Effect Of Thermal Processing Treatment On The Structure And Properties Of Al-Zn-Mg-Li Alloy For Additive Manufacturing

Al-Li alloy is widely used in the aerospace industry because of its typical low density,high strength and specific stiffness.However,the current traditional manufacturing methods（casting,etc.）for Al-Li structural parts have limitations such as long production cycles and low material utilization,which increase their manufacturing costs.As a new technology with high deposition rate,arc additive manufacturing（WAAM）is widely used for the fabrication of metal alloys.However,Al-Li alloy additive manufacturing faces serious formability control challenges.Due to the melting of metal wires involved in the WAAM process,as well as the material properties of Al-Li alloy with high solidification shrinkage,high thermal conductivity and easy hydrogen absorption,pores,coarse grains and brittle phase agglomeration are inevitably generated,leading to the strength reduction of the components.To solve the above problems,experimental studies on deformation（hot rolling,multi-directional hot forging and multi-pass variable temperature rolling）and heat treatment of Al-Zn-Mg-Li alloy fabricated by WAAM were carried out in this paper to analyze the microstructure evolution of Al Fe Cu-rich phase,T（Al Zn Cu Mg）phase and Mg Zn-rich phase in Al-Zn-Mg-Li alloy.The mechanism of grain refinement caused by dynamic reversion and dynamic recrystallization behavior during deformation was analyzed by EBSD.The mechanism of the effect of deformation and heat treatment on the brittle tissue in the matrix and the effect of brittle tissue characteristics on the mechanical properties of the alloy were investigated by SEM.The effects of deformation and heat treatment on the evolution of Mg-Zn-rich phases were revealed,which aimed to improve the porosity,coarse grains and brittle phase agglomeration inside the material and to maximize the mechanical properties of the alloy.The main work and conclusions of this paper are as follows:1.The microstructure morphological characteristics of Al-Zn-Mg-Li alloy made by WAAM during hot rolling and heat treatment and its influence law on mechanical properties were systematically investigated:due to strong extrusion,there was obvious dynamic reversion and dynamic recrystallization in the sample,columnar grains were improved,and the average grain size was 16.7μm;the coarse Al₇Cu₂Fe phase in the matrix and Al Fe Cu phases will be severely broken and the brittle phase aggregation is improved;during the hot rolling treatment,the T-phase particles precipitate out from the saturated solid solution,which reduces the supersaturation of the solid solution;these brittle phases are easy to cause stress concentration during the deformation process,which deteriorates the mechanical properties of the alloy.During the solid solution process,a large amount of T-phase andη-phase in the matrix is re-dissolved into the matrix,which improves the precipitation of the strengthening phaseη′during the subsequent aging process,and the best mechanical properties of the alloy can be obtained after hot rolling and solid solution aging.2.Experiments on the multidirectional forging（MF）and deformation heat treatment process of Al-Zn-Mg-Li alloy were carried out.The evolution of the microstructure of the alloy during MF process and its influence on the mechanical properties were investigated,and the grain refinement behavior and pore elimination process during MF process were revealed.The evolution of brittle T-phase in MF after aging and solid solution aging and its influence on mechanical properties were analyzed:during MF,the coarse brittle phase in the matrix will be severely fragmented,and the larger the forging press,the more fragmented and dispersed the brittle phase is,and the area fraction decreases to a minimum of 0.7%;due to the high dislocation density and sub-structure,the precipitation of T-phase and S-phase is more obvious during the forging process;the strong thermal deformation causes the healing of pores distributed in the form of strips in the alloy,and the porosity decreases significantly;the severe extrusion deformation causes the alloy to have obvious dynamic recovery and dynamic recrystallization,producing lots of fine grains,and the average grain size decreases significantly from 23.1μm to 16.9μm with the increase of forging pressure from 30%to 50%;with the increase of forging pressure,aging temperature and time,theη′phase coarsens significantly,and the spherical T-phase size becomes smaller;many of S-phase and T-phase will dissolve back into the matrix during the solid solution process,and the saturation of the solid solution increases significantly.The highest strength can be obtained by solid solution aging of the alloy after 50%multidirectional forging.3.Multi-pass variable temperature rolling and low-temperature aging treatment experiments were carried out to investigate the mechanism of the effect of ultra-low temperature rolling and medium temperature rolling（CR+WR）on the brittle phase of the alloy.The low-temperature aging behavior of the CR+WR alloy was explored,and the mechanism of plasticity and strength enhancement of the alloy was revealed:cold deformation inhibited the dynamic reversion,meanwhile high-density dislocations were introduced into the alloy,which led to a dramatic increase in dislocation density and stress concentration around the brittle particles and accelerated their fragmentation;after the CR+WR specimens were aged,the matrix precipitated a high-density nano-sizedη′phase and atomic clusters GP region.They are crucial for the enhancement of strength and plasticity of the alloy.The highest tensile strength of 732 MPa was obtained at 100°C/30h aging by tuning the multi-pass variable temperature rolling and low temperature aging process,which was 55.4%higher than that of the normal rolled aging sample,meanwhile the elongation of the sample was 9.45%.The samples obtained a maximum elongation of 13%at 120°C/5 h aging,and the mechanical properties of the alloy met the use requirements.