Wear-Resisting Titanium Matrix Composite Coatings Fabricated On TC4 Substrate By TIG Cladding With Flux Cored Wire

Titanium（Ti）alloys are restricted to be used in wear environment due to low hardness and inferior wear resistance.By surfacing high-hardness and wear-resisting coating on Ti alloy via cladding technology,the hardness and wear resistance of Ti alloy can be effectively improved while the plasticity and toughness are basically unchanged.Moreover,the shortages of the widely-used powder cladding method,such as low material utilization and fabrication efficiency,can be solved by wire-feed cladding method.In this study,Ti-TiB₂ cored wire containing TiB₂,Al60V40,Ti6Al4V mixed powders and Ti-B₄C cored wire containing B₄C powders were employed as cladding materials to fabricate TiB reinforced and（TiB₂+TiC+TiB）reinforced TMCs coatings on Ti6Al4V substrate by wire-feed TIG cladding.The as-fabricated coatings could effectively improve the hardness and wear resistance of Ti6Al4V alloy.The melting characteristics and reaction mechanisms of the wires were analyzed.The fabrication process,microstructure characteristics and formation mechanisms of the coatings were analyzed.The wear and anti-wear mechanisms of coatings were discussed.Adopting the combined process of ball milling,powder loading,tube sealing and drawing,（37）1.4mm Ti-TiB₂ cored wires with different TiB₂ content（5.9-17.6 wt.%）were successfully fabricated using TiB₂,Al60V40 and Ti6Al4V powders as raw materials.The analysis on wire melting characteristic showed that,as the Ti-TiB₂cored wire wire touched the arc,the first-melted Al60V40 powders would react with other powders via a series of liquid-solid chemical reactions,and then all the powders along with pure Ti sheath were melted by the arc,forming nonuniform molten droplets.Due to insufficient element diffusion caused by transient wire-arc interacton,the center region of molten droplet was rich in Al,V element and contained lagging-melted TiB₂ particles.The lagging melt of TiB₂ was mainly attributed to its high melting point.Besides,the enrichment of Al,V in center region would impede TiB₂from dissolving into Ti.The TIG cladding process of TiB/Ti composite coating was studied and showed that,with increasing the distance of wire and tungsten electrode（DWE）,the transfer mode of the wire changed from interrupted bridging transfer（IBT）（0-4 mm）to slag column bridging transfer（SCBT）（4-5 mm）before continuous contact transfer（CCT）（>5 mm）,accompanied by the improvement of arc voltage stability.CCT was bad for the melt of wire;SCBT was liable to transform to CCT when being affected because of its narrow DWE;A relatively continuous and smooth clad layer could be obtained under IBT if a large DWE（3-4 mm）was used.With the increase of cladding current（110-140 A）under IBT mode with DWE=3-4 mm,the transfer frequency of molten droplet was increased while the“balling”effect of melt was alleviated,resulting in the improvement in the continuity of coating.Besides,as the cladding current was increased,the uniformity of TiB whiskers in coating was improved,but the content of TiB reduced for the enhancement of dilution ratio.As wire feeding speed（WFS）was increased(0.8-1.2 m·min^-1),the transfer frequency of molten droplet was accelerated while the dilution rate of coating was reduced,so that the content of TiB was increased.But as WFS reached 1.4 m·min^-1,the surface quality of coating got worse,since the wire was not fully melted by arc.The optimized processing parameters were as follows:DWE=3-4 mm,cladding current=140 A,WFS=1.0m·min^-1.Microstructure analysis showed that the matrix of TiB/Ti composite coating was composed ofα/βlamellae.The width of lamella was very fine（0.5-1μm）,due to the rapid solidification of TIG cladding process.The reinforcements of TiB/Ti composite coating consisted of rod-like primary TiB whiskers and needle-like eutectic TiB whiskers,formed by hypereutectic solidification process.With the increase of TiB content（10-30 vol.%）,the size and content of primary TiB whiskers increased,while the content of eutectic TiB whiskers decreased but their size was unchanged.Nanoindentation results illuminated that,the hardness and modulus was respectively28.2 GPa and 451.7 GPa for primary TiB whiskers,and 6.5 GPa and 160 GPa for eutectic region（eutectic TiB whiskers and coating matrix）,and both of them were higher than that of substrate（4.7 GPa and 140 GPa）.Moreover,the plastic deformation resistance of coating was also higher than that of substrate.Hardness testing showed that the hardness of TiB/Ti composite coating presented an increasing trend with the increase of TiB content,and when TiB content reached 30 vol.%,the hardness of coating was HRC 52.5,which was 70%higher than that of substrate（HRC 31.2）.The hardening mechanisms of coating were the load transfer strengthening and second phase strengthening of TiB whiskers and the grain refinement strengthening of coating matrix.The increase of TiB content improved the wear resistance of coating,and the wear volume of 30 vol.%TiB/Ti coating was only51.4%that of substrate.It was found that the content of primary TiB whiskers influenced the wear behavior of coating.As normal load was increased from 5 N to20 N,the coating containing more primary TiB whiskers presented a larger reduction in wear rate and friction coefficient compared to the coating lacking in primary TiB whiskers.The main reason was that the primary TiB whiskers could block or trap wear debris,leading to the formation of tribo-oxidation layer on wear surface.The anti-wear mechanisms of coating were the support and protection effect of primary TiB whiskers,and the anti-friction and lubrication of tribo-oxidation layer.The wear mechanism of coating was mainly abrasive wear.To further enhance the hardness and wear resistance of TMCs coating,（TiB₂+TiC+TiB）reinforced TMCs coating was fabricated by TIG cladding of Ti-B₄C cored wire.As the Ti-B₄C cored wire touched the arc,a periodic self-propagating combustion phenomenon of the wire was observed,which was triggered by the capillary action of melted Ti sheath into the inside B₄C powders.During combustion reaction,B₄C experienced decarbonization process in addition to being dissolved by Ti melt.The lattice constants of B₄C were decreased after decarbonization.Morover,it was revealed that the decarbonizing phenomenon was more serious for the B₄C particles in combustion front region.The as-deposited（TiB₂+TiC+TiB）reinforced TMCs coating consisted of two layers showing various microstructures,the upper layer was TiB₂-TiC ceramic formed by the self-propagating product of wire,while the lower layer was（TiB+TiC）/Ti composites transition layer formed due to dissolution-precipitation mechanism.In ceramic layer,a specific orientation relationship between adjacient TiB₂ and TiC grains was found as（111）_TiC//（0001）_TiBand（110）_TiC//（11 2（?）0）_TiB2,which was formed during solidification related to hetergeneous nucleation.Owing to the arc-assisted heating remarkably enhanced the reaction temperature of Ti/B₄C system,the microstructure of coating was dense.Hardness testing showed that the Vickers hardness of TiB₂-TiC ceramic layer was as high as 2140.The high hardness was ascribed to the dense microstructure of ceramic layer and the phase boundary toughening of TiB₂/TiC interface.Wear testing showed that the wear rate of the coating was only 12.7%of the substrate,and the wear mechanism was abrasive wear characterized by grain fracture.