Fabrication of in-situ Ni/Ni3Ti-NiTi-Fe2Ti-βCr2Ti reinforced Ti-based composite coating using tungsten inert gas processing

Highlights: • Coating of Ti substrate with Ni/Ni₃Ti-NiTi-Fe₂Ti-βCr₂Ti phases during TIG cladding technique. • Evaluating the effects of heat input on the microstructure of prepared composite coatings. • Studying the influences of heat input on micro-hardness and electrochemical properties of coatings. • Analysis of residual stresses on prepared coating by Sin²ψ method. -- Abstract: This study is an attempt to improve the surface micro-hardness and corrosion resistance of pure Ti by fabricating in-situ Ni/Ni₃Ti-NiTi-Fe₂Ti-βCr₂Ti composite using TIG process and Inconel 718 filler alloy. Surface coating prepared by depositing of the filler rods on the surface of pure Ti substrate at various heat inputs ranging from 2.750 to 3.125 kJ/cm. Field emission scanning electron microscopy (FESEM) and X-ray diffraction spectroscopy (XRD) were carried out for detailed characterization of the elemental composition, surface morphology and phase analysis. X-ray diffraction via sin²ψ analysis and ASTM E384-HV were used to evaluate the residual stresses at the interface of prepared coating and micro-hardness of prepared samples, respectively. The corrosion behavior of the coatings was also investigated in NaCl 3.5 wt% electrolyte using potentiostat analysis. Results manifested that the Ni₃Ti dendritic phase, NiTi needle-like phase, Fe₂Ti and β-Cr₂Ti rode-like phases mainly formed at the heat inputs ranging from 2.750 and 3 kJ/cm, whereas the Ni₂Nb and Ni₈Nb phases mainly generated at the higher heat input equal to 3.125 kJ/cm and the creation of micro-crack and micro-segregation was occurred. By increasing in the heat input up to 3 kJ/cm, creating of unreacted Ti particles and increasing in the size of dendrite and rode-like particles have been observed in the fusion zone. All of the prepared samples have much higher hardness than that of the pure Ti (in the range of 2.3–5.3 times higher than Ti). In this condition, based on the polarization and electrochemical impedance spectroscopy (EIS) tests, the prepared sample at the heat input equal to 2.875 kJ/cm has the highest corrosion resistance. The results of residual stresses determination of the optimum sample exhibited the tensile residual stress equal to 215 ± 40 MPa at the centerline of fusion zone.