【作者】 周明智；

【导师】 薛克敏；

【作者基本信息】 合肥工业大学 ， 材料加工工程， 2008， 博士

【摘要】 大塑性变形法(severe plastic deformation,SPD)是一种制备块体超细晶材料的新型塑性加工方法。作为大塑性变形法的典型代表,等通道转角挤压法(equal channel angular extrusion,ECAE)能显著细化晶粒,且具有一定的工业应用潜力,已成为材料科学与工程领域内的研究热点。粉末冶金材料是材料领域内的重要组成部分,由于孔隙的存在,使其物理和力学性能受到影响。消除孔隙、改善组织结构,提高材料综合力学性能是粉末冶金技术和塑性变形工艺的重要目标。当前,大塑性变形工艺的主要对象是致密材料,对粉末冶金材料的相关研究尚处于起步阶段。粉末材料的塑性加工能力相对致密材料而言较弱,其塑性变形、致密和细化机理尤为复杂。由于缺乏该类材料在变形过程中理论分析研究,从而限制了大塑性变形工艺在该领域内的发展和应用。为此,本文将有限元数值模拟、实验分析研究相结合,全面深入地研究了粉末材料在ECAE过程中的变形机理、致密行为及晶粒细化规律,从而为大塑性变形方法在该领域的应用提供必要的理论基础和依据。等通道转角挤压过程中,试样内部的微观组织结构同变形、温度等宏观场量参数大小及分布存在密切的联系。因此,获得在挤压变形过程中试样的流动信息和相关场量参数的分布状况,对于选择合理的工艺参数、优化模具结构,进而实现对变形过程的主动控制十分重要。本文针对粉末多孔烧结材料的特点,在基于可压缩连续介质理论的基础上,推导出可压缩性刚粘塑性热力耦合有限元列式,建立了粉末多孔烧结材料和基体材料之间的物性参数关系,从而从理论上解决了采用热力耦合方法模拟粉末材料塑性加工过程的关键问题。建立了用于分析粉末多孔烧结材料ECAE的热力耦合有限元模型,对纯铝粉末烧结材料的ECAE过程进行数值模拟分析,获得材料在ECAE过程中流动信息、变形行为、温度分布和致密过程。数值模拟表明,等通道转角挤压对粉末材料具有优良的致密效果,可有效地消除其内部的孔隙。试样所获的密度分布特征同应变分布基本一致,说明ECAE提供的大剪切变形对孔隙闭合十分有利。在上述研究基础上,进一步对不同变形条件下的挤压过程进行大面积数值模拟,总结了模具几何形状及变形工艺参数对金属流动、变形、致密及所需压力载荷的影响规律。研究结果表明,模具通道内角是影响挤压效果的关键要素,采用较小的内角有利于提高试样挤压时流动的均匀性、获得大剪切变形量以及高致密度,但过小的内角将导致外角处形成流动死区,对挤压变形效果不利。模具外圆角大小的影响主要表现在对底部区域金属的流动上,其效果随内角的减小而增强,特别当模具的内角为锐角时,将对致密效果、变形均匀性产生明显的影响。通过本文研究认为,在材料塑性可加工性能允许的条件下,模具几何形状的选择应尽量选择较小的内角,配合适当的外圆角改善外角部的金属流动,可获得满意的变形及致密效果。对不同接触摩擦状况下的数值模拟研究表明,一定大小的接触摩擦对试样整体获得大变形量、变形均匀性及致密效果都是积极有利的。通过对粉末材料不同路径进行多道次挤压过程模拟,给出了多道次挤压结果。结果表明,随着挤压道次增加和应变量的累积,试样的致密度逐步提高;采用路径A多道次可逐步减少试样端部小变形区的面积,但随着挤压道次的增加试样主要变形区分布将趋于复杂和不均匀;与路径A相比,路径C经过偶数道次挤压后可以获得更加均匀对称的变形分布。为获得试样横截面宽度(Y)方向的应变分布,本文建立了用于方形截面试件的三维有限元热力耦合有限元模型。三维有限元模拟分析结果表明,在接触摩擦的影响下,应变沿横截面宽度(Y)方向分布不均匀,其中心部较小,而靠近模具表面较大,但同摩擦对高度(Z)方向应变的影响相比较小,因此在润滑条件良好的情况下,采用二维模拟的方法仍具有较高的精度。通过对路径B_A和B_C的多道次挤压进行数值模拟,获得路径B_A、B_C对挤压效果的影响规律。研究结果表明,采用路径B_A、B_C两道次挤压后,试样横截面内部的应变分布不均匀,剪切变形主要集中于两道次的剪切面的交叉处。路径B_A随着挤压道次的增加,试样在沿其横截面和纵向截面方向的变形都将趋于不均匀;而路径B_C随着挤压道次的增加,试样的各表面都将受到剪切变形作用,其横截面的变形将逐步趋于均匀;特别是挤压4个道次时,试样刚好完成一个挤压周期,可以获得较为均匀的变形分布。在有限元模拟分析的基础上,设计了实验模具、专用挤压设备和加热装置,采用真空烧结的方法制备了实验毛坯,并对不同条件下纯铝粉末烧结试样的进行实验,制备出具有理论压实密度和超细结构块体材料。单道次挤压的实验结果表明,等通道转角挤压具有很强的致密效果,基体组织具有明显的剪切变形特征,不同区域的基体组织变形及孔隙的分布状况同有限元模拟结果相吻合,从而验证了本文所建立的有限元模型的可靠性。对挤压试样的力学性能测试表明,一道次挤压后,表面显微硬度大幅上升,说明ECAE可显著提高粉末材料的力学性能。通过光学显微镜、扫描电子显微镜、透射电镜对挤压材料在不同变形条件下的显微组织进行观察分析,对材料ECAE过程的孔隙和显微组织的演化过程进行研究。研究认为,单道次ECAE过程中的孔隙闭合效果取决于剪切变形特点和试样所处的应力状态,即大剪切塑性变形和高静水压力状态是材料得以获得良好的致密效果的关键所在。多道次挤压由于变形量的累积和不同的剪切特征不断地改变内部的孔隙形状和内部基体材料的重排使得材料进一步致密。通过对挤压试样的TEM观察,指出变形诱导机制是粉末材料细化的主要机制,而晶粒的细化效果取决于静水压力、变形量、剪切特征等关键因素。最后,本文在数值模拟和实验分析的基础上,针对粉末多孔材料在传统ECAE工艺中存在的问题进一步提出改进的粉末包套等通道转角挤压工艺(powder in tubes-equal channelangular extrusion,PITS-ECAE)和带反压的等通道转角挤压工艺(equal channel angularextrusion with back pressure,BP-ECAE)。实验研究结果表明,采用PITS-ECAE工艺可有效降低挤压过程变形材料产生破坏的可能性,从而为粉末材料获得良好的变形累积效果提供重要的保证。而BP-ECAE工艺可以在有效地降低挤压材料产生破坏的可能性的同时提高变形均匀性和致密效果,并且能在较低温度条件下实现低塑性材料大塑性变形,从而能更加有效改善材料的组织和力学性能。更多还原

【Abstract】 Severe plastic deformation (SPD) is a new method to obtain ultra-fine grained (UFG) materials. As a typical method of SPD, the equal channel angular extrusion processing (ECAE) with great potential of industrial application can refine grain effectively and now has become an attractive research field in material science and engineering.Powder metallurgy material is an important part of material family. But its mechanical characters are often affected due to pore inside. Removing inner porosity, refining microstructure and improving its mechanical characters are important aims of powder plastic deformation technology. Currently, the main object of SPD technology is full dense material, while the related study on sintered powder material is just beginning. The workability of powder material is weaker than that of full dense material, and its plastic deformation, densification and refinement mechanism are complicated. Lack of theoretical study on this kind material during deformation confines its development and application. Therefore, this paper combined finite element numerical simulation with experiment method, deeply investigated the deformation, densification behavior and grain refinement regulation of powder material during ECAE processing, and finally provided essential theoretical foundation for manufacturing the same material with SPD.During equal channel angular extrusion processing, the microstructure of metal has intimate relationship with field variables, such as strain and temperature etc. Therefore, it is important to get the flowage information of the metal and the distribution of related field variables for choosing reasonable technical parameters, optimizing die structure and realizing initiative control of the whole deformation processing. Based on the characters of porous powder material and the compressible continuous medium theories, this paper deduced the compressible rigid viscoplastic thermodynamic coupling finite element formula, built the physical property parameters relationship of porous material and full dense material, thereby solved the key problem of simulating powder material plastic deformation processing with thermodynamic coupling method from theory aspects. Thermo-coupling FE model for the simulation analysis of pure aluminum ECAE powder material processing was built and the flowage information of the material, the deformation behavior and temperature of material in ECAE were also obtained. The simulation results show that ECAE has excellent densification effect for powder material, and it can effectively remove inner pore, and the sample’s density distribution is basically concord with its strain distribution, which shows the shear deformation character offered by ECAE and is beneficial for closing pores.Based on the above research, comprehensive numerical simulation was done for pure aluminum extrusion under different conditions, and the effect rule of die geometry shape and relative technical parameter to the needed press load which make metal flowage, deformation and densification was concluded. The results show the key parameter is die channel angle, and a small channel angle is good for improving the metal flowage uniformity and getting a large strain value and high density, meanwhile an excessive small channel angle would cause flowage dead zone in outer angler of the die, which is not good for deformation. The outer round angle has an effect on metal flowage which is mainly acted on bottom metal flowage, and its effect grows as the channel angle decrease. Especially when channel angle is acute angle, it will produce extinguish effect on the whole sample densification and homogeneous deformation. Based on the investigation of this paper, under workability permission of the material, die geometry should choose as small channel angle as possible, match with proper out round angle to improve metal flowage in outer angle. Finally satisfied deformation and density can be achieved. The result of simulation under different contact friction shows a certain number of contact friction is beneficial for obtaining large strain value, homogeneous longitudinal deformation and dense material. As to powder material, multiple passes extrusion was simulated through different route, and the multiple passes extrusion results were given. The results show, as extrusion passes and accumulated strain increase, the density of sample is improved; through route A, multiple passes can diminish small sheared area at the head and tail of sample, but as extrusion passes increases, the main deformation zone distribution will become complicated and inhomogeneous; compared with route A, route C under even number passes extrusion, homogeneous and symmetrical deformation zone distribution will be acquired.To get the relative flowage information at the cross section of sample, this paper built 3D Thermo-coupling FE model for square section sample. 3D FE simulation results show, under contact friction influence, strain distribution at cross section is not homogeneous, and strain value is small at the core of the sample, while near the die surface has bigger strain with comparatively weak influence on longitudinal strain. Therefore, under the condition of small friction, 2D FEM still could have high precision. Multiple passes extrusion was simulated through extrusion route B_A and B_C, and influence of route B_A and B_C on deformation were achieved. The results show, after two passes through route B_A and B_C, the sample strain distribution turns to be inhomogeneous, and shear deformation mainly concentrates on cross position of two passes shear surface. Through route B_A, as extrusion passes increase, the deformation of sample longitudinal and cross section become inhomogeneous; while through route B_C, all surfaces are under shear, as extrusion passes increase, deformation of cross section becomes even step by step. Especially when four passes extrusion is done, with an extrusion cycle period is finished, which results in a more homogeneous strain distribution.Based on FEM analysis, this paper designed the die for ECAE experiment, the specialized extrusion equipment and heating device, fabricated extrusion roughcast with vacuum sintered method, conducted extrusion experiment for pure aluminum powder sintered material under different conditions, and successfully made bulk material with full density and ultra-fine structure. Single pass extrusion experiment results show, ECAE has powerful densification effect; grain has obvious shear deformation character; grain deformation and pore distribution at different zone are accord well with FEM simulation results, thereby testifying the reliability of FEM model built in this paper.Through optical telescope, scan electronic telescope and transmission electronic telescope, observation and analysis were done for microstructure of deformation material under different deforming conditions, and evolvement of pore and microstructure during ECAE process were studied. The results show, in single ECAE process closing effect of pore depends on shear deformation character and stress state of the sample, which is the key factors to obtain a high density material under large shear plastic deformation and high hydrostatic stress state. According to accumulated deformation and different shear deformation character multiple passes rearrange the powder, change pore shape, compact powder material further. Through TEM observation of deformation sample, pointing out deformation inducement mechanism is the main mechanism to refine powder material, and the grain refinement effect depends on some key factors such as hydrostatic stress, strain value and shear deformation character etc.Finally, based on numerical simulation and experiment analysis, aiming at problems in traditional ECAE technique of powder material, this paper proposed two improved techniques: powder in tubes-equal channel angular extrusion, PITS-ECAE, equal channel angular extrusion with back pressure, BP-ECAE, and systematical theoretical analysis was done by numerical simulation experiment method on extrusion effect. The results show, improved ECAE can effectively improve deformation homogenization and consolidation effects, meanwhile reduce the possibility of material demolishment. Especially, BP-ECAE can realize severe plastic deformation for low ductile material under relative low temperature, and consequently improve its mechanical properties and microstructure effectively.更多还原