【作者】 吴文云；

【导师】 姚寿山； 靳丽； 董杰；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2010， 博士

【摘要】 镁合金挤压管材成形容易、尺寸精度高、组织致密、机械性能高,可最大限度地实现材料结构一体化减重,是未来大规模推广镁合金应用的重点。管材的成功应用离不开弯曲、涨形、缩管等二次加工,但密排六方结构的镁合金室温塑性变形能力弱,无法照搬钢、铝、铜等的二次塑性成形工艺,所以,本文系统地研究了AZ31和AM30镁合金挤压管材的弯曲成形性能及其变形形为,希望为镁合金管材的规模应用提供一定的理论指导和实践借鉴。挤压成型了AZ31和AM30两种合金管材,并分析了它们的微观组织、室温和高温力学性能。结果表明两种合金在挤压时均发生了完全动态再结晶,形成了由大角度晶界组成的等轴晶及c轴垂直于挤压方向的丝织构,但在相同的挤压工艺下,AM30合金管材的晶粒更加细小,分布均匀。随挤压温度的升高,AM30合金管材的晶粒度增大,室温拉伸强度降低,延伸率升高;低温挤压得到的细晶管材变形时孪晶被抑制,丝织构导致基面位滑移开动困难,室温拉伸强度高,延伸率低,而高温挤压得到的粗晶管材则因孪晶、多滑移系开动,协调变形性好,延伸率高,强度低。在高温拉伸和压缩变形时,随温度的升高或应变速率的下降,AM30合金管材的屈服强度及形变硬化率降低,延伸率增大。针对镁合金的变形特点,设计并定制了一套能加热且保持相对恒温的旋转拉伸弯管机。采用该设备可以实现,加热温度范围:室温-400°C,速度范围:0-5 r/min,角度0-180°。选用两倍相对弯曲半径(弯曲半径/管外径)和90°弯曲角,以弯曲后的壁厚减薄率、截面椭圆度和回弹率作为评判标准,考察了温度、速度、管材的原始状态等工艺及材料因素对管材弯曲成形性的影响规律。结果表明,随弯曲温度的升高,管材弯曲后的截面椭圆度和释放后的回弹率均下降,当弯曲温度为200°C左右时,管壁的减薄率最小。在弯曲温度一定时,随弯曲速度的增大,管材的壁厚减薄率及截面椭圆度增大,而回弹率下降。具有细晶组织的管材在弯曲成形时,可得到较小的壁厚变化和截面形状畸变,且晶粒度的变化对弯后回弹的影响很小。管材与模具有相对滑动的接触面进行润滑处理可降低管材弯曲后的壁厚减薄率和截面椭圆度,但回弹率略有增加。实验范围内AZ31和AM30合金管材弯曲成形的最佳工艺参数为:加热温度:150-200°C,弯曲线速度8mm/s,待弯管材应具有细晶组织,管材与模具之间应做润滑处理,且在相同的工艺条件下,AM30合金管材的弯曲成形性略好于AZ31合金。利用有限元软件MSC.Marc成功实现了AM30合金管材旋转拉伸弯曲的工艺仿真。对弯曲时处于弯曲半径内外两侧的管壁分别按压缩曲线和拉伸曲线定义材料模型。有限元模拟的结果经过了实际弯曲试验结果的验证。因此该模型可对镁合金管材的弯曲工艺设计提供成形性的综合预测。通过有限元弯曲工艺模拟预测了弯曲角度、相对弯曲半径和管材的自身规格对弯曲成形性的影响规律及应力应变的分布。结果表明:随弯曲角度的增大,管材表面的最大等效应力在30°以前迅速增大,然后趋于稳定,而最大等效塑性应变一直增加,但增加速率逐渐降低。弯曲一定角度时(90°),管材表面的等效应力与等效弹性应变分布规律类似,即在弯曲半径的内外侧与中性面附近均有峰值出现,等效塑性应变的峰值出现于弯曲半径的最外侧与最内侧,在中性面附近达到最小值。随相对弯曲半径的减小,管材的弯曲成形性迅速变差,管壁的等效塑性应变增大,可成形的最小相对弯曲半径为管材外径的1.5倍。随相对壁厚(管外径/壁厚)的增大,管材的弯曲成形性变差,可成形的最大相对壁厚值为30,相对壁厚的改变对弯曲时管壁的塑性变形量影响较小。采用背散射电子衍射技术(EBSD)研究了AZ31和AM30合金管材在150°C弯曲前后不同变形量区域的管壁对应的微观组织,总结并提出了镁合金管材弯曲成形时的塑性变形机制。结果表明:处于弯曲半径外侧的管壁材料,在弯曲变形时主要发生拉伸变形,其变形机制以位错滑移为主,处于弯曲半径内侧的管壁,弯曲时主要发生压缩变形,变形时除发生位错滑移以外,{10-12}<10-11>拉伸孪晶是其重要的变形方式,且孪晶使管壁材料的宏观织构发生了近似90°的转变。另外,AM30合金管材的原始织构较AZ31合金的弱且分布相对分散,因此在弯曲时其变形协调性更好。更多还原

【Abstract】 The magnesium alloy tubes are easily to be fabricated, which have high precision of size, fine microstructure, good mechanical properties, and the replacement of tubular sections to stamped components could save weight of structure in most. The magnesium alloy tubes would be the potential industrial structuresin the future. But the application of tubes generally need further second forming, such as bending, expanding, and contraction. Magnesium, with its hexagonal close packed (hcp) crystal structure, has limited slip systems and limited formability at room temperature, and then the bending method can not copy from that for the steel and aluminum tubular sections. The aim of this paper is to investigate the bendability and deformation behavior of the popular wrought Mg alloy AZ31 and AM30 tubes, and the bending mechanisms of magnesium alloy tubes. The results would provide references and consultations to the further development and applications of the Mg alloy tubes.Two kinds of alloy AZ31 and AM30 tubes were extruded, and then the microstructure and mechanical properties of the tubes in room and high temperature were tested. The results indicate recrystallizaton following extrusion is near complete for the two kinds of alloy, and a majority of grain boundaries are high angle grain boundaries (HAGBs). A basal ring texture with the c-axes of grains perpendicular to the extrusion direction (ED) has formed. The grains of AM30 alloy tube became coarse with the raise of extrusion temperatures, and the tensile strength decreases but the elongation increases. The twinning was restricted during deformation for the fine-grain tube, and the crytal orientation is unfit for the operation of dislocation slips, so the tube has high tensile strength and low ductility. On the other hand, coarse-grain tube has low tensile strength but good ductility due to the harmony of dispersed twinning during deformation. For the tension and compression processes in high temperature, the yield strength and work hardening rate of AM30 alloy tube decrease and elongation increases with the raise of temperature and reduction of strain rate.A warmed rotary draw bender for Mg alloy tube was designed and manufactured. It can realize warmed temperature of 20-400°C, bend velocity of 0-5r/min and bend angle of 0-180°. The effects of temperature, velocity, tube microstructure etc. on the bendability have been studied using the wall thinning, cross-section ovality and spring back as evaluation criterion in the conditions of popular used two relative bend radius and 90 degree bend angle. The results indicate both the ovality and spring back decrease with the raise of temperature, but the wall thinning is smallest at 200°C. At certain temperature, the wall thinning and ovality increase, spring back decreases with the raise of bending velocity. The fine-grain tube could obtain smaller wall thinning and ovality during bending, and the grain size has little effect on the spring back. The lubrication between die and tube can make low wall thinning and ovality, but the spring back increased slightly. In the tested parameter range, the temperature of 150-200°C, the velocity of 8mm/s, fine microstructure and lubrication are most feasible for bending of AZ31 and AM30 alloy tubes. And the bendability of AM30 alloy tube is a little better than that of AZ31 alloy tube at same bending conditons.The finite element (FE) software MSC.Marc was used to simulate the bending process. The material in extrados and intrados of tube were defined with tension and compression curves respectively. The results of simulation were validated with actual experiments. The effects of bending angle degree, relative bending radius and tube size on the bendability were analyzed through FE simulation. The results suggest the maximum equivalent stress in tube surface increases shapely before 30 degree, then hold relative stable with the increase of bending angle degree. The maximum equivalent plastic strain increases all along with the raise of bending angle degree, but the increasing rate is reduced. At certain bending angle (90°), the distributions of equivalent stress and elastic strain are similar, namely, peak value appears both at neutral area, extrados and intrados. The peak value of equivalent plastic strain appears at the most far away of extrados and intrados, and the minimum value is at neutral area. The bendability of tube become worse rapidly and the equivalent plastic strain rises with the decrease of relative bending radius. The minimum bending radius is 1.5 times of tube outer diameter. With the raise of relative wall thickness (outer diameter/wall thickness), the bendability also become worse, and the maximum relative wall thickness is 30. The change of relative wall thickness has little effects on the plastic strain.The microstructure of tube wall (after bending at 150°C) at different strains were analyzed through electronic backscattered diffraction (EBSD) technology. The plastic deformation mechanisms of the two kinds of alloy during bending were summarized. The results suggest the material in extrados undergoes tension and the dislocation slip is the main deformation mechanism. And the material in intrados undergoes compression; {10-12} <10-11> tension twinning is an important deformation mechanism except for dislocation slips. The texture changes approximately 90 degree due to the extension twinning. The density of texture in AM30 alloy is smaller than that of AZ31 alloy, and the crystal orientation in AM30 is a little more dispersive, so the deformation process of AM30 alloy tube is more harmonious in bending.更多还原