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Wear Resistance of (Diamond-Ni)-Ti6Al4V Gradient Materials Prepared by Combined Selective Laser Melting and Spark Plasma Sintering Techniques

Wear Resistance of (Diamond-Ni)-Ti6Al4V Gradient Materials Prepared by Combined Selective Laser... Hindawi Advances in Tribology Volume 2019, Article ID 5415897, 12 pages https://doi.org/10.1155/2019/5415897 Research Article Wear Resistance of (Diamond-Ni)-Ti6Al4V Gradient Materials Prepared by Combined Selective Laser Melting and Spark Plasma Sintering Techniques Ramin Rahmani , Maksim Antonov, and Lauri Kollo Tallinn University of Technology, Department of Mechanical and Industrial Engineering, Ehitajate tee 5, Tallinn 19086, Estonia Correspondence should be addressed to Ramin Rahmani; ramin.rahmaniahranjani@ttu.ee Received 11 September 2018; Revised 15 January 2019; Accepted 21 February 2019; Published 4 March 2019 Guest Editor: Mikael Olsson Copyright © 2019 Ramin Rahmani et al. is Th is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. An approach of sintering 3D metal printed lattices and diamond nickel-coated particles is proposed which can be used for the production of tunnel boring machine (TBM) cutters and mining equipment blades. Nickel-coated diamond particles are mixed with titanium powder and incorporated into a lightweight Ti6Al4V (3D printed) lattice with the help of spark plasma sintering (SPS) method. Eeff ct of Ti6Al4V lattices size, diamond particles size, and nickel coating layer thickness on wear resistance of composites is discussed. Functionally graded lattice (FGL) structures were produced by selective laser melting (SLM) method, representing an increasingly growing additive manufacturing engineering area introduced in material engineering. Impact-abrasive tribo-device (IATD), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and optical surface profiler (OSP) were used to characterize samples. An ab initio design of diamond-metal composite is based on the improvement of impact and abrasive wear resistance of Ti6Al4V by adding diamond particles and by applying of gradient lattice structure. eTh specimen with larger size of the diamond particle and thicker Ni coating has better wear resistance. In addition, ANSYS sow ft are simulations were done to analyze the eeff ct of the presence of 3D printed lattice via nonlinear finite element AUTODYN solver under impact test. Diamond-based gradient composite material producedby combinedSLM-SPSmethods can be appliedin applications where resistance against impact-abrasive wear is important. 1. Introduction copper and is usually used in grinding, polishing, and boring applications with sufficient cooling and without impacts. Ti6Al4V is the most applicable titanium alloy that has Diamond-containing metal matrix composites (MMCs) are been extremely used in biomedicine, osteology, aerospace, made with the help of chemical/physical vapor deposition marine, and additive manufacturing industries due to low (CVD/PVD) techniques and they are considered because of their high thermal conductivity and mechanical properties density and high mechanical properties. Adding≈6.75 wt. %of aluminum and ≈4.5 wt. % of vanadium to titanium [3]. Selective laser melting/sintering (SLM/SLS) is one of the new additive manufacturing techniques that is applied for makes it more applicable than pure titanium for corro- production of complex metal shapes, lattice structures, and sive wear resistance applications. Additive manufactured titanium alloys have motivated in deep in vivo corrosion rapid prototyping. The SLM ability of creation lightweight metallic cellular structure with different unit cell structure, research for recovering fractures of knee and hip bones [1]. strut, and pore sizes is used as a more efficient approach to It was recently demonstrated that 3D printed materials can provide improved tribological performance (lower coefficient antishock/impact energy absorption, lightweight aerospace structure, electrothermal conductivity, fracture toughness of friction, stable performance) in sliding conditions [2]. enhancement, and acoustic insulation application [4, 5]. Diamond is the hardest known, expensive, valuable, and ver- satile material for several industries. Synthetic polycrystalline AISI 316L stainless steel, Ti6Al4V titanium, and AlSi10Mg aluminum are three highly demanded metal lattice structures diamond (PCD) is cost-effective and advantageous powder that can be coated by metals like cobalt, nickel, titanium, and duetohighstrength toweight ratio. Nowadays, Spark 2 Advances in Tribology (A) (B) (C) (D) Figure 1: (A) Low, (B) average, (C) high volume fraction lattice structures, and (D) functionally graded lattice (FGL) structure used in the current research for samples No. 5 and No. 6 exclusively (diameter of lattice structures are 20 mm). plasma sintering (SPS) is extensively used method based on results is discussed. Samples were evaluated by combined pulsed DC electric current, high heating rates, programmable impact-abrasive tribo-device (IATD), volumetric wear was measured with the help of optical surface profiler (OSP) isostatic pressures, and short heating/cooling times [6]. The SPS process enables higher heating rate and sintering at lower analyzer, and composition of obtained materials was analyzed temperature to provide consolidation of wide range of mate- by X-ray diffraction (XRD) method. rial including metals, ceramics, and cermets that is its main important advantage over conventional powder metallurgy 2. Experimental Materials and Test Methods techniques [7]. Phase degradation is almost avoided during Cellular lattice structures were fabricated from argon atom- SPS sintering. Pulsed electric current passes through a mold ized Ti6Al4V Gd5 powders with size≤ 45𝜇 mand density and conductive particles of material to be sintered. Based on pressure and temperature, it is possible to use graphite or 4429 kg/m supplied by TLS Technik GmbH., Germany. tungsten mold and different diameter or thickness to realize Polycrystalline diamond powder with 30 and 56 wt. % of desired production shape that is a suitable option in several nickel coating and fractions of 6-12, 20-30, and 40-50𝜇 m industries. Temperature and pressure can be controlled by were supplied by Van Moppes & Sons Ltd., Swiss. Nickel pyrometer/thermocouple and upper/lower punch electrode coating usually contains 8-12 % of phosphorous and has pushing force in a vacuum chamber, respectively. To avoid 1455 C melting point (according to powder producer [8]). either graphitization of diamond or metal-coating surface Realizer SLM50 3D metal printer machine (construction oxidation, simultaneous increasing of temperature and pres- volume has a diameter of 70 mm and height of 80 mm, the sure is required in SPS. Shrinkage of the powder, limited grain thickness of layer was 20-50𝜇 m, and argon consumption growth, and near-theoretical densicfi ation take place during was 30 l/h) was used for preparing low, average, and high sintering. Cellular lattice structure should be adjusted to take volume fraction (VF) Ti6Al4V lattices and FGL structures into account the shrinkage. The SLM parameters like cell shown in Figure 1. Circumferential to longitudinal (C:L) cell size, lattice layer thickness, and laser current are important size proportion for cylindrical lattices (Figures 1(A)–1(C)) for the performance of the final material in test and field with a diameter of 20 mm and initial height of 18 mm (final conditions. SLM method has been applied in recent study height of ≈10-12 mm) were 1:2 due to significant vertical due to production possibility of metallic matrix in desired shrinkage during SPS process. It was decided that final C:L shapes (for example in SPS mold size or drag bits of tunneling and VF for lattice shown in Figures 1(A), 1(B), and 1(C) machines) and possibility to fill spaces between the lattice were 2:4 and 6 %, 1:2 and 15 %, and 0.75:1.5 and 24%, rods with hard material particles (for example, diamond, respectively. FGL structure (Figure 1(D)) is composed of three cubic boron nitride, or WC-Co). equal parts as a novel applicable lattice with 20 mm diameter The main aims of this research were focused to optimize and 18 mm initial height of sections with 1:1, 1:2, and 1:3 the composition of nickel-coated diamond and titanium C:L proportion. The schematic of the desired FGL structure powders for materials with and without titanium functionally obtained by combined SLM and SPS is given in Figure 2. graded lattice (FGL) structure to provide sufficient resistance From bottom to top, reduction of Ti6Al4V lattice structure against impacts and to improve abrasive wear resistance. FGL and enhancement of diamond particles before SPSing has is a progressive multilayer lattice structure with different been shown (Figure 2). Three most important parameters volume fraction section in longitudinal or circumferential for printing of Ti6Al4V lattices by SLM method were set directions so that the sections stand on each other or are as follows: (1) laser current (LC=3000 mA, 72 W power), embedded, respectively. Another novelty of current research (2) exposure time (ET=600 𝜇 s), and (3) point distance is the combination of SLM and SPS methods to produce a new (PD=1𝜇 m). In addition, these parameters for solid parts (for generation of multicomponent structures that can be used example, 0.1 mm thick spacer between support and lattice) similarly to hard metals in various applications where wear were as follows: LC=2500 mA, ET=25𝜇 s, and PD=25𝜇 m. resistance is of high importance. In this study, the influence The thickness of every layer was 25 𝜇 mand argon was of diamond particle and lattice cell sizes along with SPS applied as a protective gas during a process inside the printing parameters (temperature, pressure, and time) on tribological chamber. Advances in Tribology 3 Surface with maximum 1 mm diamond concentration 6 mm 1:3 C/L proportion Pulsed electric current 6 mm 1:2 C/L proportion Diamond Nickel coated particles 6 mm 1:1 C/L proportion 1 mm Metallic base FGL structure Figure 2: Schematic of material with FGL structure (during preparation before SPSing). Table 1: Composition of samples and SPS sintering conditions. Extent of Ni Diamond Sintering Duration of coating on D Sample Composition, wt. % grain size, pressure, sintering, particle, wt. % 𝜇 m MPa min [8] 1 29D-37Ni-34TA 40-50 56 50 9 2 29D-37Ni-34TA 6-12 56 100 6 3 29D-13Ni-58TA 40-50 30 50 9 4 29D-13Ni-58TA 20-30 30 100 6 5 26D-32Ni-24TA-18L 6-12 56 100 6 6 26D-11Ni-45TA-18L 20-30 30 100 6 7 100TA Not-included Not-included 50 10 ∘ ∘ ∘ -Preheating was performed at 260 C, 10 MPa and 6 Min. Sintering was done at 860 C for samples No. 1-6 and 1000 Cfor No. 7. - D=diamond, TA= Ti6Al4V titanium alloy powder, and L is added in case of Ti6Al4V lattice. - For No. 5 and No. 6, the average composition is stated due to gradient configuration. In order to assess the influence of diamond particles tracing this eeff ct as well. Samples No. 1-4 and No. 7 were size, nickel coating percentage (coating thickness), and lattice lattice-free, whereas lattice-included No. 5 and No. 6 had parameters, six samples were produced and are described similar composition (diamond grain size and nickel coating in Table 1. The samples were designed so that each pair thickness) and sintering conditions as No. 2 and No. 4, of samples can be compared to trace some specific effect. respectively (Table 1). In order to provide similar diamond Sample No. 2 had finer diamond particles size than No. particle content in composite materials of No. 5 and No. 6 1; heating/sintering time was shorter and the pressure was with Ti6Al4V lattice structure, the titanium powder content higher for sample No. 2 to reduce the risk of graphite was reduced during SPS (Table 1). formation. A similar effect can be studied with the help The sintering device that was used for consolidation of samples No. 3 and No. 4 while these two samples were of nickel-coated diamond particles with titanium lattice having thinner Ni coating on diamond particles that allows structures was made by FCT Systeme GmbH. Such a device More metal More diamond 4 Advances in Tribology Temperature ( C) Pressure (MPa) 10 25 Time (Min) 3 9 12 21 25 Figure 3: SPS conditions. can enable 1000 C/min heating rate and 100 MPa (32 kN force (EDS) Zeiss EVO MA15 with INCA Energy 350 X-ray micro- for 20 mm diameter mold) pressure for samples. The machine analyser. is installed into glovebox form MBRAUN Systeme GmbH to perform all powder charging and weighting operations in a 3. Results and Discussions nitrogen atmosphere (to avoid their oxidation). Exemplary SPS sequence for sample No. 1 is presented in Figure 3. The SEM images of nickel-coated diamond particles are shown main challenge regarding diamond particles incorporation in Figure 4. Nickel as a ductile transition metal was used as into composites with a metallic or ceramic matrix with high a binder metal is produced and tested metal matrix com- melting temperature is discussed in the literature as graphiti- posite. Use of nickel-coated diamond particles was preferable zation phenomena [9]. In order to prevent graphitization, the in comparison with pure polycrystalline diamond due to SPS temperature of diamond-containing samples was kept resulting higher homogeneity of distribution of diamond in below 900 C. However, some graphitization is not a major composite after sintering (Figure 5). Theoretically, Ti6Al4V problem [10]. In order to remove the possible presence of FGL solid continuous cellular structure during SPS process water or hydrogen, the powders were preheated to 260 C should facilitate uniform heating due to its better electrical and hold at 10 MPa pressure as it is shown in Figure 3. conductivity than the set of powder particles. It was tested It was experimentally proved that such a step is improving and it can be suggested to keep maximum temperature of consolidation and resulting in a lower level of graphitization SPS procedure less than 900 C to avoid melting and escape of and better properties (porosity, strength, and hardness) of nickel from the gaps of the mold [13]. However, the volume the final composite material. Most important differences of of added nickel powder must be adjusted according to the SPS rather than conventional powder metallurgy techniques requirements of the specific application. In this research, the were fast heating, fast cooling, and simultaneous application content of Ni was varying between 11 and 37 wt. % due to of pressure and temperature. The increase in temperature variation in the thickness of Ni coating around diamond to 860 C and pressure up to 50 MPa (Figure 3) enabled particles. Higher Ni content provides better sintering and achieving high densification level ( > 98 %). densification for diamond particles. The combined impact-abrasive wear tribo-device [11] was In the SEM images (Figure 5) the dark diamond particles applied to test samples, with impact energy of 5.6 J and are visible. In Figures 5(a)–5(c) the difference of particles size frequency of impacts being 27.5 Hz provided by impact is well seen and the homogeneity of distribution is acceptable. generator (industrial hammer drill from Makita). The sample Sample No. 4 shown in Figure 5(d) has a less homogeneous was experiencing reciprocative movement and was pressed distribution of diamond particles but the appearance of the (by the dead-weight system) against the rotating wheel (made surface of the samples can be influenced by its preparation from WC-Co) with a force of 49 N; linear abrasion velocity (polishing) procedure before imaging. In addition, SEM was 1 m/s. Ottawa sand (same as used in ASTM G65 standard micrograph of lattice structure in sample No. 6 is shown in [12]) was serving as abrasive. The particle size was 0.2-0.3 mm Figure 6 after SPS and after IATD test. EDS color mapping and feeding rate was the same as used during ASTM G65 test. (Figure 6(c)) shows distribution of elements. A random The abrasive was supplied from a hopper through the pipe spectrum in tested area was chosen to define elements and nozzle into the contact region between wheel and sample. distribution. EDS elemental mapping results showed C 59.91 The duration of the test was 5 minutes corresponding to 300 %, Ti 28.28 %, Ni 7.32 %, Al 1.87 %, V 1.34 %, P 0.79 %, Sn 0.36 m of sliding distance. The surface of the samples was cleaned %, and Si 0.13 % (all results in weight percentage). by low angle incidence alumina particles jet aer ft SPSing for XRD diffraction pattern (Figure 7) of sample No. 1 shows SEM imaging. The observational study and characterization that material has diamond remained aer ft the SPS process. of samples were performed with the help of a Hitachi TM- There is also the minor presence of graphite that was either 1000 Scanning electron microscopy (SEM), 3D optical surface formed due to graphitization of diamond or appeared due to profiler (OSP) Contour GT-K0+ from Bruker, and X-ray sintering in a graphite mold. There is also the minor content diffraction(XRD)analyzerBrukerAXSD5005equippedwith of new phases formed during SPS as result of reacting of Cu-K𝛼 radiation. In addition, elements distribution in wear- initial components, namely, TiC and Al O ,that arefavorable 2 3 tested region was mapped by energy-dispersive spectroscopy for increasing of wear resistance of the composite material. Advances in Tribology 5 X 2000 30 G X 2000 30 G (a) Diamond-30 wt%Ni 20-30𝜇 m (b) Diamond-56 wt%Ni 6-12𝜇 m X 1000 100 G X 1000 100 G (c) Diamond-30 wt%Ni 40-50𝜇 m (d) Diamond-56 wt%Ni 40-50𝜇 m Figure 4: SEM micrographs of nickel-coated diamond particles. 3D OSP was applied for surface scanning of samples and 6 (26D-11Ni-45TA-18L, 20-30𝜇 m diamond grain size) has illustrated better resistance than No. 5 (26D-32Ni-24TA-18L, the profile of the area with a diameter of 17 mm around 6-12𝜇 m diamond grain size) among the samples with a lattice the tested area was obtained and analyzed by software to that is also shown in Figure 9. The best material (No. 1) had calculate the missing volume. In a photograph aer ft IATD the largest size of diamond particles and the thickest Ni layer test, 3D OSP results of sample No. 1 (top and perspective covering them. Material No. 3 (29D-13Ni-58TA, 40-50𝜇 m views) are given in Figure 8. In order to illustrate the severity diamond grain size) was also better due to larger diamond of thedamage, it is possibleto indicatethat themaximum particle size (if compared to No.4 (29D-13Ni-58TA, 20-30𝜇 m depth of the wear scar of sample No. 1 (29D-37Ni-34TA, 40- diamond grain size)). The same was valid for materials with 50𝜇 m diamond grain size) was≈300𝜇 m. The red region lattice when No. 6 was better than No. 5. It could be concluded shows intact zone while the blue zone was produced by that, in case of materials with lattice, the size of diamond sand particles passing between the wheel and sample during particles was more favorable than the thickness of Ni layer the IATD test. The results of wear rate measurements are since No. 5 had thicker layer than No. 6. shown in Figure 9. The IATD tests were repeated three times and results averaged. The best performer among the samples In aggressive impact-abrasive conditions applied during without lattice in impact-abrasive conditions was samples No. the current test, the soft material No. 7 composed of pure 1 and No. 2 (29D-37Ni-34TA, 6-12𝜇 m diamond grain size) (if titanium alloy had better resistance than No. 3 and No. missing volume of material is considered). The sample No. 4. However, materials No. 5 and No. 6 with lattice were 6 Advances in Tribology Diamond particles Matrix composition X 500 200 G X 500 200 G (a) Sample No. 1 (b) Sample No. 2 Diamond particles Matrix composition X 500 200 G X 500 200 G (c) Sample No. 3 (d) Sample No. 4 Figure 5: SEM micrograph of samples 1-4 aeft r SPSing and cleaning by alumina particles jet. having better wear resistance (than No. 7) in such impact- An important outcome of this study was to demonstrate abrasive conditions and it is expected that in conditions the effect of diamond particle size, the thickness of the Ni with lower intensity or less frequent occurrences of impacts coating, and the effect of metallic lattice structure on wear (typical for soft ground TBM applications) these materials resistance. This was proved by XRD and by SEM images will have significantly better wear resistance due to presence that diamond particles are present after the SPS process of diamond particles having extremely high resistance against and after wear testing (Figures 5, 6, and 7). Various shapes abrasion. of FGL structures can be produced by 3D printing and The comparison between material No. 2 and No. 5 and suitable SPS molds can be done in required shapes to develop between No. 4 and No. 6 provides the information about the required composite materials for industrial applications, i.e., effect of lattice on wear resistance. It is possible to conclude wear resistant parts for mining or soft ground tunnel boring that in case of fine (6-12 𝜇 m) diamond particles (No. 2 versus machines. No. 5) the addition of lattice is not favorable while in case of An advantage of FGL structure is that it is having more average (20-30𝜇 m) size of diamond particles the use of lattice metal in bottom of sample for better connection of com- (No. 4 versus No. 6) provides significant improvement of wear ponents (for example, insert to drag bit in TBM by bolting resistance of the composite material. The composite material [14]), better absorption of impact due to increased ductility with a large size of diamond particles should be investigated and gradual change of composition from prevailing very in future since it should provide the best resistance in impact- brittle diamond to prevailing ductile metal. High diamond abrasive conditions. 20 mm Sintered composition After SPS Advances in Tribology 7 Ti6Al4V lattice Remained diamond particles aer t ft est X 1000 100 G X 60 1 mm (a) Sample No. 6 (after SPSing and polishing) (b) Sample No. 6 (after SPSing, polishing and testing) CTi Al Ni X 1000 100 G (c) EDS color mapping of sample No. 6 Figure 6: SEM micrograph of sample No. 6 (a) after SPS and (b) after impact-abrasive test (c) EDS color mapping. content in top layer provides extreme hardness and higher from WC-Co cemented carbide and the lattice structure was wear resistant. modeled as a Ti6Al4V nonlinear finite element, respectively Consequently, combinability of SLM and SPS techniques (Figure 10). Samples diameter was set as 20 mm and thickness has enabled the creation of composites focused on the as 10 mm. The simulation performed for material without diversity of lattice structures, foreseeing all kind of possible lattice is shown in Figure 11. The second simulated sample future improvements in design and cost, by adjusting the having a 2×2×2 mm lattice scao ff ld is illustrated in Figure 12. fraction of desired phases to provide further improvement in During normal operation of the tribo-device (Figure 10(c)) hardness, fracture toughness, and corrosive behavior. [11] both rotation of the wheel (abrasive action) and impact between the sample and the wheel are provided but for current simulation, it was important to demonstrate the 4. Simulation Study extreme case with impacting only (Figure 10(a)). The position of impacting was also changed. During the normal operation, In order to analyze the performance of the lattice structure, an impact test has been simulated using SOLIDWORKS the contact spot is located in the center of the sample while during the simulation the center of the impact spot was design, ANSYS sow ft are, and AUTODYN solver. The rfi st sample was simulated as made from solid diamond while the located exactly at the bottom edge of the sample to provide second one was simulated as being made from a metallic the most unfavorable leading resulting in brittle chipping or fracturing. The wheel was impacting the sample with the lattice with diamond located between the elements of the lattice. The wheel of the device was simulated as being made energy of 1000 and 5000 Joules (corresponding to dieff rent Ti6Al4V lattice rods 8 Advances in Tribology A D E C F 30 40 50 60 70 80 90 2 Theta (deg) A: TiC D: Diamond B: Ti6Al4V E: !F / 2 3 C: Graphite F: Nickel Figure 7: An exemplary XRD diffraction pattern of sample No. 1 before wear testing. velocities of the wheel); the analysis time and other boundary (2) According to the results of wear testing in impact- conditions were equal for both tests (with or without lattice). abrasive conditions, either the larger size of the diamond The von Mises equivalent stress comparison between samples particle or thicker Ni coating of diamond is favorable due to without and with lattice structure is shown in Figures 11 and the reduction of the possibility of graphitization. The size of 12 for low and high impact energy. According to the results the diamond particles was found to have the strongest effect of both simulations (Figures 11 and 12), it is possible to say due to the harder removal of large diamond particles during that in case of low impact energy the maximum Von Mises the impact-abrasive process. stresses are approximately ten times higher in case of pure (3) Work on the creation of functionally graded lattice diamond material than in case of a composite material with (FGL) composites is still in progress. It was found that metallic lattice and diamond reinforcing particles. In case of diamond particles with larger size and thicker coating are high-energy impact, the pure diamond sample experienced more favorable for such materials. FGL structure provides fracturing and resulting extreme displacement of contact sur- the gradient change of metallic lattice extent from 0 % face up to≈3700𝜇 m, whilethe compositematerial had only to 100 % that improves the resistance of such materials plastic deformation. Nonlinear finite element high-velocity against impacts, allows predefining location of metallic phase contact modeling of sample’s edge is a proper approach to responsible for ductility, and can enable xfi ation of such evaluate energy absorption of materials [15]. The ability of materials by welding or by bolting that is impossible for impact energy absorption has grown up significantly with the ceramics or other ultrahard materials. addition of lattice structure as shown in Figure 13. (4) Finite element analysis was applied to illustrate bene- tfi s of lattice structure. Lattice-included material showed bet- ter response (stress, deformation, impact energy absorption, 5. Conclusions and the possibility of plastic deformation) than plain hard material against impact. The present study is an attempt to introduce a new approach toward the production of wear resistant materials by the Data Availability combination of selective laser melting and spark plasma sintering. The current work seeks to address the following The materials, techniques, mac hines, references, and simu- results. lation data used to support the findings of this study are (1) The combined approach for the production of com- included within the article. posite materials by the 3D printing of lattice (SLM) and sintering/consolidating (SPS) to incorporate metal-coated Conflicts of Interest diamond particles has been described. It was concluded that it could be used for the production of soft ground TBM or The authors declare that there are no conflicts of inter- mining parts. est regarding the publication of this paper. The founding Intensity (Arb. units) Advances in Tribology 9 (10970.1,6959.3,-293) G G -50 -100 -150 -200 -250 -323 G 0 5000 10000 15000 18035 (a) (b) (c) Figure 8: Sample No. 1 after impact-abrasive test (a) photograph, (b) 3D OSP contour micrograph, and (c) 3D OSP perspective micrograph. 119.2 78.1 53.6 48.9 27.7 11.6 8.5 No.1 No.2 No.3 No.4 No.5 No.6 Ti6Al4V Samples 3 3 Figure 9: Volumetric wear rate (missing volume) of samples during impact-abrasive test measured by 3D OSP, ×10 𝜇 m . volumetric wear rate 29D-37Ni-34TA, 40-50 m diamond size 29D-37Ni-34TA, 6-12 m diamond size 29D-13Ni-58TA, 40-50 m diamond size 29D-13Ni-58TA, 20-30 m diamond size 26D-32Ni-24TA-18L, 6-12 m diamond size 26D-32Ni-24TA-18L, 20-30 m diamond size Von Mises Stress (Pa) Von Mises Stress (Pa) Impact velocity direction 10 Advances in Tribology Abrasion-impact wheel Lattice-included sample (a) (b) Sand flow Impact impeller direction Wheel Sample (c) Figure 10: (a) Simulation mechanism, only horizontal impact motion, (b) schematic of wheel and lattice-included sample, and (c) position of sample and wheel in tribo-device laboratory [11]. 8.2e11 Max 1.89e12 Max 7.61e11 1.75e12 7.03e11 1.62e12 6.44e11 1.48e12 5.86e11 1.35e12 5.27e11 1.21e12 4.68e11 1.08e12 4.1e11 9.44e11 3.51e11 8.09e11 2.93e11 6.74e11 2.34e11 5.4e11 1.76e11 4.05e11 1.17e11 2.71e11 5.86e10 1.36e11 2.3e7 Min 1.3e9 Min 0 0.01 0.02 (m) 0 0.01 0.02 (m) 0.005 0.015 0.005 0.015 (a) (b) Figure 11: Stresses resulting from impact simulation of pure diamond sample without lattice: (a) 1000 J and (b) 5000 J impact energy. First contact point Von Mises Stress (Pa) Displacement (G) Von Mises Stress (Pa) Displacement (G) Advances in Tribology 11 9.95e10 Max 2.08e12 Max 9.24e10 1.93e12 8.53e10 1.79e12 7.82e10 1.64e12 7.11e10 1.49e12 6.4e10 1.34e12 5.69e10 1.19e12 4.98e10 1.04e12 4.26e10 8.93e11 3.55e10 7.44e11 2.84e10 5.95e11 2.13e10 4.46e11 1.42e10 2.98e11 7.11e9 1.49e11 0 Min 0 Min 0 0.01 0.02 (m) 0.02 (m) 0 0.01 0.005 0.015 0.005 0.015 (a) (b) Figure 12: Stresses resulting from impact simulation of sample consisting of diamond and Ti6Al4V lattice structure: (a) 1000 J and (b) 5000 Jimpactenergy. 385 Max 3732 Max 82.5 799.6 533.1 27.5 266.5 0 Min 0 Min 0 5e + 003 1e + 004 (um) 0 5e + 003 1e + 004 (um) 2.5e + 003 7.5e + 003 2.5e + 003 7.5e + 003 (a) (b) Figure 13: Displacement of 1000 J impact simulation of samples: (a) pure diamond and (b) diamond and Ti6Al4V lattice. sponsors had no role in the design of the study; in the project administration, and supervision,; Lauri Kollo was collection, analyses, or interpretation of data; in the writing responsible for experiments, validation, and supervision. of the manuscript; and in the decision to publish the results. Acknowledgments Authors’ Contributions The authors would like to thank Heinar Vagistr om ¨ for the Ramin Rahmani was responsible for methodology, experi- help with surface cleaning of samples via alumina nanopar- ments, software analysis, writing, investigation, and visual- ticles, Rainer Traksmaa for the help with preparation of ization,; Maksim Antonov was responsible for review, editing, XRD measurements, and Mart Viljus for the help with 12 Advances in Tribology EDS mapping. This research was supported by the Estonian [15] P. Qiao, M. Yang, and F. Bobaru, “Impact mechanics and high-energy absorbing materials: Review,” Journal of Aerospace Ministry of Higher Education and Research under Projects ¨ Engineering,vol.21, no. 4,pp. 235–248, 2008. (IUT19-29 and ETAG18012) and TTU base finance project (B56 and SS427). References [1] G.Li,L. 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Raymont, “Advanced lightweight 316L stainless steel cellular lattice struc- tures fabricated via selective laser melting,” Materials and Corrosion, vol.55, pp.533–541,2014. [6] Z.A.Munir, U.Anselmi-Tamburini, and M. Ohyanagi, “eTh effect of electric field and pressure on the synthesis and con- solidation of materials: a review of the spark plasma sintering method,” Journal of Materials Science, vol.41,no.3,pp.763–777, [7] A.Balbo and D.Sciti,“Spark plasma sintering and hot press- ing of ZrB -MoSi ultra-high-temperature ceramics,” Materials 2 2 Science and Engineering A,vol.475, no.1-2,pp.108–112, 2008. [8] http://www.vanmoppes.ch/en/. [9] L.Jaworska, M. Szutkowska, P.Klimczyk etal., “Oxidation, graphitization and thermal resistance of PCD materials with the various bonding phases of up to 800 C,” International Journal of Refractory Metals and Hard Materials, vol.45, pp.109–116,2014. [10] W.Z.Shao,V.V.Ivanov,L.Zhen,Y.S.Cui,andY.Wang,“A study on graphitization of diamond in copper-diamond composite materials,” Materials Letters, vol.58, no. 1-2,pp. 146–149, 2004. [11] M. Antonov, R. Veinthal, D.-L. Yung, D. Katuˇsin, and I. Hussainova, “Mapping of impact-abrasive wear performance of WC-Co cemented carbides,” Wear, vol. 332-333, pp. 971–978, [12] “ASTM G65-04 standard test method for measuring abrasion using the dry sand/rubber wheel apparatus, annual book of ASTM standards,” 2004, https://compass.astm.org/Standards/ HISTORICAL/G65-04.htm. [13] T. Borkar and R. Banerjee, “Influence of spark plasma sintering (SPS) processing parameters on microstructure and mechanical properties of nickel,” Materials Science and Engineering: A Structural Materials: Properties, Microstructure and Processing, vol. 618, pp. 176–181, 2014. [14] T. Camus and S. Moubarak, “Maintenance robotics in TBM tunnelling,” in Proceedings of the 32nd International Symposium on Automation and Robotics in Construction and Mining: Connected to the Future, ISARC 2015,Finland, June 2015. 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Wear Resistance of (Diamond-Ni)-Ti6Al4V Gradient Materials Prepared by Combined Selective Laser Melting and Spark Plasma Sintering Techniques

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Hindawi Advances in Tribology Volume 2019, Article ID 5415897, 12 pages https://doi.org/10.1155/2019/5415897 Research Article Wear Resistance of (Diamond-Ni)-Ti6Al4V Gradient Materials Prepared by Combined Selective Laser Melting and Spark Plasma Sintering Techniques Ramin Rahmani , Maksim Antonov, and Lauri Kollo Tallinn University of Technology, Department of Mechanical and Industrial Engineering, Ehitajate tee 5, Tallinn 19086, Estonia Correspondence should be addressed to Ramin Rahmani; ramin.rahmaniahranjani@ttu.ee Received 11 September 2018; Revised 15 January 2019; Accepted 21 February 2019; Published 4 March 2019 Guest Editor: Mikael Olsson Copyright © 2019 Ramin Rahmani et al. is Th is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. An approach of sintering 3D metal printed lattices and diamond nickel-coated particles is proposed which can be used for the production of tunnel boring machine (TBM) cutters and mining equipment blades. Nickel-coated diamond particles are mixed with titanium powder and incorporated into a lightweight Ti6Al4V (3D printed) lattice with the help of spark plasma sintering (SPS) method. Eeff ct of Ti6Al4V lattices size, diamond particles size, and nickel coating layer thickness on wear resistance of composites is discussed. Functionally graded lattice (FGL) structures were produced by selective laser melting (SLM) method, representing an increasingly growing additive manufacturing engineering area introduced in material engineering. Impact-abrasive tribo-device (IATD), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and optical surface profiler (OSP) were used to characterize samples. An ab initio design of diamond-metal composite is based on the improvement of impact and abrasive wear resistance of Ti6Al4V by adding diamond particles and by applying of gradient lattice structure. eTh specimen with larger size of the diamond particle and thicker Ni coating has better wear resistance. In addition, ANSYS sow ft are simulations were done to analyze the eeff ct of the presence of 3D printed lattice via nonlinear finite element AUTODYN solver under impact test. Diamond-based gradient composite material producedby combinedSLM-SPSmethods can be appliedin applications where resistance against impact-abrasive wear is important. 1. Introduction copper and is usually used in grinding, polishing, and boring applications with sufficient cooling and without impacts. Ti6Al4V is the most applicable titanium alloy that has Diamond-containing metal matrix composites (MMCs) are been extremely used in biomedicine, osteology, aerospace, made with the help of chemical/physical vapor deposition marine, and additive manufacturing industries due to low (CVD/PVD) techniques and they are considered because of their high thermal conductivity and mechanical properties density and high mechanical properties. Adding≈6.75 wt. %of aluminum and ≈4.5 wt. % of vanadium to titanium [3]. Selective laser melting/sintering (SLM/SLS) is one of the new additive manufacturing techniques that is applied for makes it more applicable than pure titanium for corro- production of complex metal shapes, lattice structures, and sive wear resistance applications. Additive manufactured titanium alloys have motivated in deep in vivo corrosion rapid prototyping. The SLM ability of creation lightweight metallic cellular structure with different unit cell structure, research for recovering fractures of knee and hip bones [1]. strut, and pore sizes is used as a more efficient approach to It was recently demonstrated that 3D printed materials can provide improved tribological performance (lower coefficient antishock/impact energy absorption, lightweight aerospace structure, electrothermal conductivity, fracture toughness of friction, stable performance) in sliding conditions [2]. enhancement, and acoustic insulation application [4, 5]. Diamond is the hardest known, expensive, valuable, and ver- satile material for several industries. Synthetic polycrystalline AISI 316L stainless steel, Ti6Al4V titanium, and AlSi10Mg aluminum are three highly demanded metal lattice structures diamond (PCD) is cost-effective and advantageous powder that can be coated by metals like cobalt, nickel, titanium, and duetohighstrength toweight ratio. Nowadays, Spark 2 Advances in Tribology (A) (B) (C) (D) Figure 1: (A) Low, (B) average, (C) high volume fraction lattice structures, and (D) functionally graded lattice (FGL) structure used in the current research for samples No. 5 and No. 6 exclusively (diameter of lattice structures are 20 mm). plasma sintering (SPS) is extensively used method based on results is discussed. Samples were evaluated by combined pulsed DC electric current, high heating rates, programmable impact-abrasive tribo-device (IATD), volumetric wear was measured with the help of optical surface profiler (OSP) isostatic pressures, and short heating/cooling times [6]. The SPS process enables higher heating rate and sintering at lower analyzer, and composition of obtained materials was analyzed temperature to provide consolidation of wide range of mate- by X-ray diffraction (XRD) method. rial including metals, ceramics, and cermets that is its main important advantage over conventional powder metallurgy 2. Experimental Materials and Test Methods techniques [7]. Phase degradation is almost avoided during Cellular lattice structures were fabricated from argon atom- SPS sintering. Pulsed electric current passes through a mold ized Ti6Al4V Gd5 powders with size≤ 45𝜇 mand density and conductive particles of material to be sintered. Based on pressure and temperature, it is possible to use graphite or 4429 kg/m supplied by TLS Technik GmbH., Germany. tungsten mold and different diameter or thickness to realize Polycrystalline diamond powder with 30 and 56 wt. % of desired production shape that is a suitable option in several nickel coating and fractions of 6-12, 20-30, and 40-50𝜇 m industries. Temperature and pressure can be controlled by were supplied by Van Moppes & Sons Ltd., Swiss. Nickel pyrometer/thermocouple and upper/lower punch electrode coating usually contains 8-12 % of phosphorous and has pushing force in a vacuum chamber, respectively. To avoid 1455 C melting point (according to powder producer [8]). either graphitization of diamond or metal-coating surface Realizer SLM50 3D metal printer machine (construction oxidation, simultaneous increasing of temperature and pres- volume has a diameter of 70 mm and height of 80 mm, the sure is required in SPS. Shrinkage of the powder, limited grain thickness of layer was 20-50𝜇 m, and argon consumption growth, and near-theoretical densicfi ation take place during was 30 l/h) was used for preparing low, average, and high sintering. Cellular lattice structure should be adjusted to take volume fraction (VF) Ti6Al4V lattices and FGL structures into account the shrinkage. The SLM parameters like cell shown in Figure 1. Circumferential to longitudinal (C:L) cell size, lattice layer thickness, and laser current are important size proportion for cylindrical lattices (Figures 1(A)–1(C)) for the performance of the final material in test and field with a diameter of 20 mm and initial height of 18 mm (final conditions. SLM method has been applied in recent study height of ≈10-12 mm) were 1:2 due to significant vertical due to production possibility of metallic matrix in desired shrinkage during SPS process. It was decided that final C:L shapes (for example in SPS mold size or drag bits of tunneling and VF for lattice shown in Figures 1(A), 1(B), and 1(C) machines) and possibility to fill spaces between the lattice were 2:4 and 6 %, 1:2 and 15 %, and 0.75:1.5 and 24%, rods with hard material particles (for example, diamond, respectively. FGL structure (Figure 1(D)) is composed of three cubic boron nitride, or WC-Co). equal parts as a novel applicable lattice with 20 mm diameter The main aims of this research were focused to optimize and 18 mm initial height of sections with 1:1, 1:2, and 1:3 the composition of nickel-coated diamond and titanium C:L proportion. The schematic of the desired FGL structure powders for materials with and without titanium functionally obtained by combined SLM and SPS is given in Figure 2. graded lattice (FGL) structure to provide sufficient resistance From bottom to top, reduction of Ti6Al4V lattice structure against impacts and to improve abrasive wear resistance. FGL and enhancement of diamond particles before SPSing has is a progressive multilayer lattice structure with different been shown (Figure 2). Three most important parameters volume fraction section in longitudinal or circumferential for printing of Ti6Al4V lattices by SLM method were set directions so that the sections stand on each other or are as follows: (1) laser current (LC=3000 mA, 72 W power), embedded, respectively. Another novelty of current research (2) exposure time (ET=600 𝜇 s), and (3) point distance is the combination of SLM and SPS methods to produce a new (PD=1𝜇 m). In addition, these parameters for solid parts (for generation of multicomponent structures that can be used example, 0.1 mm thick spacer between support and lattice) similarly to hard metals in various applications where wear were as follows: LC=2500 mA, ET=25𝜇 s, and PD=25𝜇 m. resistance is of high importance. In this study, the influence The thickness of every layer was 25 𝜇 mand argon was of diamond particle and lattice cell sizes along with SPS applied as a protective gas during a process inside the printing parameters (temperature, pressure, and time) on tribological chamber. Advances in Tribology 3 Surface with maximum 1 mm diamond concentration 6 mm 1:3 C/L proportion Pulsed electric current 6 mm 1:2 C/L proportion Diamond Nickel coated particles 6 mm 1:1 C/L proportion 1 mm Metallic base FGL structure Figure 2: Schematic of material with FGL structure (during preparation before SPSing). Table 1: Composition of samples and SPS sintering conditions. Extent of Ni Diamond Sintering Duration of coating on D Sample Composition, wt. % grain size, pressure, sintering, particle, wt. % 𝜇 m MPa min [8] 1 29D-37Ni-34TA 40-50 56 50 9 2 29D-37Ni-34TA 6-12 56 100 6 3 29D-13Ni-58TA 40-50 30 50 9 4 29D-13Ni-58TA 20-30 30 100 6 5 26D-32Ni-24TA-18L 6-12 56 100 6 6 26D-11Ni-45TA-18L 20-30 30 100 6 7 100TA Not-included Not-included 50 10 ∘ ∘ ∘ -Preheating was performed at 260 C, 10 MPa and 6 Min. Sintering was done at 860 C for samples No. 1-6 and 1000 Cfor No. 7. - D=diamond, TA= Ti6Al4V titanium alloy powder, and L is added in case of Ti6Al4V lattice. - For No. 5 and No. 6, the average composition is stated due to gradient configuration. In order to assess the influence of diamond particles tracing this eeff ct as well. Samples No. 1-4 and No. 7 were size, nickel coating percentage (coating thickness), and lattice lattice-free, whereas lattice-included No. 5 and No. 6 had parameters, six samples were produced and are described similar composition (diamond grain size and nickel coating in Table 1. The samples were designed so that each pair thickness) and sintering conditions as No. 2 and No. 4, of samples can be compared to trace some specific effect. respectively (Table 1). In order to provide similar diamond Sample No. 2 had finer diamond particles size than No. particle content in composite materials of No. 5 and No. 6 1; heating/sintering time was shorter and the pressure was with Ti6Al4V lattice structure, the titanium powder content higher for sample No. 2 to reduce the risk of graphite was reduced during SPS (Table 1). formation. A similar effect can be studied with the help The sintering device that was used for consolidation of samples No. 3 and No. 4 while these two samples were of nickel-coated diamond particles with titanium lattice having thinner Ni coating on diamond particles that allows structures was made by FCT Systeme GmbH. Such a device More metal More diamond 4 Advances in Tribology Temperature ( C) Pressure (MPa) 10 25 Time (Min) 3 9 12 21 25 Figure 3: SPS conditions. can enable 1000 C/min heating rate and 100 MPa (32 kN force (EDS) Zeiss EVO MA15 with INCA Energy 350 X-ray micro- for 20 mm diameter mold) pressure for samples. The machine analyser. is installed into glovebox form MBRAUN Systeme GmbH to perform all powder charging and weighting operations in a 3. Results and Discussions nitrogen atmosphere (to avoid their oxidation). Exemplary SPS sequence for sample No. 1 is presented in Figure 3. The SEM images of nickel-coated diamond particles are shown main challenge regarding diamond particles incorporation in Figure 4. Nickel as a ductile transition metal was used as into composites with a metallic or ceramic matrix with high a binder metal is produced and tested metal matrix com- melting temperature is discussed in the literature as graphiti- posite. Use of nickel-coated diamond particles was preferable zation phenomena [9]. In order to prevent graphitization, the in comparison with pure polycrystalline diamond due to SPS temperature of diamond-containing samples was kept resulting higher homogeneity of distribution of diamond in below 900 C. However, some graphitization is not a major composite after sintering (Figure 5). Theoretically, Ti6Al4V problem [10]. In order to remove the possible presence of FGL solid continuous cellular structure during SPS process water or hydrogen, the powders were preheated to 260 C should facilitate uniform heating due to its better electrical and hold at 10 MPa pressure as it is shown in Figure 3. conductivity than the set of powder particles. It was tested It was experimentally proved that such a step is improving and it can be suggested to keep maximum temperature of consolidation and resulting in a lower level of graphitization SPS procedure less than 900 C to avoid melting and escape of and better properties (porosity, strength, and hardness) of nickel from the gaps of the mold [13]. However, the volume the final composite material. Most important differences of of added nickel powder must be adjusted according to the SPS rather than conventional powder metallurgy techniques requirements of the specific application. In this research, the were fast heating, fast cooling, and simultaneous application content of Ni was varying between 11 and 37 wt. % due to of pressure and temperature. The increase in temperature variation in the thickness of Ni coating around diamond to 860 C and pressure up to 50 MPa (Figure 3) enabled particles. Higher Ni content provides better sintering and achieving high densification level ( > 98 %). densification for diamond particles. The combined impact-abrasive wear tribo-device [11] was In the SEM images (Figure 5) the dark diamond particles applied to test samples, with impact energy of 5.6 J and are visible. In Figures 5(a)–5(c) the difference of particles size frequency of impacts being 27.5 Hz provided by impact is well seen and the homogeneity of distribution is acceptable. generator (industrial hammer drill from Makita). The sample Sample No. 4 shown in Figure 5(d) has a less homogeneous was experiencing reciprocative movement and was pressed distribution of diamond particles but the appearance of the (by the dead-weight system) against the rotating wheel (made surface of the samples can be influenced by its preparation from WC-Co) with a force of 49 N; linear abrasion velocity (polishing) procedure before imaging. In addition, SEM was 1 m/s. Ottawa sand (same as used in ASTM G65 standard micrograph of lattice structure in sample No. 6 is shown in [12]) was serving as abrasive. The particle size was 0.2-0.3 mm Figure 6 after SPS and after IATD test. EDS color mapping and feeding rate was the same as used during ASTM G65 test. (Figure 6(c)) shows distribution of elements. A random The abrasive was supplied from a hopper through the pipe spectrum in tested area was chosen to define elements and nozzle into the contact region between wheel and sample. distribution. EDS elemental mapping results showed C 59.91 The duration of the test was 5 minutes corresponding to 300 %, Ti 28.28 %, Ni 7.32 %, Al 1.87 %, V 1.34 %, P 0.79 %, Sn 0.36 m of sliding distance. The surface of the samples was cleaned %, and Si 0.13 % (all results in weight percentage). by low angle incidence alumina particles jet aer ft SPSing for XRD diffraction pattern (Figure 7) of sample No. 1 shows SEM imaging. The observational study and characterization that material has diamond remained aer ft the SPS process. of samples were performed with the help of a Hitachi TM- There is also the minor presence of graphite that was either 1000 Scanning electron microscopy (SEM), 3D optical surface formed due to graphitization of diamond or appeared due to profiler (OSP) Contour GT-K0+ from Bruker, and X-ray sintering in a graphite mold. There is also the minor content diffraction(XRD)analyzerBrukerAXSD5005equippedwith of new phases formed during SPS as result of reacting of Cu-K𝛼 radiation. In addition, elements distribution in wear- initial components, namely, TiC and Al O ,that arefavorable 2 3 tested region was mapped by energy-dispersive spectroscopy for increasing of wear resistance of the composite material. Advances in Tribology 5 X 2000 30 G X 2000 30 G (a) Diamond-30 wt%Ni 20-30𝜇 m (b) Diamond-56 wt%Ni 6-12𝜇 m X 1000 100 G X 1000 100 G (c) Diamond-30 wt%Ni 40-50𝜇 m (d) Diamond-56 wt%Ni 40-50𝜇 m Figure 4: SEM micrographs of nickel-coated diamond particles. 3D OSP was applied for surface scanning of samples and 6 (26D-11Ni-45TA-18L, 20-30𝜇 m diamond grain size) has illustrated better resistance than No. 5 (26D-32Ni-24TA-18L, the profile of the area with a diameter of 17 mm around 6-12𝜇 m diamond grain size) among the samples with a lattice the tested area was obtained and analyzed by software to that is also shown in Figure 9. The best material (No. 1) had calculate the missing volume. In a photograph aer ft IATD the largest size of diamond particles and the thickest Ni layer test, 3D OSP results of sample No. 1 (top and perspective covering them. Material No. 3 (29D-13Ni-58TA, 40-50𝜇 m views) are given in Figure 8. In order to illustrate the severity diamond grain size) was also better due to larger diamond of thedamage, it is possibleto indicatethat themaximum particle size (if compared to No.4 (29D-13Ni-58TA, 20-30𝜇 m depth of the wear scar of sample No. 1 (29D-37Ni-34TA, 40- diamond grain size)). The same was valid for materials with 50𝜇 m diamond grain size) was≈300𝜇 m. The red region lattice when No. 6 was better than No. 5. It could be concluded shows intact zone while the blue zone was produced by that, in case of materials with lattice, the size of diamond sand particles passing between the wheel and sample during particles was more favorable than the thickness of Ni layer the IATD test. The results of wear rate measurements are since No. 5 had thicker layer than No. 6. shown in Figure 9. The IATD tests were repeated three times and results averaged. The best performer among the samples In aggressive impact-abrasive conditions applied during without lattice in impact-abrasive conditions was samples No. the current test, the soft material No. 7 composed of pure 1 and No. 2 (29D-37Ni-34TA, 6-12𝜇 m diamond grain size) (if titanium alloy had better resistance than No. 3 and No. missing volume of material is considered). The sample No. 4. However, materials No. 5 and No. 6 with lattice were 6 Advances in Tribology Diamond particles Matrix composition X 500 200 G X 500 200 G (a) Sample No. 1 (b) Sample No. 2 Diamond particles Matrix composition X 500 200 G X 500 200 G (c) Sample No. 3 (d) Sample No. 4 Figure 5: SEM micrograph of samples 1-4 aeft r SPSing and cleaning by alumina particles jet. having better wear resistance (than No. 7) in such impact- An important outcome of this study was to demonstrate abrasive conditions and it is expected that in conditions the effect of diamond particle size, the thickness of the Ni with lower intensity or less frequent occurrences of impacts coating, and the effect of metallic lattice structure on wear (typical for soft ground TBM applications) these materials resistance. This was proved by XRD and by SEM images will have significantly better wear resistance due to presence that diamond particles are present after the SPS process of diamond particles having extremely high resistance against and after wear testing (Figures 5, 6, and 7). Various shapes abrasion. of FGL structures can be produced by 3D printing and The comparison between material No. 2 and No. 5 and suitable SPS molds can be done in required shapes to develop between No. 4 and No. 6 provides the information about the required composite materials for industrial applications, i.e., effect of lattice on wear resistance. It is possible to conclude wear resistant parts for mining or soft ground tunnel boring that in case of fine (6-12 𝜇 m) diamond particles (No. 2 versus machines. No. 5) the addition of lattice is not favorable while in case of An advantage of FGL structure is that it is having more average (20-30𝜇 m) size of diamond particles the use of lattice metal in bottom of sample for better connection of com- (No. 4 versus No. 6) provides significant improvement of wear ponents (for example, insert to drag bit in TBM by bolting resistance of the composite material. The composite material [14]), better absorption of impact due to increased ductility with a large size of diamond particles should be investigated and gradual change of composition from prevailing very in future since it should provide the best resistance in impact- brittle diamond to prevailing ductile metal. High diamond abrasive conditions. 20 mm Sintered composition After SPS Advances in Tribology 7 Ti6Al4V lattice Remained diamond particles aer t ft est X 1000 100 G X 60 1 mm (a) Sample No. 6 (after SPSing and polishing) (b) Sample No. 6 (after SPSing, polishing and testing) CTi Al Ni X 1000 100 G (c) EDS color mapping of sample No. 6 Figure 6: SEM micrograph of sample No. 6 (a) after SPS and (b) after impact-abrasive test (c) EDS color mapping. content in top layer provides extreme hardness and higher from WC-Co cemented carbide and the lattice structure was wear resistant. modeled as a Ti6Al4V nonlinear finite element, respectively Consequently, combinability of SLM and SPS techniques (Figure 10). Samples diameter was set as 20 mm and thickness has enabled the creation of composites focused on the as 10 mm. The simulation performed for material without diversity of lattice structures, foreseeing all kind of possible lattice is shown in Figure 11. The second simulated sample future improvements in design and cost, by adjusting the having a 2×2×2 mm lattice scao ff ld is illustrated in Figure 12. fraction of desired phases to provide further improvement in During normal operation of the tribo-device (Figure 10(c)) hardness, fracture toughness, and corrosive behavior. [11] both rotation of the wheel (abrasive action) and impact between the sample and the wheel are provided but for current simulation, it was important to demonstrate the 4. Simulation Study extreme case with impacting only (Figure 10(a)). The position of impacting was also changed. During the normal operation, In order to analyze the performance of the lattice structure, an impact test has been simulated using SOLIDWORKS the contact spot is located in the center of the sample while during the simulation the center of the impact spot was design, ANSYS sow ft are, and AUTODYN solver. The rfi st sample was simulated as made from solid diamond while the located exactly at the bottom edge of the sample to provide second one was simulated as being made from a metallic the most unfavorable leading resulting in brittle chipping or fracturing. The wheel was impacting the sample with the lattice with diamond located between the elements of the lattice. The wheel of the device was simulated as being made energy of 1000 and 5000 Joules (corresponding to dieff rent Ti6Al4V lattice rods 8 Advances in Tribology A D E C F 30 40 50 60 70 80 90 2 Theta (deg) A: TiC D: Diamond B: Ti6Al4V E: !F / 2 3 C: Graphite F: Nickel Figure 7: An exemplary XRD diffraction pattern of sample No. 1 before wear testing. velocities of the wheel); the analysis time and other boundary (2) According to the results of wear testing in impact- conditions were equal for both tests (with or without lattice). abrasive conditions, either the larger size of the diamond The von Mises equivalent stress comparison between samples particle or thicker Ni coating of diamond is favorable due to without and with lattice structure is shown in Figures 11 and the reduction of the possibility of graphitization. The size of 12 for low and high impact energy. According to the results the diamond particles was found to have the strongest effect of both simulations (Figures 11 and 12), it is possible to say due to the harder removal of large diamond particles during that in case of low impact energy the maximum Von Mises the impact-abrasive process. stresses are approximately ten times higher in case of pure (3) Work on the creation of functionally graded lattice diamond material than in case of a composite material with (FGL) composites is still in progress. It was found that metallic lattice and diamond reinforcing particles. In case of diamond particles with larger size and thicker coating are high-energy impact, the pure diamond sample experienced more favorable for such materials. FGL structure provides fracturing and resulting extreme displacement of contact sur- the gradient change of metallic lattice extent from 0 % face up to≈3700𝜇 m, whilethe compositematerial had only to 100 % that improves the resistance of such materials plastic deformation. Nonlinear finite element high-velocity against impacts, allows predefining location of metallic phase contact modeling of sample’s edge is a proper approach to responsible for ductility, and can enable xfi ation of such evaluate energy absorption of materials [15]. The ability of materials by welding or by bolting that is impossible for impact energy absorption has grown up significantly with the ceramics or other ultrahard materials. addition of lattice structure as shown in Figure 13. (4) Finite element analysis was applied to illustrate bene- tfi s of lattice structure. Lattice-included material showed bet- ter response (stress, deformation, impact energy absorption, 5. Conclusions and the possibility of plastic deformation) than plain hard material against impact. The present study is an attempt to introduce a new approach toward the production of wear resistant materials by the Data Availability combination of selective laser melting and spark plasma sintering. The current work seeks to address the following The materials, techniques, mac hines, references, and simu- results. lation data used to support the findings of this study are (1) The combined approach for the production of com- included within the article. posite materials by the 3D printing of lattice (SLM) and sintering/consolidating (SPS) to incorporate metal-coated Conflicts of Interest diamond particles has been described. It was concluded that it could be used for the production of soft ground TBM or The authors declare that there are no conflicts of inter- mining parts. est regarding the publication of this paper. The founding Intensity (Arb. units) Advances in Tribology 9 (10970.1,6959.3,-293) G G -50 -100 -150 -200 -250 -323 G 0 5000 10000 15000 18035 (a) (b) (c) Figure 8: Sample No. 1 after impact-abrasive test (a) photograph, (b) 3D OSP contour micrograph, and (c) 3D OSP perspective micrograph. 119.2 78.1 53.6 48.9 27.7 11.6 8.5 No.1 No.2 No.3 No.4 No.5 No.6 Ti6Al4V Samples 3 3 Figure 9: Volumetric wear rate (missing volume) of samples during impact-abrasive test measured by 3D OSP, ×10 𝜇 m . volumetric wear rate 29D-37Ni-34TA, 40-50 m diamond size 29D-37Ni-34TA, 6-12 m diamond size 29D-13Ni-58TA, 40-50 m diamond size 29D-13Ni-58TA, 20-30 m diamond size 26D-32Ni-24TA-18L, 6-12 m diamond size 26D-32Ni-24TA-18L, 20-30 m diamond size Von Mises Stress (Pa) Von Mises Stress (Pa) Impact velocity direction 10 Advances in Tribology Abrasion-impact wheel Lattice-included sample (a) (b) Sand flow Impact impeller direction Wheel Sample (c) Figure 10: (a) Simulation mechanism, only horizontal impact motion, (b) schematic of wheel and lattice-included sample, and (c) position of sample and wheel in tribo-device laboratory [11]. 8.2e11 Max 1.89e12 Max 7.61e11 1.75e12 7.03e11 1.62e12 6.44e11 1.48e12 5.86e11 1.35e12 5.27e11 1.21e12 4.68e11 1.08e12 4.1e11 9.44e11 3.51e11 8.09e11 2.93e11 6.74e11 2.34e11 5.4e11 1.76e11 4.05e11 1.17e11 2.71e11 5.86e10 1.36e11 2.3e7 Min 1.3e9 Min 0 0.01 0.02 (m) 0 0.01 0.02 (m) 0.005 0.015 0.005 0.015 (a) (b) Figure 11: Stresses resulting from impact simulation of pure diamond sample without lattice: (a) 1000 J and (b) 5000 J impact energy. First contact point Von Mises Stress (Pa) Displacement (G) Von Mises Stress (Pa) Displacement (G) Advances in Tribology 11 9.95e10 Max 2.08e12 Max 9.24e10 1.93e12 8.53e10 1.79e12 7.82e10 1.64e12 7.11e10 1.49e12 6.4e10 1.34e12 5.69e10 1.19e12 4.98e10 1.04e12 4.26e10 8.93e11 3.55e10 7.44e11 2.84e10 5.95e11 2.13e10 4.46e11 1.42e10 2.98e11 7.11e9 1.49e11 0 Min 0 Min 0 0.01 0.02 (m) 0.02 (m) 0 0.01 0.005 0.015 0.005 0.015 (a) (b) Figure 12: Stresses resulting from impact simulation of sample consisting of diamond and Ti6Al4V lattice structure: (a) 1000 J and (b) 5000 Jimpactenergy. 385 Max 3732 Max 82.5 799.6 533.1 27.5 266.5 0 Min 0 Min 0 5e + 003 1e + 004 (um) 0 5e + 003 1e + 004 (um) 2.5e + 003 7.5e + 003 2.5e + 003 7.5e + 003 (a) (b) Figure 13: Displacement of 1000 J impact simulation of samples: (a) pure diamond and (b) diamond and Ti6Al4V lattice. sponsors had no role in the design of the study; in the project administration, and supervision,; Lauri Kollo was collection, analyses, or interpretation of data; in the writing responsible for experiments, validation, and supervision. of the manuscript; and in the decision to publish the results. Acknowledgments Authors’ Contributions The authors would like to thank Heinar Vagistr om ¨ for the Ramin Rahmani was responsible for methodology, experi- help with surface cleaning of samples via alumina nanopar- ments, software analysis, writing, investigation, and visual- ticles, Rainer Traksmaa for the help with preparation of ization,; Maksim Antonov was responsible for review, editing, XRD measurements, and Mart Viljus for the help with 12 Advances in Tribology EDS mapping. This research was supported by the Estonian [15] P. Qiao, M. Yang, and F. Bobaru, “Impact mechanics and high-energy absorbing materials: Review,” Journal of Aerospace Ministry of Higher Education and Research under Projects ¨ Engineering,vol.21, no. 4,pp. 235–248, 2008. (IUT19-29 and ETAG18012) and TTU base finance project (B56 and SS427). References [1] G.Li,L. 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