等通道弯角多道次挤压工艺累积变形均匀性研究

通过等通道弯角挤压(Equal channel angular pressing,ECAP)工艺累积足够的变形制备块状超细晶粒材料(亚微米或纳米微观结构材料)。采用节点映射法将前一次有限元分析终了结果准确映射到下一次挤压初始阶段的相应节点,实现等通道弯角多道次挤压过程有限元分析。通过有限元模拟不同模具拐角的等通道弯角多道次挤压工艺,获得相应挤压件累积等效应变分布及其规律,为ECAP挤压件晶粒均匀细化提供合理的模具设计与工艺路线。     

工艺路径对纯铝粉末材料多道次等径角挤扭变形的影响

结合实际变形过程,采用DEFORM-3D对不同变形工艺路径下纯铝粉末材料等径角挤扭(ECAPT)多道次变形过程进行了三维数值模拟。有限元模拟结果表明:A和C工艺路径可在较小的挤压载荷下实现有效的应变累积和均匀的变形分布,是两种较为理想的变形路径;BA和BC两种工艺路径虽然能够使变形材料累积的等效应变量大大增加,但却会造成试样变形分布的严重不均匀。电子背散射衍射(EBSD)实验结果表明,A路径下纯铝粉末材料经4道次等径角挤扭变形后,可获得组织均匀、性能优良的块体超细晶铝,材料平均晶粒尺寸约为0.8μm,抗压强度高达123.3MPa,验证了前述模拟结果的可靠性。     

铝合金Al5454ECAP挤压工艺模拟研究

利用Deform-3D软件对铝合金Al5454的ECAP式艺进行大量三维有限元模拟,得出了挤压过程中模具拐角、模具圆心角、摩擦条件与挤压速度对挤压件变形等效应力、等效应变的影响规律,从而为ECAP模具设计、工艺参数拟定提供有效的理论指导.     

铝-钛-硼中间合金在等径角挤压过程中的变形行为

采用等径角挤压(ECAP)技术对铝-钛-硼中间合金进行了室温挤压试验,用高温光学显微镜、扫描电镜、硬度计等分析了ECAP对合金中第二相粒子分布形态、尺寸及显微硬度的影响.结果表明:ECAP能显著改善合金中第二相粒子的分布形态,细化其尺寸;用试样绕其纵轴旋转9°.、方向不变的加工路径(Bc),经过8道次挤压后,第二相粒子由原来的散乱分布变成较为均匀分布,由原长约20μm、宽约10μm的块状粒子细化为5μm左右的小颗粒;挤压1道次后,材料硬度增加最为明显,4道次后硬度增加趋势变缓.     

连续等径通道挤压三维有限元模拟

利用三维有限元模型研究等径通道挤压工艺不同挤压路线的连续挤压过程对变形组织均匀性的影响.通过两次挤压后试样横截面等效塑性应变分布比较发现, 采用工艺路线C连续挤压两次所得的试样横截面的等效塑性应变分布似乎最均匀.但是根据不同挤压路线连续两次挤压后横截面变形分布特点推测,若采用挤压路线BC经过多次挤压后(挤压次数为4的倍数),试样横截面上的变形分布将是最均匀的,即最有利于得到微观结构均匀的超细晶材料,这一推测被四次连续挤压有限元模拟结果所证实.     

等通道弯角挤压变形机理分析与工艺路线研究

等通道弯角挤压(ECAP)过程是目前制备块状超细晶粒材料(亚微米或纳米微观结构材料)最具工业前景的工艺之一,研究ECAP变形机理从而开发出具有工程化和商品化价值的工艺具有十分重要的意义。通过塑性成形软件DEFORM-3D对目前出现的多拐角ECAP以及连续ECAP等新工艺进行数值模拟,研究了各工艺挤压过程中等效应变的历史演化以及载荷-行程曲线的变化。根据有限元模拟结果,分析了各工艺挤压过程中晶粒细化机理和变形优缺点。分析结果可对ECAP新工艺的模具设计、工艺参数拟定以及挤压工艺规划提供理论指导。     

等通道转角挤压下变形体长度对应力的影响及开裂判据分析

利用有限元软件ANSYS对2024铝合金等通道转角挤压(Equal-channel angular pressing,ECAP)过程进行计算模拟,得到等效正应力(Von Mises应力)、切应力随变形体长度及下行位移的变化,结合ECAP试验,研究ECAP形变开裂的判据.结果表明,无论变形体长度及下行位移如何变化,等效正应力都小于材料的抗拉强度,而切应力则随着变形体长度的降低总体上呈下降趋势,当变形体的长度小于某一临界值时,计算模拟表明最大切应力小于材料的抗剪强度,样品开裂倾向大幅降低,二者相当吻合.研究结果不仅说明"最大切应力不大于材料抗剪强度"可以作为材料ECAP形变过程中不出现开裂现象的判据,而且也为材料的ECAP加工提供理性研究思路,即先实测材料的应力-应变曲线,然后基于上述判据对模具结构、材料预处理工艺及样品尺寸等进行优化设计.     

ZK60镁合金往复挤压工艺参数研究

往复挤压是非常有效的晶粒细化工艺方式之一,且有望实现商业化应用。本实验以ZK60镁合金为基础,合理设计适于ZK60+x RE镁合金往复挤压的工艺参数(变形温度、挤压比、挤压道次等)。经研究,在组织均匀细小,高强且各向同性的准则要求下,可以获得适于镁合金热塑性加工的优化工艺参数,其变形温度范围为250~300℃,挤压比与挤压道次的配比是挤压比4~8、挤压道次4。     

等通道转角挤压对镁锆阻尼合金组织和性能的影响

对铸态Mg-0.6%Zr合金进行了等通道转角挤压(ECAP)变形,采用光学显微镜观察了变形后合金的显微组织,并对不同温度、不同道次变形后的合金进行了显微硬度、抗拉强度及阻尼性能测定。结果表明:合金经过ECAP变形后,发生了动态再结晶,晶粒显著细化,但随挤压温度的升高发生晶粒长大;显微硬度随挤压道次的增加而增大,抗拉强度在300℃挤压6道次时达到最大值187.80 MPa;合金经400℃挤压1道次后的阻尼性能优于铸态合金的,其他条件下均低于铸态合金的。     

不同热处理工艺对7475铝合金ECAP的影响

着重研究了退火、固溶处理、双级时效以及回归热处理4种不同的热处理工艺对7475铝合金试样在ECAP变形过程中晶粒细化的影响。退火处理后的试样在不同温度和道次挤压下呈现不同的微观组织形貌，高温下退火态铝合金试样ECAP挤压后晶粒明显细化。     

拉伸变形应变速率敏感性指数的力学涵义及其规范测量

鉴于建立超塑性变形定量力学解析理论的重要性,从分析Backofen超塑性拉伸的本构方程出发,讨论了方程中的材料参数k和应变速率敏感性指数m的力学涵义。提出了恒应变ε、恒速度υ和定载荷F三种典型变形路径条件下应变速率敏感性指数的规范测量公式。对每种变形路径又提出应变速率敏感性指数的传统测量方法和计算机模拟精确测量方法,并作了精度对比分析。给出了典型超塑Zn-5%Al合金在常态（18℃）和超塑状态（340℃）时三种变形路径下应变速率敏感性指数m ε、m υ和m F的实测结果。由于三者之间不但存在大的偏差,而且随ε的变化规律也各异。这就从试验上判明,即使在相同的应力状态下,应变速率敏感性指数也与变形路径密切相关。由此便向力学理论提出新的问题,要求从理论上解答偏差和变化规律不同的原因,进而揭示其力学本质。     

Finite element analysis of planar twist channel angular extrusion (PTCAE) as a novel severe plastic deformation method

A new severe plastic deformation (SPD) method based on equal channel angular pressing (ECAP) is introduced for producing ultrafine grains in bulk alloys, entitled as “Planar twist channel angular extrusion (PTCAE)”. In PTCAE method, there is additional angle, α, (plus φ and ψ angles in ECAP method) which represents angle associated with the lateral reversal arc of curvature in deformation zone. Three dimensional finite element method (FEM) simulations of both ECAP and PTCAE processes were performed in order to investigate the plastic deformation state of processed samples and, moreover, the effect of different die geometry (in terms of variation of planar twist angle) on plastic strain distribution and magnitude. Results revealed that PTCAE process related with ECAP process can impose higher strain values in different shear planes simultaneously in one deformation zone. Therefore, PTCAE can produce UFG or nanostructured materials better than ECAP method which has simpler design and significantly higher efficiency compared with other new SPD processes.     

等通道转角挤压Al-26%Si合金的组织和冲击性能

采用等通道转角挤压工艺(ECAP)细化了Al-26%Si合金中的组织,研究了该合金在不同温度下的冲击性能及断口形貌。结果表明:ECAP能有效细化该合金的晶粒,块状初晶硅尺寸明显减小且棱角钝化,针状共晶硅呈颗粒状弥散分布于基体中,300℃下挤压16道次后合金的室温(11℃)冲击功约为铸态的5倍;低温下,挤压16道次后合金的冲击性能随温度下降变化不大;当冲击温度由室温升高至100℃时,合金的冲击性能有所下降;铸态试样的断裂机制以脆性断裂为主,挤压后试样的断裂机制以韧性断裂为主。     

多模式超声振动等径角挤压超细晶纯铝成形机理研究

超细晶金属材料由于具有优异的力学性能,特别适合微小金属零件的塑性成形。大塑性变形法是制备超细晶金属材料的常用方法,等径角挤压法被认为是最具有发展前景的大塑性变形方法之一。传统等径角挤压需要通过多道次的应变量累积来获得超细晶材料,制备效率较低。将超声振动与等径角挤压过程相结合可以有效减小挤压成形载荷,提高等径角挤压制备超细晶的性能和效率。现有研究主要采用工具辅助超声振动模式,提出并研发基于工件辅助超声振动模式的等径角挤压成形工艺,并对不同超声振动模式1070纯铝等径角挤压成形机理进行对比研究,研究工具超声振动和工件超声振动两种不同振动方式对晶粒道次细化能力的影响规律。结果表明,随着超声功率的增大,工具超声振动和工件超声振动的超声软化效应逐渐增强,能更大幅度降低等径角挤压成形力,并提高晶粒道次细化能力。工件超声振动比工具超声振动更有利于吸收超声能量,从而能更有效提升超细晶金属的制备效率。     

Structure and microstructure evolution of Al–Mg–Si alloy processed by equal-channel angular pressing

An ultrafine grained Al–Mg–Si alloy was prepared by severe plastic deformation using the equal-channel angular pressing (ECAP) method. Samples were ECAPed through a die with an inner angle of Φ = 90° and outer arc of curvature of ψ = 37° from 1 to 12 ECAP passes at room temperature following route B_c. To analyze the evolution of the microstructure at increasing ECAP passes, X-ray diffraction and electron backscatter diffraction analyses were carried out. The results revealed two distinct processing regimes, namely (i) from 1 to 5 passes, the microstructure evolved from elongated grains and sub-grains to a rather equiaxed array of ultrafine grains and (ii) from 5 to 12 passes where no change in the morphology and average grain size was noticed. In the overall behavior, the boundary misorientation angle and the fraction of high-angle boundaries increase rapidly up to 5 passes and at a lower rate from 5 to 12 passes. The crystallite size decreased down to about 45 nm with the increase in deformation. The influence of deformation on precipitate evolution in the Al–Mg–Si alloy was also studied by differential scanning calorimetry. A significant decrease in the peak temperature associated to the 50% of recrystallization was observed at increasing ECAP passes.     

Modeling of the processing force for performing ECAP of circular cross-section materials by the UBM

Equal channel angular extrusion or pressing (ECAP) is a process used to impart severe plastic deformations (SPD) (ε?<<?1) to materials with the aim of improving their mechanical properties by reducing the grain size. In this study, an analytical modeling of the ECAP processing force required, taking circular cross-section for ECAP dies into consideration, is developed where non-strain hardening materials are processed. To obtain the equation that relates the geometry and required force, the upper bound method was used after taking an appropriate and admissible field into consideration. In addition, a comparison between analytical methods and experimental results was made. In order to perform the experimental tests, an F-1050-AA (F means as fabricated) was selected and processed at room temperature. This alloy has a yield stress of 70?MPa and an insignificant strain hardening. The experimental results obtained agree closely with those provided by the analytical formulation. With this study, it is possible to have an analytical approach to the required force for performing the ECAP process. This could help scientists and practical engineers involved in SPD processes such as ECAP, to optimize ECAP dies and the process itself thanks to the knowledge of the analytical expressions of the required force.     

A microstructural investigation of deformation in fine SiC particulate- and whisker-reinforced aluminium-matrix composites

Aluminium-based composites, reinforced with low volume fractions of whiskers and small particles, have been formed by a powder route. The materials have been tested in tension, and the microstructures examined using transmission electron microscopy. The whisker composites showed an improvement in flow stress over the particulate composites, and this was linked to an initially enhanced work-hardening rate in the whisker composites. The overall dislocation densities were estimated to be somewhat higher in the whisker composites than the particulate composites, but in the early stages of deformation the distribution was rather different, with deformation in the whisker material being far more localized and inhomogeneous. This factor, together with differences in the internal stress distribution in the materials, is used to explain the difference in mechanical properties.