Microstructure and mechanical properties of Mg-Gd-Y-Zr alloy processed by equal channel angular pressing

Microstructure evolution and texture development and their effects on mechanical properties of a Mg-Gd-Y-Zr alloy during equal channel angular pressing（ECAP） were investigated.It is found that the microstructure is still inhomogeneous after four passes,and two zones,namely the fine grain zone（FGZ） and the coarse grain zone（CGZ） are formed.The grain refinement occurs mainly by particle-stimulated nucleation（PSN） mechanism,which led to a more random texture after four passes of ECAP.In the ECAP-processed alloy,the strength did not increase while the ductility was enhanced dramatically compared with the as-received condition.The change of ductility of this alloy was discussed in terms of texture and second phase particles.

AZ31镁合金和7050铝合金ECAP过程的数值模拟研究

利用Abaqus有限元软件分别对AZ31镁合金和7050铝合金进行等通道转角挤压(ECAP)数值模拟研究。利用有限元(FE)模拟得到应力场、应变场及载荷-位移曲线,分析了在不同挤压温度、挤压速度、摩擦条件和不同截面下等通道转角挤压的变形规律。结果表明,挤压温度升高可以降低挤压载荷,这有利于AZ31镁合金挤压,在摩擦较小情况下进行变形有利于降低挤压载荷,提高变形均匀性;在不考虑变形过程中温升效应的情况下,挤压速度对挤压载荷影响较小,当挤压速度提高时,最大等效应变值有所提高;圆形截面比方形截面更适合进行等通道转角挤压。

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

Three-dimensional（3D） geometric models with different comer 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 DEFORMTM-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 important to plastic deformation. The change characteristics for distributions of effective stress and strain during an equal channel angular extrusion （ECAE） process with inner angle of 90° and without fillets at outer comer were described. Inhomogeneity index （C） was defined and deformation heterogeneity of ECAE was analyzed from the simulation and experiment results. The deformation homogeneity caused by fillets at outer comer increased compared with the die without fillets. The cumulated maximum strains decrease with increasing the fillets of outer comer in ECAE die and the inner comer angle. The analysis results show that better structures of ECAE die including appropriate outer comer fillet and the inner comer angle of 90° for the die can improve the strain and ensure plastic deformation homogenization to a certain extent. The required extrusion force drops with increasing the fillet made at outer comer in ECAE die. It is demonstrated that the prediction results are in good agreement with experiments and the theoretical calculation and the research conclusions in literatures.

The effect of Equal Channel Angular Pressing process on the microstructure of AZ31 Mg alloy strip shaped specimens

Equal Channel Angular Pressing(ECAP)is one of the most applicable Sever Plastic Deformation(SPD)processes which leads to strength and ductility improvement through the grain refining and development of a suitable texture.In this study,after designing and manufacturing a suitable die,4 pass ECAP process at route C is done on strip shaped specimens of AZ31 magnesium alloy in order to achieve desirable microstructural and mechanical properties.Microstructure then got studied through the optical microscopy.Results show that mean grain size is decreased and grain size distribution got close to normal distribution state by increasing the pass number.However,the grain size is reduced by increasing of ECAP temperature.

Microstructure evolution and mechanical behavior of AZ31 Mg alloy processed by equal-channel angular pressing

AZ31 Mg alloy bar was subjected to 8-pass equal-channel angular pressing(ECAP) at 623 K. Microstructure evolution was observed by optical microscopy(OM) on cross section and X-ray diffraction analysis. The room temperature mechanical properties of the ECAP processed specimens were also investigated. A fine-grained structure with an average sub-grain size of 9 μm is obtained after 7 ECAP passes. XRD analysis indicates that after ECAP,in placing of {1 010},planes {1 011} and {1 012} become the dominant directions that are favourable for grain refinement. ECAP processed AZ31 Mg alloy exhibits significant improvement in elongation but decrease in strength. The elongation of the specimen increases continuously up to 2 passes and then remains stable at further passes. This improvement can be related to the evolution of crystallographic texture and the scattered orientation of the basal plane(0001).

LPSO相调控对ECAP挤压Mg-Y-Zn-Al合金组织和力学性能的影响

针对含18R-LPSO相铸态Mg-Y-Zn系合金塑韧性差、变形后力学性能提高有限的问题,研究了固溶、铸态+ECAP挤压和固溶+ECAP对Mg-3.74Y-1Zn-0.5Al(at%)合金力学性能的影响。结果表明,固溶处理诱发18R→14H-LPSO相转变、Al(Y,Zn)相化合物颗粒破碎细化,改善了合金的塑性,但强度几乎不变。铸态+ECAP挤压导致18R-LPSO和Al(Y,Zn)相破碎,并诱发大量α-Mg晶粒发生再结晶。固溶+ECAP挤压使14H-LPSO相弯曲或扭折变形、细小Al(Y,Zn)颗粒均匀分散,仅少数α-Mg晶粒发生再结晶,其强度、塑韧性远高于铸态+ECAP挤压合金的。良好强度、塑韧性匹配的14H-LPSO相的大量形成、α-Mg基体再结晶被抑制及细小Al(Y,Zn)颗粒的均匀分散是其性能提高的主要原因。

DSC analysis of commercial Cu-Cr-Zr alloy processed by equal channel angular pressing

Samples of a commercial Cu-lCr-0.1Zr(mass fraction,%) alloy were subjected to equal channel angular pressing(ECAP) up to 16 passes at room temperature following route Bc.Differential scanning calorimetry(DSC) was used to highlight the precipitation sequence and to calculate the stored energy,recrystallization temperature and activation energy after each ECAP pass.On another hand,electrical properties were correlated with the dislocation density.Results show that the stored energy increases upon increasing ECAP pass numbers,while the recrystallization temperature decreases significantly.

ECAP和时效对Mg-4Zn-1Mn-0.5Ca合金组织和性能的影响

将铸态Mg-4Zn-1Mn-0.5Ca合金在300℃热挤压成棒材,对棒材进行180℃时效处理和200℃等通道转角挤压(ECAP)。研究了热挤压合金、ECAP 8道次合金和时效后ECAP 8道次合金的组织和性能。随后对试样进行室温和150℃低温单轴拉伸。结果表明:8道次ECAP后合金比热挤压态合金具有更细、更均匀的晶粒组织,同时提高了合金的室温屈服强度。时效后ECAP 8道次合金的150℃断裂伸长率比未时效ECAP8道次合金提高17.4%,说明时效处理对合金性能起着重要作用。

Observation of macroscopic shear band in aluminum-based alloy during equal-channel angular pressing

Equal-channel angular pressing(ECAP) is now recognized as an effective technique for fabricating ultrafine grained materials. The results show that prevalent macroscopic shear banding occurs in ECAP as long as the pressing passes reach 2, and the severe macroscopic shear band extends from bottom to top surface and slants to the longitudinal axis of specimen at an angle of about 45°, develops at regular intervals, with a high shear strain accommodation of about 3.7 within the band. Different families of macroscopic shear bands may cut across each other, and over 60% of sample volume is occupied by macroscopic shear band when the ECAP reaches 4 passes.

ECAP对预置孪晶AZ31镁合金组织和力学性能的影响

为了探究剪切变形对预置孪晶AZ31镁合金组织和力学性能的影响,对应变量为5%的预置孪晶试样在250℃进行了不同道次的等通道转角挤压(ECAP),随后对不同状态试样进行了显微组织观察和室温单轴拉伸测试。结果表明:预置孪晶试样经过ECAP变形后,试样组织与商用挤压态合金相比明显细化,延展性获得较大提高;且随着挤压道次的增加,合金晶粒更加细小均匀,延展性改善更加显著。其中,挤压四道次合金的室温断后伸长率较挤压态棒材提高了约163%,表明预置孪晶后引入剪切变形能有效提高镁合金棒材的室温塑性。

A new method of characterizing equivalent strain for equal channel angular processing

In order to establish the quantitative relationship between equivalent strain and the performance index of the deformed material within the range of certain passes for equal channel angular processing （ECAP）, a new approach to characterize the equivalent strain was proposed. The results show that there exists better accordance between mechanical property （such as hardness or strength） and equivalent strain after rolling and ECAP in a certain range of deformation amount, and Gauss equation can be satisfied among the equivalent strain and the mechanical properties for ECAP. Through regression analysis on the data of hardness and strength after the deformation, a more generalized expression of equivalent strain for ECAP is proposed as：ε=k0exp[-（k1M-k2）^2], where M is the strength or hardness of the material, k1 is the modified coefficient （k1∈ （0, 1））, ko and k2 are two parameters dependent on the critical strain and mechanical property that reaches saturation state for the material, respectively. In this expression the equivalent strain for ECAP is characterized novelly through the mechanical parameter relating to material property rather than the classical geometry equation.

Finite Element Analysis for Multi-pass Equal Channel Angular Pressing

Equal channel angular pressing （ECAP） is an attractive process method to produce bulk uhra-fine grained materials. There are many experiment evidences showing that the nature of the microstructural evolution in multi-pass ECAP depends on process routes. Isothermal three dimensional FEM simulations for muhi-pass ECAP were performed using DEFORM3D finite element code. The material model of 6061A1-T6 was employed. Flow nets, effective strain distribution in the workpiece and loads during multi-pass ECAP using different routes were analysed respectively, The simulations show process routes influence material flow and effective strain distri- bution in the workpiece obviously but have few influence on loads.

On the cold rolling of AZ31 Mg alloy after Equal Channel Angular Pressing

Among the various Severe Plastic Deformation(SPD)processes,Equal Channel Angular Pressing(ECAP)is one of the most applicable one which improves strength and ductility due to grain refinement and suitable texture development.In this study,cold rolling were carried out on the 4 pass ECAPed(in route A and C)strip shaped specimens of AZ31 magnesium alloy to investigate the ECAP effects on the roll-ability.Results showed that reduction in area which can be concerned as an index for roll-ability increased after ECAP.It was also seen that ECAP in route C enhanced roll-ability more than route A.

Deformation mechanism at impact test of Al-11% Si alloy processed by equal-channel angular pressing with rotary die

Al-11%Si(mass fraction)alloy was transformed into a ductile material by equal-channel angular pressing(ECAP)with a rotary die.Two mechanisms at impact test,slip deformation by dislocation motion and grain boundary sliding,were discussed.The ultrafine grains with modified grain boundaries and the high content of fine particles(<1μm)were necessary for attaining high absorbed energy.The results contradict the condition of slip deformation by dislocation motion and coincide with that of grain boundary sliding.Many fine zigzag lines like a mosaic were observed on the side surface of the tested specimens.These observed lines may show grain boundaries appeared by the sliding of grains.

Microstructural evolution of a recycled aluminum alloy deformed by equal channel angular pressing process

The microstructural evolution of a recycled aluminum alloy after equal channel angular pressing （ECAP） up to four passes was investigated using X-ray diffxaction （XRD） analysis and transmission electron microscopy （TEM）. Microhardness tests were performed to determine the associated changes in mechanical properties. An ultrafine-grained material has been obtained with a microstructure showing a mixture of highly strained crystallites. A high density of dislocations was achieved as a result of severe plastic deformation （SPD） through the die. Changes in mechanical behavior are also revealed after ECAP due to strain hardening. Thermal analysis and TEM micrographs ob- tained after annealing indicate the succession of the recovery, recrystallization, and grain growth phenomena. Moreover, the energy stored during ECAP may be related to the dislocation density introduced by SPD. We finally emphasize the role played by the precipitates in this alloy.

Equal-Channel Angular Pressing as a New Processing to Control the Microstructure and Texture of Metallic Sheets

The extended band structures of as-cold-rolled high Cr steel sheets are recrystallisation-resistant, and tend to become aggregates of the so-called grain colonies as a partially recovered state after final annealing. Such band structures diminish formability and become origin of the so-called ridging. A novel processing will be shown here, which involves strain-path change by introducing one-pass ECAP prior to cold-rolling, and facilitates recrystallisation. Indeed, the recrystallisation temperature was reduced by 100℃, compared with cold-rolling alone imposing an equivalent strain. Grain- scale microshear bands introduced during one-pass ECAP perturbed the banded structures in post-ECAP cold-rolling and enhanced the recrystallisation at the final annealing.

Microstructural approach to equal channel angular processing of commercially pure titanium——A review

A review on severe plastic deformation（SPD） technique of equal channel angular pressing（ECAP） process of commercially pure titanium（CP-Ti） alloys was presented with a major emphasize on the influence of ECAP parameters that include channel and curvature angles, processing route, temperature of operation, pressing speed, internal heating, number of pass through the die and back pressure. Various ECAP characteristics such as microstructure, strain inhomogeneity and mechanical properties are considered to achieve the maximum homogeneity, equilibrium grain refinement and mechanical improvement of CP-Ti. Investigations show that a pressing speed of 1-3 mm/s at 450 °C with route BC along with channel and curvature angles of 90° and 20° respectively with backpressure can lead to the most homogeneous ultrafine microstructure.

中碳合金钢42CrMo的ECAP工艺参数模拟及试验研究

高端滚动轴承在服役过程中存在的尺寸稳定性差和滚动疲劳寿命低等问题,严重制约着我国高端装备制造业的发展水平。因此,急需开发具有高强韧性、耐磨、耐疲劳特性的轴承钢。中碳轴承钢作为超高速高精密滚动轴承极具潜力的轴承钢材料,对不同工艺条件下的中碳合金钢ECAP(equal channel angular pressing)工艺参数开展研究,分析了ECAP过程对42CrMo钢的挤压载荷、等效应变、等效应力及温度场数值变化和分布规律。研究结果表明,适当提高挤压初始温度和挤压速度,均可以有效降低挤压载荷和等效应力;加大挤压速度可以明显提升试件挤出温度,但不利于试件ECAP过程形变的均匀性;减小摩擦系数可以显著降低挤压载荷,有助于提高晶粒细化的均匀程度。综合考虑,选定挤压温度950℃,挤压速度10 mm/s、摩擦系数0.03条件下设计试验,可以有效提高ECAP细化晶粒的能力,同时为试件进行下一步热处理提供良好初始条件。以上试验研究是对中碳钢细晶强化的全新探索,可以为其他性能相近的金属材料开展ECAP研究提供技术参考。

预置初始孪晶对ECAP变形纯镁组织和性能的影响

对预置孪晶后的纯镁进行等通道挤压(ECAP),利用光学显微镜、电子万能试验机研究1、2、4道次后组织和力学性能的变化。结果表明在250℃等通道挤压下,预压缩5%的试样比原始试样具有更细的晶粒。2道次挤压后,预孪晶的样品相较于没有预压缩的样品动态再结晶晶粒更多。4道次挤压后晶粒尺寸变得更加均匀。ECAP和预孪晶均能提高纯镁的伸长率。预孪晶4道次挤压后样品的伸长率比原始样品挤压的伸长率提高较大,说明了孪晶在纯镁ECAP变形中起着重要的作用。