Yehua Jiang

Effect of laser power and heat treatment process on microstructure and property of multi-pass Ni based alloy laser cladding coating

Ni60CuMoW alloy power was clad on 45 steel surfaces using a synchronization powder feeding method by 6kW transverse-flow CO2 laser apparatus. The effect of laser power and heat treatment process on corrosion resistance of the cladding layer was investigated. The microstructure and mechanical property were analyzed by X-ray diffractometer (XRD), scanning electron microscope (SEM), energy depressive X-ray spectroscopy (EDX), microhardness meter and PS-268A electrochemical test equipment. The results show that the cladding layer is mainly composed of γ (Ni, Fe), solid solution (Ni, Cu), compounds Ni31Si12, Cr5B3, CrB, Ni3B, FeNi3, M23C6 (Cr23C6 or (Fe,Ni) 23C6) phase and a small amount of WC or W2C. With the increase of laser power, corrosion resistance and microhardness has been greatly improved. Compared with the untreated substrate, the maximum self-corrosion potential of single-pass layer at laser power 3.2 kW in 3.5% NaCl saturated solution increases by 136.2mV, and the lowest corrosion current density decreases by 2 orders of magnitude. The mean microhardness of treated samples raises by 5.17, 4.90 and 4.89 times, respectively. The corrosion potential of multi-pass layer increases by 437.6mV and corrosion current density decreases by one order of magnitude than that of single-pass layer sample. After temper 600°C heat treatment, the primary dendrite and block (or needle) eutectic in cladding coatings become more uniform, the maximum self-corrosion potential increases by 45.5mV and corrosion current density also decreases obviously.

Microstructure and high-temperature oxidation resistance of TiN/Ti 3 Al intermetallic matrix composite coatings on Ti6Al4V alloy surface by laser cladding

A high-temperature oxidation resistant TiN embedded in Ti3Al intermetallic matrix composite coating was fabricated on titanium alloy Ti6Al4V surface by 6kW transverse-flow CO2 laser apparatus. The composition, morphology and microstructure of the laser clad TiN/Ti3Al intermetallic matrix composite coating were characterized by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). In order to evaluate the high-temperature oxidation resistance of the composite coatings and the titanium alloy substrate, isothermal oxidation test was performed in a conventional high-temperature resistance furnace at 600°C and 800°C respectively. The result shows that the laser clad intermetallic composite coating has a rapidly solidified fine microstructure consisting of TiN primary phase (granular-like, flake-like, and dendrites), and uniformly distributed in the Ti3Al matrix. It indicates that a physical and chemical reaction between the Ti powder and AlN powder occurred completely under the laser irradiation. In addition, the microhardness of the TiN/Ti3Al intermetallic matrix composite coating is 844HV0.2, 3.4 times higher than that of the titanium alloy substrate. The high-temperature oxidation resistance test reveals that TiN/Ti3Al intermetallic matrix composite coating results in the better modification of high-temperature oxidation behavior than the titanium substrate. The excellent high-temperature oxidation resistance of the laser cladding layer is attributed to the formation of the reinforced phase TiN and Al2O3, TiO2 hybrid oxide. Therefore, the laser cladding TiN/Ti3Al intermetallic matrix composite coating is anticipated to be a promising oxidation resistance surface modification technique for Ti6Al4V alloy.