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Int J Adv Manuf Technol (2016) 83:1637–1647 DOI 10.1007/s00170-015-7695-9 ORIGINAL ARTICLE Formation characteristic, microstructure, and mechanical performances of aluminum-based components by friction stir additive manufacturing Mao Yuqing 1 & Ke Liming 1,2 & Huang Chunping 2 & Liu Fencheng 2 & Liu Qiang 1 Received: 19 May 2015 / Accepted: 3 August 2015 / Published online: 18 August 2015 # Springer-Verlag London 2015 Abstract In this study, a new solid-state technique of friction
  ORIGINAL ARTICLE Formation characteristic, microstructure,and mechanical performances of aluminum-based componentsby friction stir additive manufacturing Mao Yuqing 1 &  Ke Liming 1,2 &  Huang Chunping 2 &  Liu Fencheng 2 &  Liu Qiang 1 Received: 19 May 2015 /Accepted: 3 August 2015 /Published online: 18 August 2015 # Springer-Verlag London 2015 Abstract  In this study, a new solid-state technique of frictionstir additive manufacturing (FSAM) based on friction stir welding (FSW) principle was used to build successfully a multilayered stack of an Al-based component. The resultsshow that a hook stretches into the nugget zone on advancingside, while it moves upwards to the periphery on retreatingside for a single-level welding. With manufacturing the sec-ond layer, the hooks bend outward significantly attributing tothe extrusion of above plastic material, which can avoid thehook to stretch into the stirred zone. A transition zone (TZ) isalsoformednearthe interfacebetween two layers. Inaddition,fine equiaxed grains are observed due to the dynamic recrys-tallization in the whole. However, a difference in grain sizestill existsthrough the build direction and inthe TZisformingcoarse band grains. A similar change occurs in the precipitatemorphology, size, and distribution. Form the top to the bot-tom,themicrohardnesschangesdramatically,andamaximum115 HVat the top is obtained. The tensile strength of all theslices increases and the elongation decreases slightly in com- parison of Al substrate, and the slice top has the highest me-chanical properties, which is attributed to fine grains and de-sirable precipitate characterization. Keywords  Frictionstiradditivemanufacturing .Aluminumsubstrate .Formationcharacteristic .Microstructure .Mechanicalproperties .Fracturemorphologies 1 Introduction A total of 7075 aluminum alloy, as a precipitation hardened,wrought material based on Al-Mg-Zn-Cu system, which isconsidered as one of the strongest aluminum alloys. Mean-while, due to the attractive combination of excellent weldability, high strength to weight ratio, high heat conduc-tivity, and good corrosion resistance, AA7075 has been wide-ly used to produce some components in aviation, aerospacefields, and domestic industries [1, 2]. However, a few alumi- num matrix parts pertaining to the cabin and cockpit are nor-mally in irregular and complex configurations, which are dif-ficult to fabricate by the conventional processing methodssuchascasting processes.Inaddition,duringthe conventionalcasting processes, coarser and heterogeneous microstructuresand large degrees of segregation are very easy to generate dueto the slow solidification rates [3, 4]. Therefore, such applica- tions of the traditional casting processes are limited, and somenewprocessingmethodsarehighlydesiredtocatertotheneedfor obtaining complex components with fine and uniform mi-crostructures and the stringent performance requirements laid by the aviation industry.Additive manufacturing (AM), defined as the process of  joining materials to make objects from three-dimensional(3D) model data, is a new generation of manufacturing pro-cesses in which some parts are fabricated by layer-by-layer addition of materials as opposed to traditional subtractivemanufacturing methodologies, such as machining or materialdeformation processes, and is considered to be the most sig-nificant manufacturing approach to have emerged in the last  *  Ke Limingliming_ke@126.comMao Yuqingmaoyuqing1987@mail.nwpu.edu.cn 1 State Key Laboratory of Solidification Processing, NorthwesternPolytechnical University, Xi ’ an 710072, People ’ s Republic of China  2  National Defence Key Discipline Laboratory of Light AlloyProcessing Science and Technology, Nanchang HangkongUniversity, Nanchang 330063, People ’ s Republic of China Int J Adv Manuf Technol (2016) 83:1637  –  1647DOI 10.1007/s00170-015-7695-9  few decades [5  –  7]. The AM technology attains considerablerecognitionandadoptioninvariousindustrysectorseversincethe first AM process is introduced in late 1980s [8]. Such a spur in the field of additive manufacturing is realized by thecombination of several techniques and comprehensively doc-umented in some review papers [9  –  11]. Though additivemanufacturing comes to the forefront by virtue of several in-novations that paves its way, lots of key challenges still per-sist. For the metal-based additive technologies, some specialattentions are laid on the specific characteristics such as buildrate, build volume, layer thickness, and the overlapped struc-ture between layer and layer.At present, several metal-based additive technologies in-clude mainly shaped metal deposition [12], laser additivemanufacturing techniques [13], and electron beam melting[14], which are heavily used to fabricate majority of multilay-ered metal parts. On the one hand, shaped metal depositioncan lead to higher production rates, but it is at the expense of surface finish and dimensional accuracy. Moreover, when la-ser additive manufacturing techniques is applied, though a good surface finish is obtained, the process is still limiteddue to higher operating costs, low production rate, and small build volumes. On the other hand, some components pro-duced by electron beam deposition perform better mechanical properties, but it is limited to use owing to the cost factor  pertaining to the use of inert atmosphere. Other issues includethatpowderparticles-basedsubstrateusedinallthethreetech-niques are usually contaminated; during the additive process,the substrate is melt into the liquid, and it is easy to forminternal porosity, inclusions, and other solidification defects.In addition, the microstructures of the manufactured compo-nent are extremely inhomogeneous and its structural proper-ties are spatially dependent [15, 16]. Other techniques such as ultrasonic additive manufacturing have the ability to producemulti-material components, but the microstructures are highlynonuniform and there exist great differences between the in-terfaces and interfaces or noninterfaces, so the mechanical properties are inferior compared with base material [10].Therefore, a desired requirement for any manufactured com- ponent is necessary to possess a homogeneous microstructureand better properties.Friction stir additive manufacturing (FSAM), as a newenvironmentally friendly, energy-effective solid-state processtechnology based on friction stir welding (FSW), has a huge potential to fabricate lightweight materials with high struc-tural performances [17]. In FSAM process, the metal sub-strates such as aluminum/magnesium alloys are being proc-essed but do not melt and recast, which can avoid efficientlysome internal porosities and solidification defects. The fun-damentals of FSAM procedure are easily understood, and it is similar to the FSW process while it differs in their respec-tive functions. In general, a nonconsumable rotating toolwith a special pin and shoulder is inserted into slowly theoverlapping surfaces of the plates/sheets and subsequentlymoves forward along the joint line to be joined. The sche-matic of the FSAM process is shown in Fig. 1. Furthermore,the necessary heat to weld aluminum substrates is provided by the frictional heat between the rotating shoulder and theworkpiece and the deformation heat achieved by the motionof the pin, which makes metal substrate deform to result in a circulatory flow of plasticized materials around the pin sur-face. The plasticized materials underneath the shoulder aresubjected to extruding by the rotation and the traverse move-ment of the pin from the advancing side to the retreatingside, and the weld nugget is formed, while its macro shapeis depended on the geometrical features of the pin. Currently,the reports relating to FSAM technology are extremely lim-ited. The friction joining used for the additive manufacturingis under the patent, but it is only an assumption as a possibleroute to build 3-D layers [18]. Palanivel et al. [19] reported recently that the FSAM as a potential technique can attainefficiently Mg-based components with high structural perfor-mances through controlling the microstructures. But, the re- ports of friction stir additive manufacturing Al-based partsare not pursued leading to a lack of literatures about FSAM,and some further studies such as the formation quality, de-fects, and microstructure evolution are also lacked. There-fore, the aim of this study is to investigate a multilayered build of an Al substrate obtained by FSAM technique, andsequentially, stacking layers are performed over the previ-ously processed layer. For the purpose, a sound tool is de-signed to join the Al plates, and the formation characteriza-tion, microstructure evolution in the interfaces, andnoninterfacial zones and mechanical properties of the FSAMAl-based component are further studied. Welding directionXYZTool rotationShoulderAdvancing sideStirred zoneRetreating side Fig. 1  Schematic illustration of friction stir additive manufacturing(FSAM) process1638 Int J Adv Manuf Technol (2016) 83:1637  –  1647  2 Experimental procedures In the present study, 5-mm thick plates of 7075 aluminumalloy in annealed condition were used as base material toadditively manufacture. The nominal chemical compositionsofAA7075-OarelistedinTable1,andthetensilestrengthandelongation of AA7075-O plate are 225 MPa, 17.9 %, respec-tively. The plateswitha sizeof200×40×5 mm were preparedfor longitudinally friction stir lap welding on a modifiedhorizontal-type milling machine, and the welding directionwas parallel to the rolling direction. A stack of nine layerswith a height of about 42 mm was produced by sequentially building and machining of the lap welded layers. FSAM wascarriedoutusingatoolmadeofGH4169steelwithaconcave-shaped shoulder and a left cylindrical threaded pin made. Theshoulder diameter was 30 mm, and the pin diameter and pinheight were 14 and 5.2 mm, respectively. Meanwhile, a con-stant tilt angle of the rotating tool of 2° from the vertical axisof FSW machine was used to contain the material beneath theshoulder, and a plunge depth of 0.2 mm of the pin was kept.The same welding parameters such as a constant rotationspeed of 600 rpm and a fixed traverse speed of 60 mm/minwere used during FSAM (the process parameters were opti-mized after lots of trial and error).The specimens for metallographic examination and hard-ness evaluation were cross-sectioned from the welded layers perpendicular to the welding direction, which were groundand polished following a standard metallographic process.The microstructural features in different positions were ob-served by etching in a mixed acid solution of 5 ml nitric acid,3 ml hydrochloric acid, 2 ml hydrofluoric acid, and 190 mlwater for duration of 20 s. Microstructural characterizationand secondary phase particles analysis were extensively per-formed by employing a TESCAN VEGA II-LMH scanningelectron microscope (SEM) equipped with an energy disper-sive X-ray spectroscopy (EDX) system and a JEOL 2010transmission electron microscope (TEM) operating at 200 keV. For TEM analysis, the samples of 3-mm circular disks were prepared by twin-jet electro-polish in a solutionof 25 vol% of HNO 3  and 75 vol% of methanol at   − 30 °C,and the voltage of 12 V was set up. The hardness along dif-ferent build direction from the bottom to top of the cut samplewas measured using a HX-1000 model hardness tester at a loadof0.98Nwitha10-sdwelltime,andthesemeasurementswere performed along the centerline in the cut build directionat an interval of 0.5 mm (  Z   axis). As per the ASTM, B557M-10, tensiletestsamples were cut paralleltothree directions(  X  , Y  , and  Z   axis) using the wire-cutting electrical discharge ma-chine, and subsequently machined to prepare tensile speci-mens. This was done to estimate the strength and ductility of different parts of the FSAM component in comparison of Alsubstrate. The gage dimension for the tensile sample was 40×10×6 mm, as shown in Fig. 2. The tensile tests were per-formed at room temperature using a WDS-100 testing ma-chine at an initial strain rate of 1 mm/min, and the averagevalue of three specimens was reported. The fracture surfacesof the tensile samples were analyzed by SEM technique. 3 Results and discussion 3.1 Formation characteristic Figure 3 shows the macrostructures of the cross sections of different FSAM components and particular sections of high-resolution images. Among them, a macrograph of the frictionstirlapweldedjointisshowninFig.3a .Also,thecrosssectionof the FSAM component involving nine layers stack is shownin Fig. 3b, and the local magnified figures of different parts inthe component are shown in Fig. 3c  –  f , respectively. From theimages, it is seen clearly that some obvious defects such ashook and kiss bonding are observed for a single friction stir lap welded joint. Similar results were also reported by Salariet al. [20]. Since FSAM is considered to be a multiple frictionstir lap welding process, the complex feature of the materialflow results in various kinds of defects. For the single frictionstir lap weld, one of the most common defects observed in theinterfaces between two welded plates is hook, and it movestowards to thermo-mechanically affected zone (TMAZ) or weld nugget zone (WNZ). In fact, hook is a kind of inherent feature and which is characterized as a crack-like unbondedinterface that deformed from the srcinal faying surfaces onthe advancing side (AS) and retreating side (RS), as shown inFig. 3a . Meanwhile, there is a visible difference in the move-ment direction of the hook, on the AS the hook stretches intothe WNZ, while it moves upward to the TMAZ on the RS.However, for the multiple lap joints, it is obviously seen fromFig. 3b  –  f  that the hook on two sides moves upwards and doesnot stretch into the center stirred zone. From the local magni-fications, some overlapped transition zones under each inter-faceandakissbondingadjacenttothetopperipheryarefoundin Fig. 3d, e. The main reason is resulting from the complicat-ed material flow during FSAM.In the present study, the formation of the hook in a singlelapped joint is related to the material flow in the stir zoneaffectedbythe pinwithlefthandthreadandcanbeinterpretedas the following discussions. On one hand, the stir zone isformed attributing to the cooperation of three material move-ment processes: Firstly, the stirred material of the lower platesmoves upwards and subsequently incorporate with the Table 1  Chemical composition (wt%) of AA7075-O substrateMg Zn Cu Si Fe Mn Cr Ti Al2.7 5.65 1.7 0.4 0.5 0.3 0.22 0.2 BalanceInt J Adv Manuf Technol (2016) 83:1637  –  1647 1639  material of the upper plate, secondly, the incorporated mate-rials move downwards spirally along the left thread pin, andfinally, the incorporated materials ofthe upper and lower platerelease from the bottom of the pin, and the above steps arerepeated. On the other hand, the faying surfaces of both platesare vertical to the tool, which makes the oxide layers at theinterfaces more difficult to be crumbed sufficiently than thosein butt welding. As more materials move upwards from thelower plates and incorporate with the upper plate material, thestirred zone becomes larger and larger. The srcinal interfaceson bothsides are extrudedupwards intothe stirzone, whichisassociated with changes in material flow direction, fromdownward to upward, depending on the tool geometry andwelding condition [21]. Consequently, a hook-shaped macro-structure is formed, and the schematic diagram of the hook formation is shown in Fig. 4a . During FSAM, when another  plate is additively manufactured, other incorporated materialsmove downward along the pin and extrude the hook expanding into the stir zone. In turn, the hook is bent upwardintotheTMAZduetotheupwardflowofthematerialinduced by the tool pin, and the driving force of the upward lower  plates is provided by the shoulder penetration into the surfaceof the upper plate and the pin penetration into the lower plateduring the dwell period, and the schematic diagram of hook deformation is shown in Fig. 4b. Similar results were reported by Badarinarayan et al. [22] and Yin et al. [23]. Furthermore, the formation of the kiss bonding is owing to the insufficient flowresultedfromtheincompletestirringofthepin.Whilethegeneration ofthe overlappedtransitionzone beneath the pin isassociated with welding thermal cycle due to a secondarystirring of the pin, a simple schematic is also shown in Fig. 4b. 3.2 Microstructure evolution in different regions For the FSAM components, the feature of the material flow isvery complex due to involving multiple layers lap jointswhich are suffering from varying degrees of thermal cycles.As a consequence, some differences in the microstructures indifferent noninterfaces and interfaces between layer and layer still exist. In order to observe the microstructure evolution inthe different positions, the FSAM component is convention-ally cross-sectioned, and then a center stirred zone with a sizeof 10×35×2 mm is cut off, as shown in Fig. 5b. From the picture, it is clearly seen that there is no defects in the center stirred zone. Whereafter, the microstructures of the cut struc-ture corresponding to the different positions in Fig. 5b areobserved by optical microscope, as shown in Fig. 6. For theFSAM process, the srcinal microstructure of Al substrate plays an important role in the subsequent microstructural evo-lution, which is characterized by coarser banded grains ar-ranged along the rolling direction in Fig. 6a . However,experiencing high peak temperature, strain rate, and thewelding thermal cycle, FSAM leads to dissolution, coarsen-ing, and dynamic recrystallization, and these processes could Slice tensile test Axial tensile test Fig. 2  Dimension of the tensilespecimen (f) Bending upwards (d) Hook bending (c) (e) Hook (a) 3 mm (b) 5 mm Fig. 3  Formation morphologieson the cross section:  a  a singlelapped joint,  b  a FSAMcomponentwith ninelayers stack,and  c  –  f   local magnifications in  b 1640 Int J Adv Manuf Technol (2016) 83:1637  –  1647
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