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Available online at ScienceDirect Additive Manufacturing 6 (2015) 15 Production of high strength Al85Nd8Ni5Co2alloy by selective laser melting K.G. Prashantha, H. Shakur Shahabia, H. Attara,b, V.C. Srivastavac, N. Ellendtd, V. Uhlenwinkeld, J. Eckerta,e, S. Scudinoa aIFW Dresden, Institut fr Komplexe Materialien, Postfach 27 01 16, D-01171 Dresden, Germany bSchool of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, Perth, WA 6027, Australia cMetal Extraction Aluminum alloys; Intermetallic compounds; Compression test 1. Introduction Aluminum and its alloys are among the most widely used materials for structural and functional applications, due to their high specifi c strength, high corrosion resistance and good pro- cessability 1. The strength of Al-alloys can be increased by the formation of amorphous/glassy (MG), nanocrystalline (NC) or ultrafi ne grained (UFG) structures 24. The MGs can be obtained by non-equilibrium processing techniques like melt spinning, mechanical alloying, gas atomization or copper mold casting 57. However, Al-based MGs have poor glass forma- bility and hence their size is limited, typically 1 mm 8,9. The NC/UFG alloys exhibit thermal instability due to the excess enthalpy associated with the high density of grain boundaries; thereby limiting their high temperature application spectrum 10,11. Therefore, in order to utilize the advantages of the MG/NC/UFG structured alloys, there is a strong need to develop thermally stable Al-based alloys without size limitations. Corresponding author. Tel.: +49 351 4659 685; fax: +49 351 4659 452. E-mail addresses: kgprashanth, k.g.prashanthifw-dresden.de (K.G. Prashanth). Selective Laser Melting (SLM) is an additive manufactur- ing technique capable of producing MG/NC/UFG materials 1214. It allows the fabrication of complex and intricate geometries with a high degree of accuracy, high design fl exibil- ity along with excellent process capabilities and high material utilization 1214. Although reports exist on the production of AlSi and AlSiMg based alloy systems by SLM, other systems were not explored extensively and systematically. The AlZn (7XXX) system in one of the commercial and con- ventional Al-based alloy systems that exhibits high strength. However, there are no reports on the fabrication of 7XXX alloys by SLM, which might be a result of two reasons: (1) they are extremely brittle and may lead to cracking of the samples during the fabrication process and (2) evaporation of Zn (which has low boiling point) during the SLM process, which makes it unsuit- able for the SLM process. Hence, there is a strong need to explore the various unconventional Al-based alloys (MG/NC/UFG) that may exhibit high strength at ambient temperatures and can be successfully fabricated by SLM. Recently, Li et al. have reported the production of amorphous Al86Ni6Y4.5Co2La1.5by SLM 15,16. They have performed single line scans at different laser powers on a pre-fabricated porous Al86Ni6Y4.5Co2La1.5metallic glass perform that lead /10.1016/j.addma.2015.01.001 2214-8604/ 2015 Elsevier B.V. All rights reserved. 2 K.G. Prashanth et al. / Additive Manufacturing 6 (2015) 15 to the formation of a gradient-like microstructure with crystal- lization of the amorphous phase at some places. They have also shown that the microstructure can be controlled by controlling the laser parameters. However, several cracks and micro-cracks were observed due to high thermal gradient observed during the process which caused high levels of thermal stress. Following this study Li et al. investigated the use of a re- scanning strategy to prevent the macro-cracking during the SLM process. They have claimed that a high power initial scan fol- lowed by a lower power re-scan strategy can be used to avoid cracking Al-based MG/NC alloys. However, even by adoption of such a re-melt strategy, completely amorphous and defect free Al-based samples were not possible by SLM. However, these works throw light on the potential fabrication of Al- based NC/UFG materials by SLM and furthermore there are no detailed reports on the fabrication and microstructural property correlation with the mechanical properties. The present work analyzes this aspect by focusing on the production of a thermally stable, high strength Al-based NC alloy using the SLM process. This is followed by a detailed structural and microstructural investigation, along with the mechanical properties evaluation, fracture analysis and structureproperty correlation. 2. Materials and methods Al85Nd8Ni5Co2(at.%) cylindrical specimens (3 mm diame- ter and 8 mm height) were produced by SLM from spherical gas-atomized powder (GAP) using an SLM 250 HL device (SLM Solutions GmbH, Luebeck, Germany) equipped with an Yb-YAG laser with a maximum power of 400 W and a spot size of 80 ?m. The gas atomized powder was spherical in shape with an average particle size of 48 5 ?m. The powder exhibited excellent fl owability, which is one of the important pre-requisites to be used as a raw material for the SLM pro- cess. The parameters used for the fabrication of the specimens are: power of 320 W for volume and contour, layer thickness of 50 ?m, stripe hatch with a spacing of 110 ?m between them and hatch style rotation of 73between the layers. Two differ- ent scanning speeds were used: 1455 mm/s for the volume and 1939 mm/s for the contour. The Al substrate plate was heated to 673 K during the entire SLM process to avoid the formation of cracks in the SLM samples. Structural analysis was performed by X-ray diffraction (XRD) using a D3290 PANalytical Xpert PRO (PANalytical GmbH, Kassel-Waldau, Germany) with Co- K? radiation ( = 0.17889 nm) in Bragg-Brentano confi guration. The Rietveld method was employed for estimating the crystal- lite size from the XRD patterns using the WinPlotR software package 17. The density of the consolidated samples was evaluated by the Archimedes principle. The microstructure was characterized by scanning electron microscopy (SEM) in the back scattered electron (BSE) mode using a Gemini 1530 microscope (Gt- tingen, Germany) equipped with an energy-dispersive X-ray spectroscopy (EDX) facility. The compression tests were car- ried out using an Instron 8562 testing system (Instron GmbH, Darmstadt, Germany) under quasistatic loading (strain rate 1 104s1) in the temperature range 303573 K. The strain was measured directly on the specimen using a Fiedler laser- extensometer. The samples were always tested for compression along the build direction, which implies the top of the sample after fabrication is held at the top during the compression test as well. In order to ensure the microstructural stability of the material during the high-temperature tests, the specimens were heat treated under argon atmosphere at 723 K for 4 h. The hard- ness of the individual phases was determined using an “Asmec UNAT” nano-indentor (ASMEC GmbH, Radeberg, Germany) with a Berkovich shape tip. A total number of 150 indentations were performed on a highly polished surface using a typical quadratic loading and unloading procedure. In order to have the indentation on a single phase with sub-micron size, a maximum load of 2 mN was selected. 3. Results and discussion Fig. 1 shows the SEM and EDX mapping images of the Al85Nd8Ni5Co2as-prepared (AP) SLM sample taken along its cross-section. The low magnifi cation image (Fig. 1(a) reveals the typical track morphology observed in the SLM specimens 14,18. The distance between the tracks is 100 ?m with a track overlap of 10 ?m and no visible porosity and defects are seen, which is also corroborated with the density measure- ment studies, where an average relative density of 99.75% is observed. It is to be noted that the hatch distance used for fab- ricating these samples are 110 ?m and hence the tracks should exhibit a distance of 110 ?m. However, it can be observed from Fig. 1(a) that the width of the tracks is only 100 ?m, indi- cating that there is a presence of hatch overlaps of 10 ?m. Such strategy of using hatch overlaps reduces the porosity lev- els as well as any possible discontinuities in the sample between the hatches, there by resulting in a sound sample with near to full density. It has to be noted that there is a rotational shift of around 15 deg between Fig. 1(a) and (b) as a result of the mea- surement sequence. The microstructure is non-uniform with the phases exhibiting a bimodal distribution. This is the result of the track overlap core morphology typical for the SLM samples 14, consisting of a fi ne microstructure along the track over- laps (marked as (1) in Fig. 1(b) and a coarse microstructure along the track cores (marked as (2) in Fig. 1(b). Bright platelets with different phase contrast are distributed within a dark matrix (Fig. 1(c), making it a composite-like microstructure. The elemental mapping EDX images (Fig. 1(dh), show that the dark areas are rich in Al and the bright platelets are rich in Nd, Ni and Co. The bright platelets have different contrasts suggest- ing the presence of four different phases in the Al85Nd8Ni5Co2 AP SLM samples. The presence of four different phases in the AP SLM sample was confi rmed by XRD. The diffraction pat- tern (Fig. 2) displays the existence of ?-Al (cubic, Fm3m) with a crystallite size d = 72 nm, along with the three intermetallic phases: AlNdNi4(orthorhombic, Cmc2), Al4CoNi2(cubic, Ia3d) and AlNd3(hexagonal, P63mmc). The AlNdNi4platelets (d = 29 nm) are 2.12 0.34 ?m in length and 0.41 0.13 ?m in width, whereas the Al4CoNi2(d = 42 nm) and AlNd3platelets (d = 35 nm) are 2.32 0.48 ?m and 6.01 0.74 ?m in length and 1.00 0.08 ?m and 0.89 0.14 ?m in width, respectively. K.G. Prashanth et al. / Additive Manufacturing 6 (2015) 15 3 Fig. 1. SEM images of the Al85Nd8Ni5Co2alloy showing (a) the laser tracks and (b) the bimodal microstructure. High magnifi cation SEM micrograph showing (c) the intermetallic phases with different contrasts embedded in the Al matrix and (dg) the corresponding EDX images. The XRD patterns of the different samples indicate that the phases present in the GAP, AP SLM and HT SLM samples are the same (Fig. 2). Interestingly, the XRD patterns of the AP SLM and HT SLM are very similar. The crystallite sizes of the phases in HT SLM are 84 nm for Al, 34 nm for AlNdNi4, 50 nm for Al4CoNi2and 38 nm for AlNd3, indicating that the SLM sam- ple is thermally stable with no signifi cant phase transformation and grain growth taking place during the heat-treatment process. The AP SLM specimen tested at room temperature (RT) shows a very high yield strength (YS) of 0.94 GPa and an ulti- mate compressive strength (UCS) of 1.08 GPa along with 2.45% plastic strain (Fig. 3(a and b). The HT sample shows simi- lar properties (YS = 0.81 GPa and UCS = 0.97 GPa), suggesting that the microstructure of the SLM material is thermally sta- ble at high temperatures. Such high strength levels are retained at elevated temperatures. For example, UCS of 1.05 GPa and Fig. 2. XRD patterns ( = 0.17889 nm) of the gas atomized powder along with as-prepared and heat-treated SLM samples. 0.97 GPa, and strain of 4.5 and 8.5% are observed for the sam- ples tested at 373 K and 423 K, respectively (Fig. 3(a and b). With further increase of the testing temperature, the deforma- tion exceeds 20%, where the measurements were stopped. Even at a testing temperature of 573 K, a high UCS of 0.50 GPa is observed, which indicates that the present material can be used for high strength applications at high operating temperatures. Although, intense scientifi c research has been focused on the development of high-strength amorphous and nanostruc- tured Al-based alloys, only a few reports deal with the high temperature mechanical properties of these materials. The sin- tered nano-crystalline AlFe material 19 displays the best high temperature properties among the published works on Al-based alloys. These results are compared with the mechanical proper- ties of the present AlNdNiCo SLM alloy in Fig. 3(c). It can be observed that the present alloy has similar strength levels with respect to the AlFe alloy at all test temperatures. The AlFe alloy was produced by spark plasma sintering, where the size and shape of the component is restricted, whereas processing by SLM permits the production of parts having theoretically any possible geometry with minimized need for post processing. Therefore, additive manufacturing offers the possibility to tailor the shape and corresponding properties of these high-strength Al-based parts to meet specifi c requirements, which renders this technology unique in comparison to conventional processing. The fracture surface images, as shown in Fig. 4, give further evidence for the high strength observed in the Al85Nd8Ni5Co2 alloy. The fracture takes place in a stepped morphology (Fig. 4(a), similar to the Al12Si alloy prepared by SLM 12. This can be ascribed to the bimodal microstructure (Fig. 3(b) with fi ne grains along the track-overlaps, which may act as a preferential path for crack propagation, leading to the observed stepped morphology. The superior properties observed in the present alloy can be attributed to the composite-like microstruc- ture. The fi ne microstructure, characteristic of the SLM process 12, leads to the presence of fi ne ?-Al phase surrounded by 4 K.G. Prashanth et al. / Additive Manufacturing 6 (2015) 15 Fig. 3. (a) Compressive stressstrain curves of the as-prepared (AP) and heat-treated (HT; 4 h at 723 K) SLM samples tested at room temperature (curves 1 and 2) along with the AP SLM samples tested at different temperatures, (b) corresponding mechanical data and (c) comparison of the present results with the nano-crystalline AlFe alloy produced by spark plasma sintering 19. the intermetallic phases. Since the microstructure is developed from a rapid solidifi cation process, it is expected to have an inherently strong bonding between the Al matrix and the inter- metallic reinforcements, aiding an improved and effective load transfer along the interface. This concept leads to the interfacial strengthening mechanism in the present alloy both at RT and at high temperatures. The hardness of the intermetallic phases evaluated from the nano-indentation measurements are 2.84 0.08 GPa for AlNd3, 3.74 0.10 GPa for Al4CoNi2and 5.45 0.09 GPa for AlNdNi4; which is much higher than pure ?-Al (0.33 0.03 GPa). Hence, during RT deformation, the cracks are expected to develop along the ?-Al phase and to propagate with further loading. However, the intermetallic platelets act as obstacles which leads to either the arrest or defl ection of the cracks, as observed from Fig. 4(b and c). In case of the crack arrest, further deformation proceeds through the initiation of new cracks leading to crack multiplication (Fig. 4(b). On the other hand, crack defl ection suggests that the effective mean crack path is increased leading to appreciable deformation in the material 20. Generally, at high temperatures the dislocation movement is accelerated and the magnitude of the Peierls stress is drastically reduced leading to low strength of the material 20. However, in the present alloy, the Al matrix is surrounded by the intermetallic reinforcement, which may confi ne the dislocation movement along the grain boundaries according to the confi nement theory, which leads to further strengthening of the material at elevated temperatures 21,22. All of the mech- anisms: interfacial strengthening, crack arrest and initiation of new cracks, crack defl ection phenomena and confi nement phenomena (Fig. 4(d) operate simultaneously leading to Fig. 4. Fracture surface after compression tests showing (a) stepped morphology, (b) multiple cracks and (c) crack defl ection. (d) Schematic illustrating the crack arrest and crack defl ection by the intermetallic phases in the Al85Nd8Ni5Co2alloy during compression test. K.G. Prashanth et al. / Additive Manufacturing 6 (2015) 15 5 superior room temperature as well as high temperature compressive strengths. 4. Summary Highly dense high strength Al85Nd8Ni5Co2alloy has been successfully prepared by SLM. The alloy exhibits a com- posite microstructure with the intermetallic phases AlNdNi4, Al4CoNi2and AlNd3dispersed in the Al-matrix. The inter- metallic phases are in the form of platelets and their width ranges in the sub-micron regime. The AP SLM and HT SLM samples show a UCS of 1.08 GPa and 0.97 GPa with 2.5% strain at RT. The high temperature compression tests reveal that the present system can retain their high strength due to the composite-like microstructure and confi nement phenomena, where the grain coarsening and the accelerated mobility of the dislocations at high temperatures are retarded by the intermetallic phases. Addi- tionally, interfacial strengthening, the crack arrest and crack defl ection mechanisms also contribute to the superior strength observed in the Al85Nd8Ni5Co2alloy. The present results indi- cate that SLM is one of the best options to produce high strength, thermally stable, dense and near-net shaped Al-based alloys. References 1 Lu K. The future of metals. Science 2010;328:31920. 2 Inoue A. Amorphous, nanoquasicrystalline and nanocrystalline alloys in Al-based systems. Prog Mater Sci 1998;43:365520. 3 Scudino S, Surreddi KB, Nguyen HV, Liu G, Gemming T, Sakaliyska M, et al. High-strength Al87Ni8La5bulk alloy produced by spark plasma sintering of gas atomized powders. 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