| The third generation(Gen3)Al-Li alloys have high specific strength,high specific modulus,and good weldability,so they are widely used in the manufacture of various lightweight structural parts for aviation and aerospace vehicles.This paper took 2195 Al-Li alloy profile as the object,systematically studied the evolution of peripheral coarse grain(PCG)structure in the annealing and solution treatment process,and the change of microstructure and properties during solution aging treatment,determined the best annealing and solution aging treatment system,explored the evolution of the properties of 2195 Al-Li alloy at cryogenic temperatures and the influence of PCG structure on the welding properties,to provide guidance for the industrial production process optimization of 2195 Al-Li alloy profile and its application in the storage tank.The main results are as follows:The effects of extrusion,annealing and solution treatment on the PCG structure were investigated,and it was found that the edge of profiles recrystallized during the extrusion process,the grain size was in the range of 2～5 μm and the degree of recrystallization had obvious heterogeneity along the extrusion direction,showing two kinds of recrystallization microstructure.The thickness of recrystallized microstructure at the head of the profile is 2 mm,and there are obvious boundaries;the thickness of recrystallized microstructure at the tail reaches 13 mm,and there is no obvious boundary;the recrystallized grains grow abnormally during the solution process,forming a PCG structure.The PCG structure was significantly reduced when the extrusion ratio was reduced from 20 to 14;the higher the extrusion temperature,the more serious the PCG structure.At a lower annealing temperature(320℃),almost all of the recrystallized grains after solution grew into peripheral coarse grains with grain size in the range of 65～260 μm;grain coarsening occurs during annealing when the annealing temperature reached 440℃.Annealing time had little effect on PCG structure.The solution temperature had a significant effect on the abnormal growth of recrystallization,too high(lower than 495℃)or too low(higher than 515℃)would lead to the complete development of recrystallized grains into coarse grains,but the size of coarse grains is relatively small,and the grain size is in the range of 65～260 μm.At 505℃,the recrystallized grains grew rapidly,and the coarse grains appeared in 15 min.To control the depth of PCG structure,the optimal heat treatment regime was determined to be annealing at 350℃ for 6 h,solution at 505℃ for 1.5 h.The effects of solution temperature and time on the second phase and the effects of T6 and T8 ageing treatments on the microstructure properties of the alloy were investigated.It was found that the residual second-phase components in the extrusion state of the profile were mainly copper-rich and iron-bearing phases;the area fraction of the second phase decreased significantly after solution treatment,the changes of conductivity and the redissolution pattern of the second phase were highly consistent;when the solution temperature was lower than 505℃,the area fraction of the second phase decreased rapidly with the increase of temperature.When the temperature exceeded 505℃,the re-dissolution effect of the second phase did not improve significantly;In the range of 0.5-5 h,the solution time has little effect on the second phase back solution effect;the best solution regime is 505℃/1.5 h.There was no significant difference in the second phase particles of large residual size between T6 and T8 aging treatment;for the nano-precipitation phase,the T6 state alloy contains a large amount of θ’ phase and a small amount of δ’ phase,the size of T1 phase is in the range of 40～250 nm;changing the pre-stretching amount in the range of 2%～5%has little effect on the precipitation features and mechanical properties;the size of T1 phase in the T8 state alloy is much smaller than that in the T6 state,and the distribution is 20～140 nm;increasing the aging temperature can obviously increase the aging response rate.For the alloy at the same strength level,the higher the temperature the larger the size of T1 phase and the the number of T1 was lower.The tensile properties and microstructure of 2195 Al-Li alloy at different temperatures were studied.The following conclusions were obtained:the strength of the alloy gradually increased as the temperature decreased but the elongation didn’t change much;the lower the temperature,the more obvious the increase in strength;When the temperature decreased from room temperature to 20K,the enhancement of ultimate tensile strength and yield strength were 245 MPa and 138 MPa,respectively.As the temperature decreased,the number of dimples on the fracture surface decreased,and the tearing ribs increased and deepened;the increase in strength at low temperatures was due to more areas involved in the deformation,and the deformation was more uniform.The increase in strength at low temperatures was due to more uniform deformation;the dislocation density in the material was consistent with the trend in tensile strength with temperature.The effects of PCG structure and recrystallization microstructure on the welding properties were studied.It is found that after welding the alloy which contained coarse grain and recrystallized appeared coarse grain microstructure at heat-affected zone;PCG structure had a significant impact on the material fracture location,the presence of PCG structure of the joint tensile fracture location contained both heat-affected zone and weld core area.The surface of the fracture was uneven;there were a large number of tough nests in the fracture of the joints with PCG structure,while the fracture of the joints without coarse grain microstructure was dominated by laminar deconstruction surfaces. |
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