Effect Of Micro Sc On Microstructure And Weldability Of Spray Formed 7055 Aluminum Alloy

Spray formed 7055 is an ideal material to replace steel because of its high comprehensive mechanical properties.However,the strong hot cracking during fusion welding and the softening tendency of the heat affected zone limit the application of this aluminum alloy in large structural parts.In this thesis,spray formed 7055Sc alloy containing 0.2 wt.%scandium（Sc）was prepared by spray forming method.By means of optical microscopy（OM）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,differential scanning calorimetry（DSC）,X-ray diffraction（XRD）,room temperature tensile test and microhardness test.The effects of Sc on the microstructure evolution and mechanical properties of spray formed 7055 aluminum alloy were systematically studied.The precipitation and growth of secondary Al₃（Sc,Zr）phase during different annealing processes were studied.The effect of Sc on the hot tearing susceptibility of fusion welded joints was studied.The softening reasons of weld zone and heat affected zone of spray formed 7055 aluminum alloy fusion welded joint were studied.The mechanism of Sc microalloying reducing the softening tendency of heat affected zone of spray formed 7055 alloy was studied.The results are as follows:（1）Sc mainly exists in the form of solid solution in the matrix,primary Al₃（Sc,Zr）phase,secondary Al₃（Sc,Zr）phase and W（Al Cu Sc）phase in the spray formed Al-8.0Zn-2.0Mg-2.4Cu-0.1Zr-0.2Sc alloy.Sc atoms have higher binding energy with vacancies.Therefore,when 7055Sc alloy is aged at 120℃/24h,Sc inhibits the precipitation of Mg Zn₂phase,so that the yield strength is 603.4 MPa,which is 46.9 MPa lower than that of 7055 alloy under the same aging process.The vacancies preferentially bound to Sc are released during high temperature aging,and the Mg Zn₂phase precipitates more fully.Therefore,after 160℃/24h,the yield strength of 7055Sc alloy is 576.9 MPa,which is 31.6MPa higher than that of 7055 alloy under the same aging process.（2）The size inhomogeneity of Al₃（Sc,Zr）particles increases with the increase of non-isothermal annealing temperature from 400℃ to 470℃.The precipitation and growth/coarsening of Al₃（Sc,Zr）particles occur simultaneously with the extension of homogenization annealing time at the same temperature.At the later stage of long time homogenization,the coarsening of Al₃（Sc,Zr）particles is obvious.Co MPared with the rapid heating and single-stage homogenization annealing process,the two-stage homogenization annealing process with slow heating rate can precipitate more secondary Al₃（Sc,Zr）phases with smaller size and more uniform distribution.When the heating rate is25℃/h and the two-stage homogenization annealing process is 400℃/5h+470℃/24h,the average diameter of secondary Al₃（Sc,Zr）particles in the matrix is only 14.39 nm,and the number density is as high as 68.1×10⁸/cm².（3）Compared with the commercial ER5183 and ER5356 welding wires,the use of7055 and 7055Sc welding wires which are homogeneous with the base metal can significantly reduce the hot tearing tendency of the weld.Due to the grain refinement of Sc,the hot tearing sensitivity coefficient of the weld filled with 7055Sc alloy wire is only 2.3%.When 7055 Sc welding wire is used for filling and the welding current is 160 A,the comprehensive tensile properties of the welded joint are the best.The tensile strength of the joint is 364 MPa,reaching 59.4%of the base metal,the elongation is 5.5%,and the tensile specimen is broken in the heat affected zone.（4）When the welding wire is homogeneous 7055 or 7055Sc,the weld zone will have obvious softening phenomenon.The decrease of dislocation density and the decrease of precipitated phase are the main reasons for the softening of the weld.The most serious softening area of the heat affected zone appears in the overaging zone with a thermal cycle temperature of about 250℃.At this temperature,the coherentη’phase dissolves,and the incoherentηphase grows and coarsens.After one thermal cycle,the minimum hardness value of the over-aging zone is 105 HV,the minimum tensile strength is about 410 MPa,which is 200 MPa lower than that of the base metal,and the elongation is slightly higher than that of the base metal.After the second thermal cycle,the width of the heat affected zone increases,and the most severe softening area expands to the fusion line.Theηphase is further coarsened under the secondary thermal cycle in the over-aging zone,resulting in a minimum Vickers hardness of 103 HV and a tensile strength of only 331 MPa in the over-aging zone after the secondary thermal cycle.（5）The addition of Sc in the welding wire can significantly improve the mechanical properties of the weld zone.After 7055Sc is used to weld 7055Sc-T76 base metal,the tensile strength of the weld is as high as 387 MPa,and the elongation is 16.8%,which is 86MPa and 0.8%higher than that of the weld filled with 7055 welding wire.The reasons why Sc improves the mechanical properties of the weld zone are the grain refinement of primary Al₃（Sc,Zr）and the precipitation strengthening of secondary Al₃（Sc,Zr）phase.At the same time,Sc changed the solute distribution coefficient and improved the solid solution strengthening effect of the main alloying elements.Sc can significantly improve the mechanical properties of over-aged zone.After thermal cycling at 250℃+250℃,the tensile strength of 7055Sc alloy is 364 MPa and the elongation is 16.2%,which is 33 MPa and 4%higher than that of 7055 alloy in the same state.The strengthening mechanism of Sc on the over-aged heat affected zone mainly comes from the grain refinement of the primary Al₃（Sc,Zr）phase in the base metal and the precipitation strengthening of the secondary Al₃（Sc,Zr）phase.