Research On The Mechanical Properties Of The AA7075/AA1100Multilayer Sheet Processed By The Accumulative Roll Bonding(ARB) Process

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.