| Aerospace,national defense and other fields put forward high requirements for equipment lightweight and high performance.As a representative lightweight structural material with high strength and heat resistance,rare earth magnesium alloy has become the best material for high-end equipment.Extrusion forming is the main forming method of high-performance rare earth magnesium alloy components,but it has the challenges of forming difficulty and low strength and toughness.In this paper,Mg-Gd-Y-Zn-Zr alloy strong plasticity synergistic enhancement as the research purpose,based on the regulation of LPSO phase in the crystal,carried out hot compression,the regulation of LPSO phase morphology,different morphologies of LPSO phase in the crystal extrusion,pre-control LPSO combined with ring channel Angle extrusion cylinder structure,and other experiments.The deformation coordination mechanism of LPSO phase in Mg-Gd-Y-Zn-Zr alloy was analyzed,and the microstructure evolution and strength and toughening mechanism of LPSO phase with different morphologies in the grain during heat processing were investigated.The microstructure evolution mechanism of the Mg-Gd-Y-Zn-Zr alloy during the deformation process was studied by the compression experiments of the Mg-Gd-Y-Zn-Zr alloy after the air cooling treatment at 525℃/12 h.It was found that the LPSO phase could be deformed by kinking and promoting dynamic recrystalization.The alloy was subjected to510℃,510℃/18h+480℃/8h heat treatment to obtain two kinds of alloy with the same grain size and texture,without(N sample)and with(C sample)LPSO lamellar phase.The effect of preset LPSO on the flow stress,recrystalization,dislocation density and plasticity of the alloy was studied.C sample has lower flow stress,crystallite dislocation density,number of cracks,higher degree of recrystallization and kinking,and better plasticity.The morphology of LPSO phase was regulated by heat treatment,and the evolution of LPSO phase and β phase of Mg-Gd-Y-Zn-Zr was elucidated under different precipitation temperature and time.The spacing of LPSO lamellar phase becomes larger with the prolongation of precipitation time.The β phase has a small scale at 420℃,grows up at 450℃,and does not precipitate at 480℃.Three kinds of heat treatment structures containing different morphologies of intralesional LPSO phases were obtained,namely,510℃/18h+480℃/1h intralesional LPSO lamellar phase(SN sample),510℃/18h+480℃/16 h intralesional LPSO lamellar phase(SW sample),The intergranular LPSO lamellar phase overlaps the β phase(SO sample)at 510℃/18h+420℃/1h.The positive extrusion experiments were carried out on the above three samples with different morphologies of LPSO phases to investigate the microstructure evolution and strength and toughening mechanism of the three samples.It was found that the volume fraction of dynamic recrystalization of the three samples increased with the increase of extrusion temperature,and the decrease of texture strength and dislocation density in the crystal were the main reasons for the decrease of yield strength.At the same time,the β phase size increased first and then redissolved,which caused the worst elongation at 450℃ and the best elongation at 480℃.The optimum comprehensive mechanical properties were observed when extruded at 420℃.By analyzing the microstructure and properties of the three samples at 420℃,it is found that the SN sample with a narrow lamellar phase in the grain has the highest strength and elongation after extrusion.The texture strengthening and dislocation strengthening are the main reasons for the highest strength,and the highest elongation is due to the coordination ability of the LPSO phase with less overall precipitation and narrow spacing.The combination of pre-conditioned LPSO phase and the annular channel angular extrusion(ACAE)was used to condition the initial billet to produce an intracrystalline narrow spacing LPSO phase,420℃ and 450℃ were selected as extrusion temperatures.Under 420℃ extrusion,the precipitated phase and dynamic recylization size of the cylinder wall are small,the yield strength is 212 MPa,the tensile strength is 312 MPa,and the elongation is 11.5%.At 450℃,the yield strength is 225 MPa,the tensile strength is 301 MPa,and the elongation rate is 5.7%.The thick β phase is the main reason for the rapid decline of plasticity and the low tensile strength.Subsequently,the heat treatment of the formed components showed that the fine β phase could be dissolved back into the matrix through solid solution at 420℃ and 450℃,while the coarse β phase could not be dissolved back at420℃ and 450℃.After the final heat treatment,the tube extruded at 420℃ obtained a balanced structure of strength and toughness,showing no precipitation at grain boundaries and dense nano-scale precipitation of β ’phase within the boundaries.Under the condition of weak texture strengthening in the sampling direction of the tensile rod,the tube showed good mechanical properties of 387 MPa tensile strength and 18.7% elongation.The low temperature single pass forming of high strength and toughness cylinder component was realized. |
PDF Full Text Request