Fabrication of Ti10Fe5Si5Cr3Nb Composite Coatings on Ti-6Al-4V Alloy using Laser Cladding Technique

Ti10Fe5Si5Cr3Nb composite coatings were fabricated using laser cladding technique. Laser power was varied from 800W to 1200W while the scanning speed was kept constant at 1.5 m/min. The microstructural analysis of the samples was conducted by using optical microscope, X-ray diffractometer and scanning electron microscope coupled with energy dispersive spectroscopy. Potentiodynamic polarization and diamond indenter were used to study the corrosion and microhardness properties of the alloy. The alloy exhibited a dendritic microstructure with Si-rich dispersed phase. Grain coarsening was observed to be dependent on coating depth. The microhardness was high at the top and decreased with coating depth. No improvement in corrosion resistance was noticed but the presence of Cr in the alloy promoted passivation.


Introduction
Titanium alloy (Ti-6Al-4V) continue to find considerable interest in automotive, aerospace and biomedical applications as a construction material for fabrication of structural components/ parts. This is due to its low density, high mechanical strength, biocompatibility and excellent corrosion resistance [1][2][3] . However, its poor surface hardness and wear properties limit its use in more demanding structural applications 4 . Therefore, surface modification remains key for improvement of surface properties of this alloy. These properties can be enhanced by various surface modification techniques such as laser cladding, physical and chemical vapour deposition, plasma spraying, carburizing and nitriding [5][6][7][8] . Among these technologies, laser cladding is considered the most effective since itproduces coatings with refined microstructures, good metallurgical bonding and improved mechanical properties 9,10 . The stability of these coatings make them enjoy a variety of use in applications where good structural integrity is most desirable. However, the high thermal gradients associated with laser cladding generates residual stresses which may compromise the structural soundness of the coatings during service. These stresses cause formation of microcracks which may serve as initiation sites for failure [11][12][13] .
The amount and nature of cracks formed may also be determined by the composition of the coating material. The presence of intermetallic phase forming elements such as silicon, aluminium, nickel, chromium etc. in the titanium matrix also reduces the ductility of the alloy [14][15][16] . Intermetallic phases have different thermal expansion coefficient from the matrix. The difference in thermal coefficient of expansion between the intermetallic phases and the matrix causes stresses to be generated during solidification. Since intermetallic compounds are more brittle than the matrix, they become initiation sites for propagation of cracks (cracks form or around the phases) 17 . These phases may also be un-wetted and weaken their bond with the matrix 18 . Therefore, the combination of high solidification rates and formation of intermetallic phases exarcebates formation of cracks. Titanium and aluminium silicides were reported to form when silicon was incorporated into a titanium alumnide alloy. The alloys with high Si content and fabricated at high laser scanning speeds were associated with high amount of intermetallic phases and cracks 19 . Dai et al. 20 reported similar results when Si was incorporated into Ti-Al alloy. The presence of Ti 5 Si 3 phase was found to improve the hardness of the Ti-Al coatings from 600 HV to 900 HV but formation of cracks could not be avoided at all silicon content.
To reduce generation of thermal stresses and improve the ductillity of the laser cladded coatings, variation of laser process parameters have proved critical to aid in prodduction of crack free coatings. However, Ti coatings with intermetallic phases inducers tend to be difficult to fabricate even when process parameters are varied 21,22 . Therefore, in situ substrate heating during laser has been adopted and was found to be effective in lowering thermal gradients and promote slow cooling rates. This method do not only eliminate formation of cracks but also induce grain growth and improve ductility of the coatings 23,24 . Crack free TiVCrAlSi coatings were successfully produced when Ti-6Al-4V substrate was heated to 450 o C during cladding. Slow solidification rates induced by the heating of the substrate promoted precipitation of silicide intermetallic phases but had no adverse effect on the quality of the coatings 25 . Tao et al . 6 also produced crack free TiNi/Ti 2 Ni composite coatings when the substrate was preheated. However, heating the substrate at temperatures *e-mail: doublen.malatji@gmail.com higher than 600 o C increased the diffusion of aluminium and titanium from the substrate into the coating. Even though many researchers report of variation of laser process parameters during synthesis of titanium alloys and coatings in literature, limited investigations have been conducted on in sutu heating of substrate during deposition of these materials. Few studies on synergetic effect of variation of laser process parameters and in situ heat treatment of the substrate during laser depostion are also scarce. Therefore, in this work, Ti10Fe5Si5Cr3Nb composite coatings were successfully produced on Ti-6Al-4V alloy by laser cladding at circumstance temperature of 450 o C. The microstructural, corrosion and hardness characteristics of the coatings were investigated.

Experimental
Pure commercial elemental powders of Ti, Fe, Si, Cr and Nb with a size range of between 45 to 90µm were mixed in a tubular mixer for 24 hours and used as precursor powder for production of Ti10Fe5Si5Cr3Nb alloy using laser metal deposition. An IPG Yb laser was used and the laser processing parameters are shown in Table 1. The alloy powder was deposited on a Ti-6Al-4V substrate and the argon carrier gas was used during deposition to reduce oxidation. The microstructural and phase characteristics of the coatings were examined using scanning electron microscope equipped with energy dispersive spectroscopy and x-ray diffractometer. The EDS mapping was used to study the distribution of the elements in the alloy. The microhardness of the coatings was measured using diamond indenter with dwell time of 10s and load of 500 gf. Minimum of 10 measurements were made on the cross section of the samples and averaged to obtain accurate values. Potentiodynamic polarization was used to characterize the corrosion performance of the samples in 0.5M/L H 2 SO 4 solution. The polarization measurements were carried from a potential of -1.5 to 1.5 V with a scanning rate of 0.01 V/s. Saturated calomel electrode was used as a reference and platinum served as a counter electrode. The HEA samples were used as working electrode with an exposed area of 1 cm 2 . Nova 1.8 was used to extrapolate the corrosion parameters obtained from the Tafel curves. The absence of these cracks may be due to slow cooling rates induced by heating the substrate during cladding. The heating of the substrate reduces the thermal gradient and minimizes residual stresses in the coatings 25,26 . However, the deformation of the deposits was more evident in higher laser power. High laser power is associated with high energy density and promotes increased melting of particles. However, the high temperature induced by high laser power changes melt pool flow dynamics and lead to structural deformation of the resulting deposits 27 .   and increment in laser power. The increment in laser power increases laser energy density. High laser power densities are associated with high temperatures in the melt pool which lead increased melting of particles and create inducive environment for grain growth 28 . Figure 3 reveals the SEM images that were taken from the top, middle and bottom part of the coatings. The microstructure is characterized by precipitated phase that is dispersed in a continuous matrix. The dispersion and size of the phase differ with the depth of the coating and laser power. The amount of the dispersed phase reduces with coating depth and laser power. However, the size of the dispersed phase shows an opposite behavior dispersion of the phase. Dissolution of the dispersed phase is also evident and it is subject to the same conditions as its growth. These results support the findings obtained from the optical microscope micrographs. According to EDS results shown in Table 1, the matrix (Y) is titanium based while the dispersed phase (X) is rich in silicon.

Microstructural characteristics
Distribution of elements in the coatings is shown in Figure 4 and Table 2. Uniform distribution of elements is evident in all the coatings as depicted in the EDS elemental maps. The maps also show that silicon distribution is higher in the dispersed phase than in the matrix.
The amount of all the elements is similar in all regions of the coating and no diffusion of Ti and Al from the substrate was observed. The uniform distribution of elements in the coating shows that proper mixing was achieved and that no segregation occurred during cladding. Figure 5 shows the XRD pattern of Ti10Fe5Si5Cr3Nb composite coating obtained using laser power of 800W. Phase analysis revealed the existence of β-Ti as a major phase while TiFe and Ti 3 Si 5 were identified as precipitate phases. These results are similar to those reported by other authors. Titanium matrix doped with Fe, Nb and Cr yielded alloys which were mainly characterized by β phase 29-32 .     of 1800µm for the same sample. These results correspond with the optical micrographs shown in Figure 1, where grain coarsening proved to be subject to coating depth. The reduction in solidification rate of the coating from the top to bottom region induces grain coarsening and reduces microhardness property 25 .

Electrochemical characteristics
Pontentiodynamic polarization of Ti-6Al-4V alloy, TA-800, TA-1000 and TA-1200 composite coatings are presented in Figure 7. The samples were tested in a solution containing 0.5M of sulphuric acid in room temperature. The figure reveals that Ti-6Al-4V alloy possessed the lowest corrosion potential (-483 mV) and current density (4.3 x 10 -6 A/cm 2 ) as compared to the composite coating prepared at different laser power. Despite the low corrosion potential displayed by the substrate, the difference is very low when it is compared to the potential shown by the coatings. Since the electrochemical theory states that materials with low current density are associated with better corrosion resistance, it can be concluded that the substrate possess enhanced anti-corrosive properties than the coatings 18 . The alloy had a wider passive region in a anodic branch than the composite coating showing that it has higher potential to form a protective film. However, current fluctuation was observed in this region and suggest that the protective layer formed is not stable. According to Nabhani et al. 33 , vanadium and its oxide phase decrease the resistance of this alloy to corrosion attack. Vanadium phase increase the number of vacancies in the oxide layer and allows more penetration of corrosive species. Corrosion potential and current density of -487 mV and 2.075 x 10 -5 A/cm 2 , respectively, were obtained for the alloy (TA-800) fabricated at laser power of 800W. The increase of the laser power from 800 W to 1000 W caused a negative shift in potential from -487 mV to -500 mV. The current density was also increased from 2.075 x 10 -5 A/cm 2 to 4.3 x 10 -6 A/cm 2 . However, further increase in laser power from 1000 W to 1200 W yielded results that were similar to those obtained in TA-800. TA-1200 had the highest corrosion potential (-484 mV) but the current density (3.94 x 10 -5 A/cm 2 ) was lower than that of TA-800. Even though TA-1200 had the highest corrosion potential than TA-800, a difference of 3 mV in potential is negligible. A clear evidence of passivation is noticeable on all of the curves of the samples. There is stable increase in current density in the anodic branch from a potential of around -462 mV to -284 mV for all the samples which shows that transformation from active to near passive state is achieved in this potential range. The samples re-enter an active state for a short potential range and then passivate again. The presence of Cr in the alloy can be attributed for the good passivation displayed. According to Jiang et al. 34 chromium forms a thin and compact film that hinder further interaction between the corrosive media with the surface of the alloy. The passive current density of TA-800 was also lower than that of TA-1000 and TA-1200. Therefore, from the results that were obtained, it can be deduced that TA-800 exhibited nobler corrosion characteristics than TA-1000 and TA-1200. However, no relationship between corrosion behavior of the samples with the laser power used to produce them could be established.

Conclusion
Ti composite coating was successfully produced using laser cladding on Ti-6Al-4V substrate. The heating of the substrate during cladding minimized residual stresses and enabled formation of crack free coating. Partially melted particles were observed at samples produced at laser power of 800W and disappeared with the rise in energy density. However, deformation in structure of the coating occurred as a result of increasing laser power. The predominant phase was determined to be β-Ti and precipitates of titanium silicide was found to be dispersed along the grain boundaries. The formation of the dispersed phase was also found to be subject to the laser power used. The difference in microstructural characteristics on the different regions of the coatings revealed that the solidification rate of the deposit decreases with its depth. At high laser power grain coarsening was induced and varied with coating depth. This corresponded with the hardness profiles that showed decreasing values with respect to coating depth. Despite this, the hardness values of the coating remained higher than those of the substrate. The average microhardness of the composite coating produced at laser power of 1200W (603 HV) was almost two times higher than that of Ti-6Al-4V alloy (333 HV). The alloy displayed better corrosion resistance than the composite coating. However, there was evidence that the protective film formed on the substrate was prone to pitting while the composite coating oxide was stable. Among the coating fabricated at different laser power, TA-800 exhibited improved electrochemical performance. Nevertheless, no relationship between effect of laser power and electrochemical performance of the coating could be established.