Microstructure And Friction/Corrosion Behavior Of Ti₃Al And Ti₂AlNb Coatings Prepared By Laser In-Situ Synthesis

Ti-Al intermetallic compounds?IMCs?have the characteristics of low density,high melting point and specific strength and specific elastic modulus,excellent high temperature strength and stiffness,excellent creep resistance and high temperature oxidation resistance,which make them as competitive lightweight structural materials in the aviation,marine,petrochemical and automotive industries.By cladding the Ti-Al?IMCs?coating on the Ti-6Al-4V substrate surface with good toughness and plasticity.It can overcome the insufficient oxidation resistance,tribological resistance,and corrosion resistance of titanium alloys,and can also solve the shortcomings of poor plasticity at room temperature and poor resistance at low temperature.Ti,Al,and Nb powders are uniformly mixed,and Ti₃Al??₂?coating and Ti₂AlNb?O?coating are synthesized in-situ by laser cladding on the surface of the Ti-6Al-4V substrate.The phase formation process in the coating,and the effect of this microstructure on the mechanical properties and high temperature tribology and the corrosion behavior in 3.5 wt.%NaCl solution are investigated.The main findings are as follows:The Ti₃Al??₂?phase and?-Ti???phase are formed in the coating after laser cladding with the proportion of Ti-35Al?at.%?.During the solidification,the?₂phase forms a coarse primary dendrite,and the?phase is a rapidly cooled acicular martensite structure,which grows as the primary dendrite.The ratio between the primary dendritic?₂ phase and acicular martensitic?phase is approximately 6:4.?₂phase is a hard and brittle phase.but?phase is softer and serves as a medium to connect the?₂ phase.The two phases in the coating are reasonable.which provides appropriate toughness and high hardness?680 HV?for the coating;After laser scanning of the powder with the proportion of Ti-25Al-17Nb?at.%?,the coating consisting mainly of O phase and small amount of?₂ phase is formed on the Ti-6Al-4V surface.The formation process is that O phase continuously precipitate from the?₂ phase.The first precipitated O phase continuously grows to form a slab-like structure,and the later precipitated O phase forms a needle-like structure due to the rapid cooling of the coating.In addition,some untransformed?₂ phase remained in the coating.The O phase in the coating has good plastic deformation ability.Friction test of Ti₃Al coating at 25⁵00?.The results show that the average friction coefficient between the coating and the substrate is basically in the range of0.22 to 0.37.In addition,due to the high hardness and high-temperature oxidation resistance of the?₂ phase,the wear rate of the Ti₃Al coating is 1.89-1.34 times lower than that of the Ti-6Al-4V substrate over a wide temperature range of 25°C to 500°C.The ratio between the?₂ phase and the?phase in the coating is reasonable??₂:?=6:4?,so the coating has a certain strength and toughness during the friction process,and the coating does not undergo brittle peeling;The Ti₂AlNb coating is subjected to friction experiments at 25⁸00?.The results show that when the coating is friction at 25⁴00?,fatigue peeling occurs under the effect of periodic contact stress,and the peeling abrasive debris participates in the friction process to form three-body wear.As the temperature increases,the wear debris particles decrease and the wear process weakens.The coating strength decreases when friction is performed at 600?,and the minimum shear strength decreases when friction is performed.Therefore,the coating has deeper furrows and is more severely worn.When rubbing at 800?,the coating undergoes obvious softening and creeping deformation.The continuous and smooth TiO₂ film formed on the coating surface has good lubricating effect,and the coating wear is greatly reduced.When the Ti₃Al coating is immersed in a 3.5 wt.%NaCl solution,the TiO₂ and Al₂O₃ formed on the coating surface which have a protective effect on the dissolution of the coating,making the charge transfer resistance R_ct(9.908×10¹¹?cm²)high,that is 3×10⁸ times to the substrate.This layer of oxide film formed on the coating surface can be regarded as a passivation film,which has a good resistance to solution corrosion.Moreover,the distribution of TiO₂ and Al₂O₃ passivation films is not continuous,and the?phase self-corrosion potential of the passivation film edges is low.It is dissolved during the immersion process,forming a corrosion channel of the turtle crack.Cl^-penetrates into the coating along the crack and continuously dissolves,which accelerates the corrosion process in the later period of immersion;When Ti₂AlNb coating is immersed in 3.5 wt.%NaCl solution,passivation films of Nb₂O₅,TiO₂ and Al₂O₃ are formed on the coating surface.All three oxides have good pitting resistance in NaCl solution.The in-situ laser synthesized coatings are basically O phases.During the immersion process,the potentials in the coating areas are basically the same.The passivation film is uniformly dissolved by Cl^-in the late immersion period,and no intergranular corrosion occurs.The coating has strong corrosion resistance.Both Ti₃Al and Ti₂AlNb IMCs composite coatings can be synthesized by laser in situ reaction,and both have better resistance to high temperature oxidation and wear and corrosion than the substrate.The hardness and compressive strength of Ti₃Al composite coating are better than that of Ti₂AlNb composite coating,but the plastic toughness,high temperature oxidation resistance,wear resistance and corrosion resistance of Ti₂AlNb composite coating are better than those of Ti₃Al composite coating.