding parameters such as electron beam current and welding speed, etc, Al-Li alloy EBW jointpresents a critical penetration state, and the peak longitudinal tension stress is greatly reduced nearweldment while using the welding parameters I=8mA, V=1000mm·min-1.The microstructure observation of welded joints shows that the Al-Li alloy EBW joints mainly consist of eutectic phases, and small amount of strengthing phases distribute inside the grain. Inaddition, equiaxed grains zone (EQZ) is formed near fusion line in both of the two joints, which isrelated to Li and Zr element in Al-Li alloys. During welding, the adding of circular scanning caneffectively stir the molten pool to reduce the segregation of alloying elements, decrease the size ofeutectic phase along the grain boundary and increase the amount of eutectic phase inside the grain, sothe strength of grain boundary can be improved. Moreover, circular scanning can also increase theamount of heterogeneous nucleation paricles such as Al3Zr by stirring the molten pool, as a result, thenucleation rate can be increased and the fine microstructure of weldment can be obtianed. The stirringaction of linear scanning is not so good as circular scanning, the effect of fining weld is not obviousand the eutectic phase is a little coarser along grain boundary.Results of tensile test indicate that joints with the adding of circular scanning appear higher tensilestrength, and the linear scanning intermediate, while without scanning the worst. Because circularscanning can greatly reduce the buring loss of alloying element during welding, it is beneficial toretain strengthening effect, so the higher tensile strength of welded joint can be obtained.Microhardness test results show that microhardness of weld zone is lower than that of HAZ and basemetal, and softening phenomenon occurs in HAZ. The microhardness of welded joint with circularscanning is the highest among them, the reason is that circular scanning can melt down the growncrystal by the flyback movement of electron beam, the amount of heterogeneous nucleation surfaceincrease, thus the fine microsturcture of weldment can be obtained. The fracture scanning analysesdemonstrates that welded joint without scanning presents the characteristic of brittle intergranularfracture, while welded joint with circular scanning presents the transgranular fracture characteristicswith good ductility, and there are a large number of small dimples distributed on the fracture surface.By comparison of the microstructure and mechanical properties of1420and2090Al-Li alloywelded joints under the condition of as-welded and PWHT, both of the tensile strength of two Al-Lialloys welded joints increase obviously, and the elongation of joints decrease a little after PWHT. Lotsof fine strengthening phases precipitate in the weldment after PWHT. XRD test and TEM observationof joints show that the main strengthening phase in1420Al-Li alloy weldment is spherical δ’, and in2090Al-Li alloy weldment are spherical δ’ and needlelike T1. The precipitation of strengtheningphases is advantageous to the tensile strength of welded joints. |
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