Research on Selective Laser Melting of Ti6al4v: Surface Morphologies, Optimized Processing Zone, and Ductility Improvement Mechanism

metals

Article Research on Selective Laser Melting of Ti6Al4V: Surface Morphologies, Optimized Processing Zone, and Ductility Improvement Mechanism

Di Wang 1,2 ID , Wenhao Dou 1 and Yongqiang Yang 1,*

1 School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China; [email protected] (D.W.); [email protected] (W.D.) 2 School of Metallurgy and Materials, The University of Birmingham, Birmingham B152TT, UK * Correspondence: [email protected]; Tel./Fax: +86-020-8711-1036

Received: 24 May 2018; Accepted: 15 June 2018; Published: 21 June 2018

Abstract: The quality and mechanical properties of titanium alloy fabricated using selective laser melting (SLM) are critical to the adoption of the process which has long been impeded by the lack of uniformity in SLM-fabrication parameter optimization. In order to address this problem, laser power and scanning speed were combined into linear energy density as an independent variable, while surface morphology was defined as a metric. Based on full-factor experiments, the surface quality of SLM-fabricated titanium alloy was classified into five zones: severe over-melting zone, high-energy density nodulizing zone, smooth forming zone, low-energy density nodulizing zone, and sintering zone. The mechanism resulting in the creation of each zone was analyzed. Parameter uniformity was achieved by establishing a parameter window for each zone, and it also revealed that under smooth forming conditions, the relationship of linear energy density to the quality of the formed surface is not linear. It was also found that fabrication efficiency could be improved in the condition of the formation of a smooth surface by increasing laser power and scanning speed. In addition, maximum elongation of the SLM-fabricated titanium alloy increased when the densified parts were processed using an appropriate heat treatment, from a low value of 5.79% to 10.28%. The mechanisms of change in ductility of the alloy were thoroughly analyzed from the perspectives of surface microstructure and fracture morphology. Results indicate that after heat treatment, the microcosmic structure of the alloy was converted from acicular martensite α’ phase to a layered α+β double-phase structure, the fracture type also changed from quasi-cleavage to ductile fracture.

Keywords: selective laser melting; optimized parameter zone; surface morphology; ductility improvement mechanism; energy density

1. Introduction Ti6Al4V (TC4) is the most common α+β dual-phase, high-temperature titanium alloy, and is studied more thoroughly than any other titanium alloy [1]. Due to its low density, excellent speciﬁc strength, and desirable mechanical properties, it has found widespread uses in military, aviation, space [2], and auto industries. Compared with stainless steel and CoCr alloy, Ti6Al4V is characterized by outstanding biological compatibility, corrosion resistance, and an elastic modulus more similar to bone tissues than other alloys. Hence, it is usually used for medical purpose and is of great value to biomedical research and applications [3–6]. Titanium alloy is manufactured using casting or forging traditionally. However, titanium’s strength and hardness are rather high, leading to a machining process that is rather complex and expensive, making it hard to produce complicated structures. As a result, Ti6Al4V possesses a unique combination of strength, toughness, corrosion resistance,

Metals 2018, 8, 471 ; doi:10.3390/met8070471 www.mdpi.com/journal/metals Metals 2018, 8, 471 2 of 17 low specific weight, and biocompatibility, making it very suitable for high-performance engineering solutions [7]. Selective laser melting (SLM) is a metal additive manufacturing technology which can directly fabricate parts following the design or acquisition of complex 3D models using computer techniques. This is done by directly melting metal or alloy powders using a high-energy laser beam, depending on the information of each slice of the model, and each part is manufactured in a stacked manner [8]. The density of SLM-fabricated parts has reached over 99% due to continuous development of the manufacturing technique, and a dimensional error could be controlled within 0.05 mm. Because of its ability to directly fabricate complex curved surfaces and porous structures [9,10], SLM-fabricated Ti6Al4V alloys find widespread use in the direct manufacturing of auto and aviation components [11], in addition to medical implants [12–14]. Study of the fabrication of Ti6Al4V around the world using SLM is still focused on process control and parameter optimization, but using different optimization metrics. Wang et al. [15] investigated the influence of laser scanning speed on the density and mechanical properties of SLM-fabricated titanium alloy. Experimental results demonstrated that a density of 99.57%, hardness of 407 HV, and tensile strength of 1225 MPa could be achieved, which was explained briefly after studying its microscopic structure. Thijs et al. [16] discussed the influence of parameter and scanning strategy on microscopic structure during the manufacturing process, demonstrating that rapid cooling led to the presence of martensite, a columnar crystal generated during the re-melting process of the previous layer, and that the direction of its growth was determined by the local transmission of heat. Song et al. [17] explored the influence of laser power and scanning speed on the microcosmic structure of SLM-fabricated titanium alloy using a 3D finite element model in the analysis. Their results indicated that a laser power of 110 W and scanning speed of 0.2 m/s led to an appropriate gradient of temperature within the powder bed which maximized depth of melting. The influence of laser power and scanning speed was comprehensively evaluated under the metrics of mechanical properties (e.g., density and coarseness) and microcosmic properties (e.g., hardness and structure). It was determined from these observations that optimization parameters for the fabrication of titanium alloy by SLM vary considerably and are dependent on optimization metrics. In this context, the lack of uniformity in parameter optimization is a big challenge that must be addressed for further adoption of SLM when fabricating titanium alloy. Another common challenge that impedes the SLM fabrication of titanium alloy is the low elongation that is achieved [18,19]. Vrancken et al. [20] studied the influence of heat treatment temperature, time, and cooling speed on microcosmic structure and mechanical properties of SLM-fabricated Ti6Al4V. The experimental results revealed that the maximum temperature of heat treatment had more influence on mechanical properties of the fabricated parts than any other factor, then a new heat treatment process was proposed to improve maximum elongation of titanium alloy. In this paper, the influence of linear energy density on surface morphology is discussed. A full-factor experiment was performed to classify the quality of the SLM-fabricated titanium alloy into several zones, and the principles behind each zone were analyzed. A fabricating parameter window for optimization of SLM fabrication of Ti6Al4V alloy powder was established. Based on this, tensile testing was performed and the morphology of the microcosmic structure observed. The mechanism regarding improvement in ductility of the SLM-fabricated titanium alloy after thermal treatment is discussed.

2. Experimental Methods

2.1. Equipment and Material The experiments were carried out using a self-developed selective laser melting (SLM) machine, a DiMetal-100 (SCUT, Guangzhou, China). Figure1a (reproduced from reference [ 21]) displays the principles of the DiMetal-100 SLM manufacturing equipment. The setup consisted of a ﬁber laser, optical-path transmission unit, sealing chamber (including powder recoating device), mechanical drive, control systems, and processing software. The laser was directed using a scanning galvanometer, which Metals 2018, 8, x FOR PEER REVIEW 3 of 17 which focused it through an F-θ lens for selectively melting the metallic powder on the plane, followed by layer-wise stacking into a metal part. The machine had a scanning speed in the range 10–5000 mm/s, thickness of processing layer in the range 20–100 µm, and a laser focusing spot diameterMetals 2018 ,of8, 47170 µm. The largest size of parts produced was 100 mm × 100 mm × 100 mm. Since3 ofthe 17 powder was fully melted during the process, protection of the SLM-processed parts from oxidation was essential, and achieved by completing the process in an atmosphere of argon or nitrogen with lessfocused than it 0.15% through O2. an F-θ lens for selectively melting the metallic powder on the plane, followed by layer-wiseTi6Al4V stacking powder into from a metal Falcon part. Aerotech The machine Limited had of a scanningChina, which speed satisfied in the range the requirements10–5000 mm/s of, ASTMthickness F2924, of processing was used for layer SLM in fabr theication. range 20–100 The Ti6Al4Vµm, and powder a laser viewed focusing at a spot magnification diameter of of 70 1500×µm. Theusing largest scanning size electron of parts microscopy produced was (SEM, 100 Nova mm × NanoSEM100 mm ×430,100 FEI mm. company, Since the Hillsboro, powder OR, was USA) fully ismelted shown during in Figure the 1b. process, It can protectionbe seen that of the the powder SLM-processed particles partsare almost from entirely oxidation spherical was essential, and mostly and notachieved adhered by completingto one another; the processthe mean in particle an atmosphere diameter of of argon the powder or nitrogen was with36 µm. less than 0.15% O2.

Figure 1. Principles,Principles, devices, and materials materials for for selectiv selectivee laser laser melting melting (SLM)-based (SLM)-based 3D 3D metal metal printing: printing: (a) illustration of SLM fabrication (reproduced fr fromom [21], [21], with permission from from Elsevier, Elsevier, 2018); 2018); (b) Ti-6Al-4V powderpowder viewedviewed byby scanningscanning electronelectron microscopymicroscopy (SEM).(SEM).

2.2. Experimental Methods Ti6Al4V powder (the composition of Ti6Al4V powder can be found in Table S1 in the 2.2.1.Supplementary Determination Material) of Range from for Falcon the Fabricating Aerotech Limited Parameters of China, which satisfied the requirements of ASTM F2924, was used for SLM fabrication. The Ti6Al4V powder viewed at a magnification of 1500× usingA scanning full-factor electron experiment microscopy was (SEM,performed Nova to NanoSEM obtain the 430, optimal FEI company, combination Hillsboro, of OR,parameters USA) is includingshown in Figurelaser 1power,b. It can scanning be seen thatspeed, the powderlayer thic particleskness, arescanning almost spacing, entirely sphericaland laser and spot mostly size. Modifiabilitynot adhered to of one these another; parameters the mean was particle considered diameter jointly, of theand powder the influence was 36 µofm. each factor on SLM fabrication quality analyzed. Laser power “P” and scanning speed “V” were selected for studying the2.2. influence Experimental of the Methods process parameters on the surface quality of the SLM-fabricated titanium alloy. Experimentation on the fabrication process consisted of an early stage (I) of large-range exploration and2.2.1. a Determinationlate stage (II) of Rangerange re forfinement. the Fabricating In the early Parameters stage (I), the range was 150–400 W for laser powerA and full-factor 600–1600 experiment mm/s for scanning was performed speed, as to shown obtain in theFigure optimal 2a. According combination to the offorming parameters effect duringincluding stage laser (I), power, we refined scanning and speed,optimized layer the thickness, selection scanningof parameters spacing, in order and laserto reduce spot size.the complexityModifiability of the of these experiment. parameters The narrow was considered scanning jointly,speed range and thewas influence more sustainable of each factorfor optimizing on SLM parameters,fabrication quality and we analyzed.got stage (II) Laser building power parameters. “P” and scanning The late speedstage (II) “V” of wererange selected refinement for studyingis shown inthe Figure influence 2b. In of the the experiment, process parameters the block onsize the was surface 10 mm quality × 10 mm of × the 10 SLM-fabricatedmm, 25 µm for layer titanium thickness, alloy. 0.08Experimentation mm for hatch on spacing, the fabrication and scanning process mode consisted was of S-shaped an early stageorthogonal (I) of large-rangescanning. In exploration addition, followingand a late stagethe layer (II) ofthickness range refinement. of 25 µm, Inlayer the earlythicknesses stage (I), of the30 µm, range 35 was µm, 150–400 40 µm, W and for 45 laser µm power were alsoand 600–1600used to get mm/s an optimized for scanning processing speed, as window. shown in The Figure principles2a. According of the fabrication to the forming are effectpresented during in Figurestage (I), 2c. we refined and optimized the selection of parameters in order to reduce the complexity of the experiment. The narrow scanning speed range was more sustainable for optimizing parameters, and we got stage (II) building parameters. The late stage (II) of range refinement is shown in Figure2b. In the experiment, the block size was 10 mm × 10 mm × 10 mm, 25 µm for layer thickness, 0.08 mm for hatch spacing, and scanning mode was S-shaped orthogonal scanning. In addition, following the layer thickness of 25 µm, layer thicknesses of 30 µm, 35 µm, 40 µm, and 45 µm were also used to get an optimized processing window. The principles of the fabrication are presented in Figure2c. Metals 2018, 8, 471 4 of 17 Metals 2018, 8, x FOR PEER REVIEW 4 of 17

FigureFigure 2. 2.Setting Setting parametersparameters forfor SLMSLM fabrication of Ti6Al4V: ( aa)) Fabricating Fabricating parameters parameters for for stage stage (I); (I); (b(b)) fabricating fabricating parameters parameters forfor stagestage (II);(II); ((cc)) principlesprinciples ofof S-shapedS-shaped normal scanning.

2.2.2.2.2.2. Testing Testing Methods Methods SurfaceSurface morphologymorphology ofof thethe titaniumtitanium alloy parts fabricated by SLM usin usingg different different parameters parameters neededneeded toto bebe investigatedinvestigated inin thisthis study,study, withwith featuresfeatures such as track overlap and and track track fabrication fabrication qualityquality analyzed. analyzed. To To this this end, end, the 3Dthe super 3D super depth-of-field depth-of-field microscope microscope VHX-5000 VHX-5000 (KEYENCE (KEYENCE company, Osaka,company, Japan) Osaka, was Japan) adopted was for adopted observing for surfaceobserving morphology surface morphology and quality and of quality the fabricated of the fabricated parts, and theparts, roughness and the wasroughness measured was bymeasured 3D optical by 3D Profilometer optical Profilometer from Rtec from Instruments Rtec Instruments company company (San Jose, CA,(San USA). Jose, CA, USA). Then,Then, inin order to to study study the the influence influence of heat of heat treatment treatment on the on ductility the ductility performance performance of the SLM- of the SLM-fabricatedfabricated TC4 TC4alloy, alloy, the theheat heat treatment treatment steps steps of ofour our experiment experiment were were defined defined based based on on the the phase phase diagramdiagram of of TC4 TC4 andand relevantrelevant investigationsinvestigations [[22,23].22,23]. Vacuum heat treatment treatment was was performed performed using using an an N41/HN41/H MuffleMuffle furnace from Nabertherm company, company, Lilienthal, Germany. Germany. The The heat heat treatment treatment process process required elevation of the temperature to 840 °C over 3 h, the temperature maintained for 2 h, then the required elevation of the temperature to 840 ◦C over 3 h, the temperature maintained for 2 h, then the furnace cooled until the temperature reduced to 450 °C and subsequently cooled by air. furnace cooled until the temperature reduced to 450 ◦C and subsequently cooled by air. The tensile strength of the SLM-fabricated TC4 alloy was tested. The experiment was designed The tensile strength of the SLM-fabricated TC4 alloy was tested. The experiment was designed to to be compliant with ISO 6892-1:2016, a standard for experimentation in tensile strength of metal be compliant with ISO 6892-1:2016, a standard for experimentation in tensile strength of metal materials materials at room temperature. Other parameters of performance were also tested and analyzed in at room temperature. Other parameters of performance were also tested and analyzed in accordance accordance with this standard. Samples with a diameter of 5 mm and scale distance of 40 mm were with this standard. Samples with a diameter of 5 mm and scale distance of 40 mm were selected for selected for the tensile experiment. Two groups of SLM-fabricated Ti6Al4V titanium (i.e., those with the tensile experiment. Two groups of SLM-fabricated Ti6Al4V titanium (i.e., those with and without and without heat treatment) underwent the tensile experimentation using the electronic universal heat treatment) underwent the tensile experimentation using the electronic universal testing machine testing machine CMT5105 (MTS company, Eden Prairie, MN, USA), at a rate of 0.2 mm/min. CMT5105 (MTS company, Eden Prairie, MN, USA), at a rate of 0.2 mm/min. The stress-strain curves The stress-strain curves were obtained from a software calculation of experimental data on stress and were obtained from a software calculation of experimental data on stress and distortion. Subsequently, distortion. Subsequently, tensile strength, yield strength, and elongation were computed. Tensile fracture was observed using a NanoSEM 430 (FEI company, Hillsboro, OR, USA).

Metals 2018, 8, 471 5 of 17 tensile strength, yield strength, and elongation were computed. Tensile fracture was observed using a NanoSEM 430 (FEI company, Hillsboro, OR, USA). Except for tensile property, micro-hardness is taken into discussion as well. The micro-hardness test strictly complied with ISO 6507-1:2018 on micro-hardness tester HVS-3.0 (Jinan Fangyuan Testing Machine company, Jinan, China). In this article, only the upper surface micro-hardness was considered. The imposed loading was 1 kg, and the holding time was 15 s. Similarly, both the as-built samples and heat-treated samples were considered. In order to study the inﬂuence of heat treatment on the ductility and micro-hardness of the SLM-fabricated Ti6Al4V alloy from the perspective of microcosmic structure, samples of the Ti6Al4V alloy were manufactured using SLM in compliance with ISO 4499:2011. Here, the same smooth forming parameters were adopted using a block size of 10 mm × 10 mm × 10 mm. Some samples were thermally treated using the method descried above. Finish grinding and mechanical polishing were performed on the thermally-treated and as-built samples using abrasive paper of 600, 800, 1000, and 1500 mesh in addition to a polishing cloth. The upper surface of each sample was corroded using aqua regia for approximately 60 s. The microcosmic structure of each samples’ surface metallography was observed using a Leica DMI 5000 (Leica Microsystems company, Buffalo Grove, IL, USA) at a magniﬁcation of 50–500×.

3. Results and Discussions

3.1. Classification of Surface Morphology A macroscopic view of fabricated square parts was performed and summarized. Note that for some samples, the required energy was too great to proceed with the fabrication process and the process had to be halted by human intervention via the controlling software. From the experimental results, it is clear that for a given laser power, the quality of fabricated parts varies considerably with increased scanning speed. Meanwhile, a parameter is needed as a metric to classify the fabricated regions. Surface quality of the SLM-fabricated parts strongly correlated with the parts’ density and properties. In this paper, surface quality was defined as the principal metric to observe track quality and overlapping effectiveness. Based on the definitions put forward in References [24–28], the linear energy input model in Reference [28] was used to investigate the influence of energy input on surface morphology and melting status. P ψ = , (1) V The surface morphology of parts fabricated under different parameters was observed using the VHX-5000. The observed results can be classified into five categories (i.e., severe over-melting zone, high energy density nodulizing zone, smooth forming zone, low energy density nodulizing zone and sintering zone, which can be found in Figures S1–S5 in the Supplementary Material), as shown in Figure3. It can be seen that these regions are very distinct. The serious over-melting zone is characterized by serious distortion, roughness, and some nodulizing at the surface of the parts, as seen in Figure3a. Its typical parameters are P = 150 W for laser power and V = 600 mm/s for scanning speed. For the high-energy density nodulizing region, the surface of the parts become smooth with slight nodulizing, but there were still irregular bulges and concaves, as seen in Figure3b. Its typical parameters were P = 150 W for laser power and V = 700 mm/s for scanning speed. For the smooth forming zone, the surface was smooth with continuous tracks, ideal overlapping, and elimination of nodulizing, as shown in Figure3c. Its typical parameters were P = 150 W for laser power and V = 800 mm/s for scanning speed. For the low-energy density nodulizing zone, the track at the surface of the parts was not continuous, and melting status was poor, leading to the presence of some holes between tracks. Moreover, some powders were not melted at the surface, some ball-shaped bulges were present, and the upper surface of the parts was coarse and void of metallic lustre, as shown in Figure3d. Its typical parameters were P = 150 W for laser power and V = 1000 mm/s for scanning speed. For Metals 2018, 8, 471 6 of 17 the sintering zone, the surface of the parts lacked metallic lustre, the track was not continuous, and most of the powder was sintered, as seen in Figure3e. Note the surface appears sandy with weak bonding strength between tracks. Its typical parameters were P = 150 W for laser power and V = 1100 mm/sMetals for 2018 scanning, 8, x FOR PEER speed. REVIEW Visual results indicate that nodulizing was present in both the high-energy6 of 17 (Figure3b) and low-energy density nodulizing zones (Figure3e), this result confirms the conclusions both the high-energy (Figure 3b) and low-energy density nodulizing zones (Figure 3e), this result in References [29,30]. In addition, close observation of the results reveal that spatter particles were confirms the conclusions in References [29,30]. In addition, close observation of the results reveal that insertedspatter into particles the track-overlapped were inserted into surface the track-overlapped of all samples. surface of all samples.

Figure 3. Surface morphologies of parts fabricated with parameters in different ranges: (a) serious Figure 3. Surface morphologies of parts fabricated with parameters in different ranges: (a) serious over-melting zone; (b) high-energy density nodulizing zone; (c) smooth forming zone; (d) low-energy over-melting zone; (b) high-energy density nodulizing zone; (c) smooth forming zone; (d) low-energy density nodulizing zone; (e) sintering zone. density nodulizing zone; (e) sintering zone. Furthermore, in order to fully show and distinguish the five categories of surface, the roughness andFurthermore, 3D surface morphologies in order to fully are showshown and in Figure distinguish 4. Note the that ﬁve for categories software reasons, of surface, in Figure the roughness 4, the andpeak 3D of surface the parts morphologies is the bottom are of shown the 3D insurface Figure morphology,4. Note that and for vice software versa. reasons,In addition, in Figureon the4 , theZ-axis, peak ofthe the origin parts begins is the from bottom the ofbottom the 3D of surfacethe 3D surface morphology, morphology. and vice In figure versa. 4a,b, In addition, it can clearly on the Z-axis,be observed the origin that begins the fromproportion the bottom of smooth of the 3Dformed surface tracks morphology. increased In first Figure and4a,b, then itcan decreased. clearly be observedAs mentioned that the above, proportion the bulges of smooth including formed ballings tracks and increased concaves ﬁrst which and thenwere decreased.generated due As mentionedto high- above,energy the density, bulges led including to the broken ballings scanning and concaves track. The which key difference were generated between due the to serious high-energy over-melting density, zone and high-energy density nodulizing zone is the area of normal forming plane and the depth or height of defect. Except for the height as shown in Figure 4a,b, the average roughness values Ra of multi-group samples were 20.91 µm and 16.82 µm. More smooth tracks would reduce the roughness. But in the parameter window, the roughness could not be a sufficient condition for strictly classifying

Metals 2018, 8, 471 7 of 17 led to the broken scanning track. The key difference between the serious over-melting zone and high-energy density nodulizing zone is the area of normal forming plane and the depth or height of defect. Except for the height as shown in Figure4a,b, the average roughness values Ra of multi-group samples were 20.91 µm and 16.82 µm. More smooth tracks would reduce the roughness. But in the parameterMetals 2018,window, 8, x FOR PEER the REVIEW roughness could not be a sufﬁcient condition for strictly classifying the forming7 of 17 zones. Because of some unmeasurable defects, the reliability of roughness was weakened. In the actual explorationthe forming of zones. processing Because parameters, of some unmeasurable the comprehensive defects, the evaluation reliability method of roughness is necessary. was weakened. Similarly, theIn incompletethe actual meltingexploration causes of theprocessing holes and parameters, breaks in thethe track. compre Therehensive is little evaluation regular bigmethod concaves is ornecessary. bulges in Similarly, the low-energy the incomplete density nodulizingmelting causes surface the holes and sintering and breaks surface, in the which track. isThere signiﬁcantly is little differentregular big from concaves the serious or bulges over-melting in the low-energy zone or high-energy density nodulizing density nodulizingsurface and zone.sintering Considering surface, which is significantly different from the serious over-melting zone or high-energy density nodulizing surface roughness, the average measured values of Ra in the low-energy density nodulizing zone zone. Considering surface roughness, the average measured values of Ra in the low-energy density and sintering zone were 20.37 µm and 14.51 µm, respectively. It seems that the normal forming plane nodulizing zone and sintering zone were 20.37 µm and 14.51 µm, respectively. It seems that the would increase the surface roughness. But in fact, the area of normal forming surface in the low-energy normal forming plane would increase the surface roughness. But in fact, the area of normal forming density nodulizing zone was bigger than that in the sintering zone. The sintered state of breaking surface in the low-energy density nodulizing zone was bigger than that in the sintering zone. tracks with a small thickness decreased the surface roughness. The accumulation of holes and breaking The sintered state of breaking tracks with a small thickness decreased the surface roughness. tracks exacerbated the quality of the surface. The accumulation of holes and breaking tracks exacerbated the quality of the surface.

Figure 4. 3D surface morphologies of five forming zones: (a) serious over-melting zone; (b) high- Figure 4. 3D surface morphologies of ﬁve forming zones: (a) serious over-melting zone; (b) high-energy energy density nodulizing zone; (c) smooth forming zone; (d) low-energy density nodulizing zone; density nodulizing zone; (c) smooth forming zone; (d) low-energy density nodulizing zone; (e) sintering zone. (e) sintering zone. 3.2. Fabricating Parameter Optimization Window After inspecting and summarizing the surface morphology of all square parts obtained from the full-factor experiment, the SLM-fabricated titanium alloy’s surface was classified into five zones (i.e., severe over-melting zone, high-energy density nodulizing zone, smooth forming zone, low-energy density nodulizing zone, and the sintering zone). Based on Equation (1), the fabricating parameters in the experiment were converted into linear energy density. According to their respective surface

Metals 2018, 8, 471 8 of 17

3.2. Fabricating Parameter Optimization Window After inspecting and summarizing the surface morphology of all square parts obtained from the full-factor experiment, the SLM-fabricated titanium alloy’s surface was classified into five zones (i.e., Metalssevere 2018 over-melting, 8, x FOR PEER zone, REVIEW high-energy density nodulizing zone, smooth forming zone, low-energy8 of 17 density nodulizing zone, and the sintering zone). Based on Equation (1), the fabricating parameters morphology, all fabricating parameters were allocated to a corresponding zone. Statistical data from in the experiment were converted into linear energy density. According to their respective surface the experiment was utilized to compute the linear energy density of each zone, with laser power “P” morphology, all fabricating parameters were allocated to a corresponding zone. Statistical data from as the abscissa and linear energy density “ψ” as the vertical coordinate. Given a layer thickness of 25 the experiment was utilized to compute the linear energy density of each zone, with laser power “P” as µm, the results and the relationship between the energy input and the forming zone is plotted in the abscissa and linear energy density “ψ” as the vertical coordinate. Given a layer thickness of 25 µm, Figure 5. From this figure, it can be deduced that for SLM-fabricated titanium alloy, the range of the results and the relationship between the energy input and the forming zone is plotted in Figure5. linear energy density is not constant. Rather, it varies with laser power (scanning speed). For the From this figure, it can be deduced that for SLM-fabricated titanium alloy, the range of linear energy smooth forming zone, the range of linear energy density varied from 0.17 to 0.22 J/mm at low laser density is not constant. Rather, it varies with laser power (scanning speed). For the smooth forming power (150~200 W), 0.23~0.26 J/mm at P = 300 W, and 0.27~0.29 J/mm at P = 400W. Therefore, the zone, the range of linear energy density varied from 0.17 to 0.22 J/mm at low laser power (150~200 W), linear energy density was not linearly related to the quality of the formed surface. The traditional 0.23~0.26 J/mm at P = 300 W, and 0.27~0.29 J/mm at P = 400W. Therefore, the linear energy density belief that setting the linear energy density appropriately can necessarily guarantee surface quality is was not linearly related to the quality of the formed surface. The traditional belief that setting the incorrect. It is also incorrect to establish direct correspondence between energy input and quality of linear energy density appropriately can necessarily guarantee surface quality is incorrect. It is also surface formation. On the evidence of Figure 5, after limiting the fabricating parameters to the smooth incorrect to establish direct correspondence between energy input and quality of surface formation. forming zone, the efficiency of laser forming can be improved by increasing laser power or scanning On the evidence of Figure5, after limiting the fabricating parameters to the smooth forming zone, the rate. efficiency of laser forming can be improved by increasing laser power or scanning rate.

Figure 5. FormationFormation zones zones in in energy energy density vs. energy energy input.

Samples obtained at a layer thickness of 30 µm, 35 µm, 40 µm, and 45 µm were similarly Samples obtained at a layer thickness of 30 µm, 35 µm, 40 µm, and 45 µm were similarly analyzed. analyzed. The relationship with varying layer thickness between laser energy input and parameter is The relationship with varying layer thickness between laser energy input and parameter is plotted in plotted in Figure 6. Given a layer thickness of 25 µm, 30 µm, 35 µm, 40 µm, and 45 µm, the fabricating Figure6. Given a layer thickness of 25 µm, 30 µm, 35 µm, 40 µm, and 45 µm, the fabricating parameter parameter windows of the smooth forming zone are shown in Figure 6. It can be seen that for a given windows of the smooth forming zone are shown in Figure6. It can be seen that for a given layer layer thickness, the linear energy density of the smooth forming zone varies in the same way with thickness, the linear energy density of the smooth forming zone varies in the same way with increasing increasing the laser power P. That is, the upper and lower limits of the linear energy density of the smooth forming zone increased by a large margin. Moreover, the two limits of the smooth forming zone also increased considerably with layer thickness. This can be attributed to the fact that if the quantity of powder in a layer increases, greater energy input density is required to melt the powder in order to obtain a satisfactory track and quality of formation. Therefore, the quality of SLM- fabricated titanium alloy can be guaranteed by limiting the fabricating parameters to the range of the smooth forming zone, or choosing the fabricating parameters based on the windows in Figure 6.

Metals 2018, 8, 471 9 of 17 the laser power P. That is, the upper and lower limits of the linear energy density of the smooth forming zone increased by a large margin. Moreover, the two limits of the smooth forming zone also increased considerably with layer thickness. This can be attributed to the fact that if the quantity of powder in a layer increases, greater energy input density is required to melt the powder in order to obtain a satisfactory track and quality of formation. Therefore, the quality of SLM-fabricated titanium alloy can be guaranteed by limiting the fabricating parameters to the range of the smooth forming zone,Metals 2018 or choosing, 8, x FOR PEER the fabricating REVIEW parameters based on the windows in Figure6. 9 of 17

Figure 6.6. Energy input vs. parameter rangesranges under varying thicknesses.

3.3. Analysis of Mechanical Properties

3.3.1. Tensile PropertiesProperties According to the parameterparameter window in Figure6 6,, the the tensile tensile pieces pieces were were carried carried out out with with the the optimal parameters,parameters, 200 WW forfor laserlaser powerpower ((PP),), 10001000 mm/smm/s for scanning speed (V),), 30 µmµm for layerlayer thickness,thickness, 0.080.08 mmmm forfor hatch space and S-shaped orthogonal scanning strategy. TheThe orientationorientation ofof thethe tensiletensile piecespieces waswas parallelparallel toto thethe substrate.substrate. Figure7 7 andand TableTable1 1show show thethe stress-strainstress-strain curvecurve and mechanical properties, properties, respectively, respectively, of ofSLM-fabricated SLM-fabricated Ti6Al4V Ti6Al4V alloy alloy with with and andwithout without heat heattreatment. treatment. Each set Each of data set of is datathe mean is the of mean tests ofon teststhree on samples. three samples. It can be It seen can bethat seen stress that soars stress dramatically soars dramatically with the with increasing the increasing of tensile of tensilestrain. strain.Regardless Regardless of process of process conditions, conditions, the stress–strain the stress–strain curve of curve the SLM-fabricated of the SLM-fabricated Ti6Al4V Ti6Al4V alloy can alloy be candivided be divided into three into stages: three stages:those of those elastic of deformation, elastic deformation, yield, and yield, dramatic and dramatic decrease decrease in stress. in Prior stress. to Priorheat treatment, to heat treatment, the stress–strain the stress–strain curve curvesoared soared and anddecreased decreased considerably considerably during during elongation elongation of ofTi6Al4V, Ti6Al4V, and and clear clear necking necking was was not not observed. observed. Elongation Elongation of of the the parts parts reached reached 5.79% 5.79% ± ±0.29%0.29% prior prior to tofailure, failure, as asdetailed detailed in inTable Table 1,1 ,and and the the fracture fracture type type was was brittle fracture byby inference.inference. ItIt cancan alsoalso bebe determined fromfrom Table Table1 that1 that after after heat heat treatment, treatment, the the SLM-fabricated SLM-fabricated Ti6Al4V Ti6A decreasedl4V decreased in tensile in tensile and yieldand yield strength, strength, but still but metstill themet ASTM the ASTM F136 F136 criteria crit oferia titanium of titanium alloy alloy for medical for medical purposes. purposes. After After heat treatment,heat treatment, its elongation its elongation increased increased to 10.28% to 10.28%± 0.20%. ± 0.20%.

Metals 2018, 8, 471 10 of 17 Metals 2018, 8, x FOR PEER REVIEW 10 of 17

Figure 7. Testing ofoftensile tensile properties: properties: (a ()a tensile) tensile bars bars with with no no heat heat treatment; treatment; (b) tensile(b) tensile bars bars with with heat heattreatment; treatment; (c) strain–stress (c) strain–stress curves curves of Ti6Al4V of Ti6Al4V alloy. alloy.

TableTable 1. TensileTensile properties of Ti6Al4V beforebefore and after heat treatment. Manufacturing Method Tensile Strength/MPa Elongation/% Notes Manufacturing Method Tensile Strength/MPa Elongation/% Notes SLM 1240.5 ± 7.7 5.79 ± 0.29 - SLM (Reference)SLM 1240.5 1267.0± ±7.7 5 7.28 5.79 ± ±1.120.29 [20] - SLM (Reference) 1267.0 ± 5 7.28 ± 1.12840 °C for 3 h [+20 tempering] SLM + Heat Treatment 1068.3 ± 26.7 10.28 ± 0.20 for 2840 h +◦ Cfurnace for 3 h cooling + tempering to SLM + Heat Treatment 1068.3 ± 26.7 10.28 ± 0.20 for450 2 °C h + +air furnace cooling cooling to ◦ SLM + Heat Treatment 705 °C 450for 3 Ch +air+ air cooling cooling 1082.0 ± 34.0 9.04 ± 2.03 SLM(Reference + Heat Treatment one) 705 ◦C for[20] 3 h + air cooling 1082.0 ± 34.0 9.04 ± 2.03 (Reference one) 940 °C for 1 h [+20 tempering] SLM + Heat Treatment 948.0 ± 27.0 13.59 ± 0.32 for 2 940h at◦ 650C for °C 1 h+ +air tempering cooling SLM + Heat Treatment (Reference two) 948.0 ± 27.0 13.59 ± 0.32 for 2 h at 650 ◦C + air (Reference two) [20] cooling [20] Ti6Al4V Alloy Forgings ≥895.0 ≥10.00 ASTM B381-05 Ti6Al4VTi6Al4V Alloy Alloy for Forgings Surgical ≥895.0 ≥10.00 ASTM B381-05 ≥860.0 ≥10.00 ASTM F136-12 Ti6Al4VImplant Alloy Application for Surgical ≥860.0 ≥10.00 ASTM F136-12 Implant Application Furthermore, after annealing at about 700–950 °C, UTS (ultimate tensile strength) decreased by aboutFurthermore, 200 Mpa, and after the annealing elongation at improved about 700–950 with◦ C,increasing UTS (ultimate temperature tensile strength)and was enhanced decreased by nearlyabout 2004–9%. Mpa, The andresults the show elongation that after improved heat trea withtment, increasing the titanium temperature alloy had and better was ductility enhanced while by reducingnearly 4–9%. the Thestrength, results which show is that in afterline with heat treatment,Zhang’s research the titanium [10]. alloyFrom hadTable better 1, compared ductility whilewith publishedreducing theliteratures, strength, there which is still is insome line work with for Zhang’s improvement research in[ our10]. heat From treatment Table1, results, compared although with thepublished influence literatures, of different there scanning is still some strategies work for shou improvementld be considered. in our heat According treatment to results,the principle although of maximumthe inﬂuence temperature of different and scanning cooling strategies rates [20,31], should a begroup considered. of more According refined experiments to the principle could of contributemaximum temperatureto a detailed and description cooling rates in the [20 ,microstr31], a groupucture of moreevolution. reﬁned In experiments fact, the balance could contributeof tensile strengthto a detailed and descriptionelongation through in the microstructure heat treatment evolution. is more determined In fact, the by balance an actual of tensile application’s strength need. and elongation through heat treatment is more determined by an actual application’s need.

Metals 2018, 8, 471 11 of 17

Metals 2018, 8, x FOR PEER REVIEW 11 of 17 3.3.2. Micro-Hardness 3.3.2. Micro-Hardness The three groups of micro-hardness samples were formed by the same optimal parameters of tensile pieces,The three and groups the heat of micro-hardness treatment parameter samples was were also formed the same. by the Five same testing optimal points parameters were taken of on thetensile surface pieces, of each and sample the heat for treatment micro-hardness parameter testing. was also The the spacing same. Five of the testing test points were was 2taken mm, on and thethe results surface of of micro-hardness each sample for are micro-hardness shown in Figure testin8.g. Figure The spacing8a is the of sample the test without points was heat 2 treatment,mm, and andthe Figure results8b of is micro-hardness the sample after are heat shown treatment. in Figure The 8. Figure comparison 8a is the found sample that without the surface heat treatment, indentation and Figure 8b is the sample after heat treatment. The comparison found that the surface indentation side length (D1 ≈ D2 = 72.8 µm) of the no heat treatment’s sample surface was slightly smaller side length (D1 ≈ D2 = 72.8 µm) of the no heat treatment’s sample surface was slightly smaller than than that of the surface indentation of the sample surface after heat treatment (D1 ≈ D2 = 73.5 µm). that of the surface indentation of the sample surface after heat treatment (D1 ≈ D2 = 73.5 µm). The average micro-hardness value of the upper surface of the SLM directly-made Ti6Al4V alloy was The average micro-hardness value of the upper surface of the SLM directly-made Ti6Al4V alloy was 373.67 ± 5.33 HV1. After the heat treatment, the average micro-hardness value was measured at 373.67 ± 5.33 HV1. After the heat treatment, the average micro-hardness value was measured at ± 367.67367.67 ±7.33 7.33 HV1 HV1.. Wang Wang et et al. al. [ 31[31]] chosechose aa 10001000 mm/smm/s scanning scanning speed speed to to build build a aspecimen specimen with with a a ◦ hardnesshardness of of about about 400 400 HV. HV. Vilaro Vilaro et et al. al. [ 32[32]] foundfound thatthat the sample was was annealed annealed at at 800–850 800–850 °C, C,and and thethe hardness hardness decreased decreased by by about about 20 20 HVHV afterafter cooling,cooling, which was was closely closely related related to to the the precipitation precipitation ◦ of ofα phase.α phase. Moreover, Moreover, the the hardness hardness cancan bebe reducedreduced by by about about 35 35 HV HV when when annealing annealing at at875 875 °C [31],C[31 ], althoughalthough the the hardness hardness of of the the sample sample was was slightlyslightly reducedreduced after heat heat treatment. treatment.

FigureFigure 8. 8.Measurement Measurementpoints points andand resultsresults of micro-hardness testing testing for for the the SLM SLM Ti6Al4V Ti6Al4V sample: sample: (a)( beforea) before heat heat treatment; treatment; (b ()b after) after heat heat treatment. treatment.

3.4. Mechanical Evolution Mechanism 3.4. Mechanical Evolution Mechanism Pederson et al. clarified the mechanism for the change in properties of SLM-fabricated titanium Pederson et al. clariﬁed the mechanism for the change in properties of SLM-fabricated titanium alloy following heat treatment, and they summarized the alloy phase diagram [33]. The β phase β alloytransforming following temperature heat treatment, for Ti6Al4V and they is summarized990 ± 5 °C. If thethe alloytemperature phase diagramheats up [more33]. Thethan thephase β ◦ transformingphase transforming temperature temperature, for Ti6Al4V then β is phase 990 ± would5 C. grow If the quickly temperature and become heats upa coarse more structure, than the β phaseleading transforming to the increase temperature, in plasticity then andβ a phasedecrease would in strength. grow quickly The heat and treatment become process a coarse was structure, set as leadingfollow: to temperature the increase was in plasticity heated up and to 840 a decrease °C over 3 in h, strength. the temperature The heat was treatment maintained process for 2 was h, then set as ◦ follow:the furnace temperature cooled until was heatedthe temperature up to 840 reducedC over to 3 450 h, the °C temperatureand subsequently was maintainedcooled by air. for 2 h, then ◦ the furnaceThe microcosmic cooled until structures the temperature of the SLM-fabricated reduced to 450 titaniumC and subsequentlyalloy following cooled heat treatment by air. were inspected,The microcosmic as shown in structures Figure 9. ofMorphology the SLM-fabricated of the alloy’s titanium microcosmic alloy following structure along heat treatmentthe direction were inspected,of laser scanning as shown in inthe Figure X–Y plane9. Morphology without heat of treatment the alloy’s is microcosmicpresented in Figure structure 9a. It along can be the observed direction ofthat laser prior scanning to heat in treatment, the X–Y plane it consisted without mainly heat treatmentof acicularis martensite presented α in’ and Figure β phases.9a. It canThe bereason observed for thatthis prior is that to heatduring treatment, rapid cooling it consisted of Ti6Al4V, mainly and of acicularnew columnar martensite β crystalsα’ and formed.β phases. Before The diffusion reason for thiscan is occur, that during the alloy rapid elements cooling are of converted Ti6Al4V, into and an new oversaturated columnar β solidcrystals solution formed. which Before has the diffusion same cancomposition occur, the alloyof the elements parental arephase converted but with into a different an oversaturated crystal structure solid solution (i.e., α’ phase which martensite). has the same It can be seen from Figure 9a that the acicular martensite α’ phase exists in abundance. Note that

Metals 2018, 8, 471 12 of 17

Metals 2018, 8, x FOR PEER REVIEW 12 of 17 composition of the parental phase but with a different crystal structure (i.e., α’ phase martensite). It can bemartensite seen from is Figureprincipally9a that characterized the acicular martensiteby high strengthα’ phase and exists hardness. in abundance. Its ductility Note and that tenacity martensite are isdependent principally on characterized the substructure by high of strengthmartensite and (lath hardness. martensite Its ductility is superior and tenacityto acicular are dependentmartensite onin thetenacity). substructure Therefore, of martensite SLM-fabricated (lath martensite Ti6Al4V features is superior high to acicularstrength martensite and low elongation in tenacity). prior Therefore, to heat SLM-fabricatedtreatment. As shown Ti6Al4V in Figure features 9b, high the strengthα’ phase and grows low tremendously elongation prior and to is heat dispersed treatment. in the As β shown phase inafter Figure recrystallization9b, the α’ phase and grows annealing. tremendously The reason and isis dispersed that when in thetheβ heatphase treatment after recrystallization temperature andapproximates annealing. to The the reasonβ phase is conversion that when thetemperature, heat treatment β phase temperature nucleation approximates occurs along tothe the boundaryβ phase conversionof the α’ phase. temperature, The closerβ thephase temperature nucleation is occurs to the alongphasethe change boundary temperature, of the α ’the phase. higher The the closer content the temperatureof the β phase. is to As the the phase acicular change martensite temperature, α’ phase the higher converts the content into the of thelayeredβ phase. α+βAs double-phase the acicular martensitestructure, theα’ phasematerial’s converts elongation into the improves layered α a+ccordingly.β double-phase The structure,above results the material’scan also explain elongation the improvesreason why accordingly. micro-hardness The above of SLM-ed results Ti6Al4V can also alloy explain had the a slight reason decline why micro-hardness after heat treatment; of SLM-ed it is Ti6Al4Vdue to the alloy fact that had SLM-ed a slight Ti6Al4V decline aftermicrostructures heat treatment; were itcomposed is due to of the the fact brittle that and SLM-ed tough Ti6Al4Vacicular microstructuresα’ phase martensite. were However, composed the of theα’ phase brittle martensite and tough would acicular changeα’ phase to a martensite. layered α+ However,β double-phase the α’ phaseand it martensiteleads to the would micro-hardness change to adecline. layered Sallica-Levaα+β double-phase et al. [34] and also it leads studied to thethe micro-hardness malleability of decline.SLM-fabricated Sallica-Leva titanium et al. alloys [34] also using studied differ theent malleability heat treatments of SLM-fabricated and concluded titanium that the alloys α’ phase using differentdecomposition heat treatments is critical and to concluded improvements that the inα ’malleability, phase decomposition which proves is critical our to improvementsfindings in this in malleability,experiment. whichAfter analyzing proves our fracture ﬁndings morphology in this experiment. of the SLM-fabricated After analyzing titanium fracture alloy morphology Ti6Al4V, of it the is SLM-fabricatedrevealed that due titanium to heat alloy treatment, Ti6Al4V, many it is deeper revealed dimples that due were to heat uniformly treatment, distributed many deeper across dimples a large werearea within uniformly tensile distributed fractures across of the aparts. large It area furthe withinr confirms tensile the fractures conversion of the of parts. the fabricated It further conﬁrmstitanium thealloy conversion from quasi-cleavage of the fabricated fracture titanium to ductile alloy fracture. from quasi-cleavage fracture to ductile fracture.

Figure 9. Surface micro-structure of Ti6Al4V alloy: (a) before heat treatment; (b) after heat treatment Figure 9. Surface micro-structure of Ti6Al4V alloy: (a) before heat treatment; (b) after heat treatment (α phase is bright, β phase is dark). (α phase is bright, β phase is dark). In order to further analyze the mechanisms of tensile property variation in the SLM-fabricated Ti6Al4V alloy after heat treatment, the morphologies of tensile fractures were inspected. Figure 10 presents tensile fractures of SLM-fabricated Ti6Al4V alloy. Figure 10b shows the fractured parts after heat treatment. It can be seen that the morphology of the tensile fracture resembles honeycombs with

Metals 2018, 8, 471 13 of 17

In order to further analyze the mechanisms of tensile property variation in the SLM-fabricated Ti6Al4V alloy after heat treatment, the morphologies of tensile fractures were inspected. Figure 10 Metals 2018, 8, x FOR PEER REVIEW 13 of 17 presents tensile fractures of SLM-fabricated Ti6Al4V alloy. Figure 10b shows the fractured parts after aheat number treatment. of small It can holes be seen present that at the the morphology fracture. Th ofis the is tensilethe most fracture fundamental resembles criterion honeycombs for fragile with fracturea number or ofductile small fracture. holes present In addition, at the the fracture. size an Thisd depth is the of the most dimples fundamental are related criterion to the formaterial’s fragile ductility.fracture or It ductilecan be fracture.deduced Infrom addition, Figure the10b, size followi andng depth heat of treatment, the dimples that are the related dimples to thein the material’s tensile fractureductility. are It canlarge be and deduced deep, fromcreating Figure a ductile 10b, following fracture. heatFigure treatment, 10c,d show that fractures the dimples of parts in the without tensile heatfracture treatment. are large Many and deep,cleavage creating planes a ductileand torn fracture. edges with Figure local 10c,d plastic show deformation fractures of are parts present without in thisheat fracture, treatment. in Manyaddition cleavage to a small planes number and torn of edgesdimples. with Figure local plastic10d shows deformation that clear are cleavage present inbands this arefracture, present in additionat the microcosmic to a small numbersurface ofmorphology dimples. Figure of the 10 tensiled shows fracture. that clear Some cleavage dimples bands are also are present atat thethe microcosmicedges, which surface are small morphology and shallow. of the It tensileindicates fracture. that although Some dimples the parts are alsohave present tenacity, at the edges,ductility which is limited. are small Therefore, and shallow. the fracture It indicates of the that Ti6Al4V although sample the parts without have tenacity, heat treatment the ductility is a quasi-cleavageis limited. Therefore, fracture. the In fracture fact, it of is the meaningful Ti6Al4V sampleto discuss without failure heat mechanisms treatment is from a quasi-cleavage the view of macroscopicfracture. In fact, fracture it is meaningful morphology. to discussIn recent failure work mechanisms published fromin [35,36], the view even of in macroscopic a smooth forming fracture surface,morphology. roughness In recent might work lead published to the origination in [35,36], evenof fatigue in a smooth crack which forming would surface, further roughness evolve mightinto a cracklead to initiation the origination point. of fatigue crack which would further evolve into a crack initiation point.

Figure 10. SEM images of fracture morphology of tensile testing: (a) thermally treated sample (500×); Figure 10. SEM images of fracture morphology of tensile testing: (a) thermally treated sample (500×); (b) thermally treated sample (5000×); (c) as built sample (500×); (d) as built sample (2000×). (b) thermally treated sample (5000×); (c) as built sample (500×); (d) as built sample (2000×).

3.5. Discussion 3.5. Discussion Figures 3 and 4 show that, except for the smooth forming zone, the surface morphology of the Figures3 and4 show that, except for the smooth forming zone, the surface morphology of the other zones all had defects. Track deformation and concavity occurred within the over-melting zone. other zones all had defects. Track deformation and concavity occurred within the over-melting zone. The reason is that the high density of linear energy input lengthens the cooling and solidification The reason is that the high density of linear energy input lengthens the cooling and solidiﬁcation process of the track. As a result, the melting pool absorbs the powder at both sides more easily, leading process of the track. As a result, the melting pool absorbs the powder at both sides more easily, leading to convexity. The convexity at the initial layer will not cause defects or other adverse influence on the to convexity. The convexity at the initial layer will not cause defects or other adverse inﬂuence on the fabrication of parts. But it does affect powder coating at the next layer. This problem deteriorates in fabrication of parts. But it does affect powder coating at the next layer. This problem deteriorates in subsequent layers and may well cause failure of the fabrication process. Spattering happens throughout subsequent layers and may well cause failure of the fabrication process. Spattering happens throughout the process, and its generated principle can be depicted, as Figure 11 shows, where one can see several types of spatter which formed during laser interaction with powder. As discussed in [21], particle spattering was mostly attributed to recoil pressure caused by instantaneous melting or even steaming of laser and powder. Another reason is that gas in the powder that is rapidly heated results in instantly swelling volume, propelling the powder to the solidification layer. Spattered particles are inserted at

Metals 2018, 8, 471 14 of 17 the process, and its generated principle can be depicted, as Figure 11 shows, where one can see several types of spatter which formed during laser interaction with powder. As discussed in [21], particle spattering was mostly attributed to recoil pressure caused by instantaneous melting or even steaming ofMetals laser 2018 and, 8, powder.x FOR PEER Another REVIEW reason is that gas in the powder that is rapidly heated results in instantly14 of 17 swelling volume, propelling the powder to the solidification layer. Spattered particles are inserted atthe the surface surface or orinterior interior of ofthe the SLM-fabricated SLM-fabricated part parts,s, reducing reducing the the quality quality of ofthe the powder powder coating coating in inaddition addition to tothe the density density and and performance performance of of the the fabricated fabricated parts. parts. As As for for the low-energylow-energy densitydensity nodulizing zone, there are bothboth non-meltednon-melted powderpowder andand ball-shapedball-shaped bulgesbulges atat thethe surface.surface. The reason forfor this is that the linear energy density is so low th thatat some powder does not have enough time to melt, and wettabilitywettability of the liquid track is insufficientinsufficient for thethe alreadyalready solidifiedsolidified zone. At the surface of thethe sintering zone, almost no no track track is is form formeded and and the the powder powder is is mostly mostly sintered. sintered.

Figure 11. Spatter generation mechanism when laser interact with powder in SLMSLM process.process.

Based on the research inin thisthis studystudy andand thethe experienceexperience we havehave obtainedobtained from years of experimentation, it is clear that thethe surfacesurface qualityquality ofof fabricatedfabricated parts stronglystrongly correlatescorrelates withwith suchsuch critical metrics as density and hardness ofof thethe parts.parts. High-quality fabrication of parts can be ensured by observingobserving surface quality during the fabrication process and then flexiblyflexibly adjusting the energy inputinput (i.e.,(i.e., laserlaser powerpower andand scanningscanning speed). A major bottleneck for SLM is the negative feedback in thethe fabricationfabrication process process [37 [37].]. There There are dozensare dozens of factors of factors having enormoushaving enormous influence influence on SLM fabrication on SLM quality.fabrication It is quality. thus of greatIt is thus significance of great tosignificance flexibly adjust to flexibly parameters adjust based parameters on observations based on ofobservations one metric alone.of one Themetric SLM alone. solidification The SLM andsolidification melting pool and is melting very small pool (typically is very small 100 mm (typically in diameter), 100 mm and in processingdiameter), speedand processing is usually speed in the is range usually 1–3 m/s,in the making range it1–3 extremely m/s, making difficult it extremely to perform difficult real-time to monitoringperform real-time and feedback monitoring control and on SLM.feedback In this control paper, on surface SLM. morphology In this paper, of SLM-fabricatedsurface morphology titanium of alloySLM-fabricated is classified titanium into five alloy categories. is classified A CCD into (central five categories. composite design)A CCD optical(central vision composite device design) can be adaptedoptical vision to capture device the can layer-wise be adapted surface to capture of the SLM-fabricatedthe layer-wise surface parts at of certain the SLM-fabricated frequencies, and parts then at measurecertain frequencies, the surface statusand then of each measure layer. the Subsequently, surface status defects of each in the layer. fabricated Subsequently, layers can defects be detected in the usingfabricated real-time layers image can be processing detected using techniques real-time and image the reasons processing for the techniques defects can and be the analyzed. reasons Inforthis the way,defects robustness can be analyzed. and repeatability In this way, of SLM-fabricated robustness and parts repeatability can be improved of SLM-fabricated significantly. parts Figure can 12be showsimproved the significantly. surface morphology, Figure 12 sparkingshows the comparisons surface morphology, between sparking two typical comparisons fabrication between processes. two Ittypical can be fabrication seen from processes. Figure 12 aItthat can thebe partsseen from have Figure a rough 12a surface, that the irregular parts have sparking a rough and surface, spatter. Inirregular Figure 12sparkingb, the parts and havespatter. a smooth In Figure melting 12b, surface,the parts while have the a smooth melting melting layer has surface, regular while sparking the butmelting no spatter. layer has Without regular observing sparking its but microcosmic no spatter. structure, Without observing the fabricated its microcosmic parts in Figure structure, 12b have the a denserfabricated microcosmic parts in structureFigure 12b and have stronger a denser mechanical microcosmic properties. structure and stronger mechanical properties. Another important benefit arising from the research on parameters associated with the fabrication of titanium alloy is improvement to SLM fabrication efficiency. The traditional method for greater efficiency is to adjust the parameters based on relationships with energy input. But these relationships are impossible to be clarified without considerable efforts. Our experiment reveals that given the smooth forming conditions, the energy density ratio P/V is not fixed, and that the value of P/V is not linearly correlated with the range of optimized parameters. Therefore, in order to improve

Metals 2018, 8, 471 15 of 17

Another important benefit arising from the research on parameters associated with the fabrication of titanium alloy is improvement to SLM fabrication efficiency. The traditional method for greater efficiency is to adjust the parameters based on relationships with energy input. But these relationships are impossible to be clarified without considerable efforts. Our experiment reveals that given the smooth forming conditions, the energy density ratio P/V is not fixed, and that the value of P/V is not Metals 2018, 8, x FOR PEER REVIEW 15 of 17 linearly correlated with the range of optimized parameters. Therefore, in order to improve fabrication efficiencyfabrication without efficiency compromising without compromising part quality, part the optimalquality, valuesthe optimal of the values speed needof the to speed be chosen need to from be Figurechosen6 from based Figure on layer 6 based thickness on layer and thickness power. and power.

Figure 12. Surface morphology and sparking of two typical SLM fabrication processes: (a) rough Figure 12. Surface morphology and sparking of two typical SLM fabrication processes: (a) rough surface without regular sparking and spatter formed; (b) fine surface with regular sparking and no surface without regular sparking and spatter formed; (b) ﬁne surface with regular sparking and no spatter formed. spatter formed.

4. Conclusions 4. Conclusions A full-factor experiment was performed in this study to optimize the parameters for SLM A full-factor experiment was performed in this study to optimize the parameters for SLM fabrication of titanium alloy. The SLM-fabricated parts were classified into different categories. The fabrication of titanium alloy. The SLM-fabricated parts were classiﬁed into different categories. parameter windows of the smooth forming zone were explored. The mechanisms concerning the The parameter windows of the smooth forming zone were explored. The mechanisms concerning improvement of ductility of the SLM-fabricated titanium alloy were investigated. The major the improvement of ductility of the SLM-fabricated titanium alloy were investigated. The major conclusions were as follows: conclusions were as follows: 1. From the perspective of surface morphology, a full-factor experiment was performed to classify 1. theFrom SLM-fabricated the perspective titanium of surface alloy morphology, into five zone a full-factors: severe over-melting experiment zone, was performed high-energy to density classify nodulizingthe SLM-fabricated zone, smooth titanium forming alloy zone, into ﬁve low-energy zones: severe density over-melting nodulizing zone, zone, high-energy and sintering density zone. 2. Annodulizing optimal zone,parameter smooth window forming was zone, established low-energy for densitythe smooth nodulizing forming zone, zone. and It was sintering found zone. that 2. forAn the optimal linear parameter energy density window of the was smooth established forming for zone, the smooth the upper forming and lower zone. limits It was increase found that by afor large the margin. linear energy Moreover, density the of two the limits smooth also forming increase zone, considerably the upper with and a lower rise in limits layer increase thickness. by Ita largerefutes margin. the conclusion Moreover, in thesome two studies limits that also there increase is a considerably linear relationship with a between rise in layer energy thickness. input andIt refutes quality the of conclusion fabricated inparts. some studies that there is a linear relationship between energy input 3. Theand SLM-fabricated quality of fabricated titanium parts. alloy was annealed using an appropriate heat treatment. After heat 3. treatment,The SLM-fabricated the structure titanium of the alloyalloy waswas annealedconverted using from an acicular appropriate martensite heat treatment.α’ phase to After a layered heat αtreatment,+β double-phase the structure structure. of the Fracture alloy was type converted was also from converted acicular from martensite quasi-cleavageα’ phase to fracture a layered to ductileα+β double-phase fracture. This structure. is the reason Fracture for improvement type was also in convertedductility of from SLM-fabricated quasi-cleavage titanium fracture alloy. to ductile fracture. This is the reason for improvement in ductility of SLM-fabricated titanium alloy. Author Contributions: Conceptualization, D.W., Y.Y.; Data curation, D.W., W.D.; Form analysis, D.W., W.D.; SupplementaryFunding acquisition, Materials: D.W., Y.Y.;The followingInvestigation, are availableD.W., W.D.; online Methodology, at www.mdpi.com/xxx/s1 D.W., W.D.; Project, Figure administration, S1. Severe over-meltingY.Y.; Supervision, zone, D.W., Figure Y.Y.; S2. High Visua energylization, density W.D.; nodulizing Writing—original zone, Figure draft, S3. D.W., Smooth W.D.; forming Writing—review zone, Figure S4.& Low energy density nodulizing zone, Figure S5. Sintering zone; Table S1: Composition of Ti6Al4V powder. editing, D.W., W.D., Y.Y. The authors would also want to express thanks to Master Hui Lin for his help on the Authormicrostructure Contributions: characterizationConceptualization, and mechanical D.W., testing Y.Y.; Datain this curation, experiment. D.W., W.D.; Form analysis, D.W., W.D.; Funding acquisition, D.W., Y.Y.; Investigation, D.W., W.D.; Methodology, D.W., W.D.; Project administration, Y.Y.; Supervision,Funding: This D.W., research Y.Y.; was Visualization, funded by W.D.; the National Writing—original Natural Science draft, Foundation D.W., W.D.; of Writing—review China, grant number & editing, 51775196; D.W., the Guangdong Province Science and Technology Project, grant numbers 2017B090912003, 2017A050501058, 2016B090914002, 2014B010131002, and 2015B090920002; the High-level Personnel Special Support Plan of Guangdong Province, grant number 2016TQ03X289; the Guangzhou Pearl River New Talent Project, grant number 201710010064; the Guangzhou Major Science and Technology Research for People’s livelihood through CEEUSRO Collaborative Innovation, grant number 20150802005. Di Wang thanks the China Scholarship Council for the award to study for one year at the University of Birmingham, grant number 201706155082.

Conflicts of Interest: The authors declare no conflict of interest.

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W.D., Y.Y. The authors would also want to express thanks to Master Hui Lin for his help on the microstructure characterization and mechanical testing in this experiment. Funding: This research was funded by the National Natural Science Foundation of China, grant number 51775196; the Guangdong Province Science and Technology Project, grant numbers 2017B090912003, 2017A050501058, 2016B090914002, 2014B010131002, and 2015B090920002; the High-level Personnel Special Support Plan of Guangdong Province, grant number 2016TQ03X289; the Guangzhou Pearl River New Talent Project, grant number 201710010064; the Guangzhou Major Science and Technology Research for People’s livelihood through CEEUSRO Collaborative Innovation, grant number 20150802005. Di Wang thanks the China Scholarship Council for the award to study for one year at the University of Birmingham, grant number 201706155082. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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