Post-processing of the Inconel 718 alloy parts fabricated by selective laser melting: Effects of mechanical surface treatments on surface topography, porosity, hardness and residual stress

Journal Pre-proof Post-processing of the Inconel 718 alloy parts fabricated by selective laser melting: Effects of mechanical surface treatments on surface topography, porosity, hardness and residual stress

D.A. Lesyk, S. Martinez, B.N. Mordyuk, V.V. Dzhemelinskyi, А. Lamikiz, G.I. Prokopenko PII:

S0257-8972(19)31127-2

DOI:

https://doi.org/10.1016/j.surfcoat.2019.125136

Reference:

SCT 125136

To appear in:

Surface & Coatings Technology

Received date:

20 August 2019

Revised date:

9 October 2019

Accepted date:

4 November 2019

Please cite this article as: D.A. Lesyk, S. Martinez, B.N. Mordyuk, et al., Post-processing of the Inconel 718 alloy parts fabricated by selective laser melting: Effects of mechanical surface treatments on surface topography, porosity, hardness and residual stress, Surface & Coatings Technology (2018), https://doi.org/10.1016/j.surfcoat.2019.125136

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© 2018 Published by Elsevier.

Journal Pre-proof Post-processing of the Inconel 718 alloy parts fabricated by selective laser melting: Effects of mechanical surface treatments on surface topography, porosity, hardness and residual stress D.A. Lesyka, S. Martinezb, B.N. Mordyukc, V.V. Dzhemelinskyia, А. Lamikizb, G.I. Prokopenkoc a

Laser Systems and Physical Technologies Department, National Technical University of Ukraine "Igor Sikorsky Kyiv Polytechnic Institute", 37 Peremohy ave., UA-03056 Kyiv, Ukraine b

Aeronautics Advanced Manufacturing Center, University of the Basque Country, 202 Bizkaia Science and Technology Park, SP-48170 Zamudio, Spain

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Physical Principles for Surface Engineering Department, G.V. Kurdyumov Institute for Metal Physics of the NAS of Ukraine, 36 Academician Vernadsky blvd., UA-03680 Kyiv, Ukraine

Abstract.

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The turbine blade test parts were manufactured by the selective laser melting (SLM) process using a

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nickel-based pre-alloyed Inconel (IN) 718 powder. Various mechanical post-processing techniques, such

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as barrel finishing (BF), shot peening (SP), ultrasonic shot peening (USP), and ultrasonic impact treatment (UIT), were applied to improve the surface layer properties of the SLM-built specimens. Effects

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of mechanical surface treatments on surface topography, porosity, hardness, and residual stress were

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studied. In comparison with the SLM-built state the surface roughness (Sa = 5.27 μm) of the postprocessed specimens were respectively decreased by 20.6%, 26.2%, and 57.4% after the BF, USP, and

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UIT processes except for the SP-treated ones. The Sz parameter was reduced in all treated SLM-built

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specimens except for the SP-treated ones.. The surface microhardness of the SLM-built specimen

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(~390 HV0.025) was increased after the BF (by 14.2%), USP (by 23.8%), UIT (by 50%), and SP (by 66.5%) processes. The deepest hardened layers were formed after the UIT and SP processes. Residual porosity of the SLM-built specimen was decreased by 23.1%, 40.6%, 55%, and 84% after the BF, SP, USP and UIT processes, respectively. The UIT process formed a densified subsurface layer of significantly reduced porosity (0.118%). All mechanical surface treatments successfully transformed the tensile residual stresses generated in SLM-built specimen into the compressive residual stresses (– 201.4...510.7 MPa). The thickness of hardened, densified and compressed near-surface layers ranges from ~80 μm after BF to ~140 μm after USP, and ~180 μm after SP and UIT processes, which correlates to the accumulated energy and deformation extent of the treated surface. The effect of the accumulated energy on the outcomes of the applied surface treatments is also addressed.

Journal Pre-proof Keywords: Selective laser melting, Mechanical post-processing, Barrel finishing, Shot peening, Ultrasonic shot peening, Ultrasonic impact treatment 1. Introduction. Improvement in the surface properties of the metal components manufactured by different additive techniques, such as wire and arc additive manufacturing (WAAM), laser metal deposition (LMD), electron beam melting (EBM), selective laser sintering (SLS), selective laser melting (SLM) or direct metal laser sintering (DMLS) / laser powder bed fusion (LPBF) is very relevant. The poor surface quality,

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porosity and tensile residual stresses of the fabricated parts may limit the wider application of metal-based

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additive manufacturing (AM) techniques despite their vast potential. Therefore, post-processing of AM

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components is required to eliminate the superficial and volume defects. Currently, the development of new post-processing technologies (thermal, mechanical, chemical

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treatments) is very attractive since they may improve the components fabricated by the AM methods.

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For instance, an implementation of post-processing techniques in the AM routes, such as special heat treatment, hot isostatic pressing or hot isostatic pressing combined with heat treatment [1–4],

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electrochemical polishing [5], media blasting or tumbling [6], ultrasonic excitation [7], and surface severe

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plastic deformation [8–13], could minimize or eliminate the defects (residual porosity and/or excessive

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roughness) in the metal components. The post-processing methods can be selected based on the application requirements, geometry complexity, size of parts and required surface quality. A combination of different methods can also be applied. The AM methods using a laser beam are new alternative technologies for manufacturing components instead of conventional metal forming methods [14–16]. The SLM process is one of an advanced manufacturing method, which allows the production the complex three-dimensional (3D) parts directly from 3D computer models by selectively melting successive layers of metal powder on the top of the previous one using a scanning laser beam [17]. Compared to conventional manufacturing methods, the SLM process has important advantages relating to a reduction of the production steps and high material building efficiency, as well as the possibility for manufacturing thin-walled parts from materials difficult to machine [16]. In addition to that, the manufacturing costs are weakly dependent on the

Journal Pre-proof component complexity. Trosch et al. [18] have shown that the SLM process provided mechanical properties at least as good as conventional forged or cast materials. Manufacturing of the parts by the SLM process allows the highest geometrical flexibility and accuracy as compared to the other laser-based AM processes. Moreover, in comparison with the SLS process, the SLM process leads to significant improvement in the microstructural and mechanical properties owing to full melting of the metal powder material to form the solid 3D parts [14]. Another major benefit of the SLM process relates to its high feasibility in the processing of some pure non-ferrous metal materials including Ni, Ti, Al, and Co alloys

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considered to be feasible only using the SLM process.

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[17, 18]. Thus, the production of lightweight metal parts for the aerospace and automotive sector is

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However, it should be noted that the SLM process still has some disadvantages. In particular, the SLM-fabricated metal parts suffer from a residual porosity, relatively rough and uneven surface,

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columnar grains of anisotropic properties and large tensile residual stresses in the near-surface layers [19–

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21]. These tensile residual stresses arising during the cooling stage of the manufacturing process were regarded as a key factor responsible for the distortion and even delamination of the end-products [20].

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Moreover, the dimensional and orientation heterogeneities of the grain structure of the layers were

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observed [22–24]. The effects of the treatment strategy and the SLM parameters, such as laser power,

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scanning speed, hatch spacing, build orientation, and layer thickness on the mechanical and operational properties of Inconel 718 (IN 718) alloy were also addressed [17, 25, 26]. It is known that the nickelbased IN 718 alloy widely used in aerospace, oil, and defense industries owing to its capacity to retain high mechanical properties at elevated temperatures (< 650 °C). Good corrosion resistance and prolonged fatigue life [27–29] can also be achieved by AM processes. The operational properties of nickel-based superalloys were concluded to be essentially suffered due to the occurrence of the SLM-manufacturing defects. Therefore, the improvement of the surface roughness, porosity, microstructure, and residual stress state of the IN 718 parts fabricated by the SLM process becomes especially relevant. Сurrеntlу, various mechanical surface treatments such as a laser shock peening (LSP) [30], shot peening (SP) [31], cavitation peening (CP) [32], surface mechanical attrition treatment (SMAT) [33], ultrasonic shot peening (USP) [34], ultrasonic impact treatment (UIT) [35], and barrel finishing (BF) [36]

Journal Pre-proof аrе widely used fоr surface engineering of both large-scale and small-sized metal components. Modification of thеir near-surface layers induced by surface plastic deformation allows providing the required surface microrelief, increased surface hardness, and compressive residual macrostress. For instance, the LSP involves the generation of severe surface plastic deformation on the treated specimen due to the plasma-induced shock waves. Conversely, other processes consist of random (SMAT, USP or BF) or directional (SP) bombarding the certain area of the treated surface by high-energy balls/shots. The UIT process also provides severe plastic deformation of the treated surface but by means of multiple

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impactions by several cylindrical pins. The specific energy accumulated in the modified surface layer can

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be variated in a wide interval by changing the processes’ parameters. It is also important to account for

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the possibilities of various post-processing techniques with regard to the shape and size of the processed parts/specimens. Thus, they can be applied for mechanical post-processing of the small-sized (SP, SMAT,

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LSP, USP, and BF), large-scale (SP, LSP, and UIT), and complex-shape (SP, SMAT, LSP, USP, BF, and

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CP) parts. Additionally, the BF process or vibratory/magnetic finishing allows surface treatment of the SLM-built parts without fixturing. The above-mentioned methods have some reservations but most of

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components.

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them can be applied to ensure the enhanced corrosion/wear resistance and fatigue life of the SLM-built

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Several ultrasonically driven mechanical surface treatments, such as mono-pin UIT process [8], multi-pin UIT process [11], and ultrasonic nanocrystal surface modification (UNSM) [9, 37], were recently used to enhance the surface properties of the SLM produced components made of 316L steel, Ti-6Al-4V, and ALSi10Mg alloys. It was shown that the surface roughness, residual tensile stress, and porosity can be largely reduced, the epitaxial growth of columnar grains can be prevented, and the surface hardness can be increased owing to the application of the ultrasonic surface deformation at the intermediate stages of the SLM process. The multi-pin UIT process is considered to be more effective and applicable in industrial scales, as the conventional SP process. It was shown to improve both the surface roughness and hardness of the surface layer of uniform hardening depth [38, 39]. However, based on the practical application, the LSP, BF, USP or SP techniques can be more effective for post-processing of the small-sized and complex-shape metal parts.

Journal Pre-proof An improvement of the surface characteristics of the SLM-fabricated AlSi10Mg parts after the UNSM, SP, and LSP processes was confirmed [40–42], and the SP-induced diminution in the overall relative subsurface porosity led to an increase in low- and high-cycle fatigue resistance [42]. Moreover, a reduction in surface imperfections by combined post-processing methods (heat treatment + sandblasting, heat treatment + SP, sand/glass blasting + electro/plasma polishing) can also enhance the surface properties of the SLM-built specimens [43, 44]. It was also confirmed that the wear resistance of the asbuilt specimen made of 17-4 stainless steel or AlSi10Mg alloy was improved after the SP process [12]. At

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the SLM-fabricated IN 718 alloy parts are virtually absent.

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the same time, the attempts to use the mechanical surface treatments to improve the surface properties of

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The aim of this work is to analyze the effects of the barrel finishing, shot peening, ultrasonic shot peening, and multi-pin ultrasonic impact treatment on the surface topography, hardness, porosity, and

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residual stress of the IN 718 alloy specimens manufactured by the SLM process. The effect of the

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accumulated energy on the outcomes of the applied surface treatments is also addressed.

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2. Material and Methods

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2.1.1. Powder material

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2.1. Additive manufacturing details

A nickel-based pre-alloyed IN 718 powder with the spherical shape and the particle size distribution in a range of 10…54.8 μm was selected as feedstock material (Fig. 1a). This powder was produced by argon gas atomization. Nominal chemical composition of the IN 718 powder is given in Table 1. Fig. 1b shows the particle size distribution for the initial powder. It can be observed that the diameter of a vast majority (~80%) of powder particles ranges within the interval of 17…55 μm. Table 1 Nominal chemical composition of IN 718 powder and SLM-built part (in weight percent, wt. %) Powder SLM-built part

Ni 50–55 51.34

2.1.2. Specimens

Cr 17–21 19.08

Nb 4.7–5.5 5.33

Mo 2.8–3.3 3.28

Ti 0.6–1.2 0.98

Al 0.2–0.8 0.46

C ≤0.08 0.08

Mn ≤0.35 0.27

Si ≤0.35 0.18

Fe Balance Balance

Journal Pre-proof The turbine blade test parts with base dimensions of 85.4 mm long, 32.5 mm wide, and 60 mm height were manufactured using commercially pure pre-alloyed powder (Fig. 1с). The building direction (Z axis) was perpendicular to the building platform. The chemical composition of SLM-built parts measured using is listed the energy dispersive X-ray spectroscopy (EDS) method is listed in Table 1. After that, the fabricated parts were removed from the building platform by means of a wire electrical discharge machining (EDM). The EDM had also been used for cutting the turbine blade specimens into three sections (85.4 mm x 32.5 mm x 20 mm) perpendicular to the build direction (Fig. 1d). The square

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specimens of similar slight curvature and dimensions 20 mm x 20 mm x 3.5 mm were finally cut for

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experiments (Fig. 1e).

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Fig. 1. Micrograph of morphology (a) and particle size distribution of the IN 718 powder (b), the image of the cut turbine blade parts (c,d) and experimental specimen (e), scheme of the SLM process (f) and scanning strategy (g) 2.1.3. Selective laser melting process The specimens were fabricated by the SLM process using an industrial Renishaw AM400 machine (with a maximum build volume of 248 mm × 248 mm × 285 mm). In the SLM process, a computercontrolled scanning laser beam is applied as the energy source to provide selective layer-by-layer melting of the powder particles (Fig. 1f) [15]. The machine was equipped with a scanning optics (F-theta

Journal Pre-proof objective) with integrated z-axis positioned and continuous wave ytterbium fiber laser with a maximum spot diameter of 70 µm and variable output laser power (with a maximum power of 400 W) capable to the perform scans across the build platform at a maximum scanning speed of 7000 mm/s. The chamber is filled with high purity argon gas and it is recirculated during the process to prevent/minimize oxidation of the build components. Unlike conventional scanning strategy (where the entire slice is subjected to a cross- hatching scan pattern) [21, 26], a multidirectional scanning strategy was used when the biaxial laser scanning patterns were rotated by 67° for each consecutive layer to provide uniform heat

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distribution resulting in the reduced residual stresses and porosity [24]. In addition to that, each layer was

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collected from sequentially formed stripes of 5 mm wide (Fig. 1g). The layer thickness for this build was

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set to 60 μm. The SLM process parameters of the studied nickel-based superalloy are given in Table 2.

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The SLM parameters were kept the same for all fabricated parts.

Table 2 Parameters of the SLM process 200

Spot size, μm 70

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Layer thickness, μm 60

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2.2.1. Barrel finishing

Hatch angle, °

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2.2. Post-processing techniques

Stripes spacing, mm 5

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Power, W

Scanning speed, mm/s 700

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The barrel finishing of the SLM-built specimens was carried out by means of special equipment consisted of a hexagon barrel, metal or ceramic shot media (the former in our case), and motor [45]. The scheme and parameters of the BF process are shown in Fig. 2a and listed in Table 3, respectively. During the BF process, the barrel was forcedly rotated by the motor with the rotation speed of 66 rpm to induce complex movements of the treated specimens, and their surfaces were subjected to numerous highspeed collisions with the spherical shots (diameter of 3 mm). The accumulated impact energy Ê in the treated surface layers was assessed as a product of the energy of single impact, the number of balls/pins, impact frequency, and treatment time. The rotation speed was determined based on a media type and a barrel diameter. The filling ratio of the barrel was 15%. As a result of these numerous multi-directional collisions, the surfaces of the treated specimens are severely deformed. The BF process lasted for 14400 s to accumulate high enough strain energy in the treated surface (Table 3).

Journal Pre-proof 2.2.2. Shot peening The shot peening treatment, which was implemented in this study at ambient temperature using industrial equipment, is known to produce severe plastic deformation of the treated surface due to the bombardment by metal spherical shots of small diameters (see the scheme in Fig. 2b) [31, 40]. During the SP process, the kinetic energy of the balls (made of 52100 steel, diameter of 0.5 mm and a hardness of 45…51 HRC) was driven by the compressed air (with a pressure of 0.55 MPa), the resulting impact speed was ~125 m/s. The head/nozzle feed rate was 10 mm/s, the distance between the nozzle and the work

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surface was 30 mm. The specimens were shot peened for 120 s to achieve high enough accumulated

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energy (Table 3).

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2.2.3. Ultrasonic shot peening

The USP system consisted of an ultrasonic generator with a frequency of 21.6 kHz and a power

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output of 0.8 kW, ultrasonic vibration system and a special chamber filled with the peening media

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(22 steel balls of 3.5 mm in diameter). Fig. 2c shows the schematic of the USP process, during which the electric energy is transformed into mechanical ultrasonic vibrations by means of a piezoelectric

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transducer, and the vibrations are amplified by the step-like ultrasonic horn. The horn tip vibration energy

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is transformed into acquired kinetic energy of the balls, which produced severe plastic deformation of the

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surface layer of the treated specimen [34]. The USP process was performed using the horn amplitude of 40 μm, and the distance between the specimen and the horn tip surface of 40 mm. The USP process was performed at ambient temperature and lasted for 300 s. The USP process parameters are also listed in Table 3. 2.2.4. Ultrasonic impact treatment The UIT of the SLM-built specimens also results in severe plastic deformation of the near-surface layers. A scheme and parameters of the UIT process are shown in Fig. 2d and Table 3, respectively. The UIT equipment contained an ultrasonic generator with a frequency of 21.6 kHz and a power output of 0.8 kW, an acoustic vibration system with a piezoceramic transducer, step-like horn, and multi-pin impact head [46–48]. Impact loading is caused by the vibration of the ultrasonic horn. A special impact head is positioned on the horn tip. The high-frequency impacts (1 ± 0.5 kHz) were produced by seven cylindrical

Journal Pre-proof pins of 5 mm in diameter positioned in the head that was forcedly rotated during the treatment (rotation speed of 76 rpm) to provide the lateral component of load. Pins acquire their kinetic energy from the ultrasonic horn tip and produce impacts by the treated surface providing the normal (vertical) component of load. The UIT duration was of 120 s, the amplitude of ultrasonic horn was ~18 μm, a static load on the

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acoustic system was 50 N, and a specimen feed rate was 600 mm/min.

Fig. 2. Schemes of the BF (a), SP (b), USP (c), and UIT (d) processes

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Table 3 Processing parameters of mechanical surface treatments

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Processing Energy, Quantity Impact Treatment Accumulated Sample Strain, Surface method E (mJ) of frequency, time, t (s) energy, thickness, ε = Δh/h0 strain, balls/pins fi (Hz) ΣE h (mm) (%) εS = Δh/hS (J/ mm2) (%) BF [49] 1.6 ~20 ~1 14400 1.15 3.545 0.475 8.45 SP [50] 1.5 ~100 ~70 120 3.15 3.529 0.793 14.10 USP [34] 5.6 22 ~22 300 2.03 3.540 0.484 8.60 UIT [46] 2 7 ~1∙103 120 4.2 3.519 1.074 19.10

2.3. Specimen surface characterization

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2.3.1. Topography

The 3D surface texture evolution was studied by means of a 3D Taylor Hobson Form Talysurf 120 surface profilometer. The dimension of the measured area is 3х3 mm2. An assessment of height parameters (Sa, Sp, Sv, and Sz) characterizing the 3D surface topography was conducted according to ISO 25178 standard. 2.3.2. Residual porosity The specimens were mechanically cut into vertical (section perpendicular to the deposited layers) and horizontal (section parallel to the deposited layers) cross-sections, ground and polished for examinations of superficial and volume defects according to a standard metallographic procedure. Then, the specimens were electrolytically etched at 1…2 V dc by a Lucas' reagent consisting of the lactic acid (50 ml), hydrochloric acid (150 ml) and oxalic acid (3 g) for 5 s to identify the places of formation of the

Journal Pre-proof aspherical pores. The residual porosity was observed by a Leica MEF4A optical microscope equipped with a digital camera, a TESCAN Mira 3 LMU scanning electron microscope (SEM), and the EDS measurements. Residual porosity was statistically analyzed using the Image-Pro Plus software. The area fractions and size distributions of pores in the build cross-sections of the SLM-built and post-processed specimens were estimated. 2.3.3. Microhardness and residual macrostress measurements The microhardness depth profiles in the specimen cross-sections were measured along three parallel

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paths starting from the surface towards the core material using a tester Leica VMHT with a Vickers

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indenter loaded by 0.025 kgf (HV0.025) and the dwell time of 15 s. The standard deviation for the

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microhardness measurements was ±1 HV. The surface microhardness in the near-surface layer was estimated at a depth of 15…20 μm from the top surface. In all cases, a total of five measurements were

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experimental data did not exceed by 2–5 %.

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carried out at different treated zones and the averaged magnitudes were reported. Scatter of the

The X-ray diffraction (XRD) sin2ψ based method was used to estimate the residual stress of the

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SLM-built and post-processed specimens using a Rigaku Ultima IV diffractometer in a CuKα-radiation

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with a graphite monochromator at 30 kV and 30 mA, a 2θ (20°…120°) scanning speed of 2 o/min. The

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depth distributions of residual stress were performed by a stepwise electrolytic polishing [38, 51]. The grain/crystallite analysis in the subsurface layer of about 10...20 μm was carried out by the XRD method as well.

3. Results 3.1. Surface topography The surface topography of the SLM-built and post-processed IN 718 specimens are compared in Figs. 3 and 4. The 3D assessment of surface texture parameters was estimated by the arithmetic mean height (Sa), the maximum peak height (Sp), the maximum valley height (Sv) and the maximum height (Sz) of the surface irregularities (Fig. 5).

Journal Pre-proof As usual, the morphology of the side surfaces of SLM-built specimen contains a large number of manufacturing defects, such as ellipsoidal/spherical balls or shrinkage cavities, partially melted powder particles or spattering powder particles, open pores, and signs of the laser tracks (Figs. 3a,b, and 4a-d) [57]. In addition to that, the surface is rough (Sz parameter is ~60 μm) having the irregularities of relatively high height (Fig. 3, Fig. 5). It is seen that the partially melted metal powder or spatters (indicated by arrows in Fig. 4b) are adhered to the surface specimen because of their existence inside the heat-affected zone [40]. Fig. 4c shows a general view of balling that formed alongside the surface via a

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segmentation effect of the elongated melt pool [52]. The balling phenomenon is usually formed during

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the SLM process when the molten pool becomes discontinuous and breaks into separated islands via a

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shrinking tendency to decrease the surface energy under the action of surface tension. Additionally, the unmolten powder particles were found in the near-surface layer (indicated by arrows in Fig. 4d). As a

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result, above-mentioned defects essentially deteriorate surface integrity. In order to reduce or eliminate

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surface defects, the surface mechanical methods are usually applied [45–48, 50–53]. The typical surface texture of the SLM-built specimens formed after various mechanical surface

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treatments is also shown in Figs. 3 and 4. In particular, the BF and USP processes do not result in a

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significant change in the surface texture in comparison with the SLM-built specimen. As compared to the

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Sa parameter characterizing the surface roughness of the SLM-built specimen, the Sa magnitudes evaluated after the BF and USP processes were respectively diminished just by 20% and 34% (Fig. 3, Fig. 5). Nevertheless, it should be noted that the ellipsoidal and spherical balls were successfully removed from these surfaces (Fig. 4f,j). In addition to that, the BF and USP processes provide a ~50% reduction in the total height parameter (Pt) of surface profile in comparison with the SLM-built state (Fig. 3). In contrast to the BF and USP processes, both SP and UIT led to the formation of new surface microreliefs on the post-processed SLM-built specimens (Figs. 3e,f,i, and 4g,k), reducing adverse surface defects. As compared to the SP process (Sa = 6.65 μm, Sz = 56.7 μm), UIT forms regular microrelief with a smoother surface (the Sz parameter is ~20 μm) and lower surface waviness owing to the application of the multi-pin ultrasonic tool, which produces the sliding impacts of pins by the specimen surface due to forced rotation of the impact head [38] (Figs. 3 and 5). The registered UIT-caused Sa parameter becomes

Journal Pre-proof twice lower than that of the SLM-built specimen. It is also important to point out that the dimpled surface microreliefs formed by USP and SP are significantly different due to the fact that the balls driven by the directional compressed air flow in the case of the SP process hit nearly perpendicular to the treated surface, while in the case of the USP process the balls impacted the treated surface in random angles owing to their chaotic motion inside the USP-chamber. Therefore, in contrast to USP, the SP-induced surface relief is more significantly affected by the shot speed. Additionally, a larger diameter of the shot media and multidirectional character of impacts in the case of USP can be the reason of slightly decreased

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surface texture parameters as compared to that formed after SP (Fig. 5). This result also correlates well

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with the literature data [53]. The reduction in the surface texture parameters and the modification

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homogeneity induced by SP and USP could be increased when the higher-energy shots would be applied for a longer time [54].

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In our experimental conditions, in comparison with the SLM-built state the surface roughness (Sa =

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5.27 μm) of the post-processed specimens were respectively decreased by 20.6%, 26.2%, and 57.4% after the BF, USP, and UIT processes except for the SP-treated ones (Fig. 5). It should be noted that the Sz

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parameter was reduced in all treated SLM-built specimens except for the SP-treated ones.

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Fig. 3. The surface profile and 3D texture of the SLM-built (a) and BF (b), SP (c), USP (d), and UIT (e) processed IN 718 specimens

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Fig. 4. The surface morphology and images of the near-surface layer of the SLM-built (a-d) and BF (e,f), SP (g,h), USP (i,j), and UIT (k,l) processed IN 718 specimens

Fig. 5. Surface texture parameters of the SLM-built and BF, SP, USP, and UIT processed IN 718 specimens 3.2. Residual porosity Along with the surface microrelief parameters, the residual internal porosity in the near-surface layers is also of special importance for the wear/corrosion and fatigue resistance of a material. The residual porosity near the specimen surface was studied in the build (Z-axis) and scan (XY-axis) directions (Figs. 6–8). Considering the near-surface layers, the highest area fraction of the pores is naturally seen in the SLM-built specimen, which can be explained to the inclusion of gases in the deposited layers during the

Journal Pre-proof melting and solidification processes (Fig. 6a). It is known that the SLM process is able to produce nearly full dense (98…99%) parts with some residual porosity in a form of a keyhole, spherical or shrinkage pores [40]. The subsurface porosity in the SLM-built specimen can be seen in Fig. 6a. At least two types of defects are visible here. In particular, the spherical pores (gas pores) caused by the incompleteness of the gas release from the melt during the SLM process (indicated by arrows in Fig. 7a, Fig. 8a). The gas bubbles of the spherical shape formed in a molten pool can further be 'frozen' in the solidified part as

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spherical pores. More rarely, the aspherical pores (also known as lack-of-fusion defects) might be formed

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between the previously deposited layers and scan tracks due to insufficient melting of the powder layer

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before the solidification stage [55]. The aspherical pores can be found to be oriented parallel, perpendicularly or at some angle with regard to the build direction (indicated by arrows in Fig. 7b). The

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defects in the SLM-built specimen varied in size from 1 to 50 μm.

Fig. 6. Residual porosity in the build direction of the SLM-built (a) and BF (b), SP (c), USP (d), and UIT (e) processed IN 718 specimens

Journal Pre-proof From the quality point of view of the SLM-printed metal parts of a high level of residual porosity require additional post-processing, for instance by the methods of severe plastic deformation, to eliminate or diminish the residual porosity at least in the near-surface layers. The results of a statistical analysis of the area fractions and size distributions of residual pores show that the surface porosity percentage is decreased irrespective of the treatment method used (Figs. 6 and 10). Pores of different size were observed in the specimens along the build direction. The smallest size of pores was registered after the USP and UIT processes due to effective pores closure induced by severe plastic deformation of high

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strain rate and high extent supplemented by ultrasonic action [56]. Based on the quantitative porosity analysis, the residual porosity values after USP and UIT processes were respectively decreased by 55 and

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84% as compared to the SLM-built specimen (Fig. 10). Compared to the USP process, the UIT process

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significantly decreases and virtually removes the near-surface porosity, forming a deeper densified layer

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(~200 μm). It is seen that spherical ball or unmelted powder (Fig. 8a) is deformed by the multi-pin ultrasonic tool (Fig. 8f). As a result, the smallest number of pores was observed after the UIT process

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(0.118%), which seems to be fully eliminated if the higher oscillation amplitude and/or treatment duration

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would be used (Figs. 6–8). It was also shown in our earlier studies that multi-pin UIT can result in effective pores closure in the treated surface layer of the powder metallurgy Ti6Al4V alloy [35].

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Recently, Wang et al. [9] also shown that ultrasonic surface rolling of the SLM-built Ti6Al4V alloy was able to form the pore-free surface layer of ~200 μm thick. Further thermal treatment can be expected to provide the healing of these closed pores and thus to enhance the overall material properties. In contrast to the USP and UIT processes, BF and SP led to a lesser reduction in subsurface porosity (~20…40%). In addition to that, the large oval-shape pores (Fig. 6b, Fig. 6c) or aspherical shape pores (indicated by arrows in Fig. 7c, Fig. 7e, Fig. 8e) were detected in the near-surface area. This result also correlates with the literature data, where the SP process using steel balls reduces the porosity in an aluminum alloy by 15–30% at the depth up to 500 μm [42].

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Fig. 7. SEM micrographs of residual porosity in the build direction of the SLM-built (a,b) and BF (c), USP (d), SP (e), and UIT (f) processed IN 718 specimens

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Fig. 8. SEM micrographs of residual porosity in the scan direction of the SLM-built (a,b) and BF (c), USP (d), SP (e), and UIT (f) processed IN 718 specimens

Fig. 9. Relative porosity in the build direction of the SLM-built and BF, SP, USP, and UIT processed IN 718 specimens

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3.3. Hardness A typical near-surface microhardness depth profiles after application of different post-processing techniques are shown in Fig. 10. The surface microhardness of the SLM-built specimen (HV0.025 ≈ 390) is increased after application of all post-processing techniques that is seemingly originated owing to the increasing dislocation density and some grain/crystallite refinement. The HV0.025 magnitudes achieve

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~450, ~480, ~590, and ~650 after BF, USP, UIT, and SP, respectively. The microhardness of the surface

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layers of the BF and USP processed specimens are higher than that of the SLM-built specimen by

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~15…25 %. At the same time, as compared to the SLM-built specimen (41 μm), the crystallite size in the near-surface layer was found slightly decreased after BF (~34 μm) and USP (~23 μm), respectively, The

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UIT process led to a 50% increase in the near-surface hardness, forming the crystallite size of ~15 μm, as

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compared to the untreated SLM-built specimen, while the SP-induced surface microhardness is approximately 10% higher than that of the UIT-processed specimen. Additionally, the crystallite size

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magnitude is the smallest after the SP process (~10 μm).

Fig. 10. Microhardness distribution in the build direction of the SLM-built and BF, SP, USP, and UIT processed IN 718 specimens Analyzing the HV depth profiles one can conclude that the hardening depth is the lowest (~80 μm) after the BF process. The thicker hardened layer (~140 μm) were produced by the USP process, while the deepest hardened layers (~180 μm) was respectively formed after the UIT and SP processes. These

Journal Pre-proof observations correlate well with the impact energy accumulated in the surface layers treated by different post-processing techniques (Table 3). 3.4. Residual stress From point of view of desirable prolonged operation life of the SLM-built parts, the sign and magnitude of residual stresses are additional critical characteristics. In this study, they were assessed using the X-ray stress analysis. Considering the residual stress magnitudes formed in the subsurface layers of the studied specimens, the tensile residual stresses formed in the SLM-built specimen

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(+120 MPa) because of the large thermal gradients occurred near the laser spot during rapid heating and

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cooling of the molten pool (Fig. 11) [21] were successfully eliminated by post-processing techniques.

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Moreover, the residual stresses formed by post-processing techniques are of the compressive character.

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Fig. 11. Residual stress depth profiles formed in the build direction of the SLM-built and BF, SP, USP, and UIT processed IN 718 specimens The estimated magnitude of the BF-induced stresses (−201.4 MPa) is the lowest in comparison with those registered after the USP (−313.8 MPa), multi-pin UIT (−428.7 MPa), and SP (−510.7 MPa) processes. It is also important that the SP, USP and UIT processes generate compressive residual stresses in rather deep subsurface layers (~140–180 μm). It is, therefore, could be expected that these compressive stresses would be beneficial in the improvement of any surface-related operational properties of SLMfabricated specimens/parts (fatigue life, wear, and corrosion resistance).

4. Discussion 4.1. Roughness

Journal Pre-proof The experimental results presented above (Fig. 3, Fig. 4) have indicated that the surface of SLMfabricated parts contains a large number of manufacturing defects (surface balls, partially melted or sputtered powder particles, shrinkage cavities and/or open pores), which are usual for the SLM manufactured parts [25, 40]. Various SLM parameters (laser energy density, hatch spacing, scanning speed) were shown to affect the surface topography of the IN 718 parts [25]. For instance, the balling effect (needle-like tips and deep pits) was decreased at a higher energy density. It was concluded that lower hatch spacing allowed producing the parts with smoother surfaces [57]. The results of this study,

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as well as some literature data [8, 9, 13, 14] show that mechanical post-processing of the SLM-built parts

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can be another effective technological solution. Severe plastic deformation (SPD) of the treated surfaces

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can be able to reduce the manufacturing defects, although mainly in the near-surface layers. Often, it is enough to improve the surface characteristics of the SLM-built parts to meet the stringent requirements in

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various engineering applications.

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Changes in the surface roughness parameters (Fig. 5) observed in this study well correlate to the appropriate literature data [13, 37, 40–42]. In particular, the SP process caused the almost 50% reduction

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in the surface roughness of SLM-built AlSi10Mg specimen (Ra = 11.96 μm) [40]. However, as compared

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to the machined or machined-and-polished surfaces, the SP processed surface acquired the increased

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roughness magnitudes [41]. On the other hand, such post-processing techniques as a media blasting, tumbling, LSP, and UNSM were shown to improve the surface texture of the SLM-fabricated metal components. It is also important that the multi-pin UIT process can results in a regular surface microrelief with lower surface waviness and roughness as compared to those formed after SP, and USP, which produce random impacts of high-energy balls by the components surface [38, 39, 47]. The extent of the roughness changes induced by different post-processing mechanical techniques can be illustrated with respect to the accumulated impact energy E (Table 3) using Fig. 12. Generally, it can be concluded that all mechanical surface treatments applied in this study are very effective for improving the surface microrelief parameters of the SLM-built parts – the surface roughness linearly decreases with accumulating energy, which correlates to the surface deformation extent (Table 3). Only data related to the SP-processed specimen (E ≈ 3.15 J/mm2) stand out of the linear dependence that can be

Journal Pre-proof explained by the impacting features – the SP-induced impacts are directed perpendicularly to the treated surface, and thus they induce higher acquired roughness. The presented data can be easily used to choose

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optimal regimes of other mechanical surface treatments provided the E would be known.

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Fig. 12. Dependencies of roughness Sa (1) and porosity P% (2) on the accumulated impact energy E. Dashed lines indicate the apparent trends

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Fig. 13 shows SEM images of the surface topographies of the SLM-built and post-processed specimens. It is seen that the SLM-built surface is relatively smooth without a lot of intrusions and

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extrusions (Fig. 13, a1), which correlates well to the surface topography shown in Fig.3,a. However, it

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contains numerous partially melted powder particles attached to the surface (Fig. 13, a1–a3) and some portion of the shrinkage cavities (Fig. 13, a1, a2). The surface topographies after mechanical post-

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processing (Fig. 13, b1–d1) become smoother with the increased accumulated energy applied. Still, some deformed particles are seen in the BF-processed (Fig. 13, b1) and USP processed (Fig. 13, c1) surfaces. Conversely, the SP (Fig. 13, d1) and UIT (Fig. 13, e1) modified surfaces are almost completely free from such manufacturing defects. Only some residuals of surface filaments can be seen (Fig. 13, d2, e2).

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Journal Pre-proof Fig. 13. SEM images of the surface topographies of the SLM-built (a1–a3) specimen and specimens post-processed by BF (b1–b3), USP (c1–c3), SP (d1–d3), and UIT (e1–e3) treatments. Finally, when the complex-shape or small-sized products need to be treated one should account for more convenience and higher productivity of the BF, USP, LSP, SP, media blasting and tumbling processes with regard to the multi-pin UIT process. In particular, SP, USP, and LSP are usually used to treat the turbine blades, aircraft wings, and other curved surfaces to reduce aerodynamic drag as the dimpled surfaces are known to have a lesser aerodynamic drag than that of extremely smooth surfaces. 4.2. Porosity Porosity is another feature determining the quality and operation life of the AM products. Formation of the defects in the SLM large-scale components was shown to be unavoidable [58]. According to the

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literature data, the manufacturing defects may play the role of stress risers, and they thus may reduce the fatigue life and wear resistance of the fabricated metal components [27, 41]. For instance, the SLM-built

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AlSi7Mg specimen having 1% of porosity demonstrated the fatigue life decreased by 50% in comparison with that of the dense alloy [42]. The most frequent sites for fatigue cracks’ initiations are pores (Fig. 14)

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or unmelted particles in the near-surface area irrespective of the load conditions or stress state [28]. It should be noted that as compared to the gas (spherical) pores, the aspherical pores cause a significant

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deterioration of mechanical properties of the material due to their larger size [14, 25, 26, 42].

Fig. 14. SEM image of the spherical pore in the scan direction of the SLM-built IN 718 specimen

To minimize the porosity optimum regimens for SLM process can be found and used. It was revealed that the densest IN 718 alloy specimens with a relative density of 99.73% can be fabricated with the laser power of 245 W, scanning speed of 755 mm/s and hatch spacing of 90 μm [55]. Work by Mancisidor et al. [24] showed that a combination of optimum scanning speed (600…800 mm/s) and focus offset (8…10 mm) can significantly reduce the porosity both in the hatch and borders irrespective of the scanning strategies used.

Journal Pre-proof Use of post-processing treatments is another way to improve the properties of the SLM-built parts. On the one hand, a hot isostatic pressing (HIP) treatment is able to eliminate/minimize porosity in the material volume, and thus it is one of the most effective approaches. Compared to the initial value of porosity (~0.6%) of the vacuum induction melted IN 718 superalloy, the porosity was reduced using HIP treatment, and the extent of this reduction was proportional to the soaking time decreasing to 0.136% (by ~78%) and 0.082% (by 86%) after HIP treatment for 2 and 4 hrs, respectively [59]. On the other hand, the SPD methods can also be used for the improvement purposes tough they are

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able to diminish porosity mainly in the near-surface layers. In our case, the USP and UIT processes led to

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the porosity decrease (by 55 and 84%) in the top layers of the specimens (0.317 % and 0.118 %,

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respectively) seemingly because of the specific deformation conditions operated at these processes (multidirectional sliding impacts inducing simultaneous action of both normal and lateral components of

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the load) [35]. Indeed, such conditions were concluded to be more effective for the micro-pore closure

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than only hydrostatic compressive stress [35, 60].

Curve 2 in Fig. 12 shows that the higher the accumulated impact energy of the mechanical post-

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processing treatment applied the lower the residual porosity becomes. A linear trend indicated by the

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dashed line is apparent. Less extent of the porosity decrease observed in the case of SP (1.26 kJ) than

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expected following the shown linear trend seems to be caused by a lesser lateral component of the SPinduced impacts. According to the data obtained in this study, the UIT is the most effective in the pores closure. However, it is hard to use it for porosity management in the complex-shape parts, which can be more easily treated by any of the techniques operated with spherical impacting media (BF, USP, and SP). Mechanical post-processing of the SLM-built specimens can be used either as finishing treatment or as an intermediate process between the SLM steps (applied every several molten/solidified layers). Recently, it was shown that the ultrasonic peening applied after deposition of every two SLM layers facilitated the production of full density parts [9]. It might partially substitute such expensive method as HIP. 4.3. Hardness and residual stress

Journal Pre-proof Hardness and stress state of any material (especially, in the near-surface layers) are also crucial characteristics with respect to their operational life and durability. These parameters are naturally very important in consideration of the SLM-built parts and they were analyzed in the literature. It is known that one of the possible ways to to minimize residual stress at the SLM process is a variation of the scanning strategy [17, 24, 27]. A scanning strategy is chosen depends on the part geometry. The stripe or chessboard scanning strategies are suitable for parts with thicker sections. Abovementioned scanning strategies can generate a homogeneous distribution of residual stresses via shorter

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scan vectors. The heating of the build plate during the AM process is another technique, which is able to

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reduce residual stress. While the post-process heat or mechanical surface treatments can also relieve the

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harmful stresses in already built-up AM parts.

Recently, Wang et al. reported that ultrasonic surface rolling resulted in the hardening of the surface

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layer of the SLM-built Ti-6Al-4V alloy of 100…200 μm thick, and a maximum surface hardness was

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registered to be of 410…430 HV0.1 [9]. Increase in the hardness, shear resistance, and inhibition of the delamination initiation was concluded to be the reasons for the improvement of the AM parts by

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ultrasonic surface rolling [9]. Work by Zhang et al. [8] showed that the residual stress can be largely

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reduced by the UIT during the SLM process. In addition to that, it was revealed that the tensile residual

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stresses in the SLM-built specimen (176.3 MPa) can be reduced after UIT (49.9 MPa). Recently, the LSP process combined with the SLM process was implemented to decrease/eliminate the thermal residual stresses during the SLM process [10]. Moreover, work by Damon et al. [42] showed that the SP process shifted the maximum of the compressive residual stress profile towards the deeper layers. As shown in this study, all used surface mechanical treatments resulted in the surface hardening (Fig. 10, Fig. 15) and in the formation of compressed layers (Fig. 11, Fig. 15). Naturally, both microhardness and compressive stresses increase with the accumulated energy since it correlates to the extent of the surface deformation (Table 3). However, some decrease in the HV and σR magnitudes after UIT with regard to those observed after SP seems to be related to the sliding character of UIT produced impact, which is in contrast to the normal impacts mainly occurred at the SP process.

Journal Pre-proof Since various post-processing mechanical treatments formed the hardened and compressed layers of different thickness it would be more reasonable to analyze them accounting these thicknesses, for instance, using the hardness intensity gradient Ihard. This parameter can be calculated with regard to several layer thicknesses (say, 25, 50 and 100 μm) using the following expression [51]: I hard   HVhard  HVasbuilt  / h hard ,

(1)

where HVas-built and HVhard are the microhardness magnitudes of the SLM-built and hardened specimens

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(MPa), respectively, hhard is the thickness of the considered layer possessing enhanced hardness (μm).

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Fig. 15. Dependencies of microhardness HV0.025 (1) and residual stress (2) on the accumulated impact energy E. Dashed lines indicate the apparent trends

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The dependences of the hardness intensity gradient Ihard on the accumulated impact energy Ê are

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presented in Fig. 16 (curves 2-4). They can characterize the effectiveness of the used mechanical surface treatment in a view of the hardening-to-depth ratio. It is seen that the hardness intensity after the SP (E = 1.26 kJ) and UIT (E = 1.68 kJ) processes remain high enough in the surface layers of various thicknesses up to 100 μm (Fig. 16, curves 2-4). Thus, in view of the hardness intensity gradient, the UIT and SP processes would obviously result in a longer operating life of the processed SLM-parts as compared to those undergoing BF or USP.

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Fig. 16. Dependencies of microhardness HV0.025 of the near-surface layer (1), hardness intensity gradient Ihard (2–4) for the layer thicknesses of 25 μm (2), 50μm (3) and 100 μm (4), and local plasticity characteristics δН (5) in the build direction of the SLM-built IN 718 specimens on the accumulated impact energy E Another important parameter, which can be used to assess the possibility of further improvement of

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the SLM-built part, is the local plasticity characteristics δН presenting a fraction of the plastic strain in the

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total elastoplastic strain (the ability of the material to plastic deformation). It can be determined by the

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indentation method using a Vickers pyramidal indenter according to the formula [51]:

HV , E

(2)

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 H  1  14.3 1    2  2 

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where HV is the microhardness of the surface layer (GPa); μ is the Poisson's coefficient (μ = 0.278), E is Young's modulus (E = 200 GPa). Possible variation of E due to changing porosity was assumed

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negligible. The dependence of the local plasticity characteristic δН on the accumulated impact energy E is shown in Fg. 16 (curve 5). Actually, all mechanical surface treatments insignificantly decrease the material plasticity, and thus they can be further applied for a longer time without danger of premature brittle destruction of the SLM-built part. This information could be useful when the sequence of the procedures applied during the combined AM process (for instance, consisting of SLM and intermediate mechanical surface treatments) would be chosen [7–10]. Since the application of mechanical treatments every SLM-built layer would cause more time consuming it would be better to apply them every several SLM-formed layers, and both the material quality improvement and SLM process efficiency requirements would be satisfied. Analyzing the δН parameter allows preventing undesirable overstraining at the intermediate application of mechanical

Journal Pre-proof surface treatments during the SLM manufacturing. Work by Zhang [8] confirmed that the UIT process applied after the deposition of 2–3 layers reduced both the long columnar grains and structural defects, and it formed compressive residual macrostress in the SLM-built parts. The ultrasonic peening applied after deposition of every two SLM layers was also shown to facilitate the densification of the SLM-built part up to 'full' dense of conventionally cast material [9]. Type of appropriate post-processing technique can be chosen based on the analysis of the shape and dimensions of the part to be treated. While the regimes of the chosen post-processing treatments can be

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determined, accounting for the extents of the surface roughness reduction, manufacturing defects and

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porosity eliminations, hardening/compressing, and the thickness of modified layers. Fig. 16 compares the

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relative efficiency of the used mechanical surface treatments with regard their ability to change several studied characteristics. Some of these characteristics (roughness, porosity) were decreased after

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application of the post-processing techniques, but the others (HV, residual stresses, and penetration depth)

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were increased/enhanced. An appropriate mechanical method can be chosen accounting for this data and considering the applicability of various techniques for post-processing of the small-sized, large-scale, and

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complex-shape parts (see the ‘versatility’ parameter in Fig. 17).

Fig. 17. Summarized results of surface roughness, porosity, hardness, residual stress, penetration and 3D process versatility. Marker ‘0’ indicates the SLM-built condition, and markers ‘1’–‘4’ mean the gradation of relative efficiency of the used mechanical post-processing techniques in changing various characteristics.

It is also important to take a controllability/convenience and cost/time consuming of intermediate mechanical surface treatments into account. The BF (or media blasting and tumbling) and SP (or

Journal Pre-proof cavitation peening, USP, LSP) processes are rather controllable/flexible, but they can be used predominantly for surface finishing of the SLM-built products. Conversely, the UIT, UNSM or LSP processes can provide local treatment both at the intermediate stages (every several SLM-built layers) and after the end of the SLM-process. However, they need more effort to meet the precision requirements before including them to the SLM-manufacturing routes used for complex-shape components [10]. Additionally, as compared to the SP and cavitation peening, the LSP process is known to be more expensive. Nevertheless, the LSP process can provide individually controlled shots and the deeper layers

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compressed by residual stress [9].

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Summarizing remarks

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Application of the mechanical post-processing techniques, such as SP [12, 43], LSP [10], mono-pin

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UIT [8, 61], multi-pin UIT [11, 38, 39], UNSM [19, 37], to the SLM-built parts allow reducing both superficial and structural defects, ensuring the surface hardening and compressing of the surface layers of

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the parts. Therefore, such a technological solution allows manufacturing the complex products for the

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aerospace industry without changing their design or material, but only by applying additional mechanical post-processing to improve their operational properties.

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Type of appropriate post-processing technique can be chosen based on the analysis of the shape and dimensions of the part to be treated. While the regimes of the chosen post-processing treatments can be determined, accounting for the extents of the surface roughness reduction, manufacturing defects and porosity eliminations, hardening/compressing, and the thickness of modified layers (see Fig. 17).

The BF, SP, USP, and UIT processes applied in this study to improve the IN 718 alloy specimens manufactured by the SLM process demonstrated a good efficiency for modification of the surface topography, porosity, hardness and residual stress. The obtained results allow drawing the following conclusions: 

In comparison with the SLM-built state the surface roughness (Sa = 5.27 μm) of the postprocessed specimens were respectively decreased by 20.6%, 26.2%, and 57.4% after the BF, USP, and UIT processes except for the SP-treated ones. The Sz parameter was reduced in all treated SLM-built specimens except for the SP-treated ones;

Journal Pre-proof 

Compared to the residual porosity of the SLM-built specimen (0.697 %), the porosity magnitudes were respectively decreased by 23.1%, 40.6%, 55%, and 84% after the BF, SP, USP, and UIT processes. The UIT process significantly reduced the residual porosity (0.118 %) in the nearsurface layer;



The surface microhardness of the SLM-built specimen (~390 HV0.025) was increased after the BF (by 14.2%), USP (by 23.8%), UIT (by 50%), and SP (by 66.5%) processes;



The tensile stresses observed for the SLM-built specimen (+120 MPa) was successfully transformed into compressive stresses by the BF (−201.4 MPa), USP (−313.8 MPa), multi-pin UIT (−428.7 MPa), and SP (−510.7 MPa) processes;



The thickness of hardened, densified and compressed near-surface layers ranges from ~80 μm

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after BF to ~140 μm after USP, and ~180 μm after SP and UIT processes, which correlates to the accumulated impact energy per unit surface area and to the extent of surface deformation.

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Acknowledgments. The authors would like to acknowledge O.A. Plyvak (head of the Laboratory of Measuring Equipment, National Technical University of Ukraine "Igor Sikorsky Kyiv Polytechnic Institute") for surface roughness measurements. We would like also to thank S. Faust (Otto von Guericke University Magdeburg, Germany) for the surface topography measurements and T.A. Krasovsky (head of the Laboratory of Electronics, Kyiv Academic University) provided the USP equipment.

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Figure captions: Fig. 1 Micrograph of morphology (a) and particle size distribution of the IN 718 powder (b), the image of the cut turbine blade parts (c,d) and experimental specimen (e), scheme of the SLM process (f) and scanning strategy (g) Fig. 2 Schemes of the BF (a), SP (b), USP (c), and UIT (d) processes Fig. 3 The surface roughness (green curve) and waviness (red curve) profiles, and 3D surface texture of the SLM-built (a) and BF (b), SP (c), USP (d), and UIT (e) processed IN 718 specimens

Journal Pre-proof Fig. 4 The surface morphology and images of the near-surface layer of the SLM-built (a-d) and BF (e,f), SP (g,h), USP (i,j), and UIT (k,l) processed IN 718 specimens Fig. 5 Surface roughness (a) and waviness (b) parameters of the SLM-built and BF, SP, USP, and UIT processed IN 718 specimens Fig. 6 Residual porosity in the build direction of the SLM-built (a) and BF (b), SP (c), USP (d), and UIT (e) processed IN 718 specimens Fig. 7 SEM micrographs of residual porosity in the build direction of the SLM-built (a,b) and BF (c), USP (d), SP (e), and UIT (f) processed IN 718 specimens Fig. 8 SEM micrographs of residual porosity in the scan direction of the SLM-built (a,b) and BF (c), USP (d), SP (e), and UIT (f) processed IN 718 specimens

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Fig. 9 Relative porosity in the build direction of the SLM-built and BF, SP, USP, and UIT processed IN 718 specimens

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Fig. 10 Microhardness distribution in the build direction of the SLM-built and BF, SP, USP, and UIT processed IN 718 specimens

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Fig. 11 Residual stress depth profiles formed in the build direction of the SLM-built and BF, SP, USP, and UIT processed IN 718 specimens

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Fig. 12 Dependencies of roughness Sa (1) and porosity P% (2) on the accumulated impact energy E. Dashed lines indicate the apparent trends

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Fig. 13 SEM images of the surface topographies of the SLM-built (a1–a3) specimen and specimens post processed by BF (b1–b3), USP (c1–c3), SP (d1–d3), and UIT (e1–e3) treatments

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Fig. 14 SEM image of the spherical pore in the scan direction of the SLM-built IN 718 specimen

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Fig. 15 Dependencies of microhardness HV0.025 (1) and residual stress (2) on the accumulated impact energy E. Dashed lines indicate the apparent trends

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Fig. 16 Dependencies of microhardness HV0.025 of the near-surface layer (1), hardness intensity gradient Ihard (2–4) for the layer thicknesses of 25 μm (2), 50μm (3) and 100 μm (4), and local plasticity characteristics δН (5) in the build direction of the SLM-built IN 718 specimens on the accumulated impact energy E Fig. 17 Summarized results of surface roughness, porosity, hardness, residual stress, penetration and 3D process versatility. Marker ‘0’ indicates the SLM-built condition, and markers ‘1’–‘4’ mean the gradation of relative efficiency of the used mechanical post-processing techniques in changing various characteristics.

Journal Pre-proof Highlights

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The turbine blade test IN 718 parts were manufactured by selective laser melting (SLM) Mechanical post-processing techniques were applied to improve the surface layer properties Barrel finishing (BF), shot peening (SP), ultrasonic shot peening (USP), and ultrasonic impact treatment (UIT) were applied and compared Surface texture, roughness, hardness, porosity, and residual stress were studied

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