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Evaluation on the fatigue behavior of sand-blasted AlSi10Mg obtained by DMLS

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Fatigue tests were performed on sand-blasted AlSi10Mg samples produced by Direct Metal Laser Sintering (DMLS) of powders. The effect of sand-blasting on surface properties was evaluated by roughness and residual stresses measurements, together with
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     A. Pola et alii, Frattura ed Integrità Strutturale, 49 (2019) 775-790; DOI: 10.3221/IGF-ESIS.49.69    775 Evaluation on the fatigue behavior of sand-blasted AlSi10Mg obtained by DMLS  Annalisa Pola, Davide Battini, Marialaura Tocci, Andrea Avanzini, Luca Girelli, Candida Petrogalli, Marcello Gelfi Department of Mechanical and Industrial Engineering, University of Brescia, Via Branze 38, 25123, Brescia, Italy annalisa.pola@unibs.it, http://orcid.org/0000-0002-0722-6518 davide.battini@unibs.it, http://orcid.org/0000-0002-2044-5985 m.tocci@unibs.it, http://orcid.org/0000-0002-7515-0615 andrea.avanzini@unibs.it, http://orcid.org/0000-0002-7188-7687 l.girelli005@unibs.it, http://orcid.org/0000-0002-7630-0662 candida.petrogalli@unibs.it, http://orcid.org/0000-0002-1774-3914 marcello.gelfi@unibs.it, https://orcid.org/0000-0002-8939-811X  A  BSTRACT .  Fatigue tests were performed on sand-blasted AlSi10Mg samples produced by Direct Metal Laser Sintering (DMLS) of powders. The effect of sand-blasting on surface properties was evaluated by roughness and residual stresses measurements, together with morphological analysis, in comparison with as-fabricated condition. An evident improvement of surface finishing was observed after sand-blasting, which also lead to the presence of compressive residual stress on the external surface of samples, as revealed by XRD measurements. Furthermore, defects analysis allowed the identification of a uniform distribution of porosities in the cross section whereas larger porosities seem more abundant close to the surface. It was found that the tested material exhibits good fatigue resistance, supporting the positive role of sand-blasting as a simple post-processing treatment. Superficial defects are the preferential crack initiation sites, as demonstrated by SEM analysis of fracture surfaces. K  EYWORDS .  AlSi10Mg; DMLS; Fatigue; Porosity; Sand-blasting; Scanning Electron Microscopy. Citation:  Pola, A., Battini, D., Tocci, M.,  Avanzini, A., Girelli, L., Petrogalli, C., Gelfi, M., Evaluation on the fatigue behavior of sand-blasted AlSi10Mg obtained by DMLS, Frattura ed Integrità Strutturale, 49 (2019) 775-790. Received: 16.05.2019  Accepted: 18.06.2019 Published:  01.07.2019 Copyright:  ©2019 This is an open access article under the terms of the CC-BY 4.0,  which permits unrestricted use, distribution, and reproduction in any medium, provided the srcinal author and source are credited. I NTRODUCTION dditive manufacturing (AM) for metallic materials is an innovative but already well established production technology able to merge different advantages, as such as the possibility of obtaining very complex shaped parts,  with limited mass loss during their fabrication, and characterized by excellent mechanical properties [1-3].    A. Pola et alii, Frattura ed Integrità Strutturale, 49 (2019) 775-790; DOI: 10.3221/IGF-ESIS.49.69 776  There are several AM methods available on the market and the same process can be indicated with different nomenclatures mainly according to the machine producer and brand [1, 4]. These processes can be applied with different engineering materials like stainless steel and titanium [5-8], cobalt-chromium and nickel alloys [9-11] but also copper [12-13] and aluminum alloys [14-15] or, rarely, gold [16]. Compared to other metals, the production of aluminum alloys parts via AM needs specific accuracy, due to the high reflectivity and thermal conductivity of Al, like higher laser power [14, 17] [18]. Nevertheless, the use of aluminum alloys in  AM is increasing thanks to their high strength to density ratio, that makes them attractive for automotive and aircraft sector [3, 19] as well as for space field, where many parts are already done and envisioned for future missions [20].  Among Al alloys, the most widely used and investigated is AlSi10Mg alloy, that shows interesting mechanical properties, in both as-fabricated and heat treated conditions [21-26], as well as good corrosion resistance [27-29]. Since during their life many parts undergo cycling loading conditions, several authors investigated also the fatigue behavior of this alloy in various conditions, evaluating the effect of fabrication direction or laser scanning strategy [26, 30-32].  AM products, in Al alloys as well as in other metals, can suffer from defects like porosity, residual stress and poor finishing that can negatively affect their performance [33-34]. It is known, in fact, that AM components are characterized by the presence of porosity due to many factors: melt splashing and Marangoni flow during laser scans, gas entrapment due to  vaporization of low melting point constituents in the alloy, shrinkage porosity formed during solidification, lack of fusion caused by a poor overlap of melting pools [2, 15, 35-36]. For what concerns residual stresses, they are induced by the locally concentrated energy input, which determines large thermal gradients that result, in turn, in part deformations or even cracks   [37-38] . Regarding the rough surface, this is related to the “stair step” effect, i.e. the stepped approximation by layers of curved and inclined surfaces, and by the “balling” phenomenon that causes the formation of discontinuous tracks and prevents a uniform deposition of new powder layer, also inducing porosity and delamination because of the poor inter-layer bonding [39-40]. Failure of AM parts is commonly related to the presence of the above mentioned surface and subsurface defects [41], as they are easily responsible for crack initiation during fatigue loading [26]. Therefore, post-treatments like machining and polishing, are frequently proposed on AM components in order to increase their life as they can help eliminating the   influence of surface and subsurface imperfections [42]. Hence, different authors concentrated their attention on the fatigue resistance of machined samples compared to the as-fabricated condition [26, 30, 43]. Romano et al. [41], for instance, performed a   statistical analysis of defects in AlSi10Mg, developing a model for fatigue life estimation in High Cycle Fatigue regimes taking into account the initial defects. It is worth noting that one of the main benefits of AM is the opportunity of obtaining (near)net shape parts, without the need of machining operations, which could also be hardly feasible for some geometries. Therefore, other surface treatments not involving machining or polishing can result advantageous. In this regard, some authors investigated the effect of sand-blasting or shot-peening on fatigue enhancement of AlSi10Mg alloy [44-46]. In particular, in [45] sand-blasting and shot-peening were demonstrated to remarkably improve fatigue strength compared to as-fabricated specimens. In [44] the effect of different shot-peening conditions (steel or ceramic balls) was also analyzed. The authors found that the surfaces polishing before this post-treatment or the subsequent removal of material of about 25-30 μ m from the surface improved fatigue resistance. In [46], an insight on how shot peening changes superficial and sub-superficial pore morphology and the observed increase in low-cycle and high-cycle fatigue strength was presented. However, notwithstanding the already recognized influence of surface post-treatments on fatigue resistance of AM parts, the current knowledge about its effectiveness is still incomplete. Hence, in this paper a thorough investigation of fatigue properties of Direct Metal Laser Sintered (DMLS) AlSi10Mg alloy after sand-blasting is performed, correlating the fracture mechanism to the microstructure. M  ATERIALS AND METHODS   he AM specimens were produced by DMLS method using an EOS M290 system (400 W, Yb laser fibre; F-theta lens; 30 A and 400 V power supply; 7000 hPa, 20 m 3 /h inert gas supply; 100 µm focus diameter; EOS GmbH Electro Optical System [47]). They were built along the vertical direction with a layer thickness of 30 µm, using a building platform pre-heated at 80 °C in argon atmosphere. The used powder is the commercial EOS Aluminium AlSi10Mg,  whose nominal composition is reported in Tab. 1. In order to perform a morphological analysis of powders, two samples of AlSi10Mg powder were taken. The first sample  was deposited on a tape and observed by means of LEO EVO 40 scanning electron microscope (SEM). The second sample  was mounted in an epoxy resin, polished on abrasive papers and observed by Leica DMI 5000 M optical microscope.  T     A. Pola et alii, Frattura ed Integrità Strutturale, 49 (2019) 775-790; DOI: 10.3221/IGF-ESIS.49.69    777 Si Mg Cu Fe Mn Ni Zn Pb Sn Ti  Al  AlSi10Mg 9-11 0.2-0.45 <0.05 <0.55<0.45 0.05 <0.10<0.05<0.10 <0.15 Balance    Table 1: Nominal chemical composition (wt. %) of the commercial EOS Aluminium AlSi10Mg powder.  In Fig. 1, the geometry of the specimens used for tensile (Fig.1a) and fatigue (Fig.1b) tests is shown.  After manufacturing, the samples were sand-blasted by using the B120 Microblast Ceramic Beads of Saint-Gobain Zirpro at about 10 cm from the nozzle and for an exposure time of 2 min at 5 bar. The sand particles had a size distribution in the range of 63-125 µm. Their chemical composition, performed by X-ray fluorescence on melting sample by the producer, is reported in Tab. 2.  The samples roughness (R  a  ) was measured before and after sand-blasting by a stylus profilometer (Tribotechnique) with a tip radius of 5 µm and an applied load of 1 mN. Figure 1: Specimen geometry for (a) tensile test and (b) fatigue test samples and (c) produced specimens  Min % Max % ZrO 2  60 70 SiO 2  28 33  Al 2 O 3  0 10  Table 2: Nominal chemical analysis (wt. %) of the particles used for sand-blasting.  A preliminary microstructural analysis was performed by optical (Leica DMI 5000M) and scanning electron (LEO EVO 40) microscope. To this aim, the samples were cut perpendicularly to the fabrication direction, polished up to mirror finishing and etched with Keller’s reagent (1% HF, 1.5% HCl, 2.5% HNO 3  and 95% H 2 O), applied for 30 s according to ASTM E407 standard [48].  A detailed defect analysis was carried out on four specimens to characterize the distribution of porosity. To this aim, a cylindrical specimen, with a length of 10 ± 2 mm, was obtained from the gage length of four sand-blasted AlSi10Mg fatigue samples performing a double cut, perpendicularly to the axis of the sample, with a Struers Labotom machine. The specimen  was ultrasonically cleaned in ethanol, dried and inserted in a cold mounting resin. After the solidification of the resin, the mounted specimen was grinded and polished up to mirror finishing, starting from P220 grinding paper till to a velvet cloth  with Struers colloidal silica suspension. Finally, the specimen was ultrasonically cleaned in ethanol and dried with warm air.  The optical analysis was performed by means of Leica DMI 5000 optical microscope mapping the whole polished section of the specimens with 120 ± 5 images at a magnification of 50x. The entire section was reconstructed with the support of the Leica Application Suite v4.8 software. The whole sections were processed using a custom software for defects analysis coded within NI Labview environment that evolved from previous approaches used for marker tracking [49] and crack length tracking [50] during various experimental tests. This new optical measurement code allows the processing of single or multiple images and provides a text output file containing all the data of the most significant features of each defect identified in the image(s) such as: position, bounding box, orientation, elongation, area, maximum Feret diameter, and many others. High resolutions images (over 100 Mpx) and high numbers of defects per image can be handled with ease.    A. Pola et alii, Frattura ed Integrità Strutturale, 49 (2019) 775-790; DOI: 10.3221/IGF-ESIS.49.69 778  The software operates through blob analysis of binarized images and offers the user the possibility of tailoring many processing settings in order to enhance defect identification and feature output: -   pre-binarization settings: user can extract color plane, tweak the BGC (brightness, gamma, contrast) settings or even apply a convolution filter with standard or custom kernels; -   binarization settings: global, local, or custom thresholding algorithms are available; -   post-binarization settings: basic and advanced morphological operations are available, as well as various filtering methods based on feature values or even defect aggregation algorithms. Everything is provided with a proper GUI (graphical user interface) where the user can see the processed image in real-time and save both the processed image and the features of all the defects which can also be conveniently plotted and handled  via custom Matlab code. Room temperature tensile tests were performed according to UNI EN ISO 6892-1 standard using an Instron 3369 testing machine with a 50 kN load cell. The elongation was monitored using a knife-edge extensometer (length of 25 mm) fixed to the gauge length of the samples. Four specimens were tested, at different crosshead speeds resulting in strain rates ranging from 0.000167 s -1  to 0.015 s -1 . Concerning the fatigue tests, they were performed at room temperature at different peak stress levels on a load-controlled servo-hydraulic testing machine (Instron 8501) with a stress ratio (R =  min /  max  ) set to zero and frequency 20 Hz. Eighteen tests were carried out. The specimens were loaded until failure or until 2x10 6  cycles (taken as infinite life) were reached. Experimental data were processed according to the ISO 12107 standard: the stress and life were linearly interpolated in log-log coordinates and the fatigue strength was estimated via the staircase method. In order to investigate fatigue strength for both finite and infinite life regimes, peak stress levels were selected as no preliminary knowledge of the material fatigue behavior was available. For the S-N test (cyclic stress, S, versus the number of cycles to failure, N), the guidelines reported in [51] were followed. These rules are based on the method proposed in [52] for statistical evaluation of S-N, according to which: -   a minimum of 2 tests on 4 different stress levels for the finite fatigue life regime and -   a minimum of 6 tests for a simplified staircase for the infinite fatigue life regime are needed. Specifically, 12.5 MPa for the finite life curve and 5 MPa for the infinite life curve were chosen as fixed Δσ . From this set of data, the slope of the S-N curve in the finite life region was calculated via linear regression and an estimate of fatigue strength was obtained by applying a reduced staircase method.  The fractured surfaces of fatigue test samples were observed by LEICA 300 digital microscope and LEO EVO 40 scanning electron microscope (SEM) equipped with an Oxford energy dispersive spectroscopy (EDS) probe for elemental analysis.   In order to estimate the residual stress induced by the sand-blasting treatment, which is known to affect stress state of the surface and consequently the fatigue behavior, a comparative analysis of residual stress on both   as-fabricated and sand-blasted surface was carried out for a couple of specimens by X-Ray Diffraction (XRD) technique. The measurements were performed by a Bruker D8 Discover XRD 2  diffractometer (Cu-K  α  radiation), equipped with a beam collimator of 0.5 mm in diameter. The sin 2 ψ -method was applied in omega-mode configuration on the (331) plane of aluminum with tilting angle (psi) from -60 to +60°.   R  ESULTS AND DISCUSSION    Metallographic characterization he morphology of used powders is shown in Fig. 2. Particles show a spherical shape and several satellites are present.  At times, small pores can be detected inside the particles (Fig. 2b). In Fig. 3a the microstructure of the AM samples along the building plane (xy) is shown after Keller’s etching. The melting pools elongated in the deposition plane can be observed, also revealing the underneath layers formed at different orientations as a consequence of the printing strategy. In agreement with the literature, at the pool boundary a coarser microstructure rich in Al can be detected [53].  The microstructure along the building direction (z) is reported in Fig. 3b. Similarly, the melt pools are clearly visible, in this case with the typical half cylindrical shape. Again, they are characterized by a fine core, surrounded by a coarser boundary zone. This is confirmed by the analysis performed at higher magnification (Fig. 3c), where  -Al cellular grains (dark areas) surrounded by superfine Si network (light areas) are visible, according to the literature [1, 25, 29, 54]. Small porosities were also noticed due to the AM technique [14-15, 35, 55-56].  T     A. Pola et alii, Frattura ed Integrità Strutturale, 49 (2019) 775-790; DOI: 10.3221/IGF-ESIS.49.69    779 Figure 2: SEM (a) and optical (b) images of AlSi10Mg alloy powders. Figure 3: Microstructure of AlSi10Mg DLMS samples performed by optical microscopy on (a) horizontal and (b) vertical directions and (c) by SEM after Keller’s etching  An example of the surface morphology of the as-fabricated as well as sand-blasted samples is provided in Fig. 4. The as-fabricated surface appears irregular, with a lot of satellites and/or balling, as it frequently happens in as-produced parts [43]. On the contrary, the sand-blasted sample shows a more homogeneous surface, with remarkably reduced imperfections, even though small porosities can be revealed. Figure 4: Surface morphology of (a) as-fabricated and (b) as sand-blasted samples Residual stress  As expected, sand blasting significantly affects the surface roughness R  a  of the studied material, as shown in Tab. 3. In fact, this post-treatment determines a strong decrease of the surface roughness R  a  as compared to the as-fabricated condition (approximately a reduction of 50 %). The values measured in the present study are in good agreement with the results of other authors [45],    who studied the effect of sand blasting on the same alloy. The reduction in surface roughness after sand blasting is due to the effective removal of the superficial imperfections that characterize AM materials (satellites, balling,
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