Journal of
Manufacturing and Materials Processing
Article
Porosity, Surface Quality, Microhardness and Microstructure of Selective Laser Melted 316L Stainless Steel Resulting from Finish Machining Yusuf Kaynak * and Ozhan Kitay Department of Mechanical Engineering, Marmara University, 34722 Istanbul, Turkey;
[email protected] * Correspondence:
[email protected] or
[email protected]; Tel.: +90-216-336-5770 Received: 30 April 2018; Accepted: 1 June 2018; Published: 5 June 2018
Abstract: Among additive manufacturing (AM) techniques, Selective Laser Melting (SLM) is widely used to fabricate metal components, including biocompatible bone implants made of 316L stainless steel. However, an issue with the components manufactured using this technique is the surface quality, which is generally beyond the acceptable range. Thus, hybrid manufacturing, including AM and finish machining processes, are being developed and implemented in the industry. Machining processes, particularly finish machining, are needed to improve surface quality of additively manufactured components and performance. This study focuses on the finish machining process of additively manufactured 316L stainless steel parts. Finish machining tests were carried out under dry conditions for various cutting speeds and feed rates. The experimental study reveals that finish machining resulted in up to 88% lower surface roughness of SLMed 316L stainless steel; it also had a substantial effect on microstructure and microhardness of the additively manufactured components by creating smaller grains and strain-hardened layer on the surface and subsurface of the SLMed part. The finish machining process also significantly decreased the density of porosity on the surface and subsurface, compared to an as-built sample. The created strain harden layer with less porosity is expected to increases wear and fatigue resistance of these parts. Keywords: selective laser melting; surface integrity; porosity; 316L stainless steel; finish machining
1. Introduction Additive manufacturing enables the production of complex-shaped parts and rapid prototypes that cannot be produced by machining methods [1,2]. Because of these advantages, it is a preferred method in the biomedical, aerospace, and automotive fields [3]. In addition to various AM methods [4,5], a preferred one is the selective laser melting method [6–8]. SLM technology enables the use of different materials in manufacturing and assembly of different powder materials to produce parts that meet specific needs [9]. Although the SLM process provides many advantages as compared to conventional machining, its one of the major drawbacks is the low surface quality resulting from this process [10,11]. Surface quality is greatly influenced by the “stair step” effect, which is the stepped approximation by layers of curves and inclined surfaces [12]. This effect is present, to a greater or lesser degree, in all additive layer manufacturing processes and a consequence of the additive deposition and fabrication of layers [12]. Despite the fact that layer thickness can be reduced to improve surface quality, obtaining an acceptable surface quality is a very important issue in SLM production as poor surface quality could lead to long and expensive post-finishing operations [12]. The surface quality of metallic SLMed parts is influenced by many factors; such as-built orientation [13] laser parameters—laser power, scanning speed, hatching distance, etc. [14]—and powder size [15]. In addition to surface quality, subsurface
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characteristics of additively manufactured components are also important and need to be taken into account. As a source of problems encountered in the additive manufacturing processes, internal stresses due to temperature difference during the process and shrinkage in work piece size after melting are shown [16]. It should also be noted that laser parameters substantially affect surface microstructure [17]. When studies are examined that focuses on the 316L stainless steel produced by the SLM additive manufacturing method, it is generally observed that the main concern is the only on changes in the properties after additive manufacturing. Thus, microstructure [6,18–20], fracture behavior [19,20], mechanical properties [18] fatigue performance [7,21], and residual stress [20,22], were investigated and reported in the literature. However, surface characteristics or enhancing approaches of surface characteristics have not extensively investigated as yet; although, the functionality of AM parts is highly dependent on the geometric and dimensional accuracy, as well as the surface integrity [23]. As hybrid method includes both SLM and finish machining, and finish machining is being used to improve the surface property, it is inecessary to investigate the effects of the finishing process on the surface and subsurface characteristics of additively manufactured parts. When reviewing the literature, it seems that extensive studies on the machining-induced surface integrity characteristics of 316L stainless steel produced by AM are not yet available [4]. Surface characteristics, including roughness and topography induced from processing, is vital [24–26]. Thus, their effects on the surface of SLMed components should be carefully examined. Few studies have reported that post-finishing processes such as laser re-melting [10], electrochemical and abrasive flow machining [27] and milling processes [28] have improved surface characteristics of SLMed parts. 316L stainless steel has been widely used in biomedical industries to manufacture custom-made biocompatible metal bone implants [29]. Together with AM, it is well-suited for these applications, as implants or prostheses can be individualized with very low customization costs [29]. As these implants have interactions with the human body, their surface and subsurface characteristics should be given special attention. Enhancing the surface and subsurface characteristics with post-processing will substantially help to widen the use of AM in the industry. As a post-processing operation, finish machining processes, including turning and milling, can be utilized to improve surface characteristics and control subsurface properties. Depending on the geometry of SLMed parts, either turning operation or milling operation can be used. In some cases, both needs to be used simultaneously. However, it should be noted that as a post-finishing operation the thermomechanical effect of both turning and milling on the surface and subsurface of SLMed parts is expected to be similar. In this study, the influence of the finish turning process on surface and subsurface characteristics of additively manufactured 316L stainless steel is investigated and presented. As a hybrid process is considered, no coolant or lubrication was applied during the finishing process. The effect of cutting speed and feed rate on the surface and subsurface characteristics of additively manufactured parts are also examined. Surface hardening tendency, reduced porosity on the surface and subsurface, and subsurface alteration by smaller grain sizes are observed. Surface roughness is vitally reduced by the finishing operation. 2. Materials and Methods 2.1. Work Material The workpiece used in this study was selective laser melted (SLMed) 316L stainless steel as round bars of 15 mm diameter and 80 mm length. These parts were produced using Selective Laser Melting by Renishaw AM250 under Ar atmosphere. Meander hatch pattern strategy was used to build parts used in this study. SLM laser process parameters are presented in Table 1. The as-built hardness of SLMed 316L is 260 ± 7 HV. All specimens used in this study were in as-built condition. Built direction is shown in Figure 1.
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Table 1. Selective Laser Melting process parameters. J. Manuf. Mater. Process. 2018, 2, 36 J. Manuf. Mater. Process. 2018, x FOR PEER REVIEW Particle Size of2,Powder 14–45
µm Point Distance Layer Thickness 50 µm Exposure Time Table 1. Selective Laser Melting process process parameters. parameters. LaserW Melting Power Table 1. Selective 200 Hatch Distance
Particle Size of Powder
Particle Size of Powder Layer Thickness Layer Thickness PowerPower
14–45 µm
14–45 µm 50 µm 50 µm 200 W200 W
Point Distance
Point Distance Time Exposure Exposure Time Hatch Distance Hatch Distance
60 µm 80 µs 110 µm
3 of 14 3 of 13
60 µm
6080µm µs 80 µs 110µm µm 110
Figure 1. Microstructure and building directions of as-built SLMed 316L sample. Figure 1. 1. Microstructure Microstructure and and building building directions directions of of as-built as-built SLMed Figure SLMed 316L 316L sample. sample.
2.2. Finish Machining Operation
2.2. Finish Machining Operation
The finish machining experiments were conducted on a CNC turning center, which has experiments were conducted on a CNC center, which haswhich maximum The finish finishmachining machining experiments were conducted on aturning CNC turning center, has maximum of 4500 and 18 power.120404 CNMG 120404 Grade A155 with −6° rake ◦ rake spindlespindle speed ofspeed 4500 rpm andrpm 18 kW power. Grade A155 with −6with maximum spindle speed of 4500 rpm and 18 kW kWCNMG power. CNMG 120404 Grade A155 −6°angle rake angle cutting was used experiments. Cutting speeds were as100, 50, 100, cutting tooltool was used inused allin machining experiments. CuttingCutting speeds were selected as selected 50, 100, and angle cutting tool was inallallmachining machining experiments. speeds were selected as 150 50, 150 and 200 200 m/min. In In machining tests, cutkept (ap)p)was waskept kept constant at feed 0.4 200 m/min. In machining tests, depth ofdepth cut (apof ) was constant at 0.4 mmatand rates (f ) offeed 0.08,rates 150 and m/min. machining tests, depth of cut (a constant 0.4 mmmm andand feed rates (f) of(f) 0.08, 0.16, and 0.24 mm/rev, were used. Finish machining operations were performed under 0.16, and 0.24 mm/rev, were used. Finish machining operations were performed under dry machining of 0.08, 0.16, and 0.24 mm/rev, were used. Finish machining operations were performed under dry dry conditions without any coolant and lubricant, depictedas in Figure 2. in in machining conditions without any coolant andaslubricant, Figure 2. 2. machining conditions without any coolant and lubricant, asdepicted depicted Figure
Figure 2. Experimental photography.
2.3. Measurements
Figure 2. Experimental photography. Figure 2. Experimental photography.
To examine surface characteristics (microhardness, microstructure, etc.), machined workpieces 2.3. Measurements were cut with diamond-cutting disks, following which the specimens were cold-mounted in acrylic
To examine surface characteristics (microhardness, microstructure, etc.), machined workpieces were cut with diamond-cutting disks, following which the specimens were cold-mounted in acrylic
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2.3. Measurements J. Manuf.To Mater. Process.surface 2018, 2, x characteristics FOR PEER REVIEW examine (microhardness,
4 of 13 microstructure, etc.), machined workpieces were cut with diamond-cutting disks, following which the specimens were cold-mounted in acrylic and polished using 60,60, 30 30 and 15 15 µmµm gritgrit magnetic discs. Specimens were etched using a solution of and polished using and magnetic discs. Specimens were etched using a solution 2 of mL2 HNO 3 + 4 mL Glycerine + 6 mL HCl to review the microstructure. Surface roughness values mL HNO3 + 4 mL Glycerine + 6 mL HCl to review the microstructure. Surface roughness were measured by an average four measurements using surface tester (Mitutoyo SJ210, values were measured by an of average of four measurements usingroughness surface roughness tester (Mitutoyo Kawasaki, Japan). All reported surface roughness values presented in this study are arithmetic SJ210, Kawasaki, Japan). All reported surface roughness values presented in this study are arithmetic average topography and microstructure ofofetched averagesurface surfaceroughness, roughness,Ra. Ra.Surface Surface topography and microstructure etchedspecimens specimenswere were examined byby Digital Osaka, Japan) and examined DigitalOptic OpticMicroscopy Microscopy(Keyence, (Keyence, Osaka, Japan) andscanning scanningelectron electronmicroscope microscope (SEM) (FEI Sirion XL-30, Portland State University Department of Physics, Portland, OR, USA). The (SEM) (FEI Sirion XL-30, Portland State University Department of Physics, Portland, OR, USA). microstructure of the selective laser melted 316L stainless steel is presented in Figure 3. Observed The microstructure of the selective laser melted 316L stainless steel is presented in Figure 3. Observed main microstructures are melting pool, melting pool boundary, ininthe main microstructures are melting pool, melting pool boundary,large largegrains grainsand andsub-grains sub-grains the melting pools [30,31]. Both cellular and elongated grains are observed in the microstructure of melting pools [30,31]. Both cellular and elongated grains are observed in the microstructure of as-built as-built As was in reported inbyliterature andcellular Eckertmicrostructure [32], cellular is SLMedSLMed part. Aspart. was reported literature Prashant by andPrashant Eckert [32], microstructure is observed with cell sizes within nm to µm range. Sub-grains can be observed in aor observed with cell sizes within nm to µm range. Sub-grains can be observed in a homogenous homogenous or heterogeneous structure in the melting pool, sometimes in extended form and heterogeneous structure in the melting pool, sometimes in extended form and different directions. different directions. The granules thermallyby melted by laser giveofthe appearance grainspool, in The thermally melted laser granules give the appearance slipped grainsofinslipped the melting the melting pool, as clearly shown. as clearly shown.
Figure surface and andsubsurface subsurfaceof of as-built SLMed (b) detail The detail of Figure3.3.(a)Microstructure (a)Microstructure of of surface as-built SLMed part,part, (b) The of melting melting pool and grain structure of as-built SLMed part. pool and grain structure of as-built SLMed part.
Porosity level of as-built SLM samples were also measured with Keyence digital optical Porosity level of as-built SLM samples were also measured with Keyence digital optical microscopy, using its image analysis tool. Figure 4 shows the porosities from the building direction microscopy, using its image analysis tool. Figure 4 shows the porosities from the building direction of of an as-built SLMed sample. an as-built SLMed sample.
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Figure 4. Optical images of porosities. Figure Figure4.4.Optical Opticalimages imagesof ofporosities. porosities.
Microhardness of the workpiece was measured using Future-Tech FM310e. Hardness of the Microhardness of of the the workpiece workpiece was was measured measured using using Future-Tech Future-TechFM310e. FM310e. Hardness Hardness of of the the Microhardness specimen waswas determined byby anan average withaatesting atesting testing load of 50 gf and a load specimen was determined by an averageofof offour fourmeasurements measurementswith with load of50 50gf gfand and load specimen determined average four measurements load of aaload dwelling timetime for 15 was used. dwelling time fors15 s was used. dwelling for 3. Discussion 3. Results andand Discussion 3. Results Results and Discussion 3.1. Porosity, Surface Quality and Topography
3.1. Porosity, Surface Quality Topography 3.1. Porosity, Surface Quality andand Topography Figure 55shows the density of porosity observed to the surface and subsurface of Figure shows density porosityobserved observedin inlayers layersclose close and subsurface of of Figure 5 shows the the density of of porosity in layers closetotothe thesurface surface and subsurface as-built and machined SLMed samples. It is apparent that the density of porosity is much more in as-built and machined SLMed samples. It is apparent that the density of porosity is much more in in as-built and machined SLMed samples.toItmachined is apparent that thethe density of is porosity is much1.39% more the as-built sample, when compared ones. While porosity approximately the as-built sample, when compared to machined ones. While the porosity is approximately 1.39% in the as-built sample, whenitcompared to machinedspeed ones.and While theatporosity is approximately 1.39% in in as-built sample, 0.25%atatlow-cutting low-cutting 0.48% anan as-built sample, it isis0.25% speed and 0.48% at aa higher-cutting higher-cutting speed. speed. Overall Overall an as-built sample, it is 0.25% at low-cutting speed and 0.48% at a higher-cutting speed. Overall finish finish machining machining resulted resulted in in reduction reduction of of porosity porosity density density on onthe thesurface surfaceand andsubsurface subsurface of ofSLMed SLMed finishparts. machining resulted in reduction of porosity density on the surface andone subsurface ofthat SLMed Two ofofporosities were observed in the microstructure of the is spherical parts. Twotypes types porosities were observed in the microstructure of samples: the samples: one is spherical due to gas bubbles [33], and the other is a bigger and elongated porosity that occurs between parts.occurs Two types of porosities were observed in the microstructure of the samples: one is spherical that occurs due to gas bubbles [33], and the other is a bigger and elongated porosity that occurs two melting pools [33] as shown athe red other arrow inred Figure 5. in Elongated areporosities the result of that the occurs due gas bubbles [33], and a bigger and elongated porosity that are occurs between the to two melting pools [33]with as shown with ais arrow Figure 5.porosities Elongated partially melted powder particles and can be attributed to not enough melting of the new layer and between the two melting melted pools [33] as shown with red be arrow in Figure 5. enough Elongated porosities the result of partially powder particles anda can attributed to not melting of the are not enough melting of the prior solidified material again [33]. The finish machining process eliminated new layer and not enough melting of the prior solidified material again [33]. The finish machining the result of partially melted powder particles and can be attributed to not enough melting of the partially melted powders onmelted the surface and subsurface of samples, and notably reduced process eliminated partially the surface and subsurface of samples, andporosity notably new layer and not enough melting ofpowders the prioronsolidified material again [33]. The finish machining density in these layers. This is mainly because of the elevated temperature and stress work materialand is reduced porosity density in these layers. This is mainly because of the elevated temperature process eliminated partially melted powders on the surface and subsurface of samples, and notably subjected to during cutting process.toReduced porosityprocess. resultingReduced from the porosity finish machining process is stress work material isin subjected during resulting from the reduced porosity density these layers. Thiscutting is mainly because of the elevated temperature and expected to increase fatigue resistance. finish machining process is expected to increase fatigue resistance.
stress work material is subjected to during cutting process. Reduced porosity resulting from the finish machining process is expected to increase fatigue resistance. 1.39% 0.48% 0.25% 1.39%
(a)
0.48%
0.25%
(b)
(c)
Figure5.5.(a) (a)Porosity observed on on the the surface surface and and subsurface subsurface of of as-built as-built sample sample (b) (b) Machined Machinedsample sample Figure Porosity observed c = 50 m/min); (c) Machined sample (V c = 200 m/min). (V (c) (Vc = 50(a) m/min); (c) Machined sample (Vc = 200 (b) m/min).
Figure 5. (a)Porosity observed the surface and subsurface of as-built sample (b) Machined sample Although selective laser on melting is an emerging approach due to its many advantages when c = 50 m/min); (c) Machined sample (Vc =processes, 200 m/min). (V compared to traditional manufacturing one of the limitations is to produces a surface quality that is not within an acceptable range [34]. Fabricated components are expected to have a Although selective is an emerging due to its many process advantages when smooth surface with laser lowermelting surface roughness in the approach conventional manufacturing [35,36], and AM be able to offer the same quality inone order become an alternative approach in the compared to should traditional manufacturing processes, oftothe limitations is to produces a surface
quality that is not within an acceptable range [34]. Fabricated components are expected to have a
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Although selective laser melting is an emerging approach due to its many advantages when compared to traditional manufacturing processes, one of the limitations is to produces a surface quality that is not within an acceptable range [34]. Fabricated components are expected to have J. Manuf.surface Mater. Process. 2018, 2, x FOR PEER REVIEW 6 of 13 a smooth with lower surface roughness in the conventional manufacturing process [35,36], and AM should be able to offer the same quality in order to become an alternative approach in the J. Manuf. Mater. Process. 2018, 2,Average x FOR PEER REVIEW 6 ofis13 manufacturing industry. surface roughness (Ra) of SLMed as-built 316L stainless steel manufacturing industry. Average surface roughness (Ra) of SLMed as-built 316L stainless steel is approximately 7 ± 1 µm, as shown in Figure 6. These measured values can be considered as very approximately 7 ± 1industry. µm, as to shown insurface Figure quality 6. Theseofmeasured values can be316L considered assteel veryis high, manufacturing Average roughness (Ra) of SLMed as-built stainless high, when compared the surface conventionally produced 316L stainless approximately 7 ± 1 µm, as shown in Figure 6. These measured values can be considered as very less when compared to the of conventionally produced 316L stainless steel—generally steel—generally lesssurface than 2 quality µm. Therefore, surface finishing is required to improve the surface high, when compared to the surface quality of conventionally produced 316L stainless thanquality 2 µm. of Therefore, surface finishing is required to improve the surface quality of SLMed parts. SLMed parts. steel—generally less than 2 µm. Therefore, surface finishing is required to improve the surface quality of SLMed parts.
Figure 6. Surface topography of an as-built SLMed part. Figure 6. Surface topography of an as-built SLMed part. Figure 6. topography of an as-built SLMed part.operations were carried To improve surface quality ofSurface as-built SLMed parts, finish machining To improve surface quality of as-built SLMed parts, finish machining operations were carried out. Surface topography of finish machined samples of additively manufactured 316L stainless steel out. To improve surface quality as-built parts, machining operations were carried Surface topography of finish machined samples additively manufactured 316L stainless steel at at different feed rates are shown inofFigure 7. SLMed Feed of marks onfinish the machined samples are visible at all out.rate Surface topography of finish machined samples of additively manufactured 316L stainless steel feed values. When examining the changes of surface topographies as a function of feed rate, different feed rates are shown in Figure 7. Feed marks on the machined samples are visible at all feed at differentdifferences feed rates are in Figure 7.feed Feed marks on the machined samples are visible at all withshown thechanges increasing is observed. most favorable and noticeable the rate noticeable values. When examining the of surfacerate topographies asThe a function of feed rate, feed rate values. When examining the changes of surface topographies as a function of feed rate, smoothest surface was obtained withrate the is lower feed rate. most favorable and the smoothest surface differences with the increasing observed. noticeable differences with feed the increasing feed rateThe is observed. The most favorable and the was obtained thewas lower feed rate. smoothestwith surface obtained with the lower feed rate.
(a)
(b)
(c)
Figure 7. Surface 316L stainless steel (Vc = 150 m/min). (a) f = (a) topography of finish-machined SLMed (b) (c) 0.08 mm/rev; (b) f = 0.16 mm/rev; (c) f = 0.24 mm/rev. Figure 7. Surface topography of finish-machined SLMed 316L stainless steel (Vc = 150 m/min). (a) f = Figure 7. Surface topography of finish-machined SLMed 316L stainless steel (Vc = 150 m/min). (a) 0.08 mm/rev; (b) f = 0.16 (c) samples f = 0.24 mm/rev. Surface topography of mm/rev; machined at different cutting speeds is shown in Figure 8. At
f = 0.08 mm/rev; (b) f = 0.16 mm/rev; (c) f = 0.24 mm/rev. low-cutting speeds, especially 100 m/min, debris reattachment to the machined surface is observed, Surface topography ofroughness. machined However, samples atatdifferent cutting speeds is mm/min), shown in feed Figure 8. At leading to increased surface high-cutting speeds (200 marks low-cutting speeds, especially 100 m/min, debris reattachment to the machined surface is observed, Surface topography of machined samples at different cutting speeds is shown in Figure 8. are very narrow, resulting in a much smoother surface. leading to increased surface roughness. However, at high-cutting speeds (200 mm/min), feed marks At low-cutting speeds, especially 100 m/min, debris reattachment to the machined surface is observed, Average surface roughness of machined samples as a function of feed rate is illustrated in Figure 9. narrow, resulting in a much smoother leading increased surface However, at high-cutting mm/min), Itare is to avery well-known fact thatroughness. increasing feed rate surface. results in increasedspeeds surface (200 roughness. Therefeed is anmarks Average surface roughness of machined samples a function as of feed rate is in Figure 9. increasing trend on surface when feed rate as is increased, depicted inillustrated Figure 9. Measured are very narrow, resulting in roughness a much smoother surface. It is a well-known fact that increasing feedand rate2.54 results increased roughness. Thererates, is an average roughness (Ra) are 2.14 µm in at 0.16,surface andof0.24 mm/rev feed Average surface values roughness of 0.8, machined samples as0.08, a function feed rate is illustrated in increasing trend on surface roughness when feed rate is increased, as depicted in Figure 9. Measured respectively. is a 217% fact increase in surface roughness from the lowest feed rate ofsurface 0.08 mm/rev Figure 9. It is aThere well-known that feed results inand increased roughness values (Ra)mm/rev. are 0.8,increasing 2.14 and 2.54 µmrate atfor 0.08, 0.16, 0.24 mm/rev(presented feedroughness. rates, toaverage the highest feed rate of 0.24 Topography images these cutting conditions Thererespectively. is an increasing trend on surface roughness when feed rate is increased, as depicted in Figure 9. There is a 217% increase in surface roughness from the lowest feed rate of 0.08 mm/rev in Figure 7) support these obtained values. Measured roughness values (Ra)Topography are 0.8, 2.14 and 2.54 µm cutting at 0.08,conditions 0.16, and(presented 0.24 mm/rev to the average highest feed rate of 0.24 mm/rev. images for these feed rates, respectively. There is a 217% increase in surface roughness from the lowest feed rate of in Figure 7) support these obtained values.
Measuring direction for Ra
Debris
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0.08 mm/rev to the highest feed rate of 0.24 mm/rev. Topography images for these cutting conditions (presented in Figure 7) support these obtained values. J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 7 of 13 (a)
(b)
Measuring direction for Ra
Debris
(c) (a)
(d) (b)
Figure 8. Surface topography of finish-machined SLMed 316L stainless steel (f = 0.08 mm/rev). (a) Vc = 50 m/min; (b) Vc = 100 m/min; (c) Vc = 150 m/min; (d) Vc = 200 m/min.
On the other hand, using the following equation [37], the surface roughness values for various feed rates are also calculated. =
0.0321 ×
(1)
Average Surface Roughness Ra, (µm) Average Surface Roughness Ra, (µm)
where, f indicates feed rate, and rc indicates nose radius of the cutting tool used in this finishing (c) predicted values of surface roughness (d) operation. At 0.08 and 0.16 mm/rev, shows good agreement with experimentally measured values, as shown in Figure 9; however at mm/rev (a) feed Figure 8. Surface topography of finish-machined SLMed 316L stainless steel (f =0.24 0.08 mm/rev). Vc =rate, Figure 8. Surface topography of finish-machined SLMed 316L stainless steel (f = 0.08 mm/rev). large 50 variation in between predicted measured m/min; (b) Vc = 100 m/min; (c) Vc and = 150experimentally m/min; (d) Vc = 200 m/min. is recorded. This is mainly due (a) Vc =increased 50 m/min; (b)radius Vc = 100 m/min; (c)insert, Vc = 150 m/min; (d) V m/min. c = 200 to the nose of the cutting resulting from wear occurs during cutting process. On the other hand, using the following equation [37], the surface roughness values for various feed rates are also 10 calculated. Vc=150 m/min Calculated As-built 0.0321 × (1) = 8 where, f indicates feed rate, and rc indicates nose radius of the cutting tool used in this finishing 6 operation. At 0.08 and 0.16 mm/rev, predicted values of surface roughness shows good agreement with experimentally measured values, as shown in Figure 9; however at 0.24 mm/rev feed rate, 4 large variation in between predicted and experimentally measured is recorded. This is mainly due to the increased nose radius of the cutting insert, resulting from wear occurs during cutting process. 2
10 0 8
Vc=150 m/min 0.08
0.10
0.12
0.14
Calculated 0.16
0.18
Feed Rate f, (µm)
As-built 0.20
0.22
0.24
Figure 9. Surface roughness of finish-machined SLMed 316L stainless steel as a function of the feed 6 rate.
Figure 9. Surface roughness of finish-machined SLMed 316L stainless steel as a function of the feed rate. 4
Average roughness values at equation different cutting areroughness illustratedvalues in Figure 10. On the other surface hand, using the following [37], thespeeds surface for various Roughness values do not show a uniform trend as a function of the cutting speed. Roughness feed rates are also calculated. 2 0.0321 × f 2 Ra = (1) rc 0
where, f indicates feed rate,0.08 and r0.10 nose radius of the0.20 cutting tool used in this finishing c indicates 0.12 0.14 0.16 0.18 0.22 0.24 Rateof f, (µm) operation. At 0.08 and 0.16 mm/rev, predictedFeed values surface roughness shows good agreement with experimentally measured as shownSLMed in Figure 9; however 0.24 mm/rev Figure 9. Surface roughnessvalues, of finish-machined 316L stainless steel as at a function of the feedfeed rate, large variation rate. in between predicted and experimentally measured is recorded. This is mainly due to the increased nose radius of the cutting insert, resulting from wear occurs during cutting process. Average surface roughness valuesatatdifferent different cutting cutting speeds in Figure 10. 10. Average surface roughness values speedsare areillustrated illustrated in Figure Roughness values do not show a uniform trend as a function of the cutting speed. Roughness Roughness values do not show a uniform trend as a function of the cutting speed. Roughness
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increased from a cutting speed of 50 m/min to 150 m/min, followed by a decreasing trend at a cutting from a cutting speed of 50 m/min to 150 values m/min, followed byare a decreasing trend a cutting speedincreased of 200 m/min. Average surface roughness measured 0.44, 0.81, 0.8 at and 0.57 µm at speed of 200 m/min. Average surface roughness values measured are 0.44, 0.81, 0.8 and 0.57 µm at cutting speeds of 50, 100, 150 and 200 m/min, respectively. Sharp reduction in surface roughness cutting speeds of 50, 100, 150 and 200 m/min, respectively. Sharp reduction in surface roughness value at the highest cutting speed can be attributed to tool wear at nose region of tool, resulting value at the highest cutting speed can be attributed to tool wear at nose region of tool, resulting from from an an increased increasedcutting cuttingtemperature temperature high-cutting speed. Increased radius of cutting tool is at at high-cutting speed. Increased nosenose radius of cutting tool is expected to generate much smoother surface considering 1, presented presentedabove. above.It Itshould should be expected to generate much smoother surface consideringthe theequation equation 1, also noted that the difference is 84% in between the lowest and highest roughness values within be also noted that the difference is 84% in between the lowest and highest roughness values withinthese these selected cutting speeds. selected cutting speeds. Average Surface Roughness Ra, (µm)
3.0
f=0.08 mm/rev Calculated
2.5 2.0 1.5 1.0 0.5 0.0 50
100
150
Cutting Speed Vc, (m/min)
200
Figure 10. Surface roughness of finish-machined SLMed 316L stainless steel as a function of cutting
Figure 10. Surface roughness of finish-machined SLMed 316L stainless steel as a function of speed. cutting speed.
Overall results demonstrate that the finish machining process makes a substantial contribution to enhance the surface qualitythat of selective melted 316L stainless by notablycontribution reducing to Overall results demonstrate the finishlaser machining process makessteel a substantial surface roughness of as-built components. However, it should be also noted that cutting parameters enhance the surface quality of selective laser melted 316L stainless steel by notably reducing surface including feed rate and cutting speed at theitfinishing operation playthat a dominant in controlling roughness of as-built components. However, should be also noted cutting role parameters including the surface quality of SLMed parts. feed rate and cutting speed at the finishing operation play a dominant role in controlling the surface quality of SLMed parts. 3.2. Microstructure Mechanical properties of metal parts vary significantly with microstructure, including the 3.2. Microstructure phase type, grain size and shape, dendrite, and element segregation [38]. Optical microscopy images
Mechanical properties of metal parts vary significantly with microstructure, the phase of the microstructure of SLMed parts subjected to finish machining at different feed including rates are shown type,ingrain size shape, and element segregation is [38]. Optical microscopy images Figure 11. and The effect of dendrite, finish machining on the microstructure more visible in the region that is of the microstructure of SLMed parts subjected to finish machining at different feed rates are shown close to the surface, where smaller size cells are observed. The results of the plastic deformation that in occurs in the surface zone of the material depicted in Figure 11. Whilevisible cell sizeinisthe smaller than 1 µm Figure 11. The effect of finish machining onisthe microstructure is more region that is close to the surface area that deeply from machining, is generally abovethat 1 µm to theclose surface, where smaller sizeiscells areinduced observed. The results ofcell thesize plastic deformation occurs much deeper that is aredepicted not affected by the machining process. in thewithin surface zone of thelayers material in Figure 11. While cell size is smaller than 1 µm close to The effects on the microstructure SLMed are shown in 1Figure 12. Themuch the surface area thatofis cutting deeplyspeed induced from machining, of cell size isparts generally above µm within size of cellular grain is generally less than 1 µm. Just below the machined surface, even slipped grain deeper layers that are not affected by the machining process. within the same pool showed refinement; however, more slipping is also observed in much deeper The effects of cutting speed on the microstructure of SLMed parts are shown in Figure 12. The size layers, which might be a result of both temperature and stress generated by the finish-cutting of cellular grain is slipped generally lessare than 1 µm. below thespeeds machined even200 slipped process. These grains found moreJust at the cutting of 150surface, m/min and m/min,grain within the same pool showed refinement; however, more slipping is also observed in much deeper than at cutting speeds of 50 m/min and 100 m/min. Observation of the microstructure of SLMed layers, which be amachining result of both temperature and stress generated layer. by theHowever, finish-cutting process. parts aftermight the finish process shows a clear machining-induced the depth this layer depends on cutting and speeds is generally than 20 optical Theseofslipped grains are found moreparameters at the cutting of 150less m/min andµm 200under m/min, than at microscopy. examination techniques such as TEM might make it possible to observe in after cutting speeds ofFurther 50 m/min and 100by m/min. Observation of the microstructure of SLMed parts detail the effect of machining on deeper layers of the samples. the finish machining process shows a clear machining-induced layer. However, the depth of this layer depends on cutting parameters and is generally less than 20 µm under optical microscopy. Further examination by techniques such as TEM might make it possible to observe in detail the effect of machining on deeper layers of the samples.
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ff == 0.08 0.08 mm/rev mm/rev
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ff == 0.16 0.16 mm/rev mm/rev
Figure 11. The effect of feed rate on microstructure of SLMed 316L part (Vc mm/min). = 150 mm/min). Figure Figure 11. 11. The The effect effect of of feed feed rate rate on on microstructure microstructure of of SLMed SLMed 316L 316L part part (V (Vcc == 150 150 mm/min).
V Vcc== 50 50 m/min m/min
V Vcc== 100 100 m/min m/min
V Vcc== 150 150 m/min m/min
V Vcc== 200 200 m/min m/min
Figure The effect of cutting speed on microstructure of 316L part (f(f == 0.08 mm/rev). Figure 12. 12. onon microstructure of SLMed SLMed 316L partpart 0.08 mm/rev). Figure 12. The Theeffect effectofofcutting cuttingspeed speed microstructure of SLMed 316L (f = 0.08 mm/rev).
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3.3. Microhardness of the the significant significantsurface surface characteristics of metallic material is surface hardness, One of characteristics of metallic material is surface hardness, which which influences the response capability of components subjected to various environmental, frictional, influences the response capability of components subjected to various environmental, frictional, and and contact conditions. Although determination of surface and subsurface hardness is important contact conditions. Although determination of surface and subsurface hardness is important after after post-processing of wrought this examination more vital for additively post-processing of wrought metallic metallic material, material, this examination is more vitalisfor additively manufactured manufacturedand components melted components, as highlyheat localized inputinresults components selective and laserselective melted laser components, as highly localized inputheat results large in large thermal and, consequently, high thermal stressesduring occursthe during the [39]. process thermal gradientsgradients and, consequently, high thermal stresses occurs process As [39]. the As the machining process inherently generates stress temperature surfaceofofmachined machinedparts, parts, machining process inherently generates stress andand temperature on on thethe surface this eventually influences the microhardness response of surface and subsurface of components [40]. Microhardness of of finish-machined finish-machined SLMed SLMed samples samples at at different different cutting cutting speeds speeds is illustrated in Microhardness hardness of finish-machined SLMedSLMed samplessamples at all cutting speeds increased substantially Figure 13. 13.The The hardness of finish-machined at all cutting speeds increased as compared as to as-built SLMed sample. The large difference in difference finish-machined samples andsamples as-built substantially compared to as-built SLMed sample. The large in finish-machined SLMed partsSLMed is observed at observed approximately 50 µm from50the surface. Assurface. the cutting speed increases, and as-built parts is at approximately µm from the As the cutting speed it is observed the average microhardness increases, which is generally an expected response of increases, it isthat observed that the average microhardness increases, which isnot generally not an expected 316L stainless steel. Increased leadsspeed to increased which generally to thermal response of 316L stainless steel.speed Increased leads totemperature, increased temperature, whichleads generally leads softening consequently reduced hardness wroughtinmaterial SLMed however, to thermaland softening and consequently reducedinhardness wrought[40]. material [40].parts, SLMed parts, show a completely differencedifference response,response, as illustrated. While the highest average hardness value however, show a completely as illustrated. While the highest average hardness was as 306asHV cutting speed of 200 m/min, valuemeasured was measured 306 at HVa at a cutting speed of 200 m/min,the thelowest lowestaverage average hardness hardness value was measured as 282 HV at a cutting speed of 50 m/min, at aa distance distance of of 15 15 µm µm from from the the surface. surface. m/min, at When from 50 m/min to 200 an 8% in hardness is observed. Besides, When speed speedisisincreased increased from 50 m/min to m/min, 200 m/min, anincrease 8% increase in hardness is observed. Besides, hardening at theand highest andcutting lowestspeeds cuttingisspeeds is 9%, 17%respectively, and 9%, respectively, the hardening at the highest lowest 17% and when the when hardness hardness as-builtis material is account. taken into Theprocess machining process stress and of as-builtof material taken into Theaccount. machining induces stressinduces and consequently consequently increases dislocation thesubsurface, surface andwhich subsurface, which is thefor main reason increases dislocation density on thedensity surfaceon and is the main reason increased for increased hardness on the surface of and subsurface of machined parts. It is also a hardness on the surface and subsurface machined SLMed parts. It isSLMed also a well-acknowledged well-acknowledged fact that increased speedstrain leads rates. to increased strain rates. Deformation fact that increased cutting speed leads cutting to increased Deformation with higher strain with higher strain resulting from higher cutting also the reason larger strain rate resulting fromrate higher cutting speeds might alsospeeds be themight reason forbe larger strain for hardening and hardening andincreased consequently increased on microhardness on the surface and subsurface of the SLMed consequently microhardness the surface and subsurface of the SLMed part. Moreover, part. it was the grains the surface and subsurface becomes smaller than it wasMoreover, depicted that the depicted grains onthat the surface andon subsurface becomes smaller than the bulk of material the bulk of as shown inThis the previous section.toThis also expectedmeasured to alter microhardness as shown in material the previous section. is also expected alterismicrohardness on the surface measured on theas surface and subsurface, as increased might grains, be the result of smaller and subsurface, increased microhardness might bemicrohardness the result of smaller considering the grains, considering the well-known well-known Hall-Petch model [41]. Hall-Petch model [41]. 310
Vc=50 m/min Vc= 100 m/min Vc= 150 m/min Vc= 200 m/min As-built
Microhardness, (HV)
300 290 280 270
As-built hardness
260 250 20
40
60
80
100
120
140
Depth from machined surface, (µm)
160
180
200
Figure 13. Microhardness as aa function Figure 13. Microhardness of of finish-machined finish-machined and and as-built as-built SLMed SLMed 316L 316L stainless stainless steel steel as function of of cutting speed (f (f == 0.08 cutting speed 0.08 mm/rev). mm/rev).
Hardness values of machined samples at different feed rates are shown in Figure 14. Similar to Figure 13, hardening occurs at all feed rate values; the measured hardness values also increase with
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Hardness values of2,machined J. Manuf. Mater. Process. 2018, x FOR PEERsamples REVIEW at different feed rates are shown in Figure 14. Similar 11 of to 13 Figure 13, hardening occurs at all feed rate values; the measured hardness values also increase with increased feed rate. 320 HV that is the highest average hardness values, measured at feed rates of mm/rev. The machined 0.24 mm/rev. The difference difference in in the highest and the lowest measured hardness values in machined sample is approximately 7%. In addition, hardening at the highest and the lowest feed rates are 23 14%,respectively, respectively, considering hardness of as-built material. It be should that and 14%, considering the the hardness of as-built material. It should notedbe thatnoted increased increasedleads hardness leads towear increased wearofresistance of work Thus, to it conclude is possible to hardness to increased resistance work material [42].material Thus, it [42]. is possible that conclude that post processing helps the SLMed part to increase wear resistance. post processing helps the SLMed part to increase wear resistance.
f=0.08 mm/rev f=0.16 mm/rev f=0.24 mm/rev As-built
Microhardness, (HV)
320
300
280
As-built hardness
260
20
40
60
80
100
120
140
Depth from machined surface, (µm)
160
180
200
Figure 14. 14. Microhardness of finishfinish- machined machined and and as-built as-built SLMed SLMed 316L 316L stainless stainless steel steel as as aa function function of of Figure Microhardness of feed rate rate (V (Vc == 150 m/min). feed c 150 m/min).
4. Conclusions 4. Conclusions Additive manufacturing is of interest to the biomedical industry; the main concern of additively Additive manufacturing is of interest to the biomedical industry; the main concern of additively manufactured parts, however, is surface characteristics. Finish machining to improve the surface of manufactured parts, however, is surface characteristics. Finish machining to improve the surface of components as a part of hybrid manufacturing seems to be an easily implemented approach. This components as a part of hybrid manufacturing seems to be an easily implemented approach. This study study provides experimental investigation into the effects of finish machining on the surface provides experimental investigation into the effects of finish machining on the surface enhancement enhancement capability of SLMed 316L stainless steel. The following conclusions can be drawn from capability of SLMed 316L stainless steel. The following conclusions can be drawn from this study: this study: • The finish machining process substantially reduces porosity rate on the surface and subsurface of • The finish machining process substantially reduces porosity rate on the surface and subsurface SLMed parts, as compared to as-built SLMed parts. This will eventually help to reduce the risk of of SLMed parts, as compared to as-built SLMed parts. This will eventually help to reduce the fatigue failure that originates from the surface and/or subsurface of components due to porosity. risk of fatigue failure that originates from the surface and/or subsurface of components due to • The finish machining process vitally reduces the surface roughness of SLMed 316L stainless porosity. steel and thus enhances the surface of SLMed parts. While as-built parts316L havestainless surface • The finish machining process vitallyquality reduces the surface roughness of SLMed roughness (R ), that is approximately 7 ± 1 µm, the finish machining process reduces this value a enhances the surface quality of SLMed parts. While as-built parts have surface steel and thus below 1 µm,(Rdemonstrating a significant enhancement of surface quality. roughness a), that is approximately 7 ± 1 µm, the finish machining process reduces this value Microstructure and grain size close to theenhancement surface of SLMed 316 quality. L stainless steel is affected by • below 1 µm, demonstrating a significant of surface the finish machining operation. Thetothickness of the machining-induced to by be • Microstructure and grain size close the surface of SLMed 316 L stainlesslayer steelisisfound affected approximately 10–20 µm. In that particular layer, grains become much smaller as compared the finish machining operation. The thickness of the machining-induced layer is found to be to much deeper10–20 layersµm. from of parts. addition, slipped are visible, approximately In the thatsurface particular layer, In grains become muchgrains smaller asmuch compared to induced by plastic deformation resulting from the cutting process. much deeper layers from the surface of parts. In addition, slipped grains are much visible, • The finishbymachining process leads to strain hardening on process. the surface and subsurface of SLMed induced plastic deformation resulting from the cutting parts. Considering the microhardness material, approximately 9%and to 23% increase in • The finish machining process leads of to as-built strain hardening on the surface subsurface of hardness (depending on cutting conditions) is observed. SLMed parts. Considering the microhardness of as-built material, approximately 9% to 23% increase in hardness (depending on cutting conditions) is observed. Author Contributions: Y.K. designed, supervised the experimental works, reviewing and editing the manuscript. O.K carried out the experimentations, plots the data and wrote the first draft of manuscript. Final version of manuscript was read and approved by the coauthors.
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Author Contributions: Y.K. designed, supervised the experimental works, reviewing and editing the manuscript. O.K carried out the experimentations, plots the data and wrote the first draft of manuscript. Final version of manuscript was read and approved by the coauthors. Acknowledgments: Authors acknowledge Renishaw Turkey for providing SLMed 316L stainless steel work materials for this study. Conflicts of Interest: The authors declare no conflict of interest.
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