2xxx and 7xxx series high-strength aluminum alloys have many advantages,such as high strength-to-weight ratios and excellent fatigue properties.With the rapid development of the aerospace industry,traditional processing methods are increasingly difficult to manufacture complex-shaped parts with superior mechanical properties.Laser powder bed fusion(L-PBF)additive manufacturing can realize one-time forming of complex structural parts,and has great application potential in aerospace components manufacturing.However,the fabrication of high-strength Al alloys by L-PBF technology still faces challenges.Some inherent physical properties,such as poor fluidity,high laser reflectivity,high thermal conductivity and high solidification shrinkage,would lead to the formation of hot cracks during L-PBF manufacturing of these aluminum alloys,which seriously hinders the application of L-PBF technology in the additive manufacturing of high-strength aluminum alloys.In this dissertation,hardness test,tensile test and advanced microstructural characterization techniques such as X-ray diffraction,X-ray diffraction topography,scanning electron microscopy,electron backscatter diffraction,transmission electron microscopy and high-precision X-ray energy dispersive spectroscopy were used in studying high-strength aluminum alloys fabricated by L-PBF manufacturing.The microstructure,mechanical properties and aging behavior of TiB2 modified Al-Cu-MgAg alloy,TiB2 and Si co-modified Al-Cu-Mg-Ag alloy and TiH2 modified Al-Zn-MgCu alloy were studied.The present study may provide some fundamental insights for L-PBF additive manufacturing of high strength aluminum alloy.High quality Al-Cu-Mg-Ag alloy samples were successfully prepared by L-PBF with the modification of TiB2 particles.The microstructure in as-printed samples is isotropic with an average grain size of~1 μm.A monolayer of Al3Ti two-dimensional compound s observed at the TiB2/Al interface,which makes TiB2 particles as effective heterogeneous sites to nucleatef Al grains,achieving grain refinement.During the LPBF process,a large number of uniformly distributed Ag-containing clusters and a small amount of G.P.zones were precipitated,which would interact with dislocations during tensile deformation,leading to appearance of Lüders band and C-type PLC effect during tensile deformation.The precipitation of θ’,Ω’ and Ω phases upon peakaging increases yield strength(YS)and ultimate tensile strength(UTS)to 405 MPa and 470 MPa,respectively.The peak-age samples still show an elongation(EL)about 11.6%.An excellent balance between strength and ductility is obtained for TiB2modified Al-Cu-Mg-Ag alloy fabricated by L-PBF and post heat treatment.The method of co-doping 2.7 wt.%TiB2 and 1.2 wt.%Si also successfully allows L-PBF to fabricate crack-free Al-Cu-Mg-Ag samples.The YS,UTS and EL of the asprinted samples are about 274 MPa,347 MPa and 5.4%,respectively.The average grain size was refined to only~0.6 μm.During the solidification process,Si mainly segregated at grain boundaries and surfaces of TiB2 particles.Si does not weaken the grain refinement ability of TiB2.In addition,the addition of Si changed the precipitation sequence of the Al-Cu-Mg-Ag alloy:inhibiting the precipitation of Ωprecipitates,while promoting the precipitation of Q’ precipitates.Electron microscopy and high-resolution EDS investigations showed that Mg-and Si-rich tiny cores were first formed due to coclustering of Mg and Si,followed by Ag and Cu segregation to interfaces between those tiny cores and Al matrix,resulting in formation of G.P.(Q)zones(precursor of the Q’phase).The formation of Mg-Ag clusters on {111}Al plane was inhibited,preventingthe nucleation of Ω phase.Mg and Ag segregation occurred locally at θ’/{001}Al interfaces,but did not cause structural and chemical changes in the θ’ precipitates.The microstructure of L-PBF fabricated Al-Zn-Mg-Cu(7075)alloy is composed of coarse columnar grains,showing a strong hot cracking tendency.The addition of TiH2 particles significantly improves the L-PBF printability of 7075 Al alloy.The TiH2 modified 7075 alloy(7075-Ti)is composed of fine,randomly oriented and equiaxed grains.The as-printed TiH2 modified 7075 sample has no cracks,but tiny pores are generated due to gas retention,and the porosity is 1.51 vol.%.The formation fine equiaxed grains is mainly due to the fact that a large number of in situ L12-Al3Ti particles can act as heterogeneous nucleation sites of α-Al during solidification.In addition,the solute Ti atoms in the molten pool increase the constituent supercooling,which also plays a significant role in grain growth restriction during solidification.In the subsequent heat treatment process,the supersaturated Ti solute forms E-Al18Mg3(Ti,Cr)2 dispersoids at grain boundaries,which may effectively pin the α-Al grain boundary migration during heat treatment.The YS,UTS and EL of the peak-aged TiH2 modified 7075 sample are about 496 MPa,537 MPa and 7.2%,respectively. |