Effect Of Ceramic Particles On Microstructure And Properties Of Spray Formed 7055 Aluminum Alloy TIG Welded Joints

From the perspective of reducing weight and improving protection,spray formed 7055aluminum alloy with ultra-high strength and lightweight has a good application prospect in aerospace,railway vehicles,weapons equipment and other fields.It has been successfully used to manufacture the wing truss of domestic large aircraft and the shell of Tiangong No.2compressor.However,the fusion welded joints prepared with traditional aluminum alloy welding wires have low strength and brittleness,which makes low strength and brittleness of fusion welded joint become the bottleneck restricting the application of spray formed 7055aluminum alloy.In this thesis,new aluminum alloy welding wires containing nano ceramic particles are developed,which are TiB₂-7055,ZrB₂-7055 and Sc+ZrB₂-7055 respectively.The above welding wires were used to TIG weld the spray formed 7055 aluminum alloy,and the TIG welded joints with high strength and certain plastic toughness were obtained.The effects of different kinds of ceramic particles on the microstructure and mechanical properties of TIG welded joints were discussed by XRD,SEM,TEM and tensile testing machine.The results are as follows.（1）TiB₂-7055,ZrB₂-7055 and Sc+ZrB₂-7055 welding wires were prepared by in-situ reaction method,the distribution of different ceramic particles on the welding wires matrix and their effects on the grain morphology of the matrix were studied.The results show that:（a）The grains of TiB₂-7055 welding wires are fine equiaxed,and TiB₂ particles are evenly distributed onα-Al matrix.（b）The grains of ZrB₂-7055 welding wires are fine dendrite,and ZrB₂ particles are large-area agglomeration onα-Al matrix.（c）The grains of Sc+ZrB₂-7055welding wires are fine equiaxed,and ZrB₂,Al₃（Sc,Zr）and Al₃Sc particles are evenly distributed in the form of small area agglomeration onα-Al matrix.（2）Spray formed 7055 aluminum alloys were TIG welded with TiB₂-7055 and ZrB₂-7055 welding wires respectively,the distribution of TiB₂ and ZrB₂ particles on the weld matrix and their effects on the microstructure and mechanical properties of the welded joints without heat treatment were compared.The results show that:（a）The grain size of 7055welded joint prepared with base metal as welding wires is coarse,and the tensile strength and elongation are 245MPa and 1.5%respectively.（b）TiB₂ particles with good wettability with liquid aluminum are evenly distributed on the weld matrix,and with the increase of TiB₂particles content,TiB₂-7055 weld grains gradually become fine equiaxed.The tensile strength and elongation of the welded joints first increased and then decreased.The tensile strength and elongation of 1.5 wt%TiB₂-7055 welded joint reached the maximum values,about 310MPa and 5.4%respectively.It shows ductile fracture,and the fracture of tensile specimen is in the heat affected zone.（c）ZrB₂ particles with poor wettability with liquid aluminum agglomerate in a large area on the weld matrix and are unevenly distributed.With the increase of ZrB₂ particles content,ZrB₂-7055 weld grains gradually becomes fine dendrite.The tensile strength and elongation of ZrB₂-7055 welded joints first increased and then decreased.The tensile strength and elongation of 1.5 wt%ZrB₂-7055 welded joint reached the maximum values,about 280 MPa and 2.0%respectively.The fracture of tensile specimen is in the weld.It is revealed that the effect of fine grain strengthening and precipitation strengthening produced by ceramic particles is the main reason for the improvement of mechanical properties of welded joints.（3）The spray formed 7055 aluminum alloys were TIG welded with Sc+ZrB₂-7055welding wires.The effects of Al₃（Sc,Zr）,Al₃Sc and ZrB₂ particles on the microstructure and mechanical properties of the welded joints without heat treatment were studied.The results show that Al₃（Sc,Zr）,Al₃Sc and ZrB₂ particles are mixed together and evenly distributed in the form of small agglomeration onα-Al matrix.Because Al₃Sc and Al₃（Sc,Zr）particles have good wettability with molten aluminum,they not only promote the adsorption of ZrB₂particles with higher interface energy and Al₃Sc and Al₃（Sc,Zr）particles with lower interface energy,but also increase heterogeneous nucleation ofα-Al grains.Therefore,the large-area agglomeration of ZrB₂ particles on the weld substrate is hindered.Compared with ZrB₂-7055weld,the grain of Sc+ZrB₂-7055 weld changes from dendrite to equiaxed.The tensile property of Sc+ZrB₂-7055 weld is better than that of ZrB₂-7055 weld.The tensile strength and elongation of Sc+ZrB₂-7055 welded joints are about 320 MPa and 5.0%,showing ductile fracture,which is in the heat affected zone.The problem of large-area agglomeration of ceramic particles with poor wettability with liquid metal on the weld matrix is solved.（4）7055,TiB₂-7055 and Sc+ZrB₂-7055 welded joints were heat treated by S1 process,the effects of ceramic particles on the aging precipitation dynamic of precipitates in weld matrix and the mechanical properties of welded joints were analyzed.The results show that the size ofη′phase on 7055,TiB₂-7055 and Sc+ZrB₂-7055 welds decreases and the dispersion increases in turn.The aging precipitation dynamic of precipitates in welds matrix containing Al₃（Sc,Zr）,Al₃Sc and ZrB₂ particles is greater than that of welds with only TiB₂ particles.The tensile properties of TiB₂-7055 and Sc+ZrB₂-7055 welded joints are better than those of7055.The tensile strength and elongation of 7055,TiB₂-7055 and Sc+ZrB₂-7055 welded joints are about 540 MPa and 2.5%,573 MPa and 3.2%,554 MPa and 3.5%respectively.The analysis shows that the precipitation strengthening produced by matrix precipitates,fine grain strengthening and dispersion strengthening comprehensively improve the mechanical properties of TiB₂-7055 and Sc+ZrB₂-7055 welded joints after heat treatment.（5）TiB₂-7055 and Sc+ZrB₂-7055 welded joints were heat treated by S2 and S3 processes,the effects of secondary aging time on the microstructure and mechanical properties of the welded joints were studied.The results show that with the increase of secondary aging time,the tensile strength of TiB₂-7055 and Sc+ZrB₂-7055 welded joints gradually decreases because of metastableη′phase transition to steady stateηphase,andηPhase coarsening.The strengthening effect is reduced because theηphase is not coherent with the matrix.