镁合金等通道转角挤压过程中的晶粒细化机制

采用金相显微镜、背散射电子衍射(EBSD)和透射电子显微镜(TEM)分析ZK60镁合金在等通道转角挤压(ECAP)过程中不同部位的显微组织特征。结果表明:ZK60镁合金经240℃ECAP变形1道次后,合金的晶粒得到明显细化,但组织仍不均匀。剪切变形前,合金组织主要为粗大晶粒并伴有大量孪晶,剪切区的组织主要为剪切变形带和少量再结晶组织;剪切变形后,合金的晶粒组织主要为再结晶组织;合金ECAP过程的晶粒细化主要为机械剪切和动态再结晶的综合作用。     

变形AZ31镁合金的晶粒细化

利用Gleeble-1500D热模拟机，对AZ31镁合金在300～450℃以及应变速率为0．1和1.0s^-1条件下进行了热压缩。发现在热压缩变形过程中发生了动态再结晶，其动态再结晶平均晶粒尺寸(d)的自然对数与ZenerHollomon参数(Z)的自然对数成线性关系。再利用d与Z的关系，通过较低的热挤压温度(300～350℃)，获得了动态再结晶晶粒直径在10～20μm之内的镁合金管材。     

挤压比对AZ31镁合金变通道角挤压影响的数值模拟

通过Gleeble-1500D热模拟机获得AZ31镁合金的应力-应变曲线,采用DEFORM-3D软件对其变通道角(CCAE)挤压过程进行了模拟,并分析了不同挤压比对挤压过程的应力和应变影响.结果表明,挤压比越大,所需的挤压力越大,且挤压后期挤压力减少比例越大,材料动态再结晶的程度越高.随着挤压比的增大,表面质量反而逐渐增高,试样平均等效应变成线性增大.     

Die structure optimization of equal channel angular extrusion for AZ31 magnesium alloy based on finite element method

Three-dimensional(3D)geometric models with different corner angles(90°and 120°)and with or without inner round fillets in the bottom die were designed.Some important process parameters were regarded as the calculation conditions used in DEFORMT M-3D software,such as stress—strain data of compression test for AZ31 magnesium,temperatures of die and billet,and friction coefficient.Influence of friction coefficient on deformation process was discussed.The results show that reasonable lubrication condition is im...     

AZ31镁合金板材等径角轧制变形规律研究

对等径角轧制过程中AZ31镁合金板材的应力应变状态进行了分析,采用有限元对不同通道间隙下板材的应变状态进行了模拟,研究了不同通道间隙下镁合金板材晶粒取向的演变规律及其对晶粒取向的影响。结果表明,在等径角轧制过程中,板材在模具转角处受到剪应力和压应力的作用;随通道间隙的增加,板材的变形由剪切变形演变为剪切+弯曲变形,甚至弯曲变形;由于剪应力的作用,AZ31镁合金板材的晶粒取向由普通轧制所形成的基面取向转变为等径角轧制后的非基面取向,随着剪切变形量的减小,基面沿轧制方向的偏转角度也逐渐减小。     

AZ31镁合金晶粒细化方法及机制研究现状

系统介绍了AZ31镁合金晶粒细化方法及机制,综述了6种制备细晶镁合金大塑性变形方法的工艺特点和应用,展示了大塑性变形方法在AZ31镁合金加工中的应用前景。     

等径角轧制AZ31镁合金板材的组织与性能

采用等径角轧制工艺制备了AZ31镁合金板材.结果表明:经等径角轧制后的板材,晶粒取向由等径角轧制前的(0002)基面取向演化为基面与非基面共存的取向.与等径角轧制前的板材相比,板材晶粒尺寸略有长大并有孪晶出现,但强度却明显提高,而断裂延伸率变化不大,尤其是1个道次轧制的板材其抗拉强度由等径角轧制前的240增大到275 MPa,屈服强度由193.8增大到239.2 MPa;随着等径角轧制道次的增加,板材的强度逐渐降低,至第4个道次其抗拉强度仅为250 MPa,屈服强度为207.3 MPa.     

Friction stir welding of AZ31 magnesium alloys processed by equal channel angular pressing

Equal channel angular pressing (ECAP) is an effective thermo-mechanical process to make ultrafine grains.An investigation was carried out on the friction stir welding (FSW) of ECAPed AZ31 magnesium alloys with a thickness of 15 mm.For different process parameters,the optimum FSW conditions of ECAPed AZ31 magnesium alloys were examined.The basic characterization of weld formation and the mechanical properties of the joints were discussed.The results show that the effect of welding parameters on welding quality was evident and welding quality was sensitive to welding speed.Sound joints could be obtained when the welding speed was 37.5 mm/min and the rotation speed of the stir tool was 750 r/min.The maximum tensile strength (270 MPa) of FSW was 91% that of the base materials.The value of microhardness varied between advancing side and retreating side because of the speed field near the pin of the stir tool,which weakened the deformed stress field.The value of microhardness of the welding zone was lower than that of the base materials.The maximum value was located near the heat-affected zone (HAZ).Remarkable ductile character was observed from the fracture morphologies of welded joints.     

AZ31镁合金等通道转角挤压变形均匀性有限元分析

以AZ31镁合金为研究对象,通过对不同模具外角ECAP变形过程的有限元模拟,研究不同模具外角下AZ31镁合金ECAP变形的等效应变分布.利用微观组织观察以及硬度测试,分析等效应变分布对微观组织及力学性能影响.结果表明:当模具外角ψ为20.时,工件可以获得均匀的等效应变分布.AZ31镁合金经过ECAP挤压后,微观组织显著细化,力学性能明显改善,但平均晶粒尺寸及微观维氏硬度在工件横截面上分布不均匀,等效应变分布的不均匀性是导致材料微观组织和力学性能不均匀的主要因素之一.     

挤压比对AZ31镁合金组织结构的影响

试验研究了挤压比对AZ31镁合金组织结构的影响.结果表明,258℃挤压变形镁合金在形变初期容易出现孪晶,再结晶晶粒一般出现在晶界和孪晶附近.挤压比小时动态再结晶晶粒平均尺寸为2 μm,动态结晶细化了晶粒.随着挤压比增大,晶粒尺寸减小,组织趋于均匀.挤压比达到16时,动态再结晶基本完成.挤压比为25时,能得到晶粒细小且均匀的组织,平均晶粒尺寸为7.3 μm.     

Effects of die angle on microstructures and mechanical properties of AZ31 magnesium alloy processed by equal channel angular pressing

1　INTRODUCTIONMagnesiumalloysarethelightestmetallicstruc turalmaterialsandhencetheyprovidegreatpotentialintheweightsavingofautomotiveandaerospacecomponents ,materialhandlingequipment ,portabletoolsandevensportinggoods[1,2 ] .Duetotheirhexago nalclose packed (HCP)crystalstructure ,magnesiumalloysperform poorformabilityandlimitedductilityatroomtemperature ,thustheirproductsaremainlyfabricatedbycasting ,inparticular ,die casting ,andtheapplicationsofwroughtmagnesiumalloysarelim ited .Nowit…     

等通道转角分流模挤压AZ31镁合金管材

采用等通道转角挤压(ECAP)技术改良了传统挤压模具分流孔;通过螺旋槽焊合模腔,挤出了壁厚2 mm的管材。研究了镁合金的组织变化及材料的性能。结果表明:挤压态合金组织均匀,晶粒细小(平均晶粒尺寸约为12.5μm)。等通道转角挤压的细化晶粒过程、动态再结晶以及退火再结晶使合金具有良好的组织结构和力学性能。挤压态试样断口呈现为脆性解理断裂方式,退火态试样断口则表现为脆性和韧性混合断裂机制。     

AZ31镁合金管材挤压过程的数值模拟

采用Gleeble1500热模拟机对于不同温度和变形速率下的AZ31镁合金的变形性能进行了研究。通过实验得到真实应力的关系式及真实应变关系式，进而得到真实应力-应变曲线。以此为基础，采用DEFORM-3D软件，对不同壁厚管材的成形的过程进行模拟，发现在挤压时，管材内壁的金属比外壁的金属流动快，挤压筒与圆锥面过渡处的等效应变值最大等现象，分析了产生的原因，并通过工艺试验验证了模拟分析的正确性。     

Numerical simulation of temperature field of AZ91D magnesium alloy during equal channel angular extrusion

The temperature field of AZ91D magnesium alloy extruded by equal channel angular extrusion (ECAE) has been simulated using finite element method (FEM). A series of tests were designed to obtain the simulation parameters: flow stress of AZ91D, friction factor and heat transfer coefficient. The simulated temperature agrees well with the measured one. The evolution of temperature and influencing factor was discussed in details. Furthermore, the extrusion pressure of ECAE was analyzed.     

Dynamic recrystallization and texture development during hot deformation of magnesium alloy AZ31

The dynamic recrystallization(DRX) and texture development, taking place during hot deformation of magnesium alloy AZ31 with a strong wire texture, were studied in compression at 673 K (0.73 Tm). Two kinds of samples were machined parallelly to the extruded and transverse directions of Mg alloy rods. New fine grains are evolved at original grain boundaries corrugated at low strains and develop rapidly in the medium range of strain, finally leading to a roughly full evolution of equiaxial fine grains. Kink bands are evolved at grain boundaries corrugated and also frequently in grain interiors at low strains. The boundary misorientations of kink band increase rapidly with increasing strain and approach a saturation value in high strain. The average size of the regions fragmented by kink band is almost the same as that of new grains evolved in high strain. These characteristics of new grain evolution process are not changed by the orientation of the samples, while the flow behaviors clearly depend on it. It is concluded that new grain evolution can be controlled by a deformation-induced continuous reaction, i.e. continuous dynamic recrystallization(DRX). The latter is discussed by comparing with conventional, i.e. discontinuous DRX.     

Low-temperature superplasticity of ZK40 magnesium alloy processed using equal channel angular pressing

Microstructure evolution and superplastic behaviors of ZK40 magnesium alloy were investigated in the temperature range of 473～623 K. Transmission electron microscopy (TEM) was used to study the microstructure changes, twinning occurred significantly after being processed by equal channel angular pressing (ECAP) for one pass through the die, the mean grain size was 5.6μm. Finer grains can be obtained after further processing through ECAP, the average grain size of the alloy processed by ECAP for three passes was as low as 0.8 μ_m; this alloy exhibited low temperature superplasticity at 473～523 K, elongations obtained at the same initial strain rate of 1×10~(-3) s~(-1) were 260％ at 473 K and 612％ at 523 K, respectively. Corresponding values for the ZK40 alloy processed by ECAP for only one pass were 124％ at 473 K and 212％ at 523 K, respectively; poor superplastic behavior of this material was related to the long-range stresses associated with the non-equilibrium grain boundaries within the coarse grains. The incompatibility between fine and coarse grains was thought to be unfavorable to the improvement of superplasticity.     

等通道角挤压变形AZ31镁合金的变形行为

研究挤压态和等通道角挤压(EcAE)态AZ31镁合金的变形行为与微观组织的相关性.结果表明,ECAE态AZ31镁合金的室温拉伸屈服强度与晶粒尺寸之间表现出反Hall-Petch关系,且拉压不对称性明显减弱;在室温压缩时表现出应变速率敏感性,并随变形温度升高,应变速率敏感性因子变大.挤压态合金的晶粒度为20 μm,具有典型的挤压丝织构,主要变形方式为基面位错滑移和孪生,导致了合金中明显的拉压不对称性.ECAE态合金平均品粒尺寸约为2μm,织构相对随机化,导致合金压缩时孪生比率明显下降,其他变形模式比率增加,提高了变形抗力,降低了拉压不对称性.ECAE态AZ31镁合金压缩的激活能接近其晶界扩散激活能,晶界滑移在一定程度上导致了合金的反Hall-Peteh关系的出现以及应变速率敏感性的增强.     

AZ31镁合金高温热压缩变形特性

在应变速率为0.005～5 s-1、变形温度为250～450℃条件下,在Gleeble-1500热模拟机上对AZ31镁合金的高温热压缩变形特性进行了研究.结果表明:材料流变应力行为和显微组织强烈受到变形温度的影响;变形温度低于350℃时,流变应力呈现幂指数关系;变形温度高于350℃时,流变应力呈现指数关系;变形过程中发生了动态再结晶且晶粒平均尺寸随变形参数的不同而改变,其自然对数与Zener-Hollomon(Z)参数的自然对数成线性关系;材料动态再结晶机制受变形机制的影响,随温度的不同而改变;低温下基面滑移和机械孪晶协调变形导致动态再结晶晶粒的产生;中温时Friedel-Escaig机理下位错的交滑移控制动态再结晶形核;高温时位错攀移控制整个动态再结晶过程.在本实验下,材料的最佳工艺条件是:变形温度350～400℃,应变速率为0.5～5 s-1.     

Microstructural evolution characterization of friction stirring welded AZ31 magnesium alloy

A friction stirring welding to joint 5 mm rolled AZ31 magnesium had been developed. The microstructures in various regions including the weld nugget, thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ) were investigated and compared with unaffected parent metal using optical microscopy. The results showed that the heat and mechanical process had great effect on the microstructure evolution. In weld nugget, the heat was enough to produce sufficient superplastic material flow and the mechanical effect was greatest, and the dynamic recrystallization was completed thoroughly. In TMAZ, the mechanical effect was indirectly affected by the welding tool and only some grains had undergone dynamic recrystallization. The various regious were studied in detail to better understand the microstructural evolution during friction stirring welding (FSW). The cross section near the ““““key hole““““ showed clear onion rings because the material was stirred only by the rotation of the probe and materials rotated with the probe and did not move along welding direction and in vertical direction, there was no material flow and the flow movement can be regarded as two dimensional layer flow.