| The specific strength and specific stiffness of rare earth magnesium alloys are relatively high,and the high-temperature mechanical properties are excellent.However,the poor plasticity of rare earth magnesium alloys limits its application.The plastic deformation process is usually used to refine its grains and improve the structure and texture,thereby improving the forming properties of rare earth magnesium alloys.Multi-directional forging has the advantages of grain refinement,uniform structure,and repair of internal micro-cracks.And its characteristics of simple process,high production efficiency and material utilization rate are more friendly to production and processing.Studying the hot deformation behavior of the alloy can easily obtain the better processing parameters of the alloy,and can also guide the selection of processing equipment.At present,the research on strengthening rare earth magnesium alloys through multi-directional forging process is mainly aimed at the as-cast raw material.There are few studies on improving the formability of extruded rare earth magnesium alloys by multi-directional forging process.In this paper,the extruded Mg-8.5Gd-4.5Y-0.8Zn-0.4Zr alloy was subjected to a water-cooled solution treatment after holding at 500? for 12h to improve its structure and texture.The thermal deformation behavior of the alloy at different temperatures(400?-500?)and different strain rates(0.001s-1?1s-1)is studied.Establish the constitutive equation of the alloy and draw the hot working diagram.Combined with the analysis of the alloy hot deformation structure,the range of multi-directional forging process parameters is obtained.Finally,the structure changes in the alloy multi-directional forging process are analyzed,and the alloy multi-directional forging process is explored.The main conclusions are as follows:(1)Observe the structure and texture of the extruded Mg-8.5Gd-4.5Y-0.8Zn-0.4Zr alloy before and after solid solution.It is found that a large amount of Long Period Stacking Ordered Structure(LPSO)will be precipitated at the grain boundary of the alloy after solution treatment.More rare earth elements dissolve into the matrix to play a role of solid solution strengthening.The rare earth elements that cannot be dissolved into the matrix will also form a rare earth-rich phase dispersed in the matrix to strengthen the matrix.Solution treatment will also improve the anisotropy of the material to a certain extent.The original cylindrical texture is transformed into a texture feature in which the base surface and the cylindrical surface coexist.(2)The solid solution Mg-8.5Gd-4.5Y-0.8Zn-0.4Zr alloy obtains true stress-strain curves at different deformation temperatures and strain rates through thermal simulation compression tests.Analyzing its thermal deformation behavior,it is found that the flow stress of the alloy is more sensitive to the deformation temperature and strain rate.The constitutive equation of the alloy is established based on the Arrhenius model:?=9.3979 ×1019[sinh(1.0261 ×10-2 ?)]4.5635 exp(-3.0066 × 105/RT)The constitutive equation can more accurately predict the peak stress during high temperature deformation of the alloy.Considering the influence of strain on the constitutive equation,a constitutive equation based on strain compensation is established for the alloy,which can predict the flow stress of the alloy during high temperature deformation.(3)The hot working drawing of Mg-8.5Gd-4.5Y-0.8Zn-0.4Zr alloy is drawn.It is found that the alloy has two instability zones,one is the low temperature(400?440?)region and the other is the medium high temperature(460?490?)region,and the strain rate is between 0.1s-1?1s-1.Observing the deformed structure of the alloy instability zone,it is found that there are obvious deformation streamlines in the structure,and the method of instability is rheological instability.The deformed structure of the safe zone is mainly dynamic recrystallization.Combining the alloy's hot working map and deformation structure analysis to determine the alloy's better forging process parameters,the deformation temperature range is 480?-500?,and the strain rate range is 0.01s-1?0.2s-1.(4)The solid solution Mg-8.5Gd-4.5Y-0.8Zn-0.4Zr alloy is subjected to unidirectional,bidirectional and multi-directional forging deformation.Observing the structure changes during the forging process,it is found that only part of the grains of the alloy are twinned during unidirectional forging,which is related to the grain orientation of the alloy.Increasing the load direction will cause the grains that were not conducive to twinning to be twinned under the load.During the forging process,the twins of the same variant will be coarsened to form coarse twins.Multi-directional forging can cause twisting of the alloy grain boundaries.The LPSO phase in the crystal will also deform through its own kinks and form a kink band.Twin crystals and grain boundary twists will refine the alloy grains through recrystallization during the annealing process.Annealing cannot completely eliminate the kink bands. |
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