【作者】 Kawunga Nyirenda；

【导师】 刘庆；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2013， 博士

【摘要】 在过去的十年中，晶粒尺寸在1μm以下的纳米晶和超细晶激起了人们的极大兴趣，这是因为，与传统金属材料相比它们有优异的力学性能：强度和延展性。因此，超细晶材料在汽车和航空领域中，作为高耐久性的结构组件，有极大的潜在空间。在此研究中，运用一种叫做累积叠轧的剧烈塑性变形技术加工了AA7075/AA1100多层复合板，以此通过产生超细晶的微观组织来改善其力学性能。累积叠轧是一种在金属合金中产生超细晶或亚微米晶粒的优异加工技术。在累积叠轧过程中，用钢丝刷将样品表面的氧化物除去，相互之间堆垛在一起，以厚度方向变形量为50%进行轧制。在轧制过程中结合的金属板材（叠轧的名字由此而来）和这个生产过程可以重复数次。金属材料在此过程中经受了强烈的塑性和剪切变形，导致了超细晶的微观组织的形成。与其他的剧烈塑性变形方法相比较，累积叠轧技术其中的一个优势就是可以并入到工业生产中来产生大量的超细晶金属板材，这个过程不需要对传统轧制工厂做重大的改变。平均晶粒尺寸在1μm以下的超细晶金属材料预期有更好的力学性能。为了将超细晶材料运用到实际生产中，它们必须有一定的体积尺度。对生产亚微米晶粒尺寸的块状的材料，累积叠轧是比较有前途的技术。在SPD技术产生以前，金属材料的晶粒细化都是通过传统的塑性加工和随后的退火处理导致的再结晶（不连续再结晶）得到的。通过传统方法可得到的晶粒平均尺寸在10μm左右，传统工业的冷轧总变形量在60-80%，相对应的米塞斯应变在1.06-1.86之间，然而ARB工艺的应变可能达到4.0。这就使变形金属产生大量超细晶显微结构成为可能。本研究工作的主要目的是，通过累积叠轧变形技术和后续的人工时效来同时提高AA1100/AA7075多层复合板的强度和延展性。同时，其他的一些目的有：(i)是否本实验结果的力学性能要比其他文献中报导的好或者比经过ARB变形的其他相似金属材料的性能好；(ii)是否可以通过ARB后续技术改善材料的机械性能，也就是在加工过的板材上运用热处理和其它的一些适用的技术；(iii)研究AA1100/AA7075累积叠轧板材微观组织的演变；(iv)优化AA1100/AA7075板材的累积叠轧温度，最后检验组份材料（高强AA7075铝合金和高延展性AA1100纯铝）联合作用的效果。本研究主要集中在ARB加工的AA7075/AA1100超细晶金属板材，由于它们高的比强度，对于汽车和航空领域的轻量化结构尤其有意义。本研究中的叠轧第一步是在470°C下通过第一道次加工一层复合板，然后通过后续的道次继续轧制，从初始厚度的11.4mm到1mm。在冷轧之前要进行道次退火。由AA7075和AA1100组成的复合板被用作累积叠轧技术的初始材料。累积叠轧要进行5个道次，每个道次的等效应变为0.8。研究多层复合板的机械性能和微观组织研究。本研究工作中应用的热处理技术是T6，因为它可以给予AA7075铝合金最佳的性能。由于AA1100是不能热处理的并且只能通过应变强化来增强，因此T6技术是本研究中对累积叠轧板材最好的热处理方式。本研究中通过拉伸实验和显微硬度实验分析了材料的力学性能。拉伸实验中，抗拉强度、屈服强度和延展性被作为材料的性能指标。事实就是这样，屈服强度赋予了材料的工作强度以避免在服役过程中出现灾难性的失效。抗拉强度表示材料或组件在最终突然断裂之前所能承受的最大应力。结果表明，复合板的抗拉强度、屈服应力要比初始材料的好。为了找出淬硬性是否存在AA1100/AA7075累积叠轧复合板中，分析了显微硬度。结果显示，AA1100/AA7075累积叠轧复合板的显微硬度要比单一的材料好。本研究中得到了超细晶显微组织，正如期望的那样，通过细晶强化、析出强化、超细晶材料的形成及界面反应的共同作用可以得到一种高性能的材料。由于延展性是韧度的一个性能指标，因此它的提高可以被认为是韧度的改善。在多层板复合材料中，通过与AA1100纯铝的结合，由于叠轧的内在和外在机制，AA7075铝合金的延展性得到了提高。由于纯铝是易于延展的，在铝片层之间有裂纹钝化的发生。预计会带来一个改善的韧化层，并有望减少对界面强化的需求。与纯铝相比，单一的运用AA7075铝合金是昂贵的，因此AA1100和AA7075的结合在经济效益上是吸引人地，因为与单一运用AA7075铝合金相比来说花费会降低。本研究的结果清晰地显示可以通过累积叠轧技术得到高强的超细晶块状铝。实际上这是很重要的，因为轧制是产生块状材料的最合适工艺。如果此工艺运用到实际生产，我们可以通过一个没有复杂的热机械处理的简单过程，可以不用合金元素而得到高强而简单的材料。这个符合最近社会上对循环和节省能源的需求。本课题主要结论如下：①随着累积叠轧次数的增加，铝合金7075/1100多层复合板机械性能随之增加。②在本课题中，薄板机械性能提高所涉及到的强化机制主要包括：超细晶强化，应变强化、固溶强化、位错强化、晶界强化和析出强化。③高温累积叠轧是生产轻质多层铝板的有效技术手段。高的轧制温度使得铝板在轧制过程中得到更好的变形且板材多层之间的结合也较好。④在本课题累积叠轧工艺中，存在两类变形机制的变化。一类是样品表面区域的剪切变形到中心的平面应变轧制。另一类是由于在反复的轧制过程中轧制方向不断变化导致应变路径是双向的，因此，晶粒细化的速度比常规轧制工艺快。最后，由于通过累积叠轧技术加工得到的层状复合板有较高的强度，因此建议AA7075/AA1100的运用。这需要在工程上对层状复合材料和超细晶材料的先进应用创新潜力要高，对生产这类材料在经济上是可行的。更多还原

【Abstract】 During the last decade, nanocrystalline and ultrafine-grained (UFG) materials witha grain size of less than1μm have aroused considerable interest due to their superiormechanical properties in terms of strength and/or ductility compared to conventionallygrained materials. For this reason, these UFG materials have got high potential forengineering applications in high durability structural components in the automotive andaerospace industries.In this work, the AA7075/AA1100multilayer sheet was processed by a SeverePlastic Deformation (SPD) technique called Accumulative Roll Bonding (ARB) in orderto produce an ultrafine-grained microstructure and improve the mechanical properties.Accumulative Roll-Bonding (ARB) process is an excellent and new processingtechnique for establishing ultra-fine (sub-micrometer) grains in metallic alloys.During the ARB process, the metal sheet surfaces are wire brushed in order toremove the oxide layer, stacked on top of each other and rolled together with a thicknessreduction of50%. The metal sheets bond together during rolling (hence the name "rollbonding") and the procedure can then be repeated any number of times. The material issubjected to very high plastic and shear deformation which results in the formation ofthe UFG microstructure.One of the advantages of the ARB process in comparison to other SPD methods isthat it is a continuous process which can be incorporated in industry to produce largescale UFG metallic sheets with unimportant modifications to the conventional rollingplant.Ultrafine grained (UFG) metallic materials whose mean grain size is smaller than1μm are expected to have better mechanical properties. In order to put the UFGmaterials to practical use, they must have bulky dimensions. The ARB is a promisingprocess to fabricate bulky materials with submicrometer grain sizes. Before SPDprocesses came into being, grain refinement of metallic materials was being achieved byconventional plastic working and subsequent annealing which results in recrystallizationby nucleation and growth (discontinuous recrystallization). The achievable mean grainsize in the conventional way is about10μm and the total reduction in industrialconventional cold rolling is60~80%which corresponds to von mises’ true strain (ε) of 1.06~1.86, while strains of over4.0are possible with the ARB process. This allowsUFG microstructures to form in heavily deformed materials.The main objective of this work was to find out whether both the strength andductility can be improved by deforming the AA1100/AA7075multilayer sheet using theARB process followed by artificial ageing while the minor objectives included (i) tofind out whether the mechanical properties are better than those reported in literature forsimilar metallic materials processed by the ARB technique,(ii) to find out whether themechanical properties can be improved by post ARB techniques, this means that by heattreatment processes and other suitable techniques that may be applied on the processedsheets,(iii) to find out the microstructure evolution for the AA1100/AA7075ARB sheetand (iv) to find out the optimal accumulative roll bonding temperature for theAA1100/AA7075ARB sheet and finally the work examined the combined effects of theconstituent materials, the high strength of the AA7075alloy and the ductility of purealuminum.This study focused on the ARB processed AA7075/AA1100Ultrafine-grainedmetal sheets since they are especially interesting for light weight construction in theautomobile and aerospace industries due to their high specific strength. The roll bondingin this work was done by first fabricating a clad sheet at a temperature of470°C for thefirst cycle and cold rolled for the subsequent cycles from an initial total thickness ofabout11.4mm to about1mm. Process annealing was done before cold rolling. The cladsheet consisting of AA7075and AA1100was then used as the starting material for theARB process. The ARB was done for a total number of five cycles (equivalent strain of0.8per cycle). The mechanical properties and microstructure evolution of the multilayersheet was then investigated.The heat treatment technique which was applied in this research work was the T6temper because it’s the heat treatment that gives optimal properties to the AA7075aluminum alloy. Since the AA1100is non heat treatable and only strengthened by strainhardening, this T6temper has been found to be the best for the ARB sheet to beprocessed in this work.The mechanical properties that were analyzed in this work are the tensile test andthe microhardness. In the tensile test, the ultimate tensile strength, the yield strength andthe ductility of the material were considered. This is so, because the yield stress givesthe working strength of the material so as to avoid catastrophic failure of the materialwhen in service. The ultimate tensile strength gives the maximum stress that the material or component can be able to sustain before final rapture occurs. The ductility,tensile strength and yield stress were found to be better than those for the initial material.The microhardness was analyzed since we needed to know if hardenability occursin the AA1100/AA7075multilayer ARB sheet. It was also found that the microhardnessof the multilayer AA1100/AA7075sheet was better than that for the monolithicmaterials.The UFG microstructure was obtained in this research work and as was expected,this research work showed that a high-performance material can be obtained by thecombined actions of fine-grain strengthening, precipitation strengthening, UFG materialformation and interface reaction. Since ductility is a property of toughness, itsimprovement was considered as an improvement in the toughness. This improvement inductility of AA7075alloy by combining with pure aluminum AA1100in multilayerlaminate material is due to roll bonding intrinsic and extrinsic mechanisms. Since purealuminum is very ductile, crack blunting was occurring within the aluminum layers.This brought about an improved lamination as well as a reduction in the need for theinterfacial strength. The use of monolithic AA7075is expensive as compared to purealuminum, therefore, the bonding of AA1100and AA7075will be economicallyattractive because the costs will be lowered as compared to the use of monolithicAA7075alloy.The results obtained in this research clearly showed that ultra-fine grained bulkaluminum with surprising strength can be readily obtained by ARB process. It ispractically very important because rolling is the most appropriate process to produce thebulk materials. If this process were applied to practical use, we could obtain high-strength and simple materials without alloying elements by a simple process withoutcomplicated thermo mechanical treatment. This satisfies the recent social demands ofrecycling and energy saving.The following are among the conclusions which have been made in this work:①The mechanical properties of the AA7075/AA1100multilayer sheet increasedwith the number of cycles.②The strengthening mechanisms such as, grain refinement to UFG levels, solidsolution strengthening, strain hardening, dislocation hardening, grain boundarystrengthening as well as precipitation hardening contributed to the improvement in themechanical properties of the laminate. ③Hot roll bonding can be used as a deformation and bonding method to producelight multilayer aluminum sheets with great relevance for technical applications. Thehigh processing temperatures led to better deformation of metal sheets during rolling aswell as better interlamellar bonding between the sheets.④In the present ARB process, two types of changes in deformation mode werefound to exist. One is change from shear deformation (in the surface regions) to plainstrain rolling (in the center). The other is that strain path is reversible due to a change ofrolling direction in alternate cycles. Therefore, the process of grain refinement can beaccelerated than the conventional rolling process.Finally, this work recommends the application of the AA7075/AA1100since ahigh strength material has been obtained by processing this multilayer sheet using theARB process. It is also recommended that future work should vary the volume fractionswhich were not done due to time constraint. The innovation potential of laminatedcomposite materials for advanced applications in engineering is very high and therequirements for producing such materials are economically feasible.更多还原