Strain Analysis of Ti6al4v Titanium Alloy Samples Using Digital Image Correlation

materials

Article Strain Analysis of Ti6Al4V Titanium Alloy Samples Using Digital Image Correlation

Karolina Karolewska *, Bogdan Ligaj and Dariusz Boro ´nski Faculty of Mechanical Engineering, UTP University of Science and Technology in Bydgoszcz, al. Prof. S. Kaliskiego 7, 85-796 Bydgoszcz, Poland; [email protected] (B.L.); [email protected] (D.B.) * Correspondence: [email protected]

Received: 29 May 2020; Accepted: 29 July 2020; Published: 31 July 2020

Abstract: Digital image correlation (DIC) is a non-contact optical method that allows measuring displacements on a plane used to determine the strains caused by external loads of a structural element (mechanical or thermal). Currently, digital image correlation is a widely used experimental technique to assess the mechanical behavior of materials, in particular cracking characteristics and destruction methods of various structural elements. In this paper, the DIC method is applied to determine local strains of titanium alloy Ti6Al4V specimen. The samples used in the tests were made with two different technologies: (a) from a drawn bar by machining process; and (b) by the additive manufacturing method Direct Metal Laser Sintering (DMLS). The aim of the paper is to present the mechanical properties test results of the Ti6Al4V titanium alloy produced by the DMLS additive manufacturing under static loads using the digital image correlation method. As a result of the tests carried out on the drawn bar specimens, it was concluded that the change in the measurement base affects the difference in the Young’s E modulus value in the range from 89.2 to 103.8 GPa. However, for samples formed using the DMLS method, the change in the Young’s modulus value was from 112.9 to 115.3 GPa for the same measurement base.

Keywords: additive manufacturing; DMLS technology; titanium alloy Ti6Al4V; HV10 hardness; digital image correlation (DIC)

1. Introduction Additive technologies are increasingly used in the structural elements production due to a number of advantages, i.e., short production cycle, high efficiency, and production flexibility [1]. Additive manufacturing has contributed to the widespread use of some metal alloys that are difficult to produce with conventional machining methods. Titanium alloy belongs to this materials group [2,3]. The Ti6Al4V titanium alloy is the most commonly applied material due to its excellent mechanical properties. In the production of components made using Ti6Al4V alloy powder the following additive manufacturing technologies are frequently adopted [4,5]: Laser Melting Deposition (LMD), Selective Laser Melting (SLM), and Direct Metal Laser Sintering (DMLS). These methods use a laser as an energy source to melt the metal powder. Laser energy, powder movement, and dynamic interaction between them have a significant impact on the printed material structure and its mechanical properties [4]. Chang et al. [6] presented the research results of titanium alloy Ti6Al4V samples made by the means of the SLM additive technology. The annealing treatment, which was performed favorably, affected the tensile strength (Su = 953 MPa) and the elongation (A = 17.7%). Bebei et al. [7] presented the static tests results and the microstructure characteristics of the Ti6Al4V material manufactured by the SLM method. The elements for testing were printed horizontally on the EOS M290. The printout was characterized by the following parameters: scanning speed 1200 mm/s,

Materials 2020, 13, 3398; doi:10.3390/ma13153398 www.mdpi.com/journal/materials Materials 2020, 13, 3398 2 of 19 laser power 280 W, and layer thickness 30 µm. The material was subjected to a static tensile test, which resulted in tensile strength of Su = 1261 MPa and elongation of A = 10.2%. The tensile strength of the Ti6Al4V alloy parts that were produced by selective laser melting was very similar to the strength of forged elements from the same alloy, although the elongation was slightly lower. Excellent mechanical properties in terms of tensile strength and plasticity for parts manufactured by laser melting method were attributed to almost full density. Sun et al. [8] gave the results of material print direction impact on strength under static and variable loads of the titanium alloy Ti6Al4V produced by the SLM method. Specimen print parameters were: laser power 350W, laser beam diameter 0.08 mm, scanning speed 1000 mm/s, and layer thickness 0.06 mm. The samples were manufactured in three directions: 0◦, 45◦, and 90◦. As-built specimens were annealed for 5 h at 750–850 °C, and then cooled under argon to room temperature. The material different mechanical parameters were obtained for each direction. For the 0◦ specimens, the following values were gained: E = 111.55 GPa, Su = 935 MPa, and Sy = 857 MPa. For the 45◦ samples, E = 113.26 GPa, Su = 963 MPa, and Sy = 883 MPa. For the 90◦ samples, E = 115.87 GPa, Su = 953 MPa, and Sy = 888 MPa. Liang et al. [9] presented research results concerning the various heat treatment methods impact on the material structure of samples produced by the SLM method. The material heat treatment was carried out at heightened temperature (600, 800, 850, and 900 ◦C) for 4 h and cooled in air to room temperature. The material mechanical properties values were obtained for specimens without heat treatment: Su 1230 MPa,Sy 1150 MPa, and A 8.5%. The following results were obtained for ≈ ≈ ≈ specimens after heat treatment at 600 C: Su 1210 MPa, Sy 1180 MPa, and A 8.5%. For samples after ◦ ≈ ≈ ≈ heat treatment at 800 C, the results were: Su 1080 MPa, Sy 990 MPa, and A 12,1%. For samples after ◦ ≈ ≈ ≈ heat treatment at 850 °C, the results were: Su 1040 MPa, Sy 970 MPa, and A 12.3%. For samples ≈ ≈ ≈ after heat treatment at 900 C, the results were: Su 990 MPa, Sy 900 MPa, and A 11.5%. ◦ ≈ ≈ ≈ Karolewska et al. [10] gave the research results of Ti6Al4V titanium alloy according to [11] standard. The test specimens were made using the DMLS method, and their diameter was 6 mm. The geometrical dimensions of the samples were adopted according to [12] standard. The printing process was characterized by the following parameters: laser power 200 W, minimum layer thickness 30 µm, and scanning speed up to 7 m/s. After the manufacturing process, the specimens were annealed. Based on testing under static load, the following strength parameters were achieved: E = 114.5 GPa, Su = 1127 MPa, Sy 0.2 = 1052 MPa, and A = 15.5%. Benedetti et al. [13] presented the static tests results for Ti6Al4V material produced by the Selective Laser Sintering (SLS) method using a 3DSystem ProX 300 printer. Two types of samples were adopted for testing. The first of these were tested in the as-built state, immediately after the printing process. The second part was subjected to hot isostatic pressing (HIP) treatment at 920 ◦C, aimed at removing pores and obtaining the full density of the material and modifying its microstructure. As-built specimens show a martensitic microstructure and have a higher yield strength (Sy = 1022 MPa) and tensile strength (Su = 1092 MPa) than heat-treated samples, but lower elongation in HIP condition (A = 16.5%). The properties of HIP treated samples were: elongation A = 22.5%, yield strength Sy HIP = 894.7 MPa, and tensile strength Su HIP = 962.3 MPa. The HIP heat treatment changed the parameters of the Ti6Al4V material produced by the SLS method. Benedetti et al. [14] found that mechanical properties are closely related to the material microstructure. A static tensile test was carried out for printed and untreated samples as well as for specimens subjected to HIP hot isostatic pressing. The Young’s modulus for both types of samples assumes similar values of about 110 GPa. However, there is a larger difference in terms of yield point, strength, and total elongation. Specimens with martensitic microstructure (samples without heat treatment) show higher yield point and tensile strength, but lower elongation. Quintana et al. [15] determined the mechanical properties made by the SLM additive method for two types of Ti6Al4V material: Grades 5 and 23. They examined the impact of a slight increase in oxygen content on the mechanical properties of SLM Ti6Al4V. The specimens’ microstructure and mechanical properties after the printing process and after HIP treatment with oxygen content were Materials 2020, 13, 3398 3 of 19 assessed corresponding to the Ti6Al4V ELI (Grade 5) ranges 0.10–0.11% and 0.16–0.17%. Ti6Al4V Grade 5 material showed higher yield point and strength than ELI type, in the case of both as-built samples and after HIP treatment. Grade 5 material after HIP treatment had greater elongation than Grade 23 type. Conversely, the elongation of the Grade 5 as-built material was lower than that of the ELI type. Rafi et al. [16] produced specimens from Ti6Al4V and 15-5 PH materials using SLM. Their tensile strength was determined. Strength properties were compared in relation to the orientation of the manufactured element. The samples built horizontally showed relatively better tensile properties compared to the vertically built specimens. They presented that the tensile strength of Ti6Al4V produced by SLM is higher than that of hot-machined parts due to the martensitic microstructure. However, its plasticity was lower. The tensile strength of Ti64 and PH1 samples was found to be comparable or even better than the forged material. Tensile properties affect the direction of printing. Horizontal orientation is slightly better than vertical orientation in terms of tensile strength. The test results presented in [7,13–16] indicate an increase in the tensile strength and yield point of the material produced by the additive method (in as-built state) compared to the heat-treated material. A short strength parameter analysis of the material received from additive manufacturing technology indicates that the results obtained have a large spread in regards to the material’s tensile strength Su, yield point Sy, and elongation A. Not all researchers determine and analyze changes in the Young’s modulus value. The change in strength parameters is influenced by a number of factors related to the material printing process by the additive manufacturing method, as well as to the process of formed elements heat treatment. The research methods applied to assess strength properties are the same as those adopted for testing materials produced by metallurgical processes. In the case of materials created by additive manufacturing methods, we deal with a number of layers connected together, and the weakest connection determines the entire element strength. On this assumption, the optical image digital correlation method can be selected to assess the test object’s (specimens) individual area displacement. Digital Image Correlation (DIC) is a non-contact optical method that allows measuring displacements on a plane used to determine the strains caused by external loads of a structural element (mechanical, thermal) [17]. This method uses digital images in which the pixels are the smallest observable elements. The measurement consists of taking a series of photos before and after loading the test object. The object surface being tested must have some random texture (macular structure). One of the images in the series is selected as the reference image for all subsequent analyses [18]. The reference image is divided into small rectangular areas, which are called subsets. The algorithm tracks each subset position from the reference image in all other images of the measurement series. Finding subsets is done by calculating the mutual correlation coefficient. For each subset, displacement vectors in the plane (u and v) are calculated; using triangulations, off-plane displacements (w) are determined. Through the use of interpolation methods, sub-pixel accuracy is achieved. The output is a set of displacement maps that can then be adopted to calculate strain maps in the plane (εxx, εyy). In addition, various material mechanical parameters, including Young’s modulus and Poisson’s ratio, can be determined on the basis of calculated displacement or deformation fields [19]. Currently, digital image correlation is a widely used experimental technique to assess the various materials mechanical behavior, in particular cracking characteristics and destruction methods of different structural elements. Digital image correlation is also often used to observe welded joints [20]. The material mechanical properties, and above all the Young’s modulus, are of great importance due to the calculation results under static and variable loads by the means of analytical and numerical methods [21–23]. The vast majority of structural components are designed for the extent to which Hooke’s law applies. The aim of the paper is to present the mechanical properties test results of the Ti6Al4V titanium alloy produced by the DMLS additive manufacturing under static loads using the digital image correlation method. Materials 2020, 13, x FOR PEER REVIEW 4 of 19 Materials 2020, 13, 3398 4 of 19

The study included conducting tests related to the mechanical properties (Sy, Su, A, Z, and E) and determining the hardness of the material. The study included conducting tests related to the mechanical properties (Sy,Su, A, Z, and E) and determining the hardness of the material. 2. Experimental Studies 2. Experimental Studies 2.1. Research Object 2.1. Research Object The study used titanium alloy Ti6Al4V, which belongs to two-phase alloys. The European standardThe studyISO 5832 used [11] titanium specifies alloy the Ti6Al4V,which percentage of belongsalloying to elements two-phase contained alloys. The in the European alloy (Table standard 1). ISO 5832 [11] speciﬁes the percentage of alloying elements contained in the alloy (Table1). Table 1. Chemical composition of the Ti6Al4V titanium alloy according to [11]. Table 1. Chemical composition of the Ti6Al4V titanium alloy according to [11]. Main Alloying Elements and Their Content in the Alloy Ti6Al4V, % Al Main V Alloying ElementsFe and TheirO ContentN in the AlloyC Ti6Al4V, % H Ti 5.5 ÷ 6.75 Al 3.5 ÷ 4.5 V ≤0.3 Fe ≤0.2 O ≤ N0.05 C≤0.08 H≤0.01 Ti rest 5.5 6.75 3.5 4.5 0.3 0.2 0.05 0.08 0.01 rest In this research,÷ samples÷ manufactured≤ by≤ two technologies≤ ≤were adopted:≤ a) asIn a this result research, of turning samples a drawn manufactured bar with a by diameter two technologies of 12 mm, werethe material adopted: was annealed; and b) by the use of additive technology (DMLS). a) as a result of turning a drawn bar with a diameter of 12 mm, the material was annealed; and Ti6Al4V is a two-phase alloy consisting of α and β phases. Drawn bar (DB) samples were made b) by the use of additive technology (DMLS). of 1-m drawn rods with a diameter of 12 mm. The aim of drawing process is to obtain products in the formTi6Al4V of bars isa or two-phase wires characterized alloy consisting by very of α preciseand β cross-sectionalphases. Drawn dimensions, bar (DB) samples a smooth, were bright made surface,of 1-m drawn and specific rods with mechanical a diameter properties of 12 mm. that The ca aimn only of drawingbe obtained process with is this to obtain production products method. in the Asform a ofresult bars orof wiresdrawing, characterized the geometric by very and precise mech cross-sectionalanical properties dimensions, of the a smooth,material bright change, surface, the transverseand speciﬁc dimensions mechanical are properties reduced, thatand canthe onlylength be increases, obtained without with this changing production the method.volume. As a result of of plastic drawing, deformation the geometric in the and die, mechanical the material properties also becomes of the hardened—the material change, strength the transverseproperties increasedimensions and are the reduced, plastic properties and the length decrease. increases, Throug withouth the changingrolling process, the volume. samples As were a result obtained of plastic as showndeformation in Figure in the 1. die,Then, the they material werealso subjected becomes to hardened—thean annealing process strength at propertiesthe temperature increase of and840 the°C forplastic 1 h and properties cooled decrease. in a furnace Through to ambient the rolling temperature. process, The samples use of were annealing obtained treatment as shown is into Figureobtain1 a. stableThen, theystructure were of subjected the material. to an annealing process at the temperature of 840 ◦C for 1 h and cooled in a furnaceThe test to ambientsample geometrical temperature. form The was use determined of annealing on treatment the basis is of to the obtain European a stable standard structure [24]. of Thethe material.specimen dimensions are shown in Figure 1, while the physical form is presented in Figure 2.

x R8 R8 12 12 6 ∅ ∅ ∅

y z

38 12

Figure 1. SpecimenSpecimen geometric geometric features features in in mm mm for for strength tests.

TwoThe testtypes sample of samples geometrical for strength form tests was determinedwere made. Their on the physical basis of form the European is given in standard Figure 2. [ 24 ]. The specimen dimensions are shown in Figure1, while the physical form is presented in Figure2. Two types of samples for strength tests were made. Their physical form is given in Figure2.

(a) (b)

Figure 2. Specimen for strength tests: (a) drawn bar turning; and (b) manufactured by DMLS.

Materials 2020, 13, x FOR PEER REVIEW 4 of 19

The study included conducting tests related to the mechanical properties (Sy, Su, A, Z, and E) and determining the hardness of the material.

2. Experimental Studies

2.1. Research Object The study used titanium alloy Ti6Al4V, which belongs to two-phase alloys. The European standard ISO 5832 [11] specifies the percentage of alloying elements contained in the alloy (Table 1).

Table 1. Chemical composition of the Ti6Al4V titanium alloy according to [11].

Main Alloying Elements and Their Content in the Alloy Ti6Al4V, % Al V Fe O N C H Ti 5.5 ÷ 6.75 3.5 ÷ 4.5 ≤0.3 ≤0.2 ≤0.05 ≤0.08 ≤0.01 rest

In this research, samples manufactured by two technologies were adopted: a) as a result of turning a drawn bar with a diameter of 12 mm, the material was annealed; and b) by the use of additive technology (DMLS). Ti6Al4V is a two-phase alloy consisting of α and β phases. Drawn bar (DB) samples were made of 1-m drawn rods with a diameter of 12 mm. The aim of drawing process is to obtain products in the form of bars or wires characterized by very precise cross-sectional dimensions, a smooth, bright surface, and specific mechanical properties that can only be obtained with this production method. As a result of drawing, the geometric and mechanical properties of the material change, the transverse dimensions are reduced, and the length increases, without changing the volume. As a result of plastic deformation in the die, the material also becomes hardened—the strength properties increase and the plastic properties decrease. Through the rolling process, samples were obtained as shown in Figure 1. Then, they were subjected to an annealing process at the temperature of 840 °C for 1 h and cooled in a furnace to ambient temperature. The use of annealing treatment is to obtain a stable structure of the material. The test sample geometrical form was determined on the basis of the European standard [24]. The specimen dimensions are shown in Figure 1, while the physical form is presented in Figure 2.

x R8 R8 12 12 6 ∅ ∅ ∅

y z

38 12

Figure 1. Specimen geometric features in mm for strength tests. Materials 2020, 13, 3398 5 of 19 Two types of samples for strength tests were made. Their physical form is given in Figure 2.

(a) (b) Materials 2020, 13, x FOR PEER REVIEW 5 of 19 FigureFigure 2. 2. SpecimenSpecimen for for strength strength tests: tests: ( (aa)) drawn drawn bar bar turning; turning; and and ( (bb)) manufactured manufactured by by DMLS. DMLS.

2.2.2.2. Production of Specimens by the Additive ManufacturingManufacturing MethodMethod AA commoncommon feature feature of of each each of theof the additive additive technologies technologies is the is ability the ability to produce to produce a structural a structural element ofelement any geometry, of any geometry, for which productionfor which production using other using manufacturing other manufacturin techniquesg wouldtechniques be complicated would be orcomplicated even impossible. or even Overimpossible. the years, Over many the years, additive many technologies additive technologies have been created have been and created developed and usingdeveloped various using materials various types materials in the types printing in the process—from printing process—from polymer materials polymer to materials metals. One to metals. of the printingOne of the technologies printing technologies in metal is thein metal DMLS is method. the DMLS method. ItIt is a a selective selective laser laser sintering sintering method method based based on on metallic metallic powder powder layers layers sintering sintering by means by means of a oflaser a laser beam. beam. The samples The samples manufacturing manufacturing process process by DMLS by method DMLSmethod is presented is presented as a scheme as a in scheme Figure in3. A Figure detailed3. A description detailed description of the method of the is methodgiven in is[25]. given Most in producers [ 25]. Most of producers devices for of components devices for componentsusing additive using manufacturing additive manufacturing introduce their introduce own theirtechnology own technology designations designations onto the ontomarket, the market,reserving reserving the right the toright use them to use exclusively. them exclusively. In the Incase the of case theof EOS the company, EOS company, on whose on whose printer printer our oursamples samples were were produced, produced, describes describes its itstechnology technology as as DMLS. DMLS. As As the the name name suggests, suggests, DMLS DMLS produces anan elementelement byby sinteringsintering individualindividual layers, while the SLM technology completely melts these layers. SinteringSintering processes do do not not fully fully melt melt the the powder, powder, bu butt warm warm it up it up to tothe the point point where where its particles its particles can canfusefuse together together at the at themolecular molecular level. level. With With laser laser melting, melting, we we can can get get fully fully melt melt powder powder into aa homogeneoushomogeneous part.part.

(a) (b) (c)

FigureFigure 3. SchematicSchematic presentation presentation of of the the element element manufa manufacturingcturing process process by by the the DMLS DMLS method method [25]: [25 (a]:) (initiala) initial stage; stage; (b) (inb) the in the middle middle of the of the element element production; production; and and (c) (finalc) ﬁnal stage. stage.

SpecimensSpecimens for strength teststests werewere mademade usingusing thethe EOSEOS M280 machine (Materialise, Leuven, Belgium)Belgium) withwith thethe dimensionsdimensions ofof thethe workingworking platformplatform of of 250 250 mm mm ×250 250 mmmm × 325325 mm.mm. The printing × × processprocess waswas characterizedcharacterized byby thethe followingfollowing parameters:parameters: laser power 200 W, minimum layerlayer thickness 3030 µμm,m, andand scanningscanning speedspeed upup toto 77 mm/s./s. The sample print direction was consistent with the zz-axis-axis (Figure(Figure1 ).1). Titanium Titanium powder powder with with the the following followi parametersng parameters was used:was used: grain grain size: 53–105size: 53–105µm; grain μm; shape: grain spherical;shape: spherical; grain size grain distribution: size distribution: D50 (72 µD50m); (72 internal μm); frictioninternal angle: friction 40angle:◦; grain ≤ 40°; sphericity: grain sphericity:Φ 0.95; 3 ≤ ≥ andΦ ≥ bulk0.95; density:and bulk 2.56 density: g/cm 2.56. In [g/cm10], the3. In method [10], the for method producing for samplesproducing used samples in the testsused is in described the tests inis detail.described During in detail. one technological During one technological process, a specimens process, groupa specimens adopted group to the adopted tests under to the various tests under load conditionsvarious load was conditions produced. was The resultsproduced. obtained The results in [10] andobtained included in [10] below and were included achieved below under were the sameachieved conditions. under the same conditions.

2.3. Measurement Results of Specimens Geometric Features

2.3.1. Offset Radius and Working Part Length Measurement The measurements of selected geometrical features, i.e. offset radius values and the working part length, were carried out on a Mitutoyo Formtracer SV-C3200 machine (Mitutoyo Corporation, Kanagawa, Japan). The test samples were assessed for accuracy in the scope of the required shape in accordance with [26] standard. Ten samples were estimated. The measurements concerned the determination of

Materials 2020, 13, 3398 6 of 19

2.3. Measurement Results of Specimens Geometric Features

2.3.1. Offset Radius and Working Part Length Measurement The measurements of selected geometrical features, i.e., offset radius values and the working part length, were carried out on a Mitutoyo Formtracer SV-C3200 machine (Mitutoyo Corporation, Kanagawa, Japan). MaterialsThe 2020 test, 13,samples x FOR PEER were REVIEW assessed for accuracy in the scope of the required shape in accordance6 of 19 with [26] standard. Ten samples were estimated. The measurements concerned the determination of selectedselected geometrical geometrical dimensions, dimensions, i.e. i.e., the the values values of of the the R-I R-I and and R-II R-II offset offset radius radius and and the the T1 T1 working working partpart length, length, as as well well as as the the gripping gripping parts parts distance distance T2 T2 of of the sample (Figure 44).).

R-I R-II x

z T1

FigureFigure 4. 4. TheThe test test specimen specimen with with marked marked geomet geometricalrical dimensions dimensions measurement measurement points. points.

TableTable 2 givesgives thethe geometricgeometric measurements results for thethe DBDB andand forfor thethe DMLSDMLS samples:samples: offset o ﬀset radiusradius values R-IR-I andand R-II, R-II, measuring measuring part part T1 T1 and and T2 length,T2 length, according according to Figure to Figure4. The 4. obtained The obtained results resultswere presented were presented in statistical in statistical terms using terms the using following the following parameters: parameters: mean value, mean standard value, deviation,standard deviation,minimum minimum value, and valu maximume, and maximum value. value.

Table 2. Geometric measurement results of Ti6Al4V samples produced from a drawn bar (DB specimen) Table 2. Geometric measurement results of Ti6Al4V samples produced from a drawn bar (DB and by the DMLS method (DMLS specimen). specimen) and by the DMLS method (DMLS specimen).

SelectedSelected Geometrical Geometrical Parameters Parameters GeometryGeometry of of Turned Turned DB DB Specimens Specimens GeometryGeometry of of the the DMLS DMLS Specimens Specimens

R-IR-I R-II R-II T1 T1 T2 T2 R-I R-I R-II T1 T1 T2 T2 mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm mm Mean Value 8.016 8.090 12.834 24.996 8.077 7.941 12.677 24.339 StandardMean Deviation Value 0.071 8.016 0.316 8.090 12.834 0.354 24.9960.058 8.0770.183 7.9410.176 12.6770.131 24.3390.103 MinimumStandard Value Deviation 7.835 0.071 7.933 0.316 11.9570.354 0.05824.935 0.1837.779 0.1767.643 12.424 0.131 0.10324.259 Minimum Value 7.835 7.933 11.957 24.935 7.779 7.643 12.424 24.259 Maximum Value 8.069 8.983 13.161 25.099 8.411 8.249 12.915 24.590 Maximum Value 8.069 8.983 13.161 25.099 8.411 8.249 12.915 24.590 The most important dimensions of cylindrical samples are the offset radius values, which determineThe most the value important of the dimensions stress concentration of cylindrical factor samples affecting are the the o fftestset objects radius values,damage which form. determine The lack ofthe geometric value of features the stress repeatability concentration may factor affect aff theecting value the of test the objects determined damage Ti6Al4V form. Themechanical lack of parameters.geometric features The measurements repeatability results may aff presentedect the value in ofTable the determined2 were carried Ti6Al4V out for mechanical 10 samples. parameters. Samples wereThe measurements tested in the same results manner presented that inwas Table consistent2 were carried with their out forproduction 10 samples. method. Samples were tested in the sameThe offset manner radius that wasmeasurements consistent withof turned their productionDB samples method. indicate that they are characterized by dimensionalThe off setrepeatability. radius measurements The difference of turnedbetween DB the samples average indicateR-I and R-II that radius they are is 0.007 characterized mm, which by shoulddimensional be considered repeatability. as insignificant. The difference Analyzing between the the minimum average value R-I and of R-IIR-I and radius R-II is radius, 0.007 mm,the differencewhich should in value be considered is 0.098 as mm, insignificant. which should Analyzing be considered the minimum as valueinsignificant. of R-I and Taking R-II radius, into considerationthe difference the in maximum value is 0.098 value mm, of R-I which and R-II should radius, be the considered difference as in insignificant. value is 0.914 Takingmm, which into shouldconsideration be considered the maximum a significant value difference. of R-I and The R-II difference radius, in the the di ffvalueerence of inR-I value (calculated is 0.914 as mm,the differencewhich should between be considered the maximum a significant and minimum difference. values) The is di0.234fference mm, inwh theile the value difference of R-I (calculated in the value as of the R-II radius is 1.05 mm. The offset radius measurements of samples produced using the DMLS method indicate that they are characterized by dimensional repeatability. The difference between the average R-I and R-II radius is 0.136 mm, which should be considered as insignificant. The difference in minimum value of R-I and R-II is 0.136 mm, which should be considered as insignificant. Analyzing the maximum value of R-I and R-II, it can be seen that the difference in value is 0.162 mm, which should be considered as insignificant. The difference in the value of R-I (calculated as the difference between the maximum and minimum values) is 0.632 mm, while the difference in the value of the R-II radius is 0.606 mm. Samples made using the additive method are characterized by repeatability of geometric features, which is important in the context of repeatability of the strength tests results.

Materials 2020, 13, 3398 7 of 19 the difference between the maximum and minimum values) is 0.234 mm, while the difference in the value of the R-II radius is 1.05 mm. The offset radius measurements of samples produced using the DMLS method indicate that they are characterized by dimensional repeatability. The difference between the average R-I and R-II radius is 0.136 mm, which should be considered as insignificant. The difference in minimum value of R-I and R-II is 0.136 mm, which should be considered as insignificant. Analyzing the maximum value of R-I and R-II, it can be seen that the difference in value is 0.162 mm, which should be considered as insignificant. The difference in the value of R-I (calculated as the difference between the maximum and minimum values) is 0.632 mm, while the difference in the value of the R-II radius is 0.606 mm. Samples made using the additive method are characterized by repeatability of geometric features, whichMaterials is 2020 important, 13, x FOR in PEER the REVIEW context of repeatability of the strength tests results. 7 of 19

2.3.2. Working PartPart DiameterDiameter MeasurementMeasurement For thethe testtest partpart ofof thethe specimen,specimen, thethe diameterdiameter valuevalue waswas measured,measured, andand thethe roundnessroundness waswas determined. The The measurements measurements were were carried carried out with the Mitutoyo CRYSTA-APEX S574S574 devicedevice (Mitutoyo(Mitutoyo Corporation,Corporation, Kanagawa,Kanagawa, Japan).Japan). The measurementsmeasurements werewere carriedcarried outout inin threethree sectionssections inin accordanceaccordance withwith FigureFigure5 .5.

ABC x

H1 H2 H3

Figure 5. Designation of the cross-sections in which the diameter values were measured. The distance of thethe givengiven cross-sectioncross-section (A,(A, B,B, C)C) fromfrom thethe samplesample base base was: was: H1H1= = 4040 mm,mm, H2H2 == 4343 mm,mm, H3H3 == 46 mm.

Table3 3 presents presents the the measuring measuring results results of of the the diameter diameter and and roundness roundness values values using using the the followingfollowing statisticalstatistical parameters:parameters: mean mean value, value, standard standard deviation, deviation, minimum minimum value, value, and maximumand maximum value. value. The listed The parameterslisted parameters were determined were determined on the on basis the of basis 10 samples’ of 10 samples’ test results test results in three in sections. three sections.

Table3. Measurement results for the roundness and diameter deviations of the specimen measuring part. Table 3. Measurement results for the roundness and diameter deviations of the specimen measuring part. The Specimen Measuring Part Geometric Parameters TheSpecimen Specimen DB Measuring Part SpecimenGeometric DMLS Parameters DiameterSpecimen Roundness DB Diameter Specimen Roundness DMLS

Diametermm Roundness mm Diameter mm mmRoundness mm mm mm mm Mean Value 5.994 0.009 5.949 0.019 MeanStandard Value Deviation 5.9940.007 0.004 0.009 0.004 5.949 0.004 0.019 StandardMinimum Deviation Value 0.0075.984 0.003 0.004 5.942 0.004 0.013 0.004 MinimumMaximum Value Value 5.9846.012 0.015 0.003 5.958 5.942 0.027 0.013 Maximum Value 6.012 0.015 5.958 0.027 The measuring results of the roundness in selected sections (A-A, B-B, and C-C) for a sample madeThe using measuring the DMLS results method of the are roundness shown in Figurein select6, anded sections for a turned (A-A, DB B-B, sample and C-C) are presentedfor a sample in Figuremade 7using. In Figures the DMLS6 and method7, elementary are shown measurements in Figure of6, individualand for a turned sections DB are sample illustrated are presented with black in spots.Figure They7. In areFigures in the 6 green and 7, ﬁeld elementary indicating measurem the rangeents of the of roundnessindividual tolerancesections are ﬁeld. illustrated The blue with line representsblack spots. the They specimen are in the cross green section field nominal indicating size the equal range to of 6mm. the roundness tolerance field. The blue line represents the specimen cross section nominal size equal to 6 mm.

(a) (b) (c)

Figure 6. The measurements results example of the roundness of a specimen made using the DMLS method in cross-section: (a) A-A; (b) B-B; and (c) C-C.

Materials 2020, 13, x FOR PEER REVIEW 7 of 19

2.3.2. Working Part Diameter Measurement For the test part of the specimen, the diameter value was measured, and the roundness was determined. The measurements were carried out with the Mitutoyo CRYSTA-APEX S574 device (Mitutoyo Corporation, Kanagawa, Japan). The measurements were carried out in three sections in accordance with Figure 5.

ABC x

H1 H2 H3

Figure 5. Designation of the cross-sections in which the diameter values were measured. The distance of the given cross-section (A, B, C) from the sample base was: H1 = 40 mm, H2 = 43 mm, H3 = 46 mm.

Table 3 presents the measuring results of the diameter and roundness values using the following statistical parameters: mean value, standard deviation, minimum value, and maximum value. The listed parameters were determined on the basis of 10 samples’ test results in three sections.

Table 3. Measurement results for the roundness and diameter deviations of the specimen measuring part.

The Specimen Measuring Part Geometric Parameters Specimen DB Specimen DMLS

Diameter Roundness Diameter Roundness mm mm mm mm Mean Value 5.994 0.009 5.949 0.019 Standard Deviation 0.007 0.004 0.004 0.004 Minimum Value 5.984 0.003 5.942 0.013 Maximum Value 6.012 0.015 5.958 0.027

The measuring results of the roundness in selected sections (A-A, B-B, and C-C) for a sample made using the DMLS method are shown in Figure 6, and for a turned DB sample are presented in Figure 7. In Figures 6 and 7, elementary measurements of individual sections are illustrated with Materials 13 black spots.2020, They, 3398 are in the green field indicating the range of the roundness tolerance field. The8 blue of 19 line represents the specimen cross section nominal size equal to 6 mm.

(a) (b) (c)

Figure 6. The measurements results example of the roundnessroundness of a specimen made using the DMLS Materialsmethod 2020, 13 in, cross-section:x FOR PEER REVIEW (a) A-A; (b) B-B; and (c) C-C. 8 of 19

(a) (b) (c)

Figure 7.7. TheThe measurementsmeasurements results results example example of theof the roundness roundness of a turnedof a turned DB specimen DB specimen in cross-section: in cross- section:(a) A-A; ( (ab)) A-A; B-B; and(b) B-B; (c) C-C.and (c) C-C.

The diameter measurements of the working part showed that samples made using the additive method (DMLS specimen) and turned samples from from a drawn bar (DB specimen) are within the dimensional tolerance (±0.05 ( 0.05 mm) mm) establ establishedished according according to to [27] [27] standard. standard. ± Analysis of the measuring points distribution for DMLS samples’ individual cross-sections regarding the roundness measurement indicates indicates an an ir irregularregular shape shape in in relation relation to to the the nominal nominal circle. circle. This confirms conﬁrms the distribution of points shown in FigureFigure6 6c.c. InIn thethe casecase ofof DB specimen, thethe roundness measurement results show shape regularity in the examined cross-sections.

2.3.3. Roughness Roughness Measurement Measurement of the Specimen Working Part Surface roughnessroughness tests tests of of the the specimen specimen measuring measuring part werepart were carried carried out using out theusing Mahr the MarSurf Mahr MarSurfGD 120 device GD 120 (GmbH, device Göttingen,(GmbH, Göttingen, Germany). Germany). In accordance with the recommendations and requirements of [[28]28] standard, the following roughness parameters were determined:determined: Raa,,R Rz,, and and R Rpp.. The The tests tests were were carried carried out out on on the the measuring measuring part of the sample, as shown in Figure8 8.. Ra specifies the arithmetic mean profile deviations from the average line measured along the measuring section. Rz is the arithmetic mean value of the absolute heights of the five highest profile x Ra x Rz x Rp elevations and the five lowest dimples on the elementary segment. Rp is the height of the highest profile peak. z z z The indicated roughness parameters values were determined on the basis of the same measuring (a) (b) (c) sections. The obtained test results are presented in Table4.

Figure 8. Specimens with the marking points of roughness measurement for: (a) Ra parameter; (b) Rz

parameter and (c) Rp parameter.

Ra specifies the arithmetic mean profile deviations from the average line measured along the measuring section. Rz is the arithmetic mean value of the absolute heights of the five highest profile elevations and the five lowest dimples on the elementary segment. Rp is the height of the highest profile peak. The indicated roughness parameters values were determined on the basis of the same measuring sections. The obtained test results are presented in Table 4.

Table 4. Surface roughness measurements results for the measuring part of the specimen.

Specimen Roughness Parameters Specimen DB Specimen DMLS

Ra Rz Rp Ra Rz Rp μm μm μm mm μm μm Mean Value 0.275 1.544 0.805 1.898 11.880 5.585 Standard Deviation 0.089 0.385 0.181 0.182 1.339 0.633 Minimum Value 0.161 1.132 0.617 1.607 9.988 4.593 Maximum Value 0.428 2.096 1.120 2.187 13.987 7.132

Materials 2020, 13, x FOR PEER REVIEW 8 of 19

(a) (b) (c)

Figure 7. The measurements results example of the roundness of a turned DB specimen in cross- section: (a) A-A; (b) B-B; and (c) C-C.

The diameter measurements of the working part showed that samples made using the additive method (DMLS specimen) and turned samples from a drawn bar (DB specimen) are within the dimensional tolerance (±0.05 mm) established according to [27] standard. Analysis of the measuring points distribution for DMLS samples’ individual cross-sections regarding the roundness measurement indicates an irregular shape in relation to the nominal circle. This confirms the distribution of points shown in Figure 6c. In the case of DB specimen, the roundness measurement results show shape regularity in the examined cross-sections.

2.3.3. Roughness Measurement of the Specimen Working Part Surface roughness tests of the specimen measuring part were carried out using the Mahr MarSurf GD 120 device (GmbH, Göttingen, Germany). In accordance with the recommendations and requirements of [28] standard, the following roughnessMaterials 2020 parameters, 13, 3398 were determined: Ra, Rz, and Rp. The tests were carried out on the measuring9 of 19 part of the sample, as shown in Figure 8.

x Ra x Rz x Rp

z z z (a) (b) (c)

FigureFigure 8. 8. SpecimensSpecimens with with the the marking marking points of roughness measurement for: (a)R) Ra parameter; ( b))R Rz parameterparameter and and ( cc))R Rp parameter.parameter.

Table 4. Surface roughness measurements results for the measuring part of the specimen. Ra specifies the arithmetic mean profile deviations from the average line measured along the measuring section. Rz is the arithmetic mean valueSpecimen of the Roughness absolute Parameters heights of the five highest profile elevations and the five lowest dimples on the elementary segment. Rp is the height of the highest Specimen DB Specimen DMLS profile peak. The indicated roughness parametersR valuesa Rwerez determinedRp Ra on theR basisz ofR thep same measuring sections. The obtained test results are presentedµm µ inm Tableµ m4. mm µm µm Mean Value 0.275 1.544 0.805 1.898 11.880 5.585 Table 4. SurfaceStandard roughness Deviation measurements 0.089 0.385 results 0.181for the measuring 0.182 1.339 part of the 0.633 specimen. Minimum Value 0.161 1.132 0.617 1.607 9.988 4.593 Maximum Value 0.428 2.096Specimen 1.120 Roughness 2.187 13.987Parameters7.132 Specimen DB Specimen DMLS

Ra Rz Rp Ra Rz Rp The test results analysis indicatesμm that theμ averagem valueμm of all roughnessmm parametersμm (i.e.,μ Rma ,Rz, and Rp) isMean higher Value for DMLS specimens0.275 than1.544 the values 0.805 obtained for1.898 DB specimens. 11.880 The diff 5.585erences resultStandard from the Deviation fact that the DMLS 0.089 samples were 0.385 not subjected0.181 to additional0.182 mechanical 1.339 treatment 0.633 after their production.Minimum Value This form of samples 0.161 was adopted 1.132 for testing0.617 because1.607 complex construction 9.988 elements 4.593 made usingMaximum the DMLS Value method are 0.428 not subjected2.096 to additional1.120 mechanical2.187 treatment. 13.987 The percentages 7.132 difference of the DMLS specimens’ average roughness parameters in relation to DB samples are: for Ra. 690.2%; for Rz. 769.4%; for Rp. 693.8%. The maximum values analysis of selected roughness parameters allowed determining their percentage differences: for Ra. 511.0%; for Rz. 667.3%; and for Rp. 636.8%. Analyzing the minimum values of selected parameters, their percentage differences were determined as: for Ra. 998.1%; for Rz. 882.3%; and for Rp. 744.4%. The analysis showed that the Rp parameter best describes the profile shape variability of the specimen measuring part. It results from the percentage differences of the indicated parameter, which changed within the smallest ranges of average, minimum, and maximum values.

2.4. Hardness Measurement The Vickers method was adopted to measure hardness in samples turning from a bar and printed using the DMLS additive manufacturing technology. This process involves pressing a diamond pyramid with a square base and an angle between the opposite walls of 136◦ into the metal, under load F during time t. The hardness parameter is the load ratio to the side surface durable imprint. The hardness measurement was carried out in accordance with the [29] standard. The test stand was equipped with a HUATEC Vickers HV-10 hardness tester (Huatec Group Corporation, Beijing, China). Hardness tests were carried out on three samples. During the tests, an indenter was used in the form of a four-sided shaped diamond pyramid with an apex angle of 136◦. The measuring load was about 98.07 N, which allowed determining the hardness on the HV10 scale. Hardness measurements were carried out on metallographic sections taken from broken tensile specimens made by the DMLS method and from a drawn bar by the turning process. The material hardness was tested at ﬁve points in two planes according to the diagram in Figure9a,b. Materials 2020, 13, x FOR PEER REVIEW 9 of 19

The test results analysis indicates that the average value of all roughness parameters (i.e. Ra, Rz, and Rp) is higher for DMLS specimens than the values obtained for DB specimens. The differences result from the fact that the DMLS samples were not subjected to additional mechanical treatment after their production. This form of samples was adopted for testing because complex construction elements made using the DMLS method are not subjected to additional mechanical treatment. The percentages difference of the DMLS specimens’ average roughness parameters in relation to DB samples are: for Ra. 690.2%; for Rz. 769.4%; for Rp. 693.8%. The maximum values analysis of selected roughness parameters allowed determining their percentage differences: for Ra. 511.0%; for Rz. 667.3%; and for Rp. 636.8%. Analyzing the minimum values of selected parameters, their percentage differences were determined as: for Ra. 998.1%; for Rz. 882.3%; and for Rp. 744.4%. The analysis showed that the Rp parameter best describes the profile shape variability of the specimen measuring part. It results from the percentage differences of the indicated parameter, which changed within the smallest ranges of average, minimum, and maximum values.

2.4. Hardness Measurement The Vickers method was adopted to measure hardness in samples turning from a bar and printed using the DMLS additive manufacturing technology. This process involves pressing a diamond pyramid with a square base and an angle between the opposite walls of 136° into the metal, under load F during time t. The hardness parameter is the load ratio to the side surface durable imprint. The hardness measurement was carried out in accordance with the [29] standard. The test stand was equipped with a HUATEC Vickers HV-10 hardness tester (Huatec Group Corporation, Beijing, China). Hardness tests were carried out on three samples. During the tests, an indenter was used in the form of a four-sided shaped diamond pyramid with an apex angle of 136°. The measuring load was about 98.07 N, which allowed determining the hardness on the HV10 scale. Hardness measurements were carried out on metallographic sections taken from broken tensile specimens made by the DMLS method and from a drawn bar by the turning process. The material Materials 2020, 13, 3398 10 of 19 hardness was tested at five points in two planes according to the diagram in Figure 9a,b.

z y

z5 xy

12345 z4 x z3

x (a) (b)

FigureFigure 9. 9. HardnessHardness measurement measurement method: method: (a (a) )schematic schematic designation designation of of hardness hardness measuring measuring points points alongalong the the z-axis; z-axis; and and (b ()b schematic) schematic designation designation of hardness of hardness measurement measurement points points in the in shank the shank on the on x–y-plane.the x–y-plane.

Based on the measurements carried out, the following hardness results were obtained (Table5). Columns 3 and 4 of Table5 present the hardness measurements results along the z-axis (Figure9a) for the DMLS and DB specimens. Columns 6 and 7 of Table5 contain the results achieved on the x-plane of the DMLS and DB samples gripping part (Figure9b).

Table 5. Vickers hardness measurement results.

Vickers Hardness Measurement Method (HV10) No. Along z Axis On the x–y Plane No Specimen DMLS Specimen DB No Specimen DMLS Specimen DB 1 z1 842 769 xy1 717 715 2 z2 871 785 xy2 697 680 3 z3 869 798 xy3 706 618 4 z4 881 794 xy4 701 677 5 z5 893 778 xy5 719 713

2.5. Material Strength Tests To determine the mechanical properties of the Ti6Al4V material produced in the turning process (from a drawn bar) and by the DMLS additive manufacturing technology, a static tensile test was performed in accordance with the [24] standard. Material strength tests were carried out on three samples. The stand for determining the strength parameters was an Instron 8502 hydraulic machine (Instron, Norwood, MA, USA). The control parameter during the tests was a machine piston displacement of 0.05 mm/s. The loading force and deformation were recorded during the tests. The tests were accomplished using an extensometer (Instron, Norwood, MA, USA) with a measuring base of 10 mm and a range of 1 mm. During the static tensile test, the strain value was measured in two ways. The ﬁrst method was based on the use of an extensometer mounted directly on the sample. The second way to measure sample deformation was the digital image correlation method. This method consisted in measuring Materials 2020, 13, x FOR PEER REVIEW 10 of 19

Based on the measurements carried out, the following hardness results were obtained (Table 5). Columns 3 and 4 of Table 5 present the hardness measurements results along the z-axis (Figure 9a) for the DMLS and DB specimens. Columns 6 and 7 of Table 5 contain the results achieved on the x- plane of the DMLS and DB samples gripping part (Figure 9b).

Table 5. Vickers hardness measurement results.

MaterialsDuring2020, 13 the, 3398 static tensile test, the strain value was measured in two ways. The first method11 was of 19 based on the use of an extensometer mounted directly on the sample. The second way to measure sample deformation was the digital image correlation method. This method consisted in measuring and monitoring thethe displacementsdisplacements ofof the the specimen specimen observed observed fragment, fragment, which which was was in in the the view view field field of theof the BASLER BASLER acA4024-8gm acA4024-8gm camera camera (Basler (Basler AG, Ahrensburg,AG, Ahrensburg, Germany) Germa setny) on set a tripod on a ontripod a solid on surface.a solid surface.The DIC method measurement consisted of an images series periodically recorded over a specified periodThe of DIC time. method Usually, measurement the first image consisted was the of reference an images image series [30 periodically]. The method recorded adopted over was a tospecified photograph period the of time. sample Usually, during the a tensilefirst image test was with the a camerareferenc ate image constant [30]. intervals. The method The adopted images recordedwas to photograph during the the research sample were during subjected a tensile to displacement test with a camera analysis atusing constant BASLER’s intervals. Pylon The software images (versionrecorded 6.1.0,during Basler the AG,resea Ahrensburg,rch were subjected Germany). to Thisdisplacement program comparedanalysis using all the BASLER’s photos recorded Pylon duringsoftware the (version test to the 6.1.0, reference Basler image.AG, Ahrensburg, It was based Germany). on determining This program the position compared of a given all pixelthe photos on the referencerecorded during image andthe ascertainingtest to the reference the displacement image. It was of this based point on on determining subsequent the photos. position of a given pixelFigures on the reference 10 and 11 image show and sample ascertaining photos taken the displacement with the BASLER of this acA4024-8gm point on subsequent camera photos. during a staticFigures tensile test10 and for two11 show types sample of specimens. photos Figuretaken 10with shows the BASLER a sample acA40 produced24-8gm by DMLS camera technology, during a whilestatic Figuretensile 11test shows for two a turned types sample of specimens. from a drawn Figure bar. 10 Figures shows 10 aa sample and 11a produced are reference by imagesDMLS registeredtechnology, first while during Figure the 11 static shows tensile a turned test. Figuressample 10fromb and a drawn11b are ba photosr. Figures taken 10a just and before 11a theare samplesreference were images broken. registered Figures first 10 cduring and 11 thec are static images tensile of already test. Figures broken 10b specimens. and 11b are photos ta

Materials 2020, 13, x FOR PEER REVIEW 11 of 19 (a) (b) (c)

Figure 10. PicturesPictures of of a a DMLS DMLS specimen specimen taken taken with with th thee BASLER BASLER acA4024-8gm acA4024-8gm camera camera during a static tensiletensile test: test: ( (aa)) before before starting starting the the test test (reference (reference image); image); ( (bb)) just just before before the the sample sample breaks breaks and and ( (cc)) just just afterafter breaking up.

(a) (b) (c)

Figure 11. PicturesPictures of of a a DB DB specimen specimen produced produced by by the the turning method taken with the BASLER acA4024-8gmacA4024-8gm camera camera during a static tensile test: ( (aa)) before before starting starting the the te testst (reference (reference image); image); ( (b)) just just before the sample breaks and ( c) just after breaking up.

As aa resultresult of of the the static static tensile tensile test, thetest, following the following material mechanicalmaterial mechanical properties wereproperties determined: were determined:tensile strength tensile Su, yieldstrength strength Su, yield Sy0.2 strength, Young’s Sy0.2 modulus, Young’s E, modulus elongation E, A,elongation and narrowing A, and Z.narrowing Z. Figure 12 schematically shows the area on the sample subject to displacement analysis, which was dividedFigure into 12 three schematically ranges. The shows selected the area strength on the parameter sample subject values to obtained displacement for DMLS analysis, samples which are waspresented divided in into Table three6, while ranges. the The results selected for specimens strength parameter produced values of drawn obtained bar are for given DMLS in samples Table7. areTables presented6 and7 summarizein Table 6, while the values the results obtained for specimens on the basis produced of deformation of drawn measurements bar are given made in Table with 7. Tablesan extensometer 6 and 7 summarize and using the digital values image obtained correlation on the for basis various of deformation measuring measurements ranges. The extensometer made with anmeasuring extensometer range and was using L4 = 10 digital mm, whileimage the correlation measuring for ranges various for measuring the digital ranges. image correlation The extensometer method measuringwere: L1 = range1.36 mm, was L2 L4= =2.72 10 mm, andwhile L3 the= 3.49 measuring mm. Figure ranges 13 forshows the examplesdigital image of tensile correlation charts method were: L1 = 1.36 mm, L2 = 2.72 mm, and L3 = 3.49 mm. Figure 13 shows examples of tensile charts determined using an extensometer and digital image correlation for samples made with DMLS additive manufacturing technology (Figure 13a) and from a drawn bar by turning (Figure 13b).

L4 L2 L3 L1

Figure 12. Schematic presentation of the areas analyzed using the digital image correlation method, for which the measuring ranges were: L1 = 1.36 mm, L2 = 2.72 mm, L3 = 3.49 mm, and L4 = 10 mm (measured with an extensometer).

Table 6. Selected strength parameters list of the Ti6Al4V titanium alloy manufactured by the DMLS method under static tensile loads.

Selected Strength Parameters Average Values Measurement Range Sy0.2 Su E A Z MPa MPa MPa % % DIC Range 1 L1 = 1.36 mm 1089 ± 40 1140 ± 20 115,300 ± 2037 23.5 ± 3.3 19.2 ± 3.5 DIC Range 2 L2 = 2.72 mm 1091 ± 35 1140 ± 31 115,020 ± 2805 23.3 ± 2.9 19.2 ± 3.5 DIC Range 3 L3 = 3.49 mm 1096 ± 42 1140 ± 24 112,950 ± 2350 21.2 ± 3.5 19.2 ± 3.5 Extensometer L4 = 10 mm 1086 ± 28 1121 ± 42 119,610 ± 1254 16.9 ± 4.3 19.2 ± 3.5

Materials 2020, 13, x FOR PEER REVIEW 11 of 19

Figure 10. Pictures of a DMLS specimen taken with the BASLER acA4024-8gm camera during a static tensile test: (a) before starting the test (reference image); (b) just before the sample breaks and (c) just after breaking up.

(a) (b) (c)

Figure 11. Pictures of a DB specimen produced by the turning method taken with the BASLER acA4024-8gm camera during a static tensile test: (a) before starting the test (reference image); (b) just before the sample breaks and (c) just after breaking up.

As a result of the static tensile test, the following material mechanical properties were determined: tensile strength Su, yield strength Sy0.2, Young’s modulus E, elongation A, and narrowing Z. Figure 12 schematically shows the area on the sample subject to displacement analysis, which was divided into three ranges. The selected strength parameter values obtained for DMLS samples are presented in Table 6, while the results for specimens produced of drawn bar are given in Table 7. Tables 6 and 7 summarize the values obtained on the basis of deformation measurements made with anMaterials extensometer2020, 13, 3398 and using digital image correlation for various measuring ranges. The extensometer12 of 19 measuring range was L4 = 10 mm, while the measuring ranges for the digital image correlation method were: L1 = 1.36 mm, L2 = 2.72 mm, and L3 = 3.49 mm. Figure 13 shows examples of tensile chartsdetermined determined using anusing extensometer an extensometer and digital and digi imagetal correlationimage correlation for samples for samples made with made DMLS with additive DMLS additivemanufacturing manufacturing technology technology (Figure 13 (Figurea) and 13a) from and a drawn from a bar drawn by turning bar by (Figureturning 13 (Figureb). 13b).

L4 L2 L3 L1

FigureFigure 12. 12. SchematicSchematic presentation presentation of of the the areas areas analyzed analyzed using using the the digital digital image image correlation correlation method, method, forfor which which the the measuring measuring ranges ranges were: were: L1 L1 == 1.361.36 mm, mm, L2 L2 == 2.722.72 mm, mm, L3 L3 == 3.493.49 mm, mm, and and L4 L4 == 1010 mm mm (measured(measured with with an an extensometer). extensometer).

Table 6. Selected strength parameters list of the Ti6Al4V titanium alloy manufactured by the DMLS Table 6. Selected strength parameters list of the Ti6Al4V titanium alloy manufactured by the DMLS method under static tensile loads. method under static tensile loads. Selected Strength Parameters Average Values Measurement Selected Strength Parameters Average Values MeasurementRange Range Sy0.2Sy0.2 Su Su EAZE A Z MPaMPa MPa MPa MPa MPa % % % % DIC Range 1 L1 = 1.36 mm 1089 ± 40 1140 ± 20 115,300 ± 2037 23.5 ± 3.3 19.2 ± 3.5 DIC Range 1 L1 = 1.36 mm 1089 40 1140 20 115,300 2037 23.5 3.3 19.2 3.5 DIC Range 2 L2 = 2.72 mm 1091± ± 35 1140± ± 31 115,020± ± 2805 23.3± ± 2.9 19.2± ± 3.5 DIC Range 2 L2 = 2.72 mm 1091 35 1140 31 115,020 2805 23.3 2.9 19.2 3.5 DIC Range 3 L3 = 3.49 mm 1096± ± 42 1140± ± 24 112,950± ± 2350 21.2± ± 3.5 19.2± ± 3.5 DIC Range 3 L3 = 3.49 mm 1096 42 1140 24 112,950 2350 21.2 3.5 19.2 3.5 ± ± ± ± ± ExtensometerExtensometer L4 L4 == 10 mm 1086 108628 ± 28 1121 112142 ± 42 119,610 119,6101254 ± 1254 16.9 16.94.3 ± 4.3 19.2 19.23.5 ± 3.5 ± ± ± ± ±

Table 7. Selected strength parameters list of the Ti6Al4V titanium alloy in the drawn bar form under static tensile loads.

Selected Strength Parameters Average Values Measurement Range Sy0.2 Su EAZ MPa MPa MPa % % DIC Range 1 L1 = 1.36 mm 1006 55 1044 48 103,880 4709 38.2 0.9 47.3 6.2 ± ± ± ± ± DIC Range 2 L2 = 2.72 mm 1008 47 1044 29 95,316 3897 39.4 1.3 47.3 6.2 ± ± ± ± ± DIC Range 3 L3 = 3.49 mm 1008 31 1044 46 89,234 3564 35.4 1.8 47.3 6.2 ± ± ± ± ± Extensometer L4 = 10 mm 1004 34 1042 37 106,940 4375 28.3 2.2 47.3 6.2 ± ± ± ± ±

The data obtained present that the strength parameters, such as yield point Sy0.2, tensile strength Su, and elongation, achieved for the extensometer have lower values compared to the digital image correlation method results. The tensile tests using the extensometer for DMLS and DB samples showed a difference in Young’s modulus values. They result from different material production technologies. Comparing the results of the Young’s module for DMLS samples, the values obtained for different measurement bases of the DIC method are similar. This indicates an even deformation of the layers that resulted from the element production by the DMLS method. The differences in the yield point values are small and within the limits of measurement uncertainty. For DB samples, significant differences were observed in the Young’s modulus values for different measurement bases of the DIC method. It is presumed that the results achieved are related to locally changing properties of the sample. It is also assumed that the DIC measurement method may have influenced the results of the Young’s module determined for DB samples. This method is based on images of the sample’s surface. The small surface Materials 2020, 13, x FOR PEER REVIEW 12 of 19 Materials 2020, 13, 3398 13 of 19 Table 7. Selected strength parameters list of the Ti6Al4V titanium alloy in the drawn bar form under static tensile loads. roughness could affect the readings of the displacement values, which contributed to obtaining specific Selected Strength Parameters Average Values test results. The modulusMeasurement of elasticity E obtains the highest value for the extensometer. Based on the Sy0.2 Su E A Z results, the followingRange relationship is observed: the larger is the measurement area, the greater are the MPa MPa MPa % % Young’s modulus E and elongation A, while the lower is the yield point S . The yield point S DIC Range 1 L1 = 1.36 mm 1006 ± 55 1044 ± 48 103,880 ± 4709 y0.2 38.2 ± 0.9 47.3 ± 6.2y0.2 value is close to the value obtained for the extensometer. The results obtained for turned samples DIC Range 2 L2 = 2.72 mm 1008 ± 47 1044 ± 29 95,316 ± 3897 39.4 ± 1.3 47.3 ± 6.2 behaveDIC Range in a 3 similar L3 way, = 3.49 as mm given in 1008 Table ± 317 and Figure 1044 ± 4613 b. The 89,234 only ± di 3564fference 35.4 is that ± 1.8 for range 47.3 ± 2 6.2 the highestExtensometer value of elongation L4 = 10 mm A was obtained.1004 ± 34 1042 ± 37 106,940 ± 4375 28.3 ± 2.2 47.3 ± 6.2

1200 1200

1000 1000

800 800

600 DIC range 1 600 DIC range 1 DIC range 2 DIC range 2 Stress σ, MPa 400 Stress σ, MPa 400 DIC range 3 DIC range 3 200 Extensometer 200 Extensometer

0 0 0 102030400 5 10 15 20 25 Strain ε, % Strain ε, % (a) (b)

Figure 13. An example ofof thethe stretching stretching graph graph S =S f(= εf()ε Ti6Al4V) Ti6Al4V titanium titanium alloy: alloy: (a) DMLS(a) DMLS specimen specimen and and(b) DB (b) specimen.DB specimen.

The data strength obtained tests present showed that that the the strength samples parameters, produced with such the as yield DMLS point additive Sy0.2, tensile manufacturing strength Stechnologyu, and elongation, are characterized achieved for by the higher extensometer tensile strength have lower Su and values yield pointcompared Sy0.2 comparedto the digital to turnedimage correlationDB specimens. method Similar results. values The oftensile Young’s tests modulus using the were extensometer achieved for for both DMLS sample and technologies.DB samples showedThe elongation a difference A average in Young’s value reaches modulus almost values. two Th timesey result higher from value different for specimens material made production of drawn technologies.bar in the turning Comparing process. the results of the Young’s module for DMLS samples, the values obtained for different measurement bases of the DIC method are similar. This indicates an even deformation 3. Analysis of Test Results of the layers that resulted from the element production by the DMLS method. The differences in the yield3.1. Analysis point values of Sample are Geometric small and Features within the limits of measurement uncertainty. For DB samples, significant differences were observed in the Young’s modulus values for different measurement bases of theAnalyzing DIC method. roughness It is presumed measurements, that the it results can be achieved seen that samplesare related made to locally using the changing DMLS technologyproperties ofimmediately the sample. after It is also the printingassumed process that the do DIC not me meetasurement the requirements method may of have the [ 27influenced] standard. the Itresults gives ofthe the parameter Young’s Ramodule value determined of the specimen for DB working samples. part This equal method to 0.32 is basedµm. Dueon images to the performanceof the sample’s of surface.tests under The static small loads, surface the roughness surface roughness could affect value the is readings not a factor of the that displacement significantly avalues,ffects thewhich test contributedresults. Considering to obtaining the specific results oftest specimens’ results. The geometric modulus measurements, of elasticity E obtains it should the be highest noted value that they for thedo notextensometer. all meet normative Based assumptionson the results, in as-builtthe follow state.ing The relationship surface roughness is observed: Ra average the larger value is of the the measurementmeasuring part area, for the greater tested samples are the Young’s is higher modu than thatlus E given and elongation in the standard. A, while On the the lower other is hand, the yieldthe diameters point Sy0.2 and. The roundness yield point deviations Sy0.2 value of the is close working to the part value meet obtained the normative for the conditions. extensometer. To ensure The resultscompliance obtained of geometric for turned dimensions, samples behave shape, in and a similar surface way, roughness, as given additional in Table 7 mechanical and Figure treatment 13b. The onlyshould difference be carried is that out. for range 2 the highest value of elongation A was obtained. The strength tests showed that the samples produced with the DMLS additive manufacturing 3.2. Hardness Test Results Comparison technology are characterized by higher tensile strength Su and yield point Sy0.2 compared to turned DB specimens.On the basis Similar of the values hardness of Young’s tests, amodulus difference were in theachieved results for obtained both sample for samples technologies. produced The elongationfrom a drawn A average bar and value made reaches using almost the additive two time manufacturings higher value method for specimens was found. made Accordingof drawn bar to inFigure the turning9a, five process. hardness measurements of DMLS and DB specimens were made along the z-axis. Analyzing the achieved hardness values presented in Figure 14a, it can be seen that, for the DMLS specimen, the hardness along the z-axis is higher than the hardness for the DB sample. For the DMLS specimen, the hardness results are in the range 842–893 HV10, while the hardness for the DB specimen

Materials 2020, 13, 3398 14 of 19 varies between 769 and 798 HV10. According to the measurement implementation scheme (Figure9b), the first measuring point is located in the sample gripping area, while the last measuring point is located closest to the edge of the sample crack. For the DMLS sample, it can be considered that the hardness on the z-axis increases with the approach to the crack site. The highest hardness values for the DMLS specimen (893 HV10) were obtained near the fracture site of the sample, while, for the DB specimen, the highest hardness (798 HV10) was achieved at the transition point between the shank and measuring part. The hardness at the first point located in the gripping part for the DMLS specimen is 842 HV10 and for the DB specimen 769 HV10. The increase in hardness at the DMLS fracture site may be related to the deformations occurring in this area caused by the static tensile test. The increase in hardness at the DMLS fracture site may be related to the deformations occurring in this area caused by the static tensile test. This is due to the change in the sample geometry, which affects the change in stress/strain values in specific sections. The highest hardness occurs in the area where the crack occurred. This is where the highest deformations occurred, which were close to 16.9%. In the case of a sample made of drawn bar, the change in the hardness value along the z-axis is related to the higher ductility of this material compared to the DMLS sample. In the gripping part of the sample, the rising of deformations caused by the static tensile test increases the hardness of the material. This is due to the change in the value of the sample cross-sections. On the other hand, a significant increase in these deformations in the measuring part of the sample causes significant changes in the structure of the material contributing to a decrease in its hardness. In addition, Figure 14a presents the measurement results as a polynomial function whose equation was determined for two types of samples. In each case, the determination coefficient value was R2 > 0.95, which indicates a good fit of the polynomial functionMaterials 2020 with, 13, the x FOR experimental PEER REVIEW research results. 14 of 19

(a) (b)

FigureFigure 14.14. HV10 hardness distribution: (a) along the z-axis; and (b) in the shank onon thethe x–y-plane.x–y-plane.

Figure 1414bb shows the the hardness hardness measuring measuring results results of of the the DMLS DMLS and and DB DB samples samples taken taken on onthe the x– x–y-planey-plane according according to to the the diagram diagram in in Figure Figure 9b.9b. The The hardness hardness results results for both DMLS and DBDB specimensspecimens werewere lowerlower thanthan thosethose achievedachieved alongalong thethe z-axis.z-axis. On the x-plane, higher hardness values were gained forfor thethe DMLSDMLS specimen,specimen, whichwhich werewere inin thethe rangerange ofof 697–719697–719 HV10.HV10. OnOn the other hand, the DB sample had aa hardnesshardness ofof 618–715618–715 HV10.HV10. The DB sample was characterized by the highest hardness aroundaround thethe sectionsection edgeedge PointPoint 1,1, 715715 HV10;HV10; PointPoint 5,5, 713713 HV10).HV10). As the DB sample axis approaches, a decrease inin hardnesshardness cancan bebe seenseen (Point(Point 3,3, 618618 HV10).HV10). Such hardness distribution may be aaffectedﬀected by thethe deformations value resulting from the stretching of the material outer layers. It is known that a rod with a round section loaded with tensile force has a variable distribution of deformations in the cross section. Therefore, in each of the analyzed cases, the hardness outside the sample is the highest. In the case of DB samples, this difference may be due to the producing method of the bar, i.e. pulling process. The obtained results were described by the polynomial equation shown in Figure 14b, which does not describe their variability very accurately, as evidenced by the determination coefficient R2 = 0.8024. In the case of results for DMLS specimen, the highest values were around the section edge (Point 1, 717 HV10; Point 5, 719 HV10). As with the DB sample, the hardness value was the highest on the outer layers of the sample. It can also be affected by deformations occurring in the material resulting from the stretching of the outer layers. Differences in hardness values in the printed sample were lower due to the layered manufacturing process. The difference between the highest and lowest hardness values for the DMLS sample was 22 HV10. The obtained test results were described by a polynomial equation (Figure 14b). The value of the coefficient of determination is R2 = 0.7568.

3.3. Analysis of Static Tension Test Results

Materials 2020, 13, 3398 15 of 19 deformations value resulting from the stretching of the material outer layers. It is known that a rod with a round section loaded with tensile force has a variable distribution of deformations in the cross section. Therefore, in each of the analyzed cases, the hardness outside the sample is the highest. In the case of DB samples, this difference may be due to the producing method of the bar, i.e., pulling process. The obtained results were described by the polynomial equation shown in Figure 14b, which does not describe their variability very accurately, as evidenced by the determination coefficient R2 = 0.8024. In the case of results for DMLS specimen, the highest values were around the section edge (Point 1, 717 HV10; Point 5, 719 HV10). As with the DB sample, the hardness value was the highest on the outer layers of the sample. It can also be affected by deformations occurring in the material resulting from the stretching of the outer layers. Differences in hardness values in the printed sample were lower due to the layered manufacturing process. The difference between the highest and lowest hardness values for the DMLS sample was 22 HV10. The obtained test results were described by a polynomial equation (Figure 14b). The value of the coefficient of determination is R2 = 0.7568.

3.3. Analysis of Static Tension Test Results

The tests carried out on titanium alloy Ti6Al4V allowed determining the yield point value Sy0.2 and tensile strength Su for DB and DMLS specimens. The indicated parameters have higher values for DMLS samples (Su = 1140 MPa) in relation to DB samples (Su = 1044 MPa). In the case of DB samples for the measuring ranges variable value, the quotient was S /Su 1.04, while for DMLS y0.2 ≈ specimen its value was S /Su = 1.03 1.05. Analyzing the Young’s modulus values as a function y0.2 ± of the variable measuring range value, it was found that, in the case of DB specimens, the lowest module values E = 89,234 MPa were obtained for the range L3 = 3.49 mm, while the highest module values E = 106,940 MPa for the range L4 = 10 mm. The difference in values is ∆E = 17,706 MPa (Figure 15a). In the case of DMLS samples, the lowest module values E = 112,950 MPa were gained for the range L3 = 3.49 mm, while the highest module values E = 119,610 MPa for the range L4 = 10 mm. The difference in values is ∆E = 6660 MPa (Figure 15b). The DIC method is ideal for determining local mechanical properties that may be used, e.g., in numerical calculations of construction elements. Based on the tests carried out, it can be stated that, for a Young’s modulus, the difference in the parameter E value depends on the measurement base: for samples made of drawn rod, it was about 14%, while, for samples made by the additive method, it was about 2%. In the case of DMLS samples:

for measuring base L1 = 1.36 mm, module E = 115,300 MPa; • for measuring base L2 = 2.72 mm, module E = 115,020 MPa; and • for measuring bases L3 = 3.49 mm, module E = 112,950 MPa. • Similar Young’s modulus values for DMLS samples result from sample-making technology. The specimen was made along the z-axis, which indicates that the subsequent layers’ connections of the element were tested. A small measurement base adoption reduced the number of analyzed material layers, and thus the material defects number occurring in the analyzed volume. Material defects may result from: no melting of the powder, occurring microporosity, material defects, etc. Young’s modulus is a material constant, thus its value should be the same for each of the measurement methods. Changes in the values of Young’s modulus for individual measuring ranges in the DMLS sample may be related to local changes in material properties. The individual measurement bases include small fragments of the sample in the area of the greatest deformation and subsequent cracking. As a result, the greatest local changes in material properties occur in this area. The difference in the Young’s modulus values resulted from the inaccuracy of the digital image correlation method. Analyzing the test results, it can be concluded that the DIC method is not reliable compared to the standard method using an extensometer used in engineering research. In the case of both DB and DMLS samples, the value of Young’s modulus E decreased with the increase of the L area under analysis. This may result from poor lighting of the sample during the tests (too dark or overexposed photos), Materials 2020, 13, 3398 16 of 19 which translates into “losing” points by the program during the analysis. The differences between Young’s modulus obtained for the measurement ranges L1, L2, and L3 for the DMLS sample are smaller than for the DB sample. The surface of the DMLS sample was rougher and dull, which favored taking higher quality photos, while the turned DB sample had a shiny surface, which reflected the light, causing the photos to be overexposed. As a result of the conducted research, it can be concluded that the DIC method depends on many factors, i.e., the surface of the tested element and its preparation, appropriate lighting, and the size of the analyzed area. It was found that the DIC method can be reliable for small measurement areas. Comparing the results obtained using the DIC method and the Materialsresults achieved2020, 13, x FOR using PEER an REVIEW extensometer, it was found that they differ in the range of about 5%.16 of 19

(a) (b)

Figure 15. Change of Young’s modulus value depending on changes in the measurement base value for: ( (aa)) DB DB specimen; specimen; and and ( (b)) DMLS DMLS specimen. specimen.

The changes results in elongation A for the adopted measuring ranges show that the lowest values were achieved for both types of samples for L4 = 10 mm (Figure 16). The smaller the measurement range, the greater the strain value. The smallest measuring range included a fragment of the sample in which there was a crack resulting from the occurrence of tensile forces. There was stress concentration at the crack site and a simultaneous increase in the deformation of the material layers. The average deformation value in areas L2 and L3 was lower than in the case of the L1 measurement range. The largest measuring range L4, obtained for the extensometer, was characterized by the lowest deformation value. Higher elongation values were obtained for DB specimens that range from 28.3% to 39.4%, while for DMLS samples from 16.9% to 23.5%.

(a) (b)

Figure 16. Change in the samples elongation depending on changes in the value of the measurement base for: (a) DB specimen; and (b) DMLS specimen.

4. Conclusions DIC is an optical method that allows measuring displacements that can be used to determine strains caused by external loads. In the case of material produced by the additive method, based on sintering of metallic powder layers, its strength is determined by the strength of the individual layer connections. The DIC method is ideal for determining local mechanical properties that may be used, e.g., in numerical calculations of construction elements. Based on the tests carried out, it can be stated that, for a Young’s modulus, the difference in the parameter E value depends on the measurement base: for samples made of drawn rod, it is about 14%, while, for samples made by the additive method, it is about 2%.

Materials 2020, 13, x FOR PEER REVIEW 16 of 19

(a) (b)

MaterialsFigure2020 15., 13 ,Change 3398 of Young’s modulus value depending on changes in the measurement base value17 of 19 for: (a) DB specimen; and (b) DMLS specimen.

(a) (b)

FigureFigure 16. 16. ChangeChange in in the the samples samples elongation elongation depending depending on on changes changes in in the the valu valuee of of the the measurement measurement basebase for: for: ( (aa)) DB DB specimen; specimen; and and ( (bb)) DMLS DMLS specimen. specimen.

4.4. Conclusions Conclusions DICDIC is is an optical method that allows measuring displacements that that can can be be used used to determine strainsstrains caused caused by by external external loads. loads. In In the the case case of of material material produced produced by by the the additive additive method, method, based based on on sinteringsintering of of metallic metallic powder powder layers, layers, its its strength strength is is determined determined by by the the strength strength of of the the individual individual layer layer connections.connections. The The DIC DIC method method is is ideal ideal for for determining determining local local mechanical mechanical properties properties that that may may be be used, used, e.g.,e.g., in in numerical numerical calculations calculations of construction construction elem elements.ents. Based Based on on the the tests tests ca carriedrried out, out, it it can can be be stated stated that,that, for for a a Young’s Young’s modulus, the the difference difference in the parameter E value depends on the measurement base:base: for samplessamples mademade of of drawn drawn rod, rod, it isit about is about 14%, 14%, while, while, for samples for samples made bymade the additiveby the additive method, method,it is about it 2%.is about 2%. Analysis of changes in the Young’s modulus value showed that the highest values were obtained for the measuring range L4 = 10 mm for both types of specimens. Young’s modulus for DMLS samples was 11.8% larger than module E for DB samples. The variability of the module E value for the adopted measuring ranges was much lower for DMLS specimens. It is related to the technology of sample preparation by the DMLS additive manufacturing method consisting in sintering material layers. Therefore, the structural element strength is determined by the strength of a single joint of the material in two layers. Ti6Al4V titanium alloy produced from a powder by the DMLS additive manufacturing method showed higher values of Young’s modulus E in relation to the results achieved for the drawn bar material. The assumed strain measuring ranges importantly influenced the value of Young’s modulus for specimens made of drawn bar (∆E = 17,706 MPa), while, in the case of DMLS samples, this impact was significantly smaller (∆E = 6660 MPa). The experimentally determined Young’s modulus value is important for calculations performed using numerical methods. The hardness of specimens produced using the DMLS method was higher than in the case of samples made from a drawn bar by turning. This applied to hardness along the z-axis and on the x–y-plane. Producing samples using the DMLS additive technology allowed obtaining surface roughness characterized by average values: Ra = 1.898 mm, Rz = 11.880 mm, and Rp = 5.585 mm. Materials 2020, 13, 3398 18 of 19

Comparison of the obtained results with those presented in [6–10] indicates that they are consistent with the results achieved for the annealed Ti6Al4V material. High results compliance was obtained with the results presented in this work for all analyzed parameters, i.e., Sy,Su, E, and A.

Author Contributions: Conceptualization, B.L; methodology, K.K., B.L. and D.B.; software, D.B.; validation, K.K., B.L. and D.B.; formal analysis, K.K. and B.L.; investigation, K.K. and B.L.; data curation, K.K., B.L. and D.B.; writing—original draft preparation, K.K. and B.L.; writing—review and editing, K.K and B.L.; visualization, B.L. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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