Post-Processing of Complex SLM Parts by Barrel Finishing

applied sciences

Article Post-Processing of Complex SLM Parts by Barrel Finishing

Alberto Boschetto *, Luana Bottini, Luciano Macera and Francesco Veniali

Department of mechanical and aerospace engineering, Sapienza University of Rome Via Eudossiana 18, 00184 Rome, Italy; [email protected] (L.B.); [email protected] (L.M.); [email protected] (F.V.) * Correspondence: [email protected]

Received: 24 January 2020; Accepted: 14 February 2020; Published: 19 February 2020

Featured Application: The work application is the post-processing of Selective Laser Melting parts in manufacturing industry. It provides process parameters to employ Barrel Finishing to condition Ti6Al4V and Inconel718 complex geometries.

Abstract: Selective laser melting (SLM) enables the production of metal complex shapes that are difficult or impossible to obtain with conventional production processes. However, the attainable surface quality is insufficient for most applications; thus, a secondary finishing is frequently required. Barrel finishing is an interesting candidate but is often applied without consistent criteria aimed at finding processing parameters. This work presents a methodology based on Bagnold number evaluation and bed behavior diagram, developed on experimental apparatus with different charges and process parameters. The experimentation on an industrial machine and the profilometric analysis allowed the identification of appropriate process parameters and charge media for finishing the investigated materials (Ti6Al4V and Inconel718). Two case studies, characterized by complex shapes, were considered, and consistent surface measures allowed understanding the capability of the technology.

Keywords: selective laser melting; barrel ﬁnishing; Ti6Al4V; Inconel718; post processing; roughness

1. Introduction Selective laser melting (SLM), which belongs to the category of powder bed fusion [1], is one of the fastest growing additive manufacturing (AM) technologies globally. It allows producing metal components with complex geometry, resulting in severely reduced design and production times of the entire productive process [2]. During the fabrication, metallic powders are uniformly spread on a building platform by a re-coater creating the powder bed; a focused laser beam scans it, according to the designed path, selectively melting the powders [3]. After the consolidation of this layer, the building platform lowers, and the process is repeated until the part is completed. The Wohlers report [4] indicated that 1768 metal AM systems were sold in 2017, a surge of nearly 80% compared to the previous year (983 systems in 2016). This rapid growth is due to the development of new functional applications in different fields such as aerospace, biomedical, defense, automotive, and many other industries thanks to the benefits offered by metal AM with respect to conventional manufacturing technologies. [5]. In particular, SLM permits producing fully dense parts in a direct way, thereby saving on raw material with respect to subtractive technologies [6]. It promotes the possibility of producing customized metal components in a cost-effective manner, allowing great flexibility in production; the design freedom allows improving the efficiency [7] and the functionality [8,9] of

Appl. Sci. 2020, 10, 1382; doi:10.3390/app10041382 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 1382 2 of 19 existing designs, as well as the possibility to incorporate more complex features without drastically increasing the costs. This aspect includes the possibility to create features not fabricable conventionally, such as internal structures, e.g., lattice structures [10] or channels [11]. SLM permits shortening the supply chain and the lead time by replacing some production processes with a single step or combining different sub-elements in a single component [12,13], thereby reducing or in some cases eliminating the assembly operations. Despite these advantages, the limitations for the complete implementation of this technology in the industrial framework are its low productivity and the presence of defects such as porosity, high residual stresses, cracks, inclusions, and surface irregularities that affect the structural integrity and surface quality of an SLM part [14]. In particular, the poor surface finishing and the surface anisotropy [15] are crucial aspects to consider. Typically, in order to improve the surface quality, the SLM surfaces are treated by different post-processing operations such as sand blasting, shoot peening, electropolishing, chemical polishing, and grinding [16]. These operations are skilled operator-dependent, labor-intensive, and typically hard to apply uniformly on complex-shaped parts. In Reference [17], the effect of the post-processing by machining, vibratory finishing, and drag finishing on the surface quality of SLM parts was investigated. The results showed that machining was the best choice because it permitted obtaining very high surface quality in a deterministic way, but it was limited by complex shapes. In the comparison between drag finishing and vibratory finishing, lower roughness values were observed for the former; however, the operation was limited by the need for fixturing the parts. In Reference [18], a magnetic abrasive finishing process was developed to polish SLM planar parts. The results showed that the method removes partially bonded particles and balling defects, obtaining a maximum decrease of about 76% in surface roughness. The system is suitable for planar surfaces but hardly applicable to complex shapes. In Reference [19], cavitation combined with abrasion was used as an SLM finishing process, leading to a reduction in Ra below 4 µm for external surfaces and internal channels 3 mm in diameter. Barrel finishing (BF) is the most ancient mass finishing operation, which allows conditioning the part’s surface without fixturing: this degree of freedom meets AM’s major aim of being almost independent of part geometry [20]. Notwithstanding its slow process, BF is vastly used because it is a versatile conditioning method with low-cost equipment [21]. The process is typically performed via an octagonal rotating barrel; the charge, composed of the parts, media, water, and a compound, is moved upward by rotation until a critical angle is reached; hence, a layer slides down. Within this region, namely, the active layer, a relative velocity between parts and media takes place, leading to a delicate abrasive conditioning of the part surfaces. Unfortunately, a specific motion is required for this behavior; thus, particular conditions must be satisfied by using adequate processing parameters [21]. The difficulty lies in the prediction of the flow properties in granular material motions, characterized by an intrinsic complexity of the element laws and a very large number of elements interacting with each other [22]. The elements are discontinuous and highly heterogeneous, and a dilatant behavior under shearing modifies the interactions, leading to an extreme variety of natural phenomena [23]. There is an extensive experimental database on the properties of non-cohesive granular material flow [24]. As early as 1954, Bagnold stated that the tangential stresses in an intensive shear flow are mainly caused by two factors: particle collisions and impulse interchange between different layers [25]. In his pioneering work, a relevant characterization of dynamic regimes was carried out. He found that the ratio between interparticle collision forces and interstitial viscous forces leads to two different regimes, namely, macroviscous and grain-inertial regimes. This ratio, called the Bagnold number (Ba), is defined as follows: mγ Ba = , (1) (2λeη) where m is the particle mass, γ is the average velocity gradient along the active layer depth, λe is the characteristic length, and η is the dynamic viscosity of the interstitial fluid. Appl. Sci. 2020, 10, 1382 3 of 19

In the case of small Ba (less than 40), the flow is macroviscous, such as flows of slurries or mud, where the interstitial fluid viscosity overcomes the grain inertia. For Ba greater than 450, the fluid viscosity can be neglected in comparison with the interparticle collisions; then, a grain-inertial flow takes place. A transitional regime falls between these two values [26]. Six different motions are defined in the literature, i.e., sliding, slumping, rolling, cascading, cataracting, and centrifuging, which are based on variations in the filling and the rotational speed of the barrel. No model exists for the prediction of process parameters to obtain these motions, because they are strictly dependent on the used charge in terms of material and shape, filling percentage, quantity of water and compound, etc. For this reason, the study of BF motions and process parameters is typically developed using bed behavior (BB) diagrams [27]. These are maps of the six motions and their transitions as a function of the filling percentage and the rotational speed of the barrel. These maps must be experimentally obtained because they change with the employed charge. The individuation of the motions and the relative process parameters is fundamental for the design of the BF process because only two motions, i.e., rolling and cascading, permit having an efficient material removal. These motions are characterized by a delicate continuous flow in the form of an avalanche that realizes relative motions between the media and parts being processed, causing a soft mechanical removal of material. The action was modeled in Reference [28], where the authors underlined that, during the BF process, the micro-geometrical profile is gradually cut away from peak to valley without plastic deformation. In Reference [29], this process was successfully applied to SLM parts, reaching uniform surface morphologies on planar surfaces characterized by different build orientations; however, the case of functional parts characterized by complex shapes was not investigated. In Reference [30], a turbine blade manufactured in Inconel718 by SLM was processed by BF and other post-processing treatments. The results showed that the BF application permitted a slight improvement in the surface quality compared to the other studied processes. It is our opinion that, in this paper, the potential of the BF process was not exploited because of the lack of a deep study of the charge and the process parameters. The aim of this paper was to develop a methodology for the design of the BF process specifically applied to SLM parts. The methodology consisted of the study of different charges in the exploration of various process parameter sets in order to find a suitable technological window. These sets were used to condition complex SLM parts fabricated into two different materials.

2. Materials and Methods The experimental activities reported in this paper were divided into three phases. The first was a preliminary experimentation necessary for the study of BF regimes. The output was the selection of many process parameter sets that assured the rolling and cascading regimes for the different considered charges. These parameters were used in the second phase to study the BF action on SLM specimens characterized by a simple shape and different materials. The results in terms of roughness measurements were used to select efficient BF process parameters for the finishing of two functional components (third phase).

2.1. Materials For the fabrication of the SLM specimens and the complex components, the employed machine was an EOSINT®M290 characterized by a building volume of 250 250 325 mm3 and a 400-W × × ytterbium ﬁber continuum laser, with a beam spot diameter of 100 µm. The process was carried out in an argon inert atmosphere with less than 0.1% oxygen. The building platform was preheated at 200 ◦C in order to reduce residual stresses. The ﬁrst material was Ti6Al4V, characterized by the chemical composition reported in Table1. In Figure1, the SEM image and the granulometry distribution of the employed powders are reported. The shape of the particles was highly spherical, and a large scattering of diameters could be observed (Figure1a). As shown in Figure1b, this distribution (blue curve) was asymmetrical, Appl. Sci. 2020, 10, 1382 4 of 19 and the corresponding cumulative volume (red curve) was characterized by the following metrics: dAppl.10, Sci. d50 2020, and, 10 d, x90 FOR, which PEER REVIEW were the intercepts for 10%, 50%, and 90%, paired to 27.1 µm, 42.54 ofµ m,20 and 67.8 µm, respectively.

Table 1. Chemical composition of Ti64Al4V.

Ti6Al4V (wt.%) Al V O N C H Fe Y Ti Other 5.50–6.75 3.50–4.50 0.20 0.05 0.08 0.015 0.30 0.005 Balance 0.10 maximum Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 20

Figure 1. SEM image of Ti6Al4V powders (a); granulometry distribution (b).

Table 1. Chemical composition of Ti64Al4V.

Ti6Al4V (wt.%) Al V O N C H Fe Y Ti Other 0.10 5.50–6.75 3.50–4.50 0.20 0.05 0.08 0.015 0.30 0.005 Balance maximum Figure 1. SEM image of Ti6Al4V powders (a); granulometry distributiondistribution ((b).). The used process parameters were as follows: 30 µm layer thickness, 140 µm hatch spacing, 1200 The used process parametersTable were1. Chemical as follows: composition 30 µm of layerTi64Al4V. thickness, 140 µm hatch spacing, mm/s scan speed, and 280 W laser power. The scanning strategy was the stripe type, with a stripe 1200 mm/s scan speed, and 280 W laser power. The scanning strategy was the stripe type, with a stripe length and an overlap of 5 mm and 0 mm, Ti6Al4Vrespectively. (wt.%) The scanning direction was rotated between length and an overlap of 5 mm and 0 mm, respectively. The scanning direction was rotated between consecutiveAl layers byV 67° in ordeO r to minimizeN C the out-of-planeH Fe distortion.Y The Tidetaching Other from the consecutive layers by 67 in order to minimize the out-of-plane distortion. The detaching from the building platform was ◦performed by abrasive cutting, and no other finishing operation0.10 was building5.50–6.75 platform 3.50–4.50 was performed 0.20 by abrasive0.05 0.08 cutting, 0.015 and no other0.30 ﬁnishing0.005 operationBalance was performed. performed. maximum The second considered material was Inconel718; this powder is shown in the SEM image The second considered material was Inconel718; this powder is shown in the SEM image of of Figure2a, highlighting an almost spherical shape with some oblong particles and aggregates. FigureThe 2a, used highlighting process parameters an almost were spherical as follows: shape 30 with µm layersome thickness,oblong particles 140 µm andhatch aggregates. spacing, 1200 Its Its granulometry distribution was narrower than that of Ti6Al4V (Figure2b); in this case, the d 10, granulometrymm/s scan speed, distribution and 280 wasW laser narrower power. than The that scanni of ngTi6Al4V strategy (Figure was the2b); stripe in this type, case, with the ad 10stripe, d50, d50, and d90 corresponded to 17.3 µm, 23.7 µm, and 33.4 µm respectively. Table2 displays its andlength d90 and corresponded an overlap toof 17.35 mm µm, and 23.7 0 mm, µm, respectively. and 33.4 µm The respectively. scanning direction Table 2 displayswas rotated its chemicalbetween chemical composition. composition.consecutive layers by 67° in order to minimize the out-of-plane distortion. The detaching from the building platform was performed by abrasive cutting, and no other finishing operation was performed. The second considered material was Inconel718; this powder is shown in the SEM image of Figure 2a, highlighting an almost spherical shape with some oblong particles and aggregates. Its granulometry distribution was narrower than that of Ti6Al4V (Figure 2b); in this case, the d10, d50, and d90 corresponded to 17.3 µm, 23.7 µm, and 33.4 µm respectively. Table 2 displays its chemical composition.

Figure 2. SEM image of Inconel718 powders ( a); granulometry distribution (b).

TableTable 2. Chemical composition of Ti64Al4V.

Inconel718 (wt.%) NiNi Cr Cr Nb Nb Mo Mo Ti Ti Al Co Co Cu Cu C Si, Si, Mn Mn P, P,S S B B FeFe 50–5550–55 17–2117–21 4.75–5.54.75–5.5 2.8–3.32.8–3.3 0.65–1.150.65–1.15 0.2–0.8 ≤1.01.0 ≤0.30.3 ≤0.08 ≤0.350.35 ≤0.0150.015 ≤0.0060.006 Balance Balance ≤ ≤ ≤ ≤ ≤ ≤

The applied process parameters were as follows: laser power 285 W, layer thickness 40 µm, scan Figure 2. SEM image of Inconel718 powders (a); granulometry distribution (b). speed 960 mm/s, and hatch spacing 110 µm. Furthermore, in this case the scan strategy was the stripe type, with a stripe length and anTable overlap 2. Chemical of 10 mmcomposition and 0.8 ofmm, Ti64Al4V. respectively. The BF process was carried out using an industrial equipment Rotar EMI 47 (second and third Inconel718 (wt.%) phases); it is an eccentric-flow rotating barrel with an inclined octagonal section 400 mm in diameter Ni Cr Nb Mo Ti Al Co Cu C Si, Mn P, S B Fe 50–55 17–21 4.75–5.5 2.8–3.3 0.65–1.15 0.2–0.8 ≤1.0 ≤0.3 ≤0.08 ≤0.35 ≤0.015 ≤0.006 Balance

The applied process parameters were as follows: laser power 285 W, layer thickness 40 µm, scan speed 960 mm/s, and hatch spacing 110 µm. Furthermore, in this case the scan strategy was the stripe type, with a stripe length and an overlap of 10 mm and 0.8 mm, respectively. The BF process was carried out using an industrial equipment Rotar EMI 47 (second and third Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 20 phases); it is an eccentric-flow rotating barrel with an inclined octagonal section 400 mm in diameter moved by an asynchronous motor with a variable shiftshift speed electric control system. The volume of the barrel was 47 L, and the maximum fillingfilling waswas 6464 kg.kg. The employedemployed chargescharges were were composed composed of of media media with with the the same same shape shape but dibutfferent different size; thesize; shape the wasshape angled was angled cut cylindrical cut cylindrical and the and dimensions the dimensions were 15 mmwere in 15 diameter, mm in diameter, 25 mm in height25 mm (Figure in height3a) (Figureand 3.5 3a) mm and in diameter, 3.5 mm in 10 diameter, mm in height 10 mm (Figure in height3b). (Figure The composition 3b). The composition of both media of both was amedia matrix was of a syntheticmatrix of resina synthetic and powders resin and of alumina,powders silica, of alumina, and silicon silica, carbide and silicon as abrasive. carbide The as diabrasive.fferent sizesThe weredifferent suitable sizes for were the suitable processing for ofthe complex processing shapes of complex because shapes the bigger because media the allowed bigger speeding media allowed up the speedingmachining up of the the machining external surfaces, of the external whereas surfaces the smaller, whereas media the allowed smaller reaching media the allowed internal reaching surfaces the or internaloverhang surfaces zones, whereor overhang therewas zones, not where enough there space wa fors not the enough relative space motion for of the the relative bigger motion media. of In the biggerexperimentation, media. In the three experimentation, different charge three types differe were considered:nt charge types only were big mediaconsidered: (charge only 1), onlybig media small (chargemedia (charge 1), only 3), small and media a 50:50 (charge volume 3), fraction and a mixture50:50 volume of both fraction (charge mixture 2). of both (charge 2).

FigureFigure 3. 3.Media Media employed employed in in thethe experimentation:experimentation: bi bigg (a) (a )and and small small (b) ( bangled) angled cut cutcylindrical cylindrical ceramic media. ceramic media.

In order to determine determine the the flow flow regimes regimes and and achiev achievee important important motion motion featur features,es, in in the the first first phase, phase, an experimental apparatus was employed. The barrel geometry and the internal material lining were the same same of of the the industrial industrial barrel. barrel. In In this this way, way, it itwas was expected expected that that the the same same motion motion regime regime could could be observedbe observed on onboth both barrels barrels loaded loaded with with the the same same charge, charge, employing employing the the same same process process parameters. parameters. A Atransparent transparent porthole porthole allowed allowed watching watching the the moti motionsons and and taking taking measures measures through through digital image processing. The barrel was loaded with different different fil fillingling percentages (from 30% to 60%). The The rotational rotational speed waswas controlledcontrolled using using a Yaskawaa Yaskawa VS606V7 VS606V7 programmable programmable drive; drive; it was it programmedwas programmed with awith linear a linearramp ramp in order in order to cover to cover the rangethe range speed speed of 0–110of 0–110 rpm. rpm. A A digital digital camera camera was was employed employed forfor video acquisition ofof thethe motions motions and and their their transients, transients, and and a video a video editing editing software software was usedwas used to classify to classify them. them.In the second phase, the considered specimen shape was as reported in Figure4. In order to evaluateIn the the second effect ofphase, the BF the on considered different surface specimen morphologies, shape was the as measuredreported in zones Figure were 4. theIn order surfaces to evaluatecharacterized the effect by three of the di ffBFerent on different inclinations surface (0◦, 90 morphologies,◦, and 45◦) with the respect measured to the zones stratification were the direction.surfaces characterizedThese specimens by werethree fabricated different withinclinations both considered (0°, 90°, materials.and 45°) with respect to the stratification direction.The roughnessThese specimens measurements were fabricat wereed performed with both usingconsidered a Mitutoyo materials. Surftest SJ-412 according to Reference [31]; the sampling length and the evaluation length were set to 2.5 and 12.5 mm, respectively. A 2-µm stylus, sampling every 1.5 µm, was employed. The data were processed by a spline profile filter [32] with a cut-off λc and a short wavelength cut-off λs equal to 2.5 mm and 8 µm, respectively. ISO standard roughness parameters were calculated, and the material-to-air ratio [33], previously known as the Abbott–Firestone curve (AFC), was developed. Moreover, three-dimensional (3D) maps of 5 3 mm2 were acquired, and the arithmetical mean absolute height (Sa) and the maximum height × (Sz) were calculated according to Reference [34]. The components characterized by complex shapes used in the third phase are described in Section5.

Figure 4. Specimen shape and dimensions.

The roughness measurements were performed using a Mitutoyo Surftest SJ-412 according to Reference [31]; the sampling length and the evaluation length were set to 2.5 and 12.5 mm, respectively. A 2-µm stylus, sampling every 1.5 µm, was employed. The data were processed by a spline profile filter [32] with a cut-off λc and a short wavelength cut-off λs equal to 2.5 mm and 8 µm, Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 20 moved by an asynchronous motor with a variable shift speed electric control system. The volume of the barrel was 47 L, and the maximum filling was 64 kg. The employed charges were composed of media with the same shape but different size; the shape was angled cut cylindrical and the dimensions were 15 mm in diameter, 25 mm in height (Figure 3a) and 3.5 mm in diameter, 10 mm in height (Figure 3b). The composition of both media was a matrix of a synthetic resin and powders of alumina, silica, and silicon carbide as abrasive. The different sizes were suitable for the processing of complex shapes because the bigger media allowed speeding up the machining of the external surfaces, whereas the smaller media allowed reaching the internal surfaces or overhang zones, where there was not enough space for the relative motion of the bigger media. In the experimentation, three different charge types were considered: only big media (charge 1), only small media (charge 3), and a 50:50 volume fraction mixture of both (charge 2).

Figure 3. Media employed in the experimentation: big (a) and small (b) angled cut cylindrical ceramic media.

In order to determine the flow regimes and achieve important motion features, in the first phase, an experimental apparatus was employed. The barrel geometry and the internal material lining were the same of the industrial barrel. In this way, it was expected that the same motion regime could be observed on both barrels loaded with the same charge, employing the same process parameters. A transparent porthole allowed watching the motions and taking measures through digital image processing. The barrel was loaded with different filling percentages (from 30% to 60%). The rotational speed was controlled using a Yaskawa VS606V7 programmable drive; it was programmed with a linear ramp in order to cover the range speed of 0–110 rpm. A digital camera was employed for video acquisition of the motions and their transients, and a video editing software was used to classify them. In the second phase, the considered specimen shape was as reported in Figure 4. In order to evaluate the effect of the BF on different surface morphologies, the measured zones were the surfaces characterizedAppl. Sci. 2020, 10 ,by 1382 three different inclinations (0°, 90°, and 45°) with respect to the stratification6 of 19 direction. These specimens were fabricated with both considered materials.

Figure 4. SpecimenSpecimen shape shape and dimensions.

2.2. MethodsThe roughness measurements were performed using a Mitutoyo Surftest SJ-412 according to ReferenceThe experimental [31]; the sampling barrel waslength employed and the to evaluation define granular length flow were regimes set to at2.5 di ffanderent 12.5 rotating mm, respectively. A 2-µm stylus, sampling every 1.5 µm, was employed. The data were processed by a speeds for different charge compositions. Each charge was loaded in the barrel with different filling spline profile filter [32] with a cut-off λc and a short wavelength cut-off λs equal to 2.5 mm and 8 µm, percentages, since they may affect the flow regime. For each charge, the filling percentage was varied, and a video at different speeds was recorded and analyzed. In Table3, the complete set of these experiments is reported.

Table 3. Experiments for regime analysis.

Filling Percentage Charge Rotational Speed Range 30% 40% 50% 60% 1 2–110 rpm √ √ √ √ 2 2–110 rpm √ √ √ √ 3 2–110 rpm √ √ √ √

The gradient speed and the active layer depth were measured in order to calculate the Ba number and to assess the type of regime. Afterward, the results allowed determining some conditions for grain-inertial flow, which is probably the preferred condition for efficient BF operation. The experimental apparatus was then employed to determine the motions in different processing conditions. Through a visual investigation, they were classified and, accordingly, the BBs were developed. Hence, the process parameters providing rolling and cascading regimes were properly selected.

3. First Phase of Experimentation: Results and Discussion This phase aimed to study the BF regimes in order to find suitable process parameters for the considered charges. The evaluation of Ba for several levels of the processing parameters allowed development of the contour maps shown in Figure5. The red area is related to the macro-viscous behavior ( Ba less than 40), while the green one refers to Ba greater than 450, corresponding to a grain-inertial flow; the transition between them is colored in yellow. The charge composed of only big media (Figure5a) dissipated energy in the viscous fluid only at low rotational speed and for low filling percentage. At high barrel filling, it was easy to find an inertial flow regime; however, for higher rotation, the media particles were no longer in contact and were launched toward the barrel walls, according to the definition of cataracting motion. In this condition, the Ba could not be evaluated, and a blank area is reported in the graph. For charge 2 (Figure5b), a similar behavior was observed; the granular flow was maintained at higher rotational speed with respect to charge 1 but a slight decrease in the green area was present. When charge 3 was employed, the experiment on the laboratory BF machine showed that the grain-inertial flow was limited to a combination of high filling and high rotational speed (Figure5c). Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 20

Appl. Sci. 2020, 10, 1382 7 of 19

This implies that the particle kinetic energy was mainly dissipated in the viscous ﬂuid rather than interparticle interactions. Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 20

Figure 5. at different processing parameters for charge 1 (a), charge 2 (b), and charge 3 (c).

As mentioned above, rolling and cascading motions are required for BF operation. For this purpose, morphological analysis allowed the classification of flow behavior and the development of BB. In Figure 6, the yellow and green areas indicate the six possible motions and their transitions. At low rotational speed, it is evident that charge 3 was characterized by high fluidity, i.e., the capability to move as a fluid; thus, a slipping motion was observed for a wide range of the speed. This may indicate that the charge can be placed with ease in complex geometrical features, but it could suffer FigureFigure 5. 5. Ba at at didifferentfferentprocessing processingparameters parameters forfor charge charge 1 1 ( a(a),), charge charge 2 2 ( b(b),), and and charge charge 3 3 ( c().c). from a reduction in kinetic energy. Charge 1 was characterized by slumping for speed in the range 7–20As rpm mentionedmentioned and a maximum above,above, extension rollingrolling andand of the cascadingcascading area at 45% motionsmotions filling. areare Conversely, requiredrequired charge forfor BFBF 3 operation. operation.showed a narrow For thisthis purpose,purpose,slumping morphologicalmorphological domain, highlighting analysis that allowed this charge the classificationclassifi is unablecation to ofmaintain flowflow behavior the peculiar and periodicthe development behavior, of BB.i.e., In the Figure frequency 66,, the of yellow the avalanche. and and green green areas areas indicate indicate the the six six possible possible motions motions and and their their transitions. transitions. At With regard to rolling, charge 1 and 2 had their lower limits at 50% and 40% respectively; this is Atlow low rotational rotational speed, speed, it is it isevident evident that that charge charge 3 3wa wass characterized characterized by by high high fluidity, fluidity, i.e., i.e., the the capability in accordance with the best practice of approximately 50% filling percentage and a low Froude toto movemove asas aa fluid;fluid; thus,thus, aa slippingslipping motionmotion waswas observedobserved forfor aa widewide rangerange ofof thethe speed.speed. This maymay number, i.e., on the order of 0.1. The experiment with charge 3 shows that the minimum was observed indicateindicate thatthat thethe chargecharge cancan bebe placedplaced withwith easeease inin complexcomplex geometricalgeometrical features, but it could susufferffer at the smallest filling percentage, and it tended to increase with this variable. An extension of the fromfrom aa reductionreduction inin kinetickinetic energy.energy. ChargeCharge 11 waswas characterizedcharacterized byby slumpingslumping forfor speedspeed inin thethe rangerange rolling involved cascading where the charge-free surface was affected by the inertial forces and 7–20 rpm and a maximum extension of the area at 45% filling. filling. Conversely, Conversely, charge 3 showed a narrow tended to be curved. The upper limit of charge 1 and 2 slightly decreased with filling percentage, slumping domain, highlighting that this charge is unable to maintain the peculiar periodic behavior, behavior, while it was quite constant for charge 3; it can be assessed that the cascading/cataracting transition, i.e.,i.e., thethe frequencyfrequency ofof thethe avalanche.avalanche. characterized by the undesired launching of media, was avoided at a Froude number less than 0.5. With regard to rolling, charge 1 and 2 had their lower limits at 50% and 40% respectively; this is in accordance with the best practice of approximately 50% filling percentage and a low Froude number, i.e., on the order of 0.1. The experiment with charge 3 shows that the minimum was observed at the smallest filling percentage, and it tended to increase with this variable. An extension of the rolling involved cascading where the charge-free surface was affected by the inertial forces and tended to be curved. The upper limit of charge 1 and 2 slightly decreased with filling percentage, while it was quite constant for charge 3; it can be assessed that the cascading/cataracting transition, characterized by the undesired launching of media, was avoided at a Froude number less than 0.5.

FigureFigure 6.6. Bed behavior (BB) for for charge charge 1 1 ( (aa),), charge charge 2 2 (b (b),), and and charge charge 3 3(c ().c ).

WithThe regardBB diagrams to rolling, allowed charge selecting 1 and 2process had their parameter lower limits values at 50%for the and investigated 40% respectively; charges. this The is in accordancerolling regime with was the assured best practice by adopting of approximately a 25-rpm ro 50%tational filling speed. percentage Two different and a filling low Froude percentages, number, i.e.,i.e., on 40% the and order 55%, of were 0.1. The selected, experiment as indicated with chargein Figure 3 shows 7 with that the parameter the minimum sets was3 and observed 4. The other at the smallesttwo conditions fillingpercentage, were considered and itin tended order to to provide increase a withcascading this variable. motion, i.e., An parameter extension sets of the 1 and rolling 2 involvedwere assigned cascading to filling where percentages the charge-free of 40% surface and 55 was%, respectively, affected by theat a inertialrotating forces speed and of 45 tended rpm. to be curved. The upper limit of charge 1 and 2 slightly decreased with filling percentage, while it was quite constant for chargeFigure 3; it 6. can Bed be behavior assessed (BB) that for the charge cascading 1 (a), charge/cataracting 2 (b), and transition, charge 3 characterized(c). by the undesired launching of media, was avoided at a Froude number less than 0.5. The BB diagrams diagrams allowed allowed selecting selecting process process parameter parameter values values for forthe theinvestigated investigated charges. charges. The Therolling rolling regime regime was was assured assured by byadopting adopting a 25-rpm a 25-rpm ro rotationaltational speed. speed. Two Two different different filling filling percentages, percentages, i.e.,i.e., 40%40% andand 55%,55%, werewere selected,selected, asas indicatedindicated inin FigureFigure7 7 with with the the parameter parameter sets sets 3 3 and and 4. 4. TheThe other other two conditions were considered in order to provide a cascading motion, i.e., parameter sets 1 and 2 were assigned to filling percentages of 40% and 55%, respectively, at a rotating speed of 45 rpm. Appl. Sci. 2020, 10, 1382 8 of 19 two conditions were considered in order to provide a cascading motion, i.e., parameter sets 1 and 2 were assigned to filling percentages of 40% and 55%, respectively, at a rotating speed of 45 rpm. Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 20

FigureFigure 7.7. Barrel ﬁnishingfinishing (BF) parameter parameter sets sets for for charge charge 1 1(a (a),), charge charge 2 2(b (),b ),and and charge charge 3 ( 3c) ( cwithin) within the the rollingrolling andand cascadingcascading regimes.regimes.

4.4. SecondSecond PhasePhase of Experimentation: Results Results and and Discussion Discussion TheThe industrial industrial BF BF machine machine was was used used according accordin to theg to previously the previously described described method. method. Each specimen Each (Figurespecimen4) was (Figure extracted 4) was and extracted measured and bymeasur meansed ofby roughnessmeans of roughness measurement. measurement. InIn FigureFigure8 a,8a, the the 0 0°◦ profile profileis isshown. shown. TheThe initialinitial RaRa waswas moremore thanthan 1515µ µmm withwith aa peak-to-valleypeak-to-valley Rt ofRt about of about 100 µ100m. Theµm. profileThe profile was symmetrical was symmetrical (the skewness (the skewness was close was to 0)close and to slightly 0) and platykurtic, slightly sinceplatykurtic, the kurtosis since wasthe nearkurtosis 2.5. was The highnear value2.5. The of thehigh root-mean-square value of the root-m slopeean-square of the profile, slope i.e.,of the R∆ q, indicatesprofile, i.e., a rough RΔq, indicates surface with a rough a poor surface reflectivity. with a poor If charge reflectivity. 1 was If employed charge 1 was and employed BF parameter and BF set 1 wasparameter selected, set after 1 was 12 selected, h, the profile after shown12 h, the in Figureprofile8 shownb was obtained.in Figure 8b The was average obtained. roughness The average Ra was decreasedroughness byRa 41%, was asdecreased well as theby total41%, roughnessas well as the Rt. total A cut roughness of the peaks Rt. wasA cut observed, of the peaks suggesting was aobserved, good BF mechanism;suggesting ina thisgood condition, BF mechanism; the Ba was in aboutthis condition, 500 and the the employed was motionabout 500 was and cascading. the Asemployed a measure motion of the was profile cascading. distribution As a measure shape, the of skewnessthe profile was distribution negative, shape, i.e., the the profile skewness wasmainly was madenegative, up of i.e., valleys, the profile and the was kurtosis mainly was made 3, as up in aof Gaussian valleys, distribution.and the kurtosis After was 24 h,3, theas in Ra a was Gaussian reduced bydistribution. an additional After 40%, 24 h, the the skewness Ra was reduced was 3, by and an the additional profile was 40%, markedly the skewness leptokurtic was −3, (Figure and the8c). profile was markedly leptokurtic (Figure 8c).− After 48 h, the original profile was almost deleted; only After 48 h, the original profile was almost deleted; only some scratches were present, probably the some scratches were present, probably the deepest original ones (Figure 8d). On the other hand, the deepest original ones (Figure8d). On the other hand, the employment of charge 3 with the same employment of charge 3 with the same BF parameters led to a marginal improvement. The original BF parameters led to a marginal improvement. The original profile of this specimen (Figure8e) was profile of this specimen (Figure 8e) was characterized by almost the same roughness parameters; characterized by almost the same roughness parameters; however, since, in this case, the Ba was 360, however, since, in this case, the was 360, a marginal reduction in Ra was observed with higher a marginal reduction in Ra was observed with higher peak-to-valley heights, as shown in Figure8f, peak-to-valley heights, as shown in Figure 8f, for a processing time of 12 h. At 24 h, some differences for a processing time of 12 h. At 24 h, some differences could be observed in Ra and Rt, but the could be observed in Ra and Rt, but the distribution shape was marginally modified. In fact, in Figure distribution shape was marginally modified. In fact, in Figure8g, rounded peaks can be noticed; this is 8g, rounded peaks can be noticed; this is an indication of improper BF surface actions where the anparticle indication energy of was improper probably BF surfaceused for actions local plasti wherec deformation the particle and energy wear was of the probably profile usedrather for than local plasticmicro-chipping. deformation This and led wearto a high of the processing profile rather time, thanas remarked micro-chipping. by the profile This obtained led to a at high 48 h processing (Figure time,8h), which as remarked was characterized by the profile by negligible obtained atvariations 48 h (Figure of all8h), process which parameters. was characterized by negligible variations of all process parameters. Appl. Sci. 2020, 10, 1382 9 of 19

Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 20

FigureFigure 8. 8.Ti6Al4V Ti6Al4V roughness roughness proﬁles profiles before before BFBF conditioningconditioning (a, (a, ee)) andand atat didifferentﬀerent working times times by by usingusing parameters parameters set set 1 with1 with charge charge 1 1 (b (b,c,d)–d) and and charge charge 3 (3f –(f,h ).g, h).

TheThe same same poor poor results results were were achieved achieved by by taking taking into into consideration consideration charge charge 1 with1 with BF BF parameter parameter set 3.set Here, 3. Here, the Ba thewas only was only 250and, 250 and, after after 24 h, 24 the h, the Ra Ra decreased decreased by by only only 32% 32% (Figure (Figures9a,b). 9a,b). Charge Charge 1 and1 and BF parameterBF parameter set set 1 appear 1 appear to to be be a gooda good combination. combination. These These conditionsconditions werewere applied to to a a surface surface characterizedcharacterized by by di differentfferent inclinations,inclinations, where where differe differentnt morphologies morphologies were were expected. expected. In Figure In Figure 9c, the9c, theoriginal original profile profile is is shown; shown; the the initial initial Ra Ra was was about about 4 4µm,µm, the the total total roughness roughness Rt Rt was was one-third one-third of of the the 90° surface, and the skewness was negative. This was due to the laser scans, which were prominent, 90◦ surface, and the skewness was negative. This was due to the laser scans, which were prominent, givinggiving the the profile profile this this behavior. behavior. After After 2424 h,h, thethe profileprofile (Figure(Figure9 9d)d) waswas almostalmost deleted;deleted; the Ra was markedly reduced, and only some local valleys are observed. markedly reduced, and only some local valleys are observed. When the Inconel718 material was considered, the initial Ra was lower than that of Ti6Al4V for When the Inconel718 material was considered, the initial Ra was lower than that of Ti6Al4V the same surface orientation of 90° (Figure 9e). The shape of the profile distribution was almost for the same surface orientation of 90 (Figure9e). The shape of the profile distribution was almost symmetrical and slightly platykurtic◦ as before. The employment of charge 1 and BF parameter set 1 symmetrical and slightly platykurtic as before. The employment of charge 1 and BF parameter set 1 allowed for good improvement of Ra and Rt after 24 h. The skewness was deeply reduced, and allowed for good improvement of Ra and Rt after 24 h. The skewness was deeply reduced, and kurtosis kurtosis was highly leptokurtic, suggesting that proper BF cutting conditions took place. Upon was highly leptokurtic, suggesting that proper BF cutting conditions took place. Upon observing the observing the profile (Figure 9f), this mechanism is clear since the peaks were cut away and the profile (Figure9f), this mechanism is clear since the peaks were cut away and the valleys were left valleys were left unaltered. The other BF parameter sets and charges resulted in the same previous unaltered.observations. The other BF parameter sets and charges resulted in the same previous observations. Appl. Sci. 2020, 10, 1382 10 of 19 Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 20

Figure 9.Figure Roughness 9. Roughness profiles profiles before before and and after after BF BF operationoperation with with charge charge 1 and 1 parameterand parameter set 3 for set 3 for Ti6Al4VTi6Al4V vertical vertical surfaces surfaces (a, (a ,b),b), forfor Ti6Al4V Ti6Al4V horizontal horizontal surface withsurface parameter with setparameter 1 (c,d), for Inconel718set 1 (c, d), for Inconel718vertical vertical surfaces surfaces with parameter with parameter set 1 (e,f). set 1 (e, f). The AFC allows graphing the cumulative distribution of the profile height. In Figure 10, Thethis AFC outcome allows is showngraphing for the the original cumulative profile (Figure distributi 10a) andon afterof the processing profile forheight. 24 h (Figure In Figure 10b). 10, this outcomeUnder is shown the assumption for the original that the valley profile shape (Figure is not a10a)ffected and by theafter BF conditioning,processing for the curves24 h (Figure were 10b). Under thealigned assumption according that to the the procedure valley shape described is not in Referenceaffected by [35 ].the Figure BF conditioning, 10c shows the outcome;the curves were approximately 20% of the curves overlapped and more than 50% were almost horizontal. This aligned according to the procedure described in Reference [35]. Figure 10c shows the outcome; demonstrates that a mechanism of peak cutting without valley modification took place. Moreover, approximatelythe area between20% of curves the curves represents overlapped the area removed and bymore BF operation. than 50% The valuewere was almost 15,990 horizontal.µm2/mm, This demonstratesequivalent that to a 72 mechanismµg/mm3, corresponding of peak cutting to the amount without of Ti6Al4V valley removedmodification in 24 h. took place. Moreover, the area betweenThis calculationcurves represents was repeated the area five times removed for each by BF processing operation. condition The value and for was each 15,990 charge. µm2/mm, equivalentThe to overall 72 µg/mm outcome3, iscorresponding reported in the box-and-whiskersto the amount of charts Ti6Al4V in Figure removed 11. in 24 h. It is evident that charge 3 was characterized by a low material removal rate for all BF parameter sets. Charge 1 and 2 had similar behavior; by engaging parameter sets 3 and 4, which correspond to regimes characterized by low Ba, the material removal rates were 40 µg/mm3 and 80 µg/mm3 for surfaces inclined at 0◦ and 45◦/90◦, respectively. Sets 1 and 2 allowed achieving the best results for these charges, and two observations can be provided. Firstly, charge 2, which was composed of mixed media, had the possibility to access small holes and, thus, it was preferred to charge 1. Secondly, set 2 was characterized by a higher filling percentage than set 1. A higher filling percentage results in greater active layer thickness; hence, big components are processed faster in relatively small barrel machines. Thus, charge 2 and BF parameter set 2 were chosen in this work.

Figure 10. Roughness profile and AFC for a Ti6AL4V specimen: original profile (a), same surface after 24h BF (b), aligned profiles (c). Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 20

Figure 9. Roughness profiles before and after BF operation with charge 1 and parameter set 3 for Ti6Al4V vertical surfaces (a, b), for Ti6Al4V horizontal surface with parameter set 1 (c, d), for Inconel718 vertical surfaces with parameter set 1 (e, f).

The AFC allows graphing the cumulative distribution of the profile height. In Figure 10, this outcome is shown for the original profile (Figure 10a) and after processing for 24 h (Figure 10b). Under the assumption that the valley shape is not affected by the BF conditioning, the curves were aligned according to the procedure described in Reference [35]. Figure 10c shows the outcome; approximately 20% of the curves overlapped and more than 50% were almost horizontal. This demonstrates that a mechanism of peak cutting without valley modification took place. Moreover, Appl.the area Sci. 2020 between, 10, 1382 curves represents the area removed by BF operation. The value was 15,990 µm112/mm, of 19 equivalent to 72 µg/mm3, corresponding to the amount of Ti6Al4V removed in 24 h.

Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 20 Figure 10. Roughness profile and AFC for a Ti6AL4V specimen: original profile (a), same surface ThisFigure calculation 10. Roughness was profilerepeated and five AFC times for a Ti6AL4V for each specimen: processing original condition profile and (a), samefor each surface charge. after The after 24h BF (b), aligned profiles (c). overall24h outcome BF (b), aligned is reported profiles in ( cthe). box-and-whiskers charts in Figure 11.

Figure 11.11. Box-and-whiskersBox-and-whiskers charts charts of theof materialthe material removal removal rate (MRR) rate for(MRR) diﬀerent for surfacedifferent inclinations surface inclinations (0°, 45°, 90°), using different BF process parameter sets (1, 2, 3, 4) and charge types (1, 2, (0◦, 45◦, 90◦), using diﬀerent BF process parameter sets (1, 2, 3, 4) and charge types (1, 2, 3), for Ti6AL4V 3),(a) for and Ti6AL4V Inconel718 (a) (andb). Inconel718 (b).

It is evident that charge 3 was characterized by a low material removal rate for all BF parameter sets. Charge 1 and 2 had similar behavior; by engaging parameter sets 3 and 4, which correspond to regimes characterized by low , the material removal rates were 40 µg/mm3 and 80 µg/mm3 for surfaces inclined at 0° and 45°/90°, respectively. Sets 1 and 2 allowed achieving the best results for these charges, and two observations can be provided. Firstly, charge 2, which was composed of mixed media, had the possibility to access small holes and, thus, it was preferred to charge 1. Secondly, set 2 was characterized by a higher filling percentage than set 1. A higher filling percentage results in greater active layer thickness; hence, big components are processed faster in relatively small barrel machines. Thus, charge 2 and BF parameter set 2 were chosen in this work. In the case of Inconel718, the trends were very similar; charge 3 and/or small were not able to provide adequate speed; sets 1 and 2 with charges 1 and 2 allowed good performance, albeit slightly lower than the Ti6Al4V case. This was probably related to the lower initial roughness of the parts made by this material. Hence, both materials were processed by employing charge 2 and BF processing set 2.

5. Third Phase: Case Studies In this work, two complex geometries were taken into consideration.

5.1. Case Study 1: Impeller Appl. Sci. 2020, 10, 1382 12 of 19

In the case of Inconel718, the trends were very similar; charge 3 and/or small Ba were not able to provide adequate speed; sets 1 and 2 with charges 1 and 2 allowed good performance, albeit slightly lower than the Ti6Al4V case. This was probably related to the lower initial roughness of the parts made by this material. Hence, both materials were processed by employing charge 2 and BF processing set 2.

5. Third Phase: Case Studies In this work, two complex geometries were taken into consideration.

5.1.Appl. Case Sci. Study2020, 10 1:, x FOR Impeller PEER REVIEW 12 of 20 The first case study involved the Ti6Al4V impeller shown in Figure 12a. In order to understand The first case study involved the Ti6Al4V impeller shown in Figure 12a. In order to understand how the processing time affects the surface conditioning, the six blades were shielded by steel elements, how the processing time affects the surface conditioning, the six blades were shielded by steel locally avoiding the BF action (Figure 12b). One by one, the shields were removed, and the surfaces elements, locally avoiding the BF action (Figure 12b). One by one, the shields were removed, and the were measured at different BF working times. The zones were selected according to the different surfaces were measured at different BF working times. The zones were selected according to the geometrical features shown in Figure 12c: the external side of the turbine (A), the internal surface of different geometrical features shown in Figure 12c: the external side of the turbine (A), the internal the inner cylinder (B), the top surface of the blade (C), the top blade surface (D), and the bottom blade surface of the inner cylinder (B), the top surface of the blade (C), the top blade surface (D), and the surface (E). bottom blade surface (E).

FigureFigure 12. 12.Selective Selective laser laser melting melting (SLM) (SLM) Ti6Al4V Ti6Al4V impeller impeller (a), shielded (a), shielded blades blades (b), and (b), measured and measured zones (c). zones (c). The measurements were taken after 6, 12, 24, 48, and 96 h of BF operation in order to understand the trendThe with measurements processing were time. taken At the after beginning, 6, 12, 24, the 48, component and 96 h of hadBF operation a roughness in order dependent to understand upon the localthe stratificationtrend with processing angle. A andtime. C At were the verticalbeginni wallsng, the with component a starting had roughness a roughness of about dependent 14 µm; upon D was inclinedthe local and stratification showed an angle. Ra of aboutA and 13 C µwerem; B vertical was horizontal walls with and a hadstarting the lowestroughness value of of about about 14 5.5 µm;µm. FigureD was 13 inclined shows and the showed feature an trends Ra of as about a function 13 µm; B of was the horizontal working time.and had Feature the lowest A was value external of about and marked5.5 µm. Ra Figure was observed,13 shows the as expected.feature trends At 12 as h,a function the Ra was of the reduced working by time. 67%, Feature and, at 24A was h, the external Ra was and marked Ra was observed, as expected. At 12 h, the Ra was reduced by 67%, and, at 24 h, the Ra 2.05 0.9 µm. At 48 h, the Ra decreased below 1 µm. Feature B was an internal hole 30 mm in diameter; was± 2.05 ± 0.9 µm. At 48 h, the Ra decreased below 1 µm. Feature B was an internal hole 30 mm in a lower BF action was expected since the big media were 20 mm in diameter. At 12 h, the Ra showed diameter; a lower BF action was expected since the big media were 20 mm in diameter. At 12 h, the a reduction of only 49%, and, after 96 h, the surface reached an Ra of about 5 µm. This descending Ra showed a reduction of only 49%, and, after 96 h, the surface reached an Ra of about 5µm. This speed was similar to that observed for small media and/or small Ba number; the hole slowed down the descending speed was similar to that observed for small media and/or small number; the hole media and reduced the big media hits inside. Feature C showed a roughness reduction of more than slowed down the media and reduced the big media hits inside. Feature C showed a roughness 80% at 12 h of working time, and, after 24 h, it was reduced below 1 µm. Feature D was an inclined reduction of more than 80% at 12 h of working time, and, after 24 h, it was reduced below 1 µm. surface and a partially confined zone; in this case, an 80% reduction was obtained after 48 h, with an Ra Feature D was an inclined surface and a partially confined zone; in this case, an 80% reduction was µ µ ofobtained about 3 afterm. At48 h, 96 with h, the an Ra Ra was of about lowered 3 µm. to At 2 96m. h, Feature the Ra was E was lowered an inclined to 2 µm. overhanging Feature E was surface, an which,inclined as expected,overhanging showed surface, the which, higher as Ra expected, value. The showed trend the was higher similar Ra to value. feature The D; trend after was 48 h,similar the Ra µ µ wasto feature about 3.5 D; afterm, and, 48 h, after the Ra 96 was h, the about Ra was 3.5 2.5µm, and,m, corresponding after 96 h, the toRa an was 85% 2.5 reduction. µm, corresponding All curves showto an that 85% the reduction. difference All in curves Ra decreased show that with the time. difference Notwithstanding in Ra decreased this with finding, time. the Notwithstanding behavior did not followthis finding, a first-order the behavior negative did exponential not follow function. a first-orde Itr can negative be assessed exponential that, within function. 96 h, It thecan Rabe assessed difference wasthat, less within than 96 10%, h, the highlighting Ra difference the presencewas less than of an 10%, Ra threshold highlighting nearby. the presence of an Ra threshold nearby. Appl. Sci. 2020, 10, 1382 13 of 19 Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 20

FigureFigure 13. 13.Average Average roughness roughness trend trend of theof the investigated investigated features features as aas function a function of theof the BF BF working working time. time. The profile analysis confirmed the previous observations. As shown in Figure 14a, feature A beforeThe BF profile operation analysis was characterizedconfirmed the by previous a pseudo-Gaussian observations. profile As shown (Rsk and in RkuFigure paired 14a, to feature 0.57 and A before2.77, respectively), BF operation an was Rt ofcharacterized 100 µm, and by an a Ra pseudo-Gaussian of 15.5 µm. After profile 48 h, the(Rsk valleys and Rku were paired almost to removed0.57 and 2.77,with respectively), a dramatic reduction an Rt of of 100 Ra µm, and and Rt to an 0.19 Ra ofµm 15.5 and µm. 1.2 Afterµm, respectively. 48 h, the valleys Moreover, were almost the RDq removed moved withfrom a 0.73 dramatic to 0.03, reduction which was of Ra a goodand Rt indicator to 0.19 µm of theand surface 1.2µm, reflectance.respectively. By Moreover, calculating the and RDq aligning moved fromthe AFCs, 0.73 to Figure 0.03, 14whichb was was obtained. a good Itindicator is evident of thatthe thesurface BF mechanism reflectance. was By calculating peak cutting, and suggesting aligning thethat AFCs, the process Figure was14b was correctly obtained. undertaken. It is evident In the that case the ofBF feature mechanism B (Figure was peak14c), cutting, the starting suggesting profile thatwas the similar process to the was previous correctly one undertaken. (both surfaces In the were case vertical of feature walls); B (Figure however, 14c), after the 48 starting h, the valleysprofile waswere similar still pronounced. to the previous The Rskone (both was negative surfaces andwere Rt vertical was more walls); than however, 50 µm. The after AFCs 48 h, (Figure the valleys 14d) wereshowed still a pronounced. slow action onThe the Rsk peaks, was negative which were and roundedRt was more at the than beginning 50µm. The and AFCs then partially(Figure 14d) cut showedaway. The a slow horizontal action on feature the peaks, C started which with were an rounded Ra of 5.5 at theµm, beginning and, after and 48 then h, the partially Ra decrease cut away. was Theimpressive; horizontal however, feature the C started negative with Rsk an and Ra the of 5.5 high µm, Rku and, indicate after 48 that h, somethe Ra scratches decrease werewas impressive; still present however,(Figure 14 e).the Thenegative aligned Rsk cumulative and the high distributions Rku indicate in Figure that 14somef show scratches good peak were chipping. still present The blade(Figure is 14e).an important The aligned zone cumulative of the turbine; distributions the top was in an Figure up-facing 14f show inclined good surface peak withchipping. a starting The Rablade of aboutis an important13 µm (Figure zone 14 ofg). the The turbine; BF action the prolongedtop was an for up-fac 48 hing markedly inclined modified surface with the profile. a starting Deep Ra scratchesof about 13(negative µm (Figure Rsk and14g). very The high BF action Rku) wereprolonged present, for and48 h the markedly AFCs highlighted modified the a behavior profile. Deep better scratches than the (negativeB case (Figure Rsk and 14h). very The high bottom Rku) surface were present, was overhanging and the AFCs with highlighted a starting Raa behavior and Rt ofbetter about than 15 µthem Band case 108 (Figureµm, respectively. 14h). The bottom For this surface surface, was a droppingoverhanging phenomenon with a starting was expected;Ra and Rtthe of about formation 15 µm of andballs 108 and µm, protrusions respectively. had For an ethisffect surface, on the a profile dropping amplitude phenomenon distribution, was expected; which was the movedformation to the of ballstop and and broadened. protrusions This had was an effect reflected on inthe a profile reduction amplitude of Rsk and distribution, Rku with which respect was to the moved top surface to the top(Figure and 14broadened.g). After48 This h, thewas Ra reflected was markedly in a reduction improved, of Rsk the and Rsk Rku was with negative, respect indicating to the top removed surface (Figurepeaks and 14g). deep After valleys, 48 h, the the Rku Ra was highlighted markedly a leptokurtic improved, distribution the Rsk was typical negative, of profile indicating with fewremoved peaks peaksand low and valleys, deep valleys, and the the RDq Rku was highlighted reduced by a 65%.leptok Theurtic shapes distribution of the typical aligned of AFCs, profile reported with few in peaksFigure and 14j, confirmedlow valleys, this and behavior the RDq for was every reduced BF working by 65%. time. The The shapes trend of was the closealigned to thatAFCs, of featurereported D. in Figure 14j, confirmed this behavior for every BF working time. The trend was close to that of feature D. Appl. Sci. 2020, 10, 1382 14 of 19

Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 20

Figure 14. Roughness profiles before and after processing and aligned AFC curves at different BF Figure 14. Roughness proﬁles before and after processing and aligned AFC curves at diﬀerent BF working times for features: A (a,b), B (c, d), C (e, f), D (g, h) and E (i, j). working times for features: A (a,b), B (c,d), C (e,f), D (g,h) and E (i,j). Appl. Sci. 2020, 10, x 1382 FOR PEER REVIEW 1515 of of 1920

5.2. Case Study 2: Exhaust Manifold 5.2. Case Study 2: Exhaust Manifold The second case study was an automotive exhaust manifold made of Inconel718. It conveys The second case study was an automotive exhaust manifold made of Inconel718. It conveys burnt burnt gases from three engine cylinders into a single collector. The general design restrictions require gases from three engine cylinders into a single collector. The general design restrictions require the the lowest amount of metal to occupy the least space and to have good efficiency. Usually, this lowest amount of metal to occupy the least space and to have good efficiency. Usually, this component component is made by casting, which does not allow thin pipes. Through SLM, a new efficient and is made by casting, which does not allow thin pipes. Through SLM, a new efficient and lightweight lightweight component was built (Figure 15a). For the finishing of this component, the coupling component was built (Figure 15a). For the finishing of this component, the coupling flanges were flanges were machined before the overall surface conditioning in order to avoid defects over the machined before the overall surface conditioning in order to avoid defects over the already finished already finished clamped zones. In order to shield the machined surfaces, a cover was designed and clamped zones. In order to shield the machined surfaces, a cover was designed and fabricated via fabricated via fused deposition modeling in acrylonitrile butadiene styrene (Figure 15b); in this way, fused deposition modeling in acrylonitrile butadiene styrene (Figure 15b); in this way, the BF action the BF action was avoided over these surfaces, and the finishing of the internal pipe surface was was avoided over these surfaces, and the finishing of the internal pipe surface was allowed. allowed.

FigureFigure 15. 15.Inconel718 Inconel718 automotive automotive exhaust exhaust manifold manifold (a )(a and) and covers covers used used to to shield shield machined machined zones zones (b ). (b). Six features were selected to understand how the BF operation affected the complex geometry. As shownSix features in Figure were 15a, selected zone A to was understand convex and, how thus, the exposedBF operation to the affected conditioning. the complex As demonstrated geometry. inAs Sectionshown3 in, Inconel718Figure 15a, waszone e Affi wasciently convex processed and, th byus, BF exposed with the to the selected conditioning. parameters. As demonstrated As expected, thisin Section feature 3, showedInconel718 a marked was efficiently improvement processed of the by surface;BF with inthe only selected 12 h, parameters. the initial roughnessAs expected, of aboutthis feature 11 µm showed was reduced a marked bytwo-thirds; improvement after of 24the h, surface; the Ra wasin only about 12 h, 1 µthem (Figureinitial roughness 16b). The of internal about surfaces11 µm was of the reduced pipes corresponding by two-thirds; to after features 24 h, B, C,the and Ra Dwas were about characterized 1 µm (Figure by reduced 16b). conditioning;The internal onlysurfaces 30% of was the achieved pipes corresponding after 24 h. As the to workingfeatures timeB, C, was and prolonged D were to characterized 96 h, the obtained by reduced Ra was 2.8conditioning;µm for features only 30% B and was D, achieved while the after Ra was 24 h. 3.5 Asµm the for working feature C.time This was slight prolonged deviation to 96 between h, the trendsobtained was Ra attributed was 2.8 µm to thefor difeaturesfferent inclinationsB and D, while of these the pipesRa was with 3.5 respectµm for to feature the BF C. active This regionslight flowdeviation direction. between The oppositetrends was side attributed of the header to the was different measured inclinations by means of of these the exhaust pipes internalwith respect surface to Ethe and BF the active concave region zone flow F(Figure direction. 16c). The The opposite trend of side feature of the E was header very was close measured to that of featureby means C, whichof the hadexhaust the sameinternal inclination, surface E and anthe Raconcave of 3.4 zoneµm was F (F obtainedigure 16c). after The 96 trend h. The of feature convex E zone was very F showed close ato trend that of close feature to that C, which of feature had A;the however, same inclination, a slight diandfference an Ra couldof 3.4 beµm observed, was obtained highlighting after 96 h. a The less performantconvex zone BF F action,showed as a expected. trend close to that of feature A; however, a slight difference could be observed,The 3D highlighting maps of some a less features performant are reported BF action, in as Figure expected. 17 for a morphological analysis. Zone A (Figure 17a) was characterized by a very high peak-to-valley height. An St of 105 µm was measured; this value was very close to the Rt value, highlighting a uniform fabricated surface. After 96 h, the surface was very flat, reaching an St of about 10 µm. As shown in Figure 17b, some areas were characterized by small hollows. More defects were present in the maps related to features B and E, as shown in Figure 17c,d respectively. For these areas, the St was 18 µm, and the hollows were smooth and uniformly distributed. Appl. Sci. 2020, 10, 1382 16 of 19 Appl. Sci. 2020, 10, x FOR PEER REVIEW 16 of 20

Figure 16. Investigated featuresfeatures (a (,ac,)c and) and average average roughness roughness trends trends as a as function a function of BF of working BF working time (timeb,d). (b,d). Appl. Sci. 2020, 10, x FOR PEER REVIEW 17 of 20

The 3D maps of some features are reported in Figure 17 for a morphological analysis. Zone A (Figure 17a) was characterized by a very high peak-to-valley height. An St of 105 µm was measured; this value was very close to the Rt value, highlighting a uniform fabricated surface. After 96 h, the surface was very flat, reaching an St of about 10 µm. As shown in Figure 17b, some areas were characterized by small hollows. More defects were present in the maps related to features B and E, as shown in Figures 17c,d respectively. For these areas, the St was 18 µm, and the hollows were smooth and uniformly distributed.

FigureFigure 17. 17.Three-dimensional Three-dimensional (3D) (3D) map map of featureof feature A beforeA before ﬁnishing finishing (a), (a), feature feature A afterA after 48 h48 (b h), feature(b), B afterfeature 48 hB (afterc), and 48 h feature (c), and E feature after 48 E hafter (d). 48 h (d).

6. Conclusions SLM plays an important role in producing complex parts, but the obtainable surface quality remains a crucial aspect. In this work, post-processing via BF was investigated and analyzed. In the first phase, a systematic approach based on number allowed determining the feasibility regions for BF operations. The developed BB diagrams provided the definitions of motions for different charge types as a function of the filling percentage and the rotational speed. In the second phase, the experimental activities were undertaken on simple geometries to select adequate BF process parameters and charge types for two different materials, i.e., Ti6Al4V and Inconel718. The analysis of the processed profiles demonstrated the analysis outcomes, indicating that small media are not effective for BF operation. A particular use of the AFC allowed calculating the MRR and selecting the charge and processing parameter sets. Both materials showed good machinability in the processing conditions corresponding to high number. For both materials, the mixed charge and process parameter set 2 had the desired properties, such as high MRR and good accessibility of the internal surfaces and holes. Hence, these conditions were selected for phase 3. The case studies, characterized by complex geometries, confirmed the outcomes of phase 2 in term of roughness reduction for the external surfaces. Some differences were observed over the blade and the exhaust manifold internal surfaces in case studies 1 and 2, respectively. For the former, the reduced space for the media motion determined a slower BF action as per a low number; moreover, the difference in roughness reduction was detected for the blade’s up-facing and down- facing surfaces due to the different SLM original morphologies. For the latter, the angled internal surfaces were subjected to different BF conditioning within the active layer flow; in this case, the limited media accessibility was also reflected in a slower finishing process. Apparently, the long processing time remains a limitation for BF employment on SLM parts. However, it must be considered that, in industrial use, the time is typically optimized by using large Appl. Sci. 2020, 10, 1382 17 of 19

6. Conclusions SLM plays an important role in producing complex parts, but the obtainable surface quality remains a crucial aspect. In this work, post-processing via BF was investigated and analyzed. In the first phase, a systematic approach based on Ba number allowed determining the feasibility regions for BF operations. The developed BB diagrams provided the definitions of motions for different charge types as a function of the filling percentage and the rotational speed. In the second phase, the experimental activities were undertaken on simple geometries to select adequate BF process parameters and charge types for two different materials, i.e., Ti6Al4V and Inconel718. The analysis of the processed profiles demonstrated the Ba analysis outcomes, indicating that small media are not effective for BF operation. A particular use of the AFC allowed calculating the MRR and selecting the charge and processing parameter sets. Both materials showed good machinability in the processing conditions corresponding to high Ba number. For both materials, the mixed charge and process parameter set 2 had the desired properties, such as high MRR and good accessibility of the internal surfaces and holes. Hence, these conditions were selected for phase 3. The case studies, characterized by complex geometries, confirmed the outcomes of phase 2 in term of roughness reduction for the external surfaces. Some differences were observed over the blade and the exhaust manifold internal surfaces in case studies 1 and 2, respectively. For the former, the reduced space for the media motion determined a slower BF action as per a low Ba number; moreover, the difference in roughness reduction was detected for the blade’s up-facing and down-facing surfaces due to the different SLM original morphologies. For the latter, the angled internal surfaces were subjected to different BF conditioning within the active layer flow; in this case, the limited media accessibility was also reflected in a slower finishing process. Apparently, the long processing time remains a limitation for BF employment on SLM parts. However, it must be considered that, in industrial use, the time is typically optimized by using large BF machines which have a thick active layer; consequently, a bigger surface area is processed at the same time, and many parts can also be processed together.

Author Contributions: A.B. conceived and designed the experiments, performed the experiments, analyzed the data, wrote the paper; L.B. conceived and designed the experiments, performed the experiments, analyzed the data, wrote the paper; L.M. performed the experiments and found case parts; F.V. supervised the work. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors wish to thank Eng. Michele Marano for his precious help in the experimental activities. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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