ECAP挤压L2纯铝的微观组织演化规律

用等通道转角挤压对纯铝L2进行10道次挤压,结果表明：挤压1道次后,原来晶粒尺寸为1 mm的等轴晶沿剪切方向被拉长为条带状晶,在条带状晶粒之间出现被剪切破碎的细小亚晶粒.挤压2道次后,出现了少量等轴晶.挤压4道次后,晶粒取向性变得不太明显,小角度晶界的亚晶粒逐步向大角度晶界的等轴晶演化,晶粒细化到1 μm.随挤压道次的继续增加,晶粒大小不再变化,而形状向等轴状演化.挤压10道次后,合金组织由晶粒大小为1 μm的等轴晶组成.ECAP挤压中,纯剪切变形和应变量的双重作用导致晶粒细化.当晶粒尺寸小于临界尺寸时,剪切变形对晶粒的细化起主要作用;当达到临界尺寸后,应变量起主要作用,表现在使合金组织形貌向等轴晶转变.     

等通道转角挤压后AZ31镁合金的微观结构与性能

为了进一步探讨细晶镁合金的制备方法与性能 ,采用模角φ =12 0°的模具、以BC路径对AZ31镁合金进行了等通道转角挤压试验研究 ,对挤压过程中各道次试样的微观结构及性能进行了分析测试 .结果表明 ,随着挤压道次的增加 ,晶粒得到不断细化 ,力学性能也发生显著的变化 ;当挤压 12道次时 ,总的等效应变量约为 8,晶粒得到显著细化 ,晶粒尺寸为 1～ 5μm ,但合金的抗拉强度变化不大 ,屈服强度则有所下降 ,约为 10 0MPa ,延伸率则提高到 4 5 %以上     

等径弯曲通道变形制备超细晶铝合金的组织性能

用等径弯曲通道变形(ECAP)的方法制备出超细晶铝合金材料,并研究了在不同道次条件下其显微组织的演化过程.研究表明,随着强烈塑性变形的增加,显微组织中开始形成大量晶粒尺寸小于1μm的位错胞组织,当其晶界取向差增大时,亚晶粒变为越来越细的板条状组织.当经过8道次ECAP变形后,晶粒尺寸由变形前的约50μm细化为约0.2μm.该超细晶铝合金材料在150℃的退火条件下,其晶粒尺寸稳定在0.2～0.3μm的范围内.在温度为500℃、应变速率为10-3s-1的拉伸实验中,该超细晶铝合金材料的最大延伸率高达370%,呈现出良好的超塑性.     

等径角挤压对纯铜组织与性能的影响

研究采用BC路径(即试样进入下一道次挤压时按同一方向旋转90°)对纯铜进行等径角挤压后得到的组织与性能.结果表明,通过室温下对纯铜的8道次挤压后,得到均匀、细小的等轴晶组织(晶粒尺寸约1.5μm).抗拉强度从原来的235 MPa提高到420 MPa,硬度从114 HV提高到184.3 HV,延伸率由原来的45%降低到19%.通过对不同挤压道次试样在473 K下60 min的退火处理后,其晶粒进一步细化至1μm,其抗拉强度提高到435 MPa.     

等通道转角挤压-扭转的纯铜变形行为模拟

采用DEFORM-3D有限元模拟软件对纯铜等通道转角挤压-扭转(ECAP-T)进行模拟研究,研究变形过程中试样的等效应力和等效应变分布及变形路径对等效应变分布的影响,对变形后晶粒的细化效果进行分析.结果表明:相对于单纯ECAP变形,ECAP-T具有更大的应变量和更好的晶粒细化效果;多道次挤压后试样沿Bc路径的等效应变分布最均匀,沿A路径较差;ECAP等效应变主要集中在试样的中心部位,但ECAP-T等效应变主要集中在试样表面;ECAP-T变形后沿Bc路径的晶粒细化均匀性最好,Ba路径次之,A路径最差,且变形后试样纵截面上外侧点的变形均匀性比中心处好.     

ECAP变形低碳钢的组织变化规律

采用每道次挤压后样品旋转90°进入下一道次且旋转方向不变方式(Bc)的等径弯曲通道变形(ECAP)工艺制备亚微晶Q235钢,并研究了4个面不同的组织演化. 研究表明,ECAP变形Q235钢的组织随变形道次的增加而细化,其中第1道次的细化程度最大. 4个面的形貌也不同:S面以位错胞为主;R面在第2道次出现变形带交叉现象;而T面2道次就有近似的等轴胞出现,4道次出现了晶界很清楚的等轴晶,尺寸为0.25μm左右;L面4道次后也有等轴晶出现,但晶界没有T面4道次的晶界清楚. 8道次后所有的面都已经演化成晶界清晰的等轴晶,尺寸为0.2μm左右. 采用ECAP变形可以获得亚微晶Q235钢.     

等通道转角挤压(ECAP)工艺的研究进展

对等通道转角挤压技术的基本原理和近年来的最新研究进展进行综述. 对挤压过程中晶粒细化机理和变形机理、影响挤压效果的因素分析认为,降低挤压温度、增加背压、减小入口摩擦并适当加大出口摩擦可以有效增加材料组织的均匀性. 认为ECAP加工后材料内部大角度晶界数的增加导致变形机制的改变,晶界滑移导致晶粒转动趋势的增加,这2方面的原因是产生超塑性的主要原因. 提出从单晶材料的织构层面揭示材料的微观组织演变及定量计算多晶体的宏观性质是今后的研究方向.     

晶粒组织对Al-Zn-Mg合金抗应力腐蚀性能的影响

通过四点弯曲应力腐蚀实验对Al-Zn-Mg合金挤压型材包含不同晶粒组织的试样进行应力腐蚀性能测试,并借助金相组织观察、电子背散射衍射以及透射电镜研究晶粒组织不均匀性对材料抗应力腐蚀性能的影响和作用机理。研究结果表明:Al-Zn-Mg合金挤压型材从表面至中心依次分布着粗晶组织、等轴晶组织和纤维状组织,其中粗晶组织和等轴晶组织的厚度分别约为60μm和750μm,再结晶分数和大角度晶界百分占比均从表面至中心逐渐降低;不同晶粒组织的抗应力腐蚀性能主要与晶粒粒度和晶界微观组织有关,相对于粗晶组织和等轴晶组织,纤维状组织由于再结晶行为受到抑制,晶粒粒度较小,大角度晶界较少,具有更离散的晶界析出相和较窄的无沉淀析出带,从而表现出更好的抗应力腐蚀性能。     

纯铜粉末包套-等径角挤压工艺实验研究

针对粉末材料低塑性的特点,在室温条件下采用包套-等径角挤压工艺(PITS-ECAP)将纯铜粉末颗粒直接固结成高致密度块体细晶材料.结果表明,包套-等径角挤压工艺对粉末材料具有有效的致密和细化效果.4道次PITS-ECAP工艺变形后,试样X、Y、Z面均受到剧烈剪切作用,晶粒尺寸得到明显细化,显微组织呈细长条带流线状,且分布较为均匀;试样整体组织达到完全致密,平均显微硬度高达1 470 MPa.在PITSECAP工艺变形过程中,剧烈塑性剪切变形、较高静水压力和有效应变积累是保证粉末材料致密度大幅度提高以及显微组织有效细化的主要原因.     

单级时效处理对6082铝合金型材微观组织的影响

为优化时效处理参数,探索时效处理对材料的强韧化影响机制,通过扫描电镜(SEM)、X射线衍射(XRD)和电子背散射衍射(EBSD)研究了单级时效处理对6082铝合金挤压型材微观组织演变的影响.结果表明:时效处理可以使6082铝合金棒材挤压成型材的过程中储存的残余应力得到有效去除,晶粒出现细微长大现象,平均尺寸由7.02μm增长为9.80μm;平均晶界角度降低,从25.51°降到16.06°,低角度晶界比例显著提高,从47.91%提高至67.47%;时效处理可以使晶粒{100}和{110}方向的晶粒分布发生明显的变化,明显增强了试样在{100}和{110}方向的织构.这些转变有利于6082铝合金挤压型材综合性能的提高.     

Microstructure,mechanical properties and deformation mechanisms of an Al-Mg alloy processed by the cyclical continuous expanded extrusion and drawing approach

Al-Mg alloys are an important class of non-heat treatable alloys in which Mg solute and grain size play essential role in their mechanical properties and plastic deformation behaviors.In this work,a cyclical continuous expanded extrusion and drawing(CCEED)process was proposed and implemented on an Al-3Mg alloy to introduce large plastic deformation.The results showed that the continuous expanded extrusion mainly improved the ductility,while the cold drawing enhanced the strength of the alloy.With the increased processing CCEED passes,the multi-pass cross shear deformation mechanism progressively improved the homogeneity of the hardness distributions and refined grain size.Continuous dynamic recrystallization played an important role in the grain refinement of the processed Al-3Mg alloy rods.Besides,the microstructural evolution was basically influenced by the special thermomechanical deformation conditions during the CCEED process.     

低碳钢表面机械研磨处理过程的有限元分析

运用有限元分析方法,对低碳钢表面机械研磨处理(SMAT)过程进行了数值模拟,分析计算了材料表面层的应力和应变分布情况,初步探讨了应变量和应变速率对材料塑性变形机制的影响.结果表明,经SMAT后样品表面发生了明显的塑性变形,其表面层的应变、应变速率和应力沿厚度方向均逐渐减小,这与其微观组织相对应;塑性变形时的应变量和应变速率对于样品晶粒细化和处理后最终晶粒尺寸的大小起重要作用.在系统激振频率为50 Hz时,SMAT过程中样品最表面层的应变速率最大可达681 s-1.     

On the contribution of boundaries towards strength in oxygen free high conductivity copper subjected to repetitive upsetting–extrusion

Oxygen free high conductivity (OFHC) copper was subjected to 1, 5, 10, 15 and 20 cycles of repetitive upsetting and extrusion (RUE) process at room temperature. Microstructure and microhardness in the RUE processed material were evaluated at specific locations and correlated with the equivalent plastic strain. The microhardness of the material was found to be independent of strain or number of RUE cycles at certain locations whereas it exhibited strain softening behaviour at certain locations even though significant grain refinement was achieved. This difference in behaviour is attributed to the varied strength contribution from different types of boundaries present in the material after deformation.     

基于40Cr高温动态力学性能的磨削表面动态再结晶行为

通过分离式霍普金森压杆(SHPB)实验得到了40Cr合金钢在高温高应变率下真应力-真应变曲线，据此确定了材料发生动态再结晶的临界条件.采用解析法和实验法分别求解了磨削强化层的温度场和塑性应变场分布.结果表明，磨削强化层在磨削深度方向具有较大的温度和塑性应变梯度;150μm强化层内会发生奥氏体转变;磨削表面最高温度可达1060℃.在磨削亚表面60μm内会产生剧烈的塑性变形，达到了再结晶的临界条件.较大的磨削深度使磨削强化层塑性应变增大，再结晶现象越充分，微观组织细化程度越高.     

Experimental and numerical analyses of pure copper during ECFE process as a novel severe plastic deformation method

In this paper,a new severe plastic deformation method called equal channel forward extrusion(ECFE) process has been proposed and investigated by experimental and numerical approaches on the commercial pure copper billets. The experimental results indicated that the magnitudes of yield strength,ultimate tensile strength and Vickers micro-hardness have been markedly improved from 114 MPa,204 MPa and68 HV as the annealed condition to 269 MPa,285 MPa and 126 HV after the fourth pass of ECFE process,respectively. In addition,scanning electron microscopy observation of the samples showed that the average grain size of the as-received state which is about 22 μm has been reduced to 1.4 μm after the final pass. The numerical investigation suggested that although one pass ECFE process fabricates material with the mean effective strain magnitude of about 1,the level of imposed effective plastic strain gradually diminishes from the circumference to the center of the deformed billet.     

塑性变形工艺在变形镁合金晶粒细化中的应用

综述了热挤压、轧制、大塑性变形挤压等不同塑性变形工艺在变形镁合金晶粒细化中的应用研究进展,认为目前变形镁合金发展的主要瓶颈是低加工速率导致相关产品的成本居高不下,未来将通过大尺寸半连续铸锭的多外场晶粒细化和细晶镁合金快速加工技术等予以解决。     

Effect of initial orientation on microstructure and mechanical properties of AZ31 Mg alloy sheets via accumulated extrusion bonding

In this study, effects of initial orientation on microstructure evolution and mechanical properties of AZ31 Mg alloy sheets via accumulated extrusion bonding (AEB) was systematically studied. The samples with RD and TD parallel to extrusion direction (ED) were labeled as RED and TED, respectively. RD and TD pieces alternately stacked was named as RTED. The results revealed that under three-dimensional compressive stress, {10-12} tensile twinning dominated the first stage deformation in container. As the plunger continuous press down, dynamic recrystallization (DRX) occurred, and the newly fine DRXed grains along original and twin grain boundaries gradually consumed the matrix and twin grains. The microstructure was completely transformed into recrystallized grain structure at sizing band with an average grain size of ∼0.9 ​μm in TED sample, smaller than that of RED sample. After the alloys extruded out of the die, DRXed grains significantly grew to ∼4.5 ​μm and 3.5 ​μm for RED and TED samples, respectively. A laminated microstructure was obtained with average grain sizes of∼4.4 ​μm in RD layers and 3.5 ​μm in TD layers for RTED sample. The AEB processed samples exhibited an ED-tilt double-peak basal texture with similar texture intensity. The tensile tests results indicated that attributed to the combined effect of grain refinement and texture evolution, the yield strength and fracture elongation of RED and TED samples was significantly improved. The heterogeneous microstructure in RTED sample induced an extra work hardening (HDI-work hardening) in tensile deformation and resulted in a further improved elongation of 32.0%.     

SPHC钢应变诱导相变细化铁素体晶粒

以SPHC钢为对象,在Gleeble-1500型热模拟机上进行单道次热压缩试验,通过分析变形后的应力与应变曲线及变形过程中的金相组织变化,研究应变诱导相变的基本规律及铁素体晶粒细化效果.结果表明:在750~830℃的变形中存在应变诱导铁素体相变,并获得超细晶铁素体晶粒尺寸为1.6~4.6μm;降低变形温度将增加相变所需化学驱动力,促进应变诱导铁素体相变的发生,从而细化铁素体晶粒;在一定的应变条件下,应变诱导相变获得的铁素体晶粒尺寸和体积分数均随应变速率的增加而减少.     

Surface grain refinement mechanism of SMA490BW steel cross joints by ultrasonic impact treatment

Ultrasonic impact treatment (UIT) is a postweld technique for improving the fatigue strength of welded joints. This technique makes use of ultrasonic vibration to impact and plastically deform a weld toe and can achieve surface grain refinement of the weld toe, which is considered as the main reason for the improvement of fatigue strength. In this paper, the microstructure of the surface of a treated weld toe was observed by metallographic microscopy and transmission electron microscopy (TEM). The results show that UIT could produce severe plastic deformation on the surface layer of the weld toe and the maximum depth of plastic deformation extended to approximately 260 μm beneath the treated surface. Repeated processing could exacerbate the plastic deformation on the surface layer, resulting in finer grains. We can conclude that the surface grain refinement mechanism of SMA490BW welded joints is related to the high density of dislocation tangles and dislocation walls.     

Tubular pure copper grain refining by tube cyclic extrusion–compression (TCEC) as a severe plastic deformation technique

Tube Cyclic Extrusion–Compression(TCEC) method is a novel severe plastic deformation technique developed for grain refining of cylindrical tubes to ultrafine grained(UFG)/nanostructured ones. In this method, tubes are fully constrained and deformed between chamber and mandrel with a small neck zone. The principle of TCEC technique which was adopted to impose severe plastic strains to the tubular materials was explained. Also, the deformation and grain fragmentation mechanism during TCEC was analyzed. The material deformation characteristics during TCEC were numerically simulated by FE code of ABAQUS/Explicit. The FEM results demonstrated that TCEC technique was able to impose extremely high plastic strains. The TCEC method was successfully applied to a commercially pure copper(99.99%) and significant grain refinement was achieved. TEM observation demonstrated the refinement of grains from the initial size of 45 μm to 200–350 nm after four processing cycles of TCEC. Microhardness measurements were carried out across the thickness of the initial and processed tubes. The results show good homogeneity of hardness distribution and an increase to 102 Hv from initial value of 55 Hv after four TCEC cycles. Mechanical properties of the specimens were extracted from tensile tests. The obtained results documented notable increase in the yield and ultimate strengths, whereas the uniform and total elongations decreased. Fracture surfaces after tensile tests were investigated by scanning electron microscopy(SEM), and the observed morphology indicates ductile fracture mode after four cycles of TCEC.