Y. ZHANG et al.: MICROSTRUCTURE EVOLUTION OF IN SITU COMPOSITE COATINGS FABRICATED ... 419–425 MICROSTRUCTURE EVOLUTION OF IN SITU COMPOSITE COATINGS FABRICATED BY LASER CLADDING WITH DIFFERENT POWERS RAZVOJ MIKROSTRUKTURE KOMPOZITNIH PREVLEK IZDELANIH Z RAZLI^NIMI ENERGIJAMI LASERSKEGA NATALJEVANJA Youfeng Zhang * , Guangyu Han, Shasha He, Wanwan Yang School of Materials Engineering, Shanghai University of Engineering Science, No.333 Longteng Rd., Shanghai 201620, China Prejem rokopisa – received: 2021-02-29; sprejem za objavo – accepted for publication: 2021-03-04 doi:10.17222/mit.2020.194 In situ reaction-synthesized TiB-reinforced titanium-matrix composite coatings were fabricated using the rapid, non-equilibrium synthesis technique of laser cladding. The Ti and B mixture was the original powders, while the Ti-matrix composite coatings enhanced with TiB were treated on a Ti-6Al-4V surface with different laser scan powers of 2.5 kW, 3.0 kW and 3.5 kW. The phase composition, microstructure evaluation, and microhardness of the cladding coatings were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and microhardness. The composite coatings mainly consist of black fishbone-shaped -Ti dendrites and white needle-like TiB phases. The microstructure evolution from the top to the bottom of the coatings was investigated. The TiB reinforcement dispersed homogeneously in the composite coatings and a fine microstructure was obtained in a sample fabricated with a laser power of 3.0 kW. The microhardness of the cladding coatings fabricated by different powders was over 2-fold greater than that of the Ti-6Al-4V titanium alloy substrate and achieved a maxi- mum average of 792.2 HV with the laser power of 3.0 kW. The microstructures and properties of the coatings were changed by adjusting of the laser cladding power. The effects of the laser scan power on the microstructure, hardness and friction and wear properties of the laser cladding coatings were investigated and discussed. Keywords: Laser cladding; Ti-6Al-4V alloy; Laser scanning power; Microstructure evolution; Composite coatings Avtorji pri~ujo~ega ~lanka so z in situ reakcijsko sintezo izdelali prevleke kompozitov na osnovi Ti, oja~anih s titanovim boridom (TiB). Kompozite so izdelali s pomo~jo sinteze hitrega neravnote`nega laserskega pretaljevanja. Povr{ino zlitine Ti-6Al-4V, na kateri je bila nane{ena me{anica originalnega Ti in B prahu, so skenirali oziroma pretaljevali z laserjem razli~nih mo~i (2,5 kW, 3,0 kW in 3,5 kW). Izdelane prevleke so po hitrem strjevanju in ohlajanju okarakterizirali z rentgensko difrakcijo (XRD), vrsti~nim elektronskim mikroskopom (SEM) in merilnikom mikrotrdote. Kompozitne prevleke so bile v glavnem sestavljene iz -Ti dendritov s strukturo ribjih kosti in bele igli~aste faze TiB. Ugotavljali so, kako je potekal razvoj mikrostruktur od povr{ine do spodnjega roba prevlek. Dobili so fino mikrostrukturo in homogeno disperzijo oja~itvene faze TiB po celotnem preseku prevleke na vzorcih izdelanih z mo~jo laserja 3,0 kW. Mikrotrdota prevlek, izdelanih z razmerjem 90 % Ti in 10 % B prahu, je bila ve~ kot 2-krat ve~ja od podlage iz zlitine Ti-6Al-4V. Maksimalno povpre~no trdoto prevleke 792,2 HV so dosegli pri mo~i laserja 3,0 kW. Mikrostrukture in lastnosti prevlek so se spreminjale s spreminjanjem mo~i (energije) laserskega snopa. Avtorji opisujejo vpliv skeniranja z laserjem razli~ne mo~i na nastalo mikrostrukturo izdelanih kompozitnih prevlek. Klju~ne besede: kompozitne prevleke, izdelava z laserskim pretaljevanjem, zlitina Ti-6Al-4V, skeniranje z mo~nim laserskim snopom, razvoj mikrostrukture 1 INTRODUCTION Titanium alloys are known as “space metals” because of their excellent high-temperature performance, 1–4 cor- rosion resistance and comprehensive mechanical proper- ties, which makes them popular in application fields with high requirements for workpiece surface, such as aero- space and other fields. 5–8 Due to the poor wear resistance and oxidation resistance, the safety and reliability of the structure are great influenced by titanium alloys, which limits its application in the industrial field as an impor- tant structural material. In order to improve the wear re- sistance of titanium alloys, laser cladding is often used as a surface-modification technology. The cladding material is applied on the surface of the substrate to form a sur- face-cladding layer, thus increasing the wear resistance. 9 In order to prepare high-quality in-situ reinforced com- posite coatings, TiB, TiC, 10 VC and SiC 11 were com- pared. Because of its good mechanical properties and stable thermal expansion, TiB has a modulus of elasticity of 550 GPa, a coefficient of thermal expansion of 8.6×10 –6 K –1 and a density of 4.51 g/cm 3 , 12 so TiB is the most ideal reinforcement. 13,14 Laser-cladding sur- face-modification technology can improve the surface properties of titanium alloys on the basis of saving mate- rials and reducing costs, which makes titanium alloys widely used. In previous studies, multifarious pre-placed powder systems of titanium-alloy coatings were pro- posed, such as Ti-B 4 C, 15 Ti-TiB 2 , 16,17 Ti-TiC, 18 but the in- vestigations on titanium alloys coating with pre-placed Materiali in tehnologije / Materials and technology 55 (2021) 3, 419–425 419 UDK 621.791.72:620.181:669.017.13:669.295 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 55(3)419(2021) *Corresponding author's e-mail: zhangyoufeng@sues.edu.cn (Youfeng Zhang) Ti-B powders fabricated by YAG laser system were lim- ited. 19,20 In this paper, the in situ TiB/Ti metal-matrix composite coatings in the presence of pre-placed Ti-B powders were fabricated by laser cladding with different laser scanning powers. The phase composition, microstructure evolution, hardness and friction and wear properties of the TiB/Ti composite coatings fabricated at different laser powers were studied. 2 EXPERIMENTAL PART The nominal composition of the Ti-6Al-4V alloy in w/% was Al, 6.5; V, 4.26; Fe, 0.22; N, 0.03; C, 0.07; O, 0.14; and the balance, Ti. The cylindrical specimens of Ti-6Al-4V alloy were cut to 50 mm × 10 mm. The raw materials were mixed powders of 90 w/% Ti and 10 w/% B. The purity of the Ti powder and B powder were 99.2 % and 99.9 %, respectively. The weighed powder was poured into a ball mill according to the mea- surement ratio, and the mixed powder was obtained after dry mixing with an agate ball for 2 hours. Before clad- ding, the oil stain and surface contamination were re- moved to improve the bonding effect between the coat- ing and the substrate. The mixed powder with the thickness of 0.4 mm is laid on the surface of the titanium alloy substrate. The laser-cladding process was per- formed by an IPG-YLS-5000W fiber laser with output power of 2.5 kW, 3.0 kW and 3.5 kW, a scanning speed of 5 mm/s, a spot diameter of 5.0 mm and a laser wave- length of (1075 ± 5) nm to make a single-track cladding layer. The energy-distribution mode of the laser is Gaussian. The laser processing parameters of the sam- ples are given in Table 1. Table 1: Powder composition and laser parameters of the samples Powders Laser powder /(kW) Scanning speed /(mm/s) 90%Ti+10%B 2.5 5 90%Ti+10%B 3.0 5 90%Ti+10%B 3.5 5 Samples of the laser-cladding coatings were cut, ground and then polished. The cross-sections of the sam- ples were etched in a solution of HF: HNO 3 (at the ratio of 1:2). The phase structures were measured using X-ray diffractometry (XRD) with Cu-K radiation (X’Pert PRO PANalytical). Microstructures and chemical com- positions of the cladding coatings were characterised us- ing scanning electron microscopy (SEM, Hitachi S-3400N) combined with energy-dispersive spectrometry (EDS). HXD-1000 tester was used to test the microhard- ness distribution on the cross section, the load was 500g, and the dwell time was 15 s. The friction-and-wear test was carried out by HT-600 friction-and-wear testing ma- chine. The counterpart discs were made of annealed 45# carbon steel, the roughness surface was polished with 400# SiC grit paper prior to the wear tests. The applied load was 50 N. The sliding speed was kept constant is 100 min –1 and held for 1 h. The weight loss during the wear test was measured using an electronic balance FA2004 with a resolution of ± 0.1 mg. 3 RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the composite coatings under different laser powers. It can be seen that the composite coating is composed of -Ti and TiB phases. The pre-placed powders in the coating and the substrate absorb a lot of heat energy and melt instantly to form the molten pool at high temperature when the high-energy laser beam hits the surface of the mixed powders. The Ti and B elements in the pre-placed coat- ing in situ reacted at high temperature and formed TiB. TiB is a kind of ceramic reinforcing phase with a high hardness, good wear resistance and corrosion resistance, which is beneficial to enhance the mechanical properties of a composite coating. The phase structure of the com- posite coating was not greatly affected by the different laser powers according to comparing the XRD patterns of the composite coating fabricated by different powers of 2.5 kW, 3.0 kW and 3.5 kW. Figure 2 shows the SEM micrographs of the cross-section of the composite coatings fabricated under different laser powers. It can be seen that the composite coating presents the morphology of a crescent after laser cladding because the laser was operated in Gaussian mode, showing that the laser energy density of the center area is higher than that of both sides areas. The irradia- tion energy of the laser beam of center is higher than that of the edges when the laser beam hits on the pre-placed coating surface, forming a temperature gradient in the horizontal direction and the surface tension gradient. The heat energy is transferred to the Ti alloy substrate by heat conduction, the substrate in the center area melts first due to the high energy of the laser beam and then the Y. ZHANG et al.: MICROSTRUCTURE EVOLUTION OF IN SITU COMPOSITE COATINGS FABRICATED ... 420 Materiali in tehnologije / Materials and technology 55 (2021) 3, 419–425 Figure 1: XRD patterns of the cladding composite coatings with dif- ferent laser powers molten pool rapidly cooled, which led to the morphology of crescent occurred . It can be seen in Figure 2a that the surface of the composite coating is flat and formed well with a thick- ness of 0.8–1.1 mm, and there is a clear bonding profile between the composite coating and the substrate at the laser power is 2.5 kW, indicating that they are metallur- gical bonding well. However, there are a small amounts of pores in the middle and a micro-crack on the top of the cladding coating. Figure 2b shows there is an obvi- ous white bonding band between the composite coating and the substrate at the laser power is 3.0 kW, which in- dicates that the metallurgical bonding is good. The sur- face of the composite cladding coating is flat and no cracks were observed, but pores exist. Most of the pores are distributed in the top and middle of the composite coating, and the thickness of the coating is about 1.1–1.3 mm. The gas in the molten pool is removed from the bottom to the top along with the convection of liquid metal during laser cladding process and the top of the molten pool contacts with the outside cold air first, re- sulting in that the bubbles in the top and middle of the molten pool cannot be removed in time, forming pores after cooling subsequently. As shown in Figure 2c, the composite coating formed well with the substrate by means of metallurgical bonding, but a large number of pores are scattered in the coating, and the coating thick- ness is about 1.1–1.8 mm at the laser power is 3.5 kW. Comparing the morphologies of coatings fabricated by different laser powers in Figure 2, the thickness of the composite coating increases gradually with the in- crease of the laser scanning power. This is because a part of the substrate will also be melted and enter the molten pool from the bottom of the molten pool with the con- vection of the liquid metal in the laser cladding process. Therefore, the laser energy density per unit area in- creases and the substrate melts more, resulting in the thickness of the composite coating becoming larger after cooling with the increase of the laser power. The microstructure of coating fabricated by laser power of 3.5 kW is the worst because the laser energy density is too high and the substrate melts more. The micrographs of the top of the composite coatings obtained at different laser powers are shown in Figure 3. It can be seen that the composite coating is mainly com- posed of both black rod-like and a small amount of white needle-like phase when the laser power is 2.5 kW. The composite coating is mainly composed of black fish- bone-like and white needle-like when the laser power is 3.0 kW. The former grows vertically, and the latter is evenly distributed in the coating. Compared with the coating prepared by 2.5 kW laser power, the number of white needle-like particles increased and the size de- creased significantly in the coating prepared by a laser power of 3.0 kW. A large number of black fishbone-like are distributed in the composite coating and some white needle-like particles are filled in it when the laser power is 3.5 kW. Comparing the microstructure of the three samples, it shows that the structure of the coating fabri- cated by laser power of 3.0 kW is more uniform than Y. ZHANG et al.: MICROSTRUCTURE EVOLUTION OF IN SITU COMPOSITE COATINGS FABRICATED ... Materiali in tehnologije / Materials and technology 55 (2021) 3, 419–425 421 Figure 3: SEM micrographs of the top of the cladding composite coatings: a) 2.5 kW, b) 3.0 kW and c) 3.5 kW Figure 2: SEM micrographs of the cross-sections of cladding composite coatings: a) 2.5 kW, b) 3.0 kW and c) 3.5 kW other samples fabricated by powers of 2.5 kW and 3.5 kW. The number of white needle-like phases exist and refine in the composite coating fabricated by a power of 3.0 kW. Figure 4 shows the SEM micrographs of the middle part of the laser cladding composite coatings with differ- ent laser powers. A large number of large-scale fish- bone-like grains appear in the middle of the composite coating when the laser power is 2.5 kW. The number of fishbone-like grains is obviously reduced and is refined when the laser power is 3.0 kW and 3.5 kW, because of the energy absorbed increased by molten pool with the increase of laser power, which led to more cooling time and refined grains. Figure 5 shows the SEM micrographs of the bonding area of the composite coatings with different laser pow- ers. The boundary of the bonding area of the composite coating and the substrate is clearly observed. It is straight in the coatings fabricated by a laser power of 2.5 kW and 3.5 kW and chaotic in coating of 3.0 kW. The grain grows and diffuses to the interior of the substrate in the composite coatings because the cooling effect of the Ti alloy substrate. This was attributed to the direction of the grain’s growth, depending on the direction of the heat transfer in the high-temperature molten pool. Therefore, there is not enough time for the grains growing in the bonding zone, and microstructure of the bonding zone is finer than that of the other regions. Therefore, the metal- lurgical bonding of the coating and substrate fabricated at 3.0 kW is well than other samples in the bottom of la- ser cladding coatings. The microhardness of the cross-sections of the TiB/Ti composite coatings prepared by different laser powers are given in Figure 6. It can be seen that the microhard- ness of the composite coating decreases along the direc- tion of distance from the surface, and then gradually sta- bilizes at about 350 HV. The average microhardnesses of the composite coatings are 688.02 HV, 792.17 HV and 776.87 HV, respectively. The average microhardness of the composite coating is higher than other samples when the laser power is 3.0 kW. It is consistent with the above microstructure. The microhardness of the composite coatings by laser cladding is higher than that of the tita- nium alloy, the maximum hardness of the synthesized TiB/Ti composites coating is 2.3-fold of the Ti-6Al-4V titanium alloy substrate. The specimen fabricated with laser power of 3.0 kW exhibited the highest hardness compared to the specimens of the others by 2.5 kW and 3.5 kW. In the composite coating, TiB has the highest hardness and the Ti substrate has the lowest hardness. The increased hardness is attributed to the reinforcement in composite coating. In this process, the composite coat- ing with smooth surface, fined microstructure and higher microhardness can be obtained when laser power is 3.0 kW. Generally speaking, the microhardness of coatings effects the wear resistance of materials, so the higher the Y. ZHANG et al.: MICROSTRUCTURE EVOLUTION OF IN SITU COMPOSITE COATINGS FABRICATED ... 422 Materiali in tehnologije / Materials and technology 55 (2021) 3, 419–425 Figure 4: SEM micrographs of the middle of the cladding composite coatings: a) 2.5 kW, b) 3.0 kW and c) 3.5 kW Figure 5: SEM micrographs of the bonding zone of the cladding composite coatings: a) 2.5 kW, b) 3.0 kW and c) 3.5 kW microhardness is beneficial for application as a wear-re- sistant material. In order to investigate the friction and wear proper- ties, the sample fabricated by laser power of 3.0 kW was chosen for wear test and compared with the substrate of Ti alloy. The friction coefficient of the tested coating and Ti alloy substrate as a function of time were measured and the results are shown in Figure 7.The classical me- chanics sliding friction formula is as follows: fF =⋅ N (1) where ( is the sliding friction (N), μ is the friction coef- ficient and F N is the positive pressure (N). Under the condition of ensuring that the positive pressure remains unchanged, the smaller the friction coefficient, the smaller the friction, and the stronger the friction resis- tance. As shown in Figure 7, the friction coefficient of the substrate is obviously higher than that of the cladding composite coating fabricated by laser power of 3.0 kW, and the average friction coefficient of substrate and coat- ing are about 0.84 and 0.70, respectively. Table 2 shows the result of the wear weight loss ratio tests made on coating fabricated with laser power of 3.0 kW and sub- strate. It can be seen that the wear weight loss ratio of the substrate is larger than that of the coating. The laser cladding surface exhibited good wear resistance and a Y. ZHANG et al.: MICROSTRUCTURE EVOLUTION OF IN SITU COMPOSITE COATINGS FABRICATED ... Materiali in tehnologije / Materials and technology 55 (2021) 3, 419–425 423 Figure 6: Microhardness of the laser cladding composite coatings Figure 8: SEM micrographs of the surface wear marks of substrate and cladding coating: a) substrate and b) coating Figure 7: Instantaneous friction curve of samples: a) substrate and b) coating lower wear weight loss ratio. In general, the wear resis- tance is proportional to the hardness of coatings; the presence of the TiB increases the hardness of composite coating. Both the friction coefficient and wear weight loss ratio of the composite coating decreased. Table 2: Wear weight loss ratio of substrate and cladding coating Samples Wear weight loss ratio Substrate 0.079 % Coating 0.072 % Figure 8 shows the SEM surface morphologies of the substrate and coating fabricated with laser power of 3.0 kW after the wear process. It can be observed that there are many deep fits obviously on the wear surface of the titanium alloy substrate with a large scale of peeling. On the wear surface of the cladding composite coating, there are some obvious furrows and bumps, and the furrows are small and uniform. The surface of the cladding layers with shallow wear traces is much smoother than that of substrate and there are granular grindings in the abrasive surface, which are scratched by rubbing the surface of the cladding material during the rubbing process. 4 CONCLUSIONS In summary, laser cladding surface modification was applied to a Ti-6Al-4V alloy using mixed powders of 90 w/% Ti and 10 w/% B. The effect of different laser cladding powers on the phase composition, micro- structure evaluation and wear properties of the cladding coatings was investigated. The coatings comprised black fishbone-shaped -Ti dendrites and white needle-like TiB particles when a laser scanning speed of 5 mm/s and a laser power of 2.5 kW, 3.0 kW and 3.5 kW were used. The cladding coating and the substrate combined metal- lurgically after the laser-cladding process, and no obvi- ous cracks found in the cladding coating fabricated at la- ser power of 3.0 kW. The black fishbone-shaped -Ti dendrites exist more in the top of coatings, the white nee- dle-like TiB particles structure gradually increased in the middle of coatings and the micro-needle particles mainly exist in the bottom of coatings. The TiB reinforcement dispersed homogeneously in the composite coatings fab- ricated with laser power of 3.0 kW and it is more uni- form than other samples fabricated by 2.5 kW and 3.5 kW. The microhardness of the cladding coatings fab- ricated with different powders was over two-times greater than that of the Ti-6Al-4V titanium alloy sub- strate and achieved a maximum average of 792.17 HV when laser power was 3.0 kW. The wear resistance of the composite coatings fabricated by laser power of 3.0 kW was improved obviously. The microstructures and me- chanical properties of the coatings can be controlled by adjusting the laser cladding power. High-performance in situ synthesized TiB/Ti composite coating can be ob- tained by the laser cladding process. Acknowledgment This work was supported by the Nature Science Foundation of China (No. 11604204). 5 REFERENCES 1 B. F. He, D. Y. Ma, F. Ma, K. W. Xu, Microstructures and wear prop- erties of TiC coating produced by laser cladding on Ti-6Al-4V with TiC and carbon nanotube mixed powders, Ferroelectrics, 547 (2019) 1, 217–225, doi:10.1080/00150193.2019.1592502 2 Y. J. Zhai, X. B. Liu, S. J. Qiao, M. D. Wang, X. L. Lu, Y. G. 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