【作者】 王琳；

【导师】 喻海良；

【作者基本信息】 中南大学 ， 机械工程， 2022， 博士

【摘要】 第三代铝锂合金具有密度低、强度高、刚度高、抗疲劳性能好、抗腐蚀性能好等优点,是航空航天领域理想的结构材料。提高铝锂合金力学性能、降低轧制带材各向异性等难点,至今依旧是高性能铝锂合金板材加工过程中需要面对的重大挑战。本文以2060铝锂合金板材为研究对象,研究铝锂合金轧制及后续时效过程中机械性能变化和微观组织演变机理,模拟分析了铝锂合金变形过程中微观组织演变规律,深入研究了深冷轧制（通常指在不高于-100℃温度下轧制）工艺条件下材料的变形机理及其对后续时效工艺的影响,主要研究内容和创新工作如下:（1）研究了不同温度条件下2060铝锂合金的力学性能和断裂机理。基于所研究合金板材的初始状态,建立分子动力学分析模型,对不同温度条件下的变形过程模拟计算。模拟结果中变形应力变化趋势和实验结果变化趋势一致,说明该模拟计算具有参考意义。结果表明,深冷变形时2060铝锂合金内1/2<1 1 0>全位错和1/6<1 1 2>不全位错大量增加,随着应变的增加逐渐形成以全位错为节点的网状结构,缠结程度高。较于室温变形时,深冷变形过程中晶界开始发射位错所需应变较大,说明外界施加应变在材料内分布比较均匀,位错累积能力较强。综合以上因素,深冷变形后的2060铝锂合金强塑性较好。同时,变形过程中初始晶界吸收或者发射位错后发生偏转。（2）研究了不同轧制温度、不同轧制异速比和不同压下量工况下,轧制变形参数对2060铝锂合金轧制过程中微观组织演变的影响规律。定量分析了轧后2060铝锂合金中固溶强化、位错强化和晶界强化的影响。深冷轧制后合金材料中形成大量位错胞和位错团。同时,随着应变的增加,这些位错微结构演变成晶界,使得合金材料中缺陷密度增高,导致2060铝锂合金强度增加。发现了-100℃深冷轧制过程中晶粒细化不均匀分布实现合金强韧性提高的现象,应用背应力强化模型,综合考虑背应力强化等四种强化机制准确地预测了深冷轧制后2060铝锂合金的屈服强度。（3）为了改善2060铝锂合金拉伸性能的各向异性,综合考虑了对该合金各向异性影响较大的因素,研究轧制温度、轧制方向和轧制压下量对2060铝锂合金力学性能、各向异性和微观结构的影响机理。结果表明,轧制之前初始合金材料中晶粒取向分布相对均匀,但不存在Cube织构。轧制后2060铝锂合金材料中形成了大量的小角度晶界。压下量为30%室温单向轧制后,S织构的体积分数增加,织构的最大强度值明显增加。室温交叉轧制过程中,有规律地改变轧制方向,能促进变形过程中开动更多的滑移系,导致室温交叉轧制后2060铝锂合金中织构强度降低,β纤维织构含量减少,Cube织构含量增加。同等压下量条件下深冷交叉轧制后,合金材料中的主要织构转变为Brass织构和S织构。相比室温轧制后合金材料中的晶粒取向差分布,深冷交叉轧制后合金材料中晶粒的取向差分布更加均匀,Goss织构含量相对较低,且存在一定的Copper织构,导致深冷交叉轧后2060铝锂合金断裂延伸率IPA值仅为0.23%。相比室温轧制后合金材料,深冷交叉轧制后2060铝锂合金材料中应变硬化率更高,变形过程中位错累积能力更大。当压下量增加到60%时,相比较室温轧制后的合金,深冷轧制后2060铝锂合金中Goss织构含量相对较低,导致断裂延伸率IPA因子较低。（4）研究了不同轧制参数和时效温度（140℃,150℃和160℃）工况下,2060铝锂合金时效过程中力学性能变化和微观结构演变,探明了深冷轧制对该合金板材低温时效过程中强化相的影响规律。结果表明,经过深冷温度下大塑性变形后的2060铝锂合金板材在140℃温度下时效时,峰值时效硬度可达221 HV。该工况下,较多的位错团等微缺陷能较长时间存在,导致低于常规时效温度条件下大量T₁相仍在螺位错附近形核,抑制了δ’相的析出。在-190℃条件下压下量为80%深冷轧制后的2060铝锂合金板材在140℃时效20 h后,T₁相平均长度仅为15 nm,数量密度为989μm^-2,面密度可达1.51×10⁴nm/μm²,导致该工艺条件下加工后2060铝锂合金材料强度提高。图75幅,表9个,参考文献171篇更多还原

【Abstract】 The third generation Al-Li alloy is an ideal structural material in aerospace field due to its low density,high strength,high stiffness,good fatigue resistance and good corrosion resistance.Improving mechanical properties and reducing the mechanical properties anisotropy of the rolled Al-Li alloy strips still are big challenges during deformation.This paper focuses on deformation characteristics and microstructure evolution of AA2060 sheets during rolling process and subsequence ageing treatment.Meanwhile,molecular dynamics simulation is utilized to investigate the microstructure evolution of AA2060 sheets during cryorolling（rolling under-100℃）.The main contents and innovations are as following:（1）The mechanical properties and fracture model of AA2060 alloy at different temperatures are studied by tensile tests at different temperatures.Based on the initial state of the alloy,a molecular dynamics model is established to simulate the deformation process at different temperatures.The variation trend of deformation stress of the simulation is consistent with that obtained from the experiments,which indicates that the simulation results have reference significance.1/2<1 1 0>Perfect dislocations and 1/6<1 1 2>Shockley dislocations sharply increase during the cryo-deformation.With the increase of applied strain,the mesh structure with Perfect dislocations as nodes gradually forms.The degree of dislocation entanglement is relatively high.Compared with room temperature deformation,the applied strain required by grain boundary to emit dislocations is larger during cryo-deformation,indicating that the applied strain is evenly distributed and the dislocation accumulation capacity is stronger.Based on the above factors,AA2060 alloy processed at cryogenic temperature shows higher strength and better ductility.At the same time,grain boundaries rotate after emitting or absorbing dislocations during deformation process.（2）The influences of deformation parameters,such as rolling temperatures,rolling reduction and different rolling ratios,on the microstructure evolution of AA2060 alloy are studied.The solid solution strengthening part,the dislocation strengthening part and the grain boundaries strengthening part of rolled AA2060 alloy are investigated quantitatively.A large number of dislocation cells and dislocation clusters form in the alloy after cryorolling.With the increase of applied strain,these dislocation clusters turn to grain boundaries,which increases the defect density leading to the improvement of the strength of AA2060alloy.The back stress strengthening model is applied based on the uneven grain disribution in the alloy processed by rolling at-100℃.The yield strength of rolled AA2060 alloy can be well predicted by considering the four strengthening mechanisms.（3）In order to reduce the tensile properties anisotropy of AA2060alloy,the effects of rolling temperature,rolling direction and rolling reduction on mechanical properties,the anisotropy and microstructure of AA2060 alloy are investigated.The results show that the grain orientation distribution of the alloy after solution treatment is relatively uniform,and Cube texture component does not exist.A large number of low angle grain boundaries form in the alloy after rolling.The component of S texture increases and the maximum intensity value of texture increases after room temperature rolling with the rolling reduction of 30%.Changing rolling direction regularly can promote the activation of more slip systems during room temperature cross rolling,leading to the decrease ofβfiber texture and the increase of Cube texture.The main texture component of the alloy processed by cryo-cross rolling are Brass texture and S texture.Compared with the grain misorientation distribution in the alloy processed by room temperature rolling,the grain misorientation distribution in the alloy after cryo-cross rolling is more uniform,and the Goss texture component is relatively low.There is a certain Copper texture.These result in the relatively low elongation IPA value of 0.23%.Compared with the the alloy processed by room temperature rolling,the strain hardening rate of the alloy processed by cryo-cross rolling is higher,and the dislocation accumulation capacity is larger.When the rolling reduction increase to 60%,the Goss texture component of AA2060 alloy processed by cryorolling is relatively low compared to that of the room temperature rolled alloy,resulting in the decrease of elongation IPA factor.（4）The mechanical properties and microstructure evolution of AA2060 alloy under different rolling parameters and aging temperatures（140℃,150℃and 160℃）are studied.The effects of cryorolling on precipitation of the alloy during low temperature ageing are obtained.The results show that the peak aging hardness of AA2060 alloy sheet processed by cryorolling with the rolling reduction of 80%reaches 221HV when aging at 140℃.When aging at 140℃,more dislocation clusters and other microdefects can exist for a long time,leading to the nucleation of a large number of T₁ phases near the screw dislocation.This inhibit the precipitation ofδ’phase.When ageing at 140℃for 20 h,the average length of T₁ phase in the alloy processed by cryorolling at-190℃is only 15 nm,the number density is 989μm^-2,and the planar density can reach 1.51×10⁴ nm/μm².As a result,the strength of AA2060 alloy is the highest after processing under this process condition.更多还原